Efficient removal of aqueous Cr(vi) using sulfur-modified biochar derived from anaerobic digestate: synergistic mechanism for reduction and sorption

Abstract

To realize anaerobic digestate recycling and waste control using waste, sulfur-modified digestate biochar was facilely prepared for efficient Cr(VI) removal. Characterization indicated the increased surface area and a decreased degree of aggregation for biochar under the optimized S-doping and pyrolysis temperature conditions (900-2S@DBC), resulting in more functional groups with high reducibility (e.g., phenolic –OH, C–S–C, S species with low valency) and improved electron transfer ability. The valence state analysis of sulfur element analysis confirmed the reduction of the sulfate source via carbon at high pyrolysis temperature. Under the conditions of an initial Cr(VI) concentration of 10 mg L⁻¹, a pH of 3, and a dosage of 0.5 g L⁻¹, the removal efficiency of Cr(VI) by 900-2S@DBC approached 100%, whose quasi-second-order kinetic constant was tens of times greater than that of pristine biochar. Quenching and control experiments confirmed that the S species with low valency played a more important role in Cr(VI) reduction than traditional carbon- and oxygen-based reductive groups on the carbon structure. Direct surface electron reduction made a greater contribution than the indirect solution ·O₂⁻ reduction pathway, further confirming the synergistic effect between Cr(VI) sorption and reduction. Complexation, precipitation, and ion exchange processes were involved in the final immobilization of Cr by 900-2S@DBC. Additionally, 900-2S@DBC exhibited excellent Cr(VI) removal capacity in real electroplating wastewater. This study offered a feasible approach for the utilization of anaerobic digestate as a resource and deep insights into the mechanism of Cr(VI) removal via sulfur-modified biochar.