Kapok fiber oriented-polyaniline nanofibers for efficient Cr(VI) removal

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Chemical Engineering Journal 191 (2012) 154–161
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Chemical Engineering Journal
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Kapok fiber oriented-polyaniline nanofibers for efficient Cr(VI) removal
Yian Zheng a,b , Wenbo Wang a , Dajian Huang a,b , Aiqin Wang a,∗
a
b
Center of Eco-materials and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
Graduate University of the Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history:
Received 10 January 2012
Received in revised form 28 February 2012
Accepted 28 February 2012
Keywords:
Hexavalent chromium
Removal
Kapok fiber
Polyaniline
Oriented
a b s t r a c t
Kapok fiber (KF) was combined with polyaniline (PAN) to obtain an adsorbent via in situ rapid polymerization of aniline (AN). The results indicate that KF can guide the growth orientation of PAN by which KF
oriented-PAN nanofibers were developed and used as the adsorbent to remove hexavalent chromium.
The effects of operating parameters including pH, contact time, Cr(VI) concentration and coexisting heavy
metals were studied. The pseudo-second-order equation and three adsorption isotherms including Langmuir, Freundlich and Redlich–Peterson equations were applied to determine the adsorption rate and
capacity. The results show that the as-prepared KF/PAN adsorbent has a comparable adsorption capacity
with PAN at lower initial Cr(VI) concentration. Simultaneously, KF/PAN exhibits a faster adsorption rate
as a result of its intrinsic large lumen. Coexisting heavy metals have no obvious effects on the adsorption
capacity, suggesting that KF/PAN is a highly efficient and economically viable adsorbent for selective
Cr(VI) removal.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Chromium is one of the most toxic pollutants generated by
the electroplating, leather tanning, metal finishing, steel fabrication, textile industries and chromate preparation. The fate of
chromium in the environment is closely related to its chemistry.
In aqueous environment, chromium predominantly exists in two
common oxidation states, trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)]. Cr(III) naturally occurs in the environment
and is an essential micronutrient for human beings. Conversely,
Cr(VI) is more hazardous, being carcinogenic and mutagenic to living organisms [1,2]. Moreover, Cr(VI) exhibits high mobility in most
neutral to alkaline soils, posing thus a great threat to surface water
and groundwater [3]. Due to environmental concern, the allowed
discharge limit of Cr(VI) into inland surface water is 0.1 mg/L, and
into the drinking water prescribed by the World Health Organization is 0.05 mg/L. Therefore, Cr(VI) must be substantially removed
from the environment, in order to prevent the deleterious impact
of Cr(VI) on ecosystems and public health.
There are various methods to remove Cr(VI) from the environment, such as electrocoagulation [4], electrochemical reduction [5],
chemical reduction [6], supported liquid membrane technique [7],
ion exchange [8], Donnan dialysis [9], photocatalytic reduction [10],
reverse osmosis [11], and adsorption [12,13], etc. As one of the
most promising techniques for Cr(VI) removal from wastewaters,
∗ Corresponding author. Tel.: +86 931 4968118; fax: +86 931 8277088.
E-mail address: [email protected] (A. Wang).
1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.cej.2012.02.088
adsorption technology has been employed for many years and the
effectiveness of various adsorbents has been demonstrated. During
recent years, interest has been primarily focused on the low-cost
adsorbents, and in 2010, Miretzky and Cirelli have reviewed the
Cr(VI) and Cr(III) removal from aqueous solution by raw and modified lignocellulosic materials, such as bagasse, bark, biomass, bran,
cake, coir, husk and hull, leaves, sawdust, straw and so on [2].
Polyaniline (PAN) is one of the most important and extensively studied conducting polymers. Due to its easy synthesis and
high environmental stability, PAN is becoming the most promising materials for application in many different fields. PAN carries
large amounts of amine and amine functional groups, and thus
is expected to have interactions with negatively charged anion
because of its innate cationic nature. Up to now, PAN and its composites have been explored for the removal of Hg(II) [14], sulfonated
dyes [15,16], arsenate [17] and chromium [18–20]. Due to the
increasing consciousness of cost effectiveness and public environmental protection, new composite adsorbents combining PAN and
renewable resources have attracted much attention. The combination of renewable resources and synthetic polymers usefully takes
the advantages of the biocompatibility and environmental friendliness of the renewable materials and the physical and mechanical
properties of the synthetic components. For instance, Kumar and
Chakraborty developed a polyaniline/jute fiber composite adsorbent for Cr(VI) removal [21]. Kapok fiber (KF), an agricultural
product, is fluffy, lightweight, non-allergic, non-toxic, resistant to
rot and odorless. Conventionally, KF is used as stuffing for bedding, upholstery, life preservers and other water-safety equipment
because of its excellent buoyancy and for insulation against sound
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Inc., Minneapolis, USA).
2.4. Batch experiments
and heat because of its air-filled lumen [22]. According to Hori
et al., KF is mainly composed of cellulose, xylan and lignin [23], and
shows a significantly homogeneous hollow tube shape, which is
anticipated to direct the growth orientation of PAN and accelerate
the adsorption rate for an adsorbate. Considering these information, the specific objectives of this study are (i) to prepare KF
oriented-PAN adsorbent using a facile method at room temperature, (ii) to evaluate the efficacy of the as-prepared KF/PAN for
Cr(VI) removal, and (iii) to compare the adsorption performances
of the as-prepared KF/PAN and PAN.
The batch experiments were performed by mixing 50 mg adsorbent with 25 mL Cr(VI) solution in 50 mL conical flasks in a
temperature controlled orbital shaker with constant shaking at
120 rpm. The Cr(VI) adsorption on the as-prepared adsorbents was
first studied at different pH values ranging from 4.5 to 9.0 to investigate the pH-dependence of Cr(VI) adsorption, with an initial Cr(VI)
concentration of 100 mg/L, contact time of 3 h and temperature of
30 ◦ C. The adsorption kinetics study was carried out with varying
contact time ranging from 0 to 3 h at 30 ◦ C with an initial Cr(VI)
concentration of 100 mg/L and an adsorbent amount of 2 g/L at
pH 4.5. During the adsorption process, the change in solution pH
was also measured as a function of contact time. The adsorption
isotherm study was carried out with different initial Cr(VI) concentrations ranging from 100 to 400 mg/L at pH 4.5 while maintaining
the adsorbent amount of 2 g/L, contact time of 3 h and temperature
of 30 ◦ C. The amount of Cr(VI) adsorbed was calculated using the
following equation:
2. Materials and methods
qe =
2.1. Materials
where qe is the adsorption capacity in mg/g, C0 and Ce are the initial and equilibrium Cr(VI) concentration in mg/L, m is the mass of
adsorbent used in mg, and V is the volume of Cr(VI) solution in mL.
During the non-competitive experiments, the concentration of
free Cr(VI) ions in the solution was determined spectrophotometrically using 1,5-diphenyl carbazide as a complexing agent in an
acidic solution (GB7466-87). The absorbance of the purple–violet
colored solution was recorded at 550 nm after 10 min using
a UV–visible spectrophotometer. To evaluate the adsorption
capacities of the as-prepared adsorbents under the competitive
experiments, 50 mg adsorbent was contacted with 25 mL mixture
solution containing 100 mg/L Cr(VI), 100 mg/L Cu(II) and 100 mg/L
Ni(II) in a temperature controlled orbital shaker at 30 ◦ C/120 rpm
for 3 h. The concentrations of free metal ions before and after the
adsorption were determined by inductively coupled plasma atomic
emission spectrometry (ICP-AES). Sample dilution was conducted
before measurement, if necessary. The analytical study of Cr(VI)
was carried out in triplicates, and the relative standard deviation
was less than 5%.
In the present study, the experiments were carried out for
Cr(VI) removal from aqueous solutions which were prepared by
dissolving the potassium dichromate into distilled water. Though
in principle, there was a possibility of reduction of Cr(VI) into Cr(III)
ions at low pH, the previous studies reported that at pH above 3.0,
amount of Cr(III) in solution was almost negligible [18]. To avoid
the interference of Cr(III) in the solution, the pH was adjusted to
above 3.0 during the whole adsorption process.
Fig. 1. Schematic preparation of KF/PAN.
Aniline (AN, Guangdong Xilong Chemical Co., Ltd., China) was
purified by vacuum distillation before polymerization. Kapok fiber
(KF, Shanghai Panda Industry Co., Ltd., China) and ammonium
persulfate (APS, Shanghai Sinopharm Chemical Reagent Co., Ltd.,
China) were used as received. The stock solution containing
1000 mg/L Cr(VI) was prepared by dissolving a known quantity
of potassium dichromate (K2 Cr2 O4 ) in distilled water. This stock
solution was diluted as required to obtain the working solutions
containing 100–400 mg/L Cr(VI). The solution pH was adjusted in
the range of 4.5–9.0 by adding 0.1 and 1.0 mol/L HCl or NaOH solutions. All other chemicals used were of analytical reagent grade,
and all solutions were prepared with distilled water.
2.2. Preparation of KF/PAN and PAN
KF/PAN was prepared by the chemical oxidation method as follows: 2 g aniline monomer was dissolved in 66 mL of 1.0 mol/L
HCl. The mixture was cooled in an iced bath while an appropriate
amount of KF was dispersed and stirred for 30 min. The polymerization started by introduction of pre-cooled APS solution (A designed
amount of APS in 16 mL of 1.0 mol/L HCl). The reaction mixture was
kept in an ice bath for 1 h and then kept for 16 h at room temperature. After centrifugation, the insoluble dark-green precipitate was
washed with distilled water until water became colorless. Finally,
the product was washed with industrial alcohol and dried at 50 ◦ C
in an oven. The preparation procedure of PAN was similar to that of
KF/PAN except for the addition of KF. A typical preparation process
is schemed in Fig. 1.
2.3. Central composite design (CCD)
CCD was employed for the optimization of two variables, i.e.
amounts of KF and APS. In the experimental design model, amounts
of KF (0.4–1.0 g) and APS (1.62–7.20 g) were taken as input variables and the adsorption capacity for Cr(VI) was taken as the
response. The variables obtained from the CCD model were studied
and their low and high levels were coded as −1 and +1, respectively (Table S1, Supporting information). Response surface was
(C0 − Ce )V
m
(1)
2.5. Desorption experiments
For desorption studies, 50 mg adsorbent was first contacted with
100 mg/L Cr(VI) for 3 h at 30 ◦ C. After the adsorption was complete
and the supernatant was decanted, the Cr(VI)-loaded adsorbent
was washed with distilled water and then mixed with 25 mL NaOH
solution with different concentration ranging from 1.0 to 4.0 mol/L
while maintaining the adsorbent amount of 2 g/L at 30 ◦ C for 2 h.
The desorption ratio DR (%) can be calculated from the formula:
DR =
q
qe
(2)
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Fig. 2. FTIR spectra of KF (a), PAN before and after Cr(VI) adsorption (b, c) and KF/PAN before and after Cr(VI) adsorption (d, e).
where q is the desorption capacity in mg/g, and qe is the adsorption
capacity in mg/g.
2.6. Characterization
FTIR was recorded on a Thermo Nicolet NEXUS TM spectrophotometer using KBr pellets in the range of 400–4000 cm−1 . For
morphological characterization of sample surfaces, a field emission
scanning electron microscope (FE-SEM, S-4800, Hitachi) was used.
The point of zero charge (pHPZC ) of the adsorbent was determined by an immersion technique [24]. A number of aqueous
solutions of 0.1 mol/L NaCl (blank solution, 25 mL) of varying pH
values were prepared by the addition of 0.1 and 1.0 mol/L HCl or
NaOH solutions, and then 50 mg adsorbent was added to each of
these solutions. The aqueous suspensions were equilibrated for
24 h to reach an equilibrium pH value. The pH of each suspension
was then measured using a digital pH meter (Mettler-Toledo, FE20)
standardized by buffers with pH 6.86 and pH 4.01. The change in
the pH during equilibration, pH, was then determined from the
formula of pH = pH(blank solution) − pH(suspension). The pHPZC
was identified as the pH where the minimum pH value was
obtained.
Samples prepared for immersion technique were used for zeta
potential measurements. The measurements were performed on a
Malvern Zetasizer Nano-ZS apparatus. Measurements were done
immediately after the pH measurements for the immersion technique. Samples of the suspension were collected in disposable
plastic droppers and injected directly into the electrophoresis cell.
Three measurements were recorded and an average zeta potential
was applied.
3. Results and discussion
structure related to the C N stretching vibration, respectively [26].
The absorption band at 1107 cm−1 is due to the C H in plane bending of 1,4-ring [25]. The FTIR spectrum of KF/PAN is similar to that of
PAN apart from several characteristic absorption bands originated
from KF. For the FTIR spectrum of KF, the three important ester
bands at 1741, 1374 and 1244 cm−1 are associated with carbonyl
bonds (C O ester). The absorption band at 1056 cm−1 is within the
region of polysaccharide [27]. These absorption bands from natural polymers appear in the FTIR spectrum of KF/PAN, or overlap
with absorption bands from PAN by which the absorption intensities are increased. All analysis results indicate that KF is combined
successfully with PAN.
In order to verify the adsorption, FTIR spectra are analyzed
for the as-prepared adsorbents without or with the adsorption of
Cr(VI). It is observed that after Cr(VI) adsorption, the absorption
bands assigned to C N stretching vibration of a quinonoid ring
and C C stretching vibration of a benzenoid ring have shifted from
1557–1563 cm−1 and 1470–1486 cm−1 to higher wavenumbers
1577–1585 and 1486–1500 cm−1 . The absorption bands attributed
to C N stretching vibration of secondary amine have also shown
some changes from 1290–1296 cm−1 to higher wavenumbers of
1313–1320 cm−1 . The results indicate that nitrogen atoms have
involved in the adsorption, and certain chemical bonds are formed
between nitrogen and Cr(VI) species, which causes the increase of
the vibration frequency of these surface chemical groups. In addition, some changes in the peak shape have been observed in the
region of 1000–1300 cm−1 , suggesting that the Cr(VI) adsorption
can affect and change the surrounding environment of the functional groups. The last change is the appearance of a new absorption
band at around 900 cm−1 or 940 cm−1 in the Cr-loaded adsorbents,
and this is ascribed to the presence of Cr O and Cr O bonds from
the Cr(VI) species, meaning that Cr(VI) has been adsorbed on the
surfaces [28,29].
3.1. FTIR analysis
3.2. SEM analysis
The characteristic absorption bands of KF, KF/PAN and PAN are
displayed in Fig. 2. For the FTIR spectrum of PAN, the absorption
band at 1557 cm−1 is assigned to C N stretching vibration of a
quinonoid ring, while 1470 cm−1 is associated with C C stretching vibration of a benzenoid ring. The appearance of the absorption
bands of about equal intensities at 1557 cm−1 and 1470 cm−1 suggests the presence of PAN in its 50% intrinsically oxidized base form
(Emeraldine) (Fig. S1, Supporting information) [25]. The absorption
bands at 1290 cm−1 and 1239 cm−1 are attributable to C N stretching vibration of secondary amine of PAN backbone and bipolaron
SEM micrographs of KF, KF/PAN and PAN are shown in Fig. 3.
It is observed that KF has a silky surface, while PAN particles are
aggregated densely to larger particles. SEM micrograph at 50,000×
magnification demonstrates that actually PAN is present in the form
of nanofibers (Fig. 3d). After in situ polymerization process, PAN is
oriented on the fibrous surface of KF, rendering that the surface
of KF gets coarse besides few aggregated particles. This finding is
meaningful and mirrors that KF has a guiding role in the growth of
PAN nanofibers.
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Fig. 3. SEM micrographs of (a) KF, (b) KF/PAN, and (c), (d) PAN.
SEM analysis at 15,000× magnification is undertaken to find
out the changes in morphology after adsorption (Fig. S2, Supporting information). We can observe that KF/PAN and PAN show rough
or porous surfaces initially which turn smooth after the adsorption.
It is due to that the pores and surfaces of the adsorbents are covered
by Cr(VI) molecules as a newly coated layer.
off the KF surface, while PAN particles are uniformly distributed on
the KF surface at higher KF amount (Fig. S3, Supporting information).
3.3. Response surface methodology
To understand better the adsorption process, two important values, i.e. point of zero charge (pHPZC ) and isoelectric point (pHIEP )
should be determined. It is now widely accepted pHPZC is a representation of the change in response to the net total surface charge
of the particles, while pHIEP represents the net external charge on
the surface in solution [30]. In this study, pHPZC and pHIEP were
determined using immersion technique and zeta potential measurements, as shown in Fig. 5. The results indicate that the pHPZC
is 6.2 for KF/PAN and 4.7 for PAN, while the pHIEP is 4.9 for KF/PAN
and 4.1 for PAN. Effectively, the difference between pHPZC and pHIEP
would give an indication of the surface charge distribution of the
adsorbents. Compared with pHIEP , the higher pHPZC value would
indicate more positively charged interior surface than the external
for an adsorbent. In addition, the pHPZC and pHIEP values for PAN
are low and closely matched, suggesting a more homogeneous surface, while greater difference between pHPZC and pHIEP indicates
that the surface of KF/PAN is generally more heterogeneous [31].
The pH of the aqueous solution is one of the most important
parameters in the heavy metal adsorption process. Generally, at
below pH 3.0, partial chromium ions in the solution exist as trivalent state, while above pH 3.0, chemical reduction of Cr(VI) to
Cr(III) occurs to lesser extent and anionic chromium species, such as
HCrO4 − or CrO4 2− , are the major species [32]. To prevent any chemical reduction during the adsorption process, the pH values were
investigated from 4.5 to 9.0. In this case, the equilibrium pH was
found to increase from 3.7 to 6.6 for KF/PAN and 3.4 to 5.2 for PAN, as
exhibited in Fig. 6. Generally, the adsorption of cations is favored at
pH > pHPZC , while the adsorption of anions is favored at pH < pHPZC .
As expected, the lower pH favored the Cr(VI) adsorption because
at low pH values, the as-prepared adsorbents could be protonated
For a preliminary experiment, CCD was employed for the optimization and the three-dimensional response surface is shown in
Fig. 4. According to the analysis of variance (Table S2, Supporting information), it is observed that no model terms are significant
for these values of “Prob > F” greater than 0.05. Therefore, we select
the higher KF content (1.0 g) and lower APS content (1.62 g) to
obtain an adsorbent of KF oriented-PAN nanofibers from the point
of application. In addition to this, we have also observed during
the experiments that when PAN amount is fixed, lower KF amount
would result in an adsorbent with partial aggregated PAN particles
Fig. 4. Response surface graphs of adsorption capacity for Cr (VI).
3.4. Effect of pH on adsorption
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Fig. 5. Experimental curves corresponding to immersion technique (a) and zeta potential measurements (b) for KF/PAN and PAN.
(pH < 6.2 for KF/PAN and pH < 4.7 for PAN), which was beneficial
for the adsorption of the negatively charged Cr(VI) species. As the
pH increased, the as-prepared adsorbents were increasingly deprotonated in addition to increasing competition between OH− and
Cr(VI) species, leading to a reduction in Cr(VI) adsorption. Within
the pH values studied, a gradual decrease in the adsorption capacity was observed from 44.05 to 32.25 mg/g for KF/PAN and 50.05
to 49.18 mg/g for PAN. Nevertheless, at pH > pHPZC , an appreciable
adsorption capacity was also observed, suggesting that in addition
to the electrostatic interaction, other mechanism may also coexist
during the adsorption process.
Here, it should be mentioned that after the adsorption, the equilibrium pH of Cr(VI) solution is different from the original one and
the equilibrium pH in all cases is lower than the initial pH. With
increasing initial pH from 4.5 to 9.0, a lag in pH was noted for
KF/PAN compared with PAN. PAN is a typical conducting polymer
and in weak acid solution, the further doped process of conductive
PAN nanofibers requires H+ , while the dedoped process consumes
OH− in weak alkaline solution [33], which results in the reduction
of pH for PAN within the pH range studied.
Depending on the pH and concentration of the solution, Cr(VI)
species can be found in various forms as H2 CrO4 , HCrO4 − , CrO4 2− ,
and Cr2 O7 2− . The following equilibriums are present between them
[34], from which a predominance graph is plotted in Fig. S4 (Supporting information).
H2 CrO4 H+ + HCrO4 −
−
+
HCrO4 H + CrO4
2−
pKa1 = 0.8
(3)
pKa2 = 6.5
(4)
2HCrO4 − H2 O+Cr2 O7 2−
pK = −1.52
(5)
Interaction (5) is independent of pH and depends only on the
total Cr(VI) concentration, it is, however, insignificant in this study
as the initial Cr(VI) concentration is 100 mg/L (up to 400 mg/L). As
determined, the equilibrium pH values are in the range of 3.7–6.6
for KF/PAN and 3.4–5.2 for PAN. Therefore, the Cr(VI) species are
mainly present in the form of HCrO4 − in this study.
Preliminary experiments showed that under the same experimental conditions, the adsorption capacity for Cr(VI) was 0.78,
44.05 and 50.05 mg/g using KF, KF/PAN and PAN as the adsorbent,
respectively. PAN carries large amounts of amine and amine functional groups which make great contributions for anionic HCrO4 −
removal. KF shows little adsorption for Cr(VI), and accordingly, the
introduction of KF into PAN would reduce the adsorption capacity. In spite of the decrease in adsorption capacity, the as-prepared
KF/PAN show their potential for Cr(VI) removal, as the ratio of
KF:PAN is 1:2 (KF percentage of 33.33%) in KF/PAN, while the reduction percentage in adsorption capacity is observed to be 11.99%.
3.5. Adsorption kinetics
Fig. 7a indicates the results of adsorption kinetic experiments
conducted to determine the equilibrium time required for Cr(VI)
removal. For KF/PAN, little change in the adsorption capacity is
observed after 10 min, while for PAN, this time is prolonged to
60 min. KF is a fiber with a huge hollow lumen, which provides a site
to allow these adsorbate molecules to disperse freely and quickly,
and accordingly, they would be easily encountered and captured
by KF/PAN. However, the as-prepared PAN particles are aggregated
densely to larger particles, which are not beneficial for those adsorbate molecules to enter and be adsorbed. Subsequently, more time
is required to reach the adsorption equilibrium for PAN.
The change in pH of Cr(VI) solution with contact time is shown
in Fig. 7b. It is clearly observed that with the addition of KF/PAN and
PAN, the pH value experienced a sudden decrease from 4.5 to 3.7
Fig. 6. Cr(VI) adsorption (a) and equilibrium pH (b) as a function of initial pH. Adsorption experiments: C0 , 100 mg/L; t, 3 h, adsorbent amount, 2 g/L; 30 ◦ C/120 rpm.
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Fig. 7. (a) Effect of contact time on the Cr(VI) adsorption using KF/PAN and PAN as the adsorbents, and (b) changes in solution pH with contact time. Adsorption experiments:
C0 , 100 mg/L; pH 4.5; adsorbent amount, 2 g/L; 30 ◦ C/120 rpm.
and 3.3 for KF/PAN and PAN within 10 min. After that, the pH value
showed almost a constant for KF/PAN and a slight increase for PAN.
One can speculate that the sudden decrease in pH is ascribed to H+
ions escaped from doped PAN.
In order to understand better the adsorption behaviors, the
pseudo-second-order equation was applied to test the adsorption
experimental data:
1
t
t
=
+
qt
qe
k2 q2e
(6)
where qe and qt are the amounts of Cr(VI) adsorbed (mg/g) at
equilibrium and at time t, respectively. k2 is the rate constant of
pseudo-second-order adsorption (g/mg min), which can be determined from the straight-line plot of t/qt against t. The initial
adsorption rate h (mg/g min) can be obtained using the equation
of h = k2 q2e . These parameters and correlation coefficient R2 were
calculated and listed in Table 1. It is observed that the correlation coefficient R2 for pseudo-second-order kinetic model is above
0.999 and the calculated qe values show a good agreement with
the experimental values, indicating that the pseudo-second-order
kinetic model can describe well the adsorption experimental data.
The pseudo-first-order kinetic model has also been used extensively to describe the adsorption of metal ions onto adsorbents,
but in most cases in the literature, the pseudo-first-order equation does not fit well for the whole range of contact time and is
generally applicable over the initial 20–30 min of an adsorption
process. The main disadvantages of this model are (i) that the linear equation does not give theoretical qe values that agree well with
experimental qe values, and (ii) that the plots are only linear over
the first 30 min, approximately. Beyond this initial 30 min period
the experimental and theoretical data do not correlate well. Many
researchers have tested the first-order equation for the adsorption of metal ions, but most of the fits are moderate or poor [35].
In this case, the pseudo-second-order equation shows its importance. By a careful and comparative study, we can conclude that
the rate constant of KF/PAN is twice than that of PAN, meaning its
faster adsorption rate, especially at the beginning of an adsorption
process, which is related with the large hollow structure of KF, as
observed from the SEM images.
transfer resistances of metal ions from the aqueous phase to solid
phase resulting in higher probability of collision between metal
ions and the active adsorption sites. That is, all the active adsorption
sites have been utilized at higher Cr(VI) concentrations [36].
Fitting of adsorption isotherm equations to experimental data
is an important aspect of data analysis. The Langmuir isotherm is
an indication of surface homogeneity of the adsorbent while Freundlich isotherm hints surface heterogeneity of an adsorbent. The
Redlich–Peterson model is a combination of Langmuir equation as
a limit for high concentration and Freundlich’s for low concentration, and then this model can be used extensively in a wide
concentration range and be believed to be applicable to a large class
of adsorption [37]. In this study, the efficiency of the as-prepared
adsorbents for Cr(VI) was evaluated using the Langmuir, Freundlich
and Redlich–Peterson isotherms:
Langmuir equation :
Freundlich equation :
qe =
qm bCe
1 + bCe
(7)
qe = KCe 1/n
Redlich–Peterson equation :
qe =
(8)
Kr Ce
1 + aCe b
(9)
where qe is the equilibrium adsorption capacity of Cr(VI) onto
adsorbent in mg/g, Ce is the equilibrium Cr(VI) concentration in
mg/L, and the other parameters are different isotherm constants.
Due to the inherent bias resulting from linearization, alternative
isotherm parameter sets have been determined by nonlinear
3.6. Adsorption isotherms
The Cr(VI) adsorption is significantly influenced by the initial concentration of Cr(VI) in aqueous solution. As shown in
Fig. 8, the adsorption capacity increased respectively from 44.05
to 66.16 mg/g for KF/PAN and 50.05 to 145.54 mg/g for PAN with
an increase in initial Cr(VI) concentration from 100 to 400 mg/L.
This may be due to the fact that the increased Cr(VI) concentration
provides the maximum driving force to overcome all the mass
Fig. 8. Adsorption isotherms with KF/PAN and PAN as the adsorbents. Inset shows
the plot of adsorption capacity against initial Cr(VI) concentration. Adsorption
experiments: t, 3 h; pH 4.5; adsorbent amount, 2 g/L; 30 ◦ C/120 rpm.
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Table 1
Estimated adsorption kinetic parameters for Cr(VI) removal.
Adsorbent
qe, exp (mg/g)
KF/PAN
PAN
qe, cal (mg/g)
44.05
50.05
k2 (g/mg min)
−2
1.10 × 10
5.46 × 10−3
44.05
51.02
h (mg/g min)
R2
21.37
14.20
0.9995
0.9995
Table 2
Estimated isotherm parameters for Cr(VI) removal.
Adsorbents
KF/PAN
PAN
Langmuir equation
Freundlich equation
Redlich–Peterson equation
qm (mg/g)
b (L/mg)
R2
K (L/g)
n
R2
a (L/mg)
b
Kr (L/g)
R2
65.66
122.7
0.1517
1.520
0.9574
0.7882
33.25
65.45
7.883
5.934
0.9706
0.9892
0.4527
22.79
0.9317
0.8540
20.62
1587
0.9966
0.9239
regression. The nonlinear regression approach provides a mathematically rigorous method for determining isotherm parameters
using the original form of the isotherm equation.
The estimated model parameters with correlation coefficient
R2 were summarized in Table 2. Investigation of the isothermal
characteristics shows that Cr(VI) removal by KF/PAN is in high
accordance with Redlich–Peterson isotherm, while the adsorption
data of PAN for Cr(VI) are described well by Freundlich isotherm.
Here, it should be mentioned that the adsorption capacity of KF for
Cr(VI) is negligible if compared with that of PAN. Then it is anticipated that compared with PAN, the KF oriented-PAN nanofibers
will exhibit a decreasing adsorption capacity for Cr(VI). Nevertheless, the decreased percentage is far less than the KF content in
KF/PAN. If the adsorption capacity is calculated based on per gram
of PAN, the adsorption capacity of KF/PAN for Cr(VI) is observed to
be higher than that of PAN, if the initial Cr(VI) concentration is less
than 180 mg/L.
3.7. Coexisting metal ions
Most water contaminated with heavy metals contains more
than one heavy metal ion and then it is necessary to determine
the binding ability of the as-prepared adsorbents in multi-metal
solutions. Fig. 9 shows the effects of coexisting metal ions [Cu(II)
and Ni(II)] on Cr(VI) adsorption onto the as-prepared adsorbents.
At the same initial concentration of Cr(VI), Cu(II) and Ni(II) ions,
Cr(VI) showed the highest affinity to the as-prepared adsorbents
and its adsorption was basically not affected by the presence of
Cu(II) and Ni(II) ions in the solution. So, the competition of coexisting metal ions was negligible, and the as-prepared adsorbents can
be applied to quantitative and selective removal of Cr(VI) ions in
aqueous systems containing Cu(II) and Ni(II).
Fig. 9. Adsorption capacities of KF/PAN and PAN for Cr(VI), Cu(II) and Ni(II). Adsorption experiments: C0 , 100 mg/L for each metal ion; t, 3 h; pH 4.5; adsorbent amount,
2 g/L; 30 ◦ C/120 rpm.
3.8. Desorption studies
Desorption studies help to determine the adsorption mechanism and to evaluate the feasibility of regenerating the spent
adsorbent. Since the adsorption of Cr(VI) onto the as-prepared
adsorbents is pH-dependent and the lower pH is beneficial for
the Cr(VI) adsorption, the desorption of Cr(VI) can be achieved
by increasing pH values. The results of desorption experiments
are depicted in Fig. 10. With increasing NaOH concentration from
1.0 mol/L to 4.0 mol/L, a slight increase in the desorption efficiency
was observed for both adsorbents. Nevertheless, the total desorption efficiency was found to be lower, with ∼40% for KF/PAN and
23% for PAN. As shown in Fig. S1 (Supporting information), the desorption process is accompanied by a dedoped process of PAN, which
is not favorable for the Cr(VI) adsorption as the adsorption sites are
reducing, in spite of the variation of Cr(VI) species from HCrO4 − to
CrO4 2− . As SEM images have shown, KF has a silky surface, and PAN
can be oriented on the fibrous surface of KF after in situ polymerization process, while PAN particles are aggregated densely to larger
particles. During the experiments, we have observed that similar
to KF, the KF/PAN is in the form of fluffy structure while PAN is
aggregated densely to large block size with the time. Then, it is
anticipated that compared to PAN, Cr(VI) adsorbed onto KF/PAN
can be easily desorbed.
In addition to this, the finding implies that the electrostatic
attraction is present during the whole adsorption process. If the
electrostatic attraction is in fact the main adsorption mechanism, it
is possible to regenerate the adsorbent by the strong base [38]. The
lower desorption efficiency, however, indicates that there involves
a chemical process between the adsorbents and adsorbate [39],
Fig. 10. Cr(VI) desorption efficiency from the Cr(VI)-loaded adsorbents using NaOH
solutions with different concentrations.
Author's personal copy
Y. Zheng et al. / Chemical Engineering Journal 191 (2012) 154–161
i.e. the Cr(VI) ions have formed stronger bonds with the adsorbent
surface, consistent with FTIR analysis.
4. Conclusions
KF is a renewable resource and has large hollow lumen. Due
to its unique structure, environmental friendliness and cost effectiveness, this raw material is combined with PAN to obtain a novel
adsorbent for efficient Cr(VI) removal from aqueous solution. KF
can guide the growth orientation of PAN nanofibers and the resulting adsorbent exhibits a unique KF-aligned morphology. Compared
with PAN, the as-prepared KF/PAN shows a comparable adsorption
capacity, but faster adsorption kinetics, with the rate constant is
twice than that of PAN. The coexisting Cu(II) and Ni(II) ions have
no obvious effects on Cr(VI) adsorption, suggesting that the asprepared KF/PAN can be used as highly efficient and economically
viable adsorbent for selective Cr(VI) removal. Furthermore, all the
findings in this paper demonstrate that KF has an oriented role for
designing and developing a material, and the unique structure and
properties will expand its application fields.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (no. 21107116 and no. 20877077).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cej.2012.02.088.
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