Removal efficiency and mechanisms of dissolved Cr(VI) using oak wood biochar

Environmental chromium (Cr) contamination has led to serious problems in the ecosystem owning to the carcinogenicity, toxicity, and teratogenicity of Cr. In this study, the latent application of oak wood biochar for dissolved Cr(VI) removal was investigated. Optimal treatment of Cr(VI) was achieved with a removal efficiency of 99.9% at a pH 2.0. Another critical factor influencing removal efficiency was the initial concentration of Cr(VI). The removal efficiency of Cr(VI) was >99.9% at 1-50 mg L-1; nevertheless, Cr(VI) removal rate decreased at Cr(VI) concentrations (50-600 mg L-1). Five kinetic equations were used to describe the Cr(VI) removal kinetics and the best fit was the pseudo-second-order model. Fourier transform infrared spectroscopy results showed that C-O groups of alcohol and C=O bonds may participate in the reaction. X-ray photoelectron spectroscopy analysis demonstrated that Cr is predominantly excited as Cr(III) species (similar to 82.96%). The increase in pH values and concentrations of the cations Ca2+, Na+, and K+ in the aqueous solution after the reaction indicated that ion exchange was likely responsible for Cr(III) binding with the biochar. Results indicated that the electrostatic force between Cr2O72- and biochar, Cr(VI) reduction by C-O groups in alcohol, and ion exchange and complexation between Cr(III) and C=O were the reaction mechanism.