Desalination 259 (2010) 22–28
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Rapid adsorption of Cr (VI) on modiﬁed halloysite nanotubes Wang Jinhua a, Zhang Xiang a, Zhang Bing a,b,⁎, Zhao Yafei a, Zhai Rui a, Liu Jindun a, Chen Rongfeng b a b
School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, China Henan Academy of Science, Zhengzhou 450002, China
a r t i c l e
i n f o
Article history: Received 9 December 2009 Received in revised form 20 April 2010 Accepted 21 April 2010 Available online 15 May 2010 Keywords: Heavy metal ions Adsorption Modiﬁcation Halloysite nanotubes
a b s t r a c t The halloysite nanotubes (HNTs) were modiﬁed with the surfactant of hexadecyltrimethylammonium bromide (HDTMA) to form a new adsorbent. The modiﬁed HNTs were characterized by the FTIR spectra and thermogravimetric analysis. The results show that quaternary ammonium cations were grafted on the nanotubes surface successfully. While the modiﬁed HNTs were used as adsorbent for Cr(VI) removal from its aqueous solution, they exhibited rapid adsorption rate for chromates and approached to 90% of the maximum adsorption capacity within 5 min. The effects of pH and ionic strength on the adsorption capacity were also investigated, which showed the adsorption capacity of the adsorbent decreased signiﬁcantly with the increase of ionic strength and pH. The adsorption data of Cr(VI) on the modiﬁed HNTs are well consistent with Langmuir model. The regeneration of modiﬁed HNTs could be realized by eluents and the recovered adsorbent could be used again for Cr(VI) removal. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Due to the toxicity of chromium to the ecosystem, agriculture and human health, pollution by chromium has received wide spread attention in the recent years. Chromium, a redox active metal element, usually exists as Cr(III) or Cr(VI) species in natural and industrial waters. These two oxidation states have different toxicities. Cr(III) is required by microorganisms in small quantities as an essential trace metal, while hexavalent chromium species are strong oxidants that act as carcinogens, mutagens and teratogens in biological systems. Process wastewaters from electroplating, mining operations, chromate preparation, metal-plating facilities, atomic power plants, electronic device manufacturing units and tanneries often contain hexavalent chromium at concentrations above local discharge limits. The Cr(VI) concentration in wastewater is severely restricted in many countries and the discharge of Cr(VI) to surface water is regulated to below 0.05 mg/L by the U.S. EPA. To meet environmental regulations, it is imperative for industries to reduce the chromium in their efﬂuents to an acceptable level before discharging into municipal sewers. Consequently, the removal of Cr(VI) from industrial wastewater has attracted much research interests. Among available processes used to remove hexavalent chromium, adsorption process is generally known to be one of the most promising techniques. Many adsorbents that have been tested in the removal of Cr(VI) include synthetic resin , grape waste , banana peel , wheat bran , fungi , green alga , peat , bentonite  and ⁎ Corresponding author. School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, China. Tel.: +86 371 67781724. E-mail address: [email protected]
(Z. Bing). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.04.046
boehmite . However, the adsorption capacity of natural materials to hexavalent chromium is relatively low. Therefore, the adsorbents with strong afﬁnity and high loading capacity for heavy metal ions have been developed by modifying natural materials with various organic compounds. Organic compounds include surfactant [10,11], silane coupling agent [12–14], formaldehyde, sulfuric acid  and so on. For example, prior studies [8,11] had shown that the adsorption capacity of chabazite increased signiﬁcantly after modiﬁcation with hexadecyltrimethylammonium bromide (HDTMA). Barquist and Larsen  noted that the adsorption capacity of the nanocrystalline silicalite32 nm for Cr(VI) was enhanced after the introduction of amino groups. The pretreatment of hazelnut shell and bentonite with formaldehyde and sulfuric acid respectively can also improve the adsorption capacity of Cr(VI) obviously . Halloysite is a kind of aluminosilicate clay with a hollow nanotubular structure and can be obtained from the natural environment , which has been used as nanocomposites [17,18], nanocontainers [19,20] and adsorbents [21,22]. In this paper, natural halloysite nanotubes (HNTs) were ﬁrst introduced as adsorbent for hexavalent chromium. Like other natural mineral materials, the adsorption capacity of HNTs to hexavalent chromium is relatively low. Therefore, there is a need to modify HNTs in order to obtain higher adsorption capacity. HNTs possess some excellent characteristics, such as large surface area, large pore volume and adequate hydroxyl groups. The adequate hydroxyl groups and large pore volume make them promising candidates for the modiﬁcation with organic matter on their surfaces. In the paper, a new adsorbent was prepared by modifying HNTs with hexadecyltrimethylammonium bromide (HDTMA). The prepared product was applied to remove Cr(VI) from aqueous solution and some inﬂuential parameters, such as contact time, ionic strength
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and initial pH of the solution were investigated by batch method. Compared with other adsorbents, the new adsorbent shows faster adsorption rate and higher adsorption capacity.
The amount of Cr(VI) adsorbed at equilibrium (qe, mg g− 1) and removal efﬁciency (%) were calculated by using the following equations, respectively.
ðC0 −Ce ÞV M
100ðC0 −Ce Þ C0
2.1. Materials and instrumentations All chemicals were analytical reagent grade without further treatment. All solutions were prepared using distilled water. The powder of halloysite nanotubes (HNTs) was reﬁned from clay minerals in Henan province, China. The powder was prepared as follows: A water suspension solution (5% in mass) was prepared by adding water to dry halloysite mineral. The suspension solution was intensively stirred for 2 h and sprayed to dry at 200 °C to obtain ﬁne powder. Before use, dry halloysite powder was sieved to eliminate aggregates. A Shimadzu ultraviolet–visible spectrophotometer (UV-2450, Japan) was used to analyze content of dichromate ion solutions. Stability temperature oscillated instrument (HZQ-F100, China) was used in preparation of adsorbent and adsorption experiments. PH value of solution was measured by microprocessor pH meter (HANNA, pH 211). The size and the morphology of halloysite powder were examined by transmission electron microscope (TEM, FEITECNA1G2). Fourier transformed infrared spectrum (FTIR) was recorded with Nicolet IR300 FTIR spectrometer. The thermal decomposition of the modiﬁed HNTs samples was recorded via TG/DSC technique by a thermogravimetric analyzer (NETZSCH, STA409PC). Surface area of the sorbent was determined with a Micromeritics BET instrument (NOVA4200e, USA). 2.2. Preparation of new composite nanotubes adsorbent The HNTs modiﬁed with hexadecyltrimethylammonium bromide (HDTMA) were prepared in the following way. In order to improve cation exchange capacity (CEC), the Na-HNTs were prepared prior to modiﬁcation. For this purpose, the halloysite nanotubes (HNTs) were washed with 1 mol/L HCl under agitation at room temperature for 2 h, kept them for 24 h in the acid solution, ﬁltrated and washed with distilled water until a pH of 6 was obtained. The acidiﬁed HNTs were agitated for 20 h in a 1 mol/L NaCl solution at room temperature and then kept for 48 h in the NaCl solution. The product was ﬁltrated, washed and dried to get Na-HNTs. As a next step 4 g sample of Na-HNTs was equilibrated for 12 h using mechanical shaker with 200 mL 0.014 M HDTMA at 60 °C. Finally, the solid was separated by ﬁltration and washed with distilled water repeatedly. The modiﬁed HNTs was dried in the oven and used in further experiments.
where C0 and Ce are initial and equilibrium concentrations in mg/L, M is the dry mass of adsorbent in grams and V is volume of solution in liters. Regeneration of modiﬁed HNTs was examined by adsorption/ desorption experiments. Adsorption experiments were performed using 0.5 g modiﬁed HNTs and Cr(VI) solution (50 mL 50 mg/L) at 25 °C for 1 h. For desorption studies, 50 mL of 1 M eluents (Na2SO4, NaCl, NaNO3 and mixed solution of NaNO3 and NaOH) was shaken with 0.50 g Cr-adsorbed HNTs for 1 h. After desorption equilibrium was reached, the adsorbents were separated and the amount of desorbed Cr(VI) was determined by the same method mentioned above. Adsorption/desorption runs were repeated for ﬁve times. 3. Results and discussion 3.1. Characterization of adsorbent The original and modiﬁed HNTs were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, transmission electron microscope and speciﬁc surface analysis. Infrared spectroscopy method was used to investigate whether the functional groups had been grafted onto the surface of nanotubes. Spectra were recorded with detector at 4 cm− 1 resolution between 400 and 4000 cm− 1 using KBr pellets. The results are shown in Fig. 1. Before the modiﬁcation (Fig. 1a), absorption bands at 3701 cm− 1 and 3628 cm− 1 are ascribed to –OH groups. The band at 910 cm− 1 is assigned to bending vibration of Al–OH. Others bands at 1000–1100 cm− 1 and 450– 550 cm− 1 are due to Si–O stretching vibration and Si–O bending vibration respectively. After the modiﬁcation (Fig. 1b), there appeared two new peaks at 2924 cm− 1 and 2854 cm− 1. The vibration bands at 2854 cm− 1 and 2924 cm− 1 are attributed to symmetric and asymmetric CH2-stretching vibration . HDTMA solid have vibrations at 2849 and 2917 cm− 1, respectively . These band positions were located at 2851 cm− 1 and 2920 cm− 1 for kaolinite modiﬁed by HDTMA  and
2.3. Dichromate ion adsorption and adsorbent regeneration Batch adsorption experiments were carried out in 250 mL Erlenmeyer ﬂasks by varying initial concentration of Cr(VI) solutions (50 mL) and adsorbent dose. Then the samples were shaked on a thermostated shaker with a shaking of 150 rpm at 25 °C. Sodium nitrate solution was used to adjust ionic strength and sodium hydroxide and nitric acid were used to adjust pH when the effects of ionic strength and pH on the adsorption capacity were studied. When reaching adsorption equilibrium, the mixture was centrifuged to get supernatant liquid and residual Cr(VI) in the supernatant liquid was detected by the diphenyl carbazide spectrophotometric method at 540 nm wavelength . Some experimental factors, such as the adsorption time (0–240 min), initial concentration of dichromate ion (25–300 mg/L), ionic strength (0–1.0 mol/L) and pH value of the solution (3–10), were chosen as the controlling parameters in the adsorption process.
Fig. 1. Infrared spectra of original HNTs (a) and modiﬁed HNTs (b).
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at 2853 cm− 1 and 2921 cm− 1 for chabazite modiﬁed by HDTMA . Compared to these results, the characteristic peaks of these vibration bands indicate the quaternary ammonium cations were grafted onto nanotubes surface successfully. Thermogravimetric analyses (TGA) of original and modiﬁed HNTs were further carried out in N2 from room temperature to 800 °C to test the grafted amount of the quaternary ammonium cations. The thermogravimetric curves that were obtained by measuring the weight loss of adsorbent from pyrolysis are shown in Fig. 2. It can be observed that the mass-loss curve decreased continuously. The weight loss of adsorbents occurs mainly in the stage of 105–800 °C, and weight loss of HNTs at temperature below 105 °C is due to the loss of free water. For the original HNTs in Fig. 2b, the weight loss of 15.42% at 650 °C is due to chemical dehydration of the HNTs. For the modiﬁed HNTs in Fig. 2a, besides mass-loss mentioned above, there is obvious weight loss of 7.14% between 210 °C and 400 °C. Comparing Fig. 2a with Fig. 2b, the weight loss between 210 °C and 400 °C can be attributed to thermal decomposition of the quaternary ammonium cations loaded on the HNTs. Furthermore, transmission electron microscope (TEM) was used to observe morphological structure of original and modiﬁed HNTs. Fig. 3 displays TEM of natural and modiﬁed HNTs. The diameter and length of natural nanotubes in Fig. 3a range from 10 to 50 nm and from 500 to 1000 nm respectively. In comparison with natural nanotubes, tube walls of modiﬁed HNTs in Fig. 3b are obviously thicker. Furthermore, all the original and modiﬁed nanotubes are straight and have open ends. The structure enables metal ion to access and adsorb on the surface easily.
Fig. 2. Thermogravimetric analysis of original HNTs (b) and modiﬁed HNTs (a).
Fig. 3. TME of original halloysite nanotubes (a) and modiﬁed HNTs (b).
In addition, the speciﬁc surface area can inﬂuence adsorption properties, so the speciﬁc surface area was also investigated. The results show that the speciﬁc surface area of original HNTs is 59.62 m2/g, while the speciﬁc surface area of the modiﬁed HNTs decreased to 41.82 m2/g, which is attributed to quaternary ammonium cations loaded on the HNTs.
3.2. Analysis of adsorption rate The adsorption time is one of the important characteristics that deﬁne the efﬁciency of sorption. In order to investigate the effect of adsorption time, modiﬁed HNTs (0.50 g) were respectively added to a series of Cr(VI) solutions at a constant concentration of 50 mg/L at 25 °C and the adsorption capacities were measured at different adsorption time from 5 to 240 min in Fig. 4. Obviously, there was a burst adsorption step within a contact time of about 10 min and then gradually to reach adsorption equilibrium. The adsorption capacities reached 4.3 mg/g (90% removal efﬁciency) for 5 min and reach 4.4 mg/g (92% removal efﬁciency) for 10 min. The results indicate the adsorbent has a rather rapid adsorption rate and a contact time of 30 min is sufﬁcient for the adsorption of Cr(VI). Table 1 shows comparison of equilibrium time of the modiﬁed HNTs with that of several adsorbents reported in the literatures [25–29]. It can be seen that the adsorption equilibrium time in present study is the shortest among these adsorbents. The rapid adsorption rate of modiﬁed halloysite nanotubes can be attributed to their unique structure. Compared with natural zeolites with pore diameters of 3–10 Å and other adsorbents, modiﬁed halloysite nanotubes have larger pore
Fig. 4. Effect of adsorption time on Cr(VI) adsorption capacity.
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Table 1 Comparison of equilibrium time of various adsorbents for Cr(VI). Adsorbents
Zeolite Y New composite chitosan biosorbent Lignocellulosic substrate extracted from wheat bran Succinylated mercerized cellulose Polyethylenimine-modiﬁed fungal biomass Halloysite nanotubes
Equilibrium time t (h)
diameter of 10–50 nm and allow Cr(VI) ion easily to access and load on their surface. 3.3. Effect of adsorbent dose Adsorbent dose is an important parameter in the determination of adsorption capacity. The effect of the adsorbent dose was investigated by addition of various amounts of HNTs in 50 mL 50 mg/L Cr(VI) aqueous solution at 25 °C for 1 h. The result is shown in Fig. 5. It is observed that the removal efﬁciency increased from 35.6% to 95.5% with an increase in adsorbent dose from 0.05 to 1.0 g. This can be attributed to the increase in the adsorbent speciﬁc surface area and availability of more adsorption sites. However, the further increasing the amount of the adsorbent causes few change of the removal efﬁciency. It is also observed that the adsorption capacity decreases from 17.6 to 3.18 mg/g as the adsorbent dose increases from 0.05 to 1.0 g. Consequently, the adsorbent dose was maintained at 0.2 g which was considered to be sufﬁcient for the removal of Cr(VI). 3.4. Effect of pH on the adsorption capacity The pH of the aqueous solution is an important controlling parameter that strongly affects the existed form of Cr(VI). In order to investigate the effect of pH value, modiﬁed HNTs (0.50 g) were respectively added to Cr(VI) solutions with concentration of 50 mg/L at 25 °C. The uptake of Cr(VI) as a function of hydrogen ion concentration was examined over a pH range of 2–10 and was shown in Fig. 6. It is evident that the adsorption capacity is highly pH dependent and the maximum adsorption is found at pH = 3. When
Fig. 6. Effect of pH on Cr(VI) adsorption capacity.
the pH value was in the range of 3–10, the adsorption capacity decreases with the increasing of pH. In aqueous solutions, Cr(VI) anion is not a simple monovalent anion but rather a series of chromate anions depending upon the pH of the solution. The chromate may be represented in various forms 2− such as H2CrO4, HCrO− and Cr2O2− in the solution phase as a 4 , CrO4 7 2− function of pH. Between pH 2 and 6, HCrO− are in 4 and Cr2O7 − equilibrium, the major specie is HCrO4 . As the pH increases, this form 2− shifts to CrO2− and Cr2O2− is the only 4 7 . At pH greater than 7.5, CrO4 chromate species in aqueous phase . So when the pH is changed, the existed form of Cr(VI) will inﬂuence the Cr(VI) uptake. At low pH, the adsorbent surfaces become positively-charged due to strong protonation, electrostatic force between the positively-charged 2− surface and the negatively-charged HCrO− 4 and Cr2O7 , as well as the interaction between quaternary ammonium cations and HCrO− 4 and Cr2O2− 7 in the internal nanotubes, will enhance the Cr(VI) adsorp2− tion. However, when the pH value is less than 2, HCrO− 4 and Cr2O7 can transform H2Cr2O7 and electrostatic interaction will reduce accordingly. With increase of pH, the degree of protonation of the surface reduces gradually and hence adsorption capacity decrease in the pH range of 3–6. Furthermore, the lower afﬁnity of Cr(VI) adsorption above pH 7 can be attributed to the strong competition between OH− with CrO2− since more OH− anions are present. Similar result 4 has also been previously reported in prior studies [30–32]. 3.5. Effect of ionic strength on the adsorption capacity In addition, the adsorption capacity of the modiﬁed HNTs is inhibited signiﬁcantly with increase of ionic strength. In order to investigate the ionic strength effect, modiﬁed HNTs (0.50 g) were respectively added to Cr(VI) solutions with concentration of 100 mg/L at 25 °C. The results (Fig. 7) indicate that the inorganic electrolyte suppresses distinctly the adsorption of dichromate. The ionic strength concentration increases from 0 to 1.0 mol/L, the Cr(VI) adsorption capacity of modiﬁed HNTs decreases from 6.611 mg/g to 0.297 mg/g correspondingly. There are two reasons for the above results. First, the inorganic electrolyte (NO− 3 ) can shield electrostatic attraction between Cr(VI) ion and modiﬁed HNTs. Second, the NO− 3 contest the surface adsorption position with Cr(VI) ion. The part of the HCrO− 4 and Cr2O2− adsorbed on the adsorbent were displaced by the NO− 7 3 ions in solution, which can be expressed as:
Fig. 5. Effect of adsorbent dose on adsorption of Cr(VI) onto modiﬁed HNTs.
þ − 2− − HNTs− C16 H33 −NðCH3 Þ3 HCrO4 = Cr2 O7 + NO3 ⇌HNTs þ − − 2− − C16 H33 −NðCH3 Þ3 NO3 + HCrO4 = Cr2 O7
W. Jinhua et al. / Desalination 259 (2010) 22–28 Table 2 The desorption capacity and efﬁciency of various eluents. Eluents
Desorption capacity (mg/g)
Desorption efﬁciency %
Na2SO4 NaCl NaNO3 Mixed solution of NaNO3 and NaOH
1.311 2.979 4.166 4.525
27.5 62.4 87.3 94.8
The Langmuir isotherm model is applied to establish the relationship between the amount of Cr(VI) uptake and their equilibrium concentration in aqueous solution. The experimental data is ﬁtted with the linear form of the Langmuir equation : Ce = qe = 1 = Q max b + Ce = Q max
Fig. 7. Effect of ionic strength on Cr(VI) adsorption capacity.
As the NO− 3 in the solution increased, the reaction would be shifted to the right, resulting in the reduction of Cr(VI) adsorption. Hence, more Cr(VI) was desorbed by increasing the concentration of NO− 3 ions. 3.6. Adsorption isotherm The effect of initial concentration of Cr(VI) on the adsorption capacity is shown in Fig. 8a. The results indicate that adsorption capacity increases with increase of Cr(VI) initial concentration and achieves maximum adsorption capacity ﬁnally.
Fig. 8. Effect of initial concentration of Cr(VI) on adsorption capacity (a) and Langmuir sorption isotherms model (b).
where qe is the equilibrium adsorption capacity, Q max is the maximum amount of metal ion per weight unit, Ce is equilibrium metal ion concentration. The Langmuir plot for Cr(VI) adsorption using the adsorbent is shown in Fig. 8b. The results reveal that the adsorption is well ﬁtted the linear Langmuir model. From the linear ﬁt of the plot, the maximum adsorption capacity (Q max) can be calculated as 6.9 mg/g. The linear relation coefﬁcient r2 is also calculated. The high value of r2 (0.999) indicates the applicability of the Langmuir isotherm model in this work. 3.7. Adsorbent regeneration Since Cr(VI) adsorption onto modiﬁed HNTs is a reversible process, it is possible for regeneration of the adsorbent to reuse. The primary objective of regeneration is to restore the adsorption capacity of exhausted adsorbent while the secondary objective is to recover valuable Cr(VI) in the adsorbed phase, if any . The adsorption of Cr (VI) on modiﬁed HNTs is highly ionic strength and pH dependent; hence the desorption of Cr(VI) can be accomplished by increasing the ionic strength and pH. To ﬁnd the most potential eluent for the desorption of Cr(VI), 50 mL of 1 M eluents (Na2SO4, NaCl, NaNO3 and mixed solution of NaNO3 and NaOH) was mixed with 0.50 g Cradsorbed adsorbent for 1 h. The results are shown in Table 2. The desorption capacity of Na2SO4, NaCl, NaNO3 and mixed solution of NaNO3 and NaOH was found to be 1.311 mg/g, 2.979 mg/g, 4.166 mg/g and 4.525 mg/g, respectively. Thus, mixed solution of NaNO3 and NaOH as a most effective eluent was used for the desorption process. The
Fig. 9. The regeneration efﬁciency of modiﬁed HNTs.
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Fig. 10. Reaction mechanism of adsorption.
regeneration efﬁciency of modiﬁed HNTs undergoing ﬁve cycles is illustrated in Fig. 9. It can be noticed that the adsorption capacity decreases with increasing of regeneration cycle numbers and the regeneration efﬁciency still keep above 60% after ﬁve adsorption/ desorption experiments. The results indicate that modiﬁed HNTs can be regenerated and reused. 3.8. Analysis of adsorption mechanisms In the work, the major reactions responsible for the chromate anions adsorption are shown in Fig. 10. The halloysite nanotubes used in this study are mainly hollow tube with a length of 500–1000 nm and a diameter of approximately 10–50 nm. The Na-HNTs were prepared prior to modiﬁcation in order to improve cation exchange capacity (CEC). Then the Na-HNTs were modiﬁed cation exchange with hexadecyltrimethylammonium bromide (HDTMA). Through surface 2− modiﬁcation, anions of HCrO− 4 and Cr2O7 can be bound to quaternary ammonium cations adsorbed on the halloysite nanotubes surface. 4. Conclusion To summarize, the halloysite nanotubes (HNTs) were modiﬁed with the surfactant of hexadecyltrimethylammonium bromide (HDTMA) to form a new adsorbent. The adsorption experimental results demonstrate that the adsorption capacity of the adsorbent (modiﬁed HNTs) decrease signiﬁcantly by increasing ionic strength and pH. The adsorption data of Cr(VI) on the modiﬁed HNTs are well consistent with Langmuir model. Particularly, The experimental results show that the modiﬁed HNTs are evaluated as efﬁcient adsorbent for chromates with adsorption rapid rate and modiﬁed HNTs can also be regenerated and reused, which implies potential application for removing Cr(VI) pollutants from waste waters. Acknowledgments This work was supported by NSFC (20871105), National High Technology Research and Development Program of China (863 Program, Grant No.2008AA06Z330) and Henan Outstanding Youth Science Fund (0612002400).
References  T.H. Shi, Z.C. Wang, Y. Liu, S.G. Jia, C.M. Du, Removal of hexavalent chromium from aqueous solutions by D301, D314 and D354 anion-exchange resins, Journal of Hazardous Materials 161 (2009) 900–906.  R. Chand, K. Narimura, H. Kawakita, K. Ohto, T. Watari, K. Inoue, Grape waste as a biosorbent for removing Cr(VI) from aqueous solution, Journal of Hazardous Materials 163 (2009) 245–250.  J.R. Memon, S.Q. Memon, M.I. Bhanger, A. El-Turki, K.R. Hallam, G.C. Allen, Banana peel: a green and economical sorbent for the selective removal of Cr(VI) from industrial wastewater, Colloids and Surfaces B: Biointerfaces 70 (2009) 232–237.  K.K. Singh, S.H. Hasan, M. Talat, V.K. Singh, S.K. Gangwar, Removal of Cr (VI) from aqueous solutions using wheat bran, Chemical Engineering Journal 151 (2009) 113–121.  V. Prigione, M. Zerlottin, D. Refosco, V. Tigini, A. Anastasi, G.C. Varese, Chromium removal from a real tanning efﬂuent by autochthonous and allochthonous fungi, Bioresource Technology 100 (2009) 2770–2776.  V.K. Gupta, A. Rastogi, Biosorption of hexavalent chromium by raw and acidtreated green alga Oedogonium hatei from aqueous solutions, Journal of Hazardous Materials 163 (2009) 396–402.  M.P. Koivula, K. Kujala, H. Rönkkömäki, M. Mäkelä, Sorption of Pb(II), Cr(III), Cu(II), As(III) to peat, and utilization of the sorption properties in industrial waste landﬁll hydraulic barrier layers, Journal of Hazardous Materials 164 (2009) 345–352.  O. Maryuk, S. Pikus, E. Olszewska, M. Majdan, H. Skrzypek, E. Zieba, Benzyldimethyloctadecylammonium bentonite in chromates adsorption, Materials Letters 59 (2005) 2015–2017.  F. Granados-Correa, J. Jiménez-Becerril, Chromium (VI) adsorption on boehmite, Journal of Hazardous Materials 162 (2009) 1178–1184.  M. Noroozifar, M. Khorasani-Motlagh, M.N. Gorgij, H.R. Naderpour, Adsorption behavior of Cr(VI) on modiﬁed natural zeolite by a new bolaform N,N,N,N′,N′,N′hexamethyl-1,9-nonanediammonium dibromide reagent, Journal of Hazardous Materials 155 (2008) 566–571.  M. Majdan, S. Pikus, Z. Rzączyńska, M. Iwan, O. Maryuk, R. Kwiatkowski, H. Skrzypek, Characteristics of chabazite modiﬁed by hexadecyltrimethylammonium bromide and of its afﬁnity toward chromates, Journal of Molecular Structure 791 (2006) 53–60.  K. Barquist, S.C. Larsen, Chromate adsorption on amine-functionalized nanocrystalline silicalite-1, Microporous and Mesoporous Materials 116 (2008) 365–369.  L. Bois, A. Bonhommé, A. Ribes, B. Pais, G. Rafﬁn, F. Tessier, Functionalized silica for heavy metal ions adsorption, Colloids and Surfaces A: Physicochemical and Engineering Aspects 221 (2003) 221–230.  J.S. Li, X.Y. Miao, Y.X. Hao, J.Y. Zhao, X.Y. Sun, L.J. Wang, Synthesis, aminofunctionalization of mesoporous silica and its adsorption of Cr(VI), Journal of Colloid and Interface Science 318 (2008) 309–314.  Y. Bayrak, Y. Yesiloglu, U. Gecgel, Adsorption behavior of Cr(VI) on activated hazelnut shell ash and activated bentonite, Microporous and Mesoporous Materials 91 (2006) 107–110.  Y.B. Fu, L.D. Zhang, Simultaneous deposition of Ni nanoparticles and wires on a tubular halloysite template: a novel metallized ceramic microstructure, Journal of Solid State Chemistry 178 (2005) 3595–3600.
W. Jinhua et al. / Desalination 259 (2010) 22–28
 H. Ismail, P. Pasbakhsh, M.N.A. Fauzi, Morphological, thermal and tensile properties of halloysite nanotubes ﬁlled ethylene propylene diene monomer (EPDM) nanocomposites, Polymer Tesing 27 (2008) 841–850.  B.C. Guo, Q.L. Zou, Y.D. Lei, Crystallization behavior of polyamide 6/halloysite nanotubes nanocomposites, Thermochimica Acta 484 (2009) 48–56.  D.G. Shchukin, S.V. Lamaka, K.A. Yasakau, Active anticorrosion coatings with halloysite nanocontainers, Journal of Physical Chemistry 112 (2008) 958–964.  E. Abdullayev, R. Price, D. Shchukin, Halloysite tubes as nanocontainers for anticorrosion coating with benzotriazole, Acs Applied Materials & Interfaces 1 (2009) 1437–1443.  A. Kilislioglu, B. Bilgin, Adsorption of uranium on halloysite, Radiochemical Acta 90 (2002) 155–160.  M.F. Zhao, P. Liu, Adsorption behavior of methylene blue on halloysite nanotubes, Microporous and Mesoporous Materials 112 (2008) 419–424.  H.L. Hong, W.T. Jiang, X.L. Zhang, L.Y. Tie, Z.H. Li, Adsorption of Cr(VI) on STACmodiﬁed rectorite, Applied Clay Science 42 (2008) 292–299.  Z.H. Li, L. Gallus, Surface conﬁguration of sorbed hexadecyltrimethylammonium on kaolinite as indicated by surfactant and counterion sorption, cation desorption, and FTIR, Colloids and Surfaces A: Physicochemical and Engineering Aspects 264 (2005) 61–67.  A.M. Yusof, N.A. Malek, Removal of Cr(VI) and As(V) from aqueous solutions by HDTMA-modiﬁed zeolite Y, Journal of Hazardous Materials 162 (2009) 1019–1024.
 V.M. Boddu, K. Abburi, J.L. Talbott, E.D. Smith, Removal of hexavalent chromium from wastewater using a new composite chitosan biosorbent, Environment Science Technology 37 (2003) 4449–4456.  L. Dupont, E. Guillon, Removal of hexavalent chromium with a lignocellulosic substrate extracted from wheat bran, Environment Science Technology 37 (2003) 4235–4241.  S.B. Deng, Y.P. Ting, Polyethylenimine-modiﬁed fungal biomass as a high-capacity biosorbent for Cr(VI) anions: sorption capacity and uptake mechanisms, Environment Science Technology 39 (2005) 8490–8496.  L.V.A. Gurgel, J.C.P. Melo, J.C. Lena, L.F. Gil, Adsorption of chromium (VI) ion from aqueous solution by succinylated mercerized cellulose functionalized with quaternary ammonium groups, Bioresource Technology 100 (2009) 3214–3220.  S.S. Baral, S.N. Dasa, P. Rath, G.R. Chaudhury, Chromium(VI) removal by calcined bauxite, Biochemical Engineering Journal 34 (2007) 69–75.  A.K. Bhattacharya, T.K. Naiya, S.N. Mandal, S.K. Das, Adsorption, kinetics and equilibrium studies on removal of Cr(VI) from aqueous solutions using different low-cost adsorbents, Chemical Engineering Journal 137 (2008) 529–541.  J.Y. Qiu, Z.Y. Wang, H.B. Li, L. Xu, J. Peng, M.L. Zhai, Adsorption of Cr(VI) using silicabased adsorbent prepared by radiation-induced grafting, Journal of Hazardous Materials 166 (2009) 270–276.  J. Hu, G.H. Chen, I.M.C. Lo, Removal and recovery of Cr(VI) from wastewater by maghemite nanoparticles, Water Research 39 (2005) 4528–4536.