Objective  The study aimed to explore the optimal conditions for adsorption of Cr by bagasse biochar and reveal the underlying mechanisms.

Method  Under the preservation of N₂, three types of bagasse biochar were prepared with the temperature precisely controlled at 350, 450 and 550 ℃, respectively. The changes in morphology of bagasse before and after carbonization were compared by using scanning electron microscope(SEM). The effects of preparation temperature, dosage of biochar, initial concentration of Cr and reaction time on the adsorption of Cr(Ⅲ) and Cr(Ⅵ)were investigated.

Result  The porosity of bagasse biochar expanded with the increase in preparation temperature. Bagasse biochar prepared at low temperature exhibited good efficiency for Cr(Ⅵ)removal, when the mass concentration of Cr(Ⅵ)and carbon dosage were 50 mg/L and 15 g/L, the maximum amount of adsorbed Cr(Ⅵ)by bagasse biochar prepared at 350 ℃ was 5.703 mg/g after reaction for 7 d, with the removal rate of 92.39%. The bagassebiochar prepared at high temperature showed optimal efficiency for Cr(Ⅲ)removal rate. When the mass concentration of Cr(Ⅵ)and carbon dosage were 75 mg/L and 10 g/L, the maximum amount of adsorbed Cr(Ⅵ)by bagasse biochar prepared at 550 ℃ was 9.158 mg/g after reaction for 7 d, with the removal rate of 97.06%. The adsorption isotherm of Cr obeyed the Langmuir model, and the kinetics curves of Cr(Ⅵ)and Cr(Ⅲ)fitted the pseudo-first-order kinetic model and pseudo-second-order kinetic model, respectively.

Conclusion  The efficiency of Cr(Ⅲ)and Cr(Ⅵ)removal was largely dependent on the temperature for biochar preparation. Based on the results above, the biochar prepared at 350 ℃ showed optimal efficiency for Cr(Ⅵ)adsorption, and 550 ℃ for Cr(Ⅲ)adsorption.