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“Devil tree saw dust”; a novel biosorbent has been utilised successfully for the removal of hexavalent chromium from contaminated water. Batch adsorption procedure is utilised to test the ability of saw dust as an adsorbent for hexavalent chromium (reduction coupled adsorption). The contribution of various parameters on sorption, such as contact time, sorbate concentration, pH of the medium and temperature were estimated and maximum uptake of hexavalent chromium from contaminated water was 333.33 mg g−1 at pH 2.0 and temperature of 35°C. Hexavalent chromium uptake from contaminated water followed the pseudo‐first‐order rate expression. The standard free energy change (ΔG0), standard enthalpy change (ΔH0) and standard entropy change (ΔS0) have also been evaluated and it has been concluded that the sorption was feasible, spontaneous and endothermic in nature. The process follows well Langmuir isotherm. Fourier Transform Infra‐Red (FTIR) spectroscopy and scanning electron microscopy (SEM) of hexavalent chromium loaded and unloaded saw dust were performed, SEM clearly indicates chromium adsorption. FTIR spectroscopy revealed the involvement of carbonyl, hydroxyl and amide groups on the cell surfaces in chromium binding. Very good adsorption capacity and low cost or cost free of devil tree saw dust makes this biosorbent as one of the best adsorbents for removal of hexavalent chromium from contaminated water. © 2012 Canadian Society for Chemical Engineering

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Bioremediation is an efficient and cheap process for the removal of heavy metals. Solid agricultural wastes are frequently used as biosorbent for the removal of Cr(VI). In this study, water extract of Azadiracta indica sawdust is used to reduce Cr(VI) to Cr(III) and thereby lowering the toxicity. Water extract of A. indica sawdust effectively reduces hexavalent chromium at acidic pH (pH = 2). The reaction is irreversible. Trivalent chromium produced after reduction of Cr(VI) forms soluble organo-Cr(III) complex. Coordination of Cr(III) in the water extract occurs via the chelating coordination of -OH, -NH, etc, functional groups. The presence of these functional groups were confirmed from the IR spectra of the freeze-dried water extract, and the formation of Cr(III) is proved from the UV-vis spectra of the reaction mixture after the completion of reaction. But the rate of this reduction is very slow. Addition of nonfunctional surfactants increases the rate of reduction. Anionic surfactant sodium dodecylsulphate produces maximum effect.