Two fold modified chitosan for enhanced adsorption of hexavalent chromium from simulated wastewater and industrial effluents
Introduction
At present, heavy metal pollution in environment is of great concern towards the health of human beings (Flora, Gupta, & Tiwari, 2012). Chromium is not an omission. Chromium exists in trivalent and hexavalent states. Cr(III) is an essential nutrient for glucose, lipid and protein metabolism in mammals (Mertz, 1976) whereas Cr(VI) is highly toxic. Cr(VI) can easily diffuse in cell membranes and has tendency to cause adverse effect on human health (Katz & Salem, 2006). Exposure to Cr(VI) may lead to skin irritation, kidney, liver and gastric damage. It may cause lung cancer and is known as potential carcinogen. Effluents from various industries such as electroplating, tannery, petroleum, paints and dyeing industries contain Cr(VI) (Ma et al., 2012). Permissible limit of Cr(VI) in drinking water according to WHO guidelines is 0.05 mg/ L (WHO, 1996). Therefore, for detoxification of Cr(VI), there is a need for an affirmative remediation. Conventional methods for the removal of Cr(VI) include chemical precipitation (Mirbagheri & Hosseini, 2005), redox reaction (Ölmez, 2009), mechanical filtration (Muthukrishnan & Guha, 2008), membrane separation (Korus & Loska, 2009), ion exchange (Zhang, Xia, Liu, & Zhang, 2015), and adsorption (Mohan, Singh, & Singh, 2006). Chemical methods require large amounts of chemicals and generate toxic sludge that needs further treatment. The adsorption method is preferred for the removal of heavy metals as it is economically most favourable. Use of biopolymers including cellulose and chitosan is a common practice for chromium detoxification. Properties like biocompatibility, biodegradability and good adsorption tendency enhance the utility of chitosan in pharmaceuticals, waste water treatment, flocculation etc (Alves and Mano, 2008, Dabrowski, 2001, Muzzarelli, 1973, Rinaudo, 2006).
Chitosan has also gained pronounced attention as biosorbents due to its physicochemical properties like chemical stability, high reactivity, excellent chelation behaviour and high selectivity towards pollutants (Elwakeel, 2010a, Elwakeel, 2010b, Owlad et al., 2009). A modification of chitosan mainly involves the free amine group on deacetylated units and hydroxyl groups on C3 and C6 carbons of monomers units. These groups of chitosan can be grafted or crosslinked with organic and inorganic moieties to enhance the adsorption efficiency. Porous chitosan beads (Wan Ngah, Kamari, Fatinathan, & Ng, 2006), crosslinked chitosan beads (Zimmermann, Mecab̂o, Fagundes, Rodrigues, 2010), grafted chitosan (Sharma & Mishra, 2010), chitosan resin (Tan, He, & Du, 2001), magnetic chitosan resin (Elwakeel, 2010a, Elwakeel, 2010b) and chitosan coated onto ceramic alumina (Boddu, Abburi, Talbott, & Smith, 2003) have been reported to show higher adsorption efficiency compared to untreated chitosan. Kahu et al. prepared a sulphate cross-linked chitosan for detoxification of chromium (Kahu, Saravanan, & Jugade, 2014). Anchoring of trialkyl amines on adsorbent has been reported in literature for the solid phase extraction of Cr(VI) (Kalidhasan, Kumar, Rajesh, & Rajesh, 2012; Kumar & Rajesh, 2013; Pinkert, Marsh, Pang, & Staiger, 2009; Shekhawat, Kahu, Saravanan, & Jugade, 2015).
Modification of adsorbent materials is based on two aspects- structural and functional. Both of these have been proved to enhance the adsorption capacity of the materials. In present study, we report two-fold modification on chitosan for admirable adsorption of Cr(VI). It involves crosslinking the polymeric chains with phosphate and functional modification with ionic solid ethylhexadecyldimethylammonium bromide. Crosslinking gives enhanced structural stability and porosity to the material while modification with ammonium salt leads to enhanced ionic interaction with the hydrogen-chromate species.
Section snippets
Materials
Diphenylcarbazide, sodium hydroxide, dipotassium hydrogen phosphate, ethylhexadecyldimethylammonium bromide, and potassium dichromate were procured from Merck, India. Chitosan with molecular weight of 120 kDa and degree of deacetylation 85% was procured from Uniloid Bio-Chemicals India Limited, Hyderabad, India. All the reagents were of analytical grade and used without further purification.
Preparation of ionic solid impregnated phosphated chitosan (ISPC)
2.0 g chitosan powder was taken in a round bottom flask. About 10 mL distilled water was added and 0.1 M
FT-IR spectral analysis
The FT-IR spectral analysis of chitosan (Fig. 1) showed distinct broad peaks corresponding to the OH and NH stretching vibrations in the region 3808 cm−1 and 3276 cm−1, the NH bending vibration around 1545 cm−1, CN bending vibration at 1374 cm−1and the CH and CO stretching bands around 2867 and 1017 cm−1 respectively. In ISPC, the peak due to P = O asymmetric stretching appeared at around 1220 cm−1 and PO stretching was observed around 926 cm−1 (Amaral, Granja, & Barbosa, 2005). Also, one distinct peak
Conclusion
Modification of a material for enhancement of efficiency is the prime concern from industrial point of view. Dual modification in terms of structural and functional modification in amine group as well as hydroxyl group of chitosan makes it a wonderful material for enhanced adsorption of Cr(VI). The ISPC adsorbent has shown admirable adsorption capacity of 266.67 mg/g for hexavalent chromium which is much higher compared to most of the reported adsorbents. Experimental data showed adsorption
Acknowledgements
We acknowledge University Grants Commission, New Delhi, India for the start-up grant and RTM Nagpur University for University Research Project.
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