Efficient Removal of Aqueous Cr(VI) Using Sulfur-Modified Biochar Derived from Anaerobic Digestate: Synergistic Mechanism for Reduction and Sorption

Abstract

Combined with the separation and dehydration treatment of digestate from anaerobic digestion and acidification of food waste, sulfur-modified digestate biochar was facilely prepared for efficient Cr(VI) removal to realize anaerobic digestate recycling and waste control by waste. Characterization indicated the increased surface area and mesopores and the decreased aggregation degree for biochar under optimized S doping and pyrolysis temperature (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 forms transformation analysis confirmed the reduction of sulfate source via carbon under high pyrolysis temperature. Under the conditions of an initial Cr(VI) concentration of 10 mg/L, a pH of 3, and a dosage of 0.5 g/L, the removal efficiency of Cr(VI) by 900-2S@DBC approaches 100%. 900-2S@DBC showed apparent higher Cr(VI) removal performance than other biochar. Its quasi-second-order kinetic constant was tens of times than pristine biochar. Quenching and control experiments confirmed the S species with low valency played a more important role in Cr(VI) reduction than traditional carbon and oxygen based reductive groups on carbon structure. Direct surface electron reduction made more contribution than indirect solution •O2- reduction pathway, further confirming the synergistic effect between Cr(VI) sorption and reduction. Complexation, precipitation, and ion exchange processes participated 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 resource utilization of anaerobic digestate and deep insights for Cr(VI) removal mechanism via sulfur-modified biochar.