Adsorption Characteristics and Mechanisms of Mn/Zn Modified Chestnut Shell Biochar for Cr(VI) in Water

Abstract

In this study, raw biochar (BC600) was prepared via KOH-activated pyrolysis of chestnut shell powder, followed by Zn/Mn co-modification to synthesize modified biochar (MBC600). The variations in surface microstructure and chemical composition of the biochar were investigated through characterization techniques. Adsorption experiments were conducted to evaluate the Cr(VI) removal performance of MBC600 in aqueous solutions, with adsorption kinetic and thermodynamic models fitted to elucidate the underlying mechanisms. Cycling experiments were performed to assess its stability and regenerability.The results demonstrated that MBC600 exhibited abundant functional groups dispersed on its surface and within pores, with its specific surface area and pore volume increased by approximately 4.5-fold, significantly enhancing adsorption capacity. At a dosage exceeding 1.5 g/L and an initial solution pH of 3, over 95% Cr(VI) removal efficiency was achieved within 2 h, with a maximum adsorption capacity of 44.6 mg/g. The adsorption process was spontaneous and dominated by chemisorption, following a multilayer heterogeneous diffusion mechanism, and characterized as exothermic with increased entropy. After five cycles, the removal efficiency remained at 74.4%. The primary removal mechanisms included electrostatic attraction, redox reactions, and complexation.