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Surfactant-modified clay as adsorbent for chromate
Applied Clay Science 20 Ž2001. 65–71 www.elsevier.comrlocaterclay
Surfactant-modified clay as adsorbent for chromate B.S. Krishna a , D.S.R. Murty b, B.S. Jai Prakash a,) b
a Department of Chemistry, Bangalore Institute of Technology, KR Road, Bangalore-560 004, India Chemical Laboratory, Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, NagarabhaÕi, Bangalore-560 072, India
Received 10 January 2000; received in revised form 15 September 2000; received in revised form 28 November 2000; accepted 31 January 2001
Abstract The adsorption of oxyanion of chromate was not given much attention, perhaps due to the fact that clays are negatively charged and unmodified clay mineral surface shows no affinity for chromate. After modification by cationic surfactant, clay was found to adsorb considerable amounts of chromate. Hexadecyl trimethylammonium ŽHDTMA. bromide was used to modify the surface of clay minerals such as kaolinite, montmorillonite, and pillared montmorillonite. Montmorillonite adsorbed a quantity of HDTMA equivalent to the cation exchange capacity. Kaolinite and pillared clay adsorbed relatively small amounts of the surfactant. The amount of chromate adsorbed was maximum at and below pH 1 and proportional to the amount of HDTMA fixed on motmorillonite. The amount of chromate adsorbed between pH 2 and 6 was one half of that at pH 1, and above pH f 8 the adsorption was negligible. The pH dependence of chromate adsorption was attributed to the pH dependent-equilibria 2y 2y HCr2 Oy 7 z Cr 2 O 7 z CrO4 . The adsorption data obtained was well described by Langmuir adsorption isotherm. Kinetics of chromate adsorption Ždiffusion constant. was calculated and a mechanism for the adsorption is proposed. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Clay-adsorbent; HDTMA–clay; Chromate adsorption; Surfactant modification
1. Introduction Chromium is widely used in industries such as electroplating and tannery. Untreated effluents from such industries pollute the ground water with chromium ŽLawrence, 1981; Tavani and Volzone,
)
Corresponding author. Fax: q91-80-6633706. E-mail addresses: [email protected], [email protected] ŽB.S. Jai Prakash..
1997.. The element appears primarily in two valence states—the toxic hexavalent state and the relatively benign trivalent state. The conventional treatment of effluent consists of reduction of Cr ŽVI. to Cr ŽIII., precipitation of Cr ŽIII. as Cr2 O 3 P x H 2 O at high pH ŽDavid et al., 1998. followed by the disposal of the dewatered sludge. The major shortcoming of the conventional treatment includes high cost of chemicals used for Cr ŽVI. reduction. Wastewater containing relatively low concentrations of Cr ŽVI. is usually treated with ion exchange resins to recover
0169-1317r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 0 1 . 0 0 0 3 9 - 4
66
B.S. Krishna et al.r Applied Clay Science 20 (2001) 65–71
chromate but this involves expensive resins ŽDavid et al., 1998.. Thus, there is a search for cheaper adsorbent materials such as clays to recover the chromium. Ion exchange with organic cation has been used to modify the surface properties of clay minerals and other natural materials. Such modified clay minerals have been used to adsorb organic contaminants. Clay minerals, modified with quaternary amine, can substantially enhance the removal of non-ionic organic solute from aqueous solution ŽCadena, 1987.. The adsorption of cationic surfactants on clay minerals, and the potential application of such modified clays as environmental remediation material have been studied ŽBoyd et al., 1988; Bors, 1990.. Alkyl ammonium-modified clay minerals are frequently studied because of the unique adsorptive properties of the modified materials ŽSrinivasan and Fogler, 1990; Jaynes and Boyd, 1991; Zhang et al., 1993; Xu and Boyd, 1995a,b; Sullivan, 1997.. In order to adsorb anions, the modified surface must either possess positively charged exchange sites, or there should be replacement of weakly held counter ions of the surfactant by more strongly held adsorbate counter ions. Some selective cationic surfactants provide such a modified surface for anions. Surfactant-modified zeolite were shown to have the ability to remove chromate from aqueous solution ŽHaggerty and Bowman, 1994; Li and Bowman, 1997; Li, 1998.. Haggerty and Bowman Ž1994. concluded that hexadecyl trimethylammonium ŽHDTMA. is too large to enter into the internal position of the zeolite and adsorption of amine only occurs on the zeolite’s external exchange sites. Li and Bowman Ž1997. observed that the amount of HDTMA adsorbed on the surface of zeolite increases with the initial concentration of HDTMA. When the amount of HDTMA adsorbed exceeds the external cation exchange capacity of the zeolite, adsorption of chromate occurred. This was explained by formation of surfactant bilayers; the positive charges of the bilayer are considered as adsorption sites for chromate. Montmorillonite modified with the non-ionic surfactant Tween-80 adsorbed considerable amounts of iodine ŽKrishna et al., 1998, 1999.. In this report, we studied the effect of pH on adsorption of chromate by HDTMA-modified clay minerals.
2. Materials and methods The clay minerals used in this study are: Ži. a kaolin sample with the composition, 46.0% SiO 2 , 36.5% Al 2 O 3 , 0.42% TiO 2 , 0.2% Fe 2 O 3 , 0.07% FeO, 0.65% MgO, 0.01% MnO 2 , 0.06% CaO, 0.19% Na 2 O, 0.01% K 2 O, 0.13% P2 O5 , and 14.8% LOI with a cation exchange capacity Žcec. of 0.015 meq gy1 ; Žii. a white American bentonite ŽGK-69. with the composition, 52.1% SiO 2 , 15.2% Al 2 O 3 , 1.9% Fe 2 O 3 , 0.07% FeO, 3.2% MgO, 1.92% CaO, 2.58% Na 2 O, 0.9% K 2 O, 0.26% P2 O5 , and 21.8% LOI with a cec of 0.8 meq gy1 ; and Žiii. a bentonite clay mineral from Bhuj area, Gujarat, India Žcontaining essentially montmorillonite. with the composition, 44.8% SiO 2 , 0.89% TiO 2 , 13.6% Al 2 O 3 , 11.5% Fe 2 O 3 , 0.07% FeO, 1.97% MgO, 1.69% CaO, 3.16% Na 2 O, 0.13% K 2 O, 0.24% P2 O5 , and 22.0% LOI with a cation exchange capacity of 0.8 meq gy1 . The Ceramic Technological Institute, Bangalore, India provided samples Ži. and Žii.. Ashapura International, a local commercial supplier of industrial clays, supplied sample Žiii.. The Indian montmorillonite sample was also pillared by hydroxyoligomers of aluminium at OHrAl ratio of 2.5. The pillaring procedure used by the authors is given elsewhere ŽSelvaraj et al., 1996.. 2.1. Preparation of modified clays Known amounts of hexadecyl trimethylammonium ŽHDTMA. bromide ŽQualigen Chemicals., ranging from 0.274 to 1.37 mmol, were dissolved in 100 ml of water. One gram of the clay, suspended in acetone, was added to the HDTMA solution. Acetone was used to make the suspensions less sticky and easier to handle. The contents were mixed thoroughly in a wrist-action shaker for 1 h. The mixture was centrifuged and the centrifugate was discarded. The solid was washed several times with distilled water to remove superficially held adsorbate. The amount of surfactant adsorbed was determined by C, H analysis ŽCarlo Erba CHNanalyzer.. At higher HDTMA concentrations, the surfactant solution appeared cloudy because of the low solubility of the surfactant. In addition, at high concentrations, HDTMA crystallized on the clay
B.S. Krishna et al.r Applied Clay Science 20 (2001) 65–71
mineral surface, leading to washing problems. For this reason, the maximum initial concentration was 1.37 mmol HDTMA Žin 100 ml.rg clay. At this condition, the amount of HDTMA adsorbed was equivalent to cation exchange capacity. The modified clay with a maximum amount of surfactant adsorbed Ž0.8 mmol gy1 . was used to study the kinetic of adsorption. To study the equilibrium established between the surfactant and clay, separate flasks containing 1.0-g sample of the clay treated with 1.37 mmol of HDTMA–Br were shaken for various lengths of time Ž30 min, 1 h, 5 h, 24 h, 2–7 days.. The solid was washed thoroughly Žabout 10 times. with distilled water to remove any superficially adsorbed HDTMA on the surface. The final amount adsorbed after washing was the same as after shaking for 1 h. Hence, samples treated for 1 h with HDTMA–Br and washed were used for the chromate adsorption studies. 2.2. Adsorption of chromate For the adsorption of chromate, samples of 1.0 g air-dried modified clays were used. The clay was treated with different amounts of potassium chromate ranging from 4.5 to 45 mmol of Cr. The mixture Ž100 ml. was allowed to stand for 10–60 min with periodical shaking and then centrifuged at 10,000 rpm for 5 min. The amount of chromium present in the centrifugate was analyzed by measuring the absorbance of the purple complex of Cr ŽVI. with 1,5-diphenylcarbazide ŽRand et al., 1975. at 540 nm. All trials were accompanied by blank containing only chromium solution. The amount of Cr adsorbed on the solid was also checked by ICP-AES Ž8410 Labtam, Australia.. The chromate-loaded samples were dissolved in HF–HNO 3 mixtures and aspirated into ICP-AES under optimum experimental parameters. The adsorption of chromate was determined as a function of pH ŽFig. 1.. One gram of modified bentonite sample was treated with 45 mmol of Cr and the pH was carefully adjusted between 1 and 6 with dilute hydrochloric acid. The amount of chromate adsorbed was measured after periodical shaking for 30 min; this period was found to be sufficient to attain equilibrium. The kinetics of adsorption by the
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bentonites was also studied. One gram of bentonite modified with HDTMA was treated with 100 ml of potassium chromate solution containing 45 mmol of Cr. The samples were shaken and the amount of chromate adsorbed was determined after 10, 20, 30, 40, 50, and 60 min. The amount adsorbed was plotted against square root of time. HDTMA Ž0.8 mmol. in 100 ml was treated separately with slight excess of 0.05 M potassium chromate at pH F 1.0 and pH s 4.5. The orange and yellow coloured solids obtained were filtered, washed with water and the amount of chromate present was estimated colorimetrically. 2.3. X-ray measurements The surfactant-treated and chromate-loaded Žat pH F 1.0 and pH s 4.5. samples were thoroughly washed to remove any superficially held adsorbate molecules on the surface. The samples were air-dried and the X-ray diffraction patterns were recorded on a Rigaku rotaflex diffractometer with low angle facilities ŽNi filter, Co K A -radiation..
3. Results and discussion 3.1. Adsorption of HDTMA by bentonites The amount of HDTMA adsorbed after 1-h contact time, at an initial amount of 1.37 mmolrg clay, is given in Table 1. The amount of HDTMA adsorbed increased from 0.51 mmol gy1 after 30 min to 0.80 mmol gy1 after 1 h. The sample placed for 7
Table 1 Ion exchange of the kaolin and bentonite with HDTMA cation and adsorption of chromate ŽpH F1. Clay
HDTMA Chromate bound adsorbed on Žmmol kgy1 . raw clay Žmmol kgy1 .
Kaolinite 44 Pillared clay 58 American 800 bentonite Indian bentonite 801
Chromate adsorbed on HDTMA–clay Žmmol kgy1 .
zero zero zero
30 50 795
zero
795
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B.S. Krishna et al.r Applied Clay Science 20 (2001) 65–71
days adsorbed a higher amount of HDTMA. After centrifugation and washing, the same amount of HDTMA was found. The maximum amount of HDTMA adsorbed after washing was equivalent to the cation exchange capacity Žcec.. Kaolin, having only a few exchange sites, adsorbs small amounts of HDTMA. The pillared clays showed a low adsorption of HDTMA. 3.2. Adsorption of chromate by modified bentonites The sample equilibrated for 1 h with HDTMA was used for the adsorption studies of chromate. The amount of chromate adsorbed at pH F 1 is given in Table 1. The adsorption of chromate by the raw clays was found to be lower than the measurable quantity. However, the clays modified with HDTMA adsorbed 0.795 mmol gy1 of chromate at pH F 1. The amount adsorbed depended only on the quantity of HDTMA adsorbed. Modified kaolin and pillared clays, which did not adsorb higher amounts of HDTMA, showed a low adsorption of chromate. Previous workers have made similar observations on kaolin ŽZachara et al., 1988. and modified zeolite ŽHaggerty and Bowman, 1994; Li and Bowman, 1997.. Fig. 1 shows the effect of pH on the adsorption of chromate by modified bentonite. The amount adsorbed was maximum at pH F 1 and decreased to
Fig. 2. Adsorption isotherm for chromate on modified Indian montmorillonite at pH F1.0.
almost half the quantity at pH 2, and remained constant up to pH s 6. Above pH s 8, the amount adsorbed was negligible. The variation in adsorption at different pH values is attributed to the presence of different hexavalent chromium anionic species, and is discussed under the mechanism. 3.3. Adsorption isotherm of chromate on modified montmorillonite The adsorption of chromate can be described by Langmuir isotherm ŽFig. 2.. In its linearized form, crX s 1rab q crb.
Ž 1. y1 .
X is the amount adsorbed Žmmol kg , c is the equilibrium solution concentration Žmmol Ly1 ., a is an affinity parameter ŽLangmuir constant, L mmoly1 ., and b is the maximum adsorption capacity Žmmol kgy1 .. Table 2 presents the fitted Langmuir parameters for all isotherms. Table 2 Langmuir constants for the adsorption of chromate by modified Indian montmorillonite at two different pH
Fig. 1. Effect of pH on the adsorption of chromate by modified Indian montmorillonite.
Langmuir constants
Chromate adsorption at pH s 4.2
Chromate adsorption at pH F1.0
a ŽL mmoly1 . b Žmmol kgy1 .
167 405
72 795
B.S. Krishna et al.r Applied Clay Science 20 (2001) 65–71
The value of a is indicative of the affinity of the chromate to the modified bentonite. The higher value, in comparison with the data by Haggerty and Bowman Ž1994. Ž a s 9.46 L mmoly1 , b s 4.08 mmol kgy1 ., indicates the strong interaction with the adsorbent. 3.4. Kinetics of adsorption Besides the adsorption at the surface of the adsorbent, the adsorbate molecules may also diffuse into the interior of the porous adsorbent. The diffusion constant for adsorption of chromate by modified clay was calculated by the following equation, X s Ž 2 Dt .
1r2
q C.
Ž 2. y1 .
X is the amount of chromate adsorbed Žmmol kg , D is the diffusion constant Žmmol kgy1 miny0.5 ., t is the time, and C is a constant. The amount adsorbed increased linearly with the square root of time ŽFig. 3.. The relation is typical of diffusion controlled solid state reaction, and an apparent diffusion constant can be obtained. For modified bentonite, D s 12.2 mmol kgy1 miny0 .5 was found. In the case of kaolin, such a diffusion process was not observed ŽFig. 3.. 3.5. Adsorption mechanism The amount of HDTMA adsorbed on bentonite is equivalent to the cec Ž0.8 mmol gy1 .. There was an
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Table 3 Basal spacing of bentonite clay, HDTMA–montmorillonite Žfrom India., and after reaction with chromate solution Žat pH F1.0 and pH s 4.5.
˚. dŽ001. ŽA Montmorillonite HDTMA–montmorillonite Chromate–HDTMA–clay Žat pH F1.0. Chromate–HDTMA–clay Žat pH s 4.5.
14.1 32.0 38.0 23.6
increase in the basal spacing in the c-axis, apparently due to the adsorption of amine in the interlamellar region. Several workers have noticed such an increase in the case of adsorption of alkyl-TMA by montmorillonite ŽBrindley, 1965; Johns and Sen Gupta, 1967; Lagaly, 1982; Zhang et al., 1993.. Pure dried derivative, with all the exchange sites occupied ˚ by HDTMAq cations, shows spacing less than 20 A ˚ ŽLagaly, 1982.. A large spacing of more than 30 A Ž . reported in this study Table 3 is due to the fact that the sample was not dried. Chromate species present is influenced by the solution pH. Acidification of aqueous solution of yellow chromate initiates a series of equilibria involving the formation of the orange red dichromate ŽGreenwood and Earnshaw, 1988.. pH-1
pH-7
pH)1
pH)7
2y 2y HCr2 Oy 7 Ž aq . z Cr 2 O 7 Ž aq . z CrO4 Ž aq . .
Ž 3. Below pH 1, the species present are HCr2 Oy 7 ions. The HCr2 Oy anion displaces the surfactant counter 7 ion from the exchange sites on the clays forming clay– ŽHDTMA. –HCr2 O 7 . The exchange sites on the clays are balanced by the protons present in the medium. q Mont.–HDTMAqq Hq–HCr2 Oy 7 ™Mont.–H
q HDTMAq–HCr2 Oy 7 .
Fig. 3. Kinetics of chromate adsorption by modified kaolin clay Ža. and modified Indian montmorillonite Žb. at pH F1.0.
Ž 4.
The attack of protons on the structure of clay is diminished possibly because of hydrogen bonding with oxyanion of chromate bonded to surfactant. Beyond pH 1 and below pH 6, the species present is the divalent anion Cr2 O 72y with small quantities of HCrO4y ŽGreenwood and Earnshaw, 1988.. Chro-
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mate content in the salts, obtained by interaction of HDTMA with chromate at pH F 1 and pH s 4.5, showed the salts to be ŽHDTMA.HCr2 O 7 and ŽHDTMA. 2 Cr2 O 7 , respectively. Formation of the salt ŽHDTMA. 2 Cr2 O 7 is probably the reason why the amount of chromate adsorbed at pH G 2 is only one half of that at pH F 1. The dŽ001. spacing of the chromate–HDTMA clays at both pH are found to be different ŽTable 3., indicating different intercalated species. Beyond pH 8, the adsorption of CrO42y is not favored because of the competition between the chromate and hydroxide ion.
4. Conclusions Surface modification of smectite clays by surfactants offers an easy method of adsorbing large amounts of chromate. Chromate in the acidic pH range, pH s 1–6, can be immobilized by surfactant cations in the interlayer space such as alkylammonium ions. Hexadecyl trimethylammonmium ŽHDTMA. bromide was found to hold chromate ions between the smectite layers in the form of salts. The type of salts formed depended on the pH-dependent equlibria of the chromium species, HCr2 Oy 7z Cr2 O 72yz CrO42y, and the salts possessed low solubility products in the acidic pH range. Twice the amount of chromate adsorbed at pH F 1 relative to pH G 2 may be caused by the formation of ŽHDTMA.HCr2 O 7 instead of ŽHDTMA. 2 Cr2 O 7 . Thus, immobilized interlayer surfactant species can be advantageously used to concentrate and recover toxic species present in the aqueous environment.
Acknowledgements The authors wish to acknowledge with thanks the financial assistance from the Department of Atomic Energy, Government of India, for carrying out this project. The authors are grateful to Prof. B.K. Sadashiva, Raman Research Institute, Bangalore, for carbon analysis measurements, and the Director, Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Bangalore, for chemical analysis by ICP-AES method. Grateful
thanks are also due to the Principal and the Members of the Governing Council of Bangalore Institute of Technology for the facilities provided.
References Bors, J., 1990. Adsorption of radioiodine in organo-clays and soils. Radiochim. Acta 51, 139–143. Boyd, S.A., Shaobai, S., Lee, J., Mortland, M.M., 1988. Pentochlorophenol sorption by organo-clays. Clays Clay Miner. 36, 125–130. Brindley, G.W., 1965. Complexes of primaraamines with montmorillonite and vermiculite. Clays Clay Miner. 6, 91–96. Cadena, F., 1987. Assessing exposure to toxic substances from land disposal of hazardous wastes. Proc.-APCA Annu. Meet. 1986, 79, 86r12.2, p. 22. David, K., Particia, P., Bomumil, V., 1998. Removal of trivalent and hexavalent chromium by seaweed biosrbent. Environ. Sci. Technol. 32, 2693–2698. Greenwood, N.N., Earnshaw, A., 1988. Chemistry of the Elements. Pergamon, New York, p. 1175. Haggerty, G.M., Bowman, R.S., 1994. Sorption of chromate and other inorganic anions by organo zeolite. Environ. Sci. Technol. 28, 452–458. Jaynes, W.F., Boyd, S.A., 1991. Clay mineral type and organic compound sorption by hexadecyol ammonium- exchanged clays. Soil Sci. Soc. Am. J. 55, 43. Johns, W.D., Sen Gupta, P.K., 1967. Vermiculite–alkylammonium complexes. Am. Mineral. 52, 1706–1724. Krishna, B.S., Selvaraj, S., Mohan, B.V., Murty, D.S.R., Jai Prakash, B.S., 1998. Chemically modified clays as recyclable adsorbents for iodine. Bull. Mater. Sci. 21, 355–362. Krishna, B.S., Murty, D.S.R., Jai prakash, B.S., 1999. Organo clays as recyclable adsorbent for iodine: a study of thermodynamic parameters. Clay Res. 18 Ž1., 1–10. Lagaly, G., 1982. Layer charge heterogeneity in vermiculite. Clays Clay Miner. 330, 215–222. Lawrence, M.M., 1981. Removal of hexavalent chromium from estuarine waters by model substrates and natural sediments. Environ. Sci. Technol. 15, 1482–1484. Li, Z., 1998. Chromate extraction from surfactant modified zeolite surfaces. J. Environ. Qual. 27, 240–242. Li, Z., Bowman, R.S., 1997. Counterion effects on the sorption of cationic surfactant and chromate on natural clinoptilolite. Environ. Sci. Technol. 31, 2407–2412. Rand, M.S., Greenberg, A.E., Taras, M.J., 1975. Standard Methods for the Examination of Water and Wastewater. 16th edn. American Public Health Association, New York, p. 192. Selvaraj, S., Krishna, K.N., Mohan, B.V., Jai Prakash, B.S., 1996. Pillaring of smectite using an aluminum oligomer: a study of pillar density and thermal stability. Appl. Clay Sci. 10, 439– 450. Srinivasan, K.R., Fogler, H.S., 1990. Use of inorgano-organo-clays in the removal of priority pollutants from industrial wastewater: structural aspects. Clays Clay Miner. 38, 277–287.
B.S. Krishna et al.r Applied Clay Science 20 (2001) 65–71 Sullivan, E.J., 1997. Topological and thermal properties of surfactant modified clinoptilolite studied by tapping modee atomic force microscopy and high-resolution thermogravimetric analysis. Clays Clay Miner. 45, 42. Tavani, E.L., Volzone, C., 1997. J. Soc. Leather Technol. Chem. 81, 143–148. Xu, S., Boyd, S.A., 1995a. Cationic surfactant sorption to vermiculite subsoil via hydrophobic bonding. Environ. Sci. Technol. 29, 312.
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Xu, S., Boyd, S.A., 1995b. Alternative model for cationic surfactant adsorption by layered silicates. Environ. Sci. Technol. 29, 3022. Zachara, J.M., Cowman, C.E., Schmidt, R.L., Ainsworth, C.C., 1988. Chromate adsorption by kaolin. Clays Clay Miner. 36, 317–326. Zhang, Z.Z., Sparks, D.L., Scrivener, N.C., 1993. Sorption and desorption of quaternary amine cations on clays. Environ. Sci. Technol. 27, 1625–1631.
Surfactant-modified clay as adsorbent for chromate B.S. Krishna a , D.S.R. Murty b, B.S. Jai Prakash a,) b
a Department of Chemistry, Bangalore Institute of Technology, KR Road, Bangalore-560 004, India Chemical Laboratory, Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, NagarabhaÕi, Bangalore-560 072, India
Received 10 January 2000; received in revised form 15 September 2000; received in revised form 28 November 2000; accepted 31 January 2001
Abstract The adsorption of oxyanion of chromate was not given much attention, perhaps due to the fact that clays are negatively charged and unmodified clay mineral surface shows no affinity for chromate. After modification by cationic surfactant, clay was found to adsorb considerable amounts of chromate. Hexadecyl trimethylammonium ŽHDTMA. bromide was used to modify the surface of clay minerals such as kaolinite, montmorillonite, and pillared montmorillonite. Montmorillonite adsorbed a quantity of HDTMA equivalent to the cation exchange capacity. Kaolinite and pillared clay adsorbed relatively small amounts of the surfactant. The amount of chromate adsorbed was maximum at and below pH 1 and proportional to the amount of HDTMA fixed on motmorillonite. The amount of chromate adsorbed between pH 2 and 6 was one half of that at pH 1, and above pH f 8 the adsorption was negligible. The pH dependence of chromate adsorption was attributed to the pH dependent-equilibria 2y 2y HCr2 Oy 7 z Cr 2 O 7 z CrO4 . The adsorption data obtained was well described by Langmuir adsorption isotherm. Kinetics of chromate adsorption Ždiffusion constant. was calculated and a mechanism for the adsorption is proposed. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Clay-adsorbent; HDTMA–clay; Chromate adsorption; Surfactant modification
1. Introduction Chromium is widely used in industries such as electroplating and tannery. Untreated effluents from such industries pollute the ground water with chromium ŽLawrence, 1981; Tavani and Volzone,
)
Corresponding author. Fax: q91-80-6633706. E-mail addresses: [email protected], [email protected] ŽB.S. Jai Prakash..
1997.. The element appears primarily in two valence states—the toxic hexavalent state and the relatively benign trivalent state. The conventional treatment of effluent consists of reduction of Cr ŽVI. to Cr ŽIII., precipitation of Cr ŽIII. as Cr2 O 3 P x H 2 O at high pH ŽDavid et al., 1998. followed by the disposal of the dewatered sludge. The major shortcoming of the conventional treatment includes high cost of chemicals used for Cr ŽVI. reduction. Wastewater containing relatively low concentrations of Cr ŽVI. is usually treated with ion exchange resins to recover
0169-1317r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 0 1 . 0 0 0 3 9 - 4
66
B.S. Krishna et al.r Applied Clay Science 20 (2001) 65–71
chromate but this involves expensive resins ŽDavid et al., 1998.. Thus, there is a search for cheaper adsorbent materials such as clays to recover the chromium. Ion exchange with organic cation has been used to modify the surface properties of clay minerals and other natural materials. Such modified clay minerals have been used to adsorb organic contaminants. Clay minerals, modified with quaternary amine, can substantially enhance the removal of non-ionic organic solute from aqueous solution ŽCadena, 1987.. The adsorption of cationic surfactants on clay minerals, and the potential application of such modified clays as environmental remediation material have been studied ŽBoyd et al., 1988; Bors, 1990.. Alkyl ammonium-modified clay minerals are frequently studied because of the unique adsorptive properties of the modified materials ŽSrinivasan and Fogler, 1990; Jaynes and Boyd, 1991; Zhang et al., 1993; Xu and Boyd, 1995a,b; Sullivan, 1997.. In order to adsorb anions, the modified surface must either possess positively charged exchange sites, or there should be replacement of weakly held counter ions of the surfactant by more strongly held adsorbate counter ions. Some selective cationic surfactants provide such a modified surface for anions. Surfactant-modified zeolite were shown to have the ability to remove chromate from aqueous solution ŽHaggerty and Bowman, 1994; Li and Bowman, 1997; Li, 1998.. Haggerty and Bowman Ž1994. concluded that hexadecyl trimethylammonium ŽHDTMA. is too large to enter into the internal position of the zeolite and adsorption of amine only occurs on the zeolite’s external exchange sites. Li and Bowman Ž1997. observed that the amount of HDTMA adsorbed on the surface of zeolite increases with the initial concentration of HDTMA. When the amount of HDTMA adsorbed exceeds the external cation exchange capacity of the zeolite, adsorption of chromate occurred. This was explained by formation of surfactant bilayers; the positive charges of the bilayer are considered as adsorption sites for chromate. Montmorillonite modified with the non-ionic surfactant Tween-80 adsorbed considerable amounts of iodine ŽKrishna et al., 1998, 1999.. In this report, we studied the effect of pH on adsorption of chromate by HDTMA-modified clay minerals.
2. Materials and methods The clay minerals used in this study are: Ži. a kaolin sample with the composition, 46.0% SiO 2 , 36.5% Al 2 O 3 , 0.42% TiO 2 , 0.2% Fe 2 O 3 , 0.07% FeO, 0.65% MgO, 0.01% MnO 2 , 0.06% CaO, 0.19% Na 2 O, 0.01% K 2 O, 0.13% P2 O5 , and 14.8% LOI with a cation exchange capacity Žcec. of 0.015 meq gy1 ; Žii. a white American bentonite ŽGK-69. with the composition, 52.1% SiO 2 , 15.2% Al 2 O 3 , 1.9% Fe 2 O 3 , 0.07% FeO, 3.2% MgO, 1.92% CaO, 2.58% Na 2 O, 0.9% K 2 O, 0.26% P2 O5 , and 21.8% LOI with a cec of 0.8 meq gy1 ; and Žiii. a bentonite clay mineral from Bhuj area, Gujarat, India Žcontaining essentially montmorillonite. with the composition, 44.8% SiO 2 , 0.89% TiO 2 , 13.6% Al 2 O 3 , 11.5% Fe 2 O 3 , 0.07% FeO, 1.97% MgO, 1.69% CaO, 3.16% Na 2 O, 0.13% K 2 O, 0.24% P2 O5 , and 22.0% LOI with a cation exchange capacity of 0.8 meq gy1 . The Ceramic Technological Institute, Bangalore, India provided samples Ži. and Žii.. Ashapura International, a local commercial supplier of industrial clays, supplied sample Žiii.. The Indian montmorillonite sample was also pillared by hydroxyoligomers of aluminium at OHrAl ratio of 2.5. The pillaring procedure used by the authors is given elsewhere ŽSelvaraj et al., 1996.. 2.1. Preparation of modified clays Known amounts of hexadecyl trimethylammonium ŽHDTMA. bromide ŽQualigen Chemicals., ranging from 0.274 to 1.37 mmol, were dissolved in 100 ml of water. One gram of the clay, suspended in acetone, was added to the HDTMA solution. Acetone was used to make the suspensions less sticky and easier to handle. The contents were mixed thoroughly in a wrist-action shaker for 1 h. The mixture was centrifuged and the centrifugate was discarded. The solid was washed several times with distilled water to remove superficially held adsorbate. The amount of surfactant adsorbed was determined by C, H analysis ŽCarlo Erba CHNanalyzer.. At higher HDTMA concentrations, the surfactant solution appeared cloudy because of the low solubility of the surfactant. In addition, at high concentrations, HDTMA crystallized on the clay
B.S. Krishna et al.r Applied Clay Science 20 (2001) 65–71
mineral surface, leading to washing problems. For this reason, the maximum initial concentration was 1.37 mmol HDTMA Žin 100 ml.rg clay. At this condition, the amount of HDTMA adsorbed was equivalent to cation exchange capacity. The modified clay with a maximum amount of surfactant adsorbed Ž0.8 mmol gy1 . was used to study the kinetic of adsorption. To study the equilibrium established between the surfactant and clay, separate flasks containing 1.0-g sample of the clay treated with 1.37 mmol of HDTMA–Br were shaken for various lengths of time Ž30 min, 1 h, 5 h, 24 h, 2–7 days.. The solid was washed thoroughly Žabout 10 times. with distilled water to remove any superficially adsorbed HDTMA on the surface. The final amount adsorbed after washing was the same as after shaking for 1 h. Hence, samples treated for 1 h with HDTMA–Br and washed were used for the chromate adsorption studies. 2.2. Adsorption of chromate For the adsorption of chromate, samples of 1.0 g air-dried modified clays were used. The clay was treated with different amounts of potassium chromate ranging from 4.5 to 45 mmol of Cr. The mixture Ž100 ml. was allowed to stand for 10–60 min with periodical shaking and then centrifuged at 10,000 rpm for 5 min. The amount of chromium present in the centrifugate was analyzed by measuring the absorbance of the purple complex of Cr ŽVI. with 1,5-diphenylcarbazide ŽRand et al., 1975. at 540 nm. All trials were accompanied by blank containing only chromium solution. The amount of Cr adsorbed on the solid was also checked by ICP-AES Ž8410 Labtam, Australia.. The chromate-loaded samples were dissolved in HF–HNO 3 mixtures and aspirated into ICP-AES under optimum experimental parameters. The adsorption of chromate was determined as a function of pH ŽFig. 1.. One gram of modified bentonite sample was treated with 45 mmol of Cr and the pH was carefully adjusted between 1 and 6 with dilute hydrochloric acid. The amount of chromate adsorbed was measured after periodical shaking for 30 min; this period was found to be sufficient to attain equilibrium. The kinetics of adsorption by the
67
bentonites was also studied. One gram of bentonite modified with HDTMA was treated with 100 ml of potassium chromate solution containing 45 mmol of Cr. The samples were shaken and the amount of chromate adsorbed was determined after 10, 20, 30, 40, 50, and 60 min. The amount adsorbed was plotted against square root of time. HDTMA Ž0.8 mmol. in 100 ml was treated separately with slight excess of 0.05 M potassium chromate at pH F 1.0 and pH s 4.5. The orange and yellow coloured solids obtained were filtered, washed with water and the amount of chromate present was estimated colorimetrically. 2.3. X-ray measurements The surfactant-treated and chromate-loaded Žat pH F 1.0 and pH s 4.5. samples were thoroughly washed to remove any superficially held adsorbate molecules on the surface. The samples were air-dried and the X-ray diffraction patterns were recorded on a Rigaku rotaflex diffractometer with low angle facilities ŽNi filter, Co K A -radiation..
3. Results and discussion 3.1. Adsorption of HDTMA by bentonites The amount of HDTMA adsorbed after 1-h contact time, at an initial amount of 1.37 mmolrg clay, is given in Table 1. The amount of HDTMA adsorbed increased from 0.51 mmol gy1 after 30 min to 0.80 mmol gy1 after 1 h. The sample placed for 7
Table 1 Ion exchange of the kaolin and bentonite with HDTMA cation and adsorption of chromate ŽpH F1. Clay
HDTMA Chromate bound adsorbed on Žmmol kgy1 . raw clay Žmmol kgy1 .
Kaolinite 44 Pillared clay 58 American 800 bentonite Indian bentonite 801
Chromate adsorbed on HDTMA–clay Žmmol kgy1 .
zero zero zero
30 50 795
zero
795
68
B.S. Krishna et al.r Applied Clay Science 20 (2001) 65–71
days adsorbed a higher amount of HDTMA. After centrifugation and washing, the same amount of HDTMA was found. The maximum amount of HDTMA adsorbed after washing was equivalent to the cation exchange capacity Žcec.. Kaolin, having only a few exchange sites, adsorbs small amounts of HDTMA. The pillared clays showed a low adsorption of HDTMA. 3.2. Adsorption of chromate by modified bentonites The sample equilibrated for 1 h with HDTMA was used for the adsorption studies of chromate. The amount of chromate adsorbed at pH F 1 is given in Table 1. The adsorption of chromate by the raw clays was found to be lower than the measurable quantity. However, the clays modified with HDTMA adsorbed 0.795 mmol gy1 of chromate at pH F 1. The amount adsorbed depended only on the quantity of HDTMA adsorbed. Modified kaolin and pillared clays, which did not adsorb higher amounts of HDTMA, showed a low adsorption of chromate. Previous workers have made similar observations on kaolin ŽZachara et al., 1988. and modified zeolite ŽHaggerty and Bowman, 1994; Li and Bowman, 1997.. Fig. 1 shows the effect of pH on the adsorption of chromate by modified bentonite. The amount adsorbed was maximum at pH F 1 and decreased to
Fig. 2. Adsorption isotherm for chromate on modified Indian montmorillonite at pH F1.0.
almost half the quantity at pH 2, and remained constant up to pH s 6. Above pH s 8, the amount adsorbed was negligible. The variation in adsorption at different pH values is attributed to the presence of different hexavalent chromium anionic species, and is discussed under the mechanism. 3.3. Adsorption isotherm of chromate on modified montmorillonite The adsorption of chromate can be described by Langmuir isotherm ŽFig. 2.. In its linearized form, crX s 1rab q crb.
Ž 1. y1 .
X is the amount adsorbed Žmmol kg , c is the equilibrium solution concentration Žmmol Ly1 ., a is an affinity parameter ŽLangmuir constant, L mmoly1 ., and b is the maximum adsorption capacity Žmmol kgy1 .. Table 2 presents the fitted Langmuir parameters for all isotherms. Table 2 Langmuir constants for the adsorption of chromate by modified Indian montmorillonite at two different pH
Fig. 1. Effect of pH on the adsorption of chromate by modified Indian montmorillonite.
Langmuir constants
Chromate adsorption at pH s 4.2
Chromate adsorption at pH F1.0
a ŽL mmoly1 . b Žmmol kgy1 .
167 405
72 795
B.S. Krishna et al.r Applied Clay Science 20 (2001) 65–71
The value of a is indicative of the affinity of the chromate to the modified bentonite. The higher value, in comparison with the data by Haggerty and Bowman Ž1994. Ž a s 9.46 L mmoly1 , b s 4.08 mmol kgy1 ., indicates the strong interaction with the adsorbent. 3.4. Kinetics of adsorption Besides the adsorption at the surface of the adsorbent, the adsorbate molecules may also diffuse into the interior of the porous adsorbent. The diffusion constant for adsorption of chromate by modified clay was calculated by the following equation, X s Ž 2 Dt .
1r2
q C.
Ž 2. y1 .
X is the amount of chromate adsorbed Žmmol kg , D is the diffusion constant Žmmol kgy1 miny0.5 ., t is the time, and C is a constant. The amount adsorbed increased linearly with the square root of time ŽFig. 3.. The relation is typical of diffusion controlled solid state reaction, and an apparent diffusion constant can be obtained. For modified bentonite, D s 12.2 mmol kgy1 miny0 .5 was found. In the case of kaolin, such a diffusion process was not observed ŽFig. 3.. 3.5. Adsorption mechanism The amount of HDTMA adsorbed on bentonite is equivalent to the cec Ž0.8 mmol gy1 .. There was an
69
Table 3 Basal spacing of bentonite clay, HDTMA–montmorillonite Žfrom India., and after reaction with chromate solution Žat pH F1.0 and pH s 4.5.
˚. dŽ001. ŽA Montmorillonite HDTMA–montmorillonite Chromate–HDTMA–clay Žat pH F1.0. Chromate–HDTMA–clay Žat pH s 4.5.
14.1 32.0 38.0 23.6
increase in the basal spacing in the c-axis, apparently due to the adsorption of amine in the interlamellar region. Several workers have noticed such an increase in the case of adsorption of alkyl-TMA by montmorillonite ŽBrindley, 1965; Johns and Sen Gupta, 1967; Lagaly, 1982; Zhang et al., 1993.. Pure dried derivative, with all the exchange sites occupied ˚ by HDTMAq cations, shows spacing less than 20 A ˚ ŽLagaly, 1982.. A large spacing of more than 30 A Ž . reported in this study Table 3 is due to the fact that the sample was not dried. Chromate species present is influenced by the solution pH. Acidification of aqueous solution of yellow chromate initiates a series of equilibria involving the formation of the orange red dichromate ŽGreenwood and Earnshaw, 1988.. pH-1
pH-7
pH)1
pH)7
2y 2y HCr2 Oy 7 Ž aq . z Cr 2 O 7 Ž aq . z CrO4 Ž aq . .
Ž 3. Below pH 1, the species present are HCr2 Oy 7 ions. The HCr2 Oy anion displaces the surfactant counter 7 ion from the exchange sites on the clays forming clay– ŽHDTMA. –HCr2 O 7 . The exchange sites on the clays are balanced by the protons present in the medium. q Mont.–HDTMAqq Hq–HCr2 Oy 7 ™Mont.–H
q HDTMAq–HCr2 Oy 7 .
Fig. 3. Kinetics of chromate adsorption by modified kaolin clay Ža. and modified Indian montmorillonite Žb. at pH F1.0.
Ž 4.
The attack of protons on the structure of clay is diminished possibly because of hydrogen bonding with oxyanion of chromate bonded to surfactant. Beyond pH 1 and below pH 6, the species present is the divalent anion Cr2 O 72y with small quantities of HCrO4y ŽGreenwood and Earnshaw, 1988.. Chro-
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mate content in the salts, obtained by interaction of HDTMA with chromate at pH F 1 and pH s 4.5, showed the salts to be ŽHDTMA.HCr2 O 7 and ŽHDTMA. 2 Cr2 O 7 , respectively. Formation of the salt ŽHDTMA. 2 Cr2 O 7 is probably the reason why the amount of chromate adsorbed at pH G 2 is only one half of that at pH F 1. The dŽ001. spacing of the chromate–HDTMA clays at both pH are found to be different ŽTable 3., indicating different intercalated species. Beyond pH 8, the adsorption of CrO42y is not favored because of the competition between the chromate and hydroxide ion.
4. Conclusions Surface modification of smectite clays by surfactants offers an easy method of adsorbing large amounts of chromate. Chromate in the acidic pH range, pH s 1–6, can be immobilized by surfactant cations in the interlayer space such as alkylammonium ions. Hexadecyl trimethylammonmium ŽHDTMA. bromide was found to hold chromate ions between the smectite layers in the form of salts. The type of salts formed depended on the pH-dependent equlibria of the chromium species, HCr2 Oy 7z Cr2 O 72yz CrO42y, and the salts possessed low solubility products in the acidic pH range. Twice the amount of chromate adsorbed at pH F 1 relative to pH G 2 may be caused by the formation of ŽHDTMA.HCr2 O 7 instead of ŽHDTMA. 2 Cr2 O 7 . Thus, immobilized interlayer surfactant species can be advantageously used to concentrate and recover toxic species present in the aqueous environment.
Acknowledgements The authors wish to acknowledge with thanks the financial assistance from the Department of Atomic Energy, Government of India, for carrying out this project. The authors are grateful to Prof. B.K. Sadashiva, Raman Research Institute, Bangalore, for carbon analysis measurements, and the Director, Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Bangalore, for chemical analysis by ICP-AES method. Grateful
thanks are also due to the Principal and the Members of the Governing Council of Bangalore Institute of Technology for the facilities provided.
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