• Chitosan was successively modified with oxalic acid and biochar. • The adsorption capacity was found to improve from 2.062 mg g −1 for Ch to 383.8 mg g −1 for ChOxBC. • Equilibrium, isotherm, kinetics and thermodynamics studies have been carried out. • The adsorbents could be regenerated and reused in multiple cycles. Chitosan was successively modified by crosslinking with oxalic acid and forming its composite with cotton-straw-derived biochar. Cotton straw is a huge agricultural waste in central India region where cotton is a major agriculture product while chitosan is the second largest available biopolymer. Native chitosan (Ch), oxalate crosslinked chitosan (ChOx) and the Chitosan-Oxalate-Biochar composite (ChOxBC) were synthesized and characterized by FT-IR, SEM, EDX, XRD, BET surface area analysis and pH pzc etc. In batch adsorption studies for Cr(VI) removal, all three materials showed increased adsorption capacities from 2.062 mg g −1 for Ch, 348.3 mg g −1 for ChOx, and 383.8 mg g −1 for ChOxBC after just 60 min of adsorption. The enhancement in the adsorption capacities have been attributed to better stability of crosslinked chitosan, stronger ionic interaction with hydrogen chromate at pH 3.0 and enhanced surface area of the composite. In thermodynamic studies, the spontaneity of processes was assessed across the three materials. The experimental data from kinetics studies revealed that the Cr(VI) adsorption followed pseudo-second order kinetics. A study of columns confirmed that adsorbents can be applied to large volumes of samples. The adsorbents can be regenerated and reused, which makes the study more environmentally friendly.