Preparation and characterization of micron and submicron

Applied Clay Science 72 (2013) 9–17
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Applied Clay Science
journal homepage: www.elsevier.com/locate/clay
Research paper
Preparation and characterization of micron and submicron-sized vermiculite
powders by ultrasonic irradiation
A.N. Nguyen a, b, L. Reinert a, J.-M. Lévêque a, A. Beziat b, P. Dehaudt b, J.-F. Juliaa c, L. Duclaux a,⁎
a
b
c
LCME, Université de Savoie, 73376 Le Bourget du Lac Cedex, France
CEA/DEN/DTEC Laboratoire d'Étanchéité, F30207 Bagnols sur Cèze, France
TECHNETICS GROUP France, 90 Rue de la roche du geai, 42029 Saint-Etienne, France
a r t i c l e
i n f o
Article history:
Received 5 November 2011
Received in revised form 10 December 2012
Accepted 19 December 2012
Available online xxxx
Keywords:
K-vermiculite
Ultrasounds
Hydrogen peroxide
Size reduction
a b s t r a c t
Micron and submicron-sized vermiculite lamellar particles with nanometric thickness (b 10 nm) were prepared by ultrasonic treatments (b12 h) of aqueous and hydrogen peroxide suspensions of thermally exfoliated vermiculite. Laser granulometry characterizations showed that the particles size distribution was
dependent on the treatment time and that the use of H2O2 afforded smaller particles than H2O. In both
media, an exfoliation and a size reduction were observed after only 1 h of ultrasonic treatment by Scanning
Electron Microscopy, X-ray diffraction, and Nitrogen Adsorption Measurements at 77 K. X-ray diffraction
studies showed the absence of damage in crystals structure after sonication and also a reduction of crystallites size along the basal direction (00l). The different ultrasonic treatments also induced modifications of
the surface properties of the vermiculite particles, brought out by BET surface measurements, infrared spectroscopy, pH modifications of the materials and zeta potential analyses. Sonication of the vermiculites yielded
to the formation of carbonate anions from the dissolved CO2 and hydroxide anions released from the clay
layers. The long ultrasound irradiation of the vermiculite in hydrogen peroxide (>5 h) generated the decrease of the surface charge, pointed out by pH and zeta potential modifications, allowing an aggregation
of the submicron particles in the suspensions.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Vermiculite, a lamellar hydrated aluminium iron magnesium silicate,
has the unusual property of exfoliating, when submitted to a thermal
shock, due to the inter-lamellar generation of steam. In its exfoliated
state, vermiculite shows very interesting properties such as low bulk
density, low thermal conductivity and comparatively high melting
point (1240–1430 °C), high absorbency, high specific surface area and
cation exchange properties. Its other benefits are chemical inertness, endurance, and environmental safety. For all these appealing features, exfoliated vermiculite remains topically one of most used mineral material in
the development of R&D solutions; as evidenced by its number of applications reported in the literature, such as the development of dedicated
adsorbent for water treatment (Kehal et al., 2010), the design of
nano-composite materials (Du et al., 2003), the making of gaskets for
sealing technology (Hoyes and Bond, 2007), the production of thin inorganic films (Ballard and Rideal, 1983) or its use as lightweight porous
filler in the production of heat-insulating refractory components (El
Mouzdahir et al., 2009; Valášková et al., 2009). Thus, vermiculite based
⁎ Corresponding author at: LCME, Université de Savoie,73376 Le Bourget du Lac Cedex,
France. Tel./fax: +33 4 79 75 88 05.
E-mail address: [email protected] (L. Duclaux).
0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.clay.2012.12.007
materials display interesting physical and chemical properties which
are strongly dependent on the particles size distribution of the basic
powder. The control and reduction of particles size distribution might
also lead to the discovery of new applications.
In theory, single layers of about 1 nm thickness can be produced
when vermiculite mineral is completely exfoliated, leading to enhanced
chemical and physical properties. Several methods were thus proposed
to delaminate and/or reduce the particles size of this mineral. On the
first hand, the thermal shock remains one of the most known and used
procedure to separate the layers due to the inter-layered water molecules vaporization, leading to physical exfoliation. By a thermal shock
at 900 °C, Justo et al. (1989) prepared a nine times expanded vermiculite. It was also shown recently that microwave irradiation reduced the
time and energy required for the preparation of expanded vermiculite
particles (Marcos and Rodriguez, 2011). However, such a thermal treatment may cause dehydroxylation as well as chemical composition
changes of the raw material.
The chemical exfoliation can be used as an alternative, notably by
intercalation of alkyl ammonium ions in Li+ exchanged vermiculite
followed by a rehydration step in an appropriate aqueous medium.
This allows not only to weaken the electrostatic attractive interlayer
forces but also to separate the layer distance of several hundred
Angstroms (Walker and Garrett, 1967). Hydrogen peroxide was also
found to be an efficient exfoliating agent for vermiculite (Muromtsev
10
A.N. Nguyen et al. / Applied Clay Science 72 (2013) 9–17
et al., 1990) owing to the molecular oxygen and steam gases generation
by decomposition of the intercalated hydrogen peroxide molecules,
which separated the silicate layers apart. Muromtsev et al. suggested
that dioxygen combination with atomic oxygen of the layers led to
the breaking of the Si\OH and Al\OH bonds and the release the hydroxyl groups of vermiculite from the silicate structure, leading subsequently to a pH increase of the solution. In order to enhance existing
processes, Ballard and Rideal (1983) combined intercalation and delamination to afford in a reasonable time particles as small as possible.
They firstly exfoliated vermiculites by intercalation of aqueous tetrabutyl ammonium cations, resulting in a volume increase up to 30
times; and secondly delaminated mechanically the chemically swollen material in a high shear mixer or in a suitable colloidal mill.
Vermiculite particles with a size lower than 50 μm were obtained
after separation by sedimentation and sieved at 5 μm after the removal of largest particles (Ballard and Rideal, 1983).
On the other hand, the reduction of vermiculite particles size by
mechanical treatment such as wet or dry grinding remains one of
the simplest procedure to prepare micron-sized vermiculite though
it generates also some delamination (Pérez-Maqueda et al., 2004).
For example, within 2 min of dry grinding, some 2.5 cm sized Mgvermiculite particles (from Santa Olalla, Spain) were reduced to
40 μm average size (Pérez-Maqueda et al., 2004). Nevertheless, the
use of harsh mechanical effects leads also to structural degradation
and to a progressive amorphization, increasing together with the
treatment time.
An appropriate method for size reduction down to nanometric sizes
without denaturing the aforementioned chemical physical properties
could be the ultrasonic irradiation. The propagation of acoustic waves
in liquids generates several physical effects that can break the particles
down to very smaller ones in a short time (Doktycz and Suslick, 1990).
Indeed, Hinds et al. (1996) reported an ultrasound-assisted reduction in
micronic size of vermiculite (i.e. from 2.4 to 1.7 μm) in 25 min in a basic
50 W ultrasonic bath in water suspension.
In particular, low frequency ultrasound (20–100 kHz) favours the
emergence of physical effects such as particles size reduction, shock
waves, efficient mixing and mass transport through the unique physical phenomenon called cavitation which is the birth, growth and collapse of highly energetic micro-bubbles. Thus Pérez-Maqueda et al.
(2001) reduced Mg-vermiculite platelets (from Santa Olalla, Spain)
of 2.5 cm in length and 0.5 cm in thickness to particles of 15.5 μm average size, after a sonication period of 10 h, at 20 kHz, using a horn system
in a diluted hydrogen peroxide solution (15%). Moreover, these authors
achieved vermiculite reduction to micronic and submicronic size in
40 h by combining physical effects of cavitation and chemical exfoliation
by reaction with H2O2. Original vermiculite platelets of centimetre scale
were broken down to a median diameter of 2.4 μm, and the specific surface exhibited a significant increase from 1 to 33 m2 g−1 (Wiewiora et
al., 2003). As the sonication time was pursued beyond 40 h, the percentage of smallest particles decreased whereas the percentage of the largest
ones became predominant due to aggregation, but the crystalline structure of vermiculite was not damaged even after 100 h of sonication.
J. Poyato et al. (2009) have pointed out that size reduction by sonication
was accompanied by a change of the layer charge and of the redox state
the vermiculites. Indeed, depending on the chemistry of the pristine
vermiculite, whereas oxidation occurs in hydrogen peroxide medium,
reduction occurs in aqueous medium (J. Poyato et al., 2009).
Even if the work described by Pérez-Maqueda et al (2001) emphasises the potential of ultrasound irradiation to obtain submicronic
particles, too long durations (i.e. 40 h) were used notably in terms
of efficiency and energetic consumption. In addition, such long
times of irradiation with a titanium horn may lead to the release of
small particles of titanium into the solution and subsequently to the
poisoning and eventually to the alteration of the properties of the materials. The aim of this present work was to prepare micron/submicron
particles of vermiculite under low frequency ultrasound (20 kHz)
irradiation in water or in hydrogen peroxide solution but in a shorter
time (few hours). Thus, we explored and optimized several key parameters influencing of the ultrasonic efficiency.
2. Experimental
2.1. Material
The starting vermiculite (Granutec® E originating from Yuli
China) was purchased from CMMP French Company and was used
as received (millimetric plates). Potassium chloride (99%, Chimie Plus)
was used to saturate vermiculite before sonication (K-vermiculite).
Hydrogen peroxide (H2O2, 35%, ACROS) was used to prepare suspensions of vermiculite. After potassium exchange, the average chemical
composition of half a lattice cell calculated from elemental analysis
was (Si3 Al1)(Mg2.62 Fe0.32 Ti0.06)O10(OH)2K0.61.
2.2. Ultrasonic device
An Ultrasonic Processor (Sonics and Materials, 500 W Ultrasonic
Processor-VC505) of 350 W output with a 20 kHz converter fitted with
an ultrasonic probe (19 mm-Sonics and Materials) was used. The probe
was dipped into a cylindrical doubled-jacket reactor (for accurate temperature control) with a mean internal diameter of 80 mm ended with
a conical shape, containing 0.6 g vermiculite in hydrogen peroxide or
distilled water suspension (55 mL). The cooling liquid was kept at 5 °C
during the experiment to adjust the inside reactor temperature to ambient (about 25 °C). The sonication times were set to 1 h, 3 h, 5 h, 7 h, 9 h
and 12 h.
The calorimetric method was applied to determinate the acoustic
power. A thermocouple was dipped in the bulk distilled water to record
the temperature increase due to sonication (for few minutes). The acoustic power was calculated using the following equation (Contamine et al,
1995): Up =Cp ·M·dT/dt; where Cp is the heat capacity of solvent at a
constant pressure (J kg−1 K−1), M is the solvent mass (kg) and dT/dt
(K s−1) is the rate of temperature increase. A 56 W acoustic power
was found for our system.
2.3. Characterization
After sonication the suspensions were evaporated for 12 h at
80 °C, and the dried vermiculite powders were subsequently characterized by X-ray diffraction, Scanning Electron Microscopy, Nitrogen
adsorption measurements at 77 K, thermogravimetric analysis, infrared spectroscopy and zeta potential measurements.
The particles size distributions of the samples were measured
with a Mastersizer 2000 particle size analyzer (Malvern Instruments,
range 0.02 μm to 2000 μm).
The morphologies of the samples were observed by high resolution Scanning Electron Microscopy (SEM), using a field emission microscope (FE-SEM Zeiss Supra TM 55) coupled with an Oxford-Inca
energy dispersive spectroscopy analyzer (EDS) which was used for
local chemical analyses of the samples.
The crystalline structure of the samples was characterized by
X-ray diffraction (XRD, λCuKα1 = 154.06 pm) using a Bragg–Brentano
(θ, 2θ) mode goniometer (CGR, θ 60) equipped with an INEL XRG
3000 generator and a quartz crystal monochromator. Oriented samples were obtained by deposition of vermiculite dispersions on glass
slides. XRD patterns were collected from 2θ = 2° to 30° by step of
0.05°, 4 s per step. The Lc coherent domain sizes of crystallites
(along the c axis) were calculated using the Scherrer equation taking
into account the instrumental broadening.
The N2 adsorption–desorption isotherms were measured using an
automatic adsorption instrument (ASAP 2000, Micromeritics) at liquid
nitrogen temperature (77 K). Prior to measurements, samples were
degassed under vacuum (10−3 mbar) at 200 °C for 12 h. The specific
A.N. Nguyen et al. / Applied Clay Science 72 (2013) 9–17
12 h
exhibited a bimodal mode with a first region around 1 mm (corresponding also to the initial particles size distribution of the starting
vermiculite) and a second one located around 10 to 1 μm. This dual
mode might be due to the agglomeration of particles attributed
both to the expected increase of the surface energy and to the modification of the charge surface of the vermiculite. Indeed, the sonolytic
treatment of water under low frequency ultrasound leads to a low but
constant production of hydrogen peroxide. By increasing the reaction
time, the hydrogen peroxide concentration might increase and oxidize the surface functional groups leading subsequently to the modification of the charge surface of the material. However, the longest
ultrasonic treatment times, i.e. 9 and 12 h (Fig. 1), induced also a
clear shift towards smaller particles distributions in the second region
with respectively 620 μm and 390 μm, meaning that even if agglomeration might occur as hypothesised here above, the mechanical effects of ultrasound were yet effective to reduce the particles size.
These results could incite us to increase the ultrasonic treatment
time to afford probably even smaller particles but we made both
choices of at first the design of the shortest possible treatment methodology and at second to keep unchanged as possible the integrity of
the material. In addition, 7, 9 and 12 h experiments did not show
drastic changes in the particles size distributions in the second region
displaying the smallest particles, inciting us not to increase further
the treatment times. Moreover, the time increase of the ultrasonic
treatment led also to a growing poisoning of the vermiculite material
by titanium particles issued from the erosion of the ultrasonic probe
brought out by SEM–EDS analysis.
Fig. 1 also displays the particles size distributions obtained in 35%
aqueous solution of hydrogen peroxide according to the length of the
ultrasonic treatment such as previously in pure water. The longer the
ultrasonic treatment, the smaller the particles size distributions as
previously observed in pure water. However, at equal time of ultrasonic treatment, the particles sizes achieved in hydrogen peroxide
are smaller than their homologues obtained in pure water. In addition, the bimodal mode exhibited previously after sonication in pure
water was no more present with the disappearance of the first region
characteristic of big particles in the region of the millimetric range.
This might be explained by in situ degradation of intercalated hydrogen peroxide enhanced by the sonolysis which led to the generation
of molecular oxygen and steam released in between the layers
which helped to an enhanced exfoliation of the adjacent layers.
Thus separated, the single layers may suffer even more drastically
the mechanical effects brought up by low frequency ultrasonic irradiation and leading consequently to even smaller particles. Surprisingly, a bimodal distribution appeared again for the longer treatment
times, i.e. 5 and 12 h (Fig. 1) even if the smallest particles size distributions were obtained. This bimodal distribution is attributed to the
re-agglomeration of small particles for sonication time higher than
5 h which suggested some modification of the surface negative
charges of the vermiculite material, enabling the increasing of the attraction forces between the tactoids.
9h
3.2. X-ray diffraction
surface areas (SBET) were calculated using the BET (Brunauer–Emmett–
Teller) equation by assuming the area of the nitrogen molecule to be
0.162 nm2.
Infrared measurements were carried out on a Thermo Electron
Corporation - Nicolet 380 FTIR spectrometer from 400 to 4000 cm−1,
using an average of 64 scans and a resolution of 2 cm−1. Pellets
(pressed under 370 MPa) were made from a mixture of 1.5 mg of vermiculite sample and 200 mg of dried KBr. Spectra were obtained with
respect to a background, which was previously taken of the air and
under the same measurement conditions.
The pH of vermiculite was measured according to the French standard (NF ISO, 10390, 2005) after preparation of vermiculite suspensions
(1 volume of vermiculite with 5 volume of distilled water).
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)
analyses of the supernatant of the filtrated sonicated solutions were made
by continuous wavelength coverage from 167 to 785 nm, using a VistaPRO Simultaneous ICP-OES at a French accredited laboratory (Savoie
Labo, Le Bourget du Lac, France).
The electrophoretic mobility was measured using a Zetasizer Nano
ZS Malvern instrument. Vermiculite dried powder was dispersed in suspension with buffer solutions (pH = 4.5 and 6.5) at concentration of
1 g L−1. The suspensions were treated in an ultrasonic bath (20 kHz,
150 W) for 5 min just before measurements. The zeta potential was calculated from the Smoluchowski equation (Hunter, 1981).
Thermal properties of the materials were studied by using a NETZSCH
TG 209F1 thermogravimetric analyzer coupled with mass spectrometer
(Balzers MID) in the temperature range of 20–900 °C, under helium
atmosphere.
3. Results and discussion
3.1. Laser granulometry
Volume percentage (a.u.)
The vermiculite material was submitted to powerful low frequency ultrasounds in either pure water or in 35% H2O2 aqueous solution
from 1 to 12 h at ambient temperature. Fig. 1 displays the particles
size distributions obtained in pure water according to the length of
the ultrasonic treatment. As expected, the longer the ultrasonic treatment in water, the smaller the particles size distribution, highlighting
the effectiveness of the physical effects brought up by the cavitation
phenomenon on the vermiculite material. Thus, whereas 12 h treatment (Fig. 1) afforded the smallest particles in the region of 1 μm,
shorter treatment times gave also bigger particles size distributions
ranging from 2 to 7 μm. Noteworthy, even 1 h of ultrasonic treatment
was enough to enable the production of particles in the range of
7.5 μm starting originally from millimetric particles. Surprisingly
and whatever the irradiation time, all particles size distributions
7h
5h
3h
1h
(a)
0.01
0.1
1
10
100
1000
11
10000
Particle size (µm)
Fig. 1. Particles-size distribution of K-vermiculite (a) and vermiculite sonicated in H2O
(dotted lines) and in H2O2 (continuous lines) for 1 h, 3 h, 5 h, 7 h, 9 h, and 12 h.
X-ray diffraction patterns of sonicated vermiculites (Figs. 2 and 3)
showed that the samples were still crystalline. Whatever the solution
used for sonication, a broadening of the 002 reflection was observed
with the increase of sonication time. This broadening could be
explained by a decrease in particles size. The particle thickness of
the vermiculite platelets was calculated from the full width at half
maximum of the 002 reflection. The particle thickness decreased
from 11 nm for raw vermiculite to ~ 8.3 nm after only 1 h of ultrasonic treatment in H2O2 (Table 1). When sonication times increased, the
thickness of the particles continuously decreased. After 12 h of sonication in H2O2, the average thickness reached a minimum of ~3.2 nm, corresponding to the stacking of only 3 nanometric vermiculite sheets. For
12
A.N. Nguyen et al. / Applied Clay Science 72 (2013) 9–17
002
I (a.u.)
g
f
e
d
c
b
a
2
7
12
17
22
27
32
2θ (deg.)
Fig. 2. X-ray diffraction patterns of vermiculites sonicated at 20 kHz in H2O2 for 1 h (a), 3 h (b), 5 h (c), 7 h (d), 9 h (e), 12 h (f) and diffractogram of K-vermiculite (g).
the same sonication time but in H2O, the thickness of the particles was
~4.9 nm; the ultrasonic treatment appeared to be thus more efficient in
H2O2 than in H2O. This thickness reduction indicated that sonication
produced a delamination of the vermiculite sheets concomitant to a reduction in particles size, as observed by laser granulometry (Fig. 1).
The sonication also induced a shift of the maximum of the 002 line
towards low 2θ values with increasing sonication times (much more
visible on the H2O2 treated sample diffraction lines) (Fig. 2). This
could be attributed to different water contents (hydration ratios) for
these samples, which can be explained by a greater adsorption of
water in the interlayer space, as a result of size reduction.
gave particles of average thickness below 100 nm (Fig. 4c). After 12 h
of sonication, most of the particles displayed irregular shapes and
only few small platelets of about 400 nm length were still visible
(Fig. 4d). This shape modification confirms the efficiency of the mechanical effect of ultrasounds for reduction of the particle size, when
combined to the exfoliation capacity of hydrogen peroxide.
The particle size reduction by ultrasounds under various conditions was previously reported in the literature. Pérez-Maqueda et al.
(2001) prepared micrometric and sub-micrometric Santa Olalla vermiculite particles after 10 to 100 h of ultrasonic treatment (20 kHz,
600 W, diameter of the horn 12.7 mm). For Ojen and Santa Olalla vermiculites sonicated with a horn in a H2O/H2O2 mixture, Wiewiora et
al. (2003) also observed submicronic flakes by transmission electron
microscopy. By comparison to our work, the sonication times applied
by these authors (Pérez-Maqueda et al., 2001; and Wiewiora et al.,
2003), were longer to obtain micrometric particles. This could be obviously attributed to the bigger pristine particle size (centimetric size)
and possibly to the presence of some interstratified mica layers in the
vermiculite.
3.3. Samples morphologies
K-vermiculite is composed of platelets of about 2 mm in length and
0.1 mm in thickness. In the vermiculite prepared suspension in hydrogen peroxide (35% solution), the water molecules initially solvating the
interfoliar cations were exchanged with the H2O2 molecules. The decomposition of the H2O2 intercalated molecules led to the production of large
amounts of molecular oxygen and steam. This phenomenon, combined
to the effect of ultrasounds, induced the separation of the vermiculite
layers and the formation of disaggregated particles of reduced sizes.
After 1 h of ultrasonic treatment, the vermiculite sheets were
clearly separated (Fig. 4a). Platelets of about 100 nm in thickness
were obtained. The basal length was reduced, owing to the mechanical effect of ultrasounds. The smallest particles showed basal dimension of only 200–300 nm. Particles of smaller sizes were produced
as the ultrasonic treatment time increased but impurities of titanium
particles due to the erosion of the ultrasonic probe (Ti–6Al–4V alloy)
were observed in the prepared powder by Energy Dispersive Spectroscopy (EDS). The impurities content was increasing together
with the sonication time. After 5 h of sonication, only a few plates
of micrometric length remained visible (Fig. 4b). Most of the particles
were reduced to submicrometric length plates. 7 h sonication in H2O2
3.4. Surface area measurements
The BET surface area (SBET) of pristine K-vermiculite is low (Table 1,
SBET ~ 7 m 2 g−1), and corresponds to reported values measured for natural samples (Jiménez De Haro et al., 2004; Wiewiora et al., 2003). The
surface area rapidly increased with sonication time: SBET increased to
19 m 2 g−1 after only 1 h of treatment and reached a maximum of
~30 m2 g−1 for 5 h of sonication. This increase was directly correlated
to the reduction in particle size, as observed by SEM (Fig. 4).
For sonication times longer than 5 h in hydrogen peroxide solvent,
SBET decreased regularly. The value measured after 12 h of treatment
(Table 1, ~ 20 m 2 g −1) was very similar to that measured after 1 h of
sonication. This decrease attributed to the re-agglomeration of the
I (a.u.)
002
f
e
d
c
b
a
2
7
12
17
22
27
32
2θ (deg.)
Fig. 3. X-ray diffraction patterns of vermiculite sonicated at 20 kHz in H2O for 3 h (a), 5 h (b), 7 h (c), 9 h (d), 12 h (e) and diffractogram of K-vermiculite (f).
A.N. Nguyen et al. / Applied Clay Science 72 (2013) 9–17
Table 1
Characteristics of raw vermiculite and vermiculites sonicated in H2O2 and H2O for
various irradiation times.
Sonication time (h)
Solution
0
1
Crystallite size along c
axis Lc (nm)
Crystallite size along c
axis Lc (nm)
SBET (m2 g−1)
H2O2
11
8.3
7.0
7.6
6.9
4.8
3.2
H2O
11
–
6
5.9
5.3
5.0
4.9
7.1
19.2
5
7
9
1020
12
3720
1380
18.1
30.2
28.4
27.5
20.0
particles observed for the sonication longer than 5 h, was also confirmed by laser granulometry observations.
Absorbance (a.u.)
H2O2
3
460
13
1630
3675
680
3600
(d)
(c)
3.5. Infrared spectroscopy
The infrared spectra of the raw and sonicated vermiculites (Fig. 5) in
water and hydrogen peroxide were similar indicating that no structural
changes of the layer occurred during the irradiation. The large adsorption
band at ~1020 cm−1 was assigned to the stretching vibration of the
Si\O, whereas the large bands at ~460 cm−1 and 680 cm−1 were attributed to the stretching vibration of the Al\O bonds (Farmer, 1974).
The vibration bands of water gave a complex signal with bands identified at about 3600 cm−1 attributed to the OH asymmetric and symmetrical stretching vibrations (ν-OH). The lower frequency components were
characteristic stretching vibrations of hydrogen bound hydroxyl groups
from adsorbed interlayer water (Bishop et al., 1994; Babievskaya et al.,
2007). The two bands observed at 3675 and 3720 cm−1 (not shown)
were attributed to the stretching vibrations from the OH groups of
the layers. The OH stretching frequency at 3675 cm−1 was found typical
of the Mg3OH unit in trioctahedral minerals such as vermiculite
(Fernandez et al., 1970; Farmer, 1974). The bands positions were not
shifted by sonication suggesting that the structural OH groups of the
layers were not mainly modified by this treatment.
The bending band of OH (δ-OH) from water exhibited two components in the raw vermiculite at 1630 and 1640 cm−1 (not shown), previously explained by two kinds of water molecules interacting or not
interacting with the interlayer cation (Sposito and Prost, 1982). This
band was broadened after sonication in water or hydrogen peroxide
whatever the sonication time.
(b)
(a)
400
900
1400
1900
2400
2900
3400
3900
Wavenumbers / cm-1
Fig. 5. Infrared spectrum of K-vermiculite (a), vermiculite sonicated in H2O for 5 h (b), and
spectra of vermiculite sonicated in H2O2 for 5 h (c) and 12 h (d).
In the spectra of the sonicated vermiculites (in water or hydrogen peroxide), the presence of three additional bands was observed compared to
the spectra of raw material: a band at 877–880 cm−1 (not shown), a narrow band at 1380 cm−1 (Figs. 6 and 7), and a very broad band at 1440–
1460 cm−1 typical of carbonate species. The band at 877–880 cm−1
was attributed to the ν2 bending modes of the (CO3)2− units. The bands
at 1380 cm−1 and ~1450 cm−1 were assigned to the ν3 antisymmetric
(CO3)2− stretching modes. The observation of two bands for these
modes suggested the presence of two independent carbonate units possibly in different phases. The bands at 1380 cm−1 was attributed to free
carbonate ions in inorganic salts (D3h symmetry) at the clay surface,
as its asymmetric ν(CO) vibration (ν3) was previously reported at
1415 cm−1 (Farmer, 1974). In the case of surface carbonates, a splitting
of this weak asymmetric ν (CO) vibration (ν3 mode) into two peaks of a
a
b
c
d
Fig. 4. Scanning electron micrographs of vermiculite sonicated in H2O2 for 1 h (a), 5 h (b), 7 h (c) and 12 h (d).
A.N. Nguyen et al. / Applied Clay Science 72 (2013) 9–17
symmetric and an asymmetric C\O stretch of the unbound oxygen is
caused by a lowering of the D3h symmetry of the carbonate ion. This
splitting, Δν3, is related to the structure of the surface (adsorbed) carbonate (e.g., monodentate, bidentate, or bridged) (Prescott et al.,
2005; Du et al., 2010). Weak bands at 1480 and 1270 cm−1 (indicated
in Figs. 6 and 7) might be assigned to the splitting of the ν3 mode into
two peaks. The determined Δν3 splitting was 210 cm−1 and might be
due to monodentate carbonate species observed at the surface of
the vermiculites sonicated in water or in H2O2 as Δν3 splitting of
170 cm−1 and 225 cm−1 was reported for adsorption of this carbonate
species on MgO (Evans and Whateley, 1967) and Al2O3 (Busca and
Lorenzelli, 1982), respectively. Very weak bands at the same positions
were also visible in the raw vermiculite (Fig. 7) indicating the presence
of very small amount of monodentate carbonates adsorbed at the layer
edges.
The broad band at ~ 1450 cm −1, also assigned to ν3 vibration in
carbonate, was not clearly observed in the spectra of the vermiculites
irradiated in hydrogen peroxide, except for the 5 h sonicated sample.
This band was attributed to a second crystalline carbonate form (possibly
hydrated and lattice form) at the surface of the vermiculite which was
more favourably obtained by sonication in water. This form might be hydrated K2CO3 as the maximum absorption bands for the infrared spectrum of this solid were reported at 1379 and 1464 cm−1 (Schutte and
HBuijs, 1961), in agreement with the positions of the main observed signatures: sharp peak and broad band.
The formation of the carbonate deposits identified by infrared at
the surface of sonicated vermiculites was attributed to the salt precipitation due to the evaporation of the solution. Indeed, similar signal at
1390 cm −1, attributed to the vibration mode of CO32− ions, was also
reported for nanosized nickel aluminate spinel (Jeevanandam et al.,
2002) or nanosized copper aluminate powders (Weizhong et al.,
2010) prepared by sonochemistry. Moreover, the characteristic infrared bands of carbonates were found to disappear after washing the
sonicated vermiculite samples in acidic medium, confirming the carbonates elimination by dissolution.
The sonication led to the exfoliation of the potassium exchanged vermiculite so that the exchangeable potassium cations were dissolved into
the solution (water or hydrogen peroxide). Indeed, Inductively Coupled
Plasma (ICP) titrations of the supernatant obtained from filtrated sonicated suspensions showed the presence of higher concentrations of K+
(47 mg L−1, Table 2) compared to other cations (Mg2+, etc.). The carbonate ions were formed during the ultrasound treatment possibly
from dissolved carbon dioxide gas and hydroxide anions (OH−). The
hydroxide anions were formed during the exfoliation process as previously reported (Kehal et al., 2010; Muromtsev et al., 1990; Obut and
1380
1270
1480
(e)
Absorbance (a.u.)
1450
(d)
(c)
(b)
1270
1480
1380
(e)
Absorbance (a.u.)
14
(d)
1450
(c)
(b)
(a)
1200
1250
1300
1350
1400
1450
1500
1550
1600
Wavenumbers / cm-1
Fig. 7. Infrared spectra of vermiculite sonicated in H2O2 for 3 h (b), 5 h (c), 9 h (d), 12 h
(e) and spectrum of K-vermiculite (a) between 1200 and 1600 cm−1.
Girgin, 2002). The carbonates precipitated with leached interlayer cations during the drying of the solution to recover the solid vermiculite
material according to the following proposed reaction:
þ
−
2MðaqÞ þ CO2ðaqÞ þ 2OH →M2 CO3 ðH2 OÞx þ ð1−xÞH2 O
The carbonates formed were expected to be mainly K2CO3(H2O)x
and Na2CO3(H2O)x as the leached cations identified by ICP were
mainly K + (dissolved into the solution after the separation of the
layers), and Na + (possibly originating from the glass reactor leaching
owing to the sonication).
3.6. Thermogravimetric analyses
The TGA curve of the K-vermiculite (Fig. 8a) showed no weight loss
due to dehydration below 200 °C as the K-vermiculite from which the
interlayer cations were exchanged with potassium cations is known to
be less hydrated than Na-vermiculite or Ca-vermiculite (Kawano and
Tomita, 1991; Tardy et al., 1985). Due to the ionic size of the K+ ions
which exactly fit the cavity of the layers, the hydration of the interlayer
spacing is hindered. In Fig. 8 (a), a first small weight loss of the heated
K-vermiculite (0.25%) is observed in the range [275–350 °C] which can
be attributed to adsorbed organic impurities, no weight loss was observed for the K-vermiculite by increasing the temperature from above
350 °C to about 450 °C. The weight loss of the K-vermiculite between
450 °C and 850 °C is due to dehydroxylation. The large weight loss at
T>850 °C is related to structural changes of the vermiculite, i.e. to the
collapse of the layers into a 3D structure and further sintering of the materials at higher temperature as previously reported (Pérez-Maqueda et
al., 2003). The absence of CO2 loss and the presence of structural water
(850 °C–900 °C) from the K-vermiculite were detected by mass spectrometry (MS) (Fig. 8b and c).
The TGA curve of the ultrasonic treated vermiculite (H2O2, 5 h)
exhibited four weight losses (Fig. 8α). The first weight loss at about
100 °C (~1.3%) was attributed to the elimination of the water molecules
absorbed on the surface of the submicronic vermiculite particles. The
second weight loss between 350 °C and 450 °C (~0.2%) corresponded
to the decomposition of the carbonates formed by sonication, as CO2
emission was identified by MS in this temperature range (Fig. 8γ).
(a)
1200
1250
1300
1350
1400
1450
1500
1550
1600
Wavenumbers / cm-1
Fig. 6. Infrared spectra of vermiculite sonicated in H2O for 3 h (b), 5 h (c), 9 h (d) and
12 h (e) and spectrum of K-vermiculite (a) between 1200 and 1600 cm−1.
Table 2
Inductively coupled plasma (ICP) analysis of filtered solution from vermiculite's suspension
(after 5 h ultrasonic treatment in H2O2).
Element
Mg
Si
Al
Fe
Ca
K
Na
Ti
Content (mg L−1)
3.3
9.4
0.06
b0.02
5.8
47
94
0.04
A.N. Nguyen et al. / Applied Clay Science 72 (2013) 9–17
Ioncurrent x 1013 / A
TGA / %
24
100
99
(α ) TG US Verm.
98
(β ) MS-H 2O US Verm.
19
(a) TG K-Verm.
14
9
97
(γ) MS-CO2 US Verm.
(b) MS-H2O
96
(c) MS-CO2
95
0
200
400
600
800
4
-1
1000
Temperature / °C
Fig. 8. Thermograms (TG) of K-exchanged vermiculite (K-Verm.) (a), and sonicated (5 h, in
H2O2) vermiculite (US Verm.) (α). Mass spectrometry (MS) signals of steam evolving gas
from K-vermiculite (b), and sonicated (5 h, in H2O2) vermiculite (US Verm.) (β). MS signals
of CO2 evolving gas from K-vermiculite (c), and sonicated (5 h, in H2O2) vermiculite
(US Verm.) (γ).
A third weight loss (~1.4%) was attributed to dehydroxylation in the
range 450–850 °C (Marcos et al., 2009). MS peaks of evolving steam
at 500 °C and 850 °C (Fig. 8β), brought out that the dehydroxylation
occurred in two steps. The first loss of water at 500 °C was attributed
to the presence of interstratified mica as the dehydroxylation of the
interstratified mica layers in vermiculite was reported in the range of
550–575 °C (Justo et al., 1993). The second loss of water at 850 °C
was related to the loss of hydroxyl anions from the vermiculite layers
(Walker and Cole, 1957) and subsequent structural rearrangement.
Finally a fourth weight loss observed at T > 850 °C, was attributed
to sintering, similarly to raw vermiculite. The TGA characterization
pointed out the high hydration rate of the sample prepared by sonication, explained by its high specific surface area according to the
small-sized particle (submicronic platelets of less than 10 nm thickness) enhancing the water adsorption sites. Moreover, the presence
of carbonates previously observed by infrared was confirmed by the
MS signal observed for the vermiculite sonicated in hydrogen peroxide
(Fig. 8γ).
3.7. pH and zeta potential
3.7.1. pH of the vermiculite dispersions and of the sonicated solutions
Whatever the sonication time, the pH of the aqueous dispersions
(Table 3) of vermiculites prepared by sonication in hydrogen peroxide
was in the range of 7.7–8.7, thus lower than the pH of raw vermiculite
(i.e. 9.0). This drop could not be attributed to the thermal decomposition (by thermal shock treatment) or dissolution of some carbonates
(in hydrogen peroxide solution) associated to vermiculite, as traces
were also identified by infrared spectroscopy and TGA in sonicated vermiculite samples exhibiting lower pH.
The irradiation of vermiculites in H2O or H2O2 solutions led to the
dissolution of the interlayer cations (mainly K + as proved by the ICP
titrations in Table 2) into the solution. Probably OH − ions were liberated from the surface to restore the charges equilibrium (Üçgül and
Gírgín, 2002; Kehal et al., 2010). Similarly, Muromtsev et al. (1990)
showed the release in solution of magnesium and calcium exchangeable cations of vermiculite after chemical exfoliation in H2O2 and
proved the presence of Mg 2+ and Ca 2 + ions in the peroxide solution.
As a consequence, the pH of aqueous dispersions of modified vermiculites was lower than the initial pH of the raw vermiculite dispersion
due to the lack of basic OH − groups at the surface of vermiculite. The
formation of traces of carbonates, highlighted by infrared observations, suggested also that dissolved carbon dioxide was transformed
15
into CO32− ions to counterbalance the excess of positive charge in
solution.
The change of the negative charge of the surface of vermiculite
concomitant to exfoliation might also influence the pH of the dispersions. Thus, the pHs of the hydrogen peroxide or water solutions were
measured as a function of the irradiation time (Table 3).
Whatever the liquid medium, the pH increased together with the
sonication time. This increase of pH in the hydrogen peroxide solution (from 3.65 to 6.66 after 5 h of sonication) was related to the decomposition of the H2O2 molecule under ultrasounds and to the
increase of the OH − concentration in the solution. This confirmed
the exfoliation and the release of hydroxide ions from the vermiculite
layers during the delamination. The pH was also slightly increased in
water sonicated suspensions (from 9 to 9.5 after 5 h of sonication)
due to the formation of hydroxide anions concomitant to the exfoliation process, as previously reported by measuring the pH of vermiculite powders obtained by sonication from the water dispersions
(Table 3).
3.7.2. Zeta potential
In order to investigate the changes in the surface charges of vermiculites owing to the exfoliation process, the zeta potentials of the
vermiculites sonicated in H2O and in H2O2 were determined (Fig. 9).
Measurements were made at pH = 4.5 and 6.5 as these values were
close to the pH of the point zero net proton charge of the materials,
i.e. ~ 4.6 for raw vermiculite and ~ 6 for vermiculites sonicated in similar conditions (Kehal et al., 2010).
As expected, the surface charge of the particles were negative and
in the same range as previously published values measured for similar materials (Friend and Hunter, 1970; Duman and Tunç, 2009). For
both sonication media (Fig. 9), zeta potentials decrease when the
pH increases, which corresponds to classical trends for clay minerals
(Barros et al., 2008). The zeta potential measured for the raw vermiculite are in agreement with the values (− 40 mV at pH = 6.5,-25 mV
at pH = 4.5) measured for a thermally exfoliated vermiculite at the
same pHs by Duman and Tunç (2009).
The zeta potential of vermiculite sonicated in H2O2 was clearly increased with sonication time indicating a decrease of the surface negative charge of vermiculite particles. This change in the surface charge
is more pronounced after 5 h of treatment in H2O2 (e.g., at pH = 6.5:
average value of − 227 mV after more than 5 h of treatment instead
of − 40 mV before 5 h sonication) and can be related to the release
of the hydroxide anions from the vermiculite layers. These results
were in agreement with the work of Poyato et al. (2009) showing
that sonication of two vermiculites for size reduction were accompanied by a change of the redox state of the Fe 2+/Fe 3+ cations from the
layers and the layer charge of the material. These authors also found
that the ultrasound activated milling of the Ojén vermiculite in hydrogen peroxide to the smallest size particles (thickness of 20 nm)
yielded to a decrease of the global charge of the layers whereas the
sonication in water gave an increase of the layer charge.
The smaller change in the surface charge obtained by sonication in
water than by sonication in hydrogen peroxide (Fig. 9) is consistent
with the absence of aggregation (constant decrease of the particle size
together with the sonication time) observed in irradiated dispersion
in water medium (Fig. 1, granulometry). By contrast, the sonication in
Table 3
pH of vermiculites and of the H2O or H2O2 suspensions measured after sonication of for
various times.
Treatment time (h)
0
1
2
3
4
5
7
9
12
pH of vermiculitea
pH of H2O2 suspension
pH of H2O suspension
9.00
3.65
9.00
8.64
5.59
9.39
–
5.97
9.50
8.57
6.15
9.53
–
6.28
9.57
7.88
6.66
9.60
8.41
–
–
7.78
–
–
8.39
–
–
a
pH of the material measured according to the NF ISO 10390 standard.
16
A.N. Nguyen et al. / Applied Clay Science 72 (2013) 9–17
Zeta potential (mV)
-10
H2O pH=6.5
H2O pH=4.5
H2O2 pH=6.5
H2O2 pH=4.5
decrease together with sonication time, was enhanced by sonication
treatments in hydrogen peroxide instead of water.
Acknowledgements
-20
The authors thank the National Research Agency (ANR) for funding
the CELAJOAS project (ANR-08-MAPR-0011). An Ngoc Nguyen thanks
the “Commissariat à l'Energie Atomique et aux Energies Alternatives”
for a PhD grant.
-30
-40
References
-50
0
3
6
9
12
Sonication time (h)
Fig. 9. Zeta potential measured at pH = 6.5 and 4.5 of the vermiculites sonicated in H2O
or in H2O2, as a function of sonication time.
H2O2 medium for time higher than 5 h, led to the aggregation of the
submicronic platelets, because of the lowering of the inter-particles repulsions due to the surface charge decrease. This aggregation was
brought out by the bimodal size distribution and the BET surface area
increase. These results suggested that reaching a threshold particle
small size (submicronic size) by efficient size reduction and exfoliation
(as using H2O2 medium for sonication), gave the possibility of particle
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at the surface to link the particles thanks to water adsorbed molecules.
4. Conclusion
Whatever the sonication medium (H2O or H2O2), a delamination
of the raw vermiculite layers associated to a significant decrease in
particle sizes was observed. Micron and submicron-sized platelets of
vermiculite were obtained after short treatment times (b 12 h).
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sizes (about 1 μm) were obtained for the longest ultrasonic treatment
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that the vermiculite size distribution was narrower by sonication in
H2O2 than in H2O, but still sonication-time dependent. The crystallographic structure of vermiculite was not damaged during the process.
Scanning Electron Microscope observations and X-ray diffraction
studies exhibited nanometric (stacking of 5 layers for 12 h. sonication) thickness and submicrometric (or micrometric) basal dimensions for the platelets of vermiculite for sonication times lower than
5 h.
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ultrasound activated reaction of the dissolved carbon dioxide (from
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from the layers in order to counterbalance the dissolution of
interlayer cations (mainly K +) into the solution. The modification of
the surface charge of the particles brought out by zeta potential
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