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Preparation and Characterization of Silane Coupling Agent Modified Halloysite for Cr(VI) Removal Peng Luo,† Jiang-shan Zhang,‡ Bing Zhang,*,† Jin-hua Wang,† Ya-fei Zhao,† and Jin-dun Liu*,† † ‡
School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, People's Republic of China College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang 455002, People's Republic of China ABSTRACT: Natural halloysite nanotubes (HNTs) were modified with a silane coupling agent, N-β-aminoethyl-γ-aminopropyl trimethoxysilane (KH-792), to form a new adsorbent for Cr(VI) removal. The as-prepared product was characterized by FTIR spectroscopy, TGA, TEM, and specific surface analysis. The results showed that KH-792 was successfully grafted onto the halloysite surface. Modified HNTs exhibited a rapid adsorption rate for Cr(VI) and approached 95% of the maximum adsorption capacity within 5 min. The effects of initial Cr(VI) concentration, temperature, pH, and ionic strength on the adsorption capacity were investigated in batch experiments. The results showed that low temperature was favorable to improve adsorption efficiency, and the adsorption capacity decreased significantly with the increase of pH and ionic strength. The optimum pH was found to be 35. The main adsorption mechanism was considered to be electrostatic interaction between protonated amino groups on the adsorbent surface and negatively charged Cr(VI). The results above confirmed that modified HNTs had the potential to be utilized as a low-cost and relatively effective adsorbent for Cr(VI) removal.
1. INTRODUCTION In a natural system chromium exists in two stable oxidation states: Cr(III) and Cr(VI). Cr(III) is considered to be essential to mammals for the maintenance of glucose, lipid, and protein metabolism, while Cr(VI) species, existing in the effluents of electroplating, tanning, mining, and fertilizer industries, are strong oxidizing agents which act as carcinogens, mutagens, and teratogens in biological systems. Low-level exposure to Cr(VI) ions is known to be toxic to mammals and capable of permeating cell membranes. Cr(VI) adsorption on aquifer minerals is limited because of the negative charges, making it more mobile in subsurface soils and aquifers. Therefore, the development of technologies to prevent further discharge and remediate Cr(VI) contamination is of great importance. The conventional methods to remove Cr(VI) from wastewater involves the reduction of Cr(VI) to the less mobile and toxic Cr(III) species, followed by chemical precipitation, reverse osmosis, ion exchange, foam flotation, electrolysis, and adsorption.1 A major drawback of precipitation is sludge production. Ion exchange and reverse osmosis are not economically attractive because they usually require sophisticated equipment and expensive chemical reagents, which therefore result in high operating costs and masses of toxic byproducts. Among the methods, adsorption is considered as an efficient and economically feasible alternative for the removal of Cr(VI) due to easy operation and development of various available adsorbents. The adsorbent should be inexpensive, accessible, and environmentally friendly. Clay minerals have been reported to be promising adsorbents because of their large specific surface areas, because they are cheap, and because they are abundant resources. Furthermore, the regeneration of these low-cost substitutes is always not necessary. Varied clays have been used to remove Cr(VI) from wastewater including bentonite, rectorite, kaolinite, montmorillonite, hydrotalcite, wollastonite, and boehmite.28 However, the adsorption capacity r 2011 American Chemical Society
of these mineral materials for Cr(VI) is still relatively low because of negatively charged surfaces, which have little or no affinity for anions. In order to adsorb anions, it is suggested that the modified surfaces possess positively charged exchange sites or there should be replacement of weakly held counterions by more strongly held counterions.9 Halloysite, a type of naturally occurring clay mineral with a nanotubular structure, is available in abundance in many countries such as China, the United States, Brazil, and France. Halloysite nanotubes (HNTs) have huge specific surface area, plentiful micropores, high length-to-diameter (L/D) ratio, abundant hydroxyl groups, environmental friendliness, and biocompatibility,1012 making them potentially useful as adsorbents for ammonium ions, reactive dyes, and heavy metals.13,14 HNTs possess a net negative structural charge resulting from the hydroxyl groups, and the major mechanism for the adsorption of ionic pollutants is electrostatic interaction or ion exchange. Recent studies have shown that there has been a growing interest in the application of natural minerals modified by quaternary alkylammonium salts for the removal of Cr(VI) because the modification can increase the surface positive charge and thus enhance the adsorption capacity for anionic pollutants.3,15 The functional groups of cationic surfactants have been found to be responsible for metal sorption. Among them, the amino group is very effective in removing heavy metals. It not only chelates cationic metal ions, but also adsorbs anionic metal species through electrostatic interaction or hydrogen bonding.16 The most commonly used cationic surfactants in the modification of natural minerals for Cr(VI) adsorption is hexadecyltrimethylammonium Received: May 3, 2011 Accepted: August 2, 2011 Revised: August 2, 2011 Published: August 02, 2011 10246
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Industrial & Engineering Chemistry Research bromide (HDTMA), which is rather broadly described in the literature.15,17 N-β-Aminoethyl-γ-aminopropyl trimethoxysilane (KH-792) is composed of a large number of primary and secondary amino groups in a molecule. The amino groups can be protonated when they are cross-linked on the adsorbent surface and exhibit good adsorption ability for heavy metals. Until now, we have not found any studies on the modification of natural clays with KH-792. Moreover, the utilization of HNTs is an attractive alternative to traditional separation methods due to their high specific surface area which leads to a high adsorption rate and capacity compared to other types of materials such as resins, foams, and conventional fibers. Therefore, it would be interesting to investigate the possibility of HNTs with KH-792 on the surface for adsorbing Cr(VI). In the present work, we prepared a novel adsorbent by grafting KH-792 onto HNTs and studied the ability of modified HNTs to remove Cr(VI) from simulated wastewater. The factors affecting the Cr(VI) adsorption capacity were investigated in detail, and the adsorption mechanisms were elucidated through adsorption behavior.
2. EXPERIMENTAL SECTION 2.1. Materials. Halloysite clay from Henan Province (China) was milled and sieved followed by oven drying at 373 K for 24 h. The halloysite was analyzed for its chemical composition and was found to contain 46.16% SiO2, 38.70% Al2O3, 0.033% MgO, 0.191% CaO, 0.05% Fe2O3, 0.03% K2O, 0.04% Na2O, 0.004% TiO2, and 14.60% loss on ignition, which was nearly similar to previous reports.18,19 Potassium dichromate (K 2 Cr 2 O 7 , analytical grade) and N-(β-aminoethyl)-γ-aminopropyl trimethoxysilane (KH-792, NH2CH2CH2NHCH2CH2CH2Si(OCH3)3) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Other inorganic chemicals used in this study such as toluene (C 6 H5 CH3 ), sodium nitrate (NaNO 3 ), sodium hydroxide (NaOH), and hydrochloric acid (HCl) were all analytical grade reagents and were used without further treatment. A stock solution (1000 mg/L) was prepared by dissolving K2Cr2O7 in distilled water; desired concentrations were obtained when needed by diluting the stock solution with distilled water. HCl and NaOH solutions were used for pH adjustment. NaNO3 was used to adjust the ionic strength. 2.2. Preparation of Silane Coupling Agent Modified HNTs (m-HNTs). Natural HNTs were modified by the following procedures.20 (1) The dry halloysite was immersed in 1.0 mol/L HCl solution under agitation at room temperature for 4 days in order to activate the surface groups, and then filtrated and washed with distilled water until the pH reached a constant value close to neutrality. (2) The acidified HNTs were converted to a multifunctional ligand by further reaction with KH-792. Typical synthesis included dispersion of 0.5 g of the acidified HNTs in 25 mL of toluene followed by addition of an appropriate amount of KH-792 (5-fold excess). The mixture was kept under refluxing conditions (383 ( 1 K) for 24 h. Next, the as-synthesized powder was filtered out and washed many times with toluene to remove an excessive amount of modifier and possible products of hydrolysis. Then, the powder was dried overnight in an oven under vacuum at 368373 K. (3) An additional extraction for 20 h was performed in the Soxhlet extractor apparatus to control the thermal stability of the attached KH-792 and check the resistance for washing. Then, the powder was dried in the oven at
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318323 K under vacuum for 12 h. (4) The resulting material was stirred in hydrochloric acid to convert amino groups into ammonium salts. The functionalized material, designated as m-HNTs, was dried in an oven at 383 ( 1 K for 6 h and used in further experiments. 2.3. Characterization Techniques. The Fourier transform infrared spectrum (FTIR; Thermo., NEXUS FTIR) was recorded using KBr pellets at a resolution of 4 cm1 between 400 and 4000 cm1 to observe surface functional groups. Thermogravimetric analysis (TGA; Netzsch, STA409PC) was carried out from 298 to 1073 K at a heating rate of 10 K/min in N2 atmosphere. A transmission electron microscope (TEM; Philips CM 120 Biofilter) was utilized to identify the morphology and microscopic structure of natural and modified HNTs. The N2 adsorption isotherm (Quantachrome NOVA 4200e) was measured at 77 K to determine the specific surface area. 2.4. Adsorption Experiments. The Cr(VI) adsorption experiments were performed according to the batch method in stopper conical flasks containing 50 mL of varying initial concentration of Cr(VI) and the given dose of adsorbent. Then the samples were agitated on a thermostatted shaker with a shaking of 180 rpm at 288308 K. On reaching equilibrium the adsorbent was centrifuged for 15 min at 3000 rpm and then filtered with 0.45 μm membranes. The concentration of the remaining Cr(VI) in supernatant was detected by the diphenyl carbazide spectrophotometric method using a UVvis spectrophotometer (Shimadzu, UV-3000) at a λmax of 540 nm.21,22 The removal efficiency (R, %) and the amounts of Cr(VI) adsorbed at time t (qt, mg/g) and at equilibrium (qe, mg/g) were calculated by using the following equations: qt ¼ ðC0 Ct ÞðV =MÞ
ð1Þ
qe ¼ ðC0 Ce ÞðV =MÞ
ð2Þ
R ¼ 100½ðC0 Ce Þ=C0
ð3Þ
where C0, Ct, and Ce (mg/L) are the initial, time t, and equilibrium concentrations of Cr(VI) solution, respectively; V (L) is the volume of Cr(VI) solution and M (g) is the weight of adsorbent.
3. RESULTS AND DISCUSSION 3.1. Structural and Morphological Characterization. The FTIR spectra are shown in Figure 1. Most band positions do not change after modification, suggesting that the basic crystal structures of HNTs and m-HNTs remain constant. However, the vibration bands at 2927 and 2852 cm1 are newly observed, which can be attributed to CH asymmetric and symmetric stretching vibrations, respectively. The in-plane deformation bands at 1509 (scissoring) and 1457 cm1 (wagging) can further confirm the existence of CH. The 3361 cm1 band is assigned to the stretching vibration of NH. Multiple peaks between 1668 and 1642 cm1 can be explained by the incomplete superposition of in-plane NH and interlayer water’s deformation vibration, while there is only a single band at 1646 cm1 for natural halloysite. The above results may indicate that silane coupling agent, the sole source for CH and NH, has been successfully grafted onto halloysite. The results of thermogravimetric analysis (TGA) are displayed in Figure 2. The weight losses of 1.59 wt % for original halloysite and 2.07 wt % for modified halloysite, respectively, are 10247
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Figure 3. TEM images of (a) HNTs and (b) m-HNTs.
Figure 1. FTIR spectra of (a) HNTs and (b) m-HNTs.
Figure 4. Effect of contact time for Cr(VI) adsorption capacity onto m-HNTs.
Figure 2. TGA graphs of (a) HNTs and (b) m-HNTs.
observed up to 378 K, which can be attributed to the loss of free interlayer water. The TGA curve of original halloysite shows that a weight loss of 15.42 wt % occurred between 378 and 1073 K which is caused by the dehydroxylation of surface OH, and a drastic decrease can be observed approximately from 700 to 800 K. As for modified halloysite, a weight loss of 17.74 wt % between 378 and 1073 K is displayed, 2.32 wt % larger than natural halloysite, which can be attributed to the extra thermal decomposition of loaded KH-792. One may conclude that the grafting ratio (w/w) is approximately 2.32%. Its TGA curve is smoother than that of the original sample between 378 and 800 K. Two stages of thermal decomposition account for this process, including KH-792 species grafted onto Si/AlOH on the external surface of the lumen, the oligomerized KH-792, and KH-792 grafted onto AlOH of the internal surface. The decomposition of different KH-792 species seems to be a gradual process and overlap partially, so a smooth and complex multistep weight loss is observed.23 A transmission electron microscope (TEM) was used to observe the morphological structures of original and modified HNTs. Figure 3 displays TEM contrast images of HNTs and m-HNTs. The inner diameter and lumen length of natural HNTs in Figure 3a range from 40 to 50 nm and from 500 to 1000 nm, respectively. In Figure 3b, the tube wall of modified HNTs becomes less distinguished, namely thicker, than that of natural
HNTs. Furthermore, both the natural and modified HNTs are straight and have open ends, rather than being blocked only at the ends, indicating that the lumen has been completely filled by grafted KH-792.23,24 The specific surface area of natural HNTs is 59.62 m2/g, while the specific surface area of modified HNTs decreases to 46.66 m2/g, which can also prove the existence of grafted KH-792. These hollow and open-ended structures enable metal ions to easily access and be adsorbed onto the surface, and the relatively large specific surface area may provide a number of adsorption sites. 3.2. Analysis of Adsorption Rate. The effect of contact time on the adsorption capacity of Cr(VI) was investigated with 0.15 g of m-HNTs in 50 mL of 100 mg/L Cr(VI) solution at 298 K. The results are shown in Figure 4. The plot clearly displays that there was an adsorption step burst within a contact time of about 5 min and then it leveled off. The adsorption capacity of Cr(VI) reached 31.57 mg/g (removal efficiency >95%) within the first 5 min, and the equilibrium adsorption capacity was 32.27 mg/g. The photograph in Figure 4 also shows that the color of solution changed from orange to colorless after removal of Cr(VI) from the solution. The results indicate that the adsorption process had a rather rapid adsorption rate and a contact time of 30 min was sufficient to attain saturation. The rapid adsorption of Cr(VI) is attributed to structural features of m-HNTs which have a large specific surface area and pore volume. At the beginning the removal rate is high because there are abundant active groups available for Cr(VI) adsorption. Then the rate of removal becomes almost insignificant owing to a quick exhaustion of the adsorption sites, and a charge balance was reached between amine groups and Cr(VI). The driving force resulting from the concentration gradient of Cr(VI) between bulk solution and the adsorbent surface is 10248
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Industrial & Engineering Chemistry Research considered to cause the adsorption. Therefore, it is easy for the adsorbate to access these active sites, thus resulting in a rapid approach to equilibrium. By comparison, the equilibrium time for the adsorption of Cr(VI) by some other adsorbents is much longer. Adsorption of Cr(VI) onto hydrous zirconium oxide needs 1 h to reach equilibrium,25 while for the Cr(VI) uptake onto cross-linked amino starch it is about 2 h.26 Other adsorbents such as HDPB-modified zeolites (4 h),27 modified fungal biomass (6 h), 16 composite chitosan (24 h),1 lignocellulosic substrate (24 h),28 and HDTMA-modified zeolite Y (48 h)17 are also reported. Also, in most cases one can observe a gradual adsorption process for Cr(VI) removal
Figure 5. Effect of Cr(VI) initial concentration and temperature for adsorption capacity onto m-HNTs.
Figure 6. Effect of pH for adsorption capacity onto m-HNTs.
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efficiency from 65 to 95%, usually taking 4060 min or longer, while in this study this process nearly disappears. 3.3. Effect of Cr(VI) Initial Concentration and Temperature. The effect of Cr(VI) initial concentration and temperature was investigated by the addition of 0.15 g of modified HNTs in 50 mL of various initial concentrations of Cr(VI) solution for 30 min at 288308 K. The results are shown in Figure 5. It is observed that the adsorption capacity was dependent on the concentration. This was to be expected due to the Cr(VI) initial concentrations that provided the essential driving force to overcome the resistances to the mass transfer of Cr(VI) between the bulk and reactive sites. For a fixed adsorbent dosage, the total available adsorption sites were limited, thus leading to an increase in adsorption capacity of Cr(VI) corresponding to an increased initial Cr(VI) concentration until the equilibrium was reached. At the temperatures investigated, there was an evident decreasing of adsorption capacity with the temperature increasing, thereby indicating that the process was exothermic in nature, which may be due to a tendency of the Cr(VI) to escape from the solid phase to the bulk phase with an increase in temperature. 3.4. Effect of pH. The uptake of Cr(VI) as a function of pH was examined over an initial pH range of 29, and the results are plotted in Figure 6. The adsorption capacity increased as the initial pH increased from 2 to 3, and then it did not significantly alter between pH 3 and 5. Afterward, a sharp decrease was observed with the pH increasing from 5 to 8. Between pH 8 and 9, the adsorption capacity did not significantly alter again. The optimum pH for removal of Cr(VI) was found to be 35. Previous researchers reported that the optimum pH was 23 for the maximum adsorption of Cr(VI) using montmorillonite and wollastonite.5,7 In the present study, the adsorbent had a relatively higher and broader working pH range. This performance may be associated with the functional amino groups on the adsorbent surface. Amino groups are of alkalescence and can be protonated even at pH above 10, thus allowing adsorption to occur through electrostatic attraction at higher pHs. Cr(VI) speciation can exist in a series of stable forms such as H2CrO4, HCrO4, Cr2O72, and CrO42, and the relative abundance of each species mainly depended upon the solution pH and Cr(VI) concentration. Generally speaking, H2CrO4 and HCrO4 were major species at pH below 2. HCrO4, Cr2O72, and CrO42 were in equilibrium in the range of pH 25, and the major species was HCrO4. As the pH further increased, the equilibrium shifted to Cr2O72 and CrO42. When the pH was higher than 7, CrO42 was the primary form.29,30 It was found that Cr(VI) uptake maintained a high value in the range of pH 35, which was attributed mainly to electrostatic attraction between the
Table 1. Comparison of Cr(VI) Adsorption Capacities with Various Clays as Adsorbents adsorption conditions adsorbents
pH
temperature (K)
equilibrium time (h)
adsorption capacity (mg/g)
ref
boehmite
5.5
298
1
0.214
8
wollastonite
2.5
303
2
0.686
7
rectorite (OREC3)
6.0
299
1
3.57
3
kaolinite (K2)
4.6
303
4
8.0
4
bentonite (QB-Aq2) montmorillonite (Mag)
5.0 2.5
310 298
1 2
14.64 20.16
2 5
hydrotalcite (CHT)
6.07.0
298
6
34.344.7
6
halloysite (m-HNTs)
5.0
298
0.5
37.25
this study
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Industrial & Engineering Chemistry Research negatively charged HCrO4 and the positively charged amino functional groups caused by strong protonation on the adsorbent surface. However, when the pH was reduced to less than 3, electrostatic interaction was reduced as Cr(VI) mainly existed in the form of H2Cr2O7. At pH higher than 5, hydroxyl groups competed with the Cr(VI) ions and deprotonation processes occurred on the adsorbent, resulting in the decrease of Cr(VI) uptake. Moreover, considering the valence of Cr(VI) species, the Cr(VI) species was mostly in the univalent form (HCrO4) at lower pH and thus just required one exchangeable site for one molecule of Cr(VI) species. However, with pH increasing, the divalent forms of Cr(VI) species (Cr2O72 and CrO42) were abundantly present and necessitated two exchangeable sites for the adsorption to occur. This resulted in a higher removal capacity of Cr(VI) species at lower pH than that at higher pH. Similar results have also been reported elsewhere.16 Adsorption capacities for Cr(VI) using different clays as adsorbents are shown in Table 1. In addition, various adsorbents for Cr(VI) uptake were also summarized by Weng et al.,31 which ranged from 0.15 to 294 mg/g. In a comparison of these values, the modified HNTs displayed a medium level, which, however, was much higher than that of similar minerals. It should be noted that even when the pH ranges between 2 and 3 (strong acidic condition) and between 8 and 9, the adsorbent still displays sound Cr(VI) uptakes of 34.11 and 17.13 mg/g, respectively. The high resistance for strong acidic and alkaline conditions and wide-ranging pHs with maximum adsorption capacity provided the new adsorbent a far-ranging industrial application.
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3.5. Effect of Ionic Strength. Since industrial effluents were always contaminated by various additives such as inorganic salts, it was important to study the effect of these ions on the adsorption property of Cr(VI). Modified HNTs (0.15 g) were added into 50 mL of 100 mg/L Cr(VI) solution with various NaNO3 concentrations at 298 K. The results are depicted in Figure 7. The adsorption capacities of Cr(VI) on modified HNTs were significantly reduced by the presence of competitive nitrate anion (NO3). When the ionic strength concentration increased from 0 to 1.0 mol/L, the adsorption capacity of Cr(VI) by m-HNTs decreased from 31.23 to 5.28 mg/g correspondingly. NO3 ions contest the surface adsorption position with Cr(VI) anions. The part of HCrO4 and Cr2O72 adsorbed on the adsorbent is displaced by the NO3 in solution, which can be expressed as follows:
HNTs NH4 þ HCrO4 =Cr2 O7 2 þ NO3 h HNTs NH4 þ NO3 þ HCrO4 =Cr2 O7 2
ð4Þ
As the NO3 in the solution increases, the reaction will be shifted from the left to the right, resulting in the reduction of Cr(VI) adsorption. On the other hand, Cr(VI) species are surrounded by an electrically diffused double layer, the thickness of which can be expanded by the presence of NaNO3. This expansion makes it difficult for Cr(VI) species to reach the adsorbent surface, thus leading the electrostatic attraction to weaken and the uptake of Cr(VI) to decrease.32 3.6. Adsorption Mechanisms. The adsorption process was usually determined by adsorbent surface properties, especially by the functional groups on the adsorbent surface. The most commonly reported mechanisms for adsorption included electrostatic interaction, ion exchange, chelation, hydrogen band, precipitation, and complexation.16 The amine groups (NH2, NH, N< ) could be protonated easily under acidic conditions, which was favorable for anion adsorption via electrostatic interaction. The proposed mechanism of Cr(VI) adsorption is illustrated in Figure 8. Taking into account the effect of pH discussed above, the major reaction responsible for the chromium anions adsorption was presumed as follows: HNTs NH4 þ þ HCrO4 =Cr2 O7 2 h HNTs HCrO4 =Cr2 O7 2 þ NH4 þ
Figure 7. Effect of ionic strength for adsorption capacity onto m-HNTs.
ð5Þ
At lower pH, amine groups were strongly protonated and the high adsorption capacity was caused by the electrostatic attraction between the negatively charged surface and the positively charged amino groups. However, the amount adsorbed
Figure 8. Schematic diagram illustrating Cr(VI) adsorption onto HNTs surface grafted with KH-792. 10250
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Industrial & Engineering Chemistry Research decreased with pH decreasing from 3, which was related to the formation of nonionic H2Cr2O7. At higher pH, the protonation effect of amine groups decreased, resulting in the decrease of Cr(VI) uptake. Also, the Cr(VI) anion was unstable in the presence of electron donors and demonstrated a very high positive redox potential.16 Therefore, the reduction of hexavalent chromium to trivalent chromium may also account for the improved removal of chromium at low pH. The reduced Cr(III) ions could form Cr(OH)3 and precipitate onto the adsorbent surface. A desorption study was conducted using nitric and hydrochloric acids, but the results were poor. This may indicate that the adsorption process of Cr(VI) onto m-HNTs was partially irreversible even using strong acids as eluents, and it may indicate the existence of reduction of hexavalent chromium to trivalent chromium.
4. CONCLUSIONS Functionalization of natural halloysite nanotubues can be achieved by modification with organosilane KH-792. The characteristic results confirmed that the HNTs were modified with KH-792 successfully. Then the adsorption properties of the adsorbent were studied. The adsorption results demonstrated that Cr(VI) adsorption by m-HNTs studied in batch experiments was strongly dependent on contact time, initial pH, temperature, and ionic strength. The Cr(VI) removal efficiency of 95% was reached within the first 5 min, and a contact time of 30 min was sufficient to attain saturation. High initial concentration and low temperature favored the Cr(VI) adsorption on m-HNTs. The optimum pH was 35, which was a relatively higher and broader working pH range. The experimental results showed that electrostatic interaction between adsorbent and Cr(VI) played an important role in the process of adsorption. On the basis of these results, it was concluded that m-HNTs could be used as a low-cost and relatively effective adsorbent for the removal of Cr(VI) from wastewater. Further experiments were in progress in order to validate the efficiency of this material in the treatment of industrial effluents. ’ AUTHOR INFORMATION Corresponding Author
* Tel./fax: +86-371-67781724. E-mail:
[email protected] (B.Z.);
[email protected] (J.L.).
’ ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 20871105) and the Henan Outstanding Youth Science Fund (No. 0612002400). ’ REFERENCES (1) Boddu, V. M.; Abburi, K.; Talbott, J. L.; Smith, E. D. Removal of Hexavalent Chromium from Wastewater Using a New Composite Chitosan Biosorbent. Environ. Sci. Technol. 2003, 37, 4449. (2) Sarkar, B.; Xi, Y. F.; Megharaj, M.; Krishnamurti, G. S. R.; Rajarathnam, D.; Naidu, R. Remediation of Hexavalent Chromium through Adsorption by Bentonite Based ArquadÒ 2HT-75 Organoclays. J. Hazard. Mater. 2010, 183, 87. (3) Huang, Y.; Ma, X. Y.; Liang, G. Z.; Yan, Y. X.; Wang, S. H. Adsorption Behavior of Cr(VI) on Organic-Modified Rectorite. Chem. Eng. J. 2008, 138, 187. (4) Bhattacharyya, K. G.; Gupta, S. S. Adsorption of Chromium(VI) from Water by Clays. Ind. Eng. Chem. Res. 2006, 45, 7232.
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