In this study, pillared layered clays were prepared by modifying Vietnamese bentonite with polymeric Al and Fe. The obtained materials were characteristic of X-ray diffraction analysis, thermal analysis, and nitrogen adsorption/desorption isotherms. The results indicated that hydroxy-aluminum ([Al13O4(OH)24(H2O)12]7+) and poly-hydroxyl-Fe or polyoxo-Fe cations were intercalated into layers of clay, resulting in an increase of
Arsenic is a potentially toxic metal that is said to be one of the most concerned contaminants in aquatic sources. Many approaches have been reported for the removal of arsenic including membrane dialysis, oxidation/reduction, precipitation/coprecipitation, filtration and adsorption, and ion exchange [
In the present article, the preparation of pillared layered clays by modification of bentonite with polymeric Al and Fe and the removal of As(V) from aqueous solution were demonstrated.
Bentonite was obtained from Vietnamese mining company and purified by the sedimentation combined with sonication and centrifugation. Aluminum chloride (AlCl3·6H2O, 99%), ferric nitrate nonahydrate (Fe(NO3)3·9H2O, 99%), silver nitrate (AgNO3, 98%), and sodium hydroxide (NaOH, 98%) were obtained from Guangzhou company, China. Stock solution of H3AsO4 100 mg/L was purchased from Merck. Chemical composition in mass analyzed by EDX of SiO2; Al2O3; Fe2O3; TiO2; MgO; CaO; K2O; and Na2O in mass is 69.1; 18.7; 4.4; 0.37; 4.19; 2.93; 0.25; and 0.07, respectively. The cation exchange capacity (CEC) was found to be 0.75 mmol/g.
X-ray diffraction (XRD) patterns were obtained on D8-Advance Brucker, Germany with CuK
The purified bentonite was noted as B. The Fe pillaring solution, prepared from Fe(NO3)·9H2O and NaOH with OH−/Fe3+ molar ratio of 0.3 was stirred for 2 h and then aged in 24 h at ambient temperature. The mixture of B (1.0 g) and 100 mL of deionized water was vigorously stirred for 1 h. After that, the Fe pillaring solution was added slowly into the suspension containing the bentonite (the ratio of 10 mmol Fe/g dry bentonite); the mixture was stirred for 24 h at room temperature. The solid was separated by centrifugation and dried at 100°C for 10 h. The obtained polymeric Fe-modified bentonite was denoted as Fe-B. The Al pillaring solution was prepared by adding 0.1 M NaOH to 0.1 M AlCl3 solution with vigorous stirring to obtain the [OH−]/[Al3+] molar ratio of 2.4. Then, the pillaring solution was vigorously stirred for 7 h at 70°C and aged for 24 h at ambient temperature. After the aging process, the solution was slowly dropped under vigorous stirring to bentonite suspension for 24 h (24 mmol Al/g of dry bentonite). The final solid was obtained by filtration and washed with distilled water until free of chlorides (using the AgNO3 test). The solid was dried at 100°C for 10 h. The obtained polymeric Al-modified bentonite was denoted as Al-B.
Batch adsorption experiments were performed in 100 mL flask. 0.05 g of modified bentonite was added into the 100 mL flasks containing 50 mL solution with various concentrations of As(V). The pH solution was adjusted to the desired value by adding amounts of 0.01 M NaOH or 0.01 M HCl, and the bottles were shaken by magnetic stirrer for 4 h to attain equilibrium. The adsorbent was separated by centrifugation. Then, the concentration of As(V) was analyzed by the AAS method. Blank control tests were carried out for the sake of comparison.
The adsorption capacity of arsenate was calculated by using the following equation:
The effect of pH on arsenic adsorption was investigated in the pH ranges from 2 to 9 at ambient temperature.
For kinetic experiments, 0.2 g of adsorbent was added to 250 mL of known initial concentration in the pH = 3.0 (for the Fe-B sample) or pH = 4.0 (for the Al-B sample), and the mixture was stirred at an identical stirring speed of 600 rpm. At given time intervals, about 5 mL of solution was withdrawn and then centrifuged, and the equilibrium concentrations of the adsorbate were analyzed by the AAS method. The adsorption kinetics experiments were conducted at 10°C, 20°C, 30°C, and 40°C.
The XRD patterns of B, Fe-B, and Al-B samples are shown in Figure
XRD patterns of B, Fe-B, and Al-B.
The XRD patterns of B, Fe-B, and Al-B samples are shown in Figure
The FTIR spectra of the samples are depicted in Figure
FTIR spectra of B, Fe-B, and Al-B.
The peak at 3549 cm−1 of B sample was assigned to Al-Fe-OH vibration [
FT-IR spectra of the Fe-B and Al-B show that some characteristic bands of the initial bentonite were changed. Peak related to the
TG and DTA curves of B, Fe-B, and Al-B samples are presented in Figure
TG and DTA curves of B (a), Fe-B (b), and Al-B (c).
TG and DTA curves of B, Fe-B, and Al-B samples are presented in Figure
The nitrogen adsorption and desorption isotherms of B, Fe-B, and Al-B materials are shown in Figure
N2 adsorption/desorption isotherms of B, Fe-B, and Al-B.
The nitrogen adsorption/desorption isotherms of the samples were all of type III according to IUPAC classification which indicated that the samples possessed mesoporous structure. The shape of hysteresis loops around from 0.4 to 0.9 of relative pressure was attributed to the slit-shaped pores, and their sizes are not well proportioned [
Textural parameters of B, Fe-B, and Al-B samples.
Adsorbent |
|
|
|
|
|
---|---|---|---|---|---|
B | 114.44 | 44.72 | 69.72 | 0.020 | 0.187 |
Fe-B | 146.07 | 41.03 | 105.04 | 0.018 | 0.159 |
Al-B | 170.13 | 119.61 | 50.52 | 0.057 | 0.111 |
The pH effect on the arsenate adsorption capacity of polymeric Al/Fe-modified bentonite is shown in Figure
Effect of pH on the adsorption capacity of As(V) onto Fe-B sample (
It was found that the best adsorption of As(V) onto modified bentonite was in the range of pH 2.0–4.0. When increasing pH from 4.0 to 9.0, the amount of arsenate adsorbed (
Point of zero charge plots of Fe-B (a) and Al-B (b).
In order to get more understanding of mechanism for arsenate absorption onto modified bentonite, the As(V) absorption at different pH was carried out. The pH values before and after the absorption are shown in Table
pH of As(V) solution before and after the arsenate absorption onto Fe-B (
pH before the arsenate absorption | 2.0 | 3.0 | 4.0 | 5.0 | 6.0 | 7.0 | 8.0 | 9.0 |
---|---|---|---|---|---|---|---|---|
pH after the arsenate absorption onto Fe-B | 2.0 | 2.9 | 3.3 | — | 4.0 | 4.8 | 7.0 | 7.2 |
pH after the arsenate absorption onto Al-B | 2.7 | 3.7 | 5.4 | 5.8 | 6.4 | 7.9 | 8.1 | 9.4 |
From Table
The liberation of H+ ions decreased pH of the solution.
The pH increased after the As(V) absorption onto Al-B sample due to the following reaction:
The liberation of OH− ions increased pH of the solution.
So, in this pH range, the main adsorption mechanisms could be considered as the electrostatic interactions and ion exchange.
FTIR spectra of Fe-B before and after the As(V) absorption are depicted in Figure
FT-IR spectra of Fe-B before and after the As(V) adsorption.
Figure
The SEM images of Fe-B, Fe-B after the As(V) adsorption, Al-B, and Al-B after the As(V) adsorption are shown in Figure
SEM images of Fe-B (a), Fe-B after the As(V) adsorption (b), Al-B (c), and Al-B after the As(V) adsorption (d).
The morphology of modified bentonite changed clearly by the As(V) adsorption. The morphology of Fe-B included plates with diameter of several
Experimental kinetic data were evaluated by using pseudo-first-order and pseudo-second-order kinetic models. The pseudo-first-order kinetic model in linear form is presented in the following equation:
The pseudo-second-order kinetic model is given by [
The goodness of fit for the compatible model is assessed based on determination coefficient
The plots of pseudo-first-order and pseudo-second-order kinetic models are illustrated in Figures
The pseudo-first-order kinetic model (a) and pseudo-second-order kinetic model (b) of As(V) adsorption by Fe-B at different temperatures.
The pseudo
Parameters of pseudo
Adsorbent |
|
|
First-order kinetic model | Second-order kinetic model | ||||
---|---|---|---|---|---|---|---|---|
|
|
|
|
|
|
|||
Fe-B | 283 | 14.38 | 0.010 | 15.19 | 0.942 | 0.35 × 10−3 | 21.14 | 0.924 |
293 | 14.42 | 0.013 | 12.44 | 0.977 | 0.68 × 10−3 | 19.80 | 0.973 | |
303 | 14.42 | 0.027 | 9.80 | 0.962 | 4.20 × 10−3 | 15.53 | 0.999 | |
313 | 14.44 | 0.021 | 6.57 | 0.992 | 7.30 × 10−3 | 14.97 | 0.998 | |
|
||||||||
Al-B | 283 | 18.47 | 1.6 × 10−3 | 16.35 | 0.956 | 1.0 × 10−3 | 22.22 | 0.984 |
293 | 20.75 | 1.9 × 10−3 | 13.01 | 0.948 | 2.9 × 10−3 | 22.22 | 0.998 | |
303 | 20.80 | 7.0 × 10−3 | 8.56 | 0.949 | 4.2 × 10−3 | 20.83 | 0.992 | |
313 | 21.05 | 9.0 × 10−3 | 7.09 | 0.752 | 5.8 × 10−3 | 21.28 | 0.996 |
Table
The activation energy can be computed by using the Arrhenius equation:
(a) Arrhenius plots and (b) Eyring plots of As(V) adsorption onto Fe-B and Al-B.
Besides calculating the activation energy, the Gibbs energy Δ
The linear plot of
The values of the activation energy were found to be 80.29 kJ/mol and 41.90 kJ/mol for the arsenate adsorption onto Fe-B and Al-B, respectively (Table
Activation energy and activation parameters of adsorption process.
|
|
∆ |
Δ |
Δ |
---|---|---|---|---|
Fe-B | ||||
283 | 80.29 | 77.82 | −0.037 | 88.41 |
293 | 88.79 | |||
303 | 89.16 | |||
313 | 89.53 | |||
|
||||
Al-B | ||||
41.90 | 39.32 | −0.162 | 85.16 | |
86.78 | ||||
88.40 | ||||
90.02 |
The free energy of activation (Δ
Equilibrium studies were carried out to obtain the adsorption capacity of modified bentonite at different temperatures. Two adsorption isotherms, namely, the Langmuir [
In addition, a separation factor,
The Freundlich isotherm was applied to the adsorption on a heterogeneous surface with uniform energy. The linear form of this model can be expressed as follows:
The plots for he Langmuir and Freundlich isotherm models are shown in Figures
Plots of Langmuir (a) and Freundlich (b) isotherms in linear form for the adsorption of As(V) onto Fe-B at several temperatures.
Plots of Langmuir (a) and Freundlich (b) isotherms in linear form for the adsorption of As(V) onto Al-B at several temperatures.
As seen from Table
The parameters of isotherm models in linear form for the adsorption of As(V) onto modified bentonite.
|
Langmuir model | Freundlich model | |||||
---|---|---|---|---|---|---|---|
|
|
|
|
|
|
|
|
|
|||||||
283 | 17.76 | 1.08 | 0.976 | 0.016–0.053 | 8.81 | 4.86 | 0.561 |
293 | 18.05 | 5.18 | 0.972 | 0.003–0.011 | 9.99 | 5.36 | 0.391 |
303 | 18.98 | 7.87 | 0.976 | 0.002–0.007 | 10.69 | 5.44 | 0.405 |
|
|||||||
|
|||||||
283 | 29.41 | 0.32 | 0.984 | 0.046–0.111 | 21.477 | 7.41 | 0.795 |
293 | 32.59 | 0.41 | 0.970 | 0.037–0.090 | 16.912 | 6.10 | 0.779 |
303 | 35.71 | 0.70 | 0.984 | 0.022–0.055 | 14.069 | 5.29 | 0.875 |
Specific surface area of modified bentonite was calculated at the temperature of 283 K as follows:
The effective surface area values of the studied samples were less than
The maximum As(V) adsorption capacities (
Comparison of adsorption capacity with other adsorbents.
Adsorbent | Arsenate adsorption capacity (mg/g) | References |
---|---|---|
SMB3 | 0.288 | [ |
Polymeric Al/Fe-modified montmorillonite | 21.233 | [ |
Al13-Mont | 5.008 | [ |
C-Fe-M | 8.85 | [ |
Fe-M | 15.15 | [ |
Al-B | 35.71 | Present study |
Fe-B | 18.98 | Present study |
Gibb’s free energy (∆
The values of ∆
Plot
Thermodynamic parameters for the As(V) adsorption onto Fe-B and Al-B materials.
Adsorbent | ∆ |
∆ |
∆ | ||
---|---|---|---|---|---|
283 K | 293 K | 303 K | |||
Fe-B | −26.59 | −31.35 | −33.47 | 71.23 | 0.347 |
Al-B | −23.76 | −25.16 | −27.38 | 27.34 | 0.180 |
For the both cases, the obtained ∆
In the reusability test, 0.5 g of used modified bentonites saturated with As(V) (initial Al-B or initial Fe-B) was added into thirty milliliters of 0.01 M HCl solution. The mixture was shaken at a temperature of 30°C using a magnetic stirrer for 24 h. The solids were centrifuged, rinsed for several times with distilled water, dried at 100°C, and investigated for As(V) adsorption capacity at the first run.
Figure
The dependence of adsorption capacity through the reuse of adsorbent (
In this study, the polymeric Al- and Fe-modified bentonites were applied to remove arsenate ions from aqueous solution. The pseudo-second-order kinetic model fit well with the arsenate adsorption kinetic data for the two modified bentonites. The maximum monolayer adsorption capacities of As(V) at 303 K derived from the Langmuir model were 35.71 mg/g for Al-bentonite and 18.98 mg/g for Fe-bentonite, which were higher than that compared to other adsorbents reported previously. The negative values of ∆
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.