aplicación de hidrotalcitas y sólidos mesoporosos ordenados como

UNIVERSIDAD DE CORDOBA
DEPARTAMENTO DE QUÍMICA INORGÁNICA E INGENIERÍA QUÍMICA
APLICACIÓN DE HIDROTALCITAS Y SÓLIDOS
MESOPOROSOS ORDENADOS COMO ADSORBENTES
DE HERBICIDAS
Memoria presentada por Dña. Rocío Otero
Izquierdo para aspirar al Grado de Doctor
con
mención
internacional
Universidad de Córdoba
Fdo. Rocío Otero Izquierdo
Córdoba 2015
por
la
TITULO: Aplicación de hidrotalcitas y sólidos mesoporosos ordenados como
adsorbentes de herbicidas
AUTOR: Rocío Otero Izquierdo
© Edita: Servicio de Publicaciones de la Universidad de Córdoba. 2015
Campus de Rabanales
Ctra. Nacional IV, Km. 396 A
14071 Córdoba
www.uco.es/publicaciones
[email protected]
TÍTULO DE LA TESIS: APLICACIÓN DE HIDROTALCITAS Y SÓLIDOS MESOPOROSOS
ORDENADOS COMO ADSORBENTES DE HERBICIDAS
DOCTORANDO/A: ROCÍO OTERO IZQUIERDO
INFORME RAZONADO DEL DIRECTOR Y DE LA CODIRECTORA DE LA TESIS
La Lcda. Rocío Otero Izquierdo ha llevado a cabo con éxito el desarrollo de la Tesis
Doctoral “Aplicación de hidrotalcitas y sólidos mesoporosos ordenados como
adsorbentes de herbicidas.”, bajo la dirección de los doctores abajo firmantes en los
laboratorios del Departamento de Química Inorgánica e Ingeniería Química. La
doctoranda ha experimentado una formación continua tanto en la práctica
experimental como en la discusión de los resultados obtenidos. Los objetivos
marcados en el Proyecto inicial de la Tesis Doctoral se han completado de manera
sobresaliente, obteniendo resultados relevantes que han dado lugar a la publicación
de cinco artículos científicos en revistas internacionales de prestigio (Q1) y diez
comunicaciones en congresos científicos.
Por todo ello, se autoriza la presentación de la tesis doctoral.
Córdoba,14 de Abril 2015
Firma del director
Fdo.: José María Fernández Rodríguez
Firma de la codirectora
Fdo : Mª Ángeles Ulibarri Cormenzana
D. José María Fernández Rodríguez, Catedrático de Escuelas Universitarias de Química
Inorgánica del Departamento de Química Inorgánica e Ingeniería Química de la
Universidad de Córdoba y Dña. Mª Ángeles Ulibarri Cormenzana, Catedrática de
Universidad de Química Inorgánica.
CERTIFICAN
Que el presente Trabajo de Investigación, titulado Aplicación de hidrotalcitas y sólidos
mesoporosos ordenados como adsorbentes de herbicidas y que constituye la
Memoria que presenta la Lda. Dña. Rocío Otero Izquierdo para aspirar al Grado de
Doctor por la Universidad de Córdoba en Ciencias Químicas, ha sido realizado en los
laboratorios del Departamento de Química Inorgánica e Ingeniería Química, bajo
nuestra dirección, y reúne las condiciones exigidas para ser presentado como Tesis
Doctoral.
Y para que conste, expedimos y firmamos el presente certificado en Córdoba, a 14 de
Abril de 2015.
Fdo.: José María Fernández Rodríguez
Fdo.: Mª Ángeles Ulibarri Cormenzana
Mediante la defensa de esta Memoria se pretende optar a la mención de Doctor
Europeo, habida cuenta de que la doctoranda reúne los requisitos exigidos por tal
mención, a saber:
1- Informes de dos doctores pertenecientes a Instituciones de Enseñanza
Superiores de otros países:
- Silvia Real. INIFTA, Universidad de la Plata, Argentina.
- Jeriffa De Clercq, Universidad de Gante, COMOC, Bélgica.
2- Un miembro del tribunal que ha de evaluar la Tesis pertenece a un centro de
Enseñanza Superior de otro país.
-Dr. Els de Canck, researcher of COMOC (Center for Ordered Materials,
Organometallics and Catalysis).
3- La defensa de parte de esta Memoria se realizará en la lengua oficial de otro
país europeo: inglés.
4- Estancia de cuatro meses en un centro de investigación de otro país europeo:
-Departamento de Química Inorgánica y Química Física de la Universidad de
Gante, Bélgica.
A Mi Familia.
AGRADECIMIENTOS
Poniendo punto y seguido a esta etapa de mi carrera y haciendo una
introspectiva tanto a mi trabajo como a mi vida, llego a la conclusión de que sin el
apoyo necesario cualquier meta que te propongas por pequeña que ésta sea,
puede ser una auténtica odisea. Por eso y porque es de bien nacido ser
agradecidos, estas palabras son para todas las personas que me han apoyado de
una manera u otra.
Quiero expresar mi más sincero agradecimiento a mis directores de esta
Tesis Doctoral, D. José María Fernández Rodríguez y Dª Mª Ángeles Ulibarri
Cormenzana por todo su tiempo dedicado, ayuda y conocimientos aportados para
la realización de este trabajo los cuales han sido fundamentales para mi formación
como investigadora.
Me gustaría agradecer a todos los miembros pertenecientes al grupo FQM
214. A las Doctoras Cristi e Ivana por todos los consejos y enseñanzas recibidas
durante estos años. A mis compañeras de laboratorio por todos los buenos ratos
que hemos pasado y en especial a Dikra que me inició en el campo de la
investigación y con la que pasé las mejores tardes aprendiendo síntesis mientras
intercambiábamos nuestras diferentes opiniones culturales.
Agradecer también a todos los Doctores, licenciados o colaboradores del
Departamento de Química Inorgánica por todo el compañerismo y apoyo recibido.
A todos mis compañeros becarios y contratados que han pasado o están pasando
por esta etapa y que me han ayudado siempre a resolver todas mis dudas y
problemas. También a todos los que me brindan su amistad día a día y
compartimos ratos de charla en los pasillos y laboratorios.
Al profesor Francisco Romero Salguero del Departamento de Química
Orgánica que sin tener ninguna obligación me ayudó como a cualquiera de sus
doctorandos.
Al Servicio Central de Apoyo a la Investigación (SCAI) de la Universidad de
Córdoba por toda su ayuda prestada para la realización de diversas técnicas
necesarias para el desarrollo de esta Tesis Doctoral.
Aprovecho la ocasión para hacer una especial mención a todas las
personas que hicieron que me sintiera como en casa durante mi estancia en
Bélgica. Al Profesor Pascal Van der Voort del Departamento de Química Inorgánica
(COMOC) de la Universidad de Gante por sus conocimientos, paciencia, motivación
y sobretodo enseñarme una manera de trabajar la cual hizo cambiar mi concepto
de investigación. A Dolores por ayudarme en todo desde ese primer día que pensé
que en lugar de aterrizar en Bélgica había llegado al Polo Norte. Hiciste mi estancia
más cómoda dentro y fuera del laboratorio. El día que me fui, sé que me deje una
amiga. A todos los componentes del Grupo COMOC con los cuales aprendí mucho y
pasé buenos momentos.
A los proyectos AGL2008-0403-C02-02/AGR y CTM2011-25325 del
Ministerio de Ciencia y Tecnología, mediante el grupo de investigación FQM-214,
así como una beca de Formación de Profesorado Univesritario (FPU) finaciada por
el Ministerio de Ciencia e Innovación.
A mis amigas de dentro y fuera de la Facultad por estar ahí cuando más os
he necesitado y que a su modo han sabido siempre ayudarme.
Todo esto nunca hubiera sido posible sin mi familia, fuente de apoyo
constante e incondicional en toda mi vida. A mi madre por darme siempre su mano
cuando más lo he necesitado, por estar siempre a mi lado, por ser mi confidente y
mi mejor amiga. Gracias por ese amor incondicional, que como ya sé, solo una
madre puede dar. A mi padre por ser mi ejemplo, por sus consejos y sobretodo por
mostrarme el mejor camino a seguir. Gracias por todo lo que me has dado e
inculcado en la vida. A mi hermana Marí por creer siempre en mí y darme dos de
las mejores alegrías de mi vida, Aitana y Paula, que siempre me han sacado una
sonrisa hasta en los malos momentos.
Finalmente a las dos personas más importantes, los pilares de mi vida,
Juan y Rocío. A Juan por su amor, cariño y comprensión a lo largo de estos años
que a veces han sido duros pero que hemos sabido superar sin ningún problema.
Gracias por todo el apoyo incondicional, por ser mi amigo leal y principalmente por
lo mejor que me ha pasado en la vida, Rocío, quien ha sido mi motivación para
culminar este Doctorado.
Para el mejor resultado obtenido en esta Tesis Doctoral, Rocío
Índice
CAPÍTULO I. HIPOTESIS Y OBJETIVOS ............................................................................... 1
CAPÍTULO II. INTRODUCCIÓN .......................................................................................... 9
1. GENERALIDADES.................................................................................................... 11
2. DINÁMICA DE LOS PLAGUICIDAS EN EL SUELO ..................................................... 13
2.1. Procesos de transferencia.............................................................................. 15
2.1.1. Adsorción-Desorción .............................................................................. 15
2.1.2. Otros procesos de transferencia ............................................................ 16
2.2 Procesos de transformación ........................................................................... 17
3. CONTAMINACIÓN DEL AGUA POR PLAGUICIDAS.................................................. 19
4. MEDIDAS DE ADSORCIÓN. ISOTERMAS ................................................................ 20
5. ADSORBENTES EMPLEADOS .................................................................................. 24
5.1 Hidrotalcitas .................................................................................................... 24
5.1.1 Métodos de síntesis................................................................................. 28
5.1.2. Incorporación de aniones en la estructura............................................. 31
5.1.3. Aplicaciones ............................................................................................ 32
5.2 Materiales porosos ......................................................................................... 35
5.2.1 Materiales híbridos.................................................................................. 42
5.2.1.1 Síntesis de materiales híbridos de sílice........................................... 43
5.2.2 PMOs ....................................................................................................... 44
5.2.2.1 Tipos de PMOs y sus síntesis ............................................................ 48
5.2.2.2 Aplicaciones de PMOs ...................................................................... 50
5.2.3 Mesoporosos ordenados de carbón ........................................................ 53
5.2.3.1 Tipos de síntesis ............................................................................... 54
5.2.3.2 Aplicaciones de las resinas fenólicas y carbones mesoporosos
ordenados .................................................................................................... 59
BIBLIOGRAFÍA ............................................................................................................ 62
CAPÍTULO III. MATERIALES Y MÉTODOS ........................................................................ 79
1.
MATERIALES ..................................................................................................... 81
1. 1 Síntesis de adsorbentes ................................................................................. 82
1.1.1 Hidrotalcita con carbonato en la interlámina.......................................... 82
1.1.2. Hidrotalcita con anión orgánico en la interlámina ................................. 82
1.1.3 Etileno y fenileno PMOs .......................................................................... 83
1.1.4 Resina fenólica y carbon mesoporosos ordenados ................................. 84
1.2 Pesticidas ........................................................................................................ 85
1.2.1 S-Metolacloro .......................................................................................... 85
1.2.2 Mecoprop-P ............................................................................................. 87
1.2.3 Nicosulfuron ............................................................................................ 88
1.2.4 Bentazona ................................................................................................ 88
2.MÉTODOS ............................................................................................................... 91
2.1 Espectroscopía de Absorción Atómica............................................................ 91
2.2 Difracción de Rayos X...................................................................................... 92
2.3 Microscopía Electrónica .................................................................................. 93
2.3.1 Microscopía Electrónica de Transmisión ................................................. 93
2.3.2 Microscopía Electrónica de Barrido......................................................... 93
2.4. Espectroscopía Fotoelectrónica de Rayos X (Xps) ......................................... 94
2.5 Espectroscopía Infrarroja con Transformada de Fourier ................................ 95
2.6 Espectroscopía Resonancia Magnética Nuclear ............................................. 95
2.7 Análisis Termogravimétrico (TGA) .................................................................. 96
2.8 Isotermas de Adsorción-Desorción de Nitrógeno........................................... 97
2.9 Potencial Zeta ............................................................................................... 100
2.10 Espectroscopía Ultravioleta Visible............................................................. 100
2.11 Tamaño de partícula ................................................................................... 101
CAPÍTULO IV. ADSORPTION OF NON-IONIC PESTICIDE S-METOLACHLOR ON LAYERED
DOUBLE
HYDROXIDES
INTERCALATED
WITH
DODECYLSULFATE
AND
TETRADECANEDIOATE ANIONS ................................................................................... 103
CAPÍTULO V. PESTICIDES ADSORPTION-DESORPTION ON Mg-Al MIXED OXIDES. KINETIC
MODELING, COMPETING FACTORS AND RECYCLABILITY. ........................................... 127
CAPÍTULO VI. ADSORPTION OF THE HERBICIDE S-METOLACHLOR ON PERIODIC
MESOPOROUS ORGANOSILICAS .................................................................................. 153
CAPÍTULO VII. MESOPOROUS PHENOLIC RESIN AND MESOPOROUS CARBON FOR THE
REMOVAL OF S-METOLACHLOR AND BENTAZON HERBICIDES ................................... 177
CAPÍTULO VIII. CONCLUSIONES GENERALES GENERAL CONCLUSIONS ....................... 205
CAPÍTULO IX. SUMMARY ............................................................................................. 219
CAPÍTULO X. ABREVIATURAS ....................................................................................... 229
CAPÍTULO XI. PUBLICACIONES E INDICIOS DE CALIDAD .............................................. 233
CAPÍTULO I. HIPOTESIS Y OBJETIVOS
Hipótesis y objetivos
La presente memoria de Tesis Doctoral se ha encuadrado en las líneas de
investigación que desarrolla actualmente el grupo FQM-214 de la Universidad de
Córdoba. Durante el desarrollo de esta memoria se han realizado colaboraciones con
el grupo FQM-346 del Departamento de Química Orgánica de la Universidad de
Córdoba, con el grupo AGR 268 del Instituto de Recursos Naturales y Agrobiología de
Sevilla, IRNASE-CSIC, así como una estancia de cuatro meses en el Departamento de
Química Inorgánica (COMOC) de la Universidad de Gante.
La contaminación del agua se define como la introducción por el hombre en el
ambiente acuático de elementos abióticos o bióticos que causen efectos dañinos o
tóxicos, perjudiquen los recursos vivos, constituyan un peligro para la salud humana,
obstaculicen las actividades marítimas, disminuyan la calidad del agua o los valores
estéticos y de recreación [FAO, 1992]. Uno de los contaminantes que afectan
principalmente a la calidad del agua son los pesticidas químicos, los cuales son usados
por el hombre para eliminación de plagas, enfermedades y malezas que afectan a los
cultivos agrícolas. El uso excesivo de estos compuestos químicos ha provocado un gran
problema en el medioambiente. Esta Tesis Doctoral trata de aportar conocimientos
sobre la utilización de materiales para la eliminación de algunos pesticidas presentes
en el agua.
Así, el objetivo principal del trabajo presentado en esta Memoria ha sido la
preparación de diversos materiales tales como hidrotalcitas, bien calcinadas o bien
modificadas con compuestos orgánicos de cadena larga, y sólidos mesoporosos
ordenados para su posterior utilización como adsorbentes de pesticidas. La elección de
los materiales se realizó en función del carácter iónico del pesticida así como del pH
del medio en el cual se encuentran los mismos. Por ello, a pHs bajos se trabajará con
compuestos mesoporosos mientras a pHs altos con hidrotalcitas.
En la bibliografía existen numerosos estudios de adsorción de pesticidas usando
hidrotalcitas con aniones inorgánicos en la interlámina. Sin embargo, pocos estudios
abordan el uso de organohidrotalcitas para la eliminación de contaminantes orgánicos.
En la bibliografía, el estudio de la relación entre la longitud de la cadena del anión
orgánico en la interlámina de la organohidrotalcita y la adsorción de pesticidas es muy
3
Capítulo I
limitado. Por ello, el primer objetivo ha sido la búsqueda de aniones orgánicos de
cadena larga que permitan introducir en la interlámina de la hidrotalcita distintas
especies químicas contaminates.
En el estudio de la adsorción de pesticidas, el procedimiento de adsorcióndesorción es de importancia capital. Es habitual encontrar un procedimiento que tal
vez sea muy oportuno para el estudio de liberación lenta de pesticidas en cultivos, ya
que intenta reproducir las condiciones de campo, de manera que modeliza bien la
desorción del pesticida en los suelos, pero no es útil para modelizar la interpretación
de la reversibilidad o no del proceso de desorción, en definitiva no permite conocer
claramente la naturaleza del proceso de adsorción-desorción. El procedimiento suele
consistir en la eliminación de una cierta cantidad del sobrenadante por la misma
cantidad de agua destilada, dejar en contacto 24 h y repitir varias veces la misma
experiencia. El aspecto más negativo es no poder interpretar adecuadamente los casos
en los que la isoterma de desorción se encuentra por debajo de la isoterma de
adsorción, a lo cual ciertos autores denominan “artefacto experimental”. Una
explicación podría ser la pérdida de adsorbente durante su manipulación. En esta
memoria uno de los objetivos más importantes ha sido la búsqueda de un nuevo
procedimiento de adsorción-desorción, en concreto para la rama de desorción, que
permita conocer mejor la naturaleza del proceso de adsorción–desorción y que sea
comparable con el procedimiento empleado en la adsorción, siendo útil para
modelizar la interpretación de la reversibilidad o no del proceso de desorción.
Un parámetro de gran transcendencia es la temperatura a la que se realiza el
proceso de adsorción, ya que de ella va a depender la cantidad adsorbida y estará en
consonancia con la naturaleza físico-química del proceso. Por esta razón, se realizarán
isotermas de adsorción a varias temperaturas en una cámara de temperatura
constante.
Otro aspecto a tener en cuenta es el estudio de la regeneración de los
adsorbentes y sus implicaciones medioambientales. La regeneración de la hidrotalcita
calcinada ha sido muy estudiada en los últimos años mediante calcinación a 500°C. Sin
embargo, no se encuentran demasiadas referencias del reciclado de órgano4
Hipótesis y objetivos
hidrotalcitas con disolventes orgánicos. En la presente Tesis Doctoral se propondrá la
utilización de disolventes orgánicos como método de regeneración, entre otros.
En relación a los objetivos concretos, se investigará la adsorción de un pesticida
no iónico, S-Metolacloro, en dos organohidrotalcitas diferentes (OHTs), intercaladas
con aniones dodecilsulfato (HT-DDS) una y con tetradecanodioato (HT-TDD) la otra
preparados usando el método de coprecipitación.
Uno de los objetivo será estudiar los cambios estructurales como consecuencia
de la introducción de aniones voluminosos tales como DDS y TDD, así como la
variación del espaciado interlaminar por efecto de la adsorción del pesticida SMetolacloro.
Se estudiará la cinética de adsorción e isotermas de adsorción a varias
temperaturas, así como, la regeneración del adsorbente con disolventes orgánicos con
el objetivo de analizar su reciclabilidad.
La descomposición de la hidrotalcita cuando es calcinada en torno a 500°C da
lugar a una mezcla de óxidos metálicos, que se caracteriza por elevadas superficies
específicas y una dispersión homgenea de los mismos. La mezcla de óxidos metálicos
puede rehidratarse y combinarse con aniones de la solución acuosa recuperando la
estructura original.
Se abordará la adsorción de un pesticida ionizable, Nicosulfuron, y un pesticida
iónico, Mecoprop-P, en una hidrotalcita de Mg-Al calcinada a 500⁰C (HT-500).
Se realizarán experimentos de adsorción-desorción en los que se contemple la
influencia de la presencia simultánea de aniones tales como carbonato, nitrato,
sulfato, cloruro o fosfato en la adsorción de los pesticidas, así como la extracción de
éstos del complejo soporte-pesticida mediante los aniones citados para comprobar su
eficacia como desorbentes.
El estudio se completará con el análisis del proceso de regeneración de la
hidrotalcita calcinada ya que diversos estudios ponen de manifiesto que se produce
una disminución de la capacidad de adsorción después de cada ciclo de adsorción5
Capítulo I
calcinación. Esta disminución podría estar justificada por los cambios en la morfología
de las partículas después de diversos ciclos, para cuya comprobación se utilizará
microscopía electrónica de trasmisión (TEM).
Materiales tales como las organosilices mesoporosas periódicas (PMOs) tienen
propiedades únicas para ser utilizadas como adsorbentes de moléculas orgánicas en
virtud de su gran área superficial y de una estrecha distribución de mesoporos, de
manera similar a sílices mesoporosas (PMS), así como de la naturaleza orgánica de sus
puentes que pueden interactuar con ellos. Por esta razón se sintetizarán dos
organosilices (etano-PMO y benceno-PMO) y una sílice (PMS), siguiendo un
procedimiento similar.
Tanto en el caso de las hidrotalcitas como de estos nuevos adsorbentes, se
procederá a la modelización de las cinéticas e isotermas de adsorción mediante
diversas ecuaciones matemáticas. Las ecuaciones de pseudo-primer y pseudo-segundo
orden y la ecuación de Morris-Wever para el ajuste de las cinéticas, y las ecuaciones de
Freundlich, Langmuir y Dubinin-Radushkevich para las isotermas de adsorción.
Otro aspecto a considerar es la determinación de la capacidad máxima de
adsorción de ambos soportes, procediéndose si fuera necesario a realizar adsorciones
sucesivas.
Finalmente, el carbón activo es el adsorbente universal empleado en la
eliminación de una amplia variedad de contaminantes. Tiene una gran ventaja como es
su bajo coste pero también tiene inconvenientes ya que al ser un material
microporoso no permite la adsorción de moléculas grandes y además el carbón en
polvo es más difícil de manipular. Los carbones mesoporosos ordenados tienen buenas
propiedades de adsorción por lo que podrían ser utilizados en sustitución de los
carbones activos comerciales.
Se examinará la adsorción de S-Metolacloro, en función del pH, en carbón
mesoporoso ordenado (MC) y en su material de partida, una resina mesoporosa
fenólica ordenada (MPR), que se sintetizarán siguiendo un método de “soft template”,
así como en un carbón comercial (CC) para poder dilucidar las ventajas e
6
Hipótesis y objetivos
inconvenientes de éste frente a los dos nuevos materiales, procediéndose a la
modelización de las cinéticas e isotermas de adsorción mediante diversas ecuaciones
matemáticas. Previamente se caracterizaran estos materiales mediante el análisis de
su área superficial, la distribución de tamaño de poro, la distribución de tamaño de
párticula y microscopía electrónica, entre otros.
En estos materiales se evaluará la regeneración mediante calcinación bajo
atmósfera de nitrógeno contemplando aspectos como eficiencia o pérdida de material
en cada ciclo de adsorción-desorción.
Otro pesticida objeto de estudio en estos materiales será el plaguicida de postemergencia denominado Bentazona.
7
CAPÍTULO II. INTRODUCCIÓN
Introducción
1. GENERALIDADES
Los plaguicidas según el Real Decreto núm. 3349/83 de 30 de noviembre de
1983 sobre “Reglamento técnico-sanitario para la fabricación, comercialización y
utilización de plaguicidas”, establece una serie de definiciones entre las que cabe
destacar:
-
Combatir los agentes nocivos para los vegetales y productos vegetales o
prevenir su acción.
-
Favorecer o regular la producción vegetal, con excepción de los nutrientes y los
destinados a la enmienda de suelos.
-
Conservar los productos vegetales, incluida la protección de las maderas.
-
Destruir los vegetales indeseables.
-
Destruir parte de los vegetales o prevenir un crecimiento indeseable de los
mismos.
-
Hacer inofensivos, destruir o prevenir la acción de otros organismos nocivos o
indeseables distintos de los que atacan a los vegetales.
Según su naturaleza química, pueden clasificarse en inorgánicos y orgánicos.
Los primeros, no plantean un problema importante desde el punto de vista de su
toxicidad y evolución en el suelo. Sin embargo, en lo que se refiere a los orgánicos, se
ha ido desarrollando una amplia gama de productos que plantean problemas de
evolución en el complejo sistema del suelo.
La toxicidad de estos pesticidas esta relacionada con la manipulación de los
compuestos, la toxicidad residual en alimentos y su evolución en el suelo. La
evaluación de la toxicidad se lleva a cabo mediante ensayos en el laboratorio en
animales de experimentación y su expresión cuantitativa se expresa mediante la dosis
letal media DL50 (cantidad de pesticida necesario para causar la muerte al 50% de los
individuos que componen el lote de ensayo). La toxicidad residual se refiere a los
residuos en los alimentos y a la contaminación del medio biológico. Para evitar
intoxicaciones, los Organismos Internacionales, como la FAO (Organización para la
Agricultura y la Alimentación de las Naciones Unidas) y la OMS (Organización Mundial
11
Capítulo II
de la Salud) establecen unos niveles máximos admisibles respecto a la ingestión de
plaguicidas normalmente utilizados en distintos países.
Existen diversos factores que influyen en la persistencia y evolución de los
pesticidas en el suelo, los cuales se esquematizan en la Figura 1. Estos factores además
pueden verse afectados por otras variables como la humedad, la temperatura, el pH, la
materia orgánica, etc.
Figure 1. Pesticide evolution in the soil.
Debido al uso excesivo de los plaguicidas en la agricultura, en 2009 se adoptó
un paquete de medidas sobre estos compuestos por la Directiva 2009/128/CE sobre el
uso sostenible de los plaguicidas, orientada a reducir los riesgos ambientales y
sanitarios y a mantener la productividad de los cultivos mejorando el control del uso y
de distribución de estos. Los gobiernos aprobarán fitosanitarios a nivel nacional o
mediante “reconocimiento mutuo”, obligatorio dentro de la misma zona, ya que,
12
Introducción
según el nuevo Reglamento, la UE estará dividida en tres zonas (norte, centro y sur)
según las condiciones agrícolas, climatológicas y ecológicas de los países.
La Organización de las Naciones Unidas para la Alimentación y la Agricultura
(FAO) señala que la invasión de malezas, las enfermedades de las plantas y los
insectos, provocan la pérdida de entre 30% y 35% de las cosechas. Sin el uso de
plaguicidas las pérdidas serían mayores. Sin embargo, debido al uso de agroquímicos
todos los años resultan intoxicados alrededor de 25 millones de trabajadores agrícolas,
de los cuáles mueren unos 20.000. Ésto, sin considerar los errores de diagnóstico,
especialmente cuando los casos de envenenamiento, no se comunican a las
autoridades o no se registran.
Es por ello, que muchos investigadores estudian la eliminación de estos
plaguicidas mediante técnicas como fotocatálisis [1, 2], electrolisis [3, 4] y adsorción
[5-7], entre otras. Dentro de estas técnicas la que ofrece mayores ventajas es la de
adsorción ya que es mucho más económica, tiene una alta eficacia en la eliminación de
los plaguicidas y el manejo es mucho más sencillo.
2. DINÁMICA DE LOS PLAGUICIDAS EN EL SUELO
La presencia de los plaguicidas en los suelos agrícolas se produce por diversas
vías. Unas veces se debe a los tratamientos que se efectúan sobre las partes aéreas de
los cultivos para combatir sus plagas, depositándose aproximadamente un 50% del
producto en el suelo. Este es el caso de los insecticidas, fungicidas y algunos
herbicidas.
Otras veces, caso por ejemplo de los neumaticidas, desinfectantes y la
mayoría de los herbicidas, el tratamiento se efectúa directamente al suelo,
apareciendo el producto en cantidades mayores.
Los plaguicidas una vez en el suelo, entran en un ecosistema dinámico y
empiezan a moverse en el mismo, a degradarse “in situ”, a desplazarse del sistema
inicial a otros sistemas o a mantenerse en él con su estructura original o más o menos
degradada durante un período de tiempo variable.
13
Capítulo II
La desaparición de los plaguicidas en el suelo transcurre en tres etapas. Una
fase de corta duración en la que el plaguicida mantiene una determinada
concentración llamada de latencia. Una segunda etapa, la cual es relativamente rápida
en lo que respecta a su desaparición del suelo, denominada de disipación. Finalmente,
la última etapa, conocida como persistencia de plaguicida, es más lenta y puede durar
horas, días, semanas e incluso años.
Una vez el plaguicida en el suelo, puede verse afectado por diferentes
procesos (de tipo físico, químico o biológico) que determinan su dinámica y destino
final en el suelo.
Estos procesos, como se ilustra en la Figura 2, se dividen en procesos de
transferencia o transporte y en procesos de degradación o transformación.
Transferencia
o
transporte
Transformación
o
degradación
•Adsorción-desorción
•Volatilización
•Lixiviación
•Escorrentia
•Absorción
•Difusión
•Degradación química
•Biodegradación
•Fotodegradación
Figure 2. Dinamics of pesticides in soils.
Los procesos de transferencia o transporte son aquellos en los que el
compuesto se desplaza de un medio a otro o dentro de un mismo medio sin
experimentar cambio en su estructura. Dentro de estos procesos se destaca la
adsorción-desorción, volatilización, lixiviación, escorrentía y difusión (Figura 3). Sin
embargo, los procesos de transformación o degradación, implican la modificación del
plaguicida en un nuevo compuesto que puede ser de igual, menor o mayor toxicidad.
Estos
procesos
se
subdividen
en
degradación
fotodegradación.
14
química,
biodegradación
y
Introducción
De todos los mecanismos implicados en la evolución de los plaguicidas en el
suelo, la adsorción-desorción es el proceso más importante por influir directa o
indirectamente en la magnitud y efecto de los otros.
Figure 3. Principal factors affecting pesticides
2.1. Procesos de transferencia
2.1.1. Adsorción-Desorción
La adsorción del plaguicida en el suelo consiste en la acumulación del mismo
en la interfase sólido-agua o sólido-aire, siendo la desorción el proceso inverso [8]. El
adsorbato es el compuesto que se une a la fase sólida, mientras que el adsorbente es
la fase sólida en sí. El proceso de adsorción es ampliamente usado ya que es muy fácil
de diseñar y de operar así como relativamente simple a la hora de regenerar el
adsorbente [9].
Se pueden encontrar varios tipos de interacciones en la adsorción de
pesticidas. Pueden darse interacciones de Van der Waals (proceso mediante el cual los
iones de una sustancia se concentran en una superficie como resultado de la atracción
electrostática en los lugares cargados en la superficie) como es el caso del pesticida
Picloram en la montmorillonita [10]. Cuando un pesticida tiene comportamiento
15
Capítulo II
catiónico puede intercambiarse con los cationes que saturan la materia orgánica y de
esta manera quedan retenidos por fuerzas electrostáticas. Este tipo de interacción es
de cambio iónico y se puede observar en la adsorción del pesticida Paraquat en suelos
orgánicos [11]. Existen también las interacciones por puentes de hidrógeno que se
producen por la atracción electrostática entre un núcleo de H electropositivo y pares
de electrones sin compartir de átomos electronegativos. Este tipo de interacción se
produce en la adsorción del pesticida Thiazafluron en suelos [12]. Por otra parte las
interacciones hidrofóbicas son responsables de la adsorción en el caso de pesticidas
neutros tales como Metolachlor [13] o Triadimefon [14].
Existen algunos pesticidas que pueden establecer un equilibrio rápido y
reversible entre el compuesto en disolución y el compuesto adsorbido sobre la
superficie del suelo. Esto ocurre, por ejemplo, con el pesticida Fenantreno al
reaccionar con la materia orgánica del suelo e incorporarse a la misma por reacciones
de oxidación [15]. Este tipo de interacción se denomina covalente. También se pueden
observar enlaces de cambio de ligando en plaguicidas aniónicos y complejos de
transferencia de carga como en la Atrazina [16].
2.1.2. Otros procesos de transferencia
La volatilización (Figuras. 2 y 3) consiste en la pérdida de un compuesto en
forma de vapor. Una vez en fase gas, el compuesto puede desplazarse lejos desde el
sitio inicial de aplicación.
Numerosos estudios realizados con modelos explican que el movimiento de
los compuestos volátiles por el suelo se ve influenciado, en mayor medida por los
procesos de difusión [17, 18] los cuales dependen de las características físicas y
químicas del compuesto y del suelo [19].
La difusión (Figura 2) se define como el movimiento de moléculas a causa de
un gradiente de concentración. Este movimiento al azar provoca el flujo de materiales
desde las zonas de mayor concentración hacia las de menor concentración.
Existe una gran cantidad de pesticidas solubles en agua que se desplazan
fácilmente a través del suelo. Este fenómeno es conocido como lixiviación (Figuras. 2 y
16
Introducción
3) y puede provocar la contaminación de las aguas superficiales y subterráneas,
además de reducir la productividad agrícola.
La lixiviación depende de las propiedades de los pesticidas, como por ejemplo
la tendencia a ser degradados o a volatilizarse desde el suelo, del lugar de aplicación,
incluyendo la estructura, textura y contenido de materia orgánica del suelo, así como
los parámetros climáticos y finalmente también depende de la prácticas agrícolas que
determinan el tipo de pesticida aplicado, el momento de aplicación y de los cultivos
tratados.
De todos los factores de los que depende la lixiviación, los que más
contribuyen a la variabilidad de ésta son las propiedades del pesticida, seguido por las
características del lugar de aplicación, el clima, el tipo de cultivo y la corriente de flujo
[20].
La escorrentía (Figuras. 2 y 3) tiene lugar cuando la precipitación o el riego
superan la tasa de infiltración de agua en el suelo. En este proceso de transporte,
influyen diversos factores que determinan en gran medida la cuantía y características
de la concentración del soluto en el agua. Las mayores concentraciones de pesticidas
en el agua de escorrentía ocurren en la primera lluvia significativa después del
momento de aplicación. Sin embargo, lluvias muy intensas provocan escorrentías más
tempranas, con mayor velocidad de flujo y por tanto, con más energía disponible para
la extracción y transporte de plaguicidas desde la superficie del suelo.
Otro factor importante a considerar son las características del suelo ya que la
escorrentía de las sustancias puede verse afectada por la textura y contenido de
materia orgánica. La naturaleza y las características físico-químicas de los compuestos,
así como la zona de aplicación, son también otros elementos que hay que tener en
cuenta en el transporte producido en la escorrentía.
2.2 Procesos de transformación
Como se ha comentado anteriormente, la diferencia de los procesos de
transferencia con respecto a los procesos de transformación es que los primeros
implican el movimiento del plaguicida de una fase a otra mientras que en los segundos
17
Capítulo II
hay un cambio de estructura en el plaguicida. Dentro de los procesos de
transformación se encuentran la degradación química,
la biodegradación y la
fotodegradación.
La
degradación
química
depende
de
entre
otros
factores,
del
“compartimento” medioambiental, es decir aire, tierra o agua, en el cual los
compuestos son dispersados y transportados. Ésta, tiene lugar hasta la mineralización
+
-
2-
completa del pesticida dando como productos finales CO2, H2O, NH4 , NO3 , SO4 , etc.,
dependiendo de cada pesticida en particular aunque en muchas ocasiones, la
transformación no es total y el plaguicida se degrada en otros productos cuya
toxicidad, movilidad y efectos sobre el medio son prácticamente desconocidos [21]. Si
los productos obtenidos tras la degradación son menos tóxicos que la sustancia
original se trata de una inactivación o destoxificación, si por el contrario, el producto
de degradación resulta con mayor toxicidad que el original, se trata de una activación.
La biodegradación es el resultado de los procesos de digestión, asimilación y
metabolización de un compuesto orgánico llevado a cabo por protozoos, hongos y
otros organismos. En la bibliografía existen numerosos estudios de biodegradación de
plaguicidas entre los que se encuentran: biodegración de Endosulfan por el hongo
“Aspergillus niger”[22], la biodegración de Methyl Parathion por hongos de origen
marino [23] o la biodegradación de Chlorpyrifos por Pseudomonas [24] entre otros.
La fotodegradación consiste en la transformación del plaguicida por la luz
solar, que pueden tener lugar mediante reacciones de oxidación, reducción, hidrólisis,
sustitución e isomerización. El proceso de fotodegradación depende de la intensidad y
tiempo de exposición del plaguicida a la radiación solar, la presencia de catalizadores
fotoquímicos que pueden favorecer la descomposición, el pH del suelo y el tipo,
estado y grado de adsorción del plaguicida.
Existen dos efectos diferentes en la fotodegradación: la fotólisis directa y la
fotólisis indirecta. La fotólisis directa comprende la absorción directa de la luz por el
pesticida seguido de la reacción química correspondiente, sin la participación de otras
sustancias químicas. En la fotólisis indirecta, la energía de la luz es absorbida por otros
constituyentes del agua y es o bien transmitida a un plaguicida cercano o bien permite
18
Introducción
la formación de especies reactivas como radicales hidroxilo, que transforman al
plaguicida. Existen especies orgánicas e inorgánicas que pueden acelerar la
fotodegradación como los ácidos húmicos y fúlvicos o nitratos y nitritos [25].
3. CONTAMINACIÓN DEL AGUA POR PLAGUICIDAS
A lo largo de la historia, la calidad del agua potable ha sido un factor
determinante del bienestar humano. Aunque actualmente su contaminación ha
disminuido, el agua insalubre contaminada por fuentes naturales o humanas sigue
causando grandes problemas a las personas que se ven obligadas a usarla, tanto para
beber como para la irrigación de hortalizas y otras plantas comestibles.
Hoy en día, han disminuido las epidemias ocasionadas por bacterias o virus
que son causadas por agentes infecciosos transportados en el agua potable, como el
cólera, la poliomielitis y otras. Las enfermedades propagadas por ella están, en
general, bien controladas y el agua potable en los países tecnológicamente
desarrollados está ahora notablemente libre de los agentes causantes de
enfermedades que eran contaminantes muy comunes del agua hace sólo unas
décadas.
La principal preocupación sobre la seguridad del agua es la presencia de
contaminantes químicos como productos químicos orgánicos e inorgánicos y metales
pesados, procedentes de fuentes industriales, agrícolas y de la escorrentía urbana.
Dentro de los contaminantes orgánicos se encuentran los herbicidas (objeto de
estudio en esta Tesis Doctoral).
El primer plaguicida conocido como DDT (diclorodifeniltricloroetano) se
introdujo durante la Segunda Guerra Mundial y marcó el principio de un período de
crecimiento muy rápido en el uso de pesticidas. La contaminación del agua por
plaguicidas se produce al ser arrastrados por el agua de los campos de cultivo hasta los
ríos y mares donde se introducen en las cadenas alimenticias provocando la muerte de
varias formas de vida necesarias en el balance de algunos ecosistemas.
19
Capítulo II
Los herbicidas se aplican en millones de hectáreas de tierra de labranza en
todo el mundo. Éstos se encuentran en aguas superficiales y subterráneas y son
particularmente problemáticos para las fuentes de agua potable. Sus niveles varían
con la estación y con las veces que son aplicados para controlar malezas. Así, los más
solubles son los que tienen mayor probabilidad de entrar en las fuentes de agua
potable. El tratamiento con carbón activado o activo es el mejor medio para eliminar
herbicidas y sus metabolitos de las fuentes de agua potable. Sin embargo, un problema
con el carbón activo es el de la precarga, en que la materia orgánica natural en el agua
satura el carbón e impide la incorporación de compuestos orgánicos contaminantes
como los herbicidas. Es por ello, que el principal objetivo de esta Tesis Doctoral sea la
búsqueda de nuevos materiales adsorbentes.
4. MEDIDAS DE ADSORCIÓN. ISOTERMAS
En la adsorción física las moléculas están adsorbidas por fuerzas de Van der
Waals y no están fijas en un lugar específico de la superficie del sólido. Esta adsorción,
en general, predomina a temperaturas bajas. Sin embargo, cuando el adsorbato sufre
una interacción química con el adsorbente, la adsorción se denomina química o
quimisorción. Las energías de adsorción son elevadas debido a que el adsorbato forma
unos enlaces fuertes localizados en los centros activos del adsorbente. Este tipo de
adsorción, esta favorecida a alta temperatura. La mayor parte de los fenómenos de
adsorción son combinaciones de las diferentes formas de adsorción física y químicas.
La desorción es el paso del adsorbato del estado adsorbido a la fase líquida o
gaseosa. La desorción puede ser reversible cuando es total o bien parcial cuando una
parte del adsorbato permanece unido al adsorbente de manera irreversible. En la
literatura existen varios ejemplos entre los que cabe citar la adsorción de DNP y DNOC
en la hidrotalcita con cloruro en la interlámina y su producto calcinado cuyo proceso
de adsorción-desorción es irreversible [26] como también ocurre con el pesticida
Carbaryl en suelos [27].
Un procedimiento utilizado para caracterizar los procesos de adsorción es la
realización de isotermas de adsorción de un sustrato en un soporte. El procedimiento
seguido para la realización de éstas es de extrema importancia para que la información
20
Introducción
que nos aporte sea la apropiada para explicar los procesos de adsorción. En ellas se
hace interaccionar a una temperatura determinada cantidades conocidas de
adsorbente con disoluciones con diferente concentración de pesticida. Las isotermas
de adsorción corresponden al estado de equilibrio entre la concentración de pesticida
en fase líquida (Ce) y la concentración en fase sólida (Cs), de un soluto en particular,
después de un tiempo en contacto. La cantidad de pesticida adsorbido se puede
obtener por un método directo o indirecto. Con el primer método se mide extrayendo
y determinando la cantidad de pesticida presente en el adsorbente mientras que por
el segundo método se obtiene por diferencia entre la cantidad de pesticida inicial y la
concentración que hay en disolución.
Según Giles [28] las isotermas de adsorción se pueden clasificar dependiendo
de su forma y del tramo inicial en cuatro tipos (Figura 4). Mediante esta clasificación se
puede obtener información del mecanismo de adsorción, la naturaleza del adsorbato y
la superficie del adsorbente.
Figure 4. Types of adsorption isotherms and representation of
different subgroups according to the classification of Giles et al.1960.
 Isoterma de tipo S: Este tipo de isoterma presenta una primera zona donde
las moléculas de soluto compiten con las moléculas de disolvente y por tanto
21
Capítulo II
la intensidad de adsorción es baja y una segunda zona, con una mayor
pendiente, indicando la atracción que ejercen las moléculas adsorbidas por el
resto de moléculas disueltas y que favorece su adsorción.
 Isoterma de tipo L: Existe una gran afinidad entre el soluto y el adsorbente a
bajas concentraciones y va disminuyendo a medida que aumenta la
concentración. Estas isotermas se caracterizan por una disminución de la
pendiente a medida que se incrementa la concentración del soluto en la
disolución, debido a una disminución de los sitios de adsorción. Finalmente,
termina con un “plateau” cuando el adsorbente es cubierto completamente
por el soluto.
 Isoterma de tipo H: se caracterizan porque hay una alta afinidad entre el
soluto y el adsorbente. La parte inicial de la curva es completamente vertical
dado que el soluto es totalmente adsorbido. Es un caso particular de la
isoterma de tipo L y suele darse en adsorbatos de alto peso molecular como
micelas iónicas o especies poliméricas.
 Isoterma de tipo C: son isotermas lineales donde se mantiene el equilibrio
entre la cantidad de soluto adsorbido y la concentración del mismo en la
disolución de equilibrio. Este tipo de isotermas se obtiene con la mayoría de
compuestos químicos en un rango de concentraciones bajas.
Existen varias ecuaciones matemáticas o modelos de adsorción a las que se
puede ajustar las isotermas de adsorción de pesticidas en suelos, entre las que
destacaremos las ecuaciones de Freundlich, Langmuir, Dubinin-Radushkevich y
Temkin (Tabla 1).
La ecuación de Freundlich [29] es una ecuación empírica que se utiliza
principalmente para modelizar la adsorción en superficies heterogéneas y asume
que los sitios de enlace más intensos se ocupan primero y que la fuerza de enlace
disminuye con el aumento del grado de ocupación de los sitios de adsorción.
22
Introducción
La ecuación de Langmuir [30] supone que todos los sitios de adsorción son
idénticos, por lo que la adsorción se produce en una superficie homogénea del
adsorbente y que todos los sitios de adsorción son estérica y energéticamente
independientes de la cantidad adsorbida.
La ecuación de Temkin [31] se basa en que el calor de adsorción de todas las
moléculas en la lámina podría disminuir linealmente debido a las interacciones
entre adsorbente/adsorbato.
Table 1 Fittings for adsorption isotherm.
MODELO
Freundlich
FÓRMULA MATEMÁTICA
log 𝐶𝑠 = log 𝐾𝑓 + 𝑛𝑓 log 𝐶𝑒
Kf y nf dan información de la capacidad e intensidad de adsorción
respectivamente
1
1
1
=
+
𝐶𝑠 𝐶𝑚 𝐶𝑚 · 𝐿 · 𝐶𝑒
Langmuir
Cm es la capacidad máxima de adsorción
L es la constante de adsorción de Langmuir
Temkin
𝐶𝑠 = 𝐵𝑇 𝑙𝑛𝐾𝑇 + 𝐵𝑇 𝑙𝑛𝐶𝑒
KT y BT son constantes relativas a la capacidad e intensidad de
adsorción respectivamente.
Dubinin-Raduskevich
𝑙𝑛𝐶𝑠 = 𝑙𝑛𝑞𝑚 − 𝐾𝜀 2
qm es la capacidad de adsorción teórica
K es la constante relativa a la energía de adsorción.
𝜀=RTln(1+1/Ce) es el potencial Polanyi
Todos estos modelos, no permiten dilucidar el mecanismo de adsorción. Sin
embargo, mediante el uso del parámetro (K) de la ecuación de Dubinin-Radushkevich
[32], podemos obtener información sobre si la adsorción es de naturaleza química o
física aplicando la ecuación:
-0.5
E=(2K)
23
Capítulo II
-1
Cuando los valores de E están entre 0 y 8 kJ·mol , la adsorción es física, si
-1
están entre 8 y 16 kJ·mol se considera debida a intercambio de iones y cuando está
-1
entre 20 y 40 kJ·mol se asocia con la presencia de quimisorción.
5. ADSORBENTES EMPLEADOS
En el presente apartado se detallan algunos aspectos de los adsorbentes
empleados en esta Tesis Doctoral, es decir, las hidrotalcitas y compuestos
mesoporosos ordenados (benceno-PMO, etano-PMO, resina fenólica y carbón
mesoporoso), cuya comprensión es importante para la discusión de los resultados
planteados en este trabajo.
5.1 Hidrotalcitas
El mineral hidrotalcita, perteneciente al grupo de las arcillas aniónicas, se
descubrió en Suecia en torno a 1842 y es un hidroxicarbonato de magnesio y aluminio.
A partir de ahí, diversos minerales isoestructurales con la hidrotalcita, pero con otros
elementos en su composición, fueron agrupados con el nombre general de
compuestos tipo hidrotalcita.
La estructura de la hidrotalcita deriva de la brucita (Figura 5) por sustitución
isomórfica de parte de los cationes divalentes por cationes de mayor carga pero radio
3+
3+
3+
similar como Al , Fe , Co , etc. Esta sustitución parcial implica que las láminas
quedan cargadas positivamente. Para mantener la electroneutralidad del compuesto,
se introducen aniones en la interlámina que comparten este espacio con moléculas de
agua (Figura 6).
Las láminas se pueden apilar dando un ordenamiento hexagonal, es decir hay
dos láminas de hidróxido por celda (2H), donde cada catión metálico se encuentra
sobre el de la capa inferior, o bien en un ordenamiento romboédrico con 3 láminas de
hidróxido por celda unidad (3R) donde cada catión metálico superior se encuentra
desplazado 2/3 de a y 1/3 b respecto al catión metálico de la lámina inferior [33].
24
Introducción
Figure 5. Structure of brucite.
Las hidrotalcitas o arcillas aniónicas, también denominados hidróxidos dobles
laminares, son sólidos básicos bidimensionales de composición [M
x/n·mH2O
II
2+
2+
2+
III
3+
3+
3+
n-
II
III
n1-xM x(OH)2]A
2-
2-
-
donde M =Mg , Zn , Ni , etc., M =Al , Fe , Cr …, A =CO3 , SO4 , Cl … y x
varía entre 0.2 y 0.33 [34]. En la naturaleza, las arcillas aniónicas no son tan frecuentes
como las catiónicas pero sí son baratas y fáciles de sintetizar.
En las hidrotalcitas sus propiedades estructurales vienen determinadas por la
II
III
naturaleza de los cationes metálicos, por la relación M /M , por el anión interlaminar
y por el contenido de agua.
Naturaleza de los cationes metálicos
Existen un gran número de HDLs naturales y sintéticos conteniendo varios
cationes metálicos. Algunos de los cationes divalentes son: Mg, Mn, Ni, Cu, Zn y Ca y
trivalentes: Al, Cr, Mn, Fe, Co. Ni, La y Ga, entre otros. La incorporación de nuevos
cationes en la lámina sigue siendo uno de los retos de los procesos de síntesis en
hidrotalcitas, habiéndose descrito en los últimos años la preparación de nuevos
materiales.
Además, es posible preparar hidróxidos dobles laminares con más de dos
cationes diferentes. Zhang y col. [35] sintetizaron una hidrotalcita de ZnFeAl-Cl
mediante el método de coprecipitación usando flujo de nitrógeno para poder eliminar
25
Capítulo II
bromato de disoluciones acuosas. Guo y col. [36] prepararon una hidrotacita con los
2+
2+
metáles Cu , Mg
3+
y Fe
para poder reducir el arseniato en agua. En la bibliografía
existen numerosas publicaciones ya que de esta manera hay casi una infinidad de
combinaciones posibles.
Figure 6. Structure of hidrotalcite.
II
Relación M /M
III
II
III
En muchos casos, la relación M /M varía según las condiciones de síntresis y
la concentración de las sales [37]. Según la fórmula general el valor de x da
información del grado de sustitución del catión divalente por el trivalente y por ello el
exceso de carga positiva en la lámina. Por lo general, x varía entre 0.1≤x≤0.33. Al
alejarse de este intervalo favoreceremos la nucleación y segregación de una fase del
catión que se encuentre en exceso.
Composición del anión interlaminar
En principio, no hay limitaciones en el tipo de anión que puede ocupar el
espacio interlaminar de un HDL. No obstante, la preparación de materiales con
aniones distintos del carbonato, en diversas ocasiones presenta ciertas dificultades
debido a la gran afinidad que muestran estos materiales al contaminarse por dicho
anión, así como la posible inestabilidad de los aniones en el intervales de pH en el que
el HDL es estable.
Originalmente se sintetizaron sólidos que contenían aniones sencillos pero a
medida que avanzaron los estudios la familia de HDLs fue ampliamente extendida. En
26
Introducción
la tabla 2 se presentan algunos de los aniones que se introdcen en la interlámina de la
hidrotalcita.
Tabla 2. Some anions in the interlayer of the hydrotalcite.
ÁNIÓN INTERLAMINAR
Aniones inorgánicos
-
NO3
[29]
2CO3
[38-40]
-
[41, 42]
Cl
-
F
[42]
-
Br
OH
Aniones orgánicos
REFERENCIA
[42]
-
[42]
2SO4
[43]
Ácido p-aminobenzoico
[44]
Ácido tánico
[45]
Ácido Dodecilsulfúrico
[46, 47]
Ácido sebácico
[46]
Ácido sulfámico
[48]
Ácido dodecilbencenosulfato
[47]
Ácido ferúlico
[49]
Además de aniones inorgánicos u orgánicos, pueden intercalarse en el
espaciado interlaminar una gran variedad de especies, por ejemplo polímeros [50, 51],
ácido húmicos [52, 53], polioxometalatos [54] o antibióticos [55], entre otros.
Contenido de agua
Las moléculas de agua en la hidrotalcita pueden encontrarse en la interlámina
o en la superficie de las partículas. Es posible calcular el contenido de moléculas de
H2O en la interlámina basándose en el número de sitios presentes en la misma,
asumiendo una configuración de átomos empaquetada y restando los sitios ocupados
por el anión. Por ello, un aumento en la relación de catión trivalente y divalente
provoca un aumento de la concentración del anión interlaminar y, por consiguiente,
un descenso en el contenido de agua interlaminar.
27
Capítulo II
La hidrotalcita cuando es tratada térmicamente a temperaturas inferiores a
150ᵒC expulsa las moléculas de agua interlaminar sin alterar la estructura laminar,
entre 300-500ᵒC tiene lugar la deshidroxilación de las láminas y descomposición del
carbonato, entre 500 y 600ᵒC se forma una disolución sólida de Mg(Al)O y a
temperaturas superiores a 600ᵒC se forma MgO y la espinela MgAl2O4. Los productos
de calcinación de la hidrotalcita tienen la capacidad de reconstruir la estructura inicial
al ponerse en contacto con disoluciones que contengan al anión inicial, u otro
diferente. A temperaturas inferiores a 600ᵒC el proceso es reversible en presencia de
agua y de aniones, lo que permite usar el material como secuestrante de aniones.
Algunas de las propiedades más interesantes que caracterizan a los productos
calcinados [34, 56, 57] de las hidrotalcitas son:
-Entre 300 y 600ᵒC los productos de calcinación tienen la capacidad de
reconstruir la estructura laminar al ponerse en contacto con disoluciones
acuosas que contienen aniones. A esta propiedad se le conoce como efecto
memoria.
2
-Área superficial específica elevada (entre 100-300m /g)
-Distribución homogénea de los iones metálicos en los productos de
calcinación, térmicamente estables, con formación mediante reducción de
metales cristalinos muy pequeños y estables.
-Alta capacidad de intercambio aniónico.
- Propiedades ácido-base ajustables en función de la presencia de diferentes
metales en la lámina.
5.1.1 Métodos de síntesis
Los compuestos tipo hidrotalcitas son poco comunes en la naturaleza,
mientras que a nivel de laboratorio e industrial su síntesis es relativamente sencilla y
de bajo costo [37].
28
Introducción
Alguno de los métodos empleados para la síntesis de los hidróxidos dobles
laminares son coprecipitación [58, 59], método de la urea [60, 61], intercambio iónico
[62, 63], reconstrucción [64, 65], hidrólisis inducida [37], técnica sol-gel [66, 67],
electrosíntesis [68] y métodos asistidos con tratamiento hidrotermal [69-71],
ultrasonido [72, 73] y microondas [74, 75].
La coprecipitación es el método de síntesis más común en las arcillas
aniónicas. Consiste en la adicción paulatina y por goteo de una disolución que contiene
2+
3+
la mezcla de dos sales metálicas (M y M ) con anión común, en proporción adecuada
a una disolución que contiene un medio acuoso. Se añade disolución alcalina
simultáneamente con el fin de fijar el pH a un valor seleccionado para producir la
coprecipitación de las dos sales metálicas.
Los aniones interlaminares se pueden adicionar de los contraiones de los
cationes constituyentes de las láminas metálicas, de los aniones hidroxilo presentes en
el medio en el caso de síntesis a pHs muy alcalinos, o bien de alguna sal presente en el
medio acuoso.
Los parámetros que influyen en las propiedades del sólido final son la
temperatura, el pH del medio, la concentración de sales metálicas en la disolución, la
velocidad de adicción de los reactivos, el lavado, el secado y el envejecimiento de
precipitado.
Crepaldi y col. [76], demostraron que los materiales preparados por este
método muestran un alto grado de cristalinidad, bajo tamaño de partícula, área
superficial específica elevada y un promedio elevado de diámetro de poro. El pH
durante la coprecipitación tiene un efecto crucial en la estructura, la química y las
propiedades texturales de la fase obtenida.
En 1998 se desarrolló un nuevo procedimiento, por Constantino y col. [77],
denominado método de la urea. Este nuevo método, a diferencia de la síntesis por
coprecipitación, parte de disoluciones homogéneas donde los iones OH- se producen
“in situ” mediante la descomposición de la urea la cual libera amoniaco (Figura 7). De
29
Capítulo II
esta manera se forma una hidrotalcita con mayor cristalinidad, grandes cristales (rango
de µm) y una distribución de tamaño de partícula muy homogénea.
Figure 7. Decomposition of urea
Cuando un óxido tal como ZnO, NiO y CuO se pone en contacto bajo agitación
con una disolución ácida de una sal trivalente como AlCl3 o CrCl3, el óxido se disuelve y
se forma el HDL siempre que el pH sea tamponado por el óxido o hidróxido en
suspensión. Esta síntesis se conoce como hidrólisis inducida.
López y col. [78] fueron los primeros en preparar hidrotalcitas mediante el
método de sol-gel. En este tipo de síntesis, el HDL tiene una gran estabilidad hasta
550ᵒC pero tienen el inconveniente de que la cristalinidad es mucho menor que la que
se alcanza para las muestras sintetizadas por el método de coprecipitación.
La síntesis de ciertas hidrotalcitas también ha sido llevada a cabo por métodos
electroquímicos, por ejemplo, a través de la reducción de óxidos de NiCo con peróxido
de hidrógeno. Aunque los resultados obtenidos en este tipo de síntesis no son muy
buenos desde el punto de vista de cristalinidad del HDL, este método puede ser muy
interesante en la preparación de electrodos modificados con HDLs.
Además, se pueden realizar tratamientos “in situ” o después de la síntesis
para controlar las propiedades texturales y estructurales. Cortas irradiaciones de
microondas dan lugar a materiales con mejor cristalinidad lo mismo que ocurre por
ultrasonidos o mediante el tratamiento hidrotermal. El método de síntesis en
microondas se usa con objeto de evitar las altas temperaturas y los largos procesos de
cristalización del método hidrotermal. En general, se obtienen compuestos de tamaño
de partícula muy pequeño.
30
Introducción
5.1.2. Incorporación de aniones en la estructura
Un aspecto de gran importancia en la síntesis de los HDLs es la naturaleza del
anión interlaminar. Basicamente, existen tres métodos para la preparación de
hidrotalcitas diferentes de carbonato y que son: coprecipitación o síntesis directa,
intercambio iónico y reconstrucción.
Mediante la síntesis directa se coprecipitan los cationes sobre una disolución
que contiene el anión que se vaya a incorporar en la estructra de la hidrotalcita
eligiendo adecuadamente el pH de trabajo. Si el pH es demasiado alto hay que evitar
2-
de forma muy cuidadosa la presencia de CO3
-
o incluso OH en el espaciado
interlaminar y la realización de la síntesis a pH muy ácido inhibe la precipitación de los
cationes metálicos.
Otro método muy utilizado es el de intercambio iónico que consiste en partir
-
-
de una hidrotalcita previamente intercalada con un anión, como Cl o NO3 , y ponerla
en contacto, en forma de suspensión con una disolución que contenga el anión que se
desea intercalar. Este método presenta la ventaja de dar hidrotalcitas más cristalinas
que las obtenidas por los demás y además, evita en parte la disolución de los cationes
laminares cuando se trabaja a pH bajo.
Con este tipo de síntesis se han intercalado una gran variedad de aniones para
tales como: ácido indol-3-butírico [79], el anión adenosintrifosfato [63], el anión
sulfato [62], medicamentos [80, 81], polioxometalatos [54], entre otros.
En 1986, Reichle propuso un método de síntesis alternativo basado en la
descomposición reversible que experimentan los HDLs cuando se calcinan a
temperaturas del orden de 400-600°C, denominado reconstrucción. La estructura de
hidrotalcitas es estable hasta 400° y se descarbonata, formando Mg6Al2O8(OH)2 tras
calcinación a aproximadamente 500-600°C, siendo este proceso reversible en
presencia de agua y aniones, particularidad que puede aprovecharse para incorporar
diferentes aniones en la interlámina.
31
Capítulo II
El método de reconstrucción es muy utizado cuando se introducen aniones
que sólo existan a pH relativamente bajo (4.5-6) ya que a pH alto producirían
materiales contaminados por la presencia del anión carbonato.
5.1.3. Aplicaciones
Como se ha comentado en el apartado anterior, una de las propiedades más
importantes de las arcillas aniónicas es su capacidad para intercambiar aniones, lo cual
permite utilizarlas para secuestrar o liberar especies en medio acuoso. Además, los
óxidos obtenidos tras calcinar estos materiales presentan excelentes propiedades tales
como, alta superficie específica, dispersión homogénea y termodinámicamente estable
de los cationes metálicos y propiedades básicas y reductoras importantes.
Es por ello, que el número de aplicaciones es extenso y esta bien establecido,
lo que ha dado lugar a patentes que inlucran a los HDLs. A continuación, se presentan
algunas de las aplicaciones más relevantes de estos compuestos.
Los hidróxidos dobles laminares pueden ser utilizados como catalizadores en
diversas reacciones químicas, soportes catalíticos o precursores de óxidos catalíticos.
La selección de metales involucrados en la composición de un HDL, determinará el tipo
de reacción que el compuesto laminar pueda catalizar. Así, una hidrotalcita de Mg/Al
con nitrato o dodecilsulfato en la interlámina es capaz de epoxidar el limoneno en
presencia de nitrilos con conversiones del 63 al 97%. Existen también reacciones en
que el catalizador se activa con luz cuando contiene metales como el Cerio, que al
estar disperso en las láminas, es capaz de catalizar la reacción de degradación de
compuestos orgánicos. Hay una gran diversidad de composiciones y efectos catalíticos
que pueden ser consultados en la bibliografía en relación a la catálisis como la
oxidación de alcoholes a aldehídos [82], oxidación de estireno [83], isomerización de
glucosa en fructosa [84], eterificación de glicerol a poligliceroles [85], hidrogenación de
fructosa [86], etc.
El campo de la medicina ha sido y sigue siendo objeto de investigación y
aplicación de los HDLs. En un principio, se descubrió que los HDLs son muy útiles como
antiácidos para tratamientos gástricos o úlceras duodenales. Su acción en el
32
Introducción
tratamiento de las úlceras gástricas se basa en que inhiben la acción del ácido
clorhídrico y la pepsina en el jugo gástrico. Años más tarde, al modificar la estructura y
propiedades se consiguieron aplicaciones más sofisticadas, utilizándose mayormente
como matrices huésped de moléculas con actividad biológica. Dentro del área
farmacéutica, las hidrotalcitas mejoran los procesos de disolución de fármacos
hidrofóbicos, participan en procesos de liberación controlada e incrementan su
estabilidad térmica y mecánica.
2+
2+
Los cationes como Ca , Mg
o Al
3+
que forman las láminas, permiten
convertir a las hidrotalcitas en biocompatibles, poco tóxicas y con gran capacidad de
protección para el fármaco. Existen muchas investigaciones en relación al intercalado
de genes en los HDLs, a la toxicidad celular y a pruebas de transferencia génica in vitro
en diferentes líneas celulares.
También han sido utilizadas como transportadores de biomacromoléculas. En
uno de los trabajos de Choy y col. [87], usaron HDLs para encapsular moléculas de ADN
mediante intercambio iónico en el agua. El compuesto resultante, se denominó bioHDL y es empleado para la liberación de las moléculas de ADN en el interior de la
célula.
Una de las aplicaciones más ingeniosas y quizá la de mayor repercusión
industrial es su uso como componente en los materiales de PVC. El principal problema
de este material es su degradación frente a la luz directa del sol, por la formación de
HCl y radicales que provocan que el PVC se vuelva amarillo. La introducción de
pequeñas cantidades de hidrotalcita en el material, captura el HCl mediante
intercambio aniónico y por tanto mantiene la fuerza y la blancura del PVC durante más
tiempo [88, 89].
Otra aplicación muy interesante es su uso como retardante de la combustión
y aislante de cables con cierta resistencia al calentamiento, fruto de sus propiedades
térmicas. En efecto, al calentar la hidrotalcita ésta se descompone liberando agua y
CO2 lo que le proporciona cierta capacidad extintora de incendios.
33
Capítulo II
La hidrotalcita debido a su estructura molecular y su capacidad para
intercambiar aniones tienen propiedades para ser usadas como componente activo de
cementos y morteros [90]. Además, en el campo de la química analítica los HDLs
resultan muy interesantes como modificadores de la superficie de los electrodos con el
fin de crear nuevos sensores amperométricos [91-93].
Desde el punto de vista medioambiental, los HDLs actúan como
intercambiadores y adsorbentes. Las hidrotalcitas debido a que tiene una alta
capacidad de adsorción de especies aniónicas posee una alta capacidad de intercambio
aniónico y un espacio interlaminar que permite adsorber moléculas polares y una
amplia gama de especies aniónicas. Sin embargo, la principal dificultad radica en evitar
la presencia del anión carbonato de mayor afinidad que la mayoría de las especies
-
contaminantes por lo que a veces se emplean materiales intercalados con Cl en lugar
2-
de CO3 . Otro aspecto a considerar es el pH, ya que a veces el intervalo de éste de la
especie contaminante en disolución es incompatible con el de la hdrotalcita.
Existen un gran número de publicaciones sobre la eliminación de
contaminantes inorgánicos entre las que destacan: la adsorción de perclorato [94],
2+
2+
2+
nitrato [95], cationes metálicos [96], de Pb , Cu y Cd [53, 97], adsorción de nitrato,
VI
sulfato y fosfato [98] o Cr [99] entre otros. También se pueden encontrar en la
bibliografía trabajos de adsorción de gases como CO2 [100] o SO2 [101]. Del mismo
modo, han sido estudiados como adsorbentes de contaminantes orgánicos entre los
que se pueden citar la adsorción de los pesticidas 2,4 D, Clopyralid y Picloram usando
la hidrotalcita calcinada [102], adsorción del Linuron, 2,4 DB y Metamitron en la
hidrotalcita con caprilato en la interlámina [103], fenoles [104] glifosato en
hidrotalcitas con diferentes aniones interlaminares [105], 2,4,5 Triclorofenol en
hidrotalcita modificada con dodecilbencenosulfato de sodio [106] y de tintes sintéticos
[107, 108] para poder ser eliminados de las aguas naturales.
Las propiedades físico-químicas de los HDLs los hacen idóneos para la
adsorción de plaguicidas de tipo aniónico. Éstos tienen una movilidad en el suelo muy
alta debido a la baja capacidad de los componentes del suelo a retener moléculas
aniónicas [109]. Este problema no es tan importante para los plaguicidas catiónicos o
34
Introducción
de base débil protonable ya que interaccionan con los componentes activos del suelo,
concretamente con los minerales de la arcilla. La posibilidad de modificar la
interlámina de los HDLs permite que puedan interaccionar con los cationes orgánicos.
El efecto memoria de los HDLs los hace muy interesantes en lo que respecta a
su reciclabilidad. Así, la hidrotalcita intercalada con la especie contaminante se calcina
a 500-600°C para liberarlo. Posteriormente, el tratamiento del óxido formado con una
suspensión de agua permite recuperar la estructura laminar de la hidrotalcita
manteniéndose su capacidad de adsorción [110]. La adsorción de plaguicidas tiene un
carácter reversible que depende de las condiciones experimentales y de la naturaleza
de los aniones presentes en la disolución [111].
Existen también plaguicidas neutros que aunque son más fácilmente
degradables en el ambiente siguen presentando riesgos de contaminación. Las
hidrotalcitas tienen que ser modificadas con aniones orgánicos (anión sebacato,
dodecilsulfato, dodecilbencenosulfato…) para poder adsorber contaminantes neutros
o de baja polaridad. Existen diversos estudios de adsorción de plaguicidas en arcillas
[112-114] pero son escasos en organohidrotalcitas [13, 103, 115].
5.2 Materiales porosos
Los materiales porosos merecen una singular atención debido a sus
potenciales aplicaciones como adsorbentes, sistemas de intercambio iónico,
membranas, soportes o catalizadores. Aunque los sólidos porosos poseen una variada
composición, tienen en común el espacio accesible en el interior de su estructura.
De acuerdo con la definición de la IUPAC, los materiales porosos se clasifican
en función del tamaño de poro en: microporosos (<2nm), mesoporosos (2-50nm) y
macroporosos (>50nm).
Aunque estos materiales han sido ampliamente estudiados a lo largo del siglo
XVIII, no es hasta el siglo XIX cuando se comienza a estudiar su capacidad adsorbente.
El objetivo principal de los investigadores en relación a los materiales porosos es
controlar el tamaño, la forma, uniformidad y periodicidad de los espacios porosos, así
como los átomos o moléculas que los constituyen. El control y el ajuste preciso de
35
Capítulo II
estas propiedades permiten conseguir diferentes materiales para el desempeño de
una función deseada en una aplicación en particular.
La sílica gel [116, 117] y el carbón activado [118, 119] han sido ampliamente
utilizados como adsorbentes. El principal problema de estos materiales reside en que
al ser microporosos no permiten la adsorción de moléculas grandes. Sin embargo, los
compuestos mesoporosos ordenados que contienen sílica y carbón en su estructura
permiten la adsorción de moléculas de gran tamaño. Estos materiales poseen alta área
superficial con superficies de poro activa para su modificación o funcionalización y
grandes volúmenes de poro distribuidos de manera periódica,
Kato y col [120] sintetizaron en 1992 el primer material mesoporoso
ordenado y en este mismo año se produjo un gran avance en la preparación de estos
materiales por la Mobil Research and Development Corporation. Ellos describieron la
síntesis de los primeros materiales porosos, M41S [121], los cuales eran obtenidos a
partir de un precursor inorgánico en presencia de surfactantes catiónicos.
Inmediatamente, las posibilidades en el uso de otros moldes porógenos se amplió a
surfactantes aniónicos [122] y no iónicos [123]. Durante la primera década de los 90,
estos materiales despertaron un gran interés científico y la síntesis de nuevas
estructuras mesoporosas ordenadas ha sido objeto de numerosas citas bibliográficas.
Los poros de estos materiales se encuentran en el intervalo de 1.5 a 10nm. Al
tener poros grandes ofrecen grandes ventajas ya que estos no se bloquean tan
fácilmente y pueden ser utilizados para reacciones en fase líquida a diferencia de las
zeolitas comunes que por lo general se utilizan para gases y vapores. Para lograr una
estructura mesoporosa ordenada, se necesita partir de una solución homogénea de un
surfactante disuelto en el medio. Es muy importante conocer el comportamiento del
surfactante en disolución acuosa para entender la relación entre el surfactante y la
formación de la mesoestructura. Los surfactantes se caracterizan por ser moléculas
orgánicas y de carácter anfifílico, formados por una cabeza polar y una cadena
hidrocarbonada apolar. Cuando el surfactante se pone en contacto con agua, las
cadenas hidrofóbicas se reúnen en el interior formando micelas y los grupos polares se
disponen en la superficie en contacto con el medio acuoso.
36
Introducción
Figure 8. Ethane and methane molecules in the
hexagonal pores of MCM-41.
El miembro más interesante y estudiado de esta familia es el denominado
2
MCM-41 (Figura 8). Este sólido posee altas superficies específicas (~1000 m /g), alta
estabilidad térmica, tamaños de poros regulables en el rango 1-10 nm y una estrecha
distribución de tamaño de los mismos.
Para obtención de estos materiales se pueden seguir el proceso sol-gel o el
método cristal líquido.
Proceso sol-gel
El proceso sol-gel es una ruta química que permite fabricar materiales
amorfos y policristalinos de forma relativamente sencilla. En el estado sol hay una
suspensión estable de partículas coloidales en un líquido y en el gel se forma una red
porosa sólida tridimensional continua, que se expande a lo largo de un medio líquido.
Mediante esta síntesis se obtienen mesoestructuras inorgánicas ordenadas basándose
en la hidrólisis y policondensación de precursores de tipo alcóxido metálico M(OR)n
donde R es un grupo alquilo (Figura 9). Para la síntesis de MCM-41 los precursores más
utilizados son Si(OCH3)4 o TMOS y Si(OCH2CH3)4 o TEOS.
37
Capítulo II
Figure 9. Diagram of sol-gel process.
En esta síntesis el pH del medio juega un papel muy importante. A pHs ácidos
la condensación es la etapa determinante de la velocidad ya que el número de enlaces
siloxanos alrededor del átomo central de silicio aumenta. A estos pHs se producen
estructuras poliméricas largas débilmente ramificadas. Sin embargo, a pHs básicos la
hidrólisis es la etapa determinante y dan lugar a estructuras ramificadas. En este caso,
la velocidad de la reacción aumenta con el incremento de enlaces siloxano (Figura 10).
Método cristal líquido
Beck y col [121] propusieron el modelo de cristal líquido (Liquid cristal
templating mechanism, LCT) para la síntesis de mesoporosos. La estructura viene
definida por la organización de moléculas de surfactante en fases de tipo cristal líquido
mesostructuradas, bajo unas condiciones de temperatura y concentración
determinadas. En estas moléculas se deposita la fase inorgánica. El ion amonio
cuaternario actúa como agente director de la estructura formando las micelas que se
agregan en el cristal líquido.
En la Figura 11 se representan las diferentes estructuras que se pueden
obtener en función de las condiciones de síntesis como la temperatura, la naturaleza
de surfactante y la concentración. Así, se puede obtener una estructura hexagonal
como la MCM-41, o cúbica como la MCM-48 o laminar como la MCM-50. La
concentración más baja a la cual se observa la formación de micelas esféricas se
38
Introducción
denomina concentración micelar crítica (cmc1). La segunda concentración micelar
crítica se define como la concentración a la cual las micelas esféricas comienzan a
transformarse en micelas cilíndricas (cmc2).
Figure 10. Influence of pH in the synthesis.
Se pueden establecer dos tipos de mecanismos según las condiciones de
síntesis. Un primer mecanismo denominado “True Liquid-Crystal Templating” en el
cual las moléculas se organizan en superestructuras, antes de la formación del sólido.
Éste crece en torno a la estructura ordenada previa, es decir, la fase cristal líquido se
forma sin requerir la presencia del precursor inorgánico. En el segundo mecanismo
llamado “Cooperative Self Assembly” hay una interacción entre las moléculas orgánicas
y el precursor inorgánico induciendo así a la ordenación del sistema (Figura 12).
Años más tarde, varios investigadores tras el estudio de diversos métodos y
técnicas se posicionaron a favor del método liquid cristal templating [124].
De acuerdo con las interacciones entre las moléculas de surfactante, la
especie inorgánica y los iones presentes en el medio, se pueden distinguir diferentes
mecanismos de formación de estructuras mesoporosas [122]. El pH juega un papel
importante en la interacción entre estas moléculas. El punto isoeléctrico de la sílice, es
decir, el pH en el que su carga es cero, es de 2. Por ello, cuando el pH es mayor de 2,
39
Capítulo II
las especies de sílice tienen carga negativa. Si se emplean surfactantes catiónicos,
+ -
como por ejemplo sales de amonio cuaternarias, el mecanismo se denomina S I donde
S es el surfactante e I la especie inorgánica. Si por el contrario, se utilizan surfactantes
+
+
aniónicos como un fosfato de cadena larga, se necesita un ión mediador como Na o K
para asegurar la interacción entre ambas especies cargadas negativamente, siendo
-
+ -
S M I el mecanismo.
Figure 11. Mesoporous structure depending on the
temperature and concentration of surfactant
Por otro lado, si las condiciones de síntesis son ácidas, pH<2, la sílice estará
presente como catión. En este caso, cuando se empleen surfactantes catiónicos será
-
necesario añadir un ion molecular X que generalmente es un haluro. Este mecanismo
+ - +
- +
es expresado como S X I . Si se utiliza un surfactante aniónico, el mecanismo es S I .
40
Introducción
Figure 12. Types of mechanisms (a) True Liquid-Crystal Templating and (b) Cooperative Self
Assembly.
Las interacciones comentadas anteriormente son de naturaleza electrostática
pero se pueden producir enlaces de hidrógeno o de naturaleza dipolar. En esta ruta se
0
utilizan surfactantes no iónicos como Pluronic o neutros (N polióxido de etileno, S
0 0
0
0 0
aminas de cadena larga) con especies de sílice no cargadas, S I o N I o pares iónicos
0
0
S (XI) (Figura 13).
La última etapa en cualquier ruta sintética utilizada es la eliminación de
surfactante orgánico. Ésta se puede llevar a cabo mediante calcinación o métodos de
extracción química. En el tratamiento térmico se suelen emplear rampas de
calentamiento lentas hasta alcanzar los 550ᵒC. El problema de este tratamiento radica
en la aparición de grietas y distorsiones por contracción de la estructura. Por ello, se
suele emplear más la extracción mediante lavado con disolventes o fluidos
supercríticos [125-127], la digestión por microondas [128] o el tratamiento oxidativo
con ozono [129]. Sin embargo, estas alternativas suelen venir acompañadas de un
proceso de calcinación ya que no se consigue la eliminación completa del surfactante.
La extracción minimiza la pérdida de OH en la superficie, lo cual es de gran interés para
la funcionalización de las matrices con agentes sililantes.
41
Capítulo II
Figure 13. Possible mechanisms of interaction
between the surfactant and the inorganic species.
5.2.1 Materiales híbridos
Uno de los materiales más versátiles y estables fue presentado en 1998 por
Zhao y col. [130] con la síntesis de estructuras de sílices mesoporosas bien ordenadas,
denominadas SBA-15, con tamaño uniforme de poro de hasta 30nm. Para la síntesis de
estos materiales se empleó un surfactante de tres bloques no iónico (EOnPOmEOn) en
un medio fuertemente ácido. La calcinación a 500ᵒC produce una estructura porosa
con grandes distancias interplanares “d” de 7.5 a 32 nm entre los planos (100). En
comparación con MCM-41 proporciona mayor estabilidad térmica e hidrotérmica al
tener un mayor espesor de pared. Esto le permite tener buenas aplicaciones en los
campos de catálisis y de adsorción.
Un material híbrido incluye dos tipos de funcionalidades que comúnmente
suelen ser orgánica e inorgánica. Estos materiales generan un gran interés debido a la
posibilidad de combinar la enorme variabilidad de funciones de los compuestos
42
Introducción
orgánicos con la robustez y estabilidad térmica de los compuestos inorgánicos [131].
La integración de grupos funcionales permite ampliar su utilidad ya que han sido
utilizados como catalizadores [132-134] para una amplia variedad de transformaciones
químicas, como adsorbentes. [135, 136], soportes cromatográficos [137], agentes de
liberación de fármacos [138-140] y sensores químicos [141, 142].
5.2.1.1 Síntesis de materiales híbridos de sílice
Generalmente existen dos vías de funcionalización superficial de los
materiales de sílice: el método de post funcionalización o “grafting” y el método de
co-condensación (Figura 13). El primer método, es un proceso post-síntesis que implica
la condensación de organosilanos del grupo (R´O)3SiR siendo R el grupo orgánico, con
los grupos silanoles libres en la superficie de los poros.
Por otro lado, el método de co-condensación es un proceso de síntesis
directo en el que dos precursores, uno tetraalcoxisilano [(R’O)4Si(TEOS o TMOs)] y un
precursor híbrido trialcoxiorganosilano del tipo R-Si(OR´)3, reaccionan por cocondensación en presencia de un agente director de la estructura, generando una
sílice mesoestructurada modificada con una gran variedad de funcionalidades (de tipo
amino, tiol, alquilo, aromático o vinilo). Este proceso de funcionalización puede ocurrir
en condiciones ácidas, básicas o neutras.
El método de co-condensación tiene una serie de desventajas. El grado de
ordenamiento mesoscópico de los productos disminuye con el incremento de la
concentración (R´O)3SiR en la mezcla de reacción conduciendo a productos totalmente
desordenados [143]. El contenido de funciones orgánicas en la sílica modificada
normalmente no excede del 40% molar. Además, la proporción de grupos orgánicos
terminales que son incorporados en las paredes de los poros es generalmente más
baja que la que correspondería según la concentración de partida. Existe también una
tendencia a dar reacciones de homocondensación, debido a las diferentes velocidades
de hidrólisis y condensación de precursores estructuralmente diferentes. Puede darse
además una reducción de diámetro de poro, volumen de poro y área superficial por el
incremento en la cantidad de grupos incorporados. Por último, debe prestarse especial
cuidado para no destruir la funcionalidad orgánica durante la extracción del
43
Capítulo II
surfactante. Por ello, se suelen realizar métodos de extracción ya que la calcinación en
la mayoría de los casos no es adecuada.
Figure 14. Synthetic routes for mesoporous hybrid materials.
Debido a los problemas presentados en las rutas de “grafting” y cocondensación se ha desarrollado una tercera vía de síntesis denominada PMOs
(Periodic Mesoporous Organosílicas). Este método consiste en la hidrólisiscondensación de precursores organosilícicos puentes del tipo (R´O)3Si-R-Si(OR´)3, en
presencia de un agente director de la estructura. Estos materiales se diferencian de los
anteriores por tener incorporadas las unidades orgánicas en la estructura
tridimensional de la matriz de sílice mediante enlaces covalentes quedando éstas
homogéneamente distribuidas en las paredes de los poros. En esta Tesis Doctoral se
han sintetizado PMOs por lo que se procederá a una descripción más detallada.
5.2.2 PMOs
En 1999, tres grupos de investigación [144-146] sintetizaron los primeros
materiales periódicos mesoporosos organosilícicos, PMOs. La siguiente figura muestra
una visión simplificada de estos materiales.
44
Introducción
Figure 15. PMOs materials
El grupo de Inagaki [144] sintetizó materiales con estructura hexagonal
bidimensional o tridimensional dependiendo de la temperatura y de la longitud de la
cadena alquílica, con diámetro de poro de 31 a 27Å y alta área superficial. La síntesis
se llevó a cabo bajo condiciones ácidas y como precursor se empleó 1,2
bis(trimetoxisilil)etano y octadeciltrimetil amonio (OCTA) como agente director de
estructura. Stein y col. [145] sintetizaron una organosilica mesoporosa con alta área
superficial y puentes etilenos usando los precursores 1,2 bis (trietoxisilil)etano o
bis(trietoxisilil)etileno y hexadeciltrimetilamonio como surfactante. Finalmente, el
grupo de Ozin [146] preparó PMOs a partir del precursor 1,2 bis (trietoxisilil)eteno y
tetraortosilicato (TEOS) empleando medio básico y bromuro de hexadecilamonio como
surfactante.
Actualmente existe una gran variedad de PMOs sintetizados a partir de
diferentes precursores organosilícicos puentes (Figura 16). Hay un número limitado de
estos precursores para la síntesis de estos materiales ya que la estructura del
precursor orgánico tiene que ser rígida, tal como una cadena corta o un sistema
conjugado. Además se necesita una fuerte interacción entre el “template” o agente
director de la estructura y el precursor. Para ello hay que ajustar una serie de
parámetros como la naturaleza de “template”, el pH, las concentraciones, la
temperatura, las condiciones de envejecimiento y la presencia de ciertos aditivos.
Generalmente, el uso de surfactantes neutros da lugar a materiales de poro
de hasta 10nm. La tabla 3 presenta los surfactantes más utilizados. Por otro lado, la
adición de sales mejora el grado de orden ya que fomenta la interacción entre el
45
Capítulo II
surfactante y la sílice y además permite preparar PMOs con poros grandes y altamente
ordenados en un amplio rango de concentraciones ácidas.
F
i
g
u
r
e
1
5
Figure 16. Precursors used for the synthesis of PMOs.
46
Introducción
Table 3 Some surfactants used for the synthesis of PMOs.
CTAC/CTAB
Cetyltrimethylammonium chloride/bromide
OTAC
Octadecyltrimethylammonium chloride
CnTMACl/Br
Alkyltrimethylammonium chloride/bromide
CPCl
Cetylpyridinium chloride
FC4
Fluorocarbon surfactant
Brij-30
Polyoxyethylene (4) lauryl ether
Brij-56
Polyoxyethylene (10) cetyl ether
Brij-76
Polyoxyethylene (10) stearyl ether
Triton-X100
Polyoxyethylene (10) octylphenyl ether
P123
Pluronic
P123
poly(ethylene
glycol)–
poly(propylene glycol)–poly(ethylene glycol)
F127
Pluronic
F127
poly(ethylene
glycol)–
poly(propylene glycol)–poly(ethylene glycol)
B50-6600
Poly(ethylene
oxide)–poly(butylene
oxide)–
poly(ethylene oxide)
PEO–PLGA–
Poly(ethylene
PEO
glycolic acid)–
oxide)–poly(lactic
poly(ethylene oxide)
Cn-s-m
Divalent and gemini surfactants
47
acid-co-
Cl-/Br-
Capítulo II
Un parámetro importante en la síntesis de PMOs es la acidez o basicidad en el
medio de reacción porque determinan la interacción específica entre el surfactante y
el precursor organosilícico y afecta a la regularidad mesostructural de los PMOs [148].
Existen otros parámetros a tener en cuenta durante la síntesis de materiales
mesoporosos ordenados organosilícicos como la temperatura, el tiempo de agitación
[149] y la naturaleza del precursor.
5.2.2.1 Tipos de PMOs y sus síntesis
Como se ha podido comprobar en la Figura 16 existe una amplia variedad de
materiales periódicos mesoporosos organosilícicos (PMOs). En este apartado, sin
embargo solo se van a desarrollar los dos materiales empleados para la adsorción de SMetolacloro, es decir PMO con puentes etilénicos (denominado etano-PMO) y PMO
con puentes fenileno (denominado benceno-PMO).
Dentro de los materiales PMOs con puentes alquilo, el material con puentes
etileno ha sido uno de los más estudiados. Como precursores se emplean 1,2bis(trimetoxisilil)etano (BTME) o 1,2-bis(trietoxisilil)etano (BTEE) [150]. La estructura
de ambos se representa en la Tabla 4.
Table 4. Precursors used for synthesis of ethane-PMO.
BTME
BTEE
Inagaki y col. [144] sintetizaron este tipo de materiales PMOs usando un
surfactante catiónico de naturaleza alquiltrimetilamonio bajo. Con esta síntesis se
consiguió obtener eteno-PMO con ordenamiento mesoporoso hexagonal 2D (P6mm) o
3D (P63/mm), dependiendo de la composición molar de partida.
Generalmente, el cloruro de alquiltrimetilamonio proporciona materiales con
simetría 2D. La síntesis cuyos productos de partida son BTME, alquiltrimetilamonio,
NaOH y agua permite formar materiales de alto orden con morfologías bien definidas
48
Introducción
como barras hexagonales (2D-hexagonal), partículas esféricas (3D-hexagonal) y
decaedros (Pm3n cúbica).
Kao y col. [151] sintetizaron materiales con simetría cúbica por cocondensación de TEOS (Tetraetil ortosilicato) y BTEE y bromuro de cetiltrietilamonio
como surfactante bajo condiciones ácidas. Empleando surfactantes iónicos se había
conseguido hasta el momento un diámetro de poro menor a 4nm. Por ello, con el fin
de poder aumentar el tamaño de poro, diversos investigadores comenzaron a emplear
copolímeros de tres bloques no iónicos para la síntesis de PMOs. Como surfactantes se
han utilizado una amplia variedad de óxido de fenil(polietileno) como Triton-X100, Brij56 y Brij-76. Este último proporciona una simetría hexagonal 2-D con mejores
cualidades, produciendo metano, eteno y benceno-PMO multifuncionales [152]. Este
surfactante puede ser usado bajo condiciones neutras en presencia de fluoruro como
catalizador ya que hidroliza el precursor y promueve la formación de oligómeros de
organosílica cargados negativamente. Dependiendo del precursor utilizado y la
composición del gel, se pueden obtener materiales con simetría hexagonal 2D, más o
menos ordenados usando como template P123.
El grado de ordenamiento puede aumentarse mediante la adicción de sales
inorgánicas. Como se ha comentado anteriormente, el empleo de aditivos incrementa
la interacción entre la cabeza de surfactante y las especies organosilícicas. Así Zhao y
col. [153] sintetizaron etano-PMO bajo condiciones ácidas y usando como surfactante
F127. El empleo de KCl fue necesario para la síntesis de estos PMOs con estructura
porosa de tipo caja y simetría Fm3m.
En diversas ocasiones, algunas aplicaciones como la adsorción de
biomoléculas o la encapsulación de nanopartículas de metal requieren de materiales
con morfologías y estructuras especiales. Lu y col. [154] sintetizaron etano-PMO como
partículas huecas con diámetros entre 300 y 500nm. Sus materiales tenían cavidades
de aproximadamente 100-150nm y paredes mesoestructuradas de 100nm de espesor
con simetría hexagonal 2D. Existen además, otras morfologías interesantes que han
sido estudiadas en los últimos años [155-157].
49
Capítulo II
Para la síntesis de PMOs con puentes fenilo y derivados, al igual que para
etano-PMO, se han empleado diferentes agentes directores de estructura, es decir
templates catiónicos, copolímeros en bloque, etc.
El grupo de investigación de Ozin [158] fue el primero que descubrió estos
materiales ordenados con puentes fenilo y para ello utilizaron como precursores 1,4bis(trietoxisilil)benceno (BTEB) y 2,5-bis(trietoxisilil)tiofeno. En su síntesis emplearon
cloruro de cetilpiridino (CPCL) bajo condiciones ácidas suaves. Más tarde modificaron
el surfactante empleando otros como CTAB o C16TEABr. Además se mencionó por
primera vez el ordenamiento de los grupos aromáticos dentro de las paredes, debido
probablemente al apilamiento π-π de los mismos.
Bajo condiciones ácidas y utilizando P123 se obtiene benceno-PMO con
grandes poros, múltiples canales con ordenamiento hexagonal 2D mientras que
usando F127 se obtienen PMOs 3D en forma de caja [159] empleando en ambos casos
1,4bis(trietoxisilil)benceno como precursor. Un importante desarrollo dentro de los
PMOs fue realizado por el grupo de Inagaki quienes sintetizaron PMOs con puentes
fenilo con un ordenamiento hexagonal 2D y organización (crystal-like) de sus puentes
orgánicos fenilo dentro de las paredes de los poros. Este material alterna capas
hidrófilas e hidrófobas compuestas por sílica y benceno [160].
Los PMOs preparados bajo condiciones ácidas generalmente, carecen de
periodicidad en las paredes de los poros. Sin embargo, Sayari [161] obtuvo bencenoPMOs con un determinado orden, usando surfactantes no iónicos. La disposición 2Dhexagonal permaneció inalterada usando NaOH. Además de precursores disilánicos, se
han obtenido PMOs a partir de precursores multisilánicos. Ozin y col. [162] utilizaron
como precursor anillos de benceno simétricamente integrados mediante su unión a
tres átomos de silicio en la estructura mesoporosa.
5.2.2.2 Aplicaciones de PMOs
Los PMOs al poseer alta área superficial tienen numerosas aplicaciones ya que
pequeñas cantidades de material producen efectos significativos. Una de las
principales aplicaciones de estos materiales radica en su uso como catalizadores.
50
Introducción
Existen diferentes estructuras de los PMOs sulfonados y su acidez depende del método
de obtención, el cuál puede ser por sulfonación directa, co-condensación y postmodificación o mediante el método grafting. Hay diversos factores que afectan a la
actividad catalítica como la estructura, la hidrofobicidad o la fuerza y la estabilidad de
los sitios ácidos. Sin embargo, no se puede establecer una relación clara entre la
actividad catalítica y la cantidad de sitios ácidos [163].
Los PMOs como catalizadores se han utilizado en la reacción de acetilación de
heptanal a 1-butanol [164], dimerización de 2-fenilpropeno [165], hidroxialquilación
[166] o hidrólisis de azúcar [163] entre otros. Por otro lado, una cantidad de centros
metálicos como Ti, Sn, V o Nb han sido incorporados en la estructura de los PMOs para
que también actúen como catalizadores. Además, la introducción de compuestos
organometálicos ofrece una serie de ventajas a la “catálisis verde”, particularmente, la
recuperación del catalizador, que normalmente es tóxico y caro, se evita la
contaminación del producto y permite la posibilidad de operar a flujo continuo. Con
este objetivo, varios complejos metálicos se han incorporado a la estructura de los
PMOs.
La obtención de partículas esféricas de los PMOs para ser usadas como fases
estacionarias en cromatografía, ha sido objeto de numerosas investigaciones Para
HPLC (High-performance liquid chromatography) es muy importante el tamaño de
estas esferas (entre 3 y 10 µm) [167, 168] y el tratamiento de funcionalización postsíntesis del material silícico para producir una fase estacionaria de naturaleza apolar
[169].
Recientemente se ha preparado un nuevo material híbrido para ser usado
como fase estacionaria en HPLC, para ello los PMOs con puentes etileno y sílica pura
fueron funcionalizados con eritromicina y vancomicina[137].
Se pueden encontrar aplicaciones en microelectrónica para estos materiales.
Para ello, la permitividad relativa o constante dieléctrica k debe tener un valor bajo
para prevenir la transferencia de carga y pérdidas de señal que puedan surgir por
acumulación de carga indeseable entre los elementos interruptores y los circuitos de
corriente. Las propiedades más importantes requeridas para un material dieléctrico
51
Capítulo II
ideal, es decir, bajo k, son: elevada porosidad, estabilidad mecánica y térmica y
resistencia a la humedad. Los PMOs debido a su volumen de poro grande, su elevado
contenido orgánico y el fácil control de su hidrofobia, son en principio muy adecuados
como dieléctricos de baja permitividad.
Los PMOs con grupos funcionales orgánicos poseen elevadas áreas
superficiales y tamaño y distribución de poro más homogéneos lo que los hace idóneos
para su utilización como indicadores ópticos. Varios investigadores prepararon un
sensor para la detección de compuestos orgánicos volátiles. Este material fue
sintetizado a partir de una partícula delgada de material mesoporoso híbrido de sílice y
benceno y se insertó en la estructura de un sensor de superficie fotovoltaica [170].
Adicionalmente, si los PMOs contienen grupos cromóforos orgánicos pueden
ser adecuados como elementos fotoactivos. En este sentido, se han desarrollado
PMOs con cromóforos orgánicos tales como viológenos, azobencenos, trifenilpirilio,
diacetileno, etc. [171-173].
Las organosilicas mesoporosas ordenadas ofrecen buenas propiedades
ópticas, particularmente cuando se encuentran en forma de película delgada. Inagaki y
col.
[174]
sintetizaron
un
nuevo
PMO
usando
como
precursor
2,6-
bis(trietoxisilil)antraceno y Rodio como catalizador. En este grupo de investigación
además de su síntesis, compararon sus propiedades ópticas con el ya preparado años
atrás, el 9,10 antraceno-PMO.
Por otro lado, los PMOs son buenos soportes para la inmovilización de
biomoléculas, tales como proteínas y péptidos. Las enzimas libres debido a su
inestabilidad, su alto coste y su dificultad para ser separadas de las mezclas de
reacción para su reutilización, no son siempre catalizadores ideales. Este problema se
puede solucionar mediante su inmovilización sobre algún sólido como polímeros
orgánicos, zeolitas, materiales mesoporosos, etc.
Blanco y col. [175] compararon el uso de diferentes sílicas mesoporosas (sílica
amorfa, SBA-15 y etano-PMO) como soportes de lipasa. Ellos concluyeron, tras una
serie de investigaciones, que los materiales silícicos ordenados son excelentes
52
Introducción
soportes para la inmovilización de esta enzima cuando la superficie es bastante
hidrofóbica. Además, los PMOS tienen excelentes propiedades como biocatalizadores
nanoestructurados con una mínima pérdida de actividad. Yang y col. [176] sintetizaron
organosílices mesoporosas con puentes 1,4 dietilbenceno por co-condensación directa
usando P123 como agente director de la estructura bajo condiciones ácidas. Se
obtuvieron buenos resultados para la inmovilización de lisozima en este material.
Otra de las aplicaciones principales y la que se desarrollará en esta Tesis
Doctoral es su uso como adsorbente en este caso de pesticidas, aunque también se ha
estudiado la adsorción de metales pesados, otros contaminantes orgánicos e incluso
gases. García y col. [177] sintetizaron una organosílica mesoporosa conteniendo urea
3+
mediante el método de co-condensación y la emplearon para la adsorción de Fe .
También se han sintetizado materiales mesoporosos ordenados funcionalizados con
2+
tiol para la adsorción de Hg [178].
La adsorción de compuestos orgánicos volátiles fue estudiada en 2006 en
benceno-PMO con ordenamiento hexagonal 2D y cúbico 3D [179]. Años más tarde,
Melde y col. llevaron a cabo la adsorción de otros gases como dióxido de silicio,
cloruro de cianógeno, amoníaco y octano en etano-PMO [180]. La adsorción del gas
hidrógeno fue comparada con etileno, etenileno, fenileno y bifenilo-PMO. Los PMOs
que contienen electrones π tienen mayor capacidad de adsorción que etano-PMO
[181]. El estudio de contaminantes que tengan anillos aromáticos en su estructura es
de especial interés por las interacciones π-π entre el adsorbente y adsorbato. La tabla
5 resume los compuestos orgánicos adsorbidos en PMOs.
5.2.3 Mesoporosos ordenados de carbón
Los materiales de carbono mesoporoso están considerados por muchos
investigadores como “la siguiente generación de materiales mesoporosos” [182]. Estos
compuestos tienen un área superficial elevada, volumen de poro grande,
químicamente inertes y conducen la electricidad. Es por ello, que se han empleado en
un gran número de aplicaciones como dispositivos electrónicos, separación de
membranas, adsorbentes, catalizadores, componentes ópticos y sistemas de
conversión de energía.
53
Capítulo II
Table 5 Precursors used for the adsorption of organic compounds.
PRECURSOR
COMPUESTO ORGÁNICO
REFERENCIA
1,4 bis (trietoxisilil)benceno
Mesosulfuron
[183]
1,4 bis (trietoxisilil)benceno
Benceno, tolueno, orto y
[184]
para-xileno
1,4 bis (trietoxisilil)benceno
Hidrocarburos
policíclicos
[185]
aromáticos
1,4 bis (trietoxisilil)benceno
1,4 bis (trietoxisilil)bifenilo
Nicotina
[186]
4-clorofenol
[187]
1,4 bis(trimetoxisililetil)benceno
4-nitrofenol, 4-clorofenol, 4-
[188]
1,2-bis(trietoxisilil)etano
metilfenol
bis(trimetoxisililetil)benceno
Trinitrotolueno
bis[3(trimetoxisilil)propil]amina
1,4 bis (trietoxisilil)benceno
1,2-Bis(trietoxisilil)etano
[189]
1,4 bis (trietoxisilil)benceno
5.2.3.1 Tipos de síntesis
En 1999 se sintetizó por primera vez un carbón mesoporoso usando silica
mesoporosa como “template”. Utilizando un proceso de carbonización y como
catalizador ácido sulfúrico, se convirtió la sacarosa en carbón. La estructura del nuevo
material no fue una réplica exacta del template pero permitió la formación de una
estructura ordenada [190]. Este método se conoce como “hard template” Sin
embargo, este tipo de síntesis consume mucho tiempo y además no es posible su
producción a gran escala. Estos obstáculos hicieron a muchos investigadores buscar un
nuevo método denominado “soft template” similar a la síntesis de sílicas mesoporosas
[191-193]. Aunque se han usado principalmente resinas fenólicas como fuente de
carbón, algunos investigadores han preferido compuestos más pequeños como
resorcinol [194, 195] o phloroglucinol [196].
En el caso del método soft template la síntesis no es tan flexible como en el
caso de las sílices. Lo mismo que ocurre con las sílices mesoporosas, el éxito de esta
técnica se debe al autoensamblaje cooperativo de las moléculas de surfactante y de
54
Introducción
precursor y la polimerización del precursor alrededor del cristal líquido. El problema
radica en que no se establecen las mismas interacciones entre los precursores de
carbono con las micelas debido a la baja densidad de los grupos funcionales.
La primera síntesis de estos compuestos usando fenol-formaldehído como
precursor fue realizada por Dai y col. [196, 197]. En estos trabajos se obtuvieron
materiales mesoporosos altamente ordenados como se muestra en la Figura 17. A
partir de estas publicaciones se han realizado numerosos estudios utilizando el mismo
método y empleando resinas fenólicas como precursor.
Las síntesis de resinas fenólicas y carbones mesoporosos ordenados se
pueden dividir en cuatro etapas: formación del prepolímero mediante la adicción de
formaldehido al compuesto fenólico, formación de la mesofase, eliminación del
template y carbonización (Figura 18).
(A)
(B)
Figure 17 . Electron microscopy images of the carbon film. (A) Highresolution SEM image of the surface of the carbon film with uniform
hexagonal-pore array, (B) SEM image of the film cross section.
OH
R
OH/H+
O
H
-Polimerización
-Formación
mesofase
-Eliminación
template
-Carbonización
H
Figure 18. Schematic representation of ordered mesoporous phenolic resina and carbon using
the soft template method.
L
55
Capítulo II
a formación del prepolímero de resina fenólica es llevada a cabo por un catalizador
ácido o básico y polimerizando el formaldehido con el compuesto de carbono. Como
se ha expuesto anteriormente se usan, normalmente, tres compuestos fenólicos que
difieren en el número de grupos hidroxilo y anillos aromáticos: fenol, resorcinol y
phloroglucinol (Figura 19).
El fenol es uno de los más utilizados ya que es un compuesto de bajo coste y
su velocidad de polimerización es fácil de controlar. El phloroglucinol tiene mayor
velocidad de polimerización ya que interacciona mejor con el agente director de la
estructura debido al mayor número de grupos hidroxilo. El pH y la temperatura
afectan directamente a la velocidad de reacción y al mecanismo de reacción entre los
dos monómeros.
OH
HO
OH
HO
OH
OH
Phenol
Resorcinol
Phloroglucinol
Figure 19 Materials used as carbon sources in the synthesis of phenolic resins
and ordered mesoporous carbons.
En medio ácido se produce una sustitución electrofílica aromática entre los
monómeros mientras que a pHs altos el compuesto fenólico se desprotona formando
ión fenóxido y ocurre una sustitución en el formaldehido. Así, si se trabaja en medio
ácido se forma el prepolímero Novalac y en medio básico Resol los cuales se
representan en la Figura 20. Ambos compuestos son fácilmente transformados en
polímeros orto y para sustituidos.
Una vez formado el prepolímero, se forma la red tridimensional por
policondensación. En presencia de un agente director de la estructura, se produce un
co-ensamblado del material.
56
Introducción
OH
OH
CH2OH
HOH2C
Novalac
OH
OH
OH
H2OCH2
C
HOH2C
CH2OH
CH2OCH2
HO
OH
CH2OH
CH2OH
Resol
Figure 20. Structures Novalac and Resol prepolymers.
Si se incrementa la concentración de surfactante y/o la temperatura, se
forman mesofases laminares, hexagonales o cúbicas periódicas. Este proceso ocurre
por la organización de moléculas asimétricas en agregados supramoleculares bien
definidos mediante interacciones no covalentes siempre por encima de la
concentración y temperatura micelar crítica. Para la síntesis de estos compuestos se
emplean copolímeros en bloque ya que no son tóxicos, disponibles comercialmente y
económicos. Los más usados son P123 (EO20PO70EO20), F127 (EO106PO70EO106) y F108
(EO132PO50EO132) donde EO son óxidos de etileno y PO óxidos de propileno.
Para la síntesis de estos materiales mesoporosos se han desarrollado varias
rutas. Zhang y col. [198] sintetizaron polímeros y carbones mesoporosos altamente
ordenados con estructura cúbica bicontinua. Para ello, emplearon como fuentes de
carbono, fenol y formaldehido; como agente director de la estructura P123 y medio
básico. Cuando el material se sometió a 350ᵒC en atmósfera de nitrógeno se formó el
2
3
polímero cuya área superficial fue de 550m /g y volumen de poro 0.4 cm /g. Sin
2
embargo, a 700ᵒC el área superficial aumento a 1150m /g y el volumen de poro a
3
0.57cm /g obteniéndose el carbón mesoporoso. Un año más tarde, sintetizaron más
materiales usando F127 o P123 como agentes directores de la estructura y diferentes
fuentes de carbono como fenol, formaldehido o moléculas de hidrocarburos
57
Capítulo II
(hexadecano o decano) en medio básico [199]. Con este tipo de síntesis, se pueden
obtener materiales con diferente morfología y estructura, haciéndolos muy
prometedores en un gran número de aplicaciones.
Este mismo grupo de investigación, sintetizó polímeros y materiales con el
método EISA (ensamblaje por evaporación inducida) utilizando etanol como disolvente
[200]. El área superficial y el volumen de poro de los carbones mesoporosos
2
3
ordenados formados a 900ᵒC fueron de 968 m /g y 0.56 cm /g, respectivamente. La
síntesis de estos materiales se esquematiza en la Figura 21. Estos nuevos compuestos
exhiben un volumen de poro y un área superficial mayor que los propuestos por el
método anterior.
Figure 21. Schematic representation of the procedure used to prepare polymers and ordered mesoporous
carbons [200].
Gao y col. [201] realizaron la síntesis de estos materiales con alguna
modificación. Concretamente, el disolvente que se utilizó fue una mezcla de etanol
agua y se llevó a cabo en medio ácido. La principal diferencia con respecto a las síntesis
anteriores, radica en que una vez que se añadieron todos los reactivos, la mezcla se
58
Introducción
dejó reposar hasta que se formaron dos fases las cuales se dejaron envejecer 96h.
Pasado ese tiempo, la capa superior fue desechada y la fase rica en polímero se dejó
agitar hasta que se formó el monolito. Finalmente fue calcinada en atmosfera de
nitrógeno como en las demás síntesis. Los compuestos obtenidos después de la
calcinación tienen mayor área superficial y volumen de poro que los expuestos por los
anteriores autores.
A diferencia de los materiales mesoporosos inorgánicos, en las resinas
fenólicas y en los carbones mesoporosos se elimina el template bajo atmósfera de
nitrógeno [202]. El agente director de la estructura es completamente eliminado a
13
350ᵒC en atmósfera de nitrógeno como muestran los análisis de TGA, C NMR y FTIR.
No obstante, una pequeña cantidad de oxígeno durante el proceso de calcinación
facilita la degradación del template y los segmentos orgánicos, dando lugar a la
formación de materiales con tamaño de poro grandes, así como áreas superficiales
más elevadas [203]. Por otro lado, se debe controlar la velocidad de flujo de los gases
ya que una gran velocidad podría ocasionar una deformación en la estructura porosa
del material. Por ello, se establece una velocidad de calentamiento entre 1 y 5ᵒC/min a
350-400ᵒC durante 4-6h.
El proceso de carbonización comienza a 400ᵒC contrayéndose la estructura
ligeramente. Desde 600ᵒC no se observa ninguna contracción por lo que se puede
aumentar la velocidad de calentamiento. Los poros se vuelven asimétricos. Entre 600 y
800ᵒC se forman microporos y simultáneamente incrementa el área superficial [204].
Por encima de 800ᵒC la estructura comienza a distorsionarse un poco y los poros se
vuelven aún más asimétricos [205].
5.2.3.2 Aplicaciones de las resinas fenólicas y carbones mesoporosos ordenados
Los carbones mesoporosos ordenados al tener alta área superficial y poros
grandes y ajustables pueden ser muy prometedores para ser usados como electrodos
para baterías, pilas de combustible y supercondensadores, como adsorbentes para
procesos de separación y almacenamiento de gas y como soportes para una amplia
variedad de procesos catalíticos. Estos materiales pueden, además, ser modificados
con grupos funcionales o añadiendo metales a su estructura y pueden ser obtenidos
59
Capítulo II
directamente añadiendo el metal u otro precursor a la síntesis o empleando un
tratamiento post-síntesis como impregnación, adsorción, grafting, o intercambio
iónico.
Dai y col. [206] fabricaron películas de carbón mesoporoso conteniendo V y
Co. La conductividad de estos materiales fue superior a 20 S/cm y la capacitancia
específica para Co y V fueron de 113 F/g y 159 F/g respectivamente. En 2013, Guan y
col. sintetizaron carbón mesoporoso con poros ordenados y desordenados mediante el
método EISA bajo condiciones básicas. Estos fueron empleados como electrodos
usando H2SO4 como electrolito y empleando un simple tratamiento ácido para poder
ser usados [207]. Un año más tarde, Guo y col. [208] fabricaron un electrodo de carbón
mesoporoso dopado con nitrógeno obteniendo altas capacitancias específicas 225 F/g
mientras que el compuesto no dopado ofrecía una capacitancia de 168 F/g. Este nuevo
compuesto exhibía buena estabilidad electroquímica después de 1000 ciclos a 5 A/g.
El campo de adsorción comprende un gran número de compuestos orgánicos,
inorgánicos y gaseosos. En 2009, Deng y col. [209] prepararon varios carbones
mesoporosos ordenados por el método de soft-template dopados con los metales de
transición Pd, Pt, Ni y Ru. Demostraron que la adsorción de hidrógeno aumentaba en
todos los compuestos mesoporosos dopados exceptuando el carbón mesoporoso
dopado con Niquel que mostró una tendencia inversa y además, el metal no afectó a la
cinética de adsorción de hidrógeno. Zhang y col. [210] utilizaron el carbón mesoporoso
dopado con nitrógeno para adsorber los gases CO2 y SO2. Este compuesto exhibió
-1
-1
buena capacidad de adsorción, 2.43 mmolCO2·g y 119.1 mgSO2·g . Sin embargo,
cuando el mismo material se activó con KOH el poder de adsorción aumentó
-1
-1
significativamente a 4.3 mmolCO2·g y a 213.1 mgSO2·g .
Estos materiales también han sido empleados para eliminar metales pesados.
Así, Jean-Ping y col. [211] prepararon un carbón mesoporoso que fue obtenido
después de eliminar la sílica con HF. Cu(II) y Cr(VI) fueron los iones empleados para
realizar las pruebas de adsorción. Los resultados mostraron baja capacidad de
adsorción para el Cr(VI) pero sin embargo, fue muy selectivo para la adsorción de
Cu(II). Ese mismo año se sintetizó un material híbrido funcionalizado con tiol
60
Introducción
empleando un copolímero en bloque. Éste mostró muy buenas propiedades para ser
-1
-1
utilizado como adsorbente de Ag(I) (2.11 mmol·g ) y Pb(II) (1.6 mmol·g ) [212].
Además del carbón mesoporoso, también la resina fenólica se ha utilizado
como adsorbente para eliminar compuestos orgánicos. Zhao y col. [213] prepararon
una resina fenólica altamente ordenada con resol como precursor y varias sílicas
mesoporosas como hard template. Los materiales obtenidos tenían áreas superficiales
2
-1
3
-1
altas (1600m ·g ) y grandes volúmenes de poro (1.12 cm ·g ). Estos materiales
-1
exhibieron alta capacidad de adsorción para anilina (134.2 mg·g ) y fucsina (317.5
-1
mg·g ).
Existen también estudios de adsorción de varios alcaloides (clorhidrato de
berberina, colchicina y matrina) en carbones mesoporosos ordenados. La capacidad de
-1
-1
adsorción para una concentración de 0.1 mg·L y 298 K fue de 450, 600 y 480 mg·g ,
respectivamente [214]. También existen investigaciones para eliminar tintes como el
verde de metileno con estos compuestos híbridos conteniendo cobalto. Los resultados
demostraron que la capacidad de adsorción dependía de la concentración de Co
-1
aumentando de 2 a 90 mg·g [215].
Entre las aplicaciones a destacar de estos materiales, se encuentra su
capacidad para ser usados como catalizadores. En 2014, Liu y col. [216] prepararon
carbones mesoporosos obtenidos por los métodos hard template, soft template e
hidrotermal. Las muestras fueron además soportadas con Pd mediante el método de
impregnación. El compuesto obtenido por hard template presentaba una mejor
selectividad 82.2% y una conversión de 99.4% para pasar de la oxima ciclohexanona a
nitrociclohexano. Ese mismo año Gao y col. [217] sintetizaron carbón mesoporoso
conteniendo Nb para ser utilizado en la deshidratación de la fructosa a 5hidroximetilfurfural. Un 100% de conversión y un 76.5% de selectividad se consiguió
con el material que contenía 76.5% de Nb2O5.
Finalmente, otra aplicación importante es su uso como catalizadores en la
síntesis de biodiesel. Dong y col. [218] demostraron que los carbones mesoporosos
+
-1
ordenados sulfonados tenían una alta actividad catalítica (2.21 mmol H ·g ) a 400ᵒC
61
Capítulo II
manteniendo su estructura. Para evaluar su actividad se estudió la reacción de
esterificación de ácido oleico con metanol exhibiendo una conversión del 95%.
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78
CAPÍTULO III. MATERIALES Y MÉTODOS
Materiales y métodos
1. MATERIALES
Para la síntesis de los diferentes materiales adsorbentes empleados en esta
Tesis Doctoral se han utilizado los siguientes reactivos:
Líquidos
-Etanol; C2H6O
-Ácido clorhídrico; HCl
-1,2-bis(trietoxisilil)etano; (CH3CH2O)3SC2H4Si(CH3CH2O)3
-1,2bis(trietoxisilil)benceno; (CH3CH2O)3SiC6H4Si(CH3CH2O)3
Sólidos
-Dodecilsulfato de sodio; (C12H25NaO4S)
-Ácido tetradecanodioico; (C14H26O4)
-Nitrato de magnesio;Mg(NO3)2·6H2O
-Nitrato de aluminio; Al(NO3)3·9H2O
-Hidróxido sódico; NaOH
-Carbonato de sodio; Na2CO3
-Polioxietileno (10)-estearil éter; (Brij 76); C18H37(CH2CH2O)n n=10 aprox.
-Pluronic F127; EO106PO70EO106
-Resorcinol; C6H6O2
-Formaldehído; 36% en agua
Gases
-Nitrógeno 5.0. Envasado en botellas de acero a presión de 200 bares y con pureza de
99.99%
81
Capítulo III
1. 1 Síntesis de adsorbentes
1.1.1 Hidrotalcita con carbonato en la interlámina
La síntesis de la hidrotalcita con el anión inorgánico en la interlámina fue
preparada según el procedimiento descrito por Reichle [1]. Se seleccionó el anión
2-
2+
3+
inorgánico CO3 y como metal divalente y trivalente Mg y Al respectivamente. La
2+
3+
relación M /M fue igual a tres. Esta hidrotalcita fue calcinada a 500ᵒC dando lugar al
óxido mixto de Mg-Al, el cuál fue usado como adsorbente para los pesticidas
Mecoprop-P y Nicosulfuron.
1.1.2. Hidrotalcita con anión orgánico en la interlámina
Se
escogieron
dodecilsulfato
y
tetradecanedioato
como
aniones
interláminares para la síntesis de hidrotalcitas modificadas orgánicamente con el fin de
poder comparar el efecto del tamaño de la cadena en la adsorción de compuestos.
Para su síntesis también se utilizó el método de coprecipitación o síntesis directa y
2+
3+
Mg y Al como metales.
El dodecilsulfato de sodio C12H25NaO4S también conocido como laurilsulfato
-1
de sodio, tiene un peso molecular de 288.38 g·mol y es muy soluble en agua. La
molécula posee propiedades anfifílicas ya que tiene un grupo sulfato adosado a una
cadena de 12 átomos de carbono. La solubilidad en agua de este compuesto es
-1
independiente del pH siendo de 43 g·L . Su punto de fusión varía entre 204 y 207ᵒC.
El ácido tetradecanodioico es un sólido blanco con un peso molecular de
-1
258.35 g·mol . Es una molécula con 14 átomos de carbono y dos grupos ácido en sus
extremos. Su pKa es aproximadamente de 4.48 y su punto de fusión 127ᵒC. La
solubilidad de este compuesto depende del pH del medio. Así, a pH<5 es poco soluble
-1
-1
(0.18 g·L ), a pH 6 el compuesto tiene una solubilidad de 57g·L y a pH>6 es muy
-1
soluble (1000g·L ).
En la Figura 22 se esquematiza el procedimiento de síntesis empleado para la
obtención de las hidrotalcitas orgánicas.
82
Materiales y métodos
En la síntesis de estos materiales es muy importante establecer una atmósfera
2-
inerte durante el proceso debido a la alta afinidad del anión CO3 por la interlámina.
Este anión se forma en medio básico cuando se pone en contacto el CO2 con el H2O.
Figure 22. Schematic representation of hydrotalcite synthesis
También es muy importante obtener una fase ordenada y pura. Para ello se
añade la disolución de los metales muy lentamente sobre la disolución del anión
interlaminar. En ciertas ocasiones es necesario el control de pH.
La
fórmula
teórica
de
las
hidrotalcitas
orgánicas
son
[Mg3Al(OH)8]CH3CH2SO4·nH2O para la hidrotalcita con dodecilsulfato en la interlámina,
HTDDS y [Mg3Al(OH)8]2(CO2(CH2)12CO2 para la hidrotalcita con tetradecanodioato en la
interlámina, HTTDD.
1.1.3 Etileno y fenileno PMOs
Para la síntesis de estos dos materiales se ha seguido el procedimiento de
Burleigh y col. [2] con alguna modificación en las proporciones de ácido clohídrico y
etanol en el proceso de extracción. Los precursores fueron bis(trietoxisilil)etileno y 1,4bis(trietoxisilil)benceno (Figura 23) y como surfactante Brij 76.
Bis(trietoxisilil)etileno
1,4-bis(trietoxisilil)benceno
Si(OEt)3
Si(OEt)3
Si(OEt)3
Figure 23. Precursors for the synthesis of PMOs.
83
Si(OEt)3
Capítulo III
En la Figura 24 se esquematiza la síntesis y la extracción del surfactante
mediante la adicción de una mezcla de etanol y ácido clorhídrico.
Brij-76
B
i
Solution
H2O:HCl
Room
temperature
24 h
Organosilane
50ᵒC, 24h
90ᵒC, 24h
Filtrated
and dry
Extraction
PMO
material
Solution
EtOH:HCl
Filtrated
and dry
Surfactant
extraction
Figure 24. Experimental procedure for the preparation of periodic mesoporous organosilicas
using Brij 76 as “template”.
1.1.4 Resina fenólica y carbon mesoporosos ordenados
Para la síntesis de estos materiales se utilizó resorcinol y F127 como
precursor. El resorcinol o benceno 1-3-diol es un sólido soluble en agua con reacción
-1
ligeramente ácida cuyo pKa es 9.45 y densidad 1.27 g·mol . Su peso molecular es 110
-1
g·mol y su punto de fusión 110ᵒC. El Pluronic F127 es un copolímero en bloque cuya
estructura es EO106PO70EO106.
En la Figura 25 se esquematiza el proceso de síntesis llevado a cabo para estos
dos materiales. Cuando se calcinó en atmósfera de nitrógeno a 380ᵒC, el material
obtenido fue la resina fenólica ordenada (MPR). Sin embargo, cuando la temperatura
se aumentó a 800ᵒC, el compuesto sintetizado fue el carbón mesoporoso ordenado.
84
Materiales y métodos
HCl:ETOH
+ F127
+
≈15min
Evaporation
≈20min
MPR
Calcination ,380ᵒC
MC
Calcination ,800ᵒC
Aging 60ᵒC, 24h.
Figure 25. Representation of synthesis of ordered phenolic resin and mesoporous carbon.
1.2 Pesticidas
Los pesticidas empleados fueron S-Metolacloro, Mecoprop-P, Nicosulfuron y
Bentazona cuyas estructuras se recogen en la Figura 26.
N
O
O
Cl
O
O
N
N
N
O
O
S
N
H
O
N
H
N
O
Nicosulfuron
S-Metolacloro
O
H
N
O
S
O
O
OH
N
Cl
O
Mecoprop-P
Bentazon
Figure 26. Pesticides used in this phD.
1.2.1 S-Metolacloro
El S-Metolacloro es el nombre comercial del 2 cloro-N-(2-etil-6-metilfenil)-N[(2S)-1-metoxi-2-propanil]acetamida
(número
85
del
CAS
87392-12-9)
que
fue
Capítulo III
suministrado por Sigma Aldrich con una pureza de aproximadamente el 100%. Es un
-1
líquido amarillento no ionizable con un peso molecular 283.20 g·mol . La solubilidad
-1
en agua es de 530mg·L y su punto fusión es aproximadamente -62.1ᵒC.
Es una cloroacetanilida con actividad herbicida de preemergencia, selectiva,
que actúa inhibiendo la división celular con lo que impide la germinación de las
semillas y dificulta el crecimiento, en especial, de las raíces. Dentro del grupo de las
clorocetanilinas, S-Metolacloro es uno de los pesticidas más usados en el mundo,
aproximadamente representa un 4.2% del pesticida global usado.
En general, la dosis empleada depende del formulado del pesticida y del tipo
de preparado de este. Puede estar como suspensión concentrada (SC), dispersión
oleosa (DO), granulado dispersable en agua (WG), concentrado emulsionable (EC),
suspo-emulsión (SE) o concentrado soluble (SL) como queda reflejado en la Tabla 6.
En Francia el isómero rac-Metolacloro fue prohibido el 30 de Diciembre de
2003. En su lugar, se comenzó a utilizar S-Metolacloro (Anexo 1 Directiva 91/414/CEE).
El paso de una sustancia a otra fue motivado por la alta eficiencia de eliminación de las
malezas lo que condujo a una reducción de las dosis aplicadas por los agricultores
-1
-1
(2.1kg·ha en 1998 a 0.5kg·ha en 2012).
Debido a su baja capacidad de adsorción y alta solubilidad en agua se detecta
fácilmente en el suelo y en la superficie del agua. Por ello, es una fuente potencial de
contaminación.
Table 6 Formulation of S-Metholachlor according to Ministry of Agriculture, Food and Environment
FORMULADO
NOMBRE
TIPO
DE DOSIS
FABRICANTE
COMERCIAL
PREPARADO EMPLEADA
Mesotriona 4% + S- Camix
SE
3-3.75 L·ha-1
SYNGENTA ESPAÑA S.A.
Metolacloro 40% (P/V)
S-Metolacloro
31.25% Primexta Liquid SC
3-4 L·ha-1
SYNGENTA ESPAÑA S.A.
terbutilazina
18.75% gold
(P/V)
Tyllanex
magnum
S-Metolacloro 96% (P/V)
Dual gold
EC
0.5-1.6 L·ha-1
SYNGENTA ESPAÑA S.A.
Dual gold 96 EC
86
Materiales y métodos
1.2.2 Mecoprop-P
El Mecoprop es un herbicida cuya sustancia activa es el ácido
fenoxipropiónico clorado que se registró en 1960. Este compuesto consiste en la
mezcla de dos isómeros en igual proporción pero solo uno de ellos es activo. Por ello,
en 1980, tras el descubrimiento de nuevas tecnologías se consiguió separar estos
isoméros y obtener así un pesticida con el isómero activo con una pureza de
aproximadamente el 95%. Este compuesto se denominó Mecoprop-P.
Table 7 Formulation of Mecoprop-P according to Ministry of Agriculture, Food and Environment.
FORMULADO
2,4-D a 7% (sal dimetilamina)
+
dicamba
2%
(sal
dimetilamina) + mcpa 7% (sal
dimetilamina) + mecoprop-p
4,2% (sal dimetilamina) P/V
Bromoxinil
12%
(ester
octanóico) + mecoprop-p 18%
(2-etil-hexil ester) P/V
Bromoxinil 12% (octanoato) +
ioxinil 12% (octanoato) +
mecoprop-P
36%
(ester
butoxietílico) P/V
Bromoxinil
7,5%
(ester
octanóico) + ioxinil 7,5% (ester
octanóico) + mecoprop-P
18,75% (ester 2-etilhexil) P/V
Carfentrazona-etil 1,5% +
mecoprop-p 60% (sal de
magnesio) P/P
Ioxinil 18% (ester octanóico) +
mecoprop-p
29%
(ester
butoxietil) P/V
MCPA 16% (sal amina) +
mecoprop-P 13% (sal amina) +
diclorprop-P 31% (sal amina)
P/V
Mecoprop-P 28% (sal potásica)
+ piraflufen-etil 0,45% P/P
Mecoprop-P
28,75%
(sal
dimetilamina) P/V
Mecoprop-P 60% (sal amina
(esp.)) P/V
Mecoprop-P 73,4% (sal de
magnesio y de dimetilamina) +
tribenuron-metil 1% P/P
NOMBRE
COMERCIAL
Dicotex
TIPO
DE
PREPARADO
SL
DOSIS
EMPLEADA
5 L·ha-1 o
1000L·ha-1
FABRICANTE
AGRIPHAR, S.A.
(consultar
www.magrama.es
)
Driwer
EC
2L·ha-1
EXCLUSIVAS SARABIA,
S.A.
Image
I-B-M Valles
EC
1-1.75 L·ha-1
NUFARM
S.A.
Brioxil super
P
EC
1.5-2 L·ha-1
ARAGONESAS AGRO,
S.A
Platform S
WG
1Kg·ha-1
FMC CHEMICAL SPRL
Certrol H
Mexiron
EC
1.3L·ha-1
NUFARM
S.A.
Duplosan
super
Optica Trio
SL
2.5 L·ha-1
NUFARM
ESPAÑA,
S.A.
NUFARM UK LIMITED
Caravan
WG
1.5-2Kg·ha-1
Herbimur
Forte
Merekal
SL
2-4 L·ha-1
SL
1.33L·ha-1
Aralis
SG
1.09 Kg·ha-1
CHEMINOVA AGRO,
S.A.
EXCLUSIVAS SARABIA,
S.A.
NUFARM
ESPAÑA,
S.A.
DU PONT IBERICA,
S.L.
ESPAÑA,
ESPAÑA,
Mecoprop-P es el nombre comercial de ácido (R)-2-(4-cloro-o-toliloxi)
propiónico con el número CAS 16484-77-8. Es un pesticida de post-emergencia con un
87
Capítulo III
-1
-1
peso molecular de 214.65 g·mol . Tiene una solubilidad en agua a 20ᵒC de 860 mg·L y
un pKa de 3.86. En condiciones aerobias tiene una vida media de 21 días en el campo.
La dosis empleada se muestra en la tabla 7.
1.2.3 Nicosulfuron
El Nicosulfuron es el nombre comercial de 2-[(4,6-Dimetoxipirimidin-2ilcarbamoil) sulfamoil]-N,Ndimetilnicotinamida (número del CAS 111991-09-4) que fue
suministrado también por Sigma-Aldrich. Es un pesticida de post-emergencia del grupo
-1
de las sulfonilureas con un peso molecular de 410.41 g·mol y un pKa de 4.78. Su
-1
solubilidad en agua a 20ᵒC es de 7500 mg·L y su punto de fusión 145ᵒC. Este pesticida
es muy usado en España para la eliminación de las malas hierbas en el cultivo del maíz.
En el suelo tiene una vida media de 26 días bajo condiciones aerobias y 63
días en ausencia de oxígeno. Este pesticida muestra una elevada movilidad en suelos
de textura franco arenosa y franco limosa. Sin embargo, algunos de sus metabolitos
son más móviles que el propio Nicosulfuron siendo sus principales productos de
degradación la piridina sulfonamida y la pirimidina amina. Además, este plaguicida es
susceptible a una fotólisis lenta en la superficie de los suelos y en agua que depende
del pH, siendo mayor la descomposición en condiciones ácidas. La dosis empleada de
este plaguicida depende de la composición del mismo como se muestra en la tabla 8.
1.2.4 Bentazona
Bentazona es el nombre comercial de 3-isopropil-1H-2,1,3-benzotiadiazin
4(3H) ona, 2,2-dióxido. Es un pesticida de post-emergencia cuyo número CAS es
25057-89-0 y un pKa de 3.28. Tiene un peso molecular de 240.3 g·mol
-1
y una
-1
solubilidad en agua a 20ᵒC de 570 mg·L . Su punto de fusión es de 140ᵒC. y es estable
en soluciones ácidas o básicas.
La Bentazona sódica es considerablemente más soluble en el agua que el
compuesto original con una solubilidad de 2300 g·L
-1
Este pesticida es estable a la hidrólisis pero se fotodegrada en el agua en
menos de 24h. También se fotodegrada en el suelo bajo condiciones aerobias, con una
vida media que fluctúa entre menos de 7 dias y 14 semanas, dependiendo de los tipos
88
Materiales y métodos
de suelo y de las condiciones. Además este pesticida es muy móvil en el suelo, y por
consiguiente tiene el potencial de contaminar las aguas superficiales. Las dosis
empleadas se especifican en la tabla 9.
Table 8. Formulation of Nicosulfuron according to Ministry of Agriculture, Food and Environment.
FORMULADO
Nicosulfuron 24% (P/V)
Nicosulfuron
3%
+
mesotriona 7.5% (P/V)
Nicosulfuron 4% P/V
NOMBRE
TIPO
DE
DOSIS
COMERCIAL
PREPARADO
EMPLEADA
Chaman Forte
Milagro 240 SC
Elumis
SC
0.25 L·ha-1
DO
1-2 L·ha-1
SYNGENTA ESPAÑA S.A.
Nicosulfuron
4% OD
Elite M
Samson
DO
1.5 L·ha-1
SHARDA EUROPE B.V.B.A
Nico
Simun
Bandera
CHEMINOVA AGRO, S.A.
ISK BIOSCIENCES EUROPE
S.A.
Nic-Sar
Nicosulfuron 4%
FABRICANTE
EXCLUSIVAS SARABIA, S.A.
1-1.5L·ha-1
SC
Nic-4
BELCHIM
CROP
PROTECTION
ESPAÑA,
S.A.
ROTAM AGROCHEMICAL
EUROPE LIMITED
SHARDA EUROPE B.V.B.A
Sajon
SAPEC AGRO S.A.U
Nicozea
Nicosulfuron
42.9%
+rimsulfuron 10.7% (P/P)
Nicosulfuron 6% P/V
Nicosulfuron 75% P/P
TRADE
CORPORATION
INTERNATIONAL, S.A.
CHEMINOVA AGRO, S.A.
ARAGONESAS AGRO, S.A.
Chaman
Tomcat-maiz SC
Tomcat-maiz
Nicovita
Nicogan
Kelvin
Principal
WG
90g·ha-1
Elite Plus 6 OD
DO
Accent 75 WG
WG
0.5-0.75
L·ha-1
53-83g·ha-1
DU PONT IBERICA, S.L.
89
-DU PONT IBERICA, S.L.
-ISK BIOSCIENCES EUROPE
S.A.
- DU PONT IBERICA, S.L.
Capítulo III
Table 9. Formulation of Bentazon according to Ministry of Agriculture, Food and Environment.
FORMULADO
Bentazona 48% (sal
sódica)
Bentazona 48% (sal
sódica) (ESP) P/V
Bentazona
48%
+
Imazamox 2.24% P/V
Bentazona 87% P/P
NOMBRE
COMERCIAL
Basagran L
Troy 480
Kaos-B
TIPO
DE
PREPARADO
SL
DOSIS
EMPLEADA
1.5-2 L·ha-1
SL
2 L·ha-1
Corum
SL
Basagran SG
SG
1.12-1.88
L·ha-1
1-1.15 Kg·ha-1
90
FABRICANTE
BASF ESPAÑOLA, S.L.
AGRIGHEM B.V.
SAPEC AGRO S.A.
BASF ESPAÑOLA S.L.
BASF ESPAÑOLA S.L.
Materiales y métodos
2.MÉTODOS
En esta sección se describen brevemente todas las técnicas llevadas a cabo
para la caracterización de los materiales utilizados como adsorbentes para poder
conocer la composición y sus características estructurales. Así mismo, se han
empleado técnicas para poder determinar la concentración de pesticida adsorbido o
bien observar las diferencias tras la adsorción.
2.1 Espectroscopía de Absorción Atómica
Esta técnica data del siglo XIX, aunque no fue hasta la década de los 50
cuando fue desarrollada en profundidad por un equipo de químicos de Australia,
dirigidos por Alan Walsh. Esta técnica ha permitido junto con el análisis
termogravimétrico conocer la composición de las hidrotalcitas.
El análisis químico de los metales se ha llevado a cabo mediante
espectroscopía de absorción atómica previa disolución de las muestras con HCl
concentrado (50%). En esta técnica los átomos, en el atomizador, pasan a estado
gaseoso fundamental y sus electrones son promovidos a orbitales más altos por un
instante mediante la absorción de una cierta cantidad de energía. Esta cantidad de
energía se corresponde a una transición electrónica típica de cada elemento.
Se puede calcular a partir de la ley de Beer-Lambert cuántas de estas
transiciones tienen lugar y así obtener una señal que es proporcional a la
concentración del elemento, ya que la cantidad de energía absorbida en la llama
puede medirse con un detector.
El análisis elemental de Mg y Al se realizó en un espectrofotómetro de Perkin
Elmer 3100 después de disolver las muestras en HCl. Para determinar Mg se emplea un
atomizador de llama con una cabeza de quemador de 10 cm, que utiliza flujo de
acetileno 2.5L·min-1 como combustible y de aire 4L·min-1 como oxidante. Sin embargo,
para Al se emplea un atomizador de llama con una cabeza de quemador de 5 cm, con
-1
un flujo de protóxido de nitrógeno de 3.5L·min como combustible y de aire a 4 L·min
91
-1
Capítulo III
como oxidante. El análisis de S, C y N fue llevado a cabo en un analizador Eurovector
3A 2010.
2.2 Difracción de Rayos X
La difracción de rayos X es la técnica básica para el análisis cualitativo y
cuantitativo de fases cristalinas de cualquier tipo de material, tanto natural como
sintético. La difracción de rayos X se basa en la dispersión coherente de un haz
monocromático de rayos X por parte de la materia y en la interferencia constructiva de
las ondas que están en fase y que se dispersan en determinadas direcciones del
espacio.
Este fenómeno se puede explicar mediante la ley de Bragg que predice la
dirección en la que se da la interferencia constructiva de las ondas que están en fase y
que se dispersan en determinadas direcciones del espacio:
𝑛𝜆 = 2𝑑𝑠𝑒𝑛𝜃
Esta ecuación nos indica la relación que existe entre el espaciado entre dos
planos de átomos (d), la longitud de onda de la radiación X utilizada (𝜆) y el ángulo de
incidencia de los rayos X (𝜃), siendo n un número entero.
Los estudios de difracción de rayos X fueron realizados en un difractómetro
Siemens modelo D5000, controlado por el sistema informático DiffracPlus BASIC 4.0,
provisto de un monocromador de grafito para el haz difractado y detector de tipo
proporcional. La radiación utilizada fue Cu Kα1,2 (λ1 = 1.54056 Å y λ2 = 1.54439 Å) y las
condiciones de registro fueron 40 kV y 30 mA.
Para poder obtener difractogramas a bajo ángulo se empleó el difractómetro
ARL X´TRA Thermo Scientific. En este caso la radiación fue también Cu Kα a 45kV y
44mA.
92
Materiales y métodos
2.3 Microscopía Electrónica
2.3.1 Microscopía Electrónica de Transmisión
El microscopio electrónico de transmisión (TEM) es un instrumento que
aprovecha los fenómenos físico-atómicos que se producen cuando un haz de
electrones
suficientemente
acelerado
colisiona
con
una
muestra
delgada
convenientemente preparada.
A partir del modelo construido por Knoll y Ruska en 1931, varias casas
comerciales han fabricado diversos modelos de microscopios electrónicos modernos,
pero todos ellos basados en el modelo original.
La obtención de las imágenes se llevo a cabo con dos microscopios
electrónicos pertenecientes a los Servicios Centrales de Apoyo a la Investigación (SCAI)
de la Universidad de Córdoba: el JEOL JEM 1400 con resolución de 0.38nm entre
puntos con voltaje de aceleración 80, 100 y 120 kV y el microscopio JEOL JEM 2010 de
alta resolución (resolución de 0.194 nm entre puntos) y un voltaje de aceleración de
80, 100, 120, 160 y 200Kv. Previo a su análisis, las muestras se dispersaron en etanol,
colocando la mezcla en un baño de ultrasonidos y, transcurridos 15 minutos, una
microgota de suspensión se depositó sobre una rejilla de cobre recubierta de carbón
suministrado por la empresa ANAME.
2.3.2 Microscopía Electrónica de Barrido
El microscopio electrónico de barrido (SEM) fue inventado en 1937 por
Manfred von Ardenne. Este aparato utiliza un haz de electrones para generar una
imagen.
Esta técnica nos permite observar y estudiar superficies de un área muy
reducida de cualquier tipo de material, bien sea orgánico o inorgánico, sin necesidad
de pasar por procesos de preparación muy complejos, ya que los requerimientos más
importantes son que la muestra esté deshidratada y sea eléctricamente conductora.
El equipamiento actual consta de un detector de electrones secundarios (SE)
que proporciona imágenes topográficas de la superficie de la muestra, un detector de
electrones retrodispersados (BSE) que proporciona imágenes en escala de grises
93
Capítulo III
relativas a los elementos constituyentes de la muestra y un detector de energías
dispersivas de rayos X (EDX) que permite el análisis cualitativo y semicuantitativo de
los elementos constituyentes de la muestra.
El microscopio utilizado, un Jeol modelo JMS 6400 del SCAI de la Universidad
de Córdoba, emplea un voltaje de aceleración del haz de electrones de 20 kV. Las
muestras colocadas sobre cinta adhesiva de cobre se unen a un portamuestras
también de cobre de 15 mm de diámetro. Después se recubren, solo los sistemas no
conductores, con una capa fina de oro.
2.4. Espectroscopía Fotoelectrónica de Rayos X (Xps)
La espectroscopía de fotoelectrones de rayos X fue desarrollada por Kai
Siegbahn a partir de 1957 y se utiliza para estudiar los niveles de energía de los
electrones atómicos básicos, principalmente en sólidos.
Es una espectroscopía semicuantitativa y de baja resolución espacial que se
emplea para determinar la estequiometría, estado químico y estructura electrónica de
los elementos que existen en la superficie de un material. Es una técnica superficial
debido al corto rango de los fotoelectrones que son excitados en el sólido, alcanzando
profundidades de 10 a 100Å. La energía de los fotoelectrones que abandonan la
muestra es determinada mediante un analizador CHA (Concentric Hemispherical
Analyser) que proporciona un espectro con una serie de picos de fotoelectrones. Así,
cada elemento tiene una energía de unión de los picos (Binding Energy) y el área de
éstos puede ser utilizada para determinar la composición de la superficie del material.
La espectroscopia XPS trabaja a ultravacío para prever la contaminación de la
muestra. Además, la muestra puede ser bombardeada con iones Argon para eliminar
la posible contaminación superficial. Los espectros fueron registrados en un
espectrofotómetro PHI-5700 fabricado por Physical Electronic, utilizando como fuente
de excitación Mg Kα (1253.6 eV). Para la adquisición de datos el equipo cuenta con un
software PHI ACCESS-ESCA V0.6F. El espectrofotómetro se calibró corrigiendo la señal
con la energía de enlace de los electrones C 1s a 248.8 eV, respecto del nivel de Fermi.
94
Materiales y métodos
2.5 Espectroscopía Infrarroja con Transformada de Fourier
Es una técnica que se emplea para identificar moléculas a través del análisis
de sus enlaces químicos. Cada enlace químico en una molécula vibra a una frecuencia
característica y, generalmente, esta frecuencia se encuentra dentro del intervalo de la
radiación infrarroja. Cuando una molécula absorbe un fotón, salta de un estado
fundamental a un estado excitado, y da lugar a una vibración.
La espectroscopía infrarroja ha sido de gran utilidad para la caracterización de
los adsorbentes así como confirmar la presencia de los plaguicidas en los productos de
adsorción.
Los espectros de absorción de infrarrojo de las distintas muestras se
registraron con un espectrofotómetro de infrarrojo con transformada de Fourier
-1
Perkin-Elmer Spectrum One 2000 en un rango de número de onda entre 400 cm y
-1
-1
4000cm y con una resolución de 4cm . Los espectros obtenidos son el promedio de
64 registros mejorando así la relación señal/ruido. Las muestras, antes de realizar el
espectro, se dejan en estufa a 60⁰C durante 24h. Se utiliza como blanco una pastilla de
KBr.
2.6 Espectroscopía Resonancia Magnética Nuclear
La espectroscopía de resonancia magnética nuclear (RMN) fue desarrollada a
finales de los años cuarenta para estudiar los núcleos atómicos. En 1951, se descubrió
que esta técnica podía ser utilizada para determinar las estructuras de los compuestos
orgánicos. Puede utilizarse sólo para estudiar núcleos atómicos con un número impar
de protones o neutrones (o de ambos).
Cuando una muestra se coloca en un campo magnético, los núcleos con espín
positivo se orientan en la misma dirección del campo, en un estado de mínima energía
denominado estado de espín α, mientras que los núcleos con espín negativo se
orientan en dirección opuesta a la del campo magnético, en un estado de mayor
energía denominado estado de espín β.
Existen más núcleos en el estado de espín α que en el β pero aunque la
diferencia de población no es enorme sí que es suficiente para establecer las bases de
95
Capítulo III
la espectroscopia de RMN. La diferencia de energía entre los dos estados de espín α y
β, depende de la fuerza del campo magnético aplicado H0. Cuanto mayor sea el campo
magnético, mayor diferencia energética habrá entre los dos estados de espín.
Cuando una muestra que contiene un compuesto orgánico es irradiada
brevemente por un pulso intenso de radiación, los núcleos en el estado de espín α son
promovidos al estado de espín β. Esta radiación se encuentra en la región de las
radiofrecuencias del espectro electromagnético.
Cuando los núcleos vuelven a su estado inicial emiten señales cuya frecuencia
depende de la diferencia de energía (∆E) entre los estados de espín α y β. El
espectrómetro de RMN detecta estas señales y las registra como una gráfica de
frecuencias frente a intensidad, que es el llamado espectro de RMN. El término
resonancia magnética nuclear procede del hecho de que los núcleos están en
resonancia con la radiofrecuencia de la radiación. Es decir, los núcleos pasan de un
estado de espín a otro como respuesta a la radiación a la que son sometidos.
El análisis de
29
Si MAS RMN ha sido empleado para estudiar la sílice y
organosilices mesoporosas periódicas ordenadas proporcionando información acerca
29
del grado de condensación del material. Los espectros de RMN Si se han obtenido a
temperatura ambiente en un espectrómetro Bruker ACP-400 a una velocidad de giro
de 8 o 12 KHz y frecuencias de resonancias de 79.49 MHz, respectivamente. El pulso de
excitación y tiempo de retardo para silicio fue 6 μs y 60 s. Los espectros de 29Si MAS
RMN han sido deconvolucionados utilizando el programa Dmfit2006.
2.7 Análisis Termogravimétrico (TGA)
Es una técnica que mide la variación de masa de un compuesto en función de
la temperatura. Las variaciones de temperatura no siempre implican un cambio en la
masa de la muestra; existen sin embargo, cambios térmicos que sí se acompañan de
un cambio de masa, como la descomposición, la sublimación, la reducción, la
desorción, la absorción y la vaporización. Estos cambios pueden ser medidos con un
analizador termogravimétrico.
96
Materiales y métodos
El análisis de las muestras se realizó mediante un equipo SETARAM Setsys
-1
Evolution 16/18 en un rango de temperatura entre 30-1000⁰C a 5⁰C·min .
2.8 Isotermas de Adsorción-Desorción de Nitrógeno
La adsorción física o fisisorción de gases es una técnica de análisis de
propiedades texturales (superficie específica, volumen y tamaño de poros) basada en
la interacción que tiene lugar entre un gas (adsorbato) y el sólido que se quiere
caracterizar (adsorbente). La interpretación de estas isotermas mediante diferentes
modelos matemáticos permite obtener valores para las propiedades texturales. Las
propiedades que caracterizan texturalmente a un sólido son varias, aunque las más
empleadas suelen ser:
2
-1
-Superficie específica: suele expresarse en m ·g . Para la medida de esta
propiedad se emplean modelos matemáticos como Langmuir o B.E.T
(Brunauer-Emmett-Teller), siendo este último el más empleado.
3
-1
-Volumen total de poros: se suele expresar en cm ·g . Hace referencia al
volumen ocupado por el adsorbato, dentro del adsorbente a una presión
determinada, normalmente P/P0=1. En este caso, se trata de una medida
directa y expresa el volumen que ocupan los poros en una unidad másica de
sólido.
-Distribución de tamaño de poro: consiste en expresar el volumen de poro
frente al tamaño de poro al que se adscribe. No es una medida directa, sino la
consecuencia de aplicar modelos matemáticos más o menos complejos. Los
modelos más utilizados para el rango de los microporos son Horvath-Kawazoe
(HK) y Dubinin mientras que para el rango de los mesoporos el más empleado
es Barret-Joyner-Halenda (BJH).
La isoterma de adsorción es el resultado de la representación gráfica de la
cantidad adsorbida, a temperatura constante, en función de la presión (o
concentración) del adsorbato. La IUPAC clasifica las isotermas en seis tipos (Figura
27.):
97
Capítulo III
Figure 27. Clasification of adsorption isotherm of gases.
-Tipo I: la isoterma es cóncava respecto al eje de la presión relativa ( P/P0 ),
-3
aumenta rápidamente a baja presión ( P/P0<1x10
) y posteriormente alcanza un
plateau de saturación horizontal. Este tipo de isotermas la muestran los sólidos
microporosos.
-Tipo II: esta clase de isoterma es característica de sólidos no-porosos o de
adsorbentes macroporosos. La total reversibilidad de la isoterma de adsorcióndesorción, es decir, la ausencia de histéresis, es una condición que se cumple en este
tipo de sistemas.
-Tipo III: es característico de procesos de adsorción en sólidos no porosos en
los que la interacción adsorbente-adsorbato es débil. En la práctica no es común este
tipo de isotermas.
-Tipo IV: a bajas presiones se comporta como la isoterma de Tipo II. Es
característica de los sólidos mesoporosos. Ésta presenta un incremento de la cantidad
adsorbida importante a presiones relativas intermedias y ocurre mediante un
mecanismo de llenado en multicapas.
-Tipo V: del mismo modo que las de Tipo III, esta clase de isotermas se
obtiene cuando las interacciones entre el adsorbato y el adsorbente son débiles. La
presencia de histéresis está asociada con el mecanismo de llenado y vaciado de los
poros. En la práctica es poco usual encontrase con este tipo de isotermas.
98
Materiales y métodos
-Tipo VI: Es muy poco frecuente. Es característico de la adsorción en
multicapa de gases nobles sobre superficies altamente uniformes. Este tipo de
adsorción en escalones ocurre sólo para sólidos con una superficie no porosa muy
uniforme.
La histéresis que aparece en el rango de multicapa de las isotermas de
fisisorción se asocia normalmente con la condensación capilar en la estructura de
mesoporos.
Los diferentes tipos de histéresis se muestran en la Figura 28. La histéresis H1
se encuentra en materiales formados por aglomerados, agregados de partículas
esféricas ordenadas de una manera uniforme así como materiales con poros cilíndricos
y con un elevado grado de uniformidad del tamaño de poro. En este tipo de histéresis
las ramas de adsorción-desorción son paralelas y casi verticales.
Figure 28. Types of hysteresis loops
En la histéresis tipo H2 la rama de desorción es completamente vertical y se
observa en materiales con poros dispuestos como canales. Sin embargo, la histéresis
H3 se asocia a materiales formando agregados de partículas planas, con sus poros en
99
Capítulo III
forma de platos. Además esta histéresis no se estabiliza a presiones relativas a la
presión de saturación. Finalmente, la histéresis H4 es característica de poros estrechos
en forma de rendija, con presencia de microporosidad.
Los análisis se realizaron en un analizador Belsorp-mini II. Antes de realizar el
estudio, las muestras fueron desgasificadas a 120⁰C durante 24h para eliminar el agua
adsorbida.
2.9 Potencial Zeta
El potencial zeta es un importante indicador de la carga superficial y da
información para el entendimiento y control de los fenómenos relacionados con dicha
carga. Las medidas del potencial zeta pueden aplicarse para:
-Cálculo del punto isoeléctrico.
-Caracterización de la estructura química superficial de los sólidos.
-Determinación de la energía libre de adsorción de tensioactivos sobre los
sólidos.
-Cálculo de la densidad de carga superficial.
-Cálculo del espesor de la capa polimérica adsorbida.
En nuestro caso esta técnica fue empleada para la determinación del punto
isoeléctrico de los adsorbentes.
Las medidas se realizaron en un Zetasiser Nano Zs de Malvern y un autotitulador MPT-2 perteneciente al SCAI de la Universidad de Málaga. Previamente, las
muestras se suspendieron en una disolución de KCl 0.01M y HCl 0.1M.
2.10 Espectroscopía Ultravioleta Visible
En la espectroscopía UV-Vis se irradia con luz de una longitud de onda de
energía suficiente como para provocar transiciones electrónicas, es decir, promover un
electrón desde un orbital de baja energía a uno vacante de alta energía. El rango de
longiudes de onda de un espectro UV-Vis está aproximadamente entre 200 y 800 nm.
La luz visible o UV es absorbida por los electrones de valencia, éstos son
promovidos a estados excitados (de energía mayor). Al absorber radiación
100
Materiales y métodos
electromagnética de una frecuencia adecuada, ocurre una transición desde uno de
estos orbitales a un orbital vacío. Las diferencias de energía varían entre los diversos
orbitales. Algunos enlaces, como los dobles, provocan coloración en las moléculas ya
que absorben energía tanto en el visible como en el UV, como es el caso del βcaroteno.
Mediante la ley de Lambert Beer (A=ε·l·C) se relaciona la absorbancia medida
con la concentración de la disolución. Las medidas se llevaron a cabo en un
espectrofotómetro Perkin-Elmer UV/VIS Lambda 11.
2.11 Tamaño de partícula
La distribución de tamaño de partícula se ha determinado utilizando la técnica
de difracción de láser. El procedimiento consiste en medir la intensidad de la luz
dispersada cuando un haz láser pasa a través de una dispersión de partículas. Una
serie de detectores miden con presición la intensidad de la luz dispersada por las
partículas en un amplio rango de ángulos. Mediante un software adecuado se analizan
los datos dispersión que permiten el cálculo de la distribución del tamaño de partícula.
El equipo empleado fue un Mastersizer S (Malvern Instrument) usando etanol
como dispersante. Previo a su análisis, las muestras fueron tratadas en ultrasonidos
durante al menos 10min.
BIBLIOGRAFÍA
[1] W. T. Reichle, Synthesis of anionic clay minerals (mixed metal hydroxides,
hydrotalcite), Solid State Ionics 22 (1986) 135–141.
[2]M. C. Burleigh, S. Jayasundera, C. W. Thomas, M. S. Spector, M. A. Markowitz y B. P.
Gaber, A versatile synthetic approach to periodic mesoporous organosilicas”, Colloid
Polym. Sci., 282 (2004) 728-33
101
CAPÍTULO IV. ADSORPTION OF NON-IONIC PESTICIDE
S-METOLACHLOR ON LAYERED DOUBLE HYDROXIDES
INTERCALATED
WITH
DODECYLSULFATE
TETRADECANEDIOATE ANIONS
AND
Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double
Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions
Abstract
The objective of the present research was to investigate the adsorption of non-ionic
pesticide S-Metolachlor on two different organohydrotalcites (OHTs): one intercalated
with dodecylsulfate (HT-DDS) and the other one with tetradecanedioate (HT-TDD)
anions, both of them obtained by the coprecipitation method. The adsorbents and the
adsorption products were characterized by X-ray diffraction and FT-IR spectroscopy.
The results indicated that the adsorption of S-Metolachlor on the organohydrotalcite
HT-TDD was higher than that on HT-DDS, but desorption was lower. The amount of SMetolachlor adsorbed increased with temperature, especially for HT-TDD. SMetolachlor desorption from HT-DDS and HT-TDD was characterized using two
experimental procedures, which indicated the higher reversibility of S-Metolachlor
adsorption from HT-DDS compared to HT-TDD. The results suggest the possibility to
use both organohydrotalcites uptaking S-Metolachlor from contaminated water.
Keywords: S-Metolachlor, organohydrotalcite, dodecylsufate anion, tetradecanedioate
anion, adsorption, desorption, water treatment.
1.
Introduction
The widespread use of pesticides in modern agriculture has given rise to an
increase about the environmental concern. The contamination of soil and ground
water by pesticides applied to soil and swept by transport processes, such as leaching
and run-off, is becoming an important environmental problem (Modabber et al.,
2007). Pesticides are very hazardous pollutants, toxic to many forms of wildlife,
especially aquatic organisms, insects and mammals and can persist in the aquatic
environment for many years after their application (Shukla et al., 2006).
A strategy to reduce the pesticide transport losses after soil application consists
of the use of controlled release formulations, in which the pesticide is gradually
released over time. This limits the amount of pesticide immediately available for
transport processes. Beneficial effects related to the use of controlled release
formulations include reduction in rapid losses of pesticide by volatilization, runoff and
105
Capítulo IV
leaching. The savings in manpower and energy, by reducing the number of applications
required in comparison to conventional formulations, increase safety for the pesticide
applicator and lead to a general decrease in non-target effects (Gerstl et al., 1998; Celis
et al., 2002).
Previous research has indicated that layered double hydroxides (LDHs), also
known as anionic clays or hydrotalcite-like compounds (HTs), could be suitable to
smooth and even prevent the environmental impact caused by pesticides, i.e. they can
be used either as filters of pesticide-contaminated water, or as supports in pesticide
controlled release formulations. LDHs or HTs are brucite-like layered materials with a
II
III
x+
n−
II
III
general formula [M1−x Mx (OH)2] Xx/n ·mH2O, where M and M are divalent and
trivalent cations respectively, and X
n−
is the interlayer anion, which balances the
III
positive charge originated by the presence of M in the layers. The layer charge is
III
III
II
determined by the molar ratio x= M / (M +M ) and can vary between 0.2 and 0.4
(Cavani et al., 1991; Costantino et al., 2009). The interest of these materials is related
to their structure, high anion exchange capacity, and simplicity of their synthesis
(Rives, 2001; Reichle, 1986).
Similarly to clay minerals, the inorganic anions of HTs can be replaced with
organic anions (Meyn et al., 1990; Clearfield et al., 1991; Zhao and Vance, 1997;
Costantino et al., 2009). This simple modification changes the nature of the HT surface
from hydrophilic to hydrophobic, increasing its affinity for non-ionic organic
compounds, including many non-ionic pesticides (Pavlovic et al., 1996; Villa et al.,
1999; Celis et al., 2000; Wang et al., 2005; Bruna et al., 2006; Costa et al., 2008). These
organoclays and organo/LDHs have applications in a wide range of organic pollution
control fields (Costa et al., 2008; Celis et al., 2000; Frost et al., 2008; Zhou et al., 2008;
Laha et al., 2009; Bruna et al., 2009).
In the present paper, we report on the adsorption of a non-ionic pesticide, SMetolachlor, on two different organohydrotalcites (OHTs): one intercalated with
dodecylsulfate (HT-DDS) and the other with tetradecanedioate (HT-TDD) anions in the
interlayer space, both surfactants with the same carbon chain (twelve -CH2 units) but
differing in the nature of the chemical groups bonded to the HT layers. The aim of this
paper was to analyze the influence of the surfactant as well as the effect of
temperature on the adsorption process of S-Metolachlor to complete the study about
106
Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double
Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions
the behavior of this pesticide carried out previously (Chaara et al., 2011). A detailed
study of the reversibility of the adsorption-desorption process was also conducted
using two different experimental approaches.
2.
Materials and methods
2.1. Organic anions and pesticide
Dodecylsulfate (DDS) was supplied as sodium salt by Fluka, and tetradecanedioic acid
(TDD) and analytical standard S-Metolachlor (2-Chloro-N-(2-ethyl-6-methylphenyl)-N[(1S)-2-methoxy-1-methylethyl]acetamide) were provided by Sigma-Aldrich. The
structure of these materials is included in the following scheme:
H3C
O
CH3
CH3
N
O
Cl
H3C
S-Metolachlor
-
O
O
S
H3C
O
O
Dodecylsulfate anion
O
O-
O
Tetradecanedioate anion
O
S-Metolachlor, a chloroacetamide type compound with a water solubility of 480
mg/L (25 °C), is a selective herbicide applied as pre- and post-emergence to control
grasses and some broad-leaved weeds in a wide range of crops.
2.2. Synthesis of organohydrotalcites
Two
organohydrotalcites
(OHTs),
[Mg3Al(OH)8][(C12H25SO4)·4H2O]
and
[Mg3Al(OH)8][(C14H24O4)·4H2O], named HT-DDS and HT-TDD, respectively, were
107
Capítulo IV
obtained by the coprecipitation method (Reichle, 1986). To prepare hydrotalcite with
DDS in the interlayer, a solution containing 0.06 mol Mg(NO3)2·6H2O and 0.02 mol
Al(NO3)3·9H2O in 100 mL of distilled water was dropped to an alkaline solution (0.16
mol of NaOH in 500mL of water) containing 0.05 mol of dodecylsulfate (DDS). The
synthesis was carried out without control of pH and at room temperature. To prepare
hydrotalcite with TDD in the interlayer, a solution containing 0.06 mol Mg(NO3)2·6H2O
and 0.02 mol Al(NO3)3·9H2O in 100 mL of distilled water was dropped to an alkaline
solution of NaOH at pH=8, containing 0.01 mol of tetradecanedioic acid (TDD); during
the addition the pH was constant and the temperature 75°C. In both cases, the
syntheses were carried out in inert atmosphere (N2 bubbling), to avoid CO2 dissolution
and subsequent intercalation of carbonate into the LDH interlayer. The resultant
suspensions were hydrothermally treated at 80°C for 24 h, and the precipitate washed
with CO2-free distilled water and dried at 60°C.
2.3. Characterization of organohydrotalcites
Elemental analysis of Mg and Al was performed with an AAS Perkin Elmer 3100
spectrometer after dissolving the samples in concentrate HCl, whereas S, C and N
contents were determined with an elemental analyzer Eurovector 3A 2010. The water
content was calculated by using a thermogravimetric analyzer SETARAM Setsys
Evolution 16/18. For this purpose, 20 mg of sample was submitted to heating with a
constant temperature gradient of 5°C/min in the range of 30 to 1000°C. Powder X-ray
diffraction patterns (PXRD) were obtained on a Siemens D-5000 instrument with CuKα
radiation and FT-IR spectra were recorded by using the KBr disk method on a Perkin
Elmer Spectrum One spectrophotometer. Specific surface areas were measured from
nitrogen adsorption/desorption isotherms on a Micromeritics ASAP 2020 instrument
using N2 gas as adsorbate. The surface morphology of both samples was obtained
using a scanning electron microscope JEOL JSM 6300.
2.4. Adsorption and desorption experiments
S-Metolachlor adsorption on OHTs was studied at different initial pH values,
contact times (kinetics), and initial pesticide concentration (isotherms), by suspending
108
Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double
Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions
duplicate samples of 20 mg of each adsorbent in 30 mL of aqueous pesticide solutions.
Samples of sorbent and pesticide in water were shaken in a turn-over shaker at 52 rpm
and the supernatants were centrifuged and separated to determine the concentration
of S-Metolachlor by UV spectroscopy (Perkin Elmer UV-visible spectrophotometer,
Model Lambda 11) at 265 nm. The amounts of pesticide adsorbed were determined
from the initial and final solution concentrations.
To study the adsorption at different initial pHs (5.0, 6.6 and 9.0) and contact
times (1, 2, 5, 24 h), an initial pesticide concentration of 0.35 mM was used.
Adsorption isotherms were obtained by the batch equilibration technique as described
elsewhere (Hermosín et al., 1993; Pavlovic et al., 1997), using initial pesticide
concentrations between 0.05 mM and 0.35 mM.
Desorption isotherms were obtained using two different procedures: In the first
one, described by Bowman (1979) and Bowman and Sans (1985), desorption was
carried out immediately after adsorption from the highest equilibrium point of the
adsorption isotherm. The procedure consisted of removing 15 mL of the supernatant
liquid and replacing it with the same amount of distilled water. After shaking at room
temperature for 24 h, the suspensions were centrifuged, and the pesticide
concentration in the supernatant was determined by UV-Vis spectroscopy. This
process was repeated four times (Bruna et al., 2008). In the second procedure, which
was a modification of that described by Yang (1974), one unique desorption point was
measured from each equilibrium point of the adsorption curve by replacing a small
portion of supernatant (15 mL) with the same amount of distilled water. After shaking
for 24 h, the amount of pesticide desorbed was calculated as describe above. This
procedure avoids the difficulties mentioned by Bowman (1979) about the procedure
used by Yang (1974).
Adsorption and desorption parameters were calculated using the Freundlich equation
(Eq. 1)
1/𝑛𝑓
(1)
𝐶𝑠 = 𝐾𝑓 𝐶𝑒
where Cs(µmol/g) is the amount of S-Metolachlor adsorbed at the equilibrium
concentration Ce (µmol/L), and Kf and nf are the empirical Freundlich constants
109
Capítulo IV
(Bowman and Sans, 1985). Hysteresis coefficients, H, were calculated according to
H=nf-des/nf-ads, where nf-ads and nf-des are the Freundlich slopes of the adsorption and
desorption isotherms, respectively (Barriuso et al., 1994).
For the adsorbent regeneration and recyclability study, a preliminary experiment
was conducted to obtain the kinetics of S-Metolachlor desorption from HT-DDS and
HT-TDD using water, methanol, and ethanol as extracting solutions. Desorption kinetic
curves were obtained from the highest equilibrium point of the adsorption kinetic
curve. The extracting volume was 30 mL and the contact times 1, 2, 5, 24, and 30 h.
After shaking, the suspensions were centrifuged, the supernatant liquid was separated
and evaporated, and the residue dissolved in distilled water to determine the pesticide
concentration
by
UV-Vis
spectroscopy.
Successive
S-Metolachlor/methanol
adsorption-desorption cycles for HT-DDS were conducted to assess the potential
recyclability of this material as an adsorbent of S-Metolachlor.
3.
Results and Discussion
3.1 Characterization of the organohydrotalcite sorbents
3.1.1 Elemental analysis
According to the values of Mg/Al molar ratio (Table 1), a partial dissolution of Al
during the synthesis of HT-DDS and Mg
2+
3+
in HT-TDD seemed to occur. This could be
attributed to the differences on the synthesis conditions (temperature, pH…), as well
as, to the different acid/base properties of the organic anions (pKa = 4.46 for TDD and
-3.29 for DDS).
The molar ratio S/Al in HT-DDS (Table 1), which corresponds to the fraction of the
positive charge of HT compensated by DDS anions, indicated that most of the layer
charge (91%) was compensated by DDS anions and about 9% by other anions, probably
2-
CO3 (as it will be seen below). The elemental analysis of C in HT-TDD sample (Table 1)
suggests that, if all carbon atoms were as TDD anions, only 45% of the layer charge of
HT will be compensated. Therefore, a part of the charge must be balanced by other
2-
anions, probably CO3 , and also a small amount by nitrate anions (from the metal
salts) as suggested by the N content of the sample (Table 1). From the elemental
analysis and the TG data to obtain the water content (not included), the formulae for
both organohydrotalcites were obtained and included in Table 1.
110
Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double
Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions
Fig. 1. XRD patterns of the organohydrotalcites and their
adsorption products. (*diffraction peaks due to the Al sample
holder).
Table 1
Elemental analysis of the adsorbents and proposed formula
Adsorbent
HT-DDS
wt. %
Atomic ratios
Mg
Al
C
S
N
Mg/Al
S/Al
15.72
4.67
29.38
4.97
0
3.74
0.91
Proposed formula
[Mg0.79Al0.21(OH)2](C12H25SO4)0.20(CO3)0.00
5·0.56H2O
HT-TDD
31.39
14.67
20.22
0
0.37
2.38
0
[Mg0.71Al0.29(OH)2](NO3)0.01Xn0.28/n·0.75H2
O
3.1.2 X-ray diffraction study
Powder XRD patterns of the synthesized organohydrotalcites were recorded (Fig.
1). In both cases the diagram corresponds to a hydrotalcite-like structure with an
important expansion of the interlayer space with respect to the inorganic hydrotalcite
with carbonate anion (d003=7.8 Å) (Cavani et al., 1991). X-ray diffraction pattern of HTDDS sample (Fig. 1A) showed reflections (00l) at 26.4 Å and 13.1 Å, suggesting a
vertical monolayer arrangement of dodecylsulfate anions (Clearfield et al., 1991; Bruna
et al., 2006). X-ray diffraction pattern of HT-TDD (Fig 1B) showed (001) reflections at
17.5 Å and 8.5 Å suggesting a tilt position of the surfactant in the interlayer, since,
according to CS Chem 3D 5.0 program, TDD height is 18.3 Å, that added to the
111
Capítulo IV
hydrotalcite-layer thickness (4.8 Å) would give a value of 23.1 Å for a perpendicular
arrangement of TDD anions.
3.1.3 Textural properties
The scanning electron micrographs (Fig. 2) showed the surface morphology of
HT-DDS and HT-TDD consisting of an agglomerate of particles, although slightly more
porosity and less homogeneity in the HT-DDS particle size were observed. These
results are in accordance with the low crystallinity observed in the XRD for both
samples (Fig. 1) and with the low specific surface area values obtained by nitrogen
2
2
adsorption/desorption isotherms (not shown), 5.4 m /g for HT-DDS and < 1 m /g for
HT-TDD.
Fig. 2 SEM micrographs of samples (A) HT-DDS and (B) HT-TDD
3.2. S-Metolachlor adsorption-desorption experiments
The adsorption of S-Metolachlor was not affected by the initial pH of the
solution (not shown). This was attributed to the fact that the final pH values for the
112
Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double
Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions
equilibrated suspensions were all between 9 and 10, due to the buffer effect of
hydrotalcite-like compounds (Cavani et al., 1991). For that reason, the selected initial
pH for all subsequent adsorption experiments was 6.6, i.e. the original pH of the
water-dissolved pesticide.
The kinetic study of the S-Metolachlor adsorption on HT-DDS (Fig. 3) indicated
an almost instantaneous adsorption, reaching the equilibrium in 2 h, while the
pesticide adsorption on HT-TDD was slower and the plateau was not achieved.
Nevertheless, taking into account that in the last case the amount of pesticide
adsorbed at 24 h was only 5% greater than that adsorbed at 6 h, 24 h was assumed to
be sufficient to reach an apparent adsorption equilibrium. This different behavior in
the adsorption kinetics could be attributed to little changes in the morphology and
specific surface area of the sorbents.
Fig. 3.Time evolution of S-Metolachlor adsorption on HT-DDS and
HT-TDD (Cinitial=0.35mM)
The adsorption isotherms were obtained by plotting the amount of SMetolachlor adsorbed (Cs) versus the solute concentration (Ce) in the equilibrium
solution and were of L-2 subtype, according to the classification of Giles et al. (1960),
on both organohydrotalcites (Fig 4). The amount of S-Metolachlor adsorbed on HTDDS and HT-TDD was very similar (e.g., between 43 and 45% at an initial herbicide
concentration, Ci= 0.35mM), suggesting a direct relation between the amount of
113
Capítulo IV
pesticide adsorbed and the length of the surfactant alkyl chain (twelve –CH2 groups in
both cases), and agree with the results of a previous work (Chaara et al., 2011) where
adsorption of Metolachlor on HT-DDS was higher than that on an organohydrotalcite
modified with sebacate anion (HT-SEB), with only eight -CH2 groups. The isotherms
shape suggested that the Freundlich equation would be a good model to fitting, and
the choice is justified by the high regression coefficients (Table 2).
Table 2. Freundlich parameters for S-Metolachlor sorption (s) and desorption (d) isotherms on
organohydrotalcites and hysteresis coefficient H.
Kf
nf
R
2
H
HT-DDS (s)
29.12-16.32
0.48±0.06
0.995
HT-DDS (d*)
0.07-0.06
1.94±0.27
0.973
4.04
HT-DDS (d**)
9.46-5.81
0.61±0.05
0.990
1.27
HT-TDD (s)
40.65-27.38
0.39±0.02
0.998
2.39 ±0.36
0.967
6.13
0.90±0.19
0.914
2.31
HT-TDD (d*)
0.017-9.23·10
HT-TDD (d**)
7.55-1.55
*first desorption way
-4
**second desorption way
The Freundlich constants (Kf and nf) together with the regression coefficients
2
(R ) were very similar for both sorbents (Table 2). Kf parameters were in the range
40.6-16.3, which indicated strong sorption between the sorbate and sorbent (Liu et al.,
2010). The values of nf were less than 1, indicating a non-linear sorption of SMetolachlor on both samples. This behavior is similar to that observed by Bruna et al.
(2008) for the adsorption of terbuthylazine on organohydrotalcites and Celis et al.
(1999) for adsorption of imazamox on organoclays and organohydrotalcites.
The temperature effect on the adsorption of S-Metolachlor by HT-DDS (Fig.
5A) and HT-TDD (Fig. 5B) was also investigated. When the temperature was increased
from 10 to 30 °C, the amount of pesticide adsorbed at the initial concentration
0.35mM increased from 45.9 to 50.3% in HT-DDS and from 49.9 to 56.8% in HT-TDD. In
the case of HT-DDS, the increase in pesticide adsorption is only observed clearly at
high initial concentrations, i.e. at Ce values above 150 µmol/L. These results indicate
114
Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double
Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions
the endothermic nature of the adsorption process, as confirmed by the calculation of
the thermodynamic parameters.
Fig. 4 Adsorption and desorption isotherms of S-Metolachlor on HT-DDS (A, C) and HTTDD (B, D). Desorption was performed after adsorption from the highest equilibrium
point of the adsorption isotherm (A, B). Desorption was performed after each point of
the adsorption isotherm (C, D).
o
o
o
The change in standard free energy (ΔG ), enthalpy (ΔH ), and entropy (ΔS ) of
adsorption was calculated using the following equations (Eqs. 2-3) (Özcan et al., 2005;
Islam and Patel, 2007; Li et al., 2009; Islam et al., 2011):
o
o
o
ΔG =ΔH - TΔS
o
(2)
o
lnK = ΔS /R - ΔH /RT
(3)
where K is the equilibrium constant, R the molar gas constant and T the absolute
o
o
temperature. Values of ΔH and ΔS were calculated from the slope and intercept of
115
Capítulo IV
Van’t Hoff plots (Ln KC versus 1/T) and are summarized in Table 3. The values of K have
been obtained from value of Kf from the Freundlich equation because our isotherms
have been fitted to this equation. K has been recalculated to become dimensionless by
o
multiplying it by 1000 (Milonjic, 2007). ΔG values were calculated from Eq(2).
Fig. 5 Adsorption isotherms of S-Metolachlor at different temperaturas: (A) on HT-DDS and (B)
on HT-TDD.
o
ΔH values were positive in both cases, confirming the endothermic character
o
of the S-Metolachlor adsorption process, but ΔH for HT-TDD was an order of
magnitude higher than that for HT-DDS (Table 3). Changes in ΔS were favorable for
o
both sorbents but the ΔS value was again higher for HT-TDD compared to HT-DDS.
The larger contribution of the entropy than the enthalpy was responsible for the
o
negative value of ΔG in all cases, thus indicating that the adsorption process was
o
entropically controlled. The most negative ΔG value was observed for HT-TDD at 30°C
coincident with the greater adsorption constant.
With respect to the S-Metolachlor desorption study, the desorption isotherms
obtained according to the above-described first procedure (the more widely used
method (Hermosin et al., 1993; Liu et al., 2010; Pavlovic et al., 2005)), for both HT-DDS
and HT-TDD showed high H values, H>>1 (Table 2), indicating a high reversibility of the
pesticide adsorption process (Cruz-Guzmán et al., 2004) (Fig 4A, B). The shape of the
curves, with the desorption branch falling below the adsorption isotherm branch, is
probably due to the experimental procedure rather than to the adsorption-desorption
116
Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double
Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions
processes themselves (Koskinen et al., 1979) because that procedure, implying
successive treatments with ethanol, probably could modify the solid features.
Table 3 Thermodynamic parameters for S-Metolachlor sorption on
organohydrotalcites HT-DDS and HT-TDD determined at different temperatures
Sample
T (ºC)
K
HT-DDS
10
20
30
11000
11020
12050
HT-TDD
10
20
30
14420
20320
38470
ΔS
(KJ·mol-1·K-1)
0.090
ΔH
(KJ·mol-1)
3.53
ΔG
(KJ·mol-1)
-21.89
-22.67
-23.67
0.204
35.32
-22.53
-24.16
-26.60
To overcome some of the limitations inherent to the successive desorption
procedure, we used a second procedure to study desorption, i.e. a modification of
procedure proposed by Yang (1974), where one desorption step was performed from
every equilibrium point of the adsorption isotherm. Desorption curves thus obtained
were very different from those obtained using the conventional successive desorption
procedure (Fig. 4C, D). This procedure tries to reproduce the method carried out when
N2 adsorption on any material is performed, where the hysteresis parameter close to 1
indicates high reversibility of the process and H values higher than 1 suggest
irreversibility. The H value obtained for S-Metolachlor adsorbed on HT-DDS was very
close to 1 (Fig. 4C), according with a high reversibility, indicating that desorption is
favored. However, the hysteresis coefficient (H) for S-Metolachlor on HT-TDD is
considerably greater than 1 (2.31) (Fig 4D), that indicates a certain no reversibility of
the adsorption process. Desorption curve above the adsorption evidences the special
difficulty to remove S-Metolachlor from the substrate HT-TDD.
This new experimental procedure permits to distinguish between the
behavior of S-Metolachlor in both organohydrotalcites, so that we could conclude that
the reversibility of S-Metolachlor adsorption is higher for HT-DDS (H=1.27) than for HTTDD (2.31). However, the first procedure is still useful to simulate the conditions
occurring in soil when, after adsorption, a pesticide is sequentially desorbed by rainfall
or irrigation water.
117
Capítulo IV
3.3 Sorbent regeneration and recyclability experiments
These experiments consisted of successive adsorption-desorption stages of SMetolachlor on HT-DDS and HT-TDD with the final purpose to analyze the possibility of
sorbent regeneration.
As a preliminary step, desorption experiments were performed with
methanol, ethanol and water from the highest equilibrium point of the adsorption
kinetic curves. The process was quite fast in all the cases (Fig. 6), desorption reaching
the plateau in few hours. S-Metolachlor adsorbed on HT-DDS (Fig 6A) was totally
removed (100%) with ethanol and almost the same happened with methanol (92.8%)
while only 25.4% was desorbed using water. However, for hydrotalcite HT-TDD (Fig 6B)
desorption was not completed with ethanol (62.7%) or methanol (58.0%) and was only
20.7% with water. As expected, this pesticide showed more affinity for hydrophobic
solvents due to its non-ionic character.
Fig. 6. Desorption kinetics of S-Metolachlor with different solvents (A) HT-DDS and (B) HT-TDD
According to the above results, the study of recyclability was only performed
on hydrotalcite HT-DDS using ethanol as solvent, the unique case where desorption
was completed. HT-DDS adsorbent allowed only a stage of adsorption-desorption with
ethanol removing completely S-Metolachlor. Although the second stage was only
slightly less effective in the adsorption (95%) than the first one, only 40% of the
pesticide was desorbed after the second adsorption step. These results indicate that
118
Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double
Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions
the possibility of regeneration of the sorbent with ethanol is limited, and new
experiments should be necessary to found a more effective regeneration method.
3.4 Characterization of solids after adsorption
In order to characterize the adsorption products in the “plateau” of the
isotherms, powder XRD patterns and FT-IR spectra were registered to confirm the
adsorption of S-Metolachlor in the hydrotalcite interlayers. After adsorption of SMetolachlor, a change in the d00l-value was observed for both organohydrotalcites (Fig
1C, D). The adsorption product of S-Metolachlor on HT-DDS showed a small decrease
of the d003-value close to 1 Å (from 26.4 to 25.6 Å), (Fig. 1C). In contrast, S-Metolachlor
adsorption on HT-TDD (Fig 1D) led to a very important increase in the d003-value
compared with the pristine adsorbent (from 17.5 to 23.7 Å). This suggests a change in
the orientation of the tetradecanedioate anion from tilt position in HT-TDD to
perpendicular position after the adsorption of the pesticide, thus providing strong
evidence for sorbent deformation upon adsorption of S-Metolachlor. The d003-value of
the HT-TDD-S-Metolachlor adsorption product was only slightly higher than the sum of
the size of TDD and the thickness of the brucite layer (23.12 Å).
Taking into account that TDD anions have two negative charges versus only
one for DDS, half of TDD anions will be necessary to compensate the layer charge
producing a lower “congestion” of the interlayer space in the last case. The higher free
space in the interlayer for HT-TDD would permit S-Metolachlor to be introduced
between the organic chains causing a change in the orientation of the organic
molecules, reflected in the increase in d003 space showed in the Fig. 1C. These results
confirm those found by Chaara et al., (2011) for Alachlor adsorption on other
organohydrotalcites.
Fig. 7 shows FT-IR spectra of the adsorbents, S-Metolachlor, and adsorption
products. Characteristic features of the hydrotalcite structure are the broad bands
-1
-
around 3460 cm , corresponding to the νO-H of hydration water molecules and free OH
-1
groups of the brucite layers, and a weak band at 1650 cm corresponding to δO-H of
-1
water molecules (Fig. 7A, B). Strong peaks between 2920 cm
-1
and 2850 cm
correspond to C-H stretching modes (Bruna et al., 2008)]. The most characteristic
−1
bands of HT–DDS (Fig. 7A), are those due to the antisymmetric (1211cm ) and
119
Capítulo IV
−1
symmetric (1071 cm ) stretching S=O vibration of the sulfate group (Clearfield et al.,
1991; Bellamy, 1975). FT-IR of HT-TDD (Fig. 7B) shows characteristics bands at 1561
-1
-1
cm and 1421 cm corresponding to vibrations of carboxylate groups (antisymmetrical
and symmetrical, respectively).
Fig. 7. FT-IR spectra of (A) HT-DDS, (B) HT-TDD, (C) SMetolachlor, (D) adsorption product HT-DDS-S-Metolachlor and
(E) adsorption product HT-TDD-S-Metolachlor.
-1
The presence of the pesticide bands (1760 cm attributed to νC=O of the carbonyl
-1
group, and 1113 cm corresponding to νC-O of the ether group and/or δC-H associated
to the benzene ring) in the FT-IR spectra of the adsorption products in the “plateau” of
the isotherms (Figs. 7D, E) revealed the adsorption of S-Metolachlor in both
organohydrotalcites. These results are similar to those observed by Bruna et al., (2006)
for the adsorption of Metamitron on an organohydrotalcite with dodecylsulfate in the
interlayer.
120
Adsorption of Non-Ionic Pesticide S-Metolachlor on Layered Double
Hydroxides Intercalated with Dodecylsulfate and Tetradecanedioate Anions
4.
Conclusions
Adsorption-desorption of S-Metolachlor pesticide by two organohydrotalcites
(HT-DDS and HT-TDD) have been characterized. The organohydrotalcites presented an
expansion of the interlayer space compared to the inorganic hydrotalcite with
carbonate anion. X-ray diffraction patterns suggested a vertical monolayer
arrangement of dodecylsulfate anions and a tilt position of the tetradecanedioate
anions in the interlayer space. Adsorption of S-Metolachlor resulted in changes in the
d00l-value for both organohydrotalcites. In particular, the adsorption of S-Metolachlor
on HT-TDD led to a very important increase in the basal spacing, suggesting changes in
the orientation of the tetradecanedioate anion from tilt to perpendicular position. The
amount of adsorbed pesticide increased with the rise of the temperature from 10 to
30 °C, especially for HT-TDD.
A new experimental procedure has been used to distinguish that the reversibility
of S-Metolachlor adsorption is higher for HT-DDS than for HT-TDD. Only S-Metolachlor
adsorbed on HT-DDS was possible to be removed with ethanol. Therefore, the study of
recyclability was only performed on hydrotalcite HT-DDS using ethanol as solvent. The
results indicated that the possibility of regeneration of the sorbent with ethanol was
limited to one regeneration step, so that further experiments would be needed to fìnd
a more effective way for effective regeneration of the sorbent.
Acknowledgement
This work has been partially supported by grants AGL2008-04031, AGL201123779 and CTM2011-25325 from MICINN, cofinanced with FEDER funds, and Research
Groups FQM-214 and AGR-264 of Junta de Andalucía. R. Otero also acknowledges
financial support from MEC-ERDF. We appreciate the technical assistance received in
the SCAI (Universidad de Córdoba) from Electron Microscopy and Elemental Analysis
Units.
121
Capítulo IV
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125
CAPÍTULO V. PESTICIDES ADSORPTION-DESORPTION ON
Mg-Al MIXED OXIDES. KINETIC MODELING, COMPETING
FACTORS AND RECYCLABILITY.
Pesticide adsorption on Mg-Al mixed oxides. Kinetic
modeling, competing factors and recyclability
GRAPHICAL ABSTRACT
Abstract
The adsorption of the ionizable pesticide Nicosulfuron and the ionic pesticide
Mecoprop-P on a calcined Mg–Al hydrotalcite (HT-500) in the presence and absence of
various anions was studied with a view to assessing the potential of regenerating the
adsorbent. Kinetic tests showed the adsorption of both pesticides to involve
intraparticle diffusion, and the rate constant of intraparticle diffusion for Nicosulfuron
to increase with increasing concentration of the compound by effect of its increased
molecular size relative to Mecoprop-P. Application of the Langmuir and Dubini–
Radushkevich equations to the adsorption isotherms for the two pesticides revealed
easy adsorption of both via an ion-exchange process. Adsorption and desorption of the
two pesticides were both affected by the presence of other anions. The adsorbents
and their adsorption products were characterized by X-ray diffraction and FT-IR
spectroscopies. Based on the results of regeneration tests, calcined hydrotalcite is an
effective adsorbent for the removal of Nicosulfuron and Mecoprop-P from
contaminated water.
Keywords: Nicosulfuron, Mecoprop-P, adsorption, hydrotalcite, water treatment.
Introduction
129
Capítulo V
Pesticides are very hazardous pollutants that can persist in the aquatic
environment for many years [1]. Contamination of soil and ground water by pesticides
applied to soil and swept by transport processes such as leaching or runoff is a posing
an increasingly serious environmental problem.
Some researchers have suggested that the use of layered double hydroxides
(LDHs), also known as “anionic clays” or “hydrotalcite-like compounds” (HTs), as filters
for pesticide-contaminated water or additives in controlled-release pesticide
formulations might be effective toward partially or completely avoiding the
environmental impact of pesticides [2-4]. LDHs are brucite-like layered materials of
II
n−
x+
III
II
III
general formula [M1−x Mx (OH)2] Xx/n ·mH2O, where M and M are a divalent and a
n−
trivalent cation, respectively, and X
is an interlayer anion countering the positive
III
charge arising from the presence of M in the layers. Layer charge in an LDH is dictated
III
III
II
by the mole ratio M /(M + M ), which typically ranges from 0.2 to 0.4 [5,6]. The main
interest of these materials lies in their structure, high anion-exchange capacity and
straightforward synthesis [3,7-9].
The adsorption efficiency of LDHs is strongly affected by the properties of
their interlayer anions [10]. Thus, LDHs usually have a greater affinity for anions with a
high charge density and hence tend to adsorb multivalent anions easily in relation to
2–
monovalent ions [11,12]. In addition, LDHs exhibit preferential affinity for CO3 , which
2–
usually hinders further ion-exchange [13]. However, interlayer CO3
ions can be
removed by heating LDH at about 500 ºC to obtained calcined LDH (HT500), which can
regain the original LDH structure in the presence of dissolved anions [14–16]:
n–
n-
Mg3AlO4(OH) + 4 H2O + (1/n) X → [Mg3Al(OH)8]X1/n + OH
–
n-
where X is the anionic species present in solution.
The anion-exchange capacity (AEC) of HT500 was much higher than that of
–1
the original LDH (5.8 vs 3.3 mmol·g ) [17]; this allows the adsorption efficiency of an
LDH to be significantly increased by calcination [16,18].
The influence of various common inorganic anions on pesticide adsorption is
one other major factor to be considered since some such anions are often
encountered in pesticide-contaminated waters [10].
130
Pesticide adsorption on Mg-Al mixed oxides. Kinetic
modeling, competing factors and recyclability
Several author have studied different properties of Mecoprop and
Nicosulfuron pesticides such as their degradation or its effect in the grown of different
vegetables. It had been studied adsorption and desorption of Nicosulfuron in soils [1920] and clay minerals [21]. Furthermore, it had been studied the adsorption of
Mecoprop on calcareous and organic matter amended soils [22], in clays [23] or onto
Iron Oxides [24]. Also, Khan et al [25] have characterized the Mecoprop-LDH hybrid
synthetized by the cooprecipitation method.
In this work, we examined the adsorption of two widely used, ionizable
pesticides (Nicosulfuron and Mecoprop-P) on a calcined hydrotalcite (HT500) with a
view to elucidating their adsorption–desorption behavior and the way it is influenced
by the presence of some anions. In addition, we subjected the hydrotalcite to repeated
adsorption–calcination cycles in order to assess its reusability in the removal of
Nicosulfuron and Mecoprop-P from contaminated water.
2. Materials and Methods
2.1 Pesticides
Nicosulfuron
(2-[(4,6-dimethoxypyrimidin-2-ylcarbamoyl)sulfamoyl]-N,N-
dimethylnicotinamide) and Mecoprop-P [(R)-2-(4-chloro-o-tolyloxy)propionic acid]
were both supplied as high-purity products by Sigma–Aldrich. These pesticides are
typically used to control weeds in post-emergence treatments. Nicosulfuron is a
sulfonylurea of molecular weight 410.41 g·mol–1, a high solubility in water (7500 mg·L–1
at 20 °C) and pKa = 4.78. Mecoprop-P is an aryloxyalkanoic acid pesticide of molecular
–1
weight 214.65 g·mol , water solubility 860 mg·L
formula of these pesticides is as follows:
131
–1
and pKa = 3.86. The structural
Capítulo V
NICOSULFURON
H3C
O
N
H3C
O
O
O
O
S
O
N
N
H
CH3
N
N
H
CH3
N
MECOPROP-P
CH3
Cl
O
CO2H
CH3
2.2. Synthesis of the hydrotalcite, and characterization of the adsorbent and
adsorption products
A hydrotalcite intercalated with carbonate anions and designated MgAl-LDH
was prepared according to Reichle [7]. To this end, a solution containing 0.75 mol of
Mg(NO3)2·6H2O and 0.25 mol of Al(NO3)3·9H2O in 250 mL of distilled water was
dropped over 500 mL of another containing 1.7 mol of NaOH and 0.5 mol of Na2CO3
under vigorous stirring for 2 h. The resulting suspension was hydrothermally treated at
80 °C for 24 h, and the precipitate thus formed washed with distilled water and dried
at 60 °C to obtain the solid MgAl-LDH, calcination of which at 500 °C for 3 h gave an
Mg–Al mixed oxide that was named HT500.
The adsorbent (HT500) and adsorption products were characterized from
their XRD patterns as obtained on a Siemens D-5000 diffractometer using CuKα
radiation (λ = 1.54050 Å) and their FT-IR spectra as recorded by using the KBr disc
technique on a Perkin Elmer spectrophotometer. The materials were also
characterized microstructurally with a JEOL 200CX TEM instrument.
Elemental analyses (Mg and Al) were performed by atomic absorption
spectrometry on a Perkin Elmer AA-3100 instrument, and C and N contents
determined with a Eurovector 3A 2010 elemental analyzer.
2.3. Adsorption–desorption tests
132
Pesticide adsorption on Mg-Al mixed oxides. Kinetic
modeling, competing factors and recyclability
Kinetic adsorption and adsorption–desorption isotherms for Nicosulfuron and
Mecoprop-P were obtained by using the batch equilibration method. For kinetic
adsorption, 30 mg of adsorbent for Nicosulfuron and 20 mg for Mecoprop-P were
equilibrated in duplicate by shaking with 30 mL of pesticide solutions containing 0.25–
1.0 mM Nicosulfuron and 1.0–3.0 mM Mecoprop-P at room temperature in a turnover shaker operating at 52 rpm for variable lengths of time. For adsorptiondesorption isotherms, duplicate samples containing 30 mg of adsorbent for
Nicosulfuron and 20 mg for Mecoprop-P were equilibrated by shaking with 30 mL of
0.5–1.5 mM Nicosulfuron and 0.5–3.0 mM Mecoprop-P at room temperature for 24 h
[26]. The resulting supernatant was centrifuged and separated to determine the
concentrations of Nicosulfuron and Mecoprop-P by UV–Vis spectroscopy at 239 and
285 nm, respectively, on a Perkin Elmer Lambda 11 UV–Vis spectrophotometer.
Desorption was measured at each equilibrium point in the adsorption curve
by replacing a small portion of supernatant (15 mL for Nicosulfuron and 7.5 mL for
Mecoprop-P) with an identical volume of distilled water. After shaking for 24 h, the
amount of pesticide desorbed was calculated as described above. This procedure is
described in detail elsewhere [27]. Nicosulfuron and Mecoprop-P adsorption–
desorption isotherms were fitted to the Langmuir [28], Freundlich [29] and Dubinin–
Radushkevich (D–R) [30] equations:
𝐶𝑒
𝐶𝑠
=
1
𝐶𝑚 ·𝐿
𝐶𝑒
(1)
1/𝑛𝑓
(2)
+
𝐶𝑚
𝐶𝑠 = 𝐾𝑓 · 𝐶𝑒
–1
𝑙𝑛𝐶𝑠 = 𝑙𝑛𝑞𝑚 − 𝐾𝜀 2
(3)
where Cs (mmol·g ) is the amount of pesticide adsorbed per unit mass of adsorbent at
–1
the equilibrium concentration Ce (mmol·L ); Cm is the adsorbent monolayer capacity
-1
–1
(mmol·g ); L is the Langmuir adsorption constant (L·mmol ), which is related to the
adsorption energy; Kf (mmol
1–nf nf –1
L g ) and nf are the Freundlich parameters
incorporating all factors affecting adsorption; qm is the theoretical adsorption capacity
133
Capítulo V
–1
2
–2
of the adsorbent (mmol·g ); K (mmol ·kJ ) is a constant related to the adsorption
energy; and ε, which is equal to RT·ln [1 + (1/Ce)], is the Polanyi adsorption potential, R
being the universal gas constant and T (K) the temperature.
2.4. Effects of competing anions
–
–
2–
2–
The effects of competing anions (NO3 , Cl , SO4 , CO3
2–
and HPO4 ) on the
adsorption behavior of Nicosulfuron or Mecoprop-P were examined by mixing variable
volumes of 0.5 M solutions of the sodium salts of the anions with 50 mL of a 1 mM
pesticide solution in a total volume of 100 mL. The anion/pesticide mole ratio ranged
from 0 to 100. Duplicate volumes of 30 mL of these solutions were added to 30 mg of
adsorbent. For desorption tests, duplicate volumes of 30 mL of the anion solutions
were added to 30 mg of adsorbent containing Nicosulfuron or Mecoprop-P in
proportions such that the resulting anion/adsorbed pesticide mole ratio ranged from 0
to 100. The solutions were shaken at 52 rpm for 24 h, and the pesticide concentrations
in the supernatant then determined by UV–Vis spectroscopy.
2.5. Adsorbent regeneration and recyclability
Duplicate amounts of 30 mg of adsorbent for Nicosulfuron and 20 mg for
Mecoprop-P were equilibrated by shaking in 30 mL of solutions containing an initial
pesticide concentration of 0.5 mM at room temperature for 24 h. Then, the
supernatant was centrifuged and separated to determine the concentration of
Nicosulfuron and Mecoprop-P by UV–Vis spectroscopy at 239 and 285 nm,
respectively, the LDH–pesticide complex being calcined at 500°C for 2 h to recover the
adsorbent: HT500. The adsorbent was subjected to a total of four successive
adsorption–calcination cycles.
3. Results and discussion
3.1. Adsorption–desorption tests
3.1.1. Effect of pH and adsorption kinetics
The influence of pH on Nicosulfuron and Mecoprop-P adsorption on HT500
was studied at an initial value of 5, 7 or 9. Based on the results, the adsorption of
134
Pesticide adsorption on Mg-Al mixed oxides. Kinetic
modeling, competing factors and recyclability
Nicosulfuron was not influenced by pH and that of Mecoprop-P only slightly. This led
us to adopt pH 5 (i.e. that of the pesticide solution) for subsequent adsorption tests on
Nicosulfuron and pH 7 (i.e. that of maximum adsorption) for those on Mecoprop-P.
Table 1. Rate constants (Kad) obtained from adsorption curve for HT500 with different
initial concentrations of pesticide
Pesticide
Initial concentration
-1
2
Slope
Intercept
Kad (h )
R
0.25
-0.139
2.396
0.320
0.982
0.50
-0.130
2.384
0.300
0.972
1.00
-0.137
2.722
0.317
0.992
1.00
-0.125
3.015
0.288
0.990
2.00
-0.121
2.930
0.278
0.998
3.00
-0.124
2.968
0.286
0.995
(mmol/L)
Nicosulfuron
Mecoprop-P
Fig. 1. Adsorption kinetics of 0.5mM Nicosulfuron (A) and 1mM Mecoprop-P (B) on HT500
135
Capítulo V
Adsorption of Nicosulfuron and Mecoprop-P was fast at the start and slowed
down as equilibrium was approached. The kinetics of adsorption of the pesticides on
HT500 was examined via the kinetic order and an intraparticle diffusion kinetic model
based on the following equations was used to fit the experimental results. The
adsorption kinetics of the pesticides was elucidated by substituting the initial
concentrations of Fig.1 into the pseudo first-order rate equation of Largergren [31,32]:
log (Cse − Cs) = log Cse − K ad
t
(4)
2.303
where Cse is the maximum amount of pesticide adsorbed and Cs that adsorbed at time
–1
t, both in mmol·g . Plots of log (Cse – Cs) versus t spanning different time intervals
were all almost linear —the fit being much better for Mecoprop-P, however—, which
testifies to the applicability of the Largergren first-order rate equation here. Table 1
shows the adsorption rate constant, Kad, for the two pesticides as calculated from the
slope of each plot.
Adsorption measurements of the pesticides at different initial concentrations
made after a variable contact time were used to calculate their adsorption rate
constant, Kad, and kinetic order. In all cases, more than 50% of the adsorbed pesticide
amount occurs within 5 h., and the equilibrium was reached within 24 h in both
pesticides (see Fig. 1). Both exhibited increasing adsorption with increase in their initial
concentration.
The mechanism behind the adsorption of the two pesticides was elucidated
by analyzing the kinetic results in the light of the Morris–Weber equation for
intraparticle diffusion [32-34]:
𝐶𝑠 = 𝐾𝑝 𝑡 1/2 + 𝐶
( 5)
–1
where Cs is the amount of pesticide adsorbed at time t (mmol·g ), Kp the intraparticle
–1
diffusion rate constant (mmol·g ·h
–1/2
), t the agitation time (h) and C the intercept.
136
Pesticide adsorption on Mg-Al mixed oxides. Kinetic
modeling, competing factors and recyclability
Fig. 2. Adsorption and desorption isotherms for Nicosulfuron (A) and
Mecoprop-P (B) on calcined hydrotalcite.
At least two different types of adsorption mechanisms were to be expected
from the structural characteristics of the adsorbent and the use of stirring. One was
transfer of the pesticides from the bulk solution into HT500 pores and the other
adsorption at the outer surface of the hydrotalcite. The adsorption rate-determining
step could therefore be either adsorption or intraparticle diffusion [35]. Based on the
applied model, a plot of the amount of pesticide adsorbed, Cs, against the square root
1/2
of time, t , should be linear if intraparticle diffusion was involved in the process; also,
if the lines passed through the origin, then intraparticle diffusion would be the rate2
determining step [36,37]. The correlation coefficients R of Table 2, obtained at Cs
values spanning up to 16 h, suggest that the adsorption of Nicosulfuron and
137
Capítulo V
Mecoprop-P on calcined hydrotalcite (HT500) may involve intraparticle diffusion;
however, none of the curves passed through the origin, so intraparticle diffusion
cannot have been the rate-determining step and other factors may have operated
simultaneously to govern the adsorption process. Increasing the contact time to 24 h
led to poor correlation coefficients for the intraparticle diffusion model. Therefore,
adsorption of Nicosulfuron and Mecoprop-P on calcined hydrotalcite (HT500) may be
followed by intraparticle diffusion for up to 16 h.
Fig. 3.D-R adsorption isotherms for Nicosulfuron and Mecoprop-P on HT-500
Table 3. Freundlich and Langmuir model parameters for Nicosulfuron and Mecoprop-P adsorption onto HT500
138
Pesticide adsorption on Mg-Al mixed oxides. Kinetic
modeling, competing factors and recyclability
Kp was calculated from the slope of a plot of Cs versus t
1/2
for each pesticide
(see Table 2). Whereas the values for Mecoprop-P were all very similar, those for
Nicosulfuron increased with increasing pesticide concentration. Removing nitrate with
a Ca/Al chloride hydrotalcite [34] was previously found to result in a similar increase in
Kp with increasing concentration. The dissimilar behavior of Nicosulfuron and
Mecoprop-P in this respect can be ascribed to the greater steric hindrance in the
former —a result of its being a bulkier molecule.
3.1.2. Adsorption–desorption isotherms
Fig. 2 shows the adsorption–desorption isotherms for Nicosulfuron and
Mecoprop-P adsorbed on HT500. Adsorption isotherms are useful to determine the
adsorption capabilities of adsorbents, thermodynamic parameters such as the energy
of adsorption and the nature of host–guest interactions. Both isotherms in Fig. 2 are of
the L-type in the classification of Giles [38], therefore HT500 has a high affinity for the
two pesticides. An L-shaped isotherm suggests that, as substrate sites are filled, it
becomes increasingly difficult for a solute molecule to find a vacant site. Based on the
adsorption data, the uptake of pesticide per gram of adsorbent, Cs, was much greater
–1
–1
for Mecoprop-P (1.21 mmol·g ) than it was for Nicosulfuron (0.64 mmol·g ), probably
as a result of structural, size and ionic differences between the two pesticides.
Fig. 4.Adsorption of 0.5mM Nicosulfuron (A) and Mecoprop-P (B) by HT500 as a
function of the anion/pesticide mole ratio
139
Capítulo V
Fig. 5. Release of Nicosulfuron (A) or Mecoprop-P (B) adsorbed on HT500 by
various anions at variable anion/adsorbed pesticide ratios (0.36 and 0.47 mmol g-1
Nicosulfuron or Mecoprop-P adsorbed per gram of HT500).
The Freundlich and Langmuir equations were used to model the adsorption
isotherms in order to identify the specific factors governing adsorption of the two
pesticides. Based on the R2 values of Table 3, the Langmuir equation fitted the
experimental data much better than the Freundlich equation (see inset of Fig.2). For
this reason, the adsorption isotherms are discussed in terms of the Langmuir equation
[Eq. 1] here. The adsorption efficiency was assessed via a dimensionless separation
factor R [39,40] that was calculated from:
𝑅=
1
1+𝐿 𝐶𝑖
(6)
where Ci is the initial pesticide concentration. R values from 0 to 1 denote favorable
adsorption. The R values obtained at Nicosulfuron initial concentrations of 0.25, 0.5,
0.75, 1 and 1.5 mmol·L
–1
–2
fell in the range (31.0–7.02)·10 ; likewise, those for
140
Pesticide adsorption on Mg-Al mixed oxides. Kinetic
modeling, competing factors and recyclability
–1
Mecoprop-P at an initial concentration of 0.5, 1, 1.5 2 or 3 mmol·L spanned the range
–2
(20.0–7.69)·10 . These values are suggestive of favorable adsorption, especially at low
adsorbate concentrations.
Fig. 6. Recyclability of HT500 containing adsorbed
Nicosulfuron (A) and Mecoprop-P (B)
We used the Dubinin–Radushkevich (D–R) equation [Eq. 3] to discriminate
between physical and chemical adsorption. Parameters K and qm were calculated from
2
the slope and intercept, respectively, of a plot of ln Cs against ε (Fig. 3). The qm values
thus obtained (Table 3) were consistent with the increased adsorption of Mecoprop-P
relative Nicosulfuron noted earlier (Fig. 2). Constant K provides a measure of the mean
free energy of adsorption, E, which can be calculated from [39,41]
–1/2
E = (2K)
(7)
The mean free energy provides information about adsorption mechanisms.
Thus, adsorption is physical when 0 < E < 8 kJ/mol, due to ion-exchange if 8 < E < 16
–1
–1
kJ·mol and chemical if 20 < E < 40 kJ/mol. Our values (11.72 kJ·mol for Nicosulfuron
and 11.08 kJ·mol
–1
for Mecoprop-P) suggest that, as confirmed by the IR spectra for
the adsorption products, the two pesticides were adsorbed by ion-exchange.
141
Capítulo V
Fig. 7. TEM images of HT500 before cycling (a) and after the
3rd cycle (b)
Finally, desorption curves closely matched the adsorption curves for both
pesticides (Fig. 2), thus testifying to the high reversibility of the adsorption–desorption
process.
3.1.3. Effects of competing anions on Nicosulfuron and Mecoprop-P adsorption
The effect of competing anions on the adsorption of Nicosulfuron and
Mecoprop-P on HT500 is illustrated in Fig. 4. With an initial concentration of 0.5 mM of
both pesticides, HT500 adsorbed 0.36 mmol·g–1 Nicosulfuron and 0.47 mmol·g–1
Mecoprop-P in the absence of competing anions, but smaller amounts in their
presence. The effect was more marked on Mecoprop-P than on Nicosulfuron. The
2–
2–
adverse effect on Nicosulfuron adsorption decreased in the sequence HPO4 > CO3 ≈
–
–
2–
2–
2–
–
NO3 ≈ Cl > SO4 and that on Mecoprop-P in the sequence HPO4 > CO3 ≈ NO3 ≈ Cl
142
–
Pesticide adsorption on Mg-Al mixed oxides. Kinetic
modeling, competing factors and recyclability
2–
2–
≈ SO4 both with anion/pesticide ratios higher than 20. Therefore, HPO4 influenced
2–
the adsorption of both pesticides more markedly than did CO3
2–
and SO4 . This is
2–
2–
consistent with the sequences proposed by Forano et al. [6] (HPO4 > CrO4 > SO4
2–
2–
–
–
–
–
–
2–
–
and CO3 > SO4 > OH > F > Cl > Br > NO3 > I ) from previous results of Miyata [11]
2–
and Yamaoka et al. [42]. However, You et al. [10] found SO4 to affect the adsorption
2–
2–
of dicamba on a calcined hydrotalcite more markedly than did HPO4 or CO3 .
2–
Finally, it is worth noting the little influence of SO4 on Nicosulfuron relative
2–
to other anions, and also the —surprisingly— extremely low interference of CO3 on
Mecoprop-P adsorption at an anion/pesticide ratio of 10 (see Fig. 4).
3.1.4. Retention of the pesticides in the presence of various anions
In order to assess the stability of the host–guest complex, we measured the
retention of the pesticides by the adsorbent in the presence of various anions.
Desorption isotherms obtained by adding distilled water (Fig. 2) exhibited a high
reversibility. The influence of different anions on the release of Nicosulfuron and
Mecoprop-P previously adsorbed on HT500 is illustrated in Fig. 5. Based on the results,
both pesticides were desorbed in the presence of the studied anions, albeit to
different extent. The influence of increasing concentrations of dissolved anions was
also studied.
Goswamee et al. [16] found anions stereochemically suitable for inclusion into
LDH interlayers to boost the release of existing interlayer anions. Our results suggest
that Nicosulfuron and Mecoprop-P adsorbed on HT500 can be desorbed, whether
2–
2–
–
–
2–
partially or fully, into solutions containing HPO4 , CO3 , NO3 , Cl or SO4 ; therefore,
the nature of the ionic species present in solution affects the desorption behavior of
the pesticides. Also, increasing the anion/adsorbed pesticide mole ratio increased the
amount of pesticide removed. With Mecoprop-P, a ratio of 100/1 resulted in nearly
quantitative removal of the host–guest complex (viz. 100%, 100%, 98%, 93% and 87%
2–
–
–
2–
for CO3 , NO3 , Cl , SO4
2–
and HPO4 , respectively). Removal of the host–guest
complex for Nicosulfuron was less efficient and never complete, however; thus, it
2–
peaked at only 46% with CO3 .
The effect of the anions was different when they competed for adsorption on
HT500 (Fig. 4) and when they acted by removing the pesticides from the host–guest
143
Capítulo V
complex (Fig. 5). These results testify to the ability of the hydrotalcite to retain
Nicosulfuron and Mecoprop-P, and to remove the two pesticides from contaminated
2–
–
water in the presence of other anions. The high capacity of CO3 and NO3 to recover
Mecoprop-P suggests that the adsorbent can be reused.
3.1.5. Adsorbent recyclability
In order to assess its recyclability, the adsorbent was subjected to four
adsorption–desorption cycles on pristine HT500. As can be seen from Fig. 6, adsorption
of the two pesticides decreased after each cycle. A similar trend was previously
observed by Bruna et al. [43] in successive adsorption–desorption cycles of
carbetamide and metamitron on organohydrotalcites. Our results can be ascribed to
erosion of the adsorbent particles as seen in the TEM images after the third adsorption
cycle (Fig. 7b) relative to the pristine HT500 (Fig. 7a).
Fig. 8. XRD patterns for (a) HT500, (b) HT500-Nicosulfuron and (c)HT500-Mecoprop-P. (*Diffraction
peaks due to the Al sample holder)
3.2. Characterization of adsorption products by X-ray diffraction and FT-IR
spectroscopies
Fig. 8 shows the diffraction patterns for the adsorbent (HT500) and
adsorption products, namely: HT500-Nicosulfuron (0.03 g of HT500 and 30 mL of 0.5
144
Pesticide adsorption on Mg-Al mixed oxides. Kinetic
modeling, competing factors and recyclability
mM Nicosulfuron) and HT500-Mecoprop-P (0.02 g of HT500 and 30 mL of 1 mM
Mecoprop-P). The patterns of Fig. 8a correspond to the hydrotalcite upon calcination
at 500 °C and exhibit the typical peaks for an Al–Mg mixed oxide [44]. Once
Nicosulfuron and Mecoprop-P were adsorbed (Figs 8b and 8c, respectively), the XRD
patterns were consistent with regeneration of the hydrotalcite structure; however, d00l
exhibited no change with respect to the hydrotalcite with carbonate as interlayer
anion (d003 = 7.8 Å). By contrast, Khan et al. [25] observed an increase in interlayer
spacing (d003 = 22.54 Å) in a Mecoprop-P intercalated LDH prepared from a very high
concentration of Mecoprop dissolved in 50% methanol —where the pesticide is much
more readily soluble. Under our experimental conditions, the greatest pesticide
adsorption amounted to only 20% of the AEC of HT500 for Mecoprop-P and 10% for
Nicosulfuron; the pesticides probably adopt a flat configuration in the interlayer, on
the outer surface and at the edges of hydrotalcite particles. Based on the d003 values
derived from the X-ray diffraction patterns and FT-IR data (Fig. 9), most interlayer sites
2–
-
must have been occupied by CO3
or OH ions. Similar results were previously
obtained with other pesticides [45].
Adsorption of Nicosulfuron and Mecoprop-P on the LDH was further
confirmed by the FT-IR spectra of Fig. 9. The spectrum for the adsorbent, HT500,
–1
exhibited a broad band at ∼3500 cm
corresponding to vibrations of residual OH
–
–1
groups in the layers in addition to the bending vibration of water at 1638 cm and a
–1
band at 1383 cm
that was assigned to stretching vibrations in residual or surface
carbonate ions [46].
The spectrum for the product corresponding to the last point in the
adsorption isotherm, HT500-Mecoprop-P, exhibited the breathing bands for the
–1
benzene ring at 1496 and 1243 cm , thus confirming the presence of the pesticide.
–1
The absence of a band at 1700 cm for unionized carboxyl groups in Mecoprop-P from
the spectrum for the complex, and the presence of antisymmetric and symmetric
-1
carboxylate vibration peaks at 1600 and 1376 cm [47], confirmed that the pesticide
was adsorbed in anionic form on HT500, as expected from the LDH regeneration
mechanism established from the X-ray diffraction patterns (Fig. 8c).
145
Capítulo V
Fig. 9. FT-IR spectra for (A) HT500, (B) Nisoculfuron, (C) HT-Nicosulfuron, (D) Mecoprop-P and (E) HTMecoprop-P
The presence of Nicosulfuron among the adsorption products was apparent
from the characteristic bands at 1492 and 1375 cm–1 for C=C bonds in aromatic rings,
–1
–1
and by that at 1028 cm , due to S=O vibrations. However, the band at 1718 cm for
–1
νC=O in secondary amides was absent and a new band at 1608 cm
due to νC=O in
tertiary amides was observed instead [47]. This suggests that the pesticide may
intercalate into the LDH interlayer via the deprotonated nitrogen atom in the
secondary amide.
4. Conclusions
The adsorption–desorption behavior of the pesticides Nicosulfuron and
Mecoprop-P pesticides on calcined hydrotalcite (HT500) was studied. Based on kinetic
data, their adsorption involves intraparticle diffusion. Only with Nicosulfuron,
however, did the intraparticle diffusion rate constant increase with increasing
146
Pesticide adsorption on Mg-Al mixed oxides. Kinetic
modeling, competing factors and recyclability
concentration by effect of the dissimilar size of the pesticides. The adsorption
isotherms for both pesticides were L-shaped, and application of the Langmuir and
Dubini–Radushkevich equations revealed that adsorption was favorable and occurred
by ion-exchange. The maximum amount of Mecoprop-P adsorbed was twice than of
Nicosulfuron, which is consistent with the structural, size and ionization differences
between the two pesticides.
2–
Competing anions (particularly HPO4 ) had an adverse effect on Nicosulfuron
and Mecoprop-P adsorption. On the other hand, extraction of the pesticides from the
2–
2–
host–guest complex was especially effective with CO3 . The ability of CO3 to recover
large amounts of Mecoprop-P suggests that the adsorbent can be recycled.
Regeneration tests involving four adsorption–desorption cycles showed that
HT500 can be reused several times to remove Nicosulfuron and Mecoprop-P, albeit
with gradually decreasing efficiency.
The fact that the X-ray diffraction patterns were nearly identical with those
for hydrotalcite intercalated with carbonate ions suggests that the pesticides adopt a
flat arrangement in the interlayer and, also, that they occupy outer surfaces and edges
of the particles.
Acknowledgments
This work was partly supported by Spain’s Ministry of Science and Innovation
(Projects AGL2008-04031-CO2-2 and CTM2011-25325), and cofunded by FEDER and
the Andalusian Regional Government (Junta de Andalucía, Research Group FQM-214).
R. Otero also acknowledges funding from MEC-ERDF. The authors wish to thank the
staff at the Electron Microscopy and Elemental Analysis units of the Central Research
Support Service (SCAI) of the University of Córdoba for technical assistance.
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1975
.
CAPÍTULO VI. ADSORPTION OF THE HERBICIDE SMETOLACHLOR
ORGANOSILICAS
ON
PERIODIC
MESOPOROUS
Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas
GRAPHICAL ABSTRACT
Abstract
Due to their hydrophobic pore systems and high surface areas, periodic
mesoporous organosilicas (PMOs) offer unique properties for use as adsorbents of
organic compounds. Two organosilicas (ethane-PMO and benzene-PMO) and a silica
(PMS) were synthesized following a similar procedure. The adsorption of the herbicide
S-Metolachlor from an aqueous solution was then compared on the three solids. Both
PMOs exhibited a higher adsorption capacity than their silica counterpart. Additionally,
both PMOs showed much higher stability than the PMS, whose mesostructure was
seriously damaged under aqueous conditions. Moreover, both PMOs exhibited a very
different adsorption behavior. The adsorption of S-Metolachlor on each material was
described by different isotherms. The amount retained depended on the pH, and was
maximum at pH=2 for ethane-PMO and at pH=4 for benzene-PMO. Our results suggest
that both hydrophobic and aromatic π-π interactions play an important role in the
adsorption of this herbicide. The adsorption capacity of benzene-PMO was very high
with a loading of ca. 0.5 g herbicide per gram of adsorbent after successive adsorption
cycles.
Keywords: S-Metolachlor, herbicides, ethane-PMO, benzene-PMO, adsorption, water
treatment.
155
Capítulo VI
1. Introduction
The so-called periodic mesoporous organosilicas (PMOs) were firstly reported
in 1999 by three different research groups which synthesized them via the
condensation of bridged organosilanes of the type (R’O)3Si-R-Si(OR’)3 in the presence
of a surfactant. [1-3]. Unlike other hybrid materials prepared from mesoporous silicas
by grafting and co-condensation, the organic fragments (-R-) in PMOs materials are
abundantly and homogeneously distributed through their structure and are linked by
covalent bonds to two silicon units. Various R groups have been successfully
incorporated into these materials, including aliphatic (-CH2-, -CH2-CH2-, -CH=CH-),
aromatic (-C6H4-, -C12H8-) and even heteroaromatic fragments (-C4H2S-) [4]. Similarly to
mesoporous silicas, PMOs have a mesoporous structure with a controlled pore size
and high surface area. However, PMOs are more hydrophobic due to the existence of
organic fragments in their framework. All these facts make them excellent candidates
as adsorbents for organic molecules. In particular, ethylene- (R= -CH2-CH2-) and
phenylene- (R= -C6H4-) bridged PMOs, which are very stable under hydrothermal
conditions [5], can be very useful for the removal of organic pollutants from aqueous
effluents.
An interesting environmental application of PMOs has involved their use as
adsorbents for heavy metals. It requires the presence of certain organic functionalities
which can be present in the PMOs either by using specific organosilanes and/or
organodisilanes for their synthesis or by introducing them later through reactions with
the organic moieties present in their frameworks. Thus, PMOs with isocyanurate
bridging
groups
were
synthesized
by
co-condensation
of
TEOS,
3-
mercaptopropyltrimethoxysilane and tris[3-(trimethoxysilyl)propyl]isocyanurate and
their high adsorption capacity for Hg
2+
ions was reported [6]. Co-condensation
between 1,2-bis(triethoxysilyl)ethane and 3-(mercaptopropyl)trimethoxysilane was
used to prepare several PMOs with different pore sizes depending on the surfactant
added, i.e. Brij-76 and Pluronic P123, in the range of 1.4-3.4 and 4.2-6.4 nm,
respectively [7]. The former were quite selective for Hg
2+
2+
2+
ions whereas the latter
3+
exhibited a high affinity for Hg , Cd and Cr cations. More recently, different PMOs
have been synthesized by co-condensation of TEOS and an organodisilane precursor
156
Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas
containing disulfide units. They showed excellent efficiencies for the adsorption of Hg
2+
-1
ions (up to 716 mg g ) [8]. A different synthetic strategy involved the functionalization
of the double bonds of an ethenylene-bridged PMO with propylthiol groups [9]. This
2+
material combines the Hg adsorption ability of the thiol groups with the high stability
of the PMO, thus allowing it to be reusable after multiple regeneration cycles.
Very recently, Walcarius and Mercier have extensively revised the application
of mesoporous silicas for the removal of organic and inorganic pollutants [10]. The use
of PMO materials as adsorbents for toxic organic pollutants is limited to the work by
Borghard et al [11] .They synthesized two mesoporous polysilsesquioxanes from 1,2bis(triethoxysilyl)ethane and 1,4-bis(trimethoxysilylethyl)benzene under alkaline
conditions. Even though the latter material showed much lower specific surface area
than the former one and a non periodic structure, it adsorbed 14 times more 4nitrophenol than the former [12]. The different performance of both materials was
explained by the existence of π-π interactions between the aromatic rings of the
phenolic compounds and the arylene bridges.
S-Metolachlor is a pre- and post-emergence selective herbicide used to
control grasses and some broad-leaved weeds in a wide range of crops. Previous
papers have reported the adsorption of S-Metolachlor by different adsorbents such as
organohydrotalcites (OHTs) [13, 14], soils [15], activated charcoals [16] and clays [17].
This paper reports the adsorption of S-Metolachlor on ethylene- and phenylenebridged PMOs, i.e., ethane-PMO and benzene-PMO, respectively. The influence of
different factors, such as pH and contact time, as well as successive adsorptions,
adsorption/desorption experiments and regeneration of the adsorbent has been
studied. For comparison, a mesoporous silica has also been used as adsorbent.
2.
Materials and methods
2.1.
Materials
A periodic mesoporous silica (PMS) and two organosilicas, i.e., ethane-PMO
and benzene-PMOs, were synthesized by using Brij-76 as surfactant under acidic
157
Capítulo VI
conditions according to previously reported procedures (see supporting information)
[12].
Analytical standard S-Metolachlor (2 Chloro-N-(2-ethyl-6-methylphenyl)-N-[(1S)-2methoxy-1-methylethyl] acetamide) was supplied by Sigma Aldrich (see scheme 1). The
water solubility at 25ᵒC is 480 mg/L.
2.2 Characterization.
PMOs were characterized by different techniques. X-ray powder diffraction
(XRD) patterns were recorded on a Siemens D-5000 powder diffractometer (Cu-Kα
radiation) and FT-IR spectra were obtained with a Perkin Elmer Spectrum One
spectrophotometer using KBr disc technique. N2 isotherms were determined on a
Micromeritics ASAP 2010 analyzer at −196°C. Particle size data were obtained in a
2000F Mastersizer particle size analyzer by laser diffraction technology, with a dry
dispersion automatic system Venturi Scirocco 2000 of Malvern Instruments.
Microstructural characterization of the materials was carried out using JEOL 1400 TEM
and JEOL 2010 TEM equipments. The 29Si MAS NMR spectra were recorded at 79.49
MHz on a Bruker Avance 400 WB spectrometer at room temperature. An overall 5000
free induction decays were accumulated. The recycle time was 15 s. Chemical shifts
were measured relative to a tetramethylsilane standard. UV spectroscopy (Perkin
Elmer UV-visible spectrophotometer, Model Lambda 11) at 265 nm was used to
determine the concentration of S-Metolachlor.
2.3 Adsorption/desorption experiments.
S-Metolachlor adsorption on mesoporous materials was studied at different
initial pH values, contact times (kinetics), and initial herbicide concentration
(isotherms), by suspending duplicated samples of 20 mg of each adsorbent in 30 mL of
aqueous herbicide solutions. Samples of sorbent and herbicide in water were shaken
in a turn-over shaker at 52 rpm.
158
Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas
To study the adsorption at different initial pHs [2.0, 3.0, 4.0, 5.0 and 6.6 (pH of
herbicide solution)] and contact times (1, 2, 5, 15 and 24 h), an initial herbicide
concentration of 350 µM was used. Adsorption isotherms were obtained by the batch
equilibration technique. For that, the samples were equilibrated through shaking at 52
rpm for 24 h at room temperature, as described elsewhere [18, 19], using initial
-4
-3
herbicide concentrations between 1.0·10 M and 1.4·10 M.
Scheme 1. Adsorption of S-Metolachlor on PMOs showing the main interactions between the
herbicide and both surfaces (see text for discussion).
The supernatant liquid was centrifuged and separated to determine the
concentration of S-Metolachlor by UV spectroscopy. The amounts of herbicide
adsorbed were determined from the initial and final solution concentrations.
According to a previously described procedure [13], desorption was measured
from each equilibrium point of the adsorption curve by replacing a small portion of
supernatant (7.5 mL) with the same amount of distilled water. After shaking for 24 h,
the amount of herbicide desorbed was calculated as indicated above.
The pseudo first order [20-24] and the pseudo second order [23] models and
intraparticle diffusion equation [25, 26] were used with the goal at evaluating the
adsorption kinetic data.
log�𝐶𝑠1 − 𝐶𝑠𝑡 � = log 𝐶𝑠1 −
𝑡
𝐶𝑠𝑡
=
1
𝑘2 ·𝐶𝑆2 2
+
1
𝐶𝑠2
·𝑡
𝑘1
2.303
159
·𝑡
(1)
(2)
Capítulo VI
𝐶𝑠𝑡 = 𝑘𝑝 · 𝑡 1/2 + 𝐶
(3)
-1
where Cs1 and Cs2 are the maximum amount of pesticide adsorbed (mg·g ) for first
-1
and second-kinetic order, respectively. Cst is the amount adsorbed at time t (mg·g ), k1
-1
is the pseudo first order rate constant for the adsorption process (min ), k2 is the rate
−1
−1
constant of pseudo second order adsorption (g·mg ·min ), C is the intercept, and kp is
-1
the intraparticle diffusion rate constant (mg·g ·min
-1/2
).
S-Metolachlor adsorption isotherms were fitted to Langmuir (Eq. (4)),
Freundlich (Eq. (5)) and Dubinin-Radushkevich (D-R) (Eq. (6)) equations:
1
𝐶𝑠
=
1
𝐶𝑚 ·𝐿
·
1
𝐶𝑒
+
1
(4)
𝐶𝑚
𝑙𝑜𝑔𝐶𝑠 = 𝑙𝑜𝑔𝐾𝑓 + 𝑛𝑓 𝑙𝑜𝑔𝐶𝑒
-1
𝑙𝑛𝐶𝑠 = 𝑙𝑛𝑞𝑚 − 𝐾𝜀 2
(5)
(6)
where Cs (µmol g ) is the amount of herbicide adsorbed per unit mass adsorbent at
-1
the equilibrium concentration Ce (µmol L ), Cm is the monolayer capacity of the
-1
-1
adsorbent (µmol·g ), and L is the Langmuir adsorption constant (L·µmol ), which is
related to the adsorption energy. Kf (µmol
1-nf nf
-1
L g ) and nf are Freundlich parameters
incorporating all factors affecting the adsorption process. qm is the theoretical
-1
2
-2
adsorption capacity of the adsorbent (µmol·g ) and K (mmol ·kJ ) is the constant
related to adsorption energy. ε is the Polanyi adsorption potential, and equal to
RT·ln(1+1/Ce), where R is the universal gas constant and T is the temperature in the
Kelvin scale.
2.4 Successive adsorptions
They were carried out in the highest equilibrium point of the adsorption
isotherm curves, by suspending the solid (adsorbent-herbicide) in 30 mL of a fresh
-3
aqueous herbicide solution (1.25·10 M). The contact time was 24 h. After shaking, the
suspensions were centrifuged and the herbicide concentration determined by UV-Vis
spectroscopy. This process was repeated up to complete saturation of the adsorbent.
160
Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas
Fig. 1. Powder X-ray diffraction patterns of PMS (left), ethane-PMO (center) and benzene-PMO (right)
before, after being in contact with the solution of herbicide at different pHs and after herbicide extraction
(inset).
2.5 Regeneration of the adsorbent
In order to test the regeneration of the adsorbent, some desorption
experiments were performed with either ethanol or water from the highest
equilibrium point of the adsorption isotherms. The extracting volume was 30 mL and
the contact time was 24 h. After shaking, the suspensions were centrifuged, the
supernatant liquid was separated and evaporated (in the case of ethanol), and the
residue dissolved in the same volume of distilled water to determine the herbicide
concentration by UV-Vis spectroscopy.
3. Results and discussion
A periodic mesoporous silica (PMS) and two organosilicas, i.e., ethane-PMO
and benzene-PMO, have been tested for the adsorption of the herbicide SMetolachlor. These three materials exhibited a low angle (100) peak and two broad
161
Capítulo VI
and short second-order (110) and (200) peaks at higher incidence angles indicative of
materials with 2D hexagonal (P6mm) mesostructures (Fig. 1). The integrity of the
organic bridges was confirmed by FTIR measurements (see supporting information)
[5]. Their N2 adsorption-desorption isotherms were of type IV with a sharp increase in
the adsorption at P/P0 between 0.30-0.47 due to capillary condensation, typical of
materials with a narrow pore size distribution in the mesopore range (Fig. 2). The N2
adsorption step was shifted to higher relative pressures following the sequence:
benzene-PMO < ethane-PMO < PMS, by the effect of the increase in the pore size. The
main physical properties of the three adsorbents are summarized in Table 1. Their
mesoporous structures were confirmed by TEM images (Fig. 3); the inset in figure 3B
shows a similar pore size to that determined by the BJH method. In addition, the
structure is very similar for PMS and organosilicas (Fig. 3A-C) On the other hand, both
organosilicas showed very similar particle size distribution curves, which were centred
at around 12 and 14 µm for benzene-PMO and ethane-PMO, respectively (Fig. 4).
Fig. 2. Nitrogen adsorption-desorption isotherms and the corresponding pore size distributions
(insets) for PMS, ethane-PMO and benzene-PMO.
162
Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas
A comparison of the behavior of the three adsorbents as a function of pH (Fig.
5) revealed significant differences among them. FT-IR spectra (not shown) revealed
that the herbicide was intact after adsorption. In order to gain some insights into the
relationship between pH and adsorption, measurements of the point of zero charge
(pzc) were accomplished for the three materials. This value indicates the pH required
to give zero net surface charge. Below the pzc the hydroxyl groups at the surface of an
oxide become protonated and so positively charged whereas above the pzc they are
deprotonated and consequently negatively charged. By following the mass titration
method (Fig. 6) [27], pzc values of 3.9-4.0 were obtained for the three samples, close
to those reported for silica by other authors [28]. As a consequence, the surfaces of
the three materials were positively charged at pH=2-3. This suggest that the main
interactions between the adsorbents and S-Metolachlor at pH ≤ 3 were electrostatic,
i.e., interactions between positive charges on the surface and electron-rich regions (N,
O and Cl atoms) in the adsorbed molecules, as a result of which the values of
adsorption for samples PMS and ethane-PMO were maximum (Fig. 5). At pH=4 their
surfaces were essentially neutral and so their interactions with S-Metolachlor became
weaker. Consequently, their adsorption values decreased. Hydrophobic interactions
seem to play an important role, since the amount of adsorbed herbicide was higher for
ethane-PMO than for PMS throughout the pH range. This reasoning is based on the
assumption that both materials possess an analogous amount of Si-OH groups. 29Si
MAS NMR spectra of the three samples were recorded in order to ascertain it (Fig. 7).
They allowed determining their condensation degree (Table 2), which decreased in the
order PMS > ethane-PMO > benzene-PMO. In addition, the OH/Si ratio for benzenePMO was apparently higher than for PMS and ethane-PMO. However, the organic
moieties present in the framework of both organosilicas also contribute to the masses
of the corresponding materials. In fact, similar relative contents of silanol groups were
found in the three materials when that ratio was corrected taking into account the
mass of the Si unit for each material (Table 2).
On the other hand, the maximum adsorption for benzene-PMO occurred at
pH=4 (Fig. 5). Considering the higher hydrophobic character of ethane-PMO than that
of benzene-PMO [29], the aromatic π-π interactions between the aromatic ring of the
163
Capítulo VI
herbicide and the phenylene bridges of the hybrid material should be crucial for the
adsorption of this molecule onto benzene-PMO.
Table 1
Physicochemical properties of periodic mesoporous organosilicas.
aoa (nm)
SBETb (m2 g-1)
Smpc (m2 g-1)
Vp d (cm3 g-1)
Dpe (nm)
Wf (nm)
PMS
6.7
885
0
1.17
4.2
2.5
Ethane-PMO
7.0
1059
26
1.00
4.1
2.9
Benzene-PMO
6.2
1237
257
0.94
3.6
2.6
Material
Unit-cell dimension calculated from ao= (2d100/√3)
BET specific surface area determined in the range of relative pressures from 0.05 to 0.2
c
Micropore area, calculated by t-Plot method.
d
Single-point pore volume
e
Diameter pore average size, calculated from the adsorption branch according to the BJH method;
f
Pore wall thickness, estimated from W= ao – Dp
a
b
Both mesoporous organosilicas exhibited a higher adsorption capacity toward
the herbicide S-Metolachlor than that of the mesoporous silica, as showed by the
adsorption kinetics (Fig. 8). The amount of adsorbed herbicide significantly increased
-1
-1
from 53µmol·g on PMS to 127 and 135µmol·g on ethane-PMO and benzene-PMO,
respectively. Also, both PMOs presented a higher stability under the conditions of
adsorption than their silica counterpart (Fig. 1). The material PMS significantly
decreased its mesostructural order as the pH of the herbicide solution increased. This
effect was much less pronounced for benzene-PMO and particularly for ethane-PMO.
The decrease in the intensity of the (100) peak for PMOs can be attributed to the
presence of adsorbate molecules inside the pores which causes a decrease in the
contrast for X-rays between the organosilica walls and the pores. In fact, both PMOs
recovered their original X-rays patterns upon removing the herbicide (vide infra), what
was confirmed before and after the adsorption for benzene-PMO by TEM (Figure 3, C
and D), unlike PMS whose mesostructure was destroyed (Figure 1 inset).
164
Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas
Fig. 3. TEM images for PMS (A), ethane-PMO (B), including a TEM image of high resolution (inset), and
benzene-PMO before adsorption of S-Metolachlor (C) and after desorption (D).
Fig. 4. Particle size distribution for ethane-PMO and benzene-PMO
Fig. 5 Influence of pH on the adsorption of S-Metolachlor on PMS,
ethane-PMO and benzene-PMO
165
Capítulo VI
The adsorption kinetics of the herbicide S-Metolachlor were elucidated by
substituting the initial concentrations into the pseudo first order (Largergren equation)
and pseudo second order adsorption rate equations, respectively. Table 3 shows the
adsorption rate constants k1 and k2 calculated from the slope of each plot. The
2
correlation coefficients R for the pseudo first order kinetic model were between 0.89
2
and 0.97 and the correlation coefficients R for the pseudo second order kinetic model
were 0.99. Therefore, this adsorption system fitted better to a pseudo second order
kinetic model [22].
The mechanism behind the adsorption was studied by analyzing the kinetic
results according to the Morris–Weber equation for intraparticle diffusion (Eq.(3)). The
adsorption rate-determining step could therefore be either adsorption or intraparticle
diffusion. Based on the applied model, a plot of the amount of pesticide adsorbed, Cst,
1/2
against the square root of time, t , should be linear if intraparticle diffusion was
involved in the process; also, if the lines passed through the origin, then intraparticle
diffusion would be the rate-determining step [21, 30].
Fig. 6. Mass titration curves for PMS,ethane-PMO and benzene-PMO
2
The correlation coefficients R of Table 3, obtained at Cst values spanning up
to 24 h, suggested that the adsorption of herbicide S-Metolachlor on PMOs may
involve intraparticle diffusion; however, none of the curves passed through the origin,
so intraparticle diffusion cannot have been the rate-determining step and other factors
may have operated simultaneously to govern the adsorption process. Kp and C were
166
Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas
calculated from the plot of Cst versus t
1/2
for each adsorbent (see Table 3). The values
of kp and C for benzene-PMO were higher and lower than those for ethane-PMO,
respectively, suggesting a higher contribution of the intraparticle diffusion for
benzene-PMO.
29
Fig. 7. Si MAS NMR spectra for PMS, ethane-PMO and benzene-PMO
Table 1
29
Si MAS NMR results obtained for mesoporous materials.
Adsorption isotherms were measured for both mesoporous organosilicas due
to their significantly higher capacity compared to that of the PMS sample (Fig. 8). The
adsorption isotherms at the pH of maximum adsorption for each material are depicted
in Fig. 9. Ethane-PMO exhibited a Langmuir isotherm, specifically L2 [31]. In this case,
the adsorbed molecules are most likely to be adsorbed flat (Scheme 1), according to
the abovementioned electrostatic interactions. The plateau of the curve gave 255
-1
µmol g . By contrast, the adsorption of the herbicide on benzene-PMO followed a type
L4 isotherm. It exhibited an ill-defined first plateau which suggests a close affinity of SMetolachlor for two different regions in the surface of benzene-PMO. We tentatively
167
Capítulo VI
assume that adsorption firstly occurred in hydrophilic regions, i.e., silanol groups,
through hydrogen bonds with oxygen and nitrogen atoms of the herbicide, while the
second stage would be caused by the adsorption of S-Metolachlor on hydrophobic
regions, i.e., phenylene bridges, through aromatic π-π interactions (Scheme 2).
However, we cannot ascertain whether a partially flat or a perpendicular orientation
occurred at both stages. Also, the interaction of silanol groups (Lewis acid) of benzenePMO and aromatic rings (Lewis base) of S-Metolachlor cannot be completely ruled out
[32]. The second plateau would be reached because of the complete saturation of the
hydrophobic regions.
Fig. 1. Adsorption kinetics of S-Metolachlor on PMS,
ethane-PMO and benzene-PMO.
The adsorption results show that the amount of S-Metolachlor adsorbed on
-1
benzene-PMO (667 µmol g ) exceeded the amount adsorbed by activated charcoals
-1
-1
(480-550 µmol g ) [16], organohydrotalcites (OHTs) (356-385µmol g ) [13, 14], clays
-1
-1
(200 µmol g ) [17] and soils (50 µmol g ) [15]. The adsorption capacity of the ethanePMO was slightly smaller than that of the organohydrotalcites (OHTs) and similar to
that of clays.
168
Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas
Table 2
Kinetic parameters for the adsorption of S-Metolachlor onto PMS and PMOs
The desorption curves almost matched the adsorption ones for ethane-PMO
(Figure 9), thus indicating a high reversibility of the adsorption-desorption process.
However, the isotherm for benzene-PMO displayed a considerable hysteresis, which
must be related to the interactions between the adsorbent and S-Metolachlor.
Fig. 2. Adsorption and desorption isotherms of S-Metolachlor on ethane-PMO
and benzene-PMO
Freundlich and Langmuir equations were tested to model the adsorption
isotherms in order to determine the parameters which quantify the adsorption process
2
(Table 4). According to R values, the Langmuir equation was fitted much better to the
experimental data than the Freundlich one.
169
Capítulo VI
In order to predict the adsorption efficiency, a dimensionless separation factor R was
determined using the following equation [33, 34].
𝑅=
1
(7)
1+𝐿 𝐶𝑖
where Ci is the initial herbicide concentration. Values in the range 0 < R < 1 represent a
favorable adsorption. The R values for adsorption on ethane-PMO and benzene-PMO,
-1
with initial concentrations of S-Metolachlor 100-1400 µmol L , were found in the
range of 0.94-0.51 and 0.95-0.53, respectively. These values indicate a favorable
adsorption, and also allow to confirm the higher adsorption at low concentrations in
both cases.
Scheme 2. Plausible model for the adsorption of S-Metolachlor onto benzene-PMO through adsorption on
hydrophilic (a) and hydrophobic regions (b) of the original surface and formation of a new adsorbed layer up
to complete filling of the pores (c). The polar part and the aromatic ring of the molecule are represented by
a circle and a stick, respectively. The different colors depict different adsorption stages.
In order to distinguish between physical and chemical adsorption, parameters
K and qm from Dubinin-Radushkevich equation (D-R) were calculated from the slope
and intercept in the plot of lnCs versus ε2, respectively [24, 35]. The values of qm
(Table 4) agreed with the abovementioned higher adsorption on benzene-PMO than
on ethane-PMO (Figures 8 and 9). The constant K is related to the mean free energy of
adsorption (E), which can be calculated by Eq. 8 [33, 36]:
)-1/2
E = (2K
(8)
The mean energy provides information about the adsorption mechanism. The
-1
-1
adsorption is physical in nature when 0<E<8 kJ mol ; if it is between 8 kJ mol and 16
170
Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas
kJ mol-1, the adsorption is due to exchange of ions; and if the value of E is between
-1
20<E<40 kJ mol , a chemisorption occurs. The values found in the present study were
-1
-1
2.7 kJ mol for benzene-PMO and 3.3kJ mol for ethane-PMO, thus pointing to a
physical adsorption.
Aiming at the saturation of the adsorbents, successive adsorptions were
accomplished (Fig. 10). The maximum S-Metolachlor adsorption capacities for both
host materials were different. Ethane-PMO was saturated after two adsorptions, being
-1
the adsorbed amount of 416.63 µmol g , whereas benzene-PMO required four
-1
successive adsorptions, which accounted for 1729.35 µmol g .
Table 3
Freundlich, Langmuir and Dubinin-Radushkevich model parameters for the S-Metolachlor adsorption onto ethanePMO and benzene-PMO
Probably, this adsorption occurred on the exposed parts of the layer already
present (Scheme 2c). Taking into account that the volume of the S-Metolachlor
3
molecule is around 875 Å [37], the resulting volume occupied by the adsorbed
3
-1
herbicide was estimated to be 0.22 and 0.91 cm g for ethane-PMO and benzenePMO, respectively. Accordingly, the pores of benzene-PMO are completely filled in
with S-Metolachlor after successive adsorptions (see Table 1). This is not the case for
ethane-PMO, thus remarking the key role of the phenylene bridges to the adsorption
of S-Metolachlor. The high loading (up to 0.5 g/g) and slow release of S-Metolachlor
from benzene-PMO make it an excellent candidate as a delivery system.
171
Capítulo VI
Fig. 3. Successive adsorptions of S-Metolachlor from the highest
equilibrium point of the adsorption isotherm curve for ethanePMO and benzene-PMO.
Finally, some desorption experiments were performed with either ethanol
or water from the highest equilibrium point of the adsorption isotherms to test the
regeneration of the adsorbent. In this sense, both materials behaved similarly. SMetolachlor completely desorbed from ethane-PMO and benzene-PMO when using
ethanol. However, the herbicide was not completely removed in water. Specifically,
74.5% and 84.0% were desorbed from ethane-PMO and benzene-PMO, respectively.
As expected, the herbicide showed a higher affinity for the more hydrophobic solvent
due to its non-ionic character.
4. Conclusion
Periodic mesoporous organosilicas have unique properties to be used as
adsorbents of organic molecules in virtue of their high surface area and narrow
mesopore distribution, similarly to mesoporous silicas, as well as their bridges of
organic nature which can interact with them. We have examined a potentially practical
use of these materials with ethylene and phenylene bridges for the adsorption of a
relatively complex herbicide molecule (S-Metolachlor). While ethane-PMO behaved
172
Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas
like a mesoporous silica (PMS) but with an enhanced adsorption capacity due to its
more hydrophobic character, benzene-PMO displayed a much higher herbicide
retention at a pH close to its point of zero charge. Certainly, aromatic π-π interactions
between phenylene bridges and S-Metolachlor should play an important role. In fact,
both PMOs exhibited different isotherms, specifically types Langmuir L2 and L4 for
ethane- and benzene-PMO, respectively. In addition, the latter isotherm exhibited a
desorption hysteresis effect. Both PMOs were saturated by successive adsorptions.
-1
Thus, ethane-PMO showed an uptake of 416.63 µmol g whereas it was of 1729.35
-1
µmol g for benzene-PMO. Due to its high aqueous stability, high adsorption capacity
(higher than activated charcoals, organohydrotalcites, clays and soils) and easy
regenerability, benzene-PMO is a promising adsorbent for other complex aromatic
molecules.
Acknowledgements
The authors would like to thank Spain’s Ministry of Science and Innovation
(Projects MAT2010-18778 and CTM2011-25325), Junta de Andalucía (Project P06FQM-01741 and Research Groups FQM-214 and FQM-346) and FEDER funds. R. Otero
acknowledges Ministry of Education and Science for a research and teaching
fellowship.
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176
CAPÍTULO VII. MESOPOROUS PHENOLIC RESIN
AND MESOPOROUS CARBON FOR THE REMOVAL
OF S-METOLACHLOR AND BENTAZON HERBICIDES
Mesoporous phenolic resin and mesoporous carbon for
the removal of S-Metolachlor and Bentazon herbicides
GRAPHICAL ABSTRACT
Abstract
Two mesoporous materials, i.e., a mesoporous phenolic resin and a mesoporous
carbon, were synthesized following a soft template method to test the removal of two
pesticides, Bentazon and S-Metolachlor. The adsorbents were characterized by X-ray
diffraction analysis, nitrogen adsorption-desorption isotherms, thermogravimetric
analysis, transmission electron microscopy, particle size measurement and XPS
spectroscopy. Bentazon and S-Metolachlor adsorption kinetics and isotherm studies
were carried out at pH 2 and 4, respectively, on these mesoporous materials as well as
on a reference commercial carbon. Freundlich, Langmuir, Dubinin-Raduskevich and
Temkin models were applied to describe the adsorption behavior with the Langmuir
model showing the best fit. The regeneration of the adsorbents was carried out by
calcination under nitrogen atmosphere and the results indicated that the regeneration
of the mesoporous carbon was more efficient than that of the mesoporous phenolic
resin and the commercial carbon after being reused several times. Another
disadvantage of commercial carbon was the great loss of material that it experienced
after several adsorption-desorption cycles.
Keywords: S-Metolachlor, Bentazon, mesoporous phenolic resin, mesoporous carbon,
water treatment.
179
Capítulo VII
1. Introduction
Currently, the pollution caused by pesticides has greatly increased due to
their widespread use in agriculture. Pesticides are highly noxious, sometimes nonbiodegradable and very mobile throughout the environment [1]. Therefore, many
researchers are studying the removal of these toxic pollutants from aqueous solutions.
Photocatalysis [2-4], adsorption [5-7] and electrolysis [8-10] are some processes
involved in the removal of these pollutants. The main advantages of adsorption
compared to other techniques are its low cost, high removal efficiency and easy
operation.
Activated carbons and silica gels have been considered as excellent
adsorbents for many years. These materials are not suitable as adsorbents of large
molecules because they are mainly microporous [11]. Recently, new mesoporous
materials that show promising results for the adsorption of large pollutants have been
developed [12-17]. Ordered mesoporous materials possess tunable and uniform pore
sizes, large surface areas and large pore volumes.
Among them, the ordered mesoporous polymer resin and mesoporous carbon
(MPR and MC) are relatively new materials that combine the high porosity of
mesoporous materials with the physicochemical properties of organic polymers and
carbons, respectively [18]. MC is an excellent adsorbent and therefore, as carbon is
inert, stable, hydrophobic, light and presents a high affinity towards organic pollutants.
Ordered mesoporous carbons were firstly synthesized by a hard template method
using mesoporous silica [19-21]. However, this synthesis cannot be used for large scale
production because it is time-consuming and expensive. Later, a direct synthesis
method using phenolic resin as a carbon precursor was developed [22-24].
S-Metolachlor is a selective herbicide used in several crops. It is quite mobile
and although it is mainly found in surface water, it can contaminate groundwater.
Several authors have investigated the adsorption of this herbicide on different
materials, such as activated carbon [25], soil [26], organohydrotalcites [5] or periodic
mesoporous organosilicas (PMOs) [6]. Bentazon is a contact post-emergency herbicide
widely used in agriculture, which has a great mobility in soil and is moderately
persistent in water. That is why previous papers have reported the removal of
Bentazon mainly using activated carbon as adsorbent [1, 27, 28]. The maximum
180
Mesoporous phenolic resin and mesoporous carbon for
the removal of S-Metolachlor and Bentazon herbicides
concentration of S-Metolachlor and Bentazon admitted by the World Health
Organization (WHO) in drinking water are 0.01 mg/L and 0.03 mg/L, respectively.
Unlike activated carbons, mesoporous phenolic resins and mesoporous
carbons have been hardly used as adsorbents of pesticides. Herein we have
synthesized two mesoporous materials (MPR and MC) to be tested as regenerable
adsorbents for removing S-Metolachlor and Bentazon. MPR has been prepared by the
EISA method (evaporation induced self-assembly) [29, 30], using resorcinol and
formaldehyde in the presence of surfactant F127 (EO106-PO70-EO106) under acid
conditions. MC was synthesized by calcination of MPR. For comparison, a commercially
available activated carbon (CC) was also used as adsorbent. The adsorbents were
characterized by several structural and surface techniques. Different experiments
have been carried out to study the influence of different factors, such as pH and
contact time, adsorption/desorption behaviour and regeneration of the adsorbents.
Finally, adsorption experiments at different temperatures have been undertaken in
order to determine thermodynamic parameters.
2. Experimental section
2.1. Chemicals
Pluronic127 (EO106PO70EO106), resorcinol and commercial activated carbon
were purchased from Sigma-Aldrich.
The pesticides used as adsorbates in the experiments, Bentazon (3-(1-methylethyl)-1H2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide) and S-Metolachlor (2-chloro-N-(2-ethyl6-methylphenyl)-N-[(1S)-2-methoxy-1-methylethyl]acetamide) were purchased from
Sigma Aldrich. Some of their properties are included in Table 1.
2.2. Synthesis of adsorbents
The synthesis of phenolic resin and mesoporous carbon was carried out using
a soft-template method [18]. For that, 9 ml of 3 M HCl were added to a solution of 2.2
g of triblock copolymer Pluronic F127 and 2.2 g of resorcinol in 9 ml of ethanol. After
stirring for at least 20 min [31], a formaldehyde solution (2 ml, 36 wt%) was slowly
added to it. Then, the mixture was stirred for 20min. The resultant solution was
181
Capítulo VII
transferred to a plate to evaporate ethanol. Afterwards, the product was cured in an
oven at 60˚C overnight. Finally, the solid was calcined at 380˚C under inert atmosphere
to remove the template. The resulting material, a mesoporous phenolic resin, was
designed as MPR. Amorphous mesoporous carbon, designed as MC, was obtained by
calcination of MPR in a tubular furnace under inert atmosphere at 800˚C.
2.3. Characterization
Mesoporous phenolic resin and mesoporous carbon were characterized by
different techniques. Powder X-ray diffraction (XRD) patterns were recorded with a
Thermo Scientific ARL X’TRA Powder X-ray Diffraction System (radiation CuKα
generated by 45kV and 44mA, with slits of 2, 4, 1.5 and 0.2 for the divergence, scatter,
-1
receiving scatter and receiving slit, respectively, at 0.25° 2θ min ). Nitrogen
adsorption-desorption isotherms were obtained on a Belsorp-mini II gas analyser. Prior
to the measurements, the samples were degassed at 120ᵒC for 24 h to remove
adsorbed water. Thermogravimetric curves were recorded on a Setaram Setsys
Evolution 16/18 apparatus undernitrogen at a heating rate of 5ᵒC/min. Microstructural
characterization of the materials was carried out using JEOL 1400 TEM and JEOL 2010
TEM equipments. Particle sizes were measured in a Mastersizer S analyser (Malvern
Instruments) using ethanol as dispersant. The samples were sonicated for 10 min
before the analysis. XPS spectra were recorded with a SPECS Phoibos HAS 3500 150
-9
MCD. The residual pressure in the analysis chamber was 5·10 Pa. The X-ray source
was generated by a Mg anode (hm = 1253.6 eV) powered at 12 kV and with an
emission current of 25 mA. The powdered sample was pressed and introduced into the
spectrometer without previous thermal treatment. They were outgassed overnight
and analyzed at room temperature. Accurate binding energies (BE) have been
determined with respect to the position of the C 1s peak at 284.9 eV. The surface
atomic concentration ratios were calculated using sensitivity factors from the Casa XPS
element library. The potential zeta (ζ) has been determined with a Zetasizer Nano ZS
(Malvern Instruments), with a laser of 632.8 nm, coupled to a MPT-2 autotitrator.
182
Mesoporous phenolic resin and mesoporous carbon for
the removal of S-Metolachlor and Bentazon herbicides
Table 1.
Properties of pesticides Bentazon and S-Metolachlor.
2.4. Adsorption experiments.
All adsorption experiments were carried out in duplicate by using the batch
equilibration technique at 25±1ᵒC. Adsorption of Bentazon (25 mg/L to 250 mg/L) and
S-Metolachlor (125 mg/L to 400 mg/L) were conducted by mixing 20 mg of adsorbent
with 30 mL of pesticide solution and stirring continuously the mixture for 24h. The
suspension was centrifuged and the concentration of Bentazon and S-Metolachlor
were determined by UV-Vis spectroscopy at 232 nm and 265 nm, respectively, on a
Perkin Elmer Lambda 11 UV-Vis spectrophotometer.
The adsorption kinetics were investigated by three models, the Lagergren pseudo–
first-order model [32], the pseudo-second-order-model [33] and intraparticle diffusion
model [34].
log(𝐶𝑠 − 𝐶𝑠𝑡 ) = log 𝐶𝑠 −
𝑡
𝐶𝑠𝑡
=
1
𝑘2 ·𝐶𝑠2
+
1
𝐶𝑠
·𝑡
𝑘1
2.303
·𝑡
𝐶𝑠𝑡 = 𝑘𝑝 · 𝑡 1/2 + 𝐶
[1]
[2]
[3]
-1
where k1 is the pseudo first order rate constant for the adsorption process (h ), Cs and
-1
Cst (mg·g ) are the amounts of adsorbed pesticide at equilibrium and at time t (h),
-1
-1
respectively, k2 is the rate constant of pseudo-second-order adsorption (g·mg ·h ) and
-1
kp (mg·g ·h
-1/2
) is the intraparticle diffusion rate constant.
183
Capítulo VII
The equilibrium experimental adsorption data were fitted to Freundlich [35],
Langmuir [36], Temkin [37] and Dubinin-Raduskevich [38] models and the estimated
parameters calculated from the fitting results are reported in Table 4.
The Freundlich model assumes that the adsorption occurs on a
heterogeneous surface. The logarithmic equilibrium expression of this model is:
where Kf (mg
[4]
log 𝐶𝑠 = log 𝐾𝑓 + 𝑛𝑓 log 𝐶𝑒
1-nf nf -1
L g ) and nf are the Freundlich constants indicative of adsorption
capacity and adsorption intensity, respectively.
The Langmuir model describes the adsorbate-adsorbent interactions and is
based on the assumption that the adsorption energy is constant and independent of
the surface coverage. Its expression is:
-1
1
𝐶𝑠
=
1
𝐶𝑚
+
1
𝐶𝑚·𝐿·𝐶𝑒
[5]
-1
where Cm (mg·g ) is the monolayer capacity of the adsorbent and L (L·mg ) is the
Langmuir adsorption constant.
Temkin model describes the behavior of adsorption on heterogeneous
surfaces. This model is based on the heat of adsorption (due to the interactions
between adsorbate and adsorbent). The Temkin isotherm has generally been applied
in the following form:
[6]
𝐶𝑠 = 𝐵𝑇 𝑙𝑛𝐾𝑇 + 𝐵𝑇 𝑙𝑛𝐶𝑒
where KT and BT are constants related to adsorption capacity and intensity of
adsorption, respectively.
The isotherm of desorption was measured at each equilibrium point in the
adsorption curve by replacing 7.5 mL of supernatant with an identical volume of
distilled water. After shaking for 24 h, the amount of herbicide desorbed was
calculated as indicated above. This procedure is described in detail elsewhere [5].
The isotherms of adsorption were studied at three constant temperatures: 10, 20 and
30ᵒC for Bentazon and S-Metolachlor. To test the reversibility, the adsorbents were
subjected to three adsorption-desorption cycles. The desorption from mesoporous
phenolic resin was carried out by calcination under nitrogen at 380ᵒC for 4 hours;
mesoporous carbon and commercial carbon were treated under nitrogen at 400ᵒC for
4 h.
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Mesoporous phenolic resin and mesoporous carbon for
the removal of S-Metolachlor and Bentazon herbicides
3. Results and discussion
3.1. Characterization of the materials.
The XRD-patterns of the mesoporous phenolic resin (MPR) and the
mesoporous carbon (MC) exhibited a low angle (100) peak with d-spacing of 11.6 nm
and 10.3 nm, respectively (Fig. 1). Additional peaks at higher incidence angles were illdefined [39]. Their mesoporous structures were confirmed by TEM images (Fig. 2). The
pore channels with hexagonal arrangement suggest an ordered mesostructure. The
size of these channels was around 10 nm (see insets in figure 2b-c). However, no
ordering was observed in the commercial carbon (Fig 2a). Moreover, the N2
adsorption-desorption isotherms for MPR and MC were essentially of type IV (Fig. 3)
with a sharp capillary condensation step at P/P0= 0.45-0.57, which is typical of
mesoporous materials as defined by IUPAC [40].Nevertheless, the commercial carbon
exhibited a type I isotherm, which corresponds to a microporous material. MC had a
slightly higher surface area than MPR. This is probably due the formation of
micropores as a consequence of the decomposition of the phenolic framework of MPR
to obtain the mesoporous carbon as shown in the TGA curve between 500-750ᵒC
(Fig.4). The pore size for MPR and MC was around 9 nm (inset in Fig. 3), in agreement
with TEM. The main physicochemical properties of the two mesoporous adsorbents
(MRP and MC) along with those of the commercial carbon are listed in Table 2.
Fig. 1. Powder X-ray diffraction patterns of mesoporous phenolic resin (MPR)
and mesoporous carbon (MC)
185
Capítulo VII
The particle size distribution curves of MPR and MC are shown in Fig 5. MPR
displayed a considerably large particle size with the main peak centered at about 250
µm. Interestingly, the distribution of particle size for MC was wider and clearly shifted
to lower values (ca. 50 µm). This revealed the significant reduction of the particle size
upon carbonization of the MPR to MC. The curve for CC was very broad and centered
between 10 and 30 µm, showing a significantly lower particle size than MPR and MC.
Fig. 2. TEM images for comercial carbon (CC) (a), mesoporous
phenolic resin (MPR) (b), including a TEM image of high
resolution (inset), and mesoporous carbon (MC) (c), including a
TEM image of high resolution (inset).
Surface analyses obtained by XPS for the three adsorbents are depicted in
Table 2. MPR possessed a significant amount of oxygen, as expected from the
composition of the polymer. The fraction of oxygen decreased as a result of the
transformation of MPR into MC, which involves the deoxygenation of the phenolic
186
Mesoporous phenolic resin and mesoporous carbon for
the removal of S-Metolachlor and Bentazon herbicides
groups, among other reactions. CC also presented a considerably amount of oxygen
which was much higher than that of MC. The C 1s and O 1s XP spectra are represented
in Figure 6. The large FWHM (full width at half-maximum) values of the peaks (1.4-2.7
eV for C 1s and 2.9-3.6 eV for O 1s) suggested the existence of different carbon and
oxygen species. Obviously, MPR exhibited various types of carbon atoms, namely C-C,
C=C and C-O. MC displayed the narrowest C 1s peak due to the major presence of
aromatic carbon atoms in its framework whereas CC even showed additional small
contributions at high binding energies indicative of the existence of C=O species.
Similarly, the O 1s spectra confirmed the contribution of a higher variety of species in
CC than in MPR and MR.
Fig. 3. Nitrogen adsorptio-desorption isotherms and the corresponding pore size
distributions (insets) for mesoporous phenolic resin (MPR), mesoporous carbon
(MC) and commercial carbon (CC)
187
Capítulo VII
3.2. Adsorption Kinetics of Bentazon and S-Metolachlor
The adsorption of Bentazon and S-Metolachlor on the mesoporous materials
and the commercial carbon was studied at different pH values using 30 mL of either 50
mg/L Bentazon or 100 mg/L S-Metolachlor and 20 mg of adsorbent (Fig.7). An increase
in the pH caused a decrease in the adsorption of Bentazon probably due to the
enhancement of the electrostatic repulsion between the ionized pesticide and the
adsorbent surface [1]. The point of zero charge (pzc) was determined for the three
materials and revealed some differences among them. (see supporting information,
Figure SI1). The pzc value indicates the pH required to give zero net surface charge.
The pHs were 5.3, 3.2 and 2.7 for commercial carbon, mesoporous carbon and
phenolic resin, respectively. When the pH is higher than the pzc, the surface is
negatively charged. As pH is increased, the surface is more negatively charged. On the
other hand, Bentazon is a weak acid with pKa of 3.3; and at pH above the pKa, it exists
predominantly in anionic form. As pH is increased, the extent of dissociation of
bentazon molecules is increased and so it becomes more negatively charged. As a
result, the equilibrium adsorption of bentazon decreased with an increase in the pH of
the initial solution. This could be attributed to the increased electrostatic repulsion
between bentazon ions and the surface of the three materials [1]. Similar observations
were made for the adsorption of bentazon onto AC cloth [41]
Fig. 4. Thermogravimetric curves for mesoporous phenolic resin
(MPR) and mesoporous carbon (MC)
188
Mesoporous phenolic resin and mesoporous carbon for
the removal of S-Metolachlor and Bentazon herbicides
Table 2.
Physicochemical properties of commercial carbon, mesoporous phenolic resin and mesoporous
carbon
Given the aromatic structure of bentazon, dispersion forces should be
expected between the π cloud of the adsorbents and the aromatic ring of the
adsorbate. Also, some specific localized interactions might arise from the polar groups
(i.e., sulfoxide and carbonyl-type) [28]. The decrease in the amount adsorbed at higher
pH (anionic form predominant in solution) suggests a weaker interaction of the
surfaces with deprotonated (anionic) bentazon than with its neutral form, and
consequently that the adsorption is dominated by dispersive interactions between the
pesticide and the adsorbent surface. Similar results on the pH-dependence of
bentazon adsorption have been reported by other authors [42, 43]. In addition, the
relative drop in the adsorption was higher when the pH of the point of zero charge was
lower (see Figure 7 and Figure SI1). Thus, the phenolic resin was the most affected by
changes in the pH.
Fig. 5. Particle size distribution curves for mesoporous phenolic resin (MPR),
mesoporous carbon (MC) and commercial carbon (CC)
189
Capítulo VII
However, the adsorption of S-Metolachor on both mesoporous materials,
MPR and MC, and the commercial carbon, CC, was not affected by the pH. This could
be associated to the non-ionizable nature of S-Metolachlor in the pH range under
study. Indeed, the adsorption of S-Metolachlor would occur through π-π interactions
between the aromatic ring of the pesticide and π-electrons of the adsorbent structure
[41].
The effect of the contact time at the pH of maximum adsorption is depicted in
Fig. 8. The adsorption of Bentazon and S-Metolachlor on commercial carbon was fast,
reaching the equilibrium in 2 h, while their adsorption on the mesoporous materials
was slower (ca. 16 h). These differences in the adsorption kinetics might be explained
by the higher specific surface area (Table 2) and smaller particle size (Fig. 5) of CC with
respect to both mesoporous materials.
Fig. 6. XPS analysis for mesoporous phenolic resin (MPR), mesoporous carbon (MC) and commercial
carbon (CC)
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Mesoporous phenolic resin and mesoporous carbon for
the removal of S-Metolachlor and Bentazon herbicides
Fig. 7. Influence pf pH on the adsorption of S-Metolachlor and Bentazon on mesoporous
phenolic resin (MPR), mesoporous carbon (MC) and commercial carbon (CC)
The fitting parameters for the adsorption kinetic of Bentazon and SMetolachlor on MPR, MC and CC are listed in Table 3. The correlation coefficients R2
for the pseudo-second-order kinetics (0.999) were best fitted than for a pseudo-firstorder kinetics model (0.561-0.995). Similar results were reported by Salman et al. using
an activated carbon as adsorbent for two pesticides, Bentazon and Carbofuran [1].
Following the Morris-Weber equation for intraparticle diffusion (Eq. [3]), the
plots of Cs versus t1/2 consist of straight lines for the pesticide adsorption on the three
adsorbents. The extrapolation of these lines does not pass through the origin,
suggesting that intraparticle diffusion cannot be the rate-determining step and other
factors may have operated simultaneously to govern the adsorption process [44, 45].
Further information about the adsorption mechanism can be obtained
analysing the adsorption isotherm for the systems under study. The isotherms for the
adsorption of Bentazon and S-Metolachlor on mesoporous phenolic resin and
mesoporous carbon (Fig. 9) were compared with that of commercial carbon (Fig.10).
According to Giles classification [46], the isotherms for Bentazon adsorption on MPR,
MC and CC corresponded to a L2 type. Also, S-Metolachor on CC exhibited a L2 type
isotherm. However, the adsorption of S-Metolachlor on MPR and MC gave a L4 type
isotherm. In the case of Bentazon, the plateau was reached at Cs value of 130.74 mg·g191
Capítulo VII
1
-1
-1
, 166.72 mg·g and 218.04 mg·g for MPR, MC and CC, respectively. By contrast, the
second plateau for S-Metolachlor on MPR and MC was not reached, indicating that the
maximum adsorption capacity of the adsorbent was not achieved under these
conditions. Similar behaviour was observed in the adsorption of this pesticide on
phenylene-bridged and ethylene-bridged periodic mesoporous organosilicas [6]. The
adsorption isotherm of S-Metolachlor on CC exhibited an ill-defined plateau with a
-1
value of Cs= 340.17 mg·g .
Fig. 8. Adsorption kinetics of S-Metolachlor and Bentazon on
mesoporous phenolic resin (MPR), mesoporous carbon (MC) and
commercial carbon (CC)
The fitted parameters of adsorption were calculated from the linear
regressions of Eqs. [4]-[6]. According to the regression coefficient values, the Langmuir
model showed a much better fit than the Freundlich and Temkin models (Table 4).
Table 3.
Kinetic parameters for the adsorption of Bentazon and S-Metolachlor onto mesoporous phenolic resin and
mesoporous carbon compared with commercial carbon
192
Mesoporous phenolic resin and mesoporous carbon for
the removal of S-Metolachlor and Bentazon herbicides
These models do not give any idea about the mechanism of adsorption. To
understand the adsorption type, experimental data were fitted to the DubininRadushkevich (D-R) model:
𝑙𝑛𝐶𝑠 = ln 𝑞𝑚 − 𝐾𝜀 2
[7]
-1
2
-2
where qm is the theoretical adsorption capacity of the adsorbent (mg·g ), K (mg kJ ) is
the constant related to adsorption energy and ε=RTln(1+1/Ce) is the Polanyi potencial.
The adsorption energy (E) can be calculated using the D-R equation and the following
relationship has been used.
-0.5
E = (2K)
[8]
-1
The adsorption is physical in nature when E is between 0 and 8 kJ mol ; if it is
-1
8<E<16 kJ mol the adsorption is due to exchange of ions; and if the value of E is
-1
between 20<E<40kJ mol , a chemisorption occurs. In the case of S-Metolachlor, the
values found in the present study for the three adsorbents were around 8 KJ mol
-1
-1
whereas in the case of Bentazon were between 11 and 13 kJ mol . Consequently, the
adsorption of Bentazon could be explained by exchange of ions. In addition, E values
for S-Metolachlor were in the limit between physical adsorption and exchange of ions
and so it was not possible to explain precisely the nature of the adsorption.
The desorption curves [5] above the adsorption curves evidenced the difficulty to
remove S-Metolachlor and Bentazon from the adsorbents. The presence of hysteresis
(figs 9-10) indicates the irreversibility of the process in all cases.
3.3. The effect of temperature
In order to determine the thermodynamic parameters of the adsorption of Bentazon
and S-Metolachlor on the three adsorbents, the effect of the temperature was studied.
When the temperature was increased from 10 to 30ºC the amount of adsorbed
Bentazon slightly increased, unlike S-Metolachlor whose adsorption behaviour was
similar in the whole temperature range.
193
Capítulo VII
Fig. 9. Adsorption and desorption isotherms of S-Metolachlor and Bentazon on mesoporous
phenolic resin (MPR) and mesoporous carbon (MC).
Fig. 4 Adsorption and desorption isotherms of S-Metolachlor and Bentazon on commercial
carbon (CC).
The change in standard free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) of
adsorption was calculated using the following equations (Eqs. (9) and (10)) [47, 48]:
ΔGº = ΔHº−TΔSº
[9]
lnK = ΔSº/R−ΔHº/RT
[10]
194
Mesoporous phenolic resin and mesoporous carbon for
the removal of S-Metolachlor and Bentazon herbicides
where K is the equilibrium constant, R the molar gas constant and T the absolute
temperature. The values of ΔH° and ΔS° were calculated from the slope and intercept
of Van't Hoff plots (Ln KC versus 1/T) and are summarized in Table 5. The values of K
have been obtained from the values of KL from the Langmuir equation because our
isotherms were well fitted to this equation. K has been recalculated to become
dimensionless by multiplying it by 106 [49]. ΔG° values were calculated from Eq. (9).
The negative ΔG° values indicate that the process is thermodynamically feasible and
spontaneous. In the case of Bentazon, the ΔG° values were slighter higher as the
temperature increased indicating that the process was chemically controlled. By
contrast, the behaviour of S–Metolachlor showed two different tendencies (physical
and chemical).
Table 4. Freundlich, Langmuir, Dubinin-Raduskevich and Temkin model parameters for S-Metolachlor and
Bentazon adsorption onto mesoporous phenolic resin and mesoporous carbon compared with commercial
carbon.
3.4. Regeneration of the adsorbents
To test their reusability, the adsorbents were regenerated by calcination
under nitrogen atmosphere at 380ºC for mesoporous phenolic resin and 400ºC for
mesoporous carbon and commercial carbon. The adsorbents were subjected to three
adsorption-desorption cycles (Fig. 11). The regeneration efficiencies of the
mesoporous materials were compared with the commercial carbon. In the third cycle,
the adsorption capacity for S-Metolachlor on MPR, MC and CC were 60.5%; 69.29%
and 59.85%, respectively, while for Bentazon were 77.33% for MPR; 90.22% for MC
and 88.82% for CC. For both pesticides, the regeneration was better in mesoporous
carbon than in mesoporous phenolic resin and commercial carbon. The differences
between both mesoporous materials can be associated to the presence of more
defects in the mesostructure of the phenolic resin than in that of the mesoporous
carbon after their regeneration (see figure SI2 in supporting information). In relation to
195
Capítulo VII
CC, the lower decrease in the adsorption capacity for MC could be ascribed to its
mesoporous character and its lower fraction of micropores [50, 51]. Similar behaviour
was described by Suchithra et al. [52], who studied the regeneration of hybrid
mesoporous material compared with commercial carbon for the removal of dyes and
other organic compounds.
Table 5.
Thermodynamic parameters for S-Metolachlor and Bentazon adsorption onto mesoporous
phenolic resin and mesoporous carbon compared with commercial carbon.
7
A great disadvantage of the commercial carbon compared to the mesoporous
materials in relation to their reusability was its significant loss of material after several
adsorption-desorption cycles. After three cycles, the losses of the commercial
adsorbent were 48.2% and 47.1% for the adsorption of S-Metolachlor and Bentazon,
respectively, while it was less than 5% for the mesoporous materials (Fig. 11).
After each adsorption cycle the material was recovered with a filter of 0.45
μm of pore size, which should retain all particles according to the particle size
distribution depicted in Fig. 5. However, these results suggest that the commercial
carbon undergoes a shear degradation that decreases the particle size, thus allowing
the resulting smallest particles to pass through the filter. The loss of this material
involves serious concerns for its practical application because it reduces drastically its
durability, unlike both mesoporous materials, which proven to be more resistant to
friction.
196
Mesoporous phenolic resin and mesoporous carbon for
the removal of S-Metolachlor and Bentazon herbicides
Fig. 11. Adsorption-desorption cycles for S-Metolachlor and Bentazon.
Adsorption after first (1), second (2) and third (3) cycle. Weight lost
after three adsorption-desorption cycles (% Loss).
4. Conclusions
Mesoporous phenolic resin and mesoporous carbon exhibited good properties to be
used as adsorbents in virtue of their high surface area and narrow mesopore
distribution. The N2 adsorption-desorption isotherms for MPR and MC were
essentially of type IV. The pore size of these materials was around 9-10 nm according
to the N2 adsorption and TEM results. We have examined a potentially practical use of
these materials for the adsorption of S-Metolachlor and Bentazon. The adsorption of
these pollutants varied with the pH. The adsorption of Bentazon decreased when the
pH increased, probably due to the increase of the electrostatic repulsion between the
pesticide ion and the adsorbent surface. By contrast, the adsorption of S-Metolachor
was not affected by the pH.
The adsorption kinetics fitted better to a pseudo-second-order kinetics model.
Intraparticle diffusion was present in the adsorption process together with other
factors. The isotherms for Bentazon on MPR, MC and CC and for S-Metolachor on CC
were of L2 type while the adsorption of S-Metolachlor on MPR and MC exhibited a L4
type isotherm. In the case of Bentazon, a plateau was reached for the three
adsorbents. By contrast, the second plateau for S-Metolachlor on MPR and MC was
not reached, indicating that the maximum capacity of these adsorbents was not
197
Capítulo VII
achieved under these conditions. In all cases the Langmuir model showed a much
better fit. The adsorption of Bentazon could be explained as exchange of ions
according to the Dubinin-Radushkevich model. The presence of hysteresis in the
desorption curves indicated irreversibility of the process in all cases.
The regeneration efficiency was better in mesoporous carbon than in mesoporous
phenolic resin and commercial carbon for both pesticides. Another disadvantage of the
commercial carbon compared to the mesoporous materials for their regeneration was
the great loss of material after successive adsorption-desorption cycles.
Acknowledgements
The authors wish to acknowledge funding of this research by Ministerio de Ciencia e
Innovación (Project MAT2010-18778) and Junta de Andalucía (Project P06-FQM01741). D.E. is a postdoctoral researcher of the FWO-Vlaanderen (Fund Scientific
Research - Flanders), grant number 3E10813W. R. Otero acknowledges Ministry of
Education and Science for a research and teaching fellowship.
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203
CAPÍTULO VIII.
CONCLUSIONES GENERALES
GENERAL CONCLUSIONS
General Conclusions
A partir de las consideraciones finales desarrolladas en los correspondientes
capítulos se pueden establecer las siguientes conclusiones generales:
Hidrotalcitas con aniones orgánicos en la interlámina.
1.
Los aniones dodecilsulfato y tetradecanodioato se intercalaron en el
espacio interlaminar de la hidrotalcita aumentando el espaciado d(00l) en
comparación con la hidrotalcita con carbonato, mejorando así la capacidad
de adsorción para S-Metolacloro.
2.
Los diagramas de difracción de rayos X de las organohidrotalcitas sugirieron
una disposición vertical de aniones dodecilsulfato y una posición inclinada
de los aniones tetradecanodioato en el espacio interlaminar. La adsorción
de S-Metalocloro originó cambios en el valor d(00l) para ambas
organohidrotalcitas. En particular, la adsorción de S-Metolacloro en HT-TDD
condujo a un aumento muy importante en el espaciado basal, lo que sugirió
cambios en la orientación del anión tetradecanodiato de inclinada a
perpendicular.
3.
Se ha propuesto un nuevo procedimiento de la rama de desorción
comparable al procedimiento empleado en la adsorción. Esto ha permitido
alcanzar una interpretación del proceso físico-químico de adsorcióndesorción.
4.
La hidrotalcita con tetradecanodiato en la interlámina mostró una mayor
adsorción de S-Metolacloro con respecto a la hidrotalcita con dodecilsulfato
en la interlámina. Esto se puede asociar a la longitud de la cadena del anión
interlaminar, es decir, un anión con una cadena hidrocarbonada de mayor
longitud dará lugar a una mayor adsorción.
207
Capítulo VIII
5.
El efecto de la temperatura en la adsorción de S-Metolacloro en ambas
hidrotalcitas orgánicas mostró un descenso del valor de ΔG⁰ con el aumento
de la temperatura.
6.
Los resultados sugirieron la posibilidad de utilizar estas organohidrotalcitas
como adsorbentes de S-Metolacloro en agua contaminada. La HT-TDD
presentó una mayor adsorción de S-Metolacloro, sin embargo la HT-DDS es
posiblemente un mejor adsorbente ya que puede ser reutilizada empleando
etanol como disolvente.
Hidrotalcita calcinada
7.
Los pesticidas Nicosulfuron y Mecoprop-P mostraron una buena adsorción
en la hidrotalcita calcinada. El Nicosulfuron presentó menor adsorción que
el Mecoprop-P, lo que puede ser justificado por las diferencias
estructurales, de tamaño y de ionización de ambos pesticidas.
8.
Los diagramas de difracción de rayos X fueron casi idénticos a los de la
hidrotalcita intercalada con iones carbonato, lo que es consistente con una
disposición plana de los pesticidas en la interlámina, en la superficie externa
y/o en los bordes de las partículas del adsorbente.
9.
Los datos experimentales de las cinéticas de adsorción de Nicosulfuron y
Mecoprop-P se ajustaron a la ecuación de Largergren o pseudo primer
orden. El análisis de los datos mediante la ecuación de Morris-Weber
permitió concluir que la difusión intrapartícula no era el único factor
determinante de la adsorción para ambos pesticidas.
10. Las isotermas de adsorción de ambos pesticidas son de tipo L y la aplicación
de las ecuaciones de Langmuir y Dubini-Radushkevich a las isotermas de
adsorción para los dos pesticidas reveló que la adsorción es favorable y se
produce por un proceso de intercambio iónico.
208
General Conclusions
11. La adsorción y la desorción de los dos pesticidas se vieron afectadas por la
presencia de otros aniones. Los aniones competidores (especialmente
2-
HPO4 ) tuvieron un efecto adverso en la adsorción de Nicosulfuron y
Mecoprop-P. Por otro lado, la extracción de los pesticidas del complejo
2-
2-
soporte-pesticida fue especialmente eficaz con CO3 . La capacidad del CO3
para recuperar grandes cantidades de Mecoprop-P sugirió que el
adsorbente puede ser reciclado.
12. La reciclabilidad de HT500 realizada mediante calcinación en aire mostró
una disminución de la adsorcíon después de cada ciclo de adsorcióndesorción. Esto es debido a una deformación de las partículas del
adsorbente.
PMOs
13. Se sintetizaron dos organosilices (etano-PMO y benceno-PMO) y una sílice
(PMS). Ambos PMOs exhibieron una capacidad de adsorción de SMetalocloro superior a su homólogo de sílice (PMS).
14. Los dos PMOs sintetizados exhibieron un comportamiento de adsorción
muy diferente. Mientras etano-PMO se comportó de manera similar a una
sílice mesoporosa (PMS), aunque con una capacidad de adsorción mejorada
debido a su carácter más hidrófobo, el benceno-PMO mostró una adsorción
del herbicida mucho mayor.
15. La cantidad de S-Metolacloro adsorbida se vió influida por el pH y fue
máxima a pH 2 para etano-PMO y a pH 4 para el benceno-PMO. Los
resultados sugirieron que ambas interacciones, hidrofóbicas y aromáticas ππ entre puentes fenilo y S-Metolacloro, deben desempeñar un papel
importante en la adsorción de este herbicida.
209
Capítulo VIII
16. La ecuación de Morris-Weber mostró que existe una mayor contribución de
la difusión intrapartícula en el benceno-PMO.
17. La isoterma de adsorción de S-Metolacloro en etano-PMO es de tipo L2
-1
alcanzándose el plateau a 255 µmol·g
mientras que la isoterma de
benceno-PMO para el mismo plaguicida es de tipo L4 existiendo dos
plateaus, probablemente el primer plateau es debido a la adsorción en las
regiones hidrofílicas mientras que el segundo podría ser causado por la
adsorción de S-Metolacloro en las regiones hidrofóbicas.
18. Las adsorciones sucesivas de S-Metolacloro en los PMOs permitieron
conocer la capacidad máxima de adsorción. El etano-PMO se saturó
-1
después de la segunda adsorción (416.63 µmol·g ) mientras que el
-1
benceno-PMO después de cuatro adsorciónes (1729.35 µmol·g ).
19. Teniendo en cuenta la alta estabilidad acuosa, la elevada capacidad de
adsorción y la fácil regenerabilidad, el benceno-PMO es un adsorbente
prometedor para moléculas aromáticas.
Resinas fenólicas y carbones mesoporosos ordenados
20. Se sintetizaron dos materiales mesoporosos: una resina fenólica
mesoporosa (MPR) y un carbono mesoporoso (MC), siguiendo un método
de “soft template”. Se ha examinado el uso potencialmente práctico de
estos materiales para la adsorción de S-Metolacloro y Bentazona.
21. Los estudios de las cinéticas y de las isotermas de adsorción de Bentazona y
de S-Metolacloro se llevaron a cabo a pH 2 y 4, respectivamente. La
adsorción de Bentazona disminuyó al aumentar el pH, probablemente
debido al aumento de la repulsión electrostática entre el ion pesticida y la
superficie adsorbente. Por el contrario, la adsorción de S-Metolacloro no se
vió afectada por el pH.
210
General Conclusions
22. El equilibrio en la cinética de adsorción de Bentazona y S-Metolacloro en
carbón comercial se alcanzó a las 2 h mientras que en el caso de los
materiales mesoporosos se necesitaron 16 h. Estas diferencias pueden ser
debidas probablemente a una mayor superficie específica del carbon
comercial y un tamaño de partícula más pequeño que los presentados por
los materiales mesoporosos.
23. Las cinéticas de adsorción se ajustaron mejor a un modelo de pseudosegundo orden y la difusión intrapartícula estuvo presente en los procesos
de adsorción junto con otros factores.
24. Las isotermas de adsorción de Bentazona en MPR, MC y CC y de SMetolacloro en CC fueron de tipo L2, mientras que la adsorción de SMetolacloro en MPR y MC exhibió una isoterma tipo L4. En el caso del
Bentazona, se alcanzó una meseta para los tres adsorbentes. Por el
contrario, no se alcanzó la segunda meseta en la adsorción de SMetolacloro en MPR y MC, lo que indicó que la capacidad máxima de estos
adsorbentes no se alcanzó bajo estas condiciones.
25. En todos los casos el modelo de Langmuir mostró un mejor ajuste. La
adsorción de Bentazona podría explicarse como intercambio de iones de
acuerdo con el modelo de Dubinin-Radushkevich. La presencia de histéresis
en las curvas de desorción puso en evidencia la irreversibilidad del proceso
en todos los casos.
26. Las isotermas realizadas a diferentes temperaturas mostraron que la
adsorción de Bentazona está controlada químicamente, mientras que para
el S-Metolacloro se observaron dos tendencias distintas, química para MC y
física para MPR.
211
Capítulo VIII
27. La regeneración de los adsorbentes se llevó a cabo por calcinación bajo
atmósfera de nitrógeno y los resultados indicaron que la regeneración del
MC fue más eficiente que la del MPR y el CC.
28. Los materiales MC y MPR presentan mejores características que el CC para
ser utilizados como adsorbentes de pesticidas. En concreto, el CC
experimentó una gran pérdida de material después de varios ciclos de
adsorción-desorción.
Resumiendo, se han sintetizado diferentes materiales para ser utilizados como
adsorbentes en la eliminación de plaguicidas, dependiendo del medio en que estos
se encuentren y de la naturaleza de los mismos.
Para los pesticidas con carácter iónico o polar, la hidrotalcita calcinada (HT500) ha
mostrado buena capacidad de adsorción tanto para Nicosulfuron como Mecoprop-P.
Así mismo, los carbones mesoporosos permitieron la eliminación del pesticida
aniónico Bentazona en disolución acuosa.
Por otra parte, los pesticidas no polares tales como S-Metolacloro fueron
adsorbidos en organohidrotalcitas a pH básicos y en PMOs y carbones mesoporosos
a pHs ácidos. El mejor adsorbente a pH básico fue HT-TDD aunque HT-DDS mostró
ser un mejor candidato debido a su mayor reciclabilidad. A pHs ácidos los mejores
adsorbentes fueron el benceno-PMOs y el MC, aunque este último exhibió una
mayor capacidad de adsorción.
212
General Conclusions
The following general conclusions are set from the final considerations
developed in the corresponding chapters:
Hydrotalcites with organic anions in the interlayer
1- Dodecylsulphate and tetradecanedioate anions were intercalated in the
interlayer space of the hydrotalcite increasing of space d(00l) compared
with the carbonate hydrotalcite, thus improving the capacity of adsorption
for S-Metolachlor.
2- XRD patterns of the organohydrotalcites suggested a vertical order of
dodecylsulphate anions and an inclined order of tetradecanedioate anions
in the interlayer space. The adsorption of S-Metolachlor originated changes
in d(00l) for both organohydrotalcites. Particularly, the adsorption of SMetolachlor in HT-TDD presented an increase in the basal spacing,
suggesting changes in the orientation of the tetradecanedioate anion from
tilt to perpendicular position.
3- We have proposed a new procedure for the desorption isotherm
comparable with the procedure used in the adsorption. This has allowed a
physico-chemical interpretation of the adsorption-desorption process.
4- The Hydrotalcite with tetradecanedioate in the interlayer had a better
adsorption on S-Metolachlor than the hydrotalcite with dodecylsulphate.
This could be associated with the chain length of the interlayer anion (o
“the anion introduced in the interlayer”), i.e., an anion with a
hydrocarbonated chain will have a greater adsorption.
5- The effect of the temperature in the adsorption on S-Metolachlor showed a
decrease in the ΔG⁰ value with increasing temperature in both
organohydrotalcites.
213
Capítulo VIII
6- The results suggested the possibility of using organohydrotalcites as
adsorbents of S-Metolachlor in contaminated water. The HT-TDD showed a
higher adsorption of S-Metholachlor. However, since HT-DDS can be reused
using ethanol as a desorbent, it might be a better adsorbent.
Calcined hydrotalcite
7- Nicosulfuron and Mecoprop-P were adsorbed on calcined hydrotalcite.
Nicosulfuron had a lower adsorption than Mecoprop-P. This could be due to
structural differences, size and ionization of pesticides.
8- XRD patterns of Nicosulfuron and Mecoprop-P adsorbed in calcined
hydrotalcite were similar to XRD patterns of hydrotalcite with carbonate in
the interlayer. This fact is consistent with a flat disposal of the pesticides in
the interlayer, on the surface and at the edges of the pesticide particles.
9- The Largergren equation or pseudo first order was used for analysing the
experimental data of the adsorption kinetics of Nicosulfuron and
Mecoprop-P. Furthermore, the Morris-Weber equation showed that the
intraparticle diffusion wasn’t the determining factor for the adsorption for
both pesticides.
10- The adsorption isotherms were L type for both pesticides. According to
Langmuir and Dubini-Radushkevich equations, the adsorption was
favourable and it occurred by ion exchange.
11- Adsorption and desorption of both pesticides were affected by the
2-
presence of other anions. Competitor anions (especially HPO4 ) had an
adverse effect on the adsorption of Nicosulfuron and Mecoprop-P.
2-
Moreover, the removal of pesticides was particularly effective with CO3 .
2-
The capacity of CO3 to recover large amounts of Mecoprop-P suggested
that the adsorbent can be recycled.
214
General Conclusions
12- HT500 recyclability by calcination in air showed a decrease in the
adsorption after each adsorption-desorption cycle. This is due to a
deformation of the adsorbent particles.
PMOs
13- Two organosilicas (ethane and benzene-PMO) and a silica (PMS) were
synthesized. PMOs exhibited adsorption capacity of S-Metalocloro higher
than PMS.
14- PMOs exhibited a very different adsorption behaviour. The behaviour of
Ethane-PMO was similar to the PMS one. However, benzene-PMO
presented a much higher adsorption of the pesticide due to its
hydrophobicity.
15- The adsorption of S-Metolachlor was affected by the pH. The study of
adsorption was carried out at pH 2 on ethane-PMO and at pH 4 on benzenePMO. The results suggested that the adsorption of S-Metolachlor on
benzene-PMO would occur through π-π interactions between phenyl
bridges.
16- According to the Morris-Weber equation, the contribution of intraparticle
diffusion for benzene-PMO was higher than ethane-PMO.
17- Adsorption isotherm of S-Metolachlor on ethane-PMO is L2 and the plateau
-1
was reached at 255 µmol·g . However, the adsorption of the herbicide on
benzene-PMO followed a type L4 isotherm. This isotherm presented two
plateaus, the first one probably due to the adsorption in the hydrophilic
regions and the second one could be associated to the adsorption in the
hydrophobic regions.
215
Capítulo VIII
18- The successive adsorptions S-Metholachlor in PMOs allowed to know the
maximum adsorption capacity. Ethane-PMO was saturated after the second
-1
adsorption (416.63 µmol·g ) while the benzene-PMO was saturated after
-1
four adsorptions (1729.35 µmol·g ).
19- Considering the high aqueous stability, high adsorption capacity and easy
regenerability, benzene-PMO is a promising adsorbent to remove aromatic
pollutants.
Ordered mesoporous phenolic resin and carbon
20- Two mesoporous materials were synthesized by a method of "soft
template": a mesoporous phenolic resin (MPR) and a mesoporous carbon
(MC). The use of these materials for the adsorption of Bentazon and SMetolachlor has been assesed.
21- Kinetic studies and adsorption isotherms for Bentazon and S-Metolachlor
were conducted at pH 2 and 4, respectively. Bentazon adsorption decreased
when increasing pH, probably due to increased electrostatic repulsion
between the pesticide and the ion adsorbent surface. On the contrary, the
adsorption of S-Metolachlor was not affected by pH.
22- The equilibrium at the adsorption kinetics of Bentazon and S-Metolachlor in
CC was reached at 2 hours, while 16 hours were required in the case of
mesoporous materials. Probably, these differences are due to the greater
surface area of CC and a smaller particle size of mesoporous materials.
23- Adsorption kinetics were better fitted to a pseudo-second model. MorrisWeber equation showed that the intraparticle diffusion was present in
adsorption processes along with other factors.
216
General Conclusions
24- Adsorption isotherms of Bentazon on MPR, MC and CC and S-Metolachlor
on CC were L2 type. However, adsorption isotherms of S-Metolachlor on
MPR and MC exhibited L4 type. The plateau was reached for Bentazon in all
cases. On the contrary, the maximum capacity was not reached for SMetolachlor on MPR and MC.
25- In all cases, the Langmuir model showed a better fit. Adsorption of
Bentazon could be explained as ion exchange according to the DubininRadushkevich model. The presence of hysteresis in the desorption curves
revealed the irreversibility of the process in all cases studied.
26- The adsorption of Bentazon was chemically controlled, according to the
isotherms at differences temperatures, while for S-Metolachlor two
different tendencies were found, chemical for MC and physical for MPR.
27- Regeneration of the adsorbents was carried out by calcination under
nitrogen. The results indicated that regeneration of MC was more efficient
than the regeneration of MPR and CC.
28- MC and MPR materials have better characteristics than CC to be used as
adsorbents of pesticides. Specifically, the CC had a great loss of material
after several adsorption-desorption cycles.
In summary, different materials have been synthesized to use as adsorbents
in removing pesticides, depending on the environment in which they are located and
their nature.
Calcined hydrotalcite (HT500) shows good adsorption capacity for both
Nicosulfuron and Mecoprop-P due to their ionic or polar character. Also, the
mesoporous carbons allowed the removal of anionic Bentazon pesticide in aqueous
solution.
Moreover, nonpolar pesticides such as S-Metolachlor were adsorbed at
basic pH by organohydrotalcites and at acidic pHs by mesoporous carbons and
217
Capítulo VIII
PMOs. HT-TDD was the best adsorbent at basic pH although HT-DDS proved to be a
better option because of its greater reusability. The best adsorbents were benzenePMOs and MC at acidic pH, although the latter showed the higher adsorption
capacity.
218
CAPÍTULO IX. SUMMARY
Resume
The probability of producing pesticide-contaminated drainage water, surface
water and underground water rises due to the increase of the use of pesticides in
agriculture. Pollution of the surface and ground waters is potentially dangerous to
human health because of the contents of inorganic and organic compounds and it
can persist in the aquatic environment for many years after its application.
The removal of these toxic pollutants from aqueous solutions is the main aim
of many researches. Photocatalysis, adsorption and electrolysis are some of the
processes involved in the removal of these pollutants. Adsorption is the most
commonly used technique to remove toxic substances from wastewater due to its
low cost, ease of operation and high removal efficiency. Pesticides are frequently
detected in groundwater extraction wells all over Europe (EEA, 2004). In addition to
this, the concentration of pesticides in drinking water and groundwater should not
exceed 0.1 μg/L for a single compound or 0.5 μg/L for the sum of all pesticides
(European Parliament and Council, 2006). For these reasons, the goal of this work is
the synthesis of new materials in order to reduce the pollution.
Layered double hydroxides (LDHs) constitute a large family of materials with
x+
n−
the general formula [M(II)1−xM(III)x(OH)2] [A
x/n].mH2O,
where M(II) is a divalent
cation such as Mg, Ni, Zn, Cu or Co, M(III) is a trivalent cation such as Al, Cr, Fe or Ga
n−
and A
is an anion of charge n. The x value, generally between 0.2 and 0.4,
determines the layer charge density and the ion exchange capacity. The interest of
these materials is related to their structure, high anion exchange capacity and
simplicity of their synthesis. For this reason, they are suitable for smoothing as well
as preventing the environmental impact caused by pesticides, i.e. they can be also
used as filters for pesticide-contaminated water.
Furthermore, the adsorption efficiency of LDHs is strongly affected by the
properties of their interlayer anions. Therefore, LDHs usually have a greater affinity
with anions with a high charge density and hence tend to easily adsorb multivalent
anions rather than monovalent ions. In addition, LDHs exhibit preferential affinity
2–
with CO3 , which usually hinders further ion-exchange.
221
Capítulo IX
Similarly to clay minerals, the inorganic anions of HTs can be replaced with
organic anions. This simple modification changes the nature of the HT surface from
hydrophilic to hydrophobic, increasing its affinity for non-ionic organic compounds,
including many non-ionic pesticides. These organoclays and organo/LDHs have
applications in a wide range of organic pollution control fields.
Recently, and prior to the development of this current Ph. D. Thesis, our
group researches the synthesis of organohydrotalcites with sebacate in the
interlayer obtained by the coprecipitation method. Our original approach consisted
in comparing the adsorption of two adsorbents with the same anion chain length but
differing in the nature of the chemical groups bonded to the HT layers. We
synthesized the organohydrotalcite with dodecylsulphate and another one with
tetradecanedioate in the interlayer.
Elemental analysis was used for determining the value of both Mg/Al and S/Al
molar ratio among others. The results showed than a partial dissolution of Al
3+
seemed to occur during the synthesis of HT-TDD. Furthermore, the molar ratio S/Al
in HT-DDS indicated that most of the layer charge was compensated by DDS anions.
According to the scanning electron micrograph, an agglomerate of particles was
observed in both organohydrotalcites, although HT-DDS showed a slightly more
porosity and less homogeneity. The study of X-ray diffraction and TG analysis were
also tackled.
S-Metolachlor is the result of a breakthrough in the catalyst system of the
manufacturing process for Metolachlor. This pesticide is physically and chemically
equivalent to Metolachlor, but is more active at the site of action in susceptible
plants and allows for lower use rates. Our group considers the removing of this
pestice because it is being employed more than Metolachlor. This is due to the fact
that S-Metolachlor is more environmentally friendly than Metolachlor.
S-Metolachlor is a pre- and post-emergence selective non polar herbicide
used to control grasses and some broad-leaved weeds in a wide range of crops. It is
quite mobile and although it is mainly found in surface water, it can contaminate
groundwater. Several papers have reported the adsorption of S-Metolachlor by
222
Resume
different adsorbents such as organohydrotalcites (OHTs), soils, activated charcoals
and clays.
Adsorption-desorption of S-Metolachlor pesticide by two organohydrotalcites
(HT-DDS and HT-TDD) has been characterized. The organohydrotalcites presented an
expansion of the interlayer space compared to the inorganic hydrotalcite with
carbonate anion. X-ray diffraction patterns suggested a vertical monolayer
arrangement of dodecylsulfate anions and a tilt position of the tetradecanedioate
anions in the interlayer space. Adsorption of S-Metolachlor resulted in changes in
the d00l-value for both organohydrotalcites. In particular, the adsorption of SMetolachlor on HT-TDD led to a very important increase in the basal spacing,
suggesting changes in the orientation of the tetradecanedioate anion from tilt to
perpendicular position. The results indicated that the adsorption of S-Metolachlor on
the organohydrotalcite HT-TDD was higher than on HT-DDS, but on the contrary,
desorption was lower. The amount of adsorbed pesticide increased with the rise of
the temperature from 10 to 30°C, especially for HT-TDD.
The decomposition of hydrotalcite when calcined at around 500°C leads to
mixed metal oxides, which are characterized by high specific surface areas and
homogeneous dispersion of metal cations. The mixed metal oxides can rehydrate
and combine with anions from aqueous solution to recovery the original structure,
as it is expressed by the following two equations:
[Mg1−xAlx(OH)2](CO3)x/2·mH2O → Mg1−xAlxO1+x/2 +(x/2)CO2 + (m + 1)H2O
−
Mg1−xAlxO1+x/2 + (x/n)A + (m+1 + x/2)H2O →[Mg1−xAlx(OH)2]Ax/n·mH2O + xOH
-
The memory effect (recovery of LDH structure) takes place when a Mg–AlLDH, usually containing carbonate in the interlayer, is put in water or a solution of a
given anion. The Mg–Al-LDH is calcined at a temperature around 500°C to eliminate
most of the interlayer anion. The mixed oxide obtained by calcination is mostly
amorphous, with a high specific surface area and an ability to recover the layered
structure with water. The calcination has an anion-exchange capacity (AEC) much
–1
higher than that of the original LDH (5.8 vs 3.3 mmol·g ); this allows to increase
significantly the adsorption efficiency of an LDH by calcination. In this sense, the
223
Capítulo IX
reconstruction in pure water gives rise to the intercalation of hydroxyl anions, while
the intercalation of other anions can occur if they are present in the solution.
Calcined layered double hydroxides arouses a great interest in the environmental
community because of their high anion retention capacity and simple thermal
regeneration procedure.
Phenoxy acids, including Mecoprop-P (MCPP), are some of the most
frequently detected pesticides in groundwater, and they have been used extensively
as herbicides in agriculture. MCPP is still used in some European countries, i.e.
France, Italy and Austria. Mecoprop-p has high water solubility and poses
environmental concern because its low sorption, high mobility and slow degradation
in soil make it susceptible to leaching from soil to water. Moreover, the literature
about the effects of Mecoprop-P is poorly documented.
Sulfonylureas are a class of herbicides used for crop protection and have been
widely used to control weeds. However, the sulfonylurea herbicide residue is highly
phytotoxic for some rotational plants at only 1% or even less of the originally applied
amount. Nicosulfuron is mostly used on maize. Approximately 91,000 Kg of herbicide
after the foliar application into soil annually can run off from cropland into rivers,
ponds and lakes, causing surface and groundwater contamination.
We have studied the adsorption-desorption behavior of these pesticides and
the influence of the presence of some anions in water. Additionally, we have carried
out the recyclability of the calcined hydrotalcite by several cycles of adsorptioncalcination on the two pesticides, Nicosulfuron and Mecoprop-P.
Kinetic tests showed the adsorption of both Nicosulfuron and Mecoprop-P
pesticides, ionizable or ionic respectively, involve intraparticle diffusion, and the rate
constant of intraparticle diffusion for Nicosulfuron increases with increasing
concentration of the compound by effect of its increased molecular size relative to
Mecoprop-P. According the Giles classification, adsorption isotherms for both
pesticides were L-shaped, and application of the Langmuir and Dubini–Radushkevich
equations revealed that adsorption was favorable and occurred by ion-exchange.
Moreover, the maximum amount of Mecoprop-P adsorbed was twice the
224
Resume
Nicosulfuron one. This result is consistent with the structural, size and ionization
differences between the two pesticides.
One the other hand, the presence of others anions affected the adsorption
and desorption of Nicosulfuron and Mecoprop-P. Competing anions, (particularly
2–
HPO4 ), had an adverse effect on Nicosulfuron and Mecoprop-P adsorption, and the
extraction of the pesticides from the host–guest complex was especially effective
2–
2–
with CO3 . The ability of CO3
to recover large amounts of Mecoprop-P suggests
that the adsorbent can be recycled.
According to the results of regeneration tests, calcined hydrotalcite is an
effective adsorbent for the removal of Nicosulfuron and Mecoprop-P from
contaminated water. This adsorbent can be reused several times to remove
Nicosulfuron and Mecoprop-P, albeit with gradually decreasing efficiency.
Periodic Mesoporous Organosilicas (PMOs) are a class of mesostructured
organic–inorganic hybrid nanomaterials wherein the organic moieties are totally
incorporated in the walls. The inorganic and organic moieties placed alternately
along the mesopore walls make PMOs more hydrophobic and stable in water than
pure organized silicas. Due to their hydrophobic mesoporous structure with a
controlled pore size and high surface areas, similarly to mesoporous silicas (PMS),
these materials offer unique properties for use as adsorbents of organic compounds
in virtue of their high surface area and narrow mesopore distribution, as well as their
organic nature bridges which can interact with them.
Following the search of new materials, we tested the adsorption of benzenePMO, ethane-PMO and PMS on S-Metolachlor with the collaboration of Dr. Francisco
Romero Salguero of the group FQM 346 of the University of Cordoba. The influence
of different factors, such as the study of acid pH and contact time, as well as
successive adsorptions, adsorption/desorption experiments and regeneration of the
adsorbent has been studied. For making the comparison, a mesoporous silica has
also been used as adsorbent.
The results indicated that both PMOs exhibited a higher adsorption capacity
than their silica counterpart. Moreover, both PMOs exhibited a very different
225
Capítulo IX
adsorption behavior. While ethane-PMO behaved like a mesoporous silica (PMS) but
with an enhanced adsorption capacity due to its more hydrophobic character,
benzene-PMO displayed a much higher herbicide retention at a pH close to its point
of zero charge. Certainly, aromatic π-π interactions between phenylene bridges and
S-Metolachlor should play an important role.
Furthermore, the adsorption was affected by the pH and it was maximum at
pH=2 for ethane-PMO and at pH=4 for benzene-PMO. PMOs exhibited different
isotherms, specifically types Langmuir L2 and L4 for ethane- and benzene-PMO,
respectively. We decided to do the experiment of successive adsorptions. Ethane-1
-1
PMO showed an uptake of 416.63 µmol g whereas it was of 1729.35 µmol g for
benzene-PMO (ca. 0.5 g herbicide per gram of adsorbent after successive adsorption
cycles). These materials, ethane-PMO and benzene-PMO, have a high aqueous
stability,
high
adsorption
capacity
(higher
than
activated
charcoals,
organohydrotalcites, clays and soils) and easy regenerability. Therefore, benzenePMO is a promising adsorbent for other complex aromatic molecules.
Activated carbon is an adsorbent which is widely used in many fields. This
material is not suitable for adsorbents of large molecules since it is mainly
microporous. Recently, new mesoporous materials that show promising results for
the adsorption of large pollutants have been developed. Ordered mesoporous
polymers and carbons are an advanced class of ordered mesoporous materials,
combining the high porosity properties of mesoporous materials with the physicochemical characteristics of organic polymers and carbons. Ordered mesoporous
carbons were firstly synthesized by a hard template method using mesoporous silica.
However, this synthesis cannot be used for large scale production because it is timeconsuming and expensive. Later, a direct synthesis method using phenolic resin as a
carbon precursor was developed.
Finally, our final work was focused on the preparation of two mesoporous
materials ordered mesoporous polymer resin (MPR) and mesoporous carbon (MC) to
be tested as regenerable adsorbents. We thank Dr. Pascal Van Der Voort and his
research group at the Centre for Ordered Materials, Organometallics and Catalysis
226
Resume
(COMOC), University of Ghent, Bélgica for their indispensable collaboration on the
realization of this part of the study.
The adsorbent MPR has been prepared by the EISA method (evaporation
induced self-assembly), using resorcinol and formaldehyde under acid conditions in
the presence of surfactant F127 (EO106-PO70-EO106). MC was synthesized by
calcination of MPR at high temperature. A commercially available activated carbon
(CC) was also used as adsorbent for comparison.
The adsorbents were characterized by X-ray diffraction analysis, FT-IR
spectroscopy, and nitrogen adsorption-desorption isotherms, thermogravimetric
analysis, transmission and scanning electron microscopy, particle size measurement
and XPS spectroscopy.
We decided to study S-Metolachlor to compare with another previous
adsorbents and Bentazon because is moderately toxic by ingestion and slightly toxic
by dermal absorption and because it is moderately irritating to the skin, eyes, and
respiratory tract as well. Bentazon is a post-emergence contact diazinone herbicide
used to control annual weeds in a variety of crops, especially rice. This compound is
one of the most commonly used herbicides in developed countries.
Bentazon and S-Metolachlor adsorption kinetics and isotherm studies were
carried out at pH 2 and 4, respectively, on these mesoporous materials as well as on
a reference commercial carbon. The adsorption of Bentazon decreased when the pH
increased, probably due to the increase of the electrostatic repulsion between the
pesticide ion and the adsorbent surface. By contrast, the adsorption of S-Metolachor
was not affected by the pH.
The adsorption kinetics have been investigated by three models, the
Lagergren pseudo-first-order, the pseudo-second order and intraparticle diffusion.
The equilibrium experimental adsorption data were fitted to Freundlich, Langmuir,
Temkin and Dubinin–Raduskevich models.
The adsorption kinetics fitted better a pseudo-second-order kinetics model.
Intraparticle diffusion was present in the adsorption process together with other
factors. Furthermore, the isotherms for Bentazon on MPR, MC and CC and for S227
Capítulo IX
Metolachor on CC were of L2 type, while the adsorption of S-Metolachlor on MPR
and MC exhibited a L4 type isotherm. In the case of Bentazon, a plateau was reached
for the three adsorbents. By contrast, the second plateau for S-Metolachlor on MPR
and MC was not reached, indicating that the maximum capacity of these adsorbents
was not achieved under these conditions. In all cases, the Langmuir model showed a
much better fit. The adsorption of Bentazon could be explained as an exchange of
ions, according to the Dubinin-Radushkevich model. The presence of hysteresis in
desorption curves indicated in all cases irreversibility of the process.
The regeneration of the adsorbents was carried out by calcination under
nitrogen atmosphere because of the decomposition of these materials under air
atmosphere. The results indicated that the regeneration of the mesoporous carbon
was more efficient than those of the mesoporous phenolic resin and the commercial
carbon after being reused several times. Another disadvantage of commercial
carbon was the great loss of material that it experienced after several adsorptiondesorption cycles.
In summary, calcined hydrotalcite (HT500) shows good adsorption capacity
for both Nicosulfuron and Mecoprop-P due to their ionic or polar character. Besides,
the mesoporous carbons allowed the removal of anionic Bentazon pesticide in
aqueous solution.
Moreover, nonpolar pesticides such as S-Metolachlor were adsorbed at basic
pH by organohydrotalcites and at acidic pH by mesoporous carbons and PMOs. HTTDD was the best adsorbent at basic pH although HT-DDS proved to be a better
option because of its greater reusability. The best adsorbents were benzene-PMOs
and MC at acidic pH, although the latter showed the higher adsorption capacity.
228
CAPÍTULO X
ABREVIATURAS
230
Anexos
ABREVIATURAS
BTEB
1,4-bis(trietoxisilil)benceno
BTEE
1,2-bis(trietoxisilil)etano
BTME
1,2-bis(trimetoxisilil)etano
13
resonancia magnética nuclear de C
CC
carbón comercial
DDS
anión dodecilsulfato
DDT
dicloro difenil tricloroetano
DNOC
2 metil 4,6-nitrofenol
DNP
2,4 dinitrofenol
FTIR
espectroscopía infrarroja por transfromada de Fourier
HDLs
hidróxidos dobles laminares
HT500
hidrotalcita calcinada a 500°C
HTDDS
hidrotalcita con dodecilsulfato en la interlámina
HTTDD
hidrotalcita con tetradecanodiato en la interlámina
MC
carbón mesoporoso ordenado
MPR
resina mesoporosa fenólica ordenada
OHT
organohidrotalcita
PMOs
organosílicas mesoporosas periódicas
PMS
sílica mesoporosa periódica
TDD
anión tetradecanodiato
C NMR
13
231
Capítulo XII
TEM
microscopía electrónica de transmisión
TGA
análisis termogravimétrico
232
CAPÍTULO XI.
PUBLICACIONES
E INDICIOS DE CALIDAD
Publicaciones e Indicios de Calidad
PUBLICACIONES QUE COMPONEN ESTA TESIS DOCTORAL E INDICIOS DE CALIDAD
Los resultados obtenidos en esta Tesis Doctoral han sido recogidos en cinco artículos
científicos de gran relevancia internacional en este campo de investigación (primer
cuartil) y en varias comunicaciones nacionales e internacionales a congresos
científicos.
Las cinco publicaciones son:
-Adsorption of non-ionic pesticide S-Metolachlor on layered double hydroxides
intercalated with dodecylsulfate and tetradecanedioate anions. R. Otero, J.M.
Fernández, M.A. Ulibarri, R. Celis, F. Bruna. Applied Clay Science 65-66 (2012) 7279.
Impact Factor: 2.342
Category name: MINERALOGY 5 / 26 Q1
Category name; MATERIALS SCIENCE, MULTIDISCIPLINARY 52 / 241 Q1
Cites: 6
-Pesticides adsorption-desorption on Mg-Al mixed oxides. Kinetic modeling,
competing factors and recyclability. R. Otero, J.M. Fernández, M.A. González, I.
Pavlovic, M.A. Ulibarri. Chemical Engineering Journal 221 (2013) 214-221.
Impact Factor: 4.058
Category name: ENGINEERING, CHEMICAL 8 / 133 Q1
Category name; ENGINEERING, ENVIRONMENTAL 7 / 46 Q1
Cites: 6
-Adsorption of the herbicide S-Metolachlor on periodic mesoporous organosilicas.
R. Otero, D. Esquivel, M.A. Ulibarri, C. Jimenez-Sanchidrián, F. J. Romero-Salguero,
J. M. Fernández. Chemical Engineering Journal 205 (2013) 205-213
Impact Factor: 4.058
Category name: ENGINEERING, CHEMICAL 8 / 133 Q1
Category name; ENGINEERING, ENVIRONMENTAL 7 / 46 Q1
Cites: 6
235
Capítulo Xi
Mesoporous phenolic resin and mesoporous carbon for the removal of SMetolachlor and Bentazon herbicides. R. Otero, D. Esquivel, M.A. Ulibarri, F. J.
Romero-Salguero, P. Van Der Voort, J. M. Fernández Chemical Engineering Journal
251 (2014) 92-101.
Impact Factor: 4.058
Category name: ENGINEERING, CHEMICAL 8 / 133 Q1
Category name; ENGINEERING, ENVIRONMENTAL 7 / 46 Q1
Cites: 6
-Evaluation of different bridged organosilicas as efficient adsorbents for the
herbicide S-metolachlor. María Isabel López, Rocío Otero, Dolores Esquivel, César
Jiménez-Sanchidrian, José María Fernández and Francisco José Romero-Salguero.
RSC Adv. 5 (2015), 24158–24166,
Impact Factor: 3.708
Category name: CHEMISTRY, MULTIDISCIPLINARY 35 / 148 Q1
Cites: 0
Trabajos presentados en congresos nacionales o internacionales:
- Adsorption of herbicide Fluometuron on layered double hydroxides intercalated
by dodecylsulphate and tetradecanoate anions. Fourth International Conference on
Hybrid Materials. Sitges, Spain. 2015. R. Otero, M.A. Ulibarri, J.M. Fernández.
- Adsorption of the herbicide Fluometuron on periodic mesoporous organosilicas.
Fourth International Conference on Hybrid Materials. Sitges, Spain. 2015. Rocío
Otero, Dolores Esquivel; César Jiménez-Sanchidrián; Francisco J. Romero-Salguero;
Jose Maria Fernández-Rodríguez.
- Adsorption of the herbicide Fluometuron on periodic mesoporous organosilicas
.Nanouco V. Encuentro sobre Nanociencia y Nanotecnología de Investigadores
Andaluces. Campus de Rabanales. Universidad de Córdoba, Córdoba. 2015. Rocío
236
Publicaciones e Indicios de Calidad
Otero, Dolores Esquivel; César Jiménez-Sanchidrián; Francisco J. Romero-Salguero;
Jose Maria Fernández-Rodríguez.
- Organosílices mesoporosas periódicas (PMOs) como materiales adsorbentes de
contaminantes
orgánicos.
Nanouco
IV.
Encuentro
sobre
Nanociencia
y
Nanotecnología de Investigadores Andaluces. Campus de Rabanales. Universidad de
Córdoba, Córdoba. 2013. Rocío Otero, Dolores Esquivel, M. Ángeles Ulibarri, César
Jiménez-Sanchidrián, Francisco J. Romero-Salguero and José M. Fernández.
- Incorporación de herbicidas en hidrotalcitas modificadas. Nanouco III. Encuentro
sobre Nanociencia y Nanotecnología de Investigadores Andaluces. Campus de
Rabanales. Universidad de Córdoba, Córdoba. 2011. Rocío Otero Izquierdo; Mª Del
Rosario Pérez Pérez; Chaara, Dikra; Rosalia Extremera Jaen; Jose Maria Fernandez
Rodriguez; Cristobalina Barriga Carrasco; Maria Angeles Ulibarri Cormenzana; Ivana
Pavlovic Milicevic.
- Sorption of pesticides nicosulfuron and mecoprop-p by layered double
hydroxides. Hybrid Materials 2011. Strasbourg, France, 2011. Rocío Otero Izquierdo;
Mª Ángeles González Millán; Ivana Pavlovic Milicevic; Maria Angeles Ulibarri
Cormenzana; Cristobalina Barriga Carrasco; Jose Maria Fernandez Rodriguez.
- Adsorption of non-polar herbicide s-metolachlor on layered double hydroxides
intercalated by dodecylsulphate and tetradecanoate anions. Hybrid Materials.
Strasbourg, France, 2011. Rocío Otero Izquierdo; Ivana Pavlovic Milicevic; Maria
Ángeles Ulibarri Cormenzana; Cristobalina Barriga Carrasco; Jose Maria Fernandez
Rodríguez.
- Hidrotalcitas: Aplicaciones en Prevención y Remediación Ambiental. I Jornadas del
Campus de Excelencia Internacional Agroalimentario. Córdoba, 2010. Maria Angeles
Ulibarri Cormenzana; Cristobalina Barriga Carrasco; Jose Maria Fernandez Rodriguez;
Ivana Pavlovic Milicevic; Mª Del Rosario Pérez Pérez; Rocío Otero Izquierdo; Rosalia
Extremera Jaen.
237
Capítulo Xi
- Hidróxidos Dobles Laminares como Materiales Adsorbentes de Contaminantes en
Aguas. Nanouco (II Encuentro sobre Nanociencia y Nanotecnología de Investigadores
y Tecnólogos de la Universidad de Córdoba). Córdoba, 2010. Rocío Otero Izquierdo;
Rosalia Extremera Jaen; Chaara-Dikra; Felipe Bruna Gonzalez; Mª Del Rosario Pérez
Pérez; Ivana Pavlovic Milicevic; Cristobalina Barriga Carrasco; Maria Angeles Ulibarri
Cormenzana; Jose Maria Fernandez Rodriguez.
- Uptake of Pesticides Fluometuron and Nicosulfuron by Layered Double
Hydroxide. 2010 SEA-CSSJ-CMS TRILATERAL MEETING ON CLAYS. SEVILLA (ESPAÑA),
2010. Rocío Otero Izquierdo; Mª Del Rosario Pérez Pérez; Cristobalina Barriga
Carrasco; Maria Angeles Ulibarri Cormenzana; Ivana Pavlovic Milicevic; Jose Maria
Fernand
238
Applied Clay Science 65-66 (2012) 72–79
Contents lists available at SciVerse ScienceDirect
Applied Clay Science
journal homepage: www.elsevier.com/locate/clay
Research Paper
Adsorption of non-ionic pesticide S-Metolachlor on layered double hydroxides
intercalated with dodecylsulfate and tetradecanedioate anions
R. Otero a, J.M. Fernández a,⁎, M.A. Ulibarri a, R. Celis b, F. Bruna b
a
Departamento de Química Inorgánica e Ingeniería Química. Instituto Universitario de Química Fina y Nanoquímica (IUQFN), Campus de Rabanales, Universidad de Córdoba, Campus de
Excelencia Internacional Agroalimentario (CeiA3), Córdoba, Spain
b
Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS), CSIC, Avenida Reina Mercedes 10, Apartado 1052, 41080 Sevilla, Spain
a r t i c l e
i n f o
Article history:
Received 27 January 2012
Received in revised form 26 April 2012
Accepted 30 April 2012
Available online xxxx
Keywords:
S-Metolachlor
Organohydrotalcite
Dodecylsufate anion
Tetradecanedioate anion
Adsorption
Water treatmen
a b s t r a c t
The objective of the present research was to investigate the adsorption of non-ionic pesticide S-Metolachlor
on two different organohydrotalcites (OHTs): one intercalated with dodecylsulfate (HT-DDS) and the other
one with tetradecanedioate (HT-TDD) anions, both of them obtained by the coprecipitation method. The
adsorbents and the adsorption products were characterized by X-ray diffraction and FT-IR spectroscopy. The
results indicated that the adsorption of S-Metolachlor on the organohydrotalcite HT-TDD was higher than
that on HT-DDS, but desorption was lower. The amount of S-Metolachlor adsorbed increased with
temperature, especially for HT-TDD. S-Metolachlor desorption from HT-DDS and HT-TDD was characterized
using two experimental procedures, which indicated the higher reversibility of S-Metolachlor adsorption
from HT-DDS compared to HT-TDD. The results suggest the possibility to use both organohydrotalcites
uptaking S-Metolachlor from contaminated water.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
The widespread use of pesticides in modern agriculture has given
rise to an increase about the environmental concern. The contamination of soil and ground water by pesticides applied to soil and swept
by transport processes, such as leaching and run-off, is becoming an
important environmental problem (Modabber Ahmed et al., 2007).
Pesticides are very hazardous pollutants, toxic to many forms of
wildlife, especially aquatic organisms, insects and mammals and can
persist in the aquatic environment for many years after their
application (Shukla et al., 2006).
A strategy to reduce the pesticide transport losses after soil
application consists of the use of controlled release formulations, in
which the pesticide is gradually released over time. This limits the
amount of pesticide immediately available for transport processes.
Beneficial effects related to the use of controlled release formulations
include reduction in rapid losses of pesticide by volatilization, runoff
and leaching. The savings in manpower and energy, by reducing the
number of applications required in comparison to conventional
formulations, increase safety for the pesticide applicator and lead to
⁎ Corresponding author at: Dpto. Química Inorgánica e Ingeniería Química, Campus
de Rabanales, Universidad de Córdoba, Córdoba, Spain. Tel.: +34 957 218648; fax:
+34 957218621.
E-mail address: [email protected] (JM. Fernández).
0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.clay.2012.04.022
a general decrease in non-target effects (Celis et al., 2002; Gerstl et al.,
1998).
Previous research has indicated that layered double hydroxides
(LDHs), also known as anionic clays or hydrotalcite-like compounds
(HTs), could be suitable to smooth and even prevent the environmental impact caused by pesticides, i.e. they can be used either as
filters of pesticide-contaminated water, or as supports in pesticide
controlled release formulations. LDHs or HTs are brucite-like layered
n−
·mH2O,
materials with a general formula [M1II − xMxIII(OH)2] x+Xx/n
where M II and M III are divalent and trivalent cations respectively,
and X n− is the interlayer anion, which balances the positive charge
originated by the presence of M III in the layers. The layer charge is
determined by the molar ratio x = MIII/ (MIII + MII) and can vary
between 0.2 and 0.4 (Cavani et al., 1991; Costantino et al., 2009). The
interest of these materials is related to their structure, high anion
exchange capacity, and simplicity of their synthesis (Reichle, 1986;
Rives, 2001).
Similarly to clay minerals, the inorganic anions of HTs can be
replaced with organic anions (Clearfield et al., 1991; Costantino et al.,
2009; Meyn et al., 1990; Zhao and Vance, 1997). This simple
modification changes the nature of the HT surface from hydrophilic
to hydrophobic, increasing its affinity for non-ionic organic compounds, including many non-ionic pesticides (Bruna et al., 2006; Celis
et al., 2000; Costa et al., 2008; Pavlovic et al., 1996; Villa et al., 1999;
Wang et al., 2005). These organoclays and organo/LDHs have
applications in a wide range of organic pollution control fields
R. Otero et al. / Applied Clay Science 65-66 (2012) 72–79
(Bruna et al., 2009; Celis et al., 2000; Costa et al., 2008; Frost et al.,
2008; Laha et al., 2009; Zhou et al., 2008).
In the present paper, we report on the adsorption of a non-ionic
pesticide, S-Metolachlor, on two different organohydrotalcites
(OHTs): one intercalated with dodecylsulfate (HT-DDS) and the
other with tetradecanedioate (HT-TDD) anions in the interlayer
space, both surfactants with the same carbon chain (twelve –CH2
units) but differing in the nature of the chemical groups bonded to
the HT layers. The aim of this paper was to analyze the influence of
the surfactant as well as the effect of temperature on the adsorption
process of S-Metolachlor to complete the study about the behavior of
this pesticide carried out previously (Chaara et al., 2011). A detailed
study of the reversibility of the adsorption–desorption process was
also conducted using two different experimental approaches.
2. Materials and methods
2.1. Organic anions and pesticide
Dodecylsulfate (DDS) was supplied as sodium salt by Fluka, and
tetradecanedioic acid (TDD) and analytical standard S-Metolachlor (2Chloro-N-(2-ethyl-6-methylphenyl)-N-[(1S)-2-methoxy-1-methylethyl]
acetamide) were provided by Sigma-Aldrich. The structure of these
materials is included in the following scheme:
73
2.2. Synthesis of organohydrotalcites
Two organohydrotalcites (OHTs), [Mg3Al(OH)8][(C12H25SO4)·4H2O]
and [Mg3Al(OH)8][(C14H24O4)·4H2O], named HT-DDS and HT-TDD,
respectively, were obtained by the coprecipitation method (Reichle,
1986). To prepare hydrotalcite with DDS in the interlayer, a solution
containing 0.06 mol Mg(NO3)2·6H2O and 0.02 mol Al(NO3)3·9H2O in
100 mL of distilled water was dropped to an alkaline solution (0.16 mol
of NaOH in 500 mL of water) containing 0.05 mol of dodecylsulfate
(DDS). The synthesis was carried out without control of pH and at room
temperature. To prepare hydrotalcite with TDD in the interlayer, a
solution containing 0.06 mol Mg(NO3)2·6H2O and 0.02 mol Al(NO3)3·9H2O in 100 mL of distilled water was dropped to an alkaline solution of
NaOH at pH =8, containing 0.01 mol of tetradecanedioic acid (TDD);
during the addition the pH was constant and the temperature 75 °C. In
both cases, the syntheses were carried out in inert atmosphere (N2
bubbling), to avoid CO2 dissolution and subsequent intercalation of
carbonate into the LDH interlayer. The resultant suspensions were
hydrothermally treated at 80 °C for 24 h, and the precipitate washed
with CO2-free distilled water and dried at 60 °C.
2.3. Characterization of organohydrotalcites
Elemental analysis of Mg and Al was performed with an AAS
Perkin Elmer 3100 spectrometer after dissolving the samples in
concentrate HCl, whereas S, C and N contents were determined with
an elemental analyzer Eurovector 3A 2010. The water content was
calculated by using a thermogravimetric analyzer SETARAM Setsys
Evolution 16/18. For this purpose, 20 mg of sample was submitted to
heating with a constant temperature gradient of 5 °C/min in the range
of 30 to 1000 °C. Powder X-ray diffraction patterns (PXRD) were
obtained on a Siemens D-5000 instrument with CuKα radiation and
FT-IR spectra were recorded by using the KBr disk method on a Perkin
Elmer Spectrum One spectrophotometer. Specific surface areas were
measured from nitrogen adsorption/desorption isotherms on a
Micromeritics ASAP 2020 instrument using N2 gas as adsorbate. The
surface morphology of both samples was obtained using a scanning
electron microscope JEOL JSM 6300.
2.4. Adsorption and desorption experiments
S-Metolachlor, a chloroacetamide type compound with a water
solubility of 480 mg/L (25 °C), is a selective herbicide applied as preand post-emergence to control grasses and some broad-leaved weeds
in a wide range of crops.
S-Metolachlor adsorption on OHTs was studied at different initial
pH values, contact times (kinetics), and initial pesticide concentration
(isotherms), by suspending duplicate samples of 20 mg of each
adsorbent in 30 mL of aqueous pesticide solutions. Samples of sorbent
and pesticide in water were shaken in a turn-over shaker at 52 rpm
and the supernatants were centrifuged and separated to determine
the concentration of S-Metolachlor by UV spectroscopy (Perkin Elmer
UV-visible spectrophotometer, Model Lambda 11) at 265 nm. The
amounts of pesticide adsorbed were determined from the initial and
final solution concentrations.
To study the adsorption at different initial pHs (5.0, 6.6 and 9.0)
and contact times (1, 2, 5, 24 h), an initial pesticide concentration of
0.35 mM was used. Adsorption isotherms were obtained by the batch
equilibration technique as described elsewhere (Hermosín et al.,
1993; Pavlovic et al., 1997), using initial pesticide concentrations
between 0.05 mM and 0.35 mM.
Desorption isotherms were obtained using two different procedures: In the first one, described by Bowman (1979) and Bowman
and Sans (1985), desorption was carried out immediately after
adsorption from the highest equilibrium point of the adsorption
isotherm. The procedure consisted of removing 15 mL of the
supernatant liquid and replacing it with the same amount of distilled
water. After shaking at room temperature for 24 h, the suspensions
were centrifuged, and the pesticide concentration in the supernatant
was determined by UV–vis spectroscopy. This process was repeated
74
R. Otero et al. / Applied Clay Science 65-66 (2012) 72–79
four times ([Bruna et al., 2008). In the second procedure, which was a
modification of that described by Yang (1974), one unique desorption
point was measured from each equilibrium point of the adsorption
curve by replacing a small portion of supernatant (15 mL) with the
same amount of distilled water. After shaking for 24 h, the amount of
pesticide desorbed was calculated as describe above. This procedure
avoids the difficulties mentioned by Bowman (1979) about the
procedure used by Yang (1974).
Adsorption and desorption parameters were calculated using the
Freundlich equation (Eq. (1))
1=nf
Cs ¼ K f Ce
ð1Þ
where Cs(μmol/g) is the amount of S-Metolachlor adsorbed at the
equilibrium concentration Ce (μmol/L), and Kf and nf are the empirical
Freundlich constants (Bowman and Sans, 1985). Hysteresis coefficients, H, were calculated according to H = nf-des/nf-ads, where nf-ads
and nf-des are the Freundlich slopes of the adsorption and desorption
isotherms, respectively (Barriuso et al., 1994).
For the adsorbent regeneration and recyclability study, a preliminary experiment was conducted to obtain the kinetics of SMetolachlor desorption from HT-DDS and HT-TDD using water,
methanol, and ethanol as extracting solutions. Desorption kinetic
curves were obtained from the highest equilibrium point of the
adsorption kinetic curve. The extracting volume was 30 mL and the
contact times 1, 2, 5, 24, and 30 h. After shaking, the suspensions
were centrifuged, the supernatant liquid was separated and evaporated, and the residue dissolved in distilled water to determine the
pesticide concentration by UV–vis spectroscopy. Successive SMetolachlor/methanol adsorption–desorption cycles for HT-DDS
were conducted to assess the potential recyclability of this material
as an adsorbent of S-Metolachlor.
3. Results and discussion
3.1. Characterization of the organohydrotalcite sorbents
3.1.1. Elemental analysis
According to the values of Mg/Al molar ratio (Table 1), a partial
dissolution of Al 3 + during the synthesis of HT-DDS and Mg 2 + in HTTDD seemed to occur. This could be attributed to the differences on
the synthesis conditions (temperature, pH…), as well as, to the
different acid/base properties of the organic anions (pKa = 4.46 for
TDD and −3.29 for DSS).
The molar ratio S/Al in HT-DDS (Table 1), which corresponds to
the fraction of the positive charge of HT compensated by DDS anions,
indicated that most of the layer charge (91%) was compensated by
DDS anions and about 9% by other anions, probably CO32 − (as it will
be seen below). The elemental analysis of C in HT-TDD sample
(Table 1) suggests that, if all carbon atoms were as TDD anions, only
45% of the layer charge of HT will be compensated. Therefore, a part of
the charge must be balanced by other anions, probably CO32 −, and
also a small amount by nitrate anions (from the metal salts) as
suggested by the N content of the sample (Table 1). From the
elemental analysis and the TG data to obtain the water content (not
Fig. 1. XRD patterns of the organohydrotalcites and their adsorption products.
(⁎diffraction peaks due to the Al sample holder).
included), the formulae for both organohydrotalcites were obtained
and included in Table 1.
3.1.2. X-ray diffraction study
Powder XRD patterns of the synthesized organohydrotalcites were
recorded (Fig. 1). In both cases the diagram corresponds to a
hydrotalcite-like structure with an important expansion of the interlayer space with respect to the inorganic hydrotalcite with carbonate
anion (d003 = 7.8 Å) (Cavani et al., 1991). X-ray diffraction pattern of
HT-DDS sample (Fig. 1A) showed reflections (00 l) at 26.4 Å and 13.1 Å,
suggesting a vertical monolayer arrangement of dodecylsulfate anions
(Bruna et al., 2006; Clearfield et al., 1991). X-ray diffraction pattern of
HT-TDD (Fig. 1B) showed (001) reflections at 17.5 Å and 8.5 Å
suggesting a tilt position of the surfactant in the interlayer, since,
according to CS Chem 3D 5.0 program, TDD height is 18.3 Å, that added
to the hydrotalcite-layer thickness (4.8 Å) would give a value of 23.1 Å
for a perpendicular arrangement of TDD anions.
3.1.3. Textural properties
The scanning electron micrographs (Fig. 2) showed the surface
morphology of HT-DDS and HT-TDD consisting of an agglomerate of
particles, although slightly more porosity and less homogeneity in the
HT-DDS particle size were observed. These results are in accordance
with the low crystallinity observed in the XRD for both samples
(Fig. 1) and with the low specific surface area values obtained by
nitrogen adsorption/desorption isotherms (not shown), 5.4 m 2/g for
HT-DDS and b1 m 2/g for HT-TDD.
3.2. S-Metolachlor adsorption–desorption experiments
The adsorption of S-Metolachlor was not affected by the initial pH
of the solution (not shown). This was attributed to the fact that the
Table 1
Elemental analysis of the adsorbents and proposed formula.
Adsorbent
HT-DDS
HT-TDD
wt.%
Atomic ratios
Proposed formula
Mg
Al
C
S
N
Mg/Al
S/Al
15.72
31.39
4.67
14.67
29.38
20.22
4.97
0
0
0.37
3.74
2.38
0.91
0
X = C14H24O42 −, CO32 −.
[Mg0.79Al0.21(OH)2](C12H25SO4)0.20(CO3)0.005·0.56H2O
[Mg0.71Al0.29(OH)2](NO3)0.01Xn0.28/n·0.75H2O
R. Otero et al. / Applied Clay Science 65-66 (2012) 72–79
75
Kf parameters were in the range 40.6–16.3, which indicated strong
sorption between the sorbate and sorbent (Liu et al., 2010). The
values of nf were less than 1, indicating a non-linear sorption of SMetolachlor on both samples. This behavior is similar to that
observed by Bruna et al. (2008) for the adsorption of terbuthylazine
on organohydrotalcites and Celis et al. (1999) for adsorption of
imazamox on organoclays and organohydrotalcites.
The temperature effect on the adsorption of S-Metolachlor by HTDDS (Fig. 5A) and HT-TDD (Fig. 5B) was also investigated. When the
temperature was increased from 10 to 30 °C, the amount of pesticide
adsorbed at the initial concentration 0.35 mM increased from 45.9%
to 50.3% in HT-DDS and from 49.9% to 56.8% in HT-TDD. In the case of
HT-DDS, the increase in pesticide adsorption is only observed clearly
at high initial concentrations, i.e. at Ce values above 150 μmol/L. These
results indicate the endothermic nature of the adsorption process, as
confirmed by the calculation of the thermodynamic parameters. The
change in standard free energy (ΔG°), enthalpy (ΔH°), and entropy
(ΔS°) of adsorption was calculated using the following equations
(Eqs. (2) and (3)) (Islam and Patel, 2007, 2011; Li et al., 2009; Özcan
and Safa Özcan, 2005):
∘
∘
ΔG ¼ ΔH −TΔS
∘
∘
ð2Þ
∘
lnK ¼ ΔS =R−ΔH =RT
Fig. 2. SEM micrographs of samples (A) HT-DDS and (B) HT-TDD.
final pH values for the equilibrated suspensions were all between 9
and 10, due to the buffer effect of hydrotalcite-like compounds
(Cavani et al., 1991). For that reason, the selected initial pH for all
subsequent adsorption experiments was 6.6, i.e. the original pH of the
water-dissolved pesticide.
The kinetic study of the S-Metolachlor adsorption on HT-DDS
(Fig. 3) indicated an almost instantaneous adsorption, reaching the
equilibrium in 2 h, while the pesticide adsorption on HT-TDD was
slower and the plateau was not achieved. Nevertheless, taking into
account that in the last case the amount of pesticide adsorbed at 24 h
was only 5% greater than that adsorbed at 6 h, 24 h was assumed to
be sufficient to reach an apparent adsorption equilibrium. This
different behavior in the adsorption kinetics could be attributed to
little changes in the morphology and specific surface area of the
sorbents.
The adsorption isotherms were obtained by plotting the amount
of S-Metolachlor adsorbed (Cs) versus the solute concentration (Ce)
in the equilibrium solution and were of L-2 subtype, according to the
classification of Giles et al. (1960), on both organohydrotalcites
(Fig. 4). The amount of S-Metolachlor adsorbed on HT-DDS and HTTDD was very similar (e.g., between 43% and 45% at an initial
herbicide concentration, Ci = 0.35 mM), suggesting a direct relation
between the amount of pesticide adsorbed and the length of the
surfactant alkyl chain (twelve –CH2 groups in both cases), and agree
with the results of a previous work (Chaara et al., 2011) where
adsorption of Metolachlor on HT-DDS was higher than that on an
organohydrotalcite modified with sebacate anion (HT-SEB), with
only eight –CH2 groups. The isotherms shape suggested that the
Freundlich equation would be a good model to fitting, and the choice
is justified by the high regression coefficients (Table 2). The
Freundlich constants (Kf and nf) together with the regression
coefficients (R 2) were very similar for both sorbents (Table 2).
ð3Þ
where K is the equilibrium constant, R the molar gas constant and T
the absolute temperature. Values of ΔH° and ΔS° were calculated
from the slope and intercept of Van't Hoff plots (Ln KC versus 1/T) and
are summarized in Table 3. The values of K have been obtained from
value of Kf from the Freundlich equation because our isotherms have
been fitted to this equation. K has been recalculated to become
dimensionless by multiplying it by 1000 (Milonjic, 2007). ΔG° values
were calculated from Eq. (2).
ΔH° values were positive in both cases, confirming the endothermic character of the S-Metolachlor adsorption process, but ΔH° for
HT-TDD was an order of magnitude higher than that for HT-DDS
(Table 3). Changes in ΔS were favorable for both sorbents but the ΔS°
value was again higher for HT-TDD compared to HT-DDS. The larger
contribution of the entropy than the enthalpy was responsible for the
negative value of ΔG° in all cases, thus indicating that the adsorption
process was entropically controlled. The most negative ΔG° value was
observed for HT-TDD at 30 °C coincident with the greater adsorption
constant.
Fig. 3. Time evolution of S-Metolachlor adsorption on HT-DDS and HT-TDD
(Cinitial = 0.35 mM).
76
R. Otero et al. / Applied Clay Science 65-66 (2012) 72–79
Fig. 4. Adsorption and desorption isotherms of S-Metolachlor on HT-DDS (A, C) and HT-TDD (B, D). Desorption was performed after adsorption from the highest equilibrium point
of the adsorption isotherm (A, B). Desorption was performed after each point of the adsorption isotherm (C, D).
With respect to the S-Metolachlor desorption study, the desorption isotherms obtained according to the above-described first
procedure (the more widely used method (Hermosín et al., 1993;
Liu et al., 2010; Pavlovic et al., 2005)), for both HT-DDS and HT-TDD
showed high H values, H ≫ 1 (Table 2), indicating a high reversibility
of the pesticide adsorption process (Cruz-Guzmán et al., 2004)
(Fig. 4A, B). The shape of the curves, with the desorption branch
falling below the adsorption isotherm branch, is probably due to the
experimental procedure rather than to the adsorption–desorption
processes themselves (Koskinen et al., 1979) because that procedure,
Table 2
Freundlich parameters for S-Metolachlor sorption (s) and desorption (d) isotherms on
organohydrotalcites and hysteresis coefficient H.
HT-DDS (s)
HT-DDS (d*)
HT-DDS (d**)
HT-TDD (s)
HT-TDD (d*)
HT-TDD (d**)
Kf
nf
R2
29.12–16.32
0.07–0.06
9.46–5.81
40.65–27.38
0.017–9.23 × 10− 4
7.55–1.55
0.48 ± 0.06
1.94 ± 0.27
0.61 ± 0.05
0.39 ± 0.02
2.39 ± 0.36
0.90 ± 0.19
0.995
0.973
0.990
0.998
0.967
0.914
*first desorption way.
**second desorption way.
H
4.04
1.27
6.13
2.31
implying successive treatments with ethanol, probably could modify
the solid features.
To overcome some of the limitations inherent to the successive
desorption procedure, we used a second procedure to study desorption,
i.e. a modification of procedure proposed by Yang (1974), where one
desorption step was performed from every equilibrium point of the
adsorption isotherm. Desorption curves thus obtained were very
different from those obtained using the conventional successive
desorption procedure (Fig. 4C, D). This procedure tries to reproduce
the method carried out when N2 adsorption on any material is
performed, where the hysteresis parameter close to 1 indicates high
reversibility of the process and H values higher than 1 suggest
irreversibility. The H value obtained for S-Metolachlor adsorbed on
HT-DDS was very close to 1 (Fig. 4C), according to a high reversibility,
indicating that desorption will be favored. However, the hysteresis
coefficient (H) for S-Metolachlor on HT-TDD is considerably greater
than 1 (2.31) (Fig. 4D), which indicates a certain no reversibility of
the adsorption process. Desorption curve above the adsorption one
evidences the special difficulty to remove S-Metolachlor from the
substrate HT-TDD.
This new experimental procedure permits to distinguish between
the behavior of S-Metolachlor in both organohydrotalcites, so that we
could conclude that the reversibility of S-Metolachlor adsorption is
higher for HT-DDS (H = 1.27) than for HT-TDD (2.31). However, the
R. Otero et al. / Applied Clay Science 65-66 (2012) 72–79
77
Fig. 5. Adsorption isotherms of S-Metolachlor at different temperatures: (A) on HT-DDS and (B) on HT-TDD.
first procedure is still useful to simulate the conditions occurring in
soil when, after adsorption, a pesticide is sequentially desorbed by
rainfall or irrigation water.
3.3. Sorbent regeneration and recyclability experiments
These experiments consisted of successive adsorption–desorption
stages of S-Metolachlor on HT-DDS and HT-TDD with the final
purpose to analyze the possibility of sorbent regeneration.
As a preliminary step, desorption experiments were performed
with methanol, ethanol and water from the highest equilibrium point
of the adsorption kinetic curves. The process was quite fast in all the
cases (Fig. 6), desorption reaching the plateau in a few hours. SMetolachlor adsorbed on HT-DDS (Fig. 6A) was totally removed
(100%) with ethanol and almost the same happened with methanol
(92.8%) while only 25.4% was desorbed using water. However, for
hydrotalcite HT-TDD (Fig. 6B) desorption was not completed with
ethanol (62.7%) nor methanol (58.0%) and with water was only 20.7%.
As expected, this pesticide showed more affinity for hydrophobic
solvents due to its non-ionic character.
According to the above results, the study of recyclability was only
performed on hydrotalcite HT-DDS using ethanol as solvent, the
unique case where desorption was completed. HT-DDS adsorbent
allowed only a stage of adsorption–desorption with ethanol removing
completely S-Metolachlor. Although the second stage was only
slightly less effective in the adsorption (95%) than the first one, only
40% of the pesticide was desorbed after the second adsorption step.
These results indicate that the possibility of regeneration of the
sorbent with ethanol is limited, and new experiments should be
necessary to find a more effective regeneration method.
Table 3
Thermodynamic parameters for S-Metolachlor sorption on organohydrotalcites HTDDS and HT-TDD determined at different temperatures.
Sample
HT-DDS
HT-TDD
T (°C)
10
20
30
10
20
30
K
11000
11020
12050
14420
20320
38470
ΔS
ΔH
ΔG
(KJ·mol− 1·K− 1)
(KJ·mol− 1)
(KJ·mol− 1)
0.090
3.53
0.204
35.32
−21.89
−22.67
−23.67
−22.53
−24.16
−26.60
3.4. Characterization of solids after adsorption
In order to characterize the adsorption products in the “plateau” of
the isotherms, powder XRD patterns and FT-IR spectra were
registered to confirm the adsorption of S-Metolachlor in the
hydrotalcite interlayers. After adsorption of S-Metolachlor, a change
in the d00l-value was observed for both organohydrotalcites (Fig. 1C,
D). The adsorption product of S-Metolachlor on HT-DDS showed a
small decrease of the d003-value close to 1 Å (from 26.4 to 25.6 Å),
(Fig. 1C). In contrast, S-Metolachlor adsorption on HT-TDD (Fig. 1D) led
to a very important increase in the d003-value compared with the
pristine adsorbent (from 17.5 to 23.7 Å). This suggests a change in the
orientation of the tetradecanedioate anion from tilt position in HT-TDD
to perpendicular position after the adsorption of the pesticide, thus
providing strong evidence for sorbent deformation upon adsorption of
S-Metolachlor. The d003-value of the HT-TDD-S-Metolachlor adsorption
product was only slightly higher than the sum of the size of TDD and the
thickness of the brucite layer (23.12 Å).
Taking into account that TDD anions have two negative charges
versus only one for DDS, half of TDD anions will be necessary to
compensate the layer charge producing a lower “congestion” of the
interlayer space in the last case. The higher free space in the interlayer
for HT-TDD would permit S-Metolachlor to be introduced between
the organic chains causing a change in the orientation of the organic
molecules, reflected in the increase in d003 space showed in the
Fig. 1C. These results confirm those found by Chaara et al. (2011) for
Alachlor adsorption on other organohydrotalcites.
Fig. 7 shows FT-IR spectra of the adsorbents, S-Metolachlor, and
adsorption products. Characteristic features of the hydrotalcite
structure are the broad bands around 3460 cm − 1, corresponding to
the νO―H of hydration water molecules and free OH − groups of the
brucite layers, and a weak band at 1650 cm − 1 corresponding to δO―H
of water molecules (Fig. 7A, B). Strong peaks between 2920 cm − 1 and
2850 cm − 1 correspond to C―H stretching modes (Bruna et al.,
2008)]. The most characteristic bands of HT–DDS (Fig. 7A) are those
due to the antisymmetric (1211 cm − 1) and symmetric (1071 cm − 1)
stretching S O vibration of the sulfate group (Bellamy, 1975;
Clearfield et al., 1991). FT-IR of HT-TDD (Fig. 7B) shows characteristic
bands at 1561 cm − 1 and 1421 cm − 1 corresponding to vibrations of
carboxylate groups (antisymmetrical and symmetrical, respectively).
The presence of the pesticide bands (1760 cm − 1 attributed to νC O
of the carbonyl group, and 1113 cm − 1 corresponding to νC―O of the
ether group and/or δC―H associated to the benzene ring) in the FT-IR
spectra of the adsorption products in the “plateau” of the isotherms
78
R. Otero et al. / Applied Clay Science 65-66 (2012) 72–79
Fig. 6. Desorption kinetics of S-Metolachlor with different solvents: (A) HT-DDS and (B) HT-TDD.
(Fig. 7D, E) revealed the adsorption of S-Metolachlor in both
organohydrotalcites. These results are similar to those observed by
Bruna et al. (2006) for the adsorption of Metamitron on an
organohydrotalcite with dodecylsulfate in the interlayer.
4. Conclusions
Adsorption–desorption of S-Metolachlor pesticide by two
organohydrotalcites (HT-DDS and HT-TDD) have been characterized.
The organohydrotalcites presented an expansion of the interlayer
space compared to the inorganic hydrotalcite with carbonate anion.
X-ray diffraction patterns suggested a vertical monolayer arrangement
of dodecylsulfate anions and a tilt position of the tetradecanedioate
anions in the interlayer space. Adsorption of S-Metolachlor resulted in
changes in the d00l-value for both organohydrotalcites. In particular, the
adsorption of S-Metolachlor on HT-TDD led to a very important increase
in the basal spacing, suggesting changes in the orientation of the
tetradecanedioate anion from tilt to perpendicular position. The
amount of adsorbed pesticide increased with the rise of the temperature from 10 to 30 °C, especially for HT-TDD.
A new experimental procedure has been used to distinguish that
the reversibility of S-Metolachlor adsorption is higher for HT-DDS
than for HT-TDD. Only S-Metolachlor adsorbed on HT-DDS was
possible to be removed with ethanol. Therefore, the study of
recyclability was only performed on hydrotalcite HT-DDS using
ethanol as solvent. The results indicated that the possibility of
regeneration of the sorbent with ethanol was limited to one
regeneration step, so that further experiments would be needed to
find a more effective way for effective regeneration of the sorbent.
Acknowledgements
This work has been partially supported by grants AGL2008-04031,
AGL2011-23779 and CTM2011-25325 from MICINN, cofinanced with
FEDER funds, and Research Groups FQM-214 and AGR-264 of Junta de
Andalucía. R. Otero also acknowledges financial support from MECERDF. We appreciate the technical assistance received in the SCAI
(Universidad de Córdoba) from Electron Microscopy and Elemental
Analysis Units.
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Chemical Engineering Journal 228 (2013) 205–213
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Adsorption of the herbicide S-Metolachlor on periodic mesoporous
organosilicas
Rocío Otero a, Dolores Esquivel b,1, María A. Ulibarri a, César Jiménez-Sanchidrián b,
Francisco J. Romero-Salguero b,⇑, José M. Fernández a,⇑
a
Departamento de Química Inorgánica e Ingeniería Química, Instituto Universitario de Investigación en Química Fina y Nanoquímica (IUQFN), Campus de Rabanales, Universidad de
Córdoba, Campus de Excelencia Internacional Agroalimentario (CeiA3), Córdoba, Spain
b
Departamento de Química Orgánica, Instituto Universitario de Investigación en Química Fina y Nanoquímica (IUQFN), Campus de Rabanales, Universidad de Córdoba, Campus de
Excelencia Internacional Agroalimentario (CeiA3), Córdoba, Spain
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
PMOs are effective materials for the
adsorption of S-Metolachlor.
Phenylene bridges in PMOs provide a
very high adsorption capacity.
Hydrophobic and aromatic
interactions play a decisive role in the
adsorption.
The herbicide can be quantitatively
recovered by simple extraction with
ethanol.
PMOs are more stable than
mesoporous silicas in aqueous media.
a r t i c l e
i n f o
Article history:
Received 7 March 2013
Received in revised form 23 April 2013
Accepted 26 April 2013
Available online 7 May 2013
Keywords:
S-Metolachlor
Herbicides
Ethane-PMO
Benzene-PMO
Adsorption
Water treatment
a b s t r a c t
Due to their hydrophobic pore systems and high surface areas, periodic mesoporous organosilicas (PMOs)
offer unique properties for use as adsorbents of organic compounds. Two organosilicas (ethane-PMO and
benzene-PMO) and a silica (PMS) were synthesized following a similar procedure. The adsorption of the
herbicide S-Metolachlor from an aqueous solution was then compared on the three solids. Both PMOs
exhibited a higher adsorption capacity than their silica counterpart. Additionally, both PMOs showed
much higher stability than the PMS, whose mesostructure was seriously damaged under aqueous conditions. Moreover, both PMOs exhibited a very different adsorption behavior. The adsorption of S-Metolachlor on each material was described by different isotherms. The amount retained depended on the pH,
and was maximum at pH = 2 for ethane-PMO and at pH = 4 for benzene-PMO. Our results suggest that
both hydrophobic and aromatic p–p interactions play an important role in the adsorption of this herbicide. The adsorption capacity of benzene-PMO was very high with a loading of ca. 0.5 g herbicide per
gram of adsorbent after successive adsorption cycles.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
⇑ Corresponding authors. Tel.: +34 957212065; fax: +34 957212066 (F.J. RomeroSalguero), tel.: +34 957218648; fax: +34 957580644 (J.M. Fernández)
E-mail addresses: [email protected] (F.J. Romero-Salguero), [email protected]
(J.M. Fernández).
1
Present address: Department of Inorganic and Physical Chemistry, Center for
Ordered Materials, Organometallics and Catalysis (COMOC), Ghent University,
Krijgslaan 281, Building S3, 9000 Ghent, Belgium.
1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cej.2013.04.092
The so-called periodic mesoporous organosilicas (PMOs) were
firstly reported in 1999 by three different research groups which
synthesized them via the condensation of bridged organosilanes
of the type (R0 O)3SiARASi(OR0 )3 in the presence of a surfactant
[1–3]. Unlike other hybrid materials prepared from mesoporous
silicas by grafting and co-condensation, the organic fragments
206
R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213
(ARA) in PMOs materials are abundantly and homogeneously
distributed through their structure and are linked by covalent
bonds to two silicon units. Various R groups have been
successfully incorporated into these materials, including
aliphatic (ACH2A, ACH2ACH2A, ACH@CHA), aromatic (AC6H4A,
AC12H8A) and even heteroaromatic fragments (AC4H2SA) [4].
Similarly to mesoporous silicas, PMOs have a mesoporous structure with a controlled pore size and high surface area. However,
PMOs are more hydrophobic due to the existence of organic
fragments in their framework. All these facts make them excellent candidates as adsorbents for organic molecules. In particular, ethylene- (R = ACH2ACH2A) and phenylene- (R = AC6H4A)
bridged PMOs, which are very stable under hydrothermal conditions [5], can be very useful for the removal of organic pollutants
from aqueous effluents.
An interesting environmental application of PMOs has involved their use as adsorbents for heavy metals. It requires the
presence of certain organic functionalities which can be present
in the PMOs either by using specific organosilanes and/or
organodisilanes for their synthesis or by introducing them later
through reactions with the organic moieties present in their
frameworks. Thus, PMOs with isocyanurate bridging groups were
synthesized by co-condensation of TEOS, 3-mercaptopropyltrimethoxysilane and tris[3-(trimethoxysilyl)propyl]isocyanurate
and their high adsorption capacity for Hg2+ ions was reported
[6]. Co-condensation between 1,2-bis(triethoxysilyl)ethane and
3-(mercaptopropyl)trimethoxysilane was used to prepare several
PMOs with different pore sizes depending on the surfactant
added, i.e. Brij-76 and Pluronic P123, in the range of 1.4–3.4
and 4.2–6.4 nm, respectively [7]. The former were quite selective
for Hg2+ ions whereas the latter exhibited a high affinity for
Hg2+, Cd2+ and Cr3+ cations. More recently, different PMOs have
been synthesized by co-condensation of TEOS and an organodisilane precursor containing disulfide units. They showed excellent
efficiencies for the adsorption of Hg2+ ions (up to 716 mg g1)
[8]. A different synthetic strategy involved the functionalization
of the double bonds of an ethenylene-bridged PMO with propylthiol groups [9]. This material combines the Hg2+ adsorption
ability of the thiol groups with the high stability of the PMO,
thus allowing it to be reusable after multiple regeneration
cycles.
Very recently, Walcarius and Mercier have extensively revised
the application of mesoporous silicas for the removal of organic
and inorganic pollutants [10]. The use of PMO materials as adsorbents for toxic organic pollutants is limited to the work by
Borghard et al. [11]. They synthesized two mesoporous polysilsesquioxanes from 1,2-bis(triethoxysilyl)ethane and 1,4-bis
(trimethoxysilylethyl)benzene under alkaline conditions. Even
though the latter material showed much lower specific surface
area than the former one and a nonperiodic structure, it adsorbed
14 times more 4-nitrophenol than the former [12]. The different
performance of both materials was explained by the existence of
p–p interactions between the aromatic rings of the phenolic compounds and the arylene bridges.
S-Metolachlor is a pre- and post-emergence selective herbicide
used to control grasses and some broad-leaved weeds in a wide
range of crops. Previous papers have reported the adsorption of
S-Metolachlor by different adsorbents such as organohydrotalcites
(OHTs) [13,14], soils [15], activated charcoals [16] and clays [17].
This paper reports the adsorption of S-Metolachlor on ethyleneand phenylene-bridged PMOs, i.e., ethane-PMO and benzene-PMO,
respectively. The influence of different factors, such as pH and contact time, as well as successive adsorptions, adsorption/desorption
experiments and regeneration of the adsorbent has been studied.
For comparison, a mesoporous silica has also been used as
adsorbent.
2. Materials and methods
2.1. Materials
A periodic mesoporous silica (PMS) and two organosilicas, i.e.,
ethane-PMO and benzene-PMOs, were synthesized by using
Brij-76 as surfactant under acidic conditions according to previously reported procedures (see Supporting information) [12].
Analytical standard S-Metolachlor (2 Chloro-N-(2-ethyl-6acetamide)
methylphenyl)-N-[(1S)-2-methoxy-1-methylethyl]
was supplied by Sigma Aldrich (see Scheme 1). The water solubility
at 25 °C is 480 mg/L.
2.2. Characterization
PMOs were characterized by different techniques. X-ray powder diffraction (XRD) patterns were recorded on a Siemens
D-5000 powder diffractometer (Cu Ka radiation) and FT-IR
spectra were obtained with a Perkin Elmer Spectrum One
spectrophotometer using KBr disc technique. N2 isotherms were
determined on a Micromeritics ASAP 2010 analyzer at 196 °C.
Particle size data were obtained in a 2000 Mastersizer particle
size analyzer by laser diffraction technology, with a dry
dispersion automatic system Venturi Scirocco 2000 of Malvern
Instruments. Microstructural characterization of the materials
was carried out using JEOL 1400 TEM and JEOL 2010 TEM equipments. The 29Si MAS NMR spectra were recorded at 79.49 MHz
on a Bruker Avance 400 WB spectrometer at room temperature.
An overall 5000 free induction decays were accumulated. The
recycle time was 15 s. Chemical shifts were measured relative
to a tetramethylsilane standard. UV spectroscopy (Perkin Elmer
UV–Visible spectrophotometer, Model Lambda 11) at 265 nm
was used to determine the concentration of S-Metolachlor.
2.3. Adsorption/desorption experiments
S-Metolachlor adsorption on mesoporous materials was studied
at different initial pH values, contact times (kinetics), and initial
herbicide concentration (isotherms), by suspending duplicated
samples of 20 mg of each adsorbent in 30 mL of aqueous herbicide
solutions. Samples of sorbent and herbicide in water were shaken
in a turn-over shaker at 52 rpm.
To study the adsorption at different initial pHs [2.0, 3.0, 4.0, 5.0
and 6.6 (pH of herbicide solution)] and contact times (1, 2, 5, 15
and 24 h), an initial herbicide concentration of 350 lM was used.
Adsorption isotherms were obtained by the batch equilibration
technique. For that, the samples were equilibrated through shaking
at 52 rpm for 24 h at room temperature, as described elsewhere
[18,19], using initial herbicide concentrations between
1.0 104 M and 1.4 103 M.
Scheme 1. Adsorption of S-Metolachlor on PMOs showing the main interactions
between the herbicide and both surfaces (see text for discussion).
R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213
The supernatant liquid was centrifuged and separated to determine the concentration of S-Metolachlor by UV spectroscopy. The
amounts of herbicide adsorbed were determined from the initial
and final solution concentrations.
According to a previously described procedure [13], desorption was measured from each equilibrium point of the adsorption curve by replacing a small portion of supernatant (7.5 mL)
with the same amount of distilled water. After shaking for
24 h, the amount of herbicide desorbed was calculated as indicated above.
The pseudo first order [20–24] and the pseudo second
order [23] models and intraparticle diffusion equation [25,26]
were used with the goal at evaluating the adsorption kinetic
data.
log ðC s1 C st Þ ¼ log C s1 k1
t
2:303
ð1Þ
t
1
1
¼
þ
t
C st k2 C 2S
C s2
2
ð2Þ
C st ¼ kp t1=2 þ C
ð3Þ
where Cs1 and Cs2 are the maximum amount of pesticide adsorbed
(mg g1) for first and second-kinetic order, respectively. Cst is the
amount adsorbed at time t (mg g1), k1 is the pseudo first order rate
constant for the adsorption process (min1), k2 is the rate constant
of pseudo second order adsorption (g mg1 min1), C is the intercept, and kp is the intraparticle diffusion rate constant (mg g1 min1/2).
S-Metolachlor adsorption isotherms were fitted to Langmuir
(Eq. (4)), Freundlich (Eq. (5)) and Dubinin–Radushkevich (D–R)
(Eq. (6)) equations:
1
1
1
1
¼
þ
Cs Cm L Ce Cm
ð4Þ
log C s ¼ log K f þ nf log C e
ð5Þ
ln C s ¼ ln qm K e2
ð6Þ
1
where Cs (lmol g ) is the amount of herbicide adsorbed per unit
mass adsorbent at the equilibrium concentration Ce (lmol L1), Cm
is the monolayer capacity of the adsorbent (lmol g1), and L is
the Langmuir adsorption constant (L lmol1), which is related to
the adsorption energy. Kf (lmol1nf Lnf g1) and nf are Freundlich
parameters incorporating all factors affecting the adsorption process. qm is the theoretical adsorption capacity of the adsorbent
(lmol g1) and K (mmol2 kJ2) is the constant related to adsorption
energy. e is the Polanyi adsorption potential, and equal to RTln
(1 + 1/Ce), where R is the universal gas constant and T is the temperature in the Kelvin scale.
2.4. Successive adsorptions
They were carried out in the highest equilibrium point of the
adsorption isotherm curves, by suspending the solid (adsorbentherbicide) in 30 mL of a fresh aqueous herbicide solution
(1.25 103 M). The contact time was 24 h. After shaking, the suspensions were centrifuged and the herbicide concentration determined by UV–Vis spectroscopy. This process was repeated up to
complete saturation of the adsorbent.
2.5. Regeneration of the adsorbent
In order to test the regeneration of the adsorbent, some desorption experiments were performed with either ethanol or water
207
from the highest equilibrium point of the adsorption isotherms.
The extracting volume was 30 mL and the contact time was 24 h.
After shaking, the suspensions were centrifuged, the supernatant
liquid was separated and evaporated (in the case of ethanol), and
the residue dissolved in the same volume of distilled water to
determine the herbicide concentration by UV–Vis spectroscopy.
3. Results and discussion
A periodic mesoporous silica (PMS) and two organosilicas, i.e.,
ethane-PMO and benzene-PMO, have been tested for the adsorption of the herbicide S-Metolachlor. These three materials exhibited a low angle (1 0 0) peak and two broad and short secondorder (1 1 0) and (2 0 0) peaks at higher incidence angles indicative
of materials with 2D hexagonal (P6mm) mesostructures (Fig. 1).
The integrity of the organic bridges was confirmed by FTIR measurements (see Supporting information) [5]. Their N2 adsorption–
desorption isotherms were of type IV with a sharp increase in
the adsorption at P/P0 between 0.30 and 0.47 due to capillary condensation, typical of materials with a narrow pore size distribution
in the mesopore range (Fig. 2). The N2 adsorption step was shifted
to higher relative pressures following the sequence: benzenePMO < ethane-PMO < PMS, by the effect of the increase in the pore
size. The main physical properties of the three adsorbents are summarized in Table 1. Their mesoporous structures were confirmed
by TEM images (Fig. 3); the inset in Fig. 3B shows a similar pore
size to that determined by the BJH method. In addition, the structure is very similar for PMS and organosilicas (Fig. 3A–C). On the
other hand, both organosilicas showed very similar particle size
distribution curves, which were centered at around 12 and
14 lm for benzene-PMO and ethane-PMO, respectively (Fig. 4).
A comparison of the behavior of the three adsorbents as a function of pH (Fig. 5) revealed significant differences among them. FTIR spectra (not shown) revealed that the herbicide was intact after
adsorption. In order to gain some insights into the relationship between pH and adsorption, measurements of the point of zero
charge (pzc) were accomplished for the three materials. This value
indicates the pH required to give zero net surface charge. Below the
pzc the hydroxyl groups at the surface of an oxide become protonated and so positively charged whereas above the pzc they are
deprotonated and consequently negatively charged. By following
the mass titration method (Fig. 6) [27], pzc values of 3.9–4.0 were
obtained for the three samples, close to those reported for silica by
other authors [28]. As a consequence, the surfaces of the three
materials were positively charged at pH = 2–3. This suggest that
the main interactions between the adsorbents and S-Metolachlor
at pH 6 3 were electrostatic, i.e., interactions between positive
charges on the surface and electron-rich regions (N, O and Cl
atoms) in the adsorbed molecules, as a result of which the values
of adsorption for samples PMS and ethane-PMO were maximum
(Fig. 5). At pH = 4 their surfaces were essentially neutral and so
their interactions with S-Metolachlor became weaker. Consequently, their adsorption values decreased. Hydrophobic interactions seem to play an important role, since the amount of
adsorbed herbicide was higher for ethane-PMO than for PMS
throughout the pH range. This reasoning is based on the assumption that both materials possess an analogous amount of SiAOH
groups. 29Si MAS NMR spectra of the three samples were recorded
in order to ascertain it (Fig. 7). They allowed determining their condensation degree (Table 2), which decreased in the order
PMS > ethane-PMO > benzene-PMO. In addition, the OH/Si ratio
for benzene-PMO was apparently higher than for PMS and ethane-PMO. However, the organic moieties present in the framework
of both organosilicas also contribute to the masses of the corresponding materials. In fact, similar relative contents of silanol
208
R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213
Fig. 1. Powder X-ray diffraction patterns of PMS (left), ethane-PMO (center) and benzene-PMO (right) before, after being in contact with the solution of herbicide at different
pHs and after herbicide extraction (inset).
Table 1
Physicochemical properties of periodic mesoporous organosilicas.
Material
aoa
(nm)
SBETb
(m2 g1)
PMS
EthanePMO
BenzenePMO
6.7
7.0
885
1059
6.2
1237
Vp d
(cm3 g1)
Dpe
(nm)
Wf
(nm)
0
26
1.17
1.00
4.2
4.1
2.5
2.9
257
0.94
3.6
2.6
Smpc
(m2 g1)
p
Unit-cell dimension calculated from ao = (2d100/ 3).
BET specific surface area determined in the range of relative pressures from 0.05
to 0.2.
c
Micropore area, calculated by t-Plot method.
d
Single-point pore volume.
e
Diameter pore average size, calculated from the adsorption branch according to
the BJH method.
f
Pore wall thickness, estimated from W = ao Dp.
a
b
Fig. 2. Nitrogen adsorption–desorption isotherms and the corresponding pore size
distributions (insets) for PMS, ethane-PMO and benzene-PMO.
groups were found in the three materials when that ratio was corrected taking into account the mass of the Si unit for each material
(Table 2).
On the other hand, the maximum adsorption for benzene-PMO
occurred at pH = 4 (Fig. 5). Considering the higher hydrophobic
character of ethane-PMO than that of benzene-PMO [29], the aromatic p–p interactions between the aromatic ring of the herbicide
and the phenylene bridges of the hybrid material should be crucial
for the adsorption of this molecule onto benzene-PMO.
Both mesoporous organosilicas exhibited a higher adsorption
capacity toward the herbicide S-Metolachlor than that of the mesoporous silica, as showed by the adsorption kinetics (Fig. 8). The
amount of adsorbed herbicide significantly increased from
53 lmol g1 on PMS to 127 and 135 lmol g1 on ethane-PMO
and benzene-PMO, respectively. Also, both PMOs presented a higher stability under the conditions of adsorption than their silica
counterpart (Fig. 1). The material PMS significantly decreased its
mesostructural order as the pH of the herbicide solution increased.
This effect was much less pronounced for benzene-PMO and particularly for ethane-PMO. The decrease in the intensity of the
(1 0 0) peak for PMOs can be attributed to the presence of adsorbate
molecules inside the pores which causes a decrease in the contrast
for X-rays between the organosilica walls and the pores. In fact,
both PMOs recovered their original X-rays patterns upon removing
the herbicide (vide infra), what was confirmed before and after the
adsorption for benzene-PMO by TEM (Fig. 3C and D), unlike PMS
whose mesostructure was destroyed (Fig. 1 inset).
The adsorption kinetics of the herbicide S-Metolachlor were
elucidated by substituting the initial concentrations into the pseudo first order (Largergren equation) and pseudo second order
adsorption rate equations, respectively. Table 3 shows the adsorption rate constants k1 and k2 calculated from the slope of each plot.
The correlation coefficients R2 for the pseudo first order kinetic
model were between 0.89 and 0.97 and the correlation coefficients
R2 for the pseudo second order kinetic model were 0.99. Therefore,
this adsorption system fitted better to a pseudo second order kinetic model [22].
R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213
209
Fig. 3. TEM images for PMS (A), ethane-PMO (B), including a TEM image of high resolution (inset), and benzene-PMO before adsorption of S-Metolachlor (C) and after
desorption (D).
Fig. 6. Mass titration curves for PMS, ethane-PMO and benzene-PMO.
Fig. 4. Particle size distribution curves for ethane-PMO and benzene-PMO.
Fig. 5. Influence of pH on the adsorption of S-Metolachlor on PMS, ethane-PMO and
benzene-PMO.
The mechanism behind the adsorption was studied by analyzing the kinetic results according to the Morris–Weber equation
for intraparticle diffusion (Eq. (3)). The adsorption rate-determining step could therefore be either adsorption or intraparticle diffusion. Based on the applied model, a plot of the amount of pesticide
adsorbed, Cst, against the square root of time, t1/2, should be linear
if intraparticle diffusion was involved in the process; also, if the
lines passed through the origin, then intraparticle diffusion would
be the rate-determining step [21,30]. The correlation coefficients
R2 of Table 3, obtained at Cst values spanning up to 24 h, suggested
that the adsorption of herbicide S-Metolachlor on PMOs may involve intraparticle diffusion; however, none of the curves passed
through the origin, so intraparticle diffusion cannot have been
the rate-determining step and other factors may have operated
simultaneously to govern the adsorption process. Kp and C were
calculated from the plot of Cst versus t1/2 for each adsorbent (see
Table 3). The values of kp and C for benzene-PMO were higher
and lower than those for ethane-PMO, respectively, suggesting a
higher contribution of the intraparticle diffusion for benzene-PMO.
Adsorption isotherms were measured for both mesoporous
organosilicas due to their significantly higher capacity compared
210
R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213
Fig. 7.
29
Si MAS NMR spectra for PMS, ethane-PMO and benzene-PMO.
Table 2
Si MAS NMR results obtained for mesoporous materials.
29
a
b
c
Material
T1 (%)
T2 (%)
T3 (%)
Q2 (%)
Q3 (%)
Q4 (%)
Condensation degree (%)a
OH/Sib
OH/mass of the Si unitc
PMS
Ethane-PMO
Benzene-PMO
–
5.6
13.4
–
43.69
56.7
–
50.65
29.9
5.2
–
–
40.8
–
–
54.0
–
–
87
82
72
0.51
0.55
0.83
8.5 103
8.3 103
9.2 103
P
P
P
P
Condensation degree calculated from equation D = [ (nTn) + (nQn)]/[(3 Tn) + (4 Qn)].
Content of silanol groups per silicon atom calculated from equation OH/Si = (2T1 + T2 + 2Q2 + Q3)/(T1 + T2 + T3 + Q2 + Q3 + Q4).
The masses of the Si units are 60.1 (SiO2), 66.1 (SiO1.5CH2) and 90.1 (SiO1.5C3H2) for materials PMS, ethane-PMO and benzene-PMO, respectively.
Fig. 8. Adsorption kinetics of S-Metolachlor on PMS, ethane-PMO and benzenePMO.
to that of the PMS sample (Fig. 8). The adsorption isotherms at the
pH of maximum adsorption for each material are depicted in Fig. 9.
Ethane-PMO exhibited a Langmuir isotherm, specifically L2 [31]. In
this case, the adsorbed molecules are most likely to be adsorbed
flat (Scheme 1), according to the abovementioned electrostatic
interactions. The plateau of the curve gave 255 lmol g1. By contrast, the adsorption of the herbicide on benzene-PMO followed a
type L4 isotherm. It exhibited an ill-defined first plateau which
suggests a close affinity of S-Metolachlor for two different regions
in the surface of benzene-PMO. We tentatively assume that
adsorption firstly occurred in hydrophilic regions, i.e., silanol
groups, through hydrogen bonds with oxygen and nitrogen atoms
of the herbicide, while the second stage would be caused by the
adsorption of S-Metolachlor on hydrophobic regions, i.e., phenylene bridges, through aromatic p–p interactions (Scheme 2). However, we cannot ascertain whether a partially flat or a
perpendicular orientation occurred at both stages. Also, the interaction of silanol groups (Lewis acid) of benzene-PMO and aromatic
rings (Lewis base) of S-Metolachlor cannot be completely ruled out
[32]. The second plateau would be reached because of the complete saturation of the hydrophobic regions.
The adsorption results show that the amount of S-Metolachlor
adsorbed on benzene-PMO (667 lmol g1) exceeded the amount
adsorbed by activated charcoals (480–550 lmol g1) [16], organohydrotalcites (OHTs) (356–385 lmol g1) [13,14], clays
(200 lmol g1) [17] and soils (50 lmol g1) [15]. The adsorption
capacity of the ethane-PMO was slightly smaller than that of the
organohydrotalcites (OHTs) and similar to that of clays.
The desorption curves almost matched the adsorption ones for
ethane-PMO (Fig. 9), thus indicating a high reversibility of the
adsorption–desorption process. However, the isotherm for benzene-PMO displayed a considerable hysteresis, which must be related to the interactions between the adsorbent and S-Metolachlor.
Table 3
Kinetic parameters for the adsorption of S-Metolachlor onto PMS and PMOs.
Pseudo first order model
k1 (min1)
PMS
Ethane-PMO
Benzene-PMO
3
3.6 10
3.22 103
4.38 103
Cs1 (mg g1)
7.39
11.68
27.10
Pseudo second order model
R2
0.89
0.97
0.97
k2 (g mg1 min1)
4
8.5 10
7.5 104
2.6 104
Intraparticle diffusion
Cs2 (mg g1)
R2
kp (mg g1 min1/2)
C (mg g1)
R2
15.80
36.76
40.98
0.99
0.99
0.99
0.25
0.35
0.60
6.48
23.75
17.31
0.78
0.90
0.96
211
R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213
Fig. 9. Adsorption and desorption isotherms of S-Metolachlor on ethane-PMO and
benzene-PMO.
Fig. 10. Successive adsorptions of S-Metolachlor from the highest equilibrium point
of the adsorption isotherm curve for ethane-PMO and benzene-PMO.
Freundlich and Langmuir equations were tested to model the
adsorption isotherms in order to determine the parameters which
quantify the adsorption process (Table 4). According to R2 values,
the Langmuir equation was fitted much better to the experimental
data than the Freundlich one.
In order to predict the adsorption efficiency, a dimensionless
separation factor R was determined using the following equation
[33,34]:
agreed with the above mentioned higher adsorption on benzenePMO than on ethane-PMO (Figs. 8 and 9). The constant K is related
to the mean free energy of adsorption (E), which can be calculated
by the following equation [33,36]:
R¼
1
1 þ LC i
E ¼ ð2KÞ1=2
ð8Þ
The mean energy provides information about the adsorption
mechanism. The adsorption is physical in nature when
0 < E < 8 kJ mol1; if it is between 8 kJ mol1 and 16 kJ mol1, the
adsorption is due to exchange of ions; and if the value of E is between 20 < E < 40 kJ mol1, a chemisorption occurs. The values
found in the present study were 2.7 kJ mol1 for benzene-PMO
and 3.3 kJ mol1 for ethane-PMO, thus pointing to a physical
adsorption.
Aiming at the saturation of the adsorbents, successive adsorptions were accomplished (Fig. 10). The maximum S-Metolachlor
adsorption capacities for both host materials were different. Ethane-PMO was saturated after two adsorptions, being the adsorbed
amount of 416.63 lmol g1, whereas benzene-PMO required four
successive adsorptions, which accounted for 1729.35 lmol g1.
Probably, this adsorption occurred on the exposed parts of the
layer already present (Scheme 2c). Taking into account that the
ð7Þ
where Ci is the initial herbicide concentration. Values in the range
0 < R < 1 represent a favorable adsorption. The R values for adsorption on ethane-PMO and benzene-PMO, with initial concentrations
of S-Metolachlor 100–1400 lmol L1, were found in the range of
0.94–0.51 and 0.95–0.53, respectively. These values indicate a
favorable adsorption, and also allow to confirm the higher adsorption at low concentrations in both cases.
In order to distinguish between physical and chemical adsorption, parameters K and qm from Dubinin–Radushkevich equation
(D–R) were calculated from the slope and intercept in the plot of
ln Cs versus e2, respectively [24,35]. The values of qm (Table 4)
Scheme 2. Plausible model for the adsorption of S-Metolachlor onto benzene-PMO through adsorption on hydrophilic (a) and hydrophobic regions (b) of the original surface
and formation of a new adsorbed layer up to complete filling of the pores (c). The polar part and the aromatic ring of the molecule are represented by a circle and a stick,
respectively. The different colors depict different adsorption stages.
Table 4
Freundlich, Langmuir and Dubinin–Radushkevich model parameters for the S-Metolachlor adsorption onto ethane-PMO and benzene-PMO.
Freundlich
Ethane-PMO
Benzene-PMO
Langmuir
Dubinin–Radushkevich
nf
Kf
R2
Cm (lmol g1)
L
R2
qm (lmol g1)
K (lmol2 kJ2)
R2
0.89 ± 0.11
1.63 ± 0.18
1.06–0.26
0.032–0.0036
0.95
0.95
283.3 ± 0.2
746.27 ± 0.14
6.82 104 ± 5.43 105
5.42 104 ± 2.81 105
0.98
0.98
440.26
786.83
4.6 1010
7.1 1010
0.98
0.96
212
R. Otero et al. / Chemical Engineering Journal 228 (2013) 205–213
volume of the S-Metolachlor molecule is around 875 Å3 [37], the
resulting volume occupied by the adsorbed herbicide was estimated to be 0.22 and 0.91 cm3 g1 for ethane-PMO and benzenePMO, respectively. Accordingly, the pores of benzene-PMO are
completely filled in with S-Metolachlor after successive adsorptions (see Table 1). This is not the case for ethane-PMO, thus
remarking the key role of the phenylene bridges to the adsorption
of S-Metolachlor. The high loading (up to 0.5 g/g) and slow release
of S-Metolachlor from benzene-PMO make it an excellent candidate as a delivery system.
Finally, some desorption experiments were performed with
either ethanol or water from the highest equilibrium point of the
adsorption isotherms to test the regeneration of the adsorbent. In
this sense, both materials behaved similarly. S-Metolachlor completely desorbed from ethane-PMO and benzene-PMO when using
ethanol. However, the herbicide was not completely removed in
water. Specifically, 74.5% and 84.0% were desorbed from ethanePMO and benzene-PMO, respectively. As expected, the herbicide
showed a higher affinity for the more hydrophobic solvent due to
its non-ionic character.
4. Conclusion
Periodic mesoporous organosilicas have unique properties to be
used as adsorbents of organic molecules in virtue of their high surface area and narrow mesopore distribution, similarly to mesoporous silicas, as well as their bridges of organic nature which can
interact with them. We have examined a potentially practical use
of these materials with ethylene and phenylene bridges for the
adsorption of a relatively complex herbicide molecule (S-Metolachlor). While ethane-PMO behaved like a mesoporous silica
(PMS) but with an enhanced adsorption capacity due to its more
hydrophobic character, benzene-PMO displayed a much higher
herbicide retention at a pH close to its point of zero charge. Certainly, aromatic p–p interactions between phenylene bridges and
S-Metolachlor should play an important role. In fact, both PMOs
exhibited different isotherms, specifically types Langmuir L2 and
L4 for ethane- and benzene-PMO, respectively. In addition, the latter isotherm exhibited a desorption hysteresis effect. Both PMOs
were saturated by successive adsorptions. Thus, ethane-PMO
showed an uptake of 416.63 lmol g1 whereas it was of
1729.35 lmol g1 for benzene-PMO. Due to its high aqueous stability, high adsorption capacity (higher than activated charcoals,
organohydrotalcites, clays and soils) and easy regenerability, benzene-PMO is a promising adsorbent for other complex aromatic
molecules.
Acknowledgments
The authors would like to thank Spain’s Ministry of Science and
Innovation (Projects MAT2010-18778 and CTM2011-25325), Junta
de Andalucía (Project P06-FQM-01741 and Research Groups FQM214 and FQM-346) and FEDER funds. R. Otero acknowledges Ministry of Education and Science for a research and teaching
fellowship.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cej.2013.04.092.
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Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Pesticides adsorption–desorption on Mg–Al mixed oxides. Kinetic
modeling, competing factors and recyclability
R. Otero, J.M. Fernández ⇑, M.A. González, I. Pavlovic, M.A. Ulibarri
Departamento de Química Inorgánica e Ingeniería Química and Instituto Universitario de Química Fina y Nanoquímica (IUQFN), Campus de Rabanales,
Universidad de Córdoba and Campus de Excelencia Internacional Agroalimentario (CeiA3), Córdoba, Spain
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Adsorption of Nicosulfuron and
"
"
"
"
Mecoprop-P on calcined hydrotalcite
(HT500).
Kinetic data showed that their
adsorption involves intraparticle
diffusion.
The adsorption is favorable and
occurs via an exchange of ions.
The adsorption is affected by the
presence of ions such as
2
2
PO3
4 ; SO4 ; and CO3 .
The extraction of pesticides with
CO2
3 suggests that the adsorbent can
be recycled.
a r t i c l e
i n f o
Article history:
Received 27 December 2012
Received in revised form 5 February 2013
Accepted 5 February 2013
Available online 13 February 2013
Keywords:
Nicosulfuron
Mecoprop-P
Adsorption
Hydrotalcite
Water treatment
a b s t r a c t
The adsorption of the ionizable pesticide Nicosulfuron and the ionic pesticide Mecoprop-P on a
calcined Mg–Al hydrotalcite (HT-500) in the presence and absence of various anions was studied with
a view to assessing the potential of regenerating the adsorbent. Kinetic tests showed the adsorption of
both pesticides to involve intraparticle diffusion, and the rate constant of intraparticle diffusion for
Nicosulfuron to increase with increasing concentration of the compound by effect of its increased
molecular size relative to Mecoprop-P. Application of the Langmuir and Dubini–Radushkevich equations to the adsorption isotherms for the two pesticides revealed easy adsorption of both via an
ion-exchange process. Adsorption and desorption of the two pesticides were both affected by the
presence of other anions. The adsorbents and their adsorption products were characterized by
X-ray diffraction and FT-IR spectroscopies. Based on the results of regeneration tests, calcined hydrotalcite is an effective adsorbent for the removal of Nicosulfuron and Mecoprop-P from contaminated
water.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
Pesticides are very hazardous pollutants that can persist in the
aquatic environment for many years [1]. Contamination of soil and
ground water by pesticides applied to soil and swept by transport
⇑ Corresponding author. Tel.: +34 957 218648; fax: +34 957 218621.
E-mail address: [email protected] (J.M. Fernández).
1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cej.2013.02.007
processes such as leaching or runoff is a posing an increasingly
serious environmental problem.
Some researchers have suggested that the use of layered double
hydroxides (LDHs), also known as ‘‘anionic clays’’ or ‘‘hydrotalcitelike compounds’’ (HTs), as filters for pesticide-contaminated water
or additives in controlled-release pesticide formulations might be
effective toward partially or completely avoiding the environmental impact of pesticides [2–4]. LDHs are brucite-like layered matexþ n
II
rials of general formula ½M II1x M III
x ðOHÞ2 X x=n mH2 O, where M
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R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221
and MIII are a divalent and a trivalent cation, respectively, and Xn
is an interlayer anion countering the positive charge arising from
the presence of MIII in the layers. Layer charge in an LDH is dictated
by the mole ratio MIII/(MIII + MII), which typically ranges from 0.2 to
0.4 [5,6]. The main interest of these materials lies in their structure,
high anion-exchange capacity and straightforward synthesis [3,7–
9].
The adsorption efficiency of LDHs is strongly affected by the
properties of their interlayer anions [10]. Thus, LDHs usually
have a greater affinity for anions with a high charge density
and hence tend to adsorb multivalent anions easily in relation
to monovalent ions [11,12]. In addition, LDHs exhibit preferential
affinity for CO2
3 , which usually hinders further ion-exchange
ions can be removed by heating
[13]. However, interlayer CO2
3
LDH at about 500 °C to obtained calcined LDH (HT500), which
can regain the original LDH structure in the presence of dissolved anions [14–16]:
NICOSULFURON
H 3C
O
N
H3C
O
O
O
O
N
O
N
H
CH3
N
S
N
H
CH3
N
MECOPROP-P
CH3
Cl
O
CO2H
Mg3 AlO4 ðOHÞ þ 4H2 O þ ð1=nÞX n ! ½Mg3 AlðOHÞ8 X n
1=n þ OH
where Xn is the anionic species present in solution.
The anion-exchange capacity (AEC) of HT500 was much higher
than that of the original LDH (5.8 vs 3.3 mmol g1) [17]; this allows
the adsorption efficiency of an LDH to be significantly increased by
calcination [16,18].
The influence of various common inorganic anions on pesticide
adsorption is one other major factor to be considered since some
such anions are often encountered in pesticide-contaminated
waters [10].
Several author have studied different properties of Mecropop
and Nicosulfuron pesticides such as their degradation or its effect
in the grown of different vegetables. It had been studied adsorption
and desorption of Nicosulfuron in soils [19,20] and clay minerals
[21]. Furthermore, it had been studied the adsorption of Mecoprop
on calcareous and organic matter amended soils [22], in clays [23]
or onto Iron Oxides [24]. Also, Khan et al. [25] have characterized
the Mecoprop-LDH hybrid synthetized by the cooprecipitation
method.
In this work, we examined the adsorption of two widely used,
ionizable pesticides (Nicosulfuron and Mecoprop-P) on a calcined
hydrotalcite (HT500) with a view to elucidating their adsorption–
desorption behavior and the way it is influenced by the presence
of some anions. In addition, we subjected the hydrotalcite to repeated adsorption–calcination cycles in order to assess its reusability in the removal of Nicosulfuron and Mecoprop-P from
contaminated water.
CH3
2.2. Synthesis of the hydrotalcite, and characterization of the
adsorbent and adsorption products
A hydrotalcite intercalated with carbonate anions and designated MgAl-LDH was prepared according to Reichle [7]. To this
end, a solution containing 0.75 mol of Mg(NO3)26H2O and
0.25 mol of Al(NO3)39H2O in 250 mL of distilled water was
dropped over 500 mL of another containing 1.7 mol of NaOH
and 0.5 mol of Na2CO3 under vigorous stirring for 2 h. The resulting suspension was hydrothermally treated at 80 °C for 24 h, and
the precipitate thus formed washed with distilled water and dried
at 60 °C to obtain the solid MgAl-LDH, calcination of which at
500 °C for 3 h gave an Mg–Al mixed oxide that was named
HT500.
The adsorbent (HT500) and adsorption products were characterized from their XRD patterns as obtained on a Siemens D5000 diffractometer using Cu Ka radiation (k = 1.54050 Å) and
their FT-IR spectra as recorded by using the KBr disc technique
on a Perkin Elmer spectrophotometer. The materials were also
characterized microstructurally with a JEOL 200CX TEM
instrument.
Elemental analyses (Mg and Al) were performed by atomic
absorption spectrometry on a Perkin Elmer AA-3100 instrument,
and C and N contents determined with a Eurovector 3A 2010 elemental analyzer.
2.3. Adsorption–desorption tests
2. Materials and methods
2.1. Pesticides
Nicosulfuron (2-[(4,6-dimethoxypyrimidin-2-ylcarbamoyl)sulfamoyl]-N,N-dimethylnicotinamide) and Mecoprop-P [(R)-2-(4chloro-o-tolyloxy)propionic acid] were both supplied as
high-purity products by Sigma–Aldrich. These pesticides are typically used to control weeds in post-emergence treatments. Nicosulfuron is a sulfonylurea of molecular weight 410.41 g mol1, a
high solubility in water (7500 mg L1 at 20 °C) and pKa = 4.78.
Mecoprop-P is an aryloxyalkanoic acid pesticide of molecular
weight 214.65 g mol 1 , water solubility 860 mg L 1 and
pKa = 3.86. The structural formula of these pesticides is as follows:
Kinetic adsorption and adsorption–desorption isotherms for
Nicosulfuron and Mecoprop-P were obtained by using the batch
equilibration method. For kinetic adsorption, 30 mg of adsorbent
for Nicosulfuron and 20 mg for Mecoprop-P were equilibrated in
duplicate by shaking with 30 mL of pesticide solutions containing
0.25–1.0 mM Nicosulfuron and 1.0–3.0 mM Mecoprop-P at room
temperature in a turn-over shaker operating at 52 rpm for variable
lengths of time. For adsorption–desorption isotherms, duplicate
samples containing 30 mg of adsorbent for Nicosulfuron and
20 mg for Mecoprop-P were equilibrated by shaking with 30 mL
of 0.5–1.5 mM Nicosulfuron and 0.5–3.0 mM Mecoprop-P at
room temperature for 24 h [26]. The resulting supernatant was
centrifuged and separated to determine the concentrations of
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R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221
Nicosulfuron and Mecoprop-P by UV–Vis spectroscopy at 239 and
285 nm, respectively, on a Perkin Elmer Lambda 11 UV–Vis
spectrophotometer.
Desorption was measured at each equilibrium point in the
adsorption curve by replacing a small portion of supernatant
(15 mL for Nicosulfuron and 7.5 mL for Mecoprop-P) with an identical volume of distilled water. After shaking for 24 h, the amount
of pesticide desorbed was calculated as described above. This procedure is described in detail elsewhere [27]. Nicosulfuron and
Mecoprop-P adsorption–desorption isotherms were fitted to the
Langmuir [28], Freundlich [29] and Dubinin–Radushkevich (D–R)
[30] equations:
Ce
1
Ce
¼
þ
Cs Cm L Cm
ð1Þ
C s ¼ K f C s1=nf
ð2Þ
ln C s ¼ ln qm K e2
ð3Þ
where Cs (mmol g1) is the amount of pesticide adsorbed per unit
mass of adsorbent at the equilibrium concentration Ce (mmol L1);
Cm is the adsorbent monolayer capacity (mmol g1); L is the Langmuir adsorption constant (L mmol1), which is related to the
adsorption energy; Kf (mmol1nf Lnf g1) and nf are the Freundlich
parameters incorporating all factors affecting adsorption; qm is the
theoretical adsorption capacity of the adsorbent (mmol g1); K
(mmol2 kJ2) is a constant related to the adsorption energy; and
e, which is equal to RTln [1+(1/Ce)], is the Polanyi adsorption potential, R being the universal gas constant and T (K) the temperature.
on the results, the adsorption of Nicosulfuron was not influenced
by pH and that of Mecoprop-P only slightly. This led us to adopt
pH 5 (i.e. that of the pesticide solution) for subsequent adsorption
tests on Nicosulfuron and pH 7 (i.e. that of maximum adsorption)
for those on Mecoprop-P.
Adsorption measurements of the pesticides at different initial
concentrations made after a variable contact time were used to calculate their adsorption rate constant, Kad, and kinetic order. In all
cases, more than 50% of the adsorbed pesticide amount occurs
within 5 h, and the equilibrium was reached within 24 h in both
pesticides (see Fig. 1). Both exhibited increasing adsorption with
increase in their initial concentration.
Adsorption of Nicosulfuron and Mecoprop-P was fast at the
start and slowed down as equilibrium was approached. The kinetics of adsorption of the pesticides on HT500 was examined via the
kinetic order and an intraparticle diffusion kinetic model based on
the following equations was used to fit the experimental results.
The adsorption kinetics of the pesticides was elucidated by substituting the initial concentrations of Fig. 1 into the pseudo first-order
rate equation of Largergren [31,32]:
logðC se C s Þ ¼ log C se K ad
t
2:303
ð4Þ
where Cse is the maximum amount of pesticide adsorbed and Cs that
adsorbed at time t, both in mmol g1. Plots of log (Cse–Cs) versus t
spanning different time intervals were all almost linear – the fit
being much better for Mecoprop-P, however-, which testifies to
the applicability of the Largergren first-order rate equation here. Table 1 shows the adsorption rate constant, Kad, for the two pesticides
as calculated from the slope of each plot.
2.4. Effects of competing anions
2
2
The effects of competing anions ðNO
3 ; Cl ;SO4 ; CO3 and
2
HPO4 Þ on the adsorption behavior of Nicosulfuron or MecopropP were examined by mixing variable volumes of 0.5 M solutions
of the sodium salts of the anions with 50 mL of a 1 mM pesticide
solution in a total volume of 100 mL. The anion/pesticide mole ratio
ranged from 0 to 100. Duplicate volumes of 30 mL of these solutions
were added to 30 mg of adsorbent. For desorption tests, duplicate
volumes of 30 mL of the anion solutions were added to 30 mg of
adsorbent containing Nicosulfuron or Mecoprop-P in proportions
such that the resulting anion/adsorbed pesticide mole ratio ranged
from 0 to 100. The solutions were shaken at 52 rpm for 24 h, and the
pesticide concentrations in the supernatant then determined by
UV–Vis spectroscopy.
2.5. Adsorbent regeneration and recyclability
Duplicate amounts of 30 mg of adsorbent for Nicosulfuron and
20 mg for Mecoprop-P were equilibrated by shaking in 30 mL of
solutions containing an initial pesticide concentration of 0.5 mM
at room temperature for 24 h. Then, the supernatant was centrifuged and separated to determine the concentration of Nicosulfuron and Mecoprop-P by UV–Vis spectroscopy at 239 and 285 nm,
respectively, the LDH-pesticide complex being calcined at 500 °C
for 2 h to recover the adsorbent: HT500. The adsorbent was subjected to a total of four successive adsorption–calcination cycles.
Fig. 1. Adsorption kinetics of 0.5 mM Nicosulfuron (A) and 1 mM Mecoprop-P (B)
on HT500.
Table 1
Rate constants (Kad) obtained from adsorption curve for HT500 with different initial
concentrations of pesticide.
Pesticide
Initial concentration
(mmol/L)
Slope
Intercept
Kad (h1)
R2
Nicosulfuron
0.25
0.50
1.00
0.139
0.130
0.137
2.396
2.384
2.722
0.320
0.300
0.317
0.982
0.972
0.992
Mecoprop-P
1.00
2.00
3.00
0.125
0.121
0.124
3.015
2.930
2.968
0.288
0.278
0.286
0.990
0.998
0.995
3. Results and discussion
3.1. Adsorption–desorption tests
3.1.1. Effect of pH and adsorption kinetics
The influence of pH on Nicosulfuron and Mecoprop-P adsorption on HT500 was studied at an initial value of 5, 7 or 9. Based
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R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221
Table 2
Intraparticle diffusion rate obtained from Weber–Morris equation for different initial
concentrations of pesticides.
Pesticide
Initial
concentration
(mmol/L)
Slope
Intercept
Kp (h1)
R2
Nicosulfuron
0.25
0.50
1.00
45.242
53.211
105.721
41.696
34.395
91.729
45.242
53.211
105.721
0.998
0.960
0.970
Mecoprop-P
1.00
2.00
3.00
219.323
202.396
217.94
6.911
234.62
373.53
219.323
202.396
217.94
0.974
0.914
0.914
Fig. 3. D–R adsorption isotherms for Nicosulfuron and Mecoprop-P on HT500.
Fig. 2. Adsorption and desorption isotherms for Nicosulfuron (A) and Mecoprop-P
(B) on calcined hydrotalcite.
The mechanism behind the adsorption of the two pesticides
was elucidated by analyzing the kinetic results in the light of the
Morris–Weber equation for intraparticle diffusion [32–34]:
C s ¼ K p t 1=2 þ C
ð5Þ
where Cs is the amount of pesticide adsorbed at time t (mmol g1),
Kp the intraparticle diffusion rate constant (mmol g1 h1/2), t the
agitation time (h) and C the intercept.
At least two different types of adsorption mechanisms were to
be expected from the structural characteristics of the adsorbent
and the use of stirring. One was transfer of the pesticides from
the bulk solution into HT500 pores and the other adsorption at
the outer surface of the hydrotalcite. The adsorption rate-determining step could therefore be either adsorption or intraparticle
diffusion [35]. Based on the applied model, a plot of the amount
of pesticide adsorbed, Cs, against the square root of time, t1/2,
should be linear if intraparticle diffusion was involved in the process; also, if the lines passed through the origin, then intraparticle
diffusion would be the rate-determining step [36,37]. The correlation coefficients R2 of Table 2, obtained at Cs values spanning up to
16 h, suggest that the adsorption of Nicosulfuron and Mecoprop-P
on calcined hydrotalcite (HT500) may involve intraparticle diffusion; however, none of the curves passed through the origin, so
intraparticle diffusion cannot have been the rate-determining step
and other factors may have operated simultaneously to govern the
adsorption process. Increasing the contact time to 24 h led to poor
correlation coefficients for the intraparticle diffusion model. Therefore, adsorption of Nicosulfuron and Mecoprop-P on calcined
hydrotalcite (HT500) may be followed by intraparticle diffusion
for up to 16 h.
Kp was calculated from the slope of a plot of Cs versus t1/2 for
each pesticide (see Table 2). Whereas the values for Mecoprop-P
were all very similar, those for Nicosulfuron increased with
increasing pesticide concentration. Removing nitrate with a Ca/Al
chloride hydrotalcite [34] was previously found to result in a similar increase in Kp with increasing concentration. The dissimilar
behavior of Nicosulfuron and Mecoprop-P in this respect can be ascribed to the greater steric hindrance in the former – a result of its
being a bulkier molecule.
Table 3
Freundlich and Langmuir model parameters for Nicosulfuron and Mecoprop-P adsorption onto HT500.
Freundlich
HT500-Nicosulfuron
HT500-Mecoprop-P
Langmuir
Dubini–Radushkevich
nf
Kf
R2
Cm (mmol/g1)
L
R2
qm(mmol g1)
K(mmol2 kJ2)
R2
0.23 ± 0.08
0.36 ± 0.09
0.08–0.24
0.05–0.15
0.77
0.85
0.69 ± 0.01
1.4 ± 0.1
1.7 ± 2.3
4±9
0.99
0.99
2.74
3.48
3.64 103
4.07 103
0.98
0.94
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R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221
Fig. 6. Recyclability of HT500 containing adsorbed Nicosulfuron (A) and MecopropP (B).
Fig. 4. Adsorption of 0.5 mM Nicosulfuron (A) and Mecoprop-P (B) by HT500 as a
function of the anion/pesticide mole ratio.
thermodynamic parameters such as the energy of adsorption and
the nature of host–guest interactions. Both isotherms in Fig. 2
are of the L-type in the classification of Giles [38], therefore
HT500 has a high affinity for the two pesticides. An L-shaped isotherm suggests that, as substrate sites are filled, it becomes
increasingly difficult for a solute molecule to find a vacant site.
Based on the adsorption data, the uptake of pesticide per gram of
adsorbent, Cs, was much greater for Mecoprop-P (1.21 mmol g1)
than it was for Nicosulfuron (0.64 mmol g1), probably as a result
of structural, size and ionic differences between the two pesticides.
The Freundlich and Langmuir equations were used to model the
adsorption isotherms in order to identify the specific factors governing adsorption of the two pesticides. Based on the R2 values of
Table 3, the Langmuir equation fitted the experimental data much
better than the Freundlich equation (see inset of Fig. 2). For this
reason, the adsorption isotherms are discussed in terms of the
Langmuir equation (Eq. (1)) here. The adsorption efficiency was assessed via a dimensionless separation factor R [39,40] that was calculated from:
R¼
Fig. 5. Release of Nicosulfuron (A) or Mecoprop-P (B) adsorbed on HT500 by various
anions at variable anion/adsorbed pesticide ratios (0.36 and 0.47 mmol g1
Nicosulfuron or Mecoprop-P adsorbed per gram of HT500).
3.1.2. Adsorption–desorption isotherms
Fig. 2 shows the adsorption–desorption isotherms for Nicosulfuron and Mecoprop-P adsorbed on HT500. Adsorption isotherms
are useful to determine the adsorption capabilities of adsorbents,
1
1 þ LC i
ð6Þ
where Ci is the initial pesticide concentration. R values from 0 to 1
denote favorable adsorption. The R values obtained at Nicosulfuron
initial concentrations of 0.25, 0.5, 0.75, 1 and 1.5 mmol L1 fell in
the range (31.0–7.02) 102; likewise, those for Mecoprop-P at
an initial concentration of 0.5, 1, 1.5 2 or 3 mmol L–1 spanned the
range (20.0–7.69) 102. These values are suggestive of favorable
adsorption, especially at low adsorbate concentrations.
We used the Dubinin–Radushkevich (D–R) equation (Eq. (3)) to
discriminate between physical and chemical adsorption. Parameters K and qm were calculated from the slope and intercept, respectively, of a plot of ln Cs against e2 (Fig. 3). The qm values thus
obtained (Table 3) were consistent with the increased adsorption
of Mecoprop-P relative Nicosulfuron noted earlier (Fig. 2). Constant
K provides a measure of the mean free energy of adsorption, E,
which can be calculated from [39,41]
E ¼ ð2KÞ1=2
ð7Þ
R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221
219
presence. The effect was more marked on Mecoprop-P than on Nicosulfuron. The adverse effect on Nicosulfuron adsorption de
2
2
creased in the sequence HPO2
and
4 > CO3 NO3 Cl > SO4
2
that on Mecoprop-P in the sequence HPO4 > CO2
3 2
both with anion/pesticide ratios higher than
NO
3 Cl SO4
influenced the adsorption of both
20. Therefore, HPO2
4
pesticides more markedly than did CO3 2 and SO2
4 . This is
consistent with the sequences proposed by Forano et al. [6]
2
2
2
and CO2
(HPO2
4 > CrO4 > SO4
3 > SO4 > OH > F > Cl > Br >
NO3 > I) from previous results of Miyata [11] and Yamaoka
et al. [42]. However, You et al. [10] found SO2
4 to affect the adsorption of dicamba on a calcined hydrotalcite more markedly than did
2
HPO2
4 or CO3 .
Finally, it is worth noting the little influence of SO2
4 on Nicosulfuron relative to other anions, and also the – surprisingly – extreon Mecoprop-P adsorption at an
mely low interference of CO2
3
anion/pesticide ratio of 10 (see Fig. 4).
Fig. 7. TEM images of HT500 before cycling (a) and after the 3rd cycle (b).
The mean free energy provides information about adsorption
mechanisms. Thus, adsorption is physical when 0 < E < 8 kJ/mol,
due to ion-exchange if 8 < E < 16 kJ mol1 and chemical if
20 < E < 40 kJ/mol. Our values (11.72 kJ mol1 for Nicosulfuron
and 11.08 kJ mol1 for Mecoprop-P) suggest that, as confirmed by
the IR spectra for the adsorption products, the two pesticides were
adsorbed by ion-exchange.
Finally, the desorption curves closely matched the adsorption
curves for both pesticides (Fig. 2), thus testifying to the high
reversibility of the adsorption–desorption process.
3.1.3. Effects of competing anions on Nicosulfuron and Mecoprop-P
adsorption
The effect of competing anions on the adsorption of Nicosulfuron and Mecoprop-P on HT500 is illustrated in Fig. 4. With an initial concentration of 0.5 mM of both pesticides, HT500 adsorbed
0.36 mmol g1 Nicosulfuron and 0.47 mmol g–1 Mecoprop-P in
the absence of competing anions, but smaller amounts in their
3.1.4. Retention of the pesticides in the presence of various anions
In order to assess the stability of the host–guest complex, we
measured the retention of the pesticides by the adsorbent in the
presence of various anions. Desorption isotherms obtained by adding distilled water (Fig. 2) exhibited a high reversibility. The influence of different anions on the release of Nicosulfuron and
Mecoprop-P previously adsorbed on HT500 is illustrated in Fig. 5.
Based on the results, both pesticides were desorbed in the presence
of the studied anions, albeit to different extent. The influence of
increasing concentrations of dissolved anions was also studied.
Goswamee et al. [16] found anions stereochemically suitable for
inclusion into LDH interlayers to boost the release of existing interlayer anions. Our results suggest that Nicosulfuron and MecopropP adsorbed on HT500 can be desorbed, whether partially or fully,
2
2
into solutions containing HPO2
4 , CO3 , NO3 , Cl or SO4 ; therefore,
the nature of the ionic species present in solution affects the
desorption behavior of the pesticides. Also, increasing the anion/
adsorbed pesticide mole ratio increased the amount of pesticide
removed. With Mecoprop-P, a ratio of 100/1 resulted in nearly
quantitative removal of the host–guest complex (viz. 100%, 100%,
2
2
98%, 93% and 87% for CO2
3 ; NO3 ; Cl ; SO4 and HPO4 , respectively). Removal of the host–guest complex for Nicosulfuron was
less efficient and never complete, however; thus, it peaked at only
46% with CO2
3 .
The effect of the anions was different when they competed for
adsorption on HT500 (Fig. 4) and when they acted by removing
the pesticides from the host–guest complex (Fig. 5). These results
testify to the ability of the hydrotalcite to retain Nicosulfuron
and Mecoprop-P, and to remove the two pesticides from contaminated water in the presence of other anions. The high capacity of
CO2
3 and NO3 to recover Mecoprop-P suggests that the adsorbent
can be reused.
3.1.5. Adsorbent recyclability
In order to assess its recyclability, the adsorbent was subjected
to four adsorption–desorption cycles on pristine HT500. As can be
seen from Fig. 6, adsorption of the two pesticides decreased after
each cycle. A similar trend was previously observed by Bruna
et al. [43] in successive adsorption–desorption cycles of carbetamide and metamitron on organohydrotalcites. Our results can be
ascribed to erosion of the adsorbent particles as seen in the TEM
images after the third adsorption cycle (Fig. 7b) relative to the pristine HT500 (Fig. 7a).
3.2. Characterization of adsorption products by X-ray diffraction and
FT-IR spectroscopies
Fig. 8 shows the diffraction patterns for the adsorbent (HT500)
and adsorption products, namely: HT500-Nicosulfuron (0.03 g of
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R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221
Fig. 8. XRD patterns for (a) HT500, (b) HT500-Nicosulfuron and (c) HT500-Mecoprop-P. (Diffraction peaks due to the Al sample holder).
HT500 and 30 mL of 0.5 mM Nicosulfuron) and HT500-Mecoprop-P
(0.02 g of HT500 and 30 mL of 1 mM Mecoprop-P). The patterns of
Fig. 8a correspond to the hydrotalcite upon calcination at 500 °C
and exhibit the typical peaks for an Al–Mg mixed oxide [44]. Once
Nicosulfuron and Mecoprop-P were adsorbed (Fig. 8b and c,
respectively), the XRD patterns were consistent with regeneration
of the hydrotalcite structure; however, d00l exhibited no change
with respect to the hydrotalcite with carbonate as interlayer anion
(d003 = 7.8 Å). By contrast, Khan et al. [25] observed an increase in
interlayer spacing (d003 = 22.54 Å) in a Mecoprop-P intercalated
LDH prepared from a very high concentration of Mecoprop dissolved in 50% methanol – where the pesticide is much more readily
soluble. Under our experimental conditions, the greatest pesticide
adsorption amounted to only 20% of the AEC of HT500 for Mecoprop-P and 10% for Nicosulfuron; the pesticides probably adopt a
flat configuration in the interlayer, on the outer surface and at
the edges of hydrotalcite particles. Based on the d003 values derived
from the X-ray diffraction patterns and FT-IR data (Fig. 9), most
interlayer sites must have been occupied by CO2
3 or OH ions. Similar results were previously obtained with other pesticides [45].
Adsorption of Nicosulfuron and Mecoprop-P on the LDH was
further confirmed by the FT-IR spectra of Fig. 9. The spectrum for
the adsorbent, HT500, exhibited a broad band at 3500 cm1 corresponding to vibrations of residual OH groups in the layers in
addition to the bending vibration of water at 1638 cm1 and a band
at 1383 cm1 that was assigned to stretching vibrations in residual
or surface carbonate ions [46].
The spectrum for the product corresponding to the last point in
the adsorption isotherm, HT500-Mecoprop-P, exhibited the
breathing bands for the benzene ring at 1496 and 1243 cm1, thus
confirming the presence of the pesticide. The absence of a band at
1700 cm1 for unionized carboxyl groups in Mecoprop-P from the
spectrum for the complex, and the presence of antisymmetric and
symmetric carboxylate vibration peaks at 1600 and 1376 cm1
[47], confirmed that the pesticide was adsorbed in anionic form
on HT500, as expected from the LDH regeneration mechanism
established from the X-ray diffraction patterns (Fig. 8c).
The presence of Nicosulfuron among the adsorption products
was apparent from the characteristic bands at 1492 and
Fig. 9. FT-IR spectra for (A) HT500, (B) Nicosulfuron, (C) HT-Nicosulfuron, (D)
Mecoprop-P and (E) HT-Mecoprop-P.
1375 cm1 for C@C bonds in aromatic rings, and by that at
1028 cm1, due to S@O vibrations. However, the band at
1718 cm1 for mC@O in secondary amides was absent and a new
band at 1608 cm–1 due to mC@O in tertiary amides was observed instead [47]. This suggests that the pesticide may intercalate into the
LDH interlayer via the deprotonated nitrogen atom in the secondary amide.
4. Conclusions
The adsorption–desorption behavior of the pesticides Nicosulfuron and Mecoprop-P pesticides on calcined hydrotalcite
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R. Otero et al. / Chemical Engineering Journal 221 (2013) 214–221
(HT500) was studied. Based on kinetic data, their adsorption involves intraparticle diffusion. Only with Nicosulfuron, however,
did the intraparticle diffusion rate constant increase with increasing concentration by effect of the dissimilar size of the pesticides.
The adsorption isotherms for both pesticides were L-shaped, and
application of the Langmuir and Dubini–Radushkevich equations
revealed that adsorption was favorable and occurred by ion-exchange. The maximum amount of Mecoprop-P adsorbed was twice
than of Nicosulfuron, which is consistent with the structural, size
and ionization differences between the two pesticides.
Competing anions (particularly HPO2
4 ) had an adverse effect on
Nicosulfuron and Mecoprop-P adsorption. On the other hand,
extraction of the pesticides from the host–guest complex was
2
especially effective with CO2
3 . The ability of CO3 to recover large
amounts of Mecoprop-P suggests that the adsorbent can be
recycled.
Regeneration tests involving four adsorption–desorption cycles
showed that HT500 can be reused several times to remove Nicosulfuron and Mecoprop-P, albeit with gradually decreasing efficiency.
The fact that the X-ray diffraction patterns were nearly identical
with those for hydrotalcite intercalated with carbonate ions suggests that the pesticides adopt a flat arrangement in the interlayer
and, also, that they occupy outer surfaces and edges of the
particles.
Acknowledgments
This work was partly supported by Spain’s Ministry of Science
and Innovation (Projects AGL2008-04031-CO2-2 and CTM201125325), and cofunded by FEDER and the Andalusian Regional Government (Junta de Andalucía, Research Group FQM-214). R. Otero
also acknowledges funding from MEC-ERDF. The authors wish to
thank the staff at the Electron Microscopy and Elemental Analysis
units of the Central Research Support Service (SCAI) of the University of Córdoba for technical assistance.
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Chemical Engineering Journal 251 (2014) 92–101
Contents lists available at ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Mesoporous phenolic resin and mesoporous carbon for the removal
of S-Metolachlor and Bentazon herbicides
Rocío Otero a, Dolores Esquivel b, María A. Ulibarri a, Francisco J. Romero-Salguero c,
Pascal Van Der Voort b,⇑, José M. Fernández a,⇑
a
Departamento de Química Inorgánica e Ingeniería Química, Instituto Universitario de Química Fina y Nanoquímica (IUQFN), Campus de Rabanales, Universidad de Córdoba,
Campus de Excelencia Internacional Agroalimentario (CeiA3), Córdoba, Spain
b
Department of Inorganic and Physical Chemistry, Center for Ordered Materials, Organometallics and Catalysis (COMOC), Ghent University, Krijgslaan 281, Building S3,
9000 Ghent, Belgium
c
Departamento de Química Orgánica, Instituto Universitario de Química Fina y Nanoquímica (IUQFN), Campus de Rabanales, Universidad de Córdoba,
Campus de Excelencia Internacional Agroalimentario (CeiA3), Córdoba, Spain
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Mesoporous carbon is an efficient
adsorbent of S-Metolachlor and
Bentazon.
Mesoporous carbon can be reused
with a small loss of adsorption
capacity.
Commercial carbon undergoes the
highest loss of material during the
regeneration.
a r t i c l e
i n f o
Article history:
Received 8 February 2014
Received in revised form 5 April 2014
Accepted 10 April 2014
Available online 24 April 2014
Keywords:
S-Metolachlor
Bentazon
Mesoporous phenolic resin
Mesoporous carbon
Water treatment
a b s t r a c t
Two mesoporous materials, i.e., a mesoporous phenolic resin and a mesoporous carbon, were synthesized
following a soft template method to test the removal of two pesticides, Bentazon and S-Metolachlor. The
adsorbents were characterized by X-ray diffraction analysis, nitrogen adsorption–desorption isotherms,
thermogravimetric analysis, transmission electron microscopy, particle size measurement and XPS spectroscopy. Bentazon and S-Metolachlor adsorption kinetics and isotherm studies were carried out at pH 2
and 4, respectively, on these mesoporous materials as well as on a reference commercial carbon. Freundlich, Langmuir, Dubinin–Raduskevich and Temkin models were applied to describe the adsorption behavior with the Langmuir model showing the best fit. The regeneration of the adsorbents was carried out by
calcination under nitrogen atmosphere and the results indicated that the regeneration of the mesoporous
carbon was more efficient than that of the mesoporous phenolic resin and the commercial carbon after
being reused several times. Another disadvantage of commercial carbon was the great loss of material
that it experienced after several adsorption–desorption cycles.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
⇑ Corresponding authors. Tel.: +32 92644442; fax: +32 92644983 (P. Van Der
Voort). Tel.: +34 957218648; fax: +34 957580644 (J.M. Fernández).
E-mail addresses: [email protected] (P. Van Der Voort), um1feroj@
uco.es (J.M. Fernández).
http://dx.doi.org/10.1016/j.cej.2014.04.038
1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
Currently, the pollution caused by pesticides has greatly
increased due to their widespread use in agriculture. Pesticides
are highly noxious, sometimes non-biodegradable and very mobile
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R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101
throughout the environment [1]. Therefore, many researchers are
studying the removal of these toxic pollutants from aqueous solutions. Photocatalysis [2–4], adsorption [5–7] and electrolysis [8–
10] are some processes involved in the removal of these pollutants.
The main advantages of adsorption compared to other techniques
are its low cost, high removal efficiency and easy operation.
Activated carbons and silica gels have been considered as
excellent adsorbents for many years. These materials are not
suitable as adsorbents of large molecules because they are mainly
microporous [11]. Recently, new mesoporous materials that
show promising results for the adsorption of large pollutants have
been developed [12–17]. Ordered mesoporous materials possess
tunable and uniform pore sizes, large surface areas and large pore
volumes.
Among them, the ordered mesoporous polymer resin and mesoporous carbon (MPR and MC) are relatively new materials that
combine the high porosity of mesoporous materials with the physicochemical properties of organic polymers and carbons, respectively [18]. MC is an excellent adsorbent and therefore, as carbon
is inert, stable, hydrophobic, light and presents a high affinity
towards organic pollutants. Ordered mesoporous carbons were
firstly synthesized by a hard template method using mesoporous
silica [19–21]. However, this synthesis cannot be used for large
scale production because it is time-consuming and expensive.
Later, a direct synthesis method using phenolic resin as a carbon
precursor was developed [22–24].
S-Metolachlor is a selective herbicide used in several crops. It is
quite mobile and although it is mainly found in surface water, it
can contaminate groundwater. Several authors have investigated
the adsorption of this herbicide on different materials, such as activated carbon [25], soil [26], organohydrotalcites [5] or periodic
mesoporous organosilicas (PMOs) [6]. Bentazon is a contact postemergency herbicide widely used in agriculture, which has a great
mobility in soil and is moderately persistent in water. That is why
previous papers have reported the removal of Bentazon mainly
using activated carbon as adsorbent [1,27,28]. The maximum concentration of S-Metolachlor and Bentazon admitted by the World
Health Organization (WHO) in drinking water are 0.01 mg/L and
0.03 mg/L, respectively.
Unlike activated carbons, mesoporous phenolic resins and mesoporous carbons have been hardly used as adsorbents of pesticides.
Herein we have synthesized two mesoporous materials (MPR and
MC) to be tested as regenerable adsorbents for removing S-Metolachlor and Bentazon. MPR has been prepared by the EISA method
(evaporation induced self-assembly) [29,30], using resorcinol and
formaldehyde in the presence of surfactant F127 (EO106–PO70–
EO106) under acid conditions. MC was synthesized by calcination
of MPR. For comparison, a commercially available activated carbon
(CC) was also used as adsorbent. The adsorbents were characterized by several structural and surface techniques. Different experiments have been carried out to study the influence of different
factors, such as pH and contact time, adsorption/desorption behavior and regeneration of the adsorbents. Finally, adsorption experiments at different temperatures have been undertaken in order to
determine thermodynamic parameters.
[(1S)-2-methoxy-1-methylethyl]acetamide) were purchased from
Sigma Aldrich. Some of their properties are included in Table 1.
2.2. Synthesis of adsorbents
The synthesis of phenolic resin and mesoporous carbon was
carried out using a soft-template method [18]. For that, 9 ml of
3 M HCl were added to a solution of 2.2 g of triblock copolymer
Pluronic F127 and 2.2 g of resorcinol in 9 ml of ethanol. After stirring for at least 20 min [31], a formaldehyde solution (2 ml,
36 wt%) was slowly added to it. Then, the mixture was stirred for
20 min. The resultant solution was transferred to a plate to evaporate ethanol. Afterwards, the product was cured in an oven at 60 °C
overnight. Finally, the solid was calcined at 380 °C under inert
atmosphere to remove the template. The resulting material, a mesoporous phenolic resin, was designed as MPR. Amorphous mesoporous carbon, designed as MC, was obtained by calcination of
MPR in a tubular furnace under inert atmosphere at 800 °C.
2.3. Characterization
Mesoporous phenolic resin and mesoporous carbon were characterized by different techniques. Powder X-ray diffraction (XRD)
patterns were recorded with a Thermo Scientific ARL X’TRA Powder
X-ray Diffraction System (radiation Cu Ka generated by 45 kV and
44 mA, with slits of 2, 4, 1.5 and 0.2 for the divergence, scatter,
receiving scatter and receiving slit, respectively, at 0.25° 2h min1).
Nitrogen adsorption–desorption isotherms were obtained on a Belsorp-mini II gas analyser. Prior to the measurements, the samples
were degassed at 120 °C for 24 h to remove adsorbed water. Thermogravimetric curves were recorded on a Setaram Setsys Evolution 16/18 apparatus undernitrogen at a heating rate of 5 °C/min.
Microstructural characterization of the materials was carried out
using JEOL 1400 TEM and JEOL 2010 TEM equipments. Particle
sizes were measured in a Mastersizer S analyser (Malvern Instruments) using ethanol as dispersant. The samples were sonicated
for 10 min before the analysis. XPS spectra were recorded with a
SPECS Phoibos HAS 3500 150 MCD. The residual pressure in the
analysis chamber was 5 109 Pa. The X-ray source was generated
by a Mg anode (hm = 1253.6 eV) powered at 12 kV and with an
emission current of 25 mA. The powdered sample was pressed
and introduced into the spectrometer without previous thermal
treatment. They were outgassed overnight and analyzed at room
temperature. Accurate binding energies (BE) have been determined
with respect to the position of the C 1s peak at 284.9 eV. The surface atomic concentration ratios were calculated using sensitivity
factors from the Casa XPS element library. The potential zeta (f)
has been determined with a Zetasizer Nano ZS (Malvern Instruments), with a laser of 632.8 nm, coupled to a MPT-2 autotitrator.
Table 1
Properties of pesticides Bentazon and S-Metolachlor.
Pesticide
MW (g/mol)
S (mg/L)
pKa
Bentazon
Structure
240.3
570
3.3
S-Metolachlor
283.79
480
–
2. Experimental section
2.1. Chemicals
Pluronic127 (EO106PO70EO106), resorcinol and commercial
activated carbon were purchased from Sigma–Aldrich.
The pesticides used as adsorbates in the experiments, Bentazon
(3-(1-methylethyl)-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide) and S-Metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-
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R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101
2.4. Adsorption experiments
380 °C for 4 h; mesoporous carbon and commercial carbon were
treated under nitrogen at 400 °C for 4 h.
All adsorption experiments were carried out in duplicate by
using the batch equilibration technique at 25 ± 1 °C. Adsorption
of Bentazon (25–250 mg/L) and S-Metolachlor (125–400 mg/L)
were conducted by mixing 20 mg of adsorbent with 30 mL of pesticide solution and stirring continuously the mixture for 24 h. The
suspension was centrifuged and the concentration of Bentazon and
S-Metolachlor were determined by UV–Vis spectroscopy at 232 nm
and 265 nm, respectively, on a Perkin Elmer Lambda 11 UV–Vis
spectrophotometer.
The adsorption kinetics were investigated by three models, the
Lagergren pseudo-first-order model [32], the pseudo-secondorder-model [33] and intraparticle diffusion model [34].
logðC s C st Þ ¼ log C s k1
t
2:303
ð1Þ
t
1
1
¼
þ t
C st k2 C 2s C s
ð2Þ
C st ¼ kp t1=2 þ C
ð3Þ
where k1 is the pseudo first order rate constant for the adsorption
process (h1), Cs and Cst (mg g1) are the amounts of adsorbed pesticide at equilibrium and at time t (h), respectively, k2 is the rate
constant of pseudo-second-order adsorption (g mg1 h1) and kp
(mg g1 h1/2) is the intraparticle diffusion rate constant.
The equilibrium experimental adsorption data were fitted to
Freundlich [35], Langmuir [36], Temkin [37] and Dubinin–
Raduskevich [38] models and the estimated parameters calculated
from the fitting results are reported in Table 4.
The Freundlich model assumes that the adsorption occurs on a
heterogeneous surface. The logarithmic equilibrium expression of
this model is:
log C s ¼ log K f þ nf log C e
1nf
nf
ð4Þ
1
where Kf (mg
L g ) and nf are the Freundlich constants indicative of adsorption capacity and adsorption intensity, respectively.
The Langmuir model describes the adsorbate–adsorbent interactions and is based on the assumption that the adsorption energy
is constant and independent of the surface coverage. Its expression
is:
1
1
1
¼
þ
Cs Cm Cm L Ce
ð5Þ
where Cm (mg g1) is the monolayer capacity of the adsorbent and L
(L mg1) is the Langmuir adsorption constant.
Temkin model describes the behavior of adsorption on heterogeneous surfaces. This model is based on the heat of adsorption
(due to the interactions between adsorbate and adsorbent). The
Temkin isotherm has generally been applied in the following form:
C s ¼ BT ln K T þ BT ln C e
3. Results and discussion
3.1. Characterization of the materials
The XRD-patterns of the mesoporous phenolic resin (MPR) and
the mesoporous carbon (MC) exhibited a low angle (1 0 0) peak
with d-spacing of 11.6 nm and 10.3 nm, respectively (Fig. 1). Additional peaks at higher incidence angles were ill-defined [39]. Their
mesoporous structures were confirmed by TEM images (Fig. 2). The
pore channels with hexagonal arrangement suggest an ordered
mesostructure. The size of these channels was around 10 nm (see
insets in Fig. 2b and c). However, no ordering was observed in
the commercial carbon (Fig. 2a). Moreover, the N2 adsorption–
desorption isotherms for MPR and MC were essentially of type IV
(Fig. 3) with a sharp capillary condensation step at P/P0 = 0.45–
0.57, which is typical of mesoporous materials as defined by IUPAC
[40]. Nevertheless, the commercial carbon exhibited a type I isotherm, which corresponds to a microporous material. MC had a
slightly higher surface area than MPR. This is probably due the formation of micropores as a consequence of the decomposition of the
phenolic framework of MPR to obtain the mesoporous carbon as
shown in the TGA curve between 500 and 750 °C (Fig. 4). The pore
size for MPR and MC was around 9 nm (inset in Fig. 3), in agreement with TEM. The main physicochemical properties of the two
mesoporous adsorbents (MPR and MC) along with those of the
commercial carbon are listed in Table 2.
The particle size distribution curves of MPR and MC are shown
in Fig. 5. MPR displayed a considerably large particle size with the
main peak centered at about 250 lm. Interestingly, the distribution of particle size for MC was wider and clearly shifted to lower
values (ca. 50 lm). This revealed the significant reduction of the
particle size upon carbonization of the MPR to MC. The curve for
CC was very broad and centered between 10 and 30 lm, showing
a significantly lower particle size than MPR and MC.
Surface analyses obtained by XPS for the three adsorbents are
depicted in Table 2. MPR possessed a significant amount of oxygen,
as expected from the composition of the polymer. The fraction of
oxygen decreased as a result of the transformation of MPR into
MC, which involves the deoxygenation of the phenolic groups,
among other reactions. CC also presented a considerably amount
of oxygen which was much higher than that of MC. The C 1s and
O 1s XP spectra are represented in Fig. 6. The large FWHM (full
ð6Þ
where KT and BT are constants related to adsorption capacity and
intensity of adsorption, respectively.
The isotherm of desorption was measured at each equilibrium
point in the adsorption curve by replacing 7.5 mL of supernatant
with an identical volume of distilled water. After shaking for
24 h, the amount of herbicide desorbed was calculated as indicated
above. This procedure is described in detail elsewhere [5].
The isotherms of adsorption were studied at three constant
temperatures: 10, 20 and 30 °C for Bentazon and S-Metolachlor.
To test the reversibility, the adsorbents were subjected to three
adsorption–desorption cycles. The desorption from mesoporous
phenolic resin was carried out by calcination under nitrogen at
Fig. 1. Powder X-ray diffraction patterns of mesoporous phenolic resin (MPR) and
mesoporous carbon (MC).
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R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101
Fig. 3. Nitrogen adsorption–desorption isotherms and the corresponding pore size
distributions (insets) for mesoporous phenolic resin (MPR), mesoporous carbon
(MC) and commercial carbon (CC).
Fig. 2. TEM images for commercial carbon (CC) (a), mesoporous phenolic resin
(MPR) (b), including a TEM image of high resolution (inset), and mesoporous carbon
(MC) (c), including a TEM image of high resolution (inset).
Fig. 4. Thermogravimetric curves for mesoporous phenolic resin (MPR) and
mesoporous carbon (MC).
width at half-maximum) values of the peaks (1.4–2.7 eV for C 1s
and 2.9–3.6 eV for O 1s) suggested the existence of different carbon
and oxygen species. Obviously, MPR exhibited various types of carbon atoms, namely CAC, C@C and CAO. MC displayed the narrowest C 1s peak due to the major presence of aromatic carbon atoms
in its framework whereas CC even showed additional small contributions at high binding energies indicative of the existence of C@O
species. Similarly, the O 1s spectra confirmed the contribution of a
higher variety of species in CC than in MPR and MR.
Table 2
Physicochemical properties of commercial carbon, mesoporous phenolic resin and
mesoporous carbon.
3.2. Adsorption kinetics of Bentazon and S-Metolachlor
The adsorption of Bentazon and S-Metolachlor on the mesoporous materials and the commercial carbon was studied at different
a
b
Sample
SBET
(m2 g1)
Smicropores
(m2 g1)
Vp a
(cm3 g1)
d100
(A)
Cb
(at.%)
Ob
(at.%)
CC
MPR
MC
688
503
526
406
160
294
0.41
0.63
0.49
–
116
103
85.93
82.92
95.10
14.07
14.08
4.90
Single point total adsorption.
C and O determined by XPS.
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R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101
Fig. 5. Particle size distribution curves for mesoporous phenolic resin (MPR),
mesoporous carbon (MC) and commercial carbon (CC).
pH values using 30 mL of either 50 mg/L Bentazon or 100 mg/L SMetolachlor and 20 mg of adsorbent (Fig. 7). An increase in the
pH caused a decrease in the adsorption of Bentazon probably due
to the enhancement of the electrostatic repulsion between the ionized pesticide and the adsorbent surface [1]. The point of zero
charge (pzc) was determined for the three materials and revealed
some differences among them (see Supporting information,
Fig. SI1). The pzc value indicates the pH required to give zero net
surface charge. The pHs were 5.3, 3.2 and 2.7 for commercial carbon, mesoporous carbon and phenolic resin, respectively. When
the pH is higher than the pzc, the surface is negatively charged.
As pH is increased, the surface is more negatively charged. On
the other hand, Bentazon is a weak acid with pKa of 3.3; and at
pH above the pKa, it exists predominantly in anionic form. As pH
is increased, the extent of dissociation of Bentazon molecules is
increased and so it becomes more negatively charged. As a result,
the equilibrium adsorption of Bentazon decreased with an increase
in the pH of the initial solution. This could be attributed to the
increased electrostatic repulsion between Bentazon ions and the
surface of the three materials [1]. Similar observations were made
for the adsorption of Bentazon onto AC cloth [41].
Given the aromatic structure of Bentazon, dispersion forces
should be expected between the p cloud of the adsorbents and
the aromatic ring of the adsorbate. Also, some specific localized
interactions might arise from the polar groups (i.e., sulfoxide and
carbonyl-type) [28]. The decrease in the amount adsorbed at
higher pH (anionic form predominant in solution) suggests a
weaker interaction of the surfaces with deprotonated (anionic)
Bentazon than with its neutral form, and consequently that the
Fig. 6. XPS analysis for mesoporous phenolic resin (MPR), mesoporous carbon (MC) and commercial carbon (CC).
Fig. 7. Influence of pH on the adsorption of S-Metolachlor and Bentazon on mesoporous phenolic resin (MPR), mesoporous carbon (MC) and commercial carbon (CC).
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R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101
Fig. 8. Adsorption kinetics of S-Metolachlor and Bentazon on mesoporous phenolic
resin (MPR), mesoporous carbon (MC) and commercial carbon (CC).
adsorption is dominated by dispersive interactions between the
pesticide and the adsorbent surface. Similar results on the pHdependence of Bentazon adsorption have been reported by other
authors [42,43]. In addition, the relative drop in the adsorption
was higher when the pH of the point of zero charge was lower
(see Figs. 7 and SI1). Thus, the phenolic resin was the most affected
by changes in the pH.
However, the adsorption of S-Metolachor on both mesoporous
materials, MPR and MC, and the commercial carbon, CC, was not
affected by the pH. This could be associated to the non-ionizable
nature of S-Metolachlor in the pH range under study. Indeed, the
adsorption of S-Metolachlor would occur through p–p interactions
between the aromatic ring of the pesticide and p-electrons of the
adsorbent structure [41].
The effect of the contact time at the pH of maximum adsorption
is depicted in Fig. 8. The adsorption of Bentazon and S-Metolachlor
on commercial carbon was fast, reaching the equilibrium in 2 h,
while their adsorption on the mesoporous materials was slower
(ca. 16 h). These differences in the adsorption kinetics might be
explained by the higher specific surface area (Table 2) and smaller
particle size (Fig. 5) of CC with respect to both mesoporous
materials.
The fitting parameters for the adsorption kinetic of Bentazon
and S-Metolachlor on MPR, MC and CC are listed in Table 3. The
correlation coefficients R2 for the pseudo-second-order kinetics
(0.999) were best fitted than for a pseudo-first-order kinetics
model (0.561–0.995). Similar results were reported by Salman
et al. using an activated carbon as adsorbent for two pesticides,
Bentazon and Carbofuran [1].
Following the Morris–Weber equation for intraparticle diffusion
(Eq. (3)), the plots of Cs versus t1/2 consist of straight lines for the
pesticide adsorption on the three adsorbents. The extrapolation
of these lines does not pass through the origin, suggesting that
intraparticle diffusion cannot be the rate-determining step and
other factors may have operated simultaneously to govern the
adsorption process [44,45].
Further information about the adsorption mechanism can be
obtained analyzing the adsorption isotherm for the systems under
study. The isotherms for the adsorption of Bentazon and S-Metolachlor on mesoporous phenolic resin and mesoporous carbon
(Fig. 9) were compared with that of commercial carbon (Fig. 10).
According to Giles classification [46], the isotherms for Bentazon
adsorption on MPR, MC and CC corresponded to a L2 type. Also,
S-Metolachor on CC exhibited a L2 type isotherm. However, the
adsorption of S-Metolachlor on MPR and MC gave a L4 type isotherm. In the case of Bentazon, the plateau was reached at Cs value
of 130.74 mg g1, 166.72 mg g1 and 218.04 mg g1 for MPR, MC
and CC, respectively. By contrast, the second plateau for S-Metolachlor on MPR and MC was not reached, indicating that the maximum adsorption capacity of the adsorbent was not achieved
under these conditions. Similar behavior was observed in the
adsorption of this pesticide on phenylene-bridged and ethylenebridged periodic mesoporous organosilicas [6]. The adsorption isotherm of S-Metolachlor on CC exhibited an ill-defined plateau with
a value of Cs = 340.17 mg g1.
The fitted parameters of adsorption were calculated from the
linear regressions of Eqs. (4)–(6). According to the regression coefficient values, the Langmuir model showed a much better fit than
the Freundlich and Temkin models (Table 4).
These models do not give any idea about the mechanism of
adsorption. To understand the adsorption type, experimental data
were fitted to the Dubinin–Radushkevich (D–R) model:
ln C s ¼ ln qm K e2
ð7Þ
where qm is the theoretical adsorption capacity of the adsorbent
(mg g1), K (mg2 kJ2) is the constant related to adsorption energy
and e = RT ln(1 + 1/Ce) is the Polanyi potential.
The adsorption energy (E) can be calculated using the D–R
equation and the following relationship has been used.
E ¼ ð2KÞ0:5
ð8Þ
The adsorption is physical in nature when E is between 0 and
8 kJ mol1; if it is 8 < E < 16 kJ mol1 the adsorption is due to
exchange of ions; and if the value of E is between
20 < E < 40 kJ mol1, a chemisorption occurs. In the case of S-Metolachlor, the values found in the present study for the three adsorbents were around 8 kJ mol1 whereas in the case of Bentazon
were between 11 and 13 kJ mol1. Consequently, the adsorption
of Bentazon could be explained by exchange of ions. In addition,
E values for S-Metolachlor were in the limit between physical
adsorption and exchange of ions and so it was not possible to
explain precisely the nature of the adsorption.
The desorption curves [5] above the adsorption curves evidenced the difficulty to remove S-Metolachlor and Bentazon from
Table 3
Kinetic parameters for the adsorption of Bentazon and S-Metolachlor onto mesoporous phenolic resin and mesoporous carbon compared with commercial carbon.
Pesticide
Adsorbent
Cs
exp
Pseudo first order model
1
K1 (h
)
C S1 (mg g
1
)
Pseudo second order model
2
R
K2 (g mg
1
Bentazon
MPR
MC
CC
53.20
67.82
71.23
0.39
0.34
0.23
48.97
43.71
0.287
0.992
0.991
0.561
0.018
0.017
4.9
S-Metolachlor
MPR
MC
CC
94.28
108.4
117.9
0.21
0.32
1.22
55.40
47.28
18.28
0.983
0.995
0.941
8.4 103
0.019
0.24
h
1
)
Intraparticle diffusion
1
2
Kp (mg g1 h1/2)
54.64
69.44
71.43
0.999
0.999
0.999
4.05
5.43
0.06
30.62
38.29
70.95
0.780
0.652
0.617
97.08
109.89
117.65
0.999
0.999
0.999
6.91
5.82
0.85
54.77
76.56
113.36
0.829
0.691
0.413
)
C (mg g1)
R2
R
C S2 (mg g
98
R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101
Fig. 9. Adsorption and desorption isotherms of S-Metolachlor and Bentazon on mesoporous phenolic resin (MPR) and mesoporous carbon (MC).
Fig. 10. Adsorption and desorption isotherms of S-Metolachlor and Bentazon on commercial carbon (CC).
Table 4
Freundlich, Langmuir, Dubinin–Raduskevich and Temkin model parameters for S-Metolachlor and Bentazon adsorption onto mesoporous phenolic resin and mesoporous carbon
compared with commercial carbon.
Pesticide
Adsorbent
Freundlich
Kf
Langmuir
nf
R2
Cm
Dubinin–Raduskevich
L
R2
qm
3
Temkin
R2
K
3
BT
KT
R2
Bentazon
MPR
MC
CC
15.19–13.31
42.56–33.96
105.86–76.35
0.44 ± 0.03
0.32 ± 0.03
0.26 ± 0.06
0.980
0.964
0.774
129.20
174.82
219.30
0.05
0.11
1.04
0.994
0.998
0.998
2.1 10
1.8 103
2.6 103
3.9 10
2.6 103
2.0 103
0.986
0.968
0.815
28.39
27.56
30.17
0.45
3.09
39.84
0.984
0.947
0.893
S-Metolachlor
MPR
MC
CC
4.12–0.81
9.34–4.33
13.83–4.87
0.90 ± 0.17
0.74 ± 0.08
0.72 ± 0.12
0.900
0.921
0.922
467.28
666.67
1434.7
3.3 103
4.2 103
2.1 103
0.922
0.989
0.978
0.015
0.012
0.014
9.2 103
7.4 103
7.3 103
0.884
0.958
0.937
133.97
133.04
138.02
0.03
0.05
0.06
0.748
0.887
0.960
99
R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101
Table 5
Thermodynamic parameters for S-Metolachlor and Bentazon adsorption onto mesoporous phenolic resin and mesoporous carbon compared with commercial carbon.
Pesticide
Adsorbent
T (°C)
S-Metolachlor
MPR
10
20
30
10
20
30
10
20
30
10
20
30
MC
Bentazon
MPR
MC
DS (kJ mol1 K1)
DH (kJ mol1)
DG (kJ mol1)
979
623
542
11494
10042
9728
0.018
21.16
0.056
6.10
16.20
15.67
15.86
22.00
22.44
23.13
82782
69192
60241
36003
131520
208029
0.054
11.42
K
the adsorbents. The presence of hysteresis (Figs. 9 and 10) indicates the irreversibility of the process in all cases.
3.3. The effect of temperature
In order to determine the thermodynamic parameters of the
adsorption of Bentazon and S-Metolachlor on the three adsorbents,
the effect of the temperature was studied. When the temperature
was increased from 10 to 30 °C the amount of adsorbed Bentazon
slightly increased, unlike S-Metolachlor whose adsorption
behavior was similar in the whole temperature range.
The change in standard free energy (DG°), enthalpy (DH°), and
entropy (DS°) of adsorption was calculated using the following
equations (Eqs. (9) and (10)) [47,48]:
DG ¼ DH T DS
ln K ¼ DS =R DH =RT
ð9Þ
ð10Þ
where K is the equilibrium constant, R the molar gas constant and T
the absolute temperature. The values of DH° and DS° were calculated from the slope and intercept of Van’t Hoff plots (ln KC versus
1/T) and are summarized in Table 5. The values of K have been
obtained from the values of KL from the Langmuir equation because
our isotherms were well fitted to this equation. K has been recalculated to become dimensionless by multiplying it by 106 [49]. DG°
values were calculated from Eq. (9). The negative DG° values indicate that the process is thermodynamically feasible and spontaneous. In the case of Bentazon, the DG° values were slighter higher
as the temperature increased indicating that the process was chemically controlled. By contrast, the behavior of S-Metolachlor showed
two different tendencies (physical and chemical).
0.31
62.87
26.64
27.15
27.72
24.68
28.71
30.85
information). In relation to CC, the lower decrease in the adsorption capacity for MC could be ascribed to its mesoporous character
and its lower fraction of micropores [50,51]. Similar behavior was
described by Suchithra et al. [52], who studied the regeneration
of hybrid mesoporous material compared with commercial carbon
for the removal of dyes and other organic compounds.
A great disadvantage of the commercial carbon compared to the
mesoporous materials in relation to their reusability was its significant loss of material after several adsorption–desorption cycles.
After three cycles, the losses of the commercial adsorbent were
48.2% and 47.1% for the adsorption of S-Metolachlor and Bentazon,
respectively, while it was less than 5% for the mesoporous materials (Fig. 11). After each adsorption cycle the material was recovered with a filter of 0.45 lm of pore size, which should retain all
particles according to the particle size distribution depicted in
Fig. 5. However, these results suggest that the commercial carbon
undergoes a shear degradation that decreases the particle size,
thus allowing the resulting smallest particles to pass through the
filter. The loss of this material involves serious concerns for its
practical application because it reduces drastically its durability,
unlike both mesoporous materials, which proven to be more resistant to friction.
3.4. Regeneration of the adsorbents
To test their reusability, the adsorbents were regenerated by
calcination under nitrogen atmosphere at 380 °C for mesoporous
phenolic resin and 400 °C for mesoporous carbon and commercial
carbon. The adsorbents were subjected to three adsorption–
desorption cycles (Fig. 11). The regeneration efficiencies of the
mesoporous materials were compared with the commercial carbon. In the third cycle, the adsorption capacity for S-Metolachlor
on MPR, MC and CC were 60.5%; 69.29% and 59.85%, respectively,
while for Bentazon were 77.33% for MPR; 90.22% for MC and
88.82% for CC. For both pesticides, the regeneration was better in
mesoporous carbon than in mesoporous phenolic resin and commercial carbon. The differences between both mesoporous materials can be associated to the presence of more defects in the
mesostructure of the phenolic resin than in that of the mesoporous
carbon after their regeneration (see Fig. SI2 in Supporting
Fig. 11. Adsorption–desorption cycles for S-Metolachlor and Bentazon. Adsorption
after first (1), second (2) and third (3) cycle. Weight lost after three adsorption–
desorption cycles (% loss).
100
R. Otero et al. / Chemical Engineering Journal 251 (2014) 92–101
4. Conclusions
Mesoporous phenolic resin and mesoporous carbon exhibited
good properties to be used as adsorbents in virtue of their high surface area and narrow mesopore distribution. The N2 adsorption–
desorption isotherms for MPR and MC were essentially of type
IV. The pore size of these materials was around 9–10 nm according
to the N2 adsorption and TEM results. We have examined a potentially practical use of these materials for the adsorption of S-Metolachlor and Bentazon. The adsorption of these pollutants varied
with the pH. The adsorption of Bentazon decreased when the pH
increased, probably due to the increase of the electrostatic repulsion between the pesticide ion and the adsorbent surface. By contrast, the adsorption of S-Metolachor was not affected by the pH.
The adsorption kinetics fitted better to a pseudo-second-order
kinetics model. Intraparticle diffusion was present in the adsorption process together with other factors. The isotherms for Bentazon on MPR, MC and CC and for S-Metolachor on CC were of L2
type while the adsorption of S-Metolachlor on MPR and MC exhibited a L4 type isotherm. In the case of Bentazon, a plateau was
reached for the three adsorbents. By contrast, the second plateau
for S-Metolachlor on MPR and MC was not reached, indicating that
the maximum capacity of these adsorbents was not achieved under
these conditions. In all cases the Langmuir model showed a much
better fit. The adsorption of Bentazon could be explained as
exchange of ions according to the Dubinin–Radushkevich model.
The presence of hysteresis in the desorption curves indicated irreversibility of the process in all cases.
The regeneration efficiency was better in mesoporous carbon
than in mesoporous phenolic resin and commercial carbon for both
pesticides. Another disadvantage of the commercial carbon compared to the mesoporous materials for their regeneration was the
great loss of material after successive adsorption–desorption
cycles.
Acknowledgements
The authors wish to acknowledge funding of this research by
Ministerio de Ciencia e Innovación (Project MAT2010-18778) and
Junta de Andalucía (Project P06-FQM-01741). D.E. is a postdoctoral
researcher of the FWO-Vlaanderen (Fund Scientific Research –
Flanders), Grant Number 3E10813W. R. Otero acknowledges
Ministry of Education and Science for a research and teaching
fellowship.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cej.2014.04.038.
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RSC Advances
PAPER
Cite this: RSC Adv., 2015, 5, 24158
Evaluation of different bridged organosilicas as
efficient adsorbents for the herbicide Smetolachlor†
Marı́a Isabel López,a Rocı́o Otero,b Dolores Esquivel,a César Jiménez-Sanchidrián,a
José Marı́a Fernández*b and Francisco José Romero-Salguero*a
Three groups of porous non-ordered bridged organosilicas and silicas have been synthesized using three
different surfactants of the Tween family as porogens. One group consisted of silica materials while the
other two were organosilicas, i.e., ethanesilicas and benzenesilicas, but differing within each group in their
textural properties. The adsorption of the herbicide S-metolachlor from an aqueous solution was then
compared on all solids. The composition of the walls, i.e., the bridging group, was the main factor
governing the adsorption capacity. Broadly, the uptake of the herbicide follows the sequence: benzenesilica
> ethanesilica > silica. However, pore volume also has a significant influence on adsorption. Thus, higher
capacities result from materials with larger pore volumes and with a larger size of the mesopores.
Moreover, surface area and pore size distributions also affect the adsorption behavior, even though their
influence seems to be less pronounced. Finally, a comparison between the porous non-ordered (organo)-
Received 11th February 2015
Accepted 26th February 2015
silicas and the corresponding periodic mesoporous (organo)silicas has revealed some interesting features
for the use of these materials as adsorbents. Benzenesilicas with a large pore volume and pores in the
DOI: 10.1039/c5ra02711j
micropore and/or large mesopore regions seemed to be the best adsorbents for adsorption in a single step
www.rsc.org/advances
whereas periodic mesoporous benzenesilicas were more appropriate for repetitive adsorptions.
1. Introduction
Bridged silsesquioxanes consist of organosilica networks
prepared by hydrolysis and condensation of bis-silanes of the
type (R0 O)3Si–R–Si(OR0 )3. Usually, the bridge R is a relatively
simple hydrocarbon unit such as a methylene, an ethylene or a
phenylene group. Nevertheless, these bridges can also be of a
high complexity.1 Advantageously, all these groups are homogenously distributed along their structure. A particular case of
these materials are periodic mesoporous organosilicas
(PMOs).2–4 They are synthesized in the presence of a surfactant
template, thus giving rise to materials with long-distance
ordering and pores in the meso range.5,6
Both periodic mesoporous silicas and organosilicas
have narrow pore size distributions in the mesoporous region
(2–50 nm) and high surface areas. These properties make them
a
Departamento de Quı́mica Orgánica, Instituto de Quı́mica Fina y Nanoquı́mica
(IUIQFN), Campus de Rabanales, Universidad de Córdoba, Campus de Excelencia
Internacional Agroalimentario (ceiA3), Córdoba, Spain. E-mail: [email protected];
Fax: +34 957212066; Tel: +34 957212065
b
Departamento de Quı́mica Inorgánica e Ingenierı́a Quı́mica, Instituto de Quı́mica Fina
y Nanoquı́mica (IUIQFN), Campus de Rabanales, Universidad de Córdoba, Campus de
Excelencia Internacional Agroalimentario (ceiA3), Córdoba, Spain. E-mail: um1feroj@
uco.es; Fax: +34 957580644; Tel: +34 957218648
† Electronic supplementary
10.1039/c5ra02711j
information
24158 | RSC Adv., 2015, 5, 24158–24166
(ESI)
available.
See
DOI:
good candidates for the adsorption of different molecules.
Castricum et al.7 described the application of microporous
hybrid silica materials with alkylene and aromatic bridging
groups of different sizes in membranes for the separation of gas
mixtures such as H2/N2, CO2/H2 and CO2/CH4. The improved
hydrothermal stability of hybrid structures has also provided
membranes useful for water desalination.8 In this sense, PMOs
have been proven to be excellent adsorbents for organic
compounds due to their improved hydrophobicity. Phenol
derivatives,9,10 trinitrotoluene11 and aromatic and polyaromatic
hydrocarbons12,13 have been adsorbed on aliphatic and aromatic
bridged PMOs. The combination of inorganic and organic
moieties in their framework represented a real advantage even
for the immobilization of enzymes.14
The presence of phenylene bridges is particularly advantageous when the adsorption of aromatic pollutants is concerned
due to the pi–pi interactions between these compounds and the
organosilica. Thus, a phenylene bridge led to a higher uptake of
several phenol derivatives than an ethylene unit.10 Similarly, the
herbicide S-metolachlor was retained in a higher extent by a
phenylene-bridged PMO than an ethylene-bridged PMO and a
mesoporous silica.15 The favorable presence of aromatic units in
the pore walls was also proven for the relatively bulky pesticide
mesosulfuron-methyl.16 In this case, the introduction of
sulfonic acid groups in the phenylene-bridged PMO enhanced
the adsorption capacity via acid–base interactions with the NH
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RSC Advances
group of the sulfonamide moiety in the pesticide. Very recently,
a divinylaniline bridged PMO was employed for the removal of
hexanal through a chemisorptive process.17
S-metolachlor is a pre- and post-emergence selective herbicide used to control grasses and some broad-leaved weeds in a
wide range of crops. It is widely distributed in the environment
and, in fact, 50.3% surface water sources and 2% of groundwater sources are contaminated with metolachlor in United
States.18,19 Metolachlor is also one of the most relevant
compounds contributing to Spanish groundwater contamination with concentrations above the environmental quality
standard dened in the Directives 2006/118/EC and 2008/105/
EC.20 A similar situation has been found in other countries such
as Greece21 and Italy22 in surface water and groundwater. Recent
studies suggest that exposure to metolachlor results in
decreased cell proliferation, growth and reproductive ability of
non-target organisms.23 The WHO Water Quality Criteria limits
its concentration in 10.0 mg L1.
Herein, we report the synthesis of various silicas and organosilicas with different composition, i.e., nature of bridges, and
textural properties using surfactants of the Tween family, which
were later used as adsorbents for the herbicide S-metolachlor.
The relationship between the physicochemical properties of
these materials and the pesticide uptake was analyzed and
compared to that of the corresponding periodic mesoporous
(organo)silicas.
2.
Materials and methods
2.1.
Materials
Three ordered materials, i.e., a periodic mesoporous silica
(PMS) and two organosilicas, namely ethane-PMO (E-PMO) and
benzene-PMO (Ph-PMO), were synthesized by using Brij-76 as
surfactant under acidic conditions according to previously
reported procedures.24
Non-ordered (organo)silicas were synthesized using tetraethyl orthosilicate (TEOS) (98%), 1,2-bis(triethoxysilyl)ethane
(97%) and 1,4-bis(triethoxysilyl)benzene (96%), which were
purchased from Acros, ABCR and Aldrich, respectively, as silica
sources. The surfactants employed, all commercially available
from Aldrich, were Tween 20 [polyoxyethylene(20)sorbitan
monolaurate], Tween 40 [polyoxyethylene(20)sorbitan monopalmitate] and Tween 60 [polyoxyethylene(20)sorbitan monostearate]. HCl (37%) and ethanol (96% v/v), used to remove the
surfactant, were provided by Panreac. NH4F ($98.0%), used as
catalyst, was acquired from Sigma-Aldrich. All these chemicals
were used without further purication.
The synthesis of the non-ordered materials was performed
following a similar procedure to that described in a
previous work for silicas.25 In brief, 184 mmol of TEOS or
92 mmol of organodisilane, i.e., 1,2-bis(triethoxysilyl)ethane or
1,4-bis(triethoxysilyl)benzene, were added dropwise to 184 mL
of a solution that was 0.1 M in the corresponding surfactant
(Tween 20, 40 or 60) and 0.02 M in the catalyst (NH4F). Aer
stirring for 24 h at room temperature, the formed precipitate
was aged for 5 days at room temperature. Finally, the resulting
This journal is © The Royal Society of Chemistry 2015
solid was ltered, washed with 300 mL of distilled water and
dried in the air.
The surfactant was removed by reuxing the as-synthesized
material in an HCl solution (1 mL of 37% HCl in 50 mL of
ethanol, per gram of solid) for 12 h. Subsequently, the solid was
ltered and washed with ethanol. This process was repeated
twice. The nal material was then dried in a drying chamber at
100 C under vacuum.
The non-ordered materials, i.e., silica, ethanesilica and
benzenesilica, were named as S, EOS and PhOS, followed by a
number denoting the surfactant used, that is, 20, 40 and 60 for
Tween 20, Tween 40 and Tween 60, respectively.
Analytical standard S-metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-[(1S)-2-methoxy-1-methylethyl]acetamide) was supplied
by Sigma Aldrich. Its water solubility at 25 C is 480 mg L1.
2.2.
Characterization
All hybrid materials were characterized by different techniques.
X-ray powder diffraction (XRD) patterns were recorded on a
Siemens D-5000 powder diffractometer (Cu-Ka radiation). N2
isotherms were determined on a Micromeritics ASAP
2010 analyzer at 196 C. The specic surface area of each solid
was determined using the BET method over a relative pressure
(P/P0) range of 0.05–0.20 and the pore size distribution was
obtained by analysis of the adsorption branch of the isotherms
using the Barrett–Joyner–Halenda (BJH) method. Prior to the
measurements, the samples were outgassed at 120 C for 24 h. A
3Flex equipment from Micromeritics was used for two samples.
In this case, the porosity distribution was determined by a
density functional theory (DFT) method using a cylindrical
model for N2 on an oxide surface. Particle size data were
obtained in a Mastersizer S particle size analyzer by laser
diffraction technology with a small volume dispersion unit of
Malvern Instruments. Before the measurements, the samples
were dispersed in distilled water and sonicated for 10 min. TEM
micrographs were taken using a JEOL JEM 1400 instrument on
samples supported on copper grids with carbon coating. 29Si
MAS NMR spectra were recorded at 79.49 MHz on a Bruker
Avance 400 WB dual channel spectrometer at room temperature. An overall of 1000 free induction decays were accumulated.
The excitation pulse and recycle time were 6 ms and 60 s,
respectively. Chemical shis were measured relative to tetramethylsilane standard. Before analysis, the samples were dried
at 150 C for 24 h. Deconvolution of the spectra were performed
with PeakFit soware. The variation of zeta potential with pH
was determined using Laser Doppler Velocimetry implemented
in a Malvern Zetasizer Nano ZS equipped with a MPT-2 autotitrator. UV spectroscopy (Perkin Elmer UV-visible spectrophotometer, Model Lambda 11) at 265 nm was used to determine
the concentration of S-metolachlor.
2.3.
Adsorption experiments
S-Metolachlor adsorption on mesoporous materials was studied
suspending in triplicate samples of 20 mg of each adsorbent in
30 mL of aqueous herbicide solution (100.0 mg L1) with an
initial pH value of 2 (for the silica and ethanesilica samples) or 4
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(for the benzenesilica samples).15 Samples of adsorbent and
herbicide in water were shaken in a turn-over shaker at 52 rpm
for 24 h. Adsorption kinetics was carried out at different contact
times (Fig. S1†). Then, the supernatant liquid was centrifuged
and separated to determine the concentration of S-metolachlor
by UV-vis spectroscopy (Fig. S2†). The amounts of adsorbed
herbicide were determined from the initial and nal solution
concentrations.
Scheme 1 Schematic representation of a pore for the different
materials synthesized. R ¼ –O–, –C2H4– or –C6H4– for silicas,
ethanesilicas and benzenesilicas, respectively. The shaded area
represents the wall of the material.
Table 1 Physicochemical properties of non-ordered porous (organo)silicas and periodic mesoporous (organo)silicas
Material
SBETa (m2 g1)
Vpb (cm3 g1)
Smicroc (m2 g1)
S-20
S-40
S-60
PMS
EOS-20
EOS-40
EOS-60
E-PMO
PhOS-20
PhOS-40
PhOS-60
Ph-PMO
723
721
746
885
715
994
697
1059
942
1160
824
986
0.81
1.39
1.67
1.17
0.68
1.44
1.19
1.00
0.58
1.69
0.81
0.72
47
52
92
0
84
84
21
26
250
111
48
161
a
BET specic surface area determined in the range of relative pressures
from 0.05 to 0.20. b Single point total pore volume of pores. c Micropore
area determined by t-plot method.
Fig. 1
Successive adsorptions were carried out by suspending the
solid (adsorbent–herbicide) in 30 mL of a fresh aqueous
herbicide solution (100.0 mg L1). The contact time was 24 h.
Aer shaking, the suspensions were centrifuged and the
herbicide concentration determined by UV-vis spectroscopy.
The charged material was used again for a new adsorption cycle
up to completion of 4 successive adsorptions. The regeneration
of the adsorbent was performed in ethanol with 30 mL of
extracting volume and 24 h of contact time.
3.
Results and discussion
3.1.
(Organo)silicas as adsorbents for S-metolachlor
Different silicas, ethanesilicas and benzenesilicas have been
synthesized using three different Tween surfactants as porogens (Scheme 1). The integrity of the organic bridges was veried by FT-IR measurements (Fig. S3†).26 Thus, bands at 1277,
1414, 2899 and 2986 cm1 corresponded to vibrations of the
CH2 groups in ethane bridges. Benzene bridges showed the C–H
and C–C vibrations at 3069, and 1461 and 1636 cm1, respectively. Furthermore, intense bands between 1000 and 1110 cm1
assigned to Si–O vibrations could be observed in all (organo)silicas.
XRD analyses (Fig. S4†) revealed that all these materials were
non-periodic, that is, they lacked long distance ordering.
Nevertheless, all of them exhibited high specic surface areas
and pore volumes (Table 1). Their N2 adsorption–desorption
isotherms were of type IV with hysteresis loops exhibiting
different shapes (Fig. 1). Therefore, even though all these
materials had pores in the meso range, their porosity showed
signicant differences. In fact, all these adsorbents contained
pores over a wide range of sizes. Samples S-20 and S-40 displayed a step at a relative pressure of 0.4–0.6 and a hysteresis
loop of type H2, typical of disordered mesoporous systems.26
Their pore size distribution curves indicated the existence of
pores in the mesopore range at ca. 4 and 5 nm, respectively
(Fig. 2). Material EOS-60 also had pores around 4 nm, although
the step was hardly observed. In some cases, the adsorption step
was shied to high relative pressures (>0.6) by the effect of the
increase in pore size, which was the case for solid S-60 with
mesopores around 7 nm. On the contrary, materials EOS-20 and
Nitrogen adsorption–desorption isotherms for non-ordered porous (organo)silicas.
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Fig. 2
Pore size distribution curves for non-ordered porous (organo)silicas.
Fig. 3
TEM images for non-ordered porous (organo)silicas.
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EOS-40 did not show any hysteresis loop at P/P0 < 0.8 because
the step occurred at quite low relative pressure, thus resulting in
pores in the low mesopore range (<3 nm). Furthermore, material PhOS-20 exhibited a type H4 loop, which is usually ascribed
to slit-shaped pores in the micropore range.27 The presence of a
separate hysteresis loop above P/P0 ¼ 0.8 for several samples
(i.e., S-40 and EOS-60) was suggestive of textural porosity, which
represents the void space between randomly packed elementary
particles, in the large meso and macropore range. As can be
observed, several samples (i.e., S-60 and PhOS-40) exhibited
different overlapped hysteresis loops, thus suggesting a
combination of different porous systems. Clearly, the use of
different surfactants of the Tween family gave rise to important
differences within each group. Most of the samples exhibited
mesopores with diameters above 10 nm and/or macropores,
even though those materials prepared from Tween 20 mainly
had small mesopores and/or micropores.
Periodic mesoporous silica (PMS) and organosilicas (ethanePMO and benzene-PMO) were previously shown to exhibit
isotherms of type IV typical of mesoporous materials with a
narrow pore size distribution.15 Among the non-ordered porous
materials, only those synthesized in the presence of surfactant
Tween 20 presented an analogous pore size distribution to PMS
and PMOs, even though it was shied to smaller pore sizes for
PhOS-20 than for Ph-PMO.
The transmission electron micrographs (Fig. 3) clearly
reected the existence of small elementary particles forming
larger porous aggregates, which is consistent with the textural
porosity apparent from the nitrogen adsorption–desorption
isotherms. Also, in agreement with their isotherms, silica
materials exhibited highly disordered pores in the meso range
(4–5 nm). Although less evident, pores with sizes around 3–4 nm
were also observable in ethanesilicas. Higher magnication
images were in good agreement with the pore sizes determined
by N2 adsorption (Fig. 2).
The porous network of fused particles, particularly for silica
samples, packed more densely in the following order of
surfactants: Tween 20 > Tween 40 $ Tween 60, thus resulting in
more uniform pore systems when using the former. In the case
of benzenesilicas, the pores in sample PhOS-20 were hardly
observable because they fell in the microporous range.
The different (organo)silicas exhibited the particle size
distribution curves depicted in Fig. 4. In general, wide distributions in particle size were observed in all cases, even though
silicas presented narrower size distributions than organosilicas.
Within each group, the particle size seemed to be dependent on
the surfactant used as porogen. Thus, it increased in the
following order: Tween 20 < Tween 60 < Tween 40. Moreover,
those (organo)silicas synthesized in the presence of Tween 20
surfactant displayed a narrower particle size distribution. In
this case, the particle size decreased in the order PhOS-20 <
EOS-20 < S-20. Also, the particle size distribution curves for the
corresponding PMOs are given as a reference.
All the silicas and organosilicas were used as adsorbents for
the removal of S-metolachlor from water. The adsorption
experiments for silicas and ethane-PMOs were carried out at
pH ¼ 2, whereas those for benzenesilicas were done at pH ¼ 4,
according to the behavior previously observed in PMOs.15 The
amount of adsorbed herbicide in the three groups of materials
Fig. 4 Particle size distribution curves for non-ordered porous
(organo)silicas and periodic mesoporous (organo)silicas.
Fig. 5 Adsorption of S-metolachlor on non-ordered porous (organo)silicas and periodic mesoporous (organo)silicas.
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Adsorption capacities for S-metolachlor on different adsorbents
Table 2
Adsorbent
C0a (mg g1)
Csb (mg g1)
Adsorption (%)
Ref.
Soils
Charcoals
Clays
Organohydrotalcites (OHTs)
PMS
E-PMO
0.08
400.0
158.9
149.0
149.0
149.0
596.0
532.1 (4 times)c
149.0
596.0
150.0 (4 times)c
532.1 (4 times)c
150.0
150.0 (3 times)c
150.0
600.0
150.0
600.0
150.0
600.0
0.04
136.2–156.0
56.8
79.7
15.0
36.0
72.4
118.2
38.3
189.3
133.6
490.8
76.1
90.4
117.9
334.2
94.3
312.0
108.4
347.1
50.0
36.5
37.8
53.5
10.1
24.2
12.2
22.2
25.7
31.8
89.1
92.2
50.7
60.3
78.6
55.7
62.9
52.0
72.3
57.9
29
30
31
32
15
15
15
15
15
15
This work
15
This work
This work
33
33
33
33
33
33
Ph-PMO
PhOS-40
Commercial carbon
Mesoporous phenolic resin
Mesoporous carbon
a
Milligrams of herbicide in the starting solution per gram of adsorbent. b Milligrams of herbicide adsorbed per gram of adsorbent. c Number of
adsorption cycles performed with the same material.
revealed signicant differences among them (Fig. 5). Broadly, as
expected, the composition of the adsorbent, that is, the nature
of the bridge, was decisive. The adsorption capacity toward the
herbicide S-metolachlor was higher for benzenesilicas, followed
by ethanesilicas and nally by silicas. Unlike silicas and ethanesilicas, all benzenesilicas adsorbed herbicide. The importance of the aromaticity in the adsorption of this non-ionic
herbicide has also been evidenced for other very different
adsorbents, as was the case for maize mulch residues exposed
to microbial decomposition, whose capacity corresponded to
the lignin fraction.28 However, signicant differences could be
observed for different materials within the same group, that is,
with the same composition. Obviously, within each group, an
increase in the surface area of the materials would benet to the
uptake of S-metolachlor because the area of interaction between
the (organo)silicas and the herbicide would be more extensive.
However, although the materials with the largest surface gave
the highest uptakes, some results could not be properly
explained. Thus, sample EOS-60 had a slightly lower surface
area than sample EOS-20, but the former adsorbed much more
herbicide than the latter.
Apparently, the main factor governing the adsorption
capacity within each group was the pore volume. Thus, the
larger the pore volume, the higher the adsorption capacity was
for both silica and organosilicas. This explained the differences
between samples EOS-60 and EOS-20. By comparing the herbicide uptake for S-60, EOS-40 and PhOS-40, which were those
materials with the highest pore volumes within each group, a
higher efficiency of the organosilicas, particularly the benzenesilica, than the silica was observed. However, the distribution
of pore size must affect the herbicide uptake because otherwise
it is not possible to explain the signicant differences among
them. Material PhOS-60 gave a lower herbicide uptake than
sample PhOS-20, even though the former exhibited a higher pore
Table 3
Fig. 6
29
29
Si MAS NMR results obtained for two benzenesilicas
Material
T1 (%)
T2 (%)
T3 (%)
Da (%)
OH/Sib
PhOS-40
Ph-PMO
17.1
13.3
50.3
57.1
32.6
29.6
72
72
0.85
0.84
P
a
degree calculated from equation D ¼ [ (nTn)]/[(3 PCondensation
Tn)] 100. b Content of silanol groups per silicon atom calculated
from equation OH/Si ¼ (2T1 + T2)/(T1 + T2 + T3).
Si MAS NMR spectra for two benzenesilicas.
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volume. The signicant difference between these two benzenesilicas seemed to be caused by the much higher micropore area
in PhOS-20 than in PhOS-60. A comparison between samples S60 and S-40 as well as between EOS-40 and EOS-60 suggested
that a larger micropore area also favored the adsorption capacity
of the adsorbent. Moreover, the best adsorbents within the silica
and ethanesilica families, i.e., samples S-60 and EOS-40,
respectively, displayed pores in the upper limit of the mesopores (18–30 nm) rather than macropores (>50 nm), apart from
small mesopores. Indeed, sample PhOS-40 also exhibited an
important fraction of pores at ca. 40 nm.
Benzenesilica PhOS-40 can be considered as an efficient
adsorbent for S-metolachlor, with an uptake in the same range
as some organohydrotalcites and charcoals, although lower
than some microporous and mesoporous carbons (Table 2).
Furthermore, the herbicide S-metolachlor can be completely
desorbed from this benzenesilica by simple extraction with
ethanol.
3.2. Comparison of a non-ordered porous benzenesilica with
a periodic mesoporous benzenesilica as adsorbents for Smetolachlor
As previously stated, periodic mesoporous silica (PMS) and
organosilicas (E-PMO and Ph-PMO) exhibited a good adsorption
capacity within each family. Taking into account that the best
adsorption behavior corresponded to benzenesilicas, we
focused our study on materials PhOS-40 and Ph-PMO in order to
ascertain the key factors inuencing their differences in
adsorption.
A previous study revealed that the aromatic p–p interactions
prevailed for the adsorption of S-metolachlor on benzenesilica,15 whereas electrostatic and hydrophobic interactions played
an important role in silica and ethanesilica. Although PhOS-40
and Ph-PMO were synthesized starting from the same
precursor, both materials were obtained under rather different
conditions. Therefore, 29Si MAS NMR spectra were registered in
order to determine the hydroxylation degree in both samples
(Fig. 6). Both materials exhibited practically identical composition of Si–OH groups. In fact, their condensation degrees and
their number of OH groups per silicon atom were analogous
(Table 3). Consequently, silanol groups cannot determine the
different adsorption behavior of these materials.
Fig. 7 Variation of zeta potential as a function of pH for two
benzenesilicas.
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Fig. 8 Pore size distribution curves for two benzenesilicas.
Also, measurements of the variation of zeta potential with
pH were carried out (Fig. 7). As observed, both materials showed
very similar curves, thus indicating a similar surface charge,
even though they were synthesized under different conditions.
The point of zero charge (pzc) values for both materials were ca.
2.4, close to those reported for silica by other authors but lower
than the value calculated by us following the mass titration
method.15,34,35 Consequently, the disparities in the adsorption
behavior for Ph-PMO and PhOS-40 cannot be attributed to
differences in composition or surface charge.
Since the surface chemistry of both benzenesilicas was
similar, the different adsorption capacity of these materials
should be ascribed to their different textural properties. Again,
the pore volume, which was much higher for PhOS-40 than for
Ph-PMO, seemed to be decisive for the adsorption of the
herbicide. A comparison of their pore size distribution curves
(Fig. 8) determined by a DFT method showed clear differences
between both solids. Ph-PMO mainly exhibited mesopores with
a narrow distribution centered at ca. 4 nm as well as some
micropores, whereas PhOS-40 had pores distributed in the
whole size range with an important contribution of large
mesopores.
A previously observed characteristic of Ph-PMO as adsorbent
of S-metolachlor was its high capacity aer successive adsorptions cycles (Table 2).15 In fact, this material was able to retain
herbicide up to saturation of its pore volume. This was attributed to the existence of phenylene bridges in its walls because
ethylene bridges resulted in much less uptake and incomplete
Fig. 9 Successive
benezenesilicas.
adsorptions
of
S-metolachlor
for
two
This journal is © The Royal Society of Chemistry 2015
Paper
lling of pores. Thus, a comparison between Ph-PMO and
PhOS-40 was done by subjecting them to four adsorption cycles
(Fig. 9). Interestingly, PhOS-40 was saturated aer only two
successive adsorptions, whereas Ph-PMO was able to retain
herbicide even aer four adsorption cycles. At that point,
Ph-PMO showed an uptake of 133.6 mg g1 of S-metolachlor,
thus still far from the maximum capacity reported for this
material (490.8 mg g1).15 Therefore, the high adsorption
capacity of Ph-PMO has to be ascribed not only to the existence
of aromatic bridges but also to its mesoporous character that
allows to ll its whole pore volume.
4. Conclusion
Various non-ordered porous (organo)silicas have been synthesized using Tween surfactants as porogens. They have been
characterized and used as adsorbents for the herbicide
S-metolachlor. A combination of several factors has been shown
to determine the adsorption capacity of these materials. The
main one was the chemical nature of the bridge. Thus, benzenesilicas gave the higher uptake, followed by ethanesilicas and
nally by silicas. The surface area and particularly the pore
volume were also proven to be essential for having a high
uptake. Concerning the pore size distribution, those pores in
the micropore and/or the large mesopore range seemed to be
more favorable to adsorption. Clearly, the adsorption capacities
of materials with different textural properties for organic
molecules must be compared with caution.
The non-ordered materials were compared to the corresponding periodic mesoporous (organo)silicas. The use of the
latter assured a good uptake in all cases unlike some of the nonordered (organo)silicas. Interestingly, the phenylene-bridged
periodic mesoporous organosilica exhibited a higher capacity
than the non-ordered benzenesilica aer several successive
adsorption cycles and therefore it would be the material of
choice for multiple-pass adsorption processes.
Acknowledgements
The authors would like to thank Spanish Ministry of Economy
and Competitiveness (Projects MAT2013-44463-R and
CTM2011-25325), Junta de Andalucı́a (Project P10-FQM-6181
and Research Groups FQM-214 and FQM-346) and FEDER
funds. M.I.L. and R.O. acknowledge Spanish Ministry of
Education, Culture and Sport for their FPU research and
teaching fellowships. The authors are greatly indebted to
Micromeritics for measurements of the textural properties of
samples PhOS-40 and Ph-PMO in a 3Flex equipment.
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