BACKGROUND: This work fulfils the need to develop an eco-friendly biosorbent, elucidating the mechanism of biosorption. Removal of Cr(VI) by Rhizopus arrhizus was investigated in batch mode. Enhancement in the performance of the biosorbent was attempted by pre-treating the biomass with inorganic and organic acids, chelating agent, cross-linker and an organic solvent followed by autoclaving. The surface characterization of the biomass was carried out by potentiometric titration, surface area analysis, infrared spectroscopy, chemical modification of the biomass and scanning electron microscopy. RESULTS: All the physico-chemical treatments of the biosorbent improved Cr(VI) uptake compared with the native biomass (21.72 mg g(-1)). The highest biosorption capacity (31.52 mg g(-1)) was achieved after pre-treating the biomass with 0.5 mol L(-1) HNO(3) followed by autoclaving. Surface characterization of the biomass using pH(zpc), potentiometry and Fourier transform infrared (FTIR) analysis revealed the role of amino and carboxyl groups in Cr(VI) removal by electrostatic attraction. Chemical modification of amino and carboxyl groups significantly decreased Cr(VI) uptake capacity confirming their role in biosorption. SEM analysis showed adsorption of Cr(VI) on the biosorbent surface. CONCLUSION: Rhizopus arrhizus biomass proved to be an effective and low cost alternative biosorbent for removal of Cr(VI) from aqueous solutions. (C) 2011 Society of Chemical Industry