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Hierarchical approach towards adsorptive removal of Alizarin Red S dye using native chitosan and its successively modified versions.

In the present work, native chitosan (Ch) along with its chemically and physico-chemically modified versions, namely sulphate cross-linked chitosan (SCC) and sulphate cross-linked chitosan-bentonite composite (SCC-B), were employed as potential adsorbents for the removal of an anionic dye, Alizarin Red S (ARS) from aqueous solutions. All three adsorbents were extensively characterized using techniques such as Fourier-transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray, X-ray diffraction, Brunauer-Emmett-Teller analysis, thermogravimetric-differential thermal analysis, and pH point of zero charge. Various parameters were optimized, including pH of dye solution, contact time, adsorbent dose, initial adsorbate concentration and temperature of adsorption. Four adsorption isotherm models were studied and it was found that the Freundlich model was best-fit for all three systems. Maximum adsorption capacities towards adsorption of ARS were found to be 42.48, 109.12 and 131.58 mg g-1 for Ch, SCC and SCC-B, respectively. Kinetics of adsorption was examined by employing three well-known models in order to deduce the mechanism of adsorption. Thermodynamic studies show that the process is spontaneous and exothermic for all adsorbents employed. Furthermore, it was observed that for large sample volumes, the column adsorption method was more effective compared to the batch method.