Nanoporous biochar with high specific surface area based on rice straw digestion residue for efficient adsorption of mercury ion from water

The unreasonable disposal of residue after anaerobic digestion seriously affects the stability of the ecosystem, and the preparation of adsorbent is an effective way to value-added utilization of the residue. In this study, a high adsorption capacity (209.65 mg/g) biochar-based adsorbent was prepared by hydrothermal carbonization and alkali modification using rice straw biogas residue. The lignocellulosic structure was destroyed after anaerobic digestion, forming porous biochar with larger specific surface area (2372.51 m2/g) and richer pore structure. Besides, the mercury ion complexed on the adsorbent surface in monovalent and divalent forms and possessed favorable selectivity in the presence of other examples of interference. The adsorption process is consistent with pseudo second-order kinetics and the Langmuir isotherm, indicating a predominance of chemisorption. This study provides a methodology for use of rice straw biogas residue and treatment of mercury containing wastewater, which offers a fresh direction for resource utilization of biogas residue.