Removal of chromium (VI) using poly(methylacrylate) functionalized guar gum
Introduction
Heavy metals into aquatic ecosystem have become a matter of attention due to their toxicological effect to the ecosystem, agriculture and human health. Chromium is very common heavy metal in the environment and found in concentrations ranging from <than 0.1 μg/m3 in air to 4 g/kg in a soil (Pellerin and Booker, 2000). Naturally occurring chromium is found in the trivalent state in rocks, soil, plants and volcanic emissions. Hexavalent chromium, Cr(VI) is introduced into water through industrial applications like tanning, metallurgy, electroplating and metal finishing (Shoeib, 2002, Mohan Rao and Basava Rao, 2007).
The physiological effects of chromium on biological systems depend on its oxidation state. At low concentrations, Cr(III) is considered an essential element in mammals for maintenance and control of glucose, lipid and protein metabolism. However, at the same low concentration Cr(VI) is toxic due to its oxidizing property. Cr(VI) is reported to have damaging effect on the lungs, liver, nervous system and kidneys in mammals (EPA, 1998), The Cr(VI) may exist in the aqueous phase in different oxyanionic forms such as chromate , dichromate (), or hydrogen chromate (HCrO4-)(Gaballah and Kilbertus, 1998).
Due to the health hazards of Cr(VI), numerous studies concerning its removal from aqueous solutions have been attempted. Chemical precipitation is one of the most common conventional treatment methods to effectively decrease metal to acceptable levels. It requires large excess of chemicals and generates volumetric sludge and increases the cost (Daneshvar et al., 2000). Other available treatment methods such as ion exchange, electrolysis and reverse osmosis require high capital investment and running cost (Demir and Arisoy, 2007). Anaerobic granular biomass (Massara et al., 2008), dead biomass of marine Aspergillus niger (Khambhaty et al., in press); marine dried green alga Ulva lactuca (El-Sikaily et al., 2007), Rhizopus cohnii powder and Fe3O4 particles beads coated with alginate and polyvinyl alcohol (Li et al., 2008), Sphagnum peat moss (Seki et al., 2006) and other biomasses (Parsons et al., 2002) have been reported for the removal of Cr(VI) from aqueous solutions. In few studies, chitosan and cross linked chitosan have also been used as Cr(VI) sorbent (Schmuhl et al., 2001, Modrzejewska et al., 2006).
Guar gum (GG) is a seed gum obtained from Cyamopsis tetragonalobus, which is native to northwestern parts of India. Chemically it is a galactomannan (Xu et al., 2003), forms high viscosity colloidal dispersions in water at room temperature. Because of this property, native guar gum as well as its derivatives are commercially important and find use in diverse applications (Butt et al., 2007, Kumar, 2005). An adverse property of the guar gum is its susceptibility for quick biodegradation (Joseph, 1996) and therefore rarely used in its natural form. Potentially it can find use as adsorbent but its solubility in water limits its application. Its stability (Chowdhary et al., 2001), solubility as well as its sorbing capacity (Hegazy et al., 2001) can be changed through grafting of different vinyl monomers. Grafting can be done using redox initiators (Unnithan et al., 2004, Nayak and Singh, 2001, Srivastava and Behari, 2006, Singh et al., 2006), γ irradiation (Lokhande et al., 2003) or using microwave irradiation (Singh et al., 2004a, Singh et al., 2004b). In our very recent study (Singh et al., 2005), poly(methylmethacrylate) was grafted on Cassia marginata using microwave irradiation and the resulting graft copolymer was evaluated for Cr(VI) removal from the aqueous solutions. In the present study we for the first time report the optimization of the synthesis of guar-graft-poly(methylacylate)(GG-g-PMA) using potassium persulfate/ascorbic acid redox initiator. The copolymer synthesized under optimum condition was evaluated for Cr(VI) removal from the aqueous solutions and electroplating industry effluent using conductivity measurement where results were found as accurate as in established photometric method (Anon, 1985).
Section snippets
Methods
All the reagents used were of analytical grade. A stock Cr(VI) solution(1000 ppm) was prepared in deionized water using K2CrO4 (Merck) and all the working solutions were prepared by diluting this stock solution with deionized water. Commercially available sample of guar gum (BDH) was used after purification; methylacrylate (Loba Cheime), potassium persulfate (Merck) and ascorbic acid (Merck) were used without further purification.
The concentration of chromium ion was determined by microprocessor
Results and discussion
Varying various reaction parameters for grafting, GG-g-PMA samples having different %G and %E were synthesized using persulfate/ascorbic acid redox initiator at 35 ± 0.5 °C under atmospheric conditions. The maximum % grafting that could be reached was 406.67 % using [MA] = 22 × 10−2 M, [K2S2O8] = 20 × 10−3, [AA] = 3.0 × 10−2 M, seed gum 3 g/L and total reaction volume 25 ml and reaction time 1 h. The redox system has been used for the first time for grafting methylacrylate on to guar gum and was found to be
Conclusion
GG-g-PMA proved to be a effective sorbent in the removal of Cr(VI) from synthetic dye solutions as well as from electroplating wastewater. The sorption was found pH dependent, and was most at pH 1.0. The negative ΔH° indicated the exothermic nature of Cr(VI) sorption onto GG-g-PMA. The sorption followed pseudo-second-order kinetic model. The sorbent could be used successfully for five cycles and the adsorbed Cr(VI) ions were recovered in the same chemical form. The used conductivity method for
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Present address: Applied Science and Humanities Department, S.V. National Institute of Technology, Surat-395007, Gujarat, India.