Bicarbonate Boosts the Anaerobic Generation of Reactive Oxygen Species by Hydrochar: Surface Oxygenated Functional Groups Activation and Hexavalent Chromium Removal

Hydrochar can generate reactive oxygen species (ROS) via activating molecular oxygen under sunlight. However, whether and how hydrochar generates ROS in the dark and anaerobic environments remains unknown. Herein, we found that combining hydrochar with the co-existing bicarbonate could generate superoxide radicals (O₂^•−) under alkaline conditions, with the maximum concentration reaching 24.7 μmol L⁻¹. Interestingly, the O₂^•−generation was not affected by eliminating the molecular oxygen but highly depended on the abundance of the oxygenated functional groups (OFGs) on hydrochar surface. The results of electrochemical analysis, density functional theory calculation, and surface characterization elucidated that the bicarbonate first inner-sphere complexed with surface OFGs, followed by the electron transfer from bicarbonate to OFGs. This enabled the activation of oxygen inside OFGs into active oxygen (O*) while bicarbonate was oxidized to carbonate radical (CO₃^•-). The CO₃^•- further reacts with O* through an oxygen transfer mechanism, resulting in the generation of O₂^•-. The generated O₂^•− was used for the Cr(VI) treatment, which could efficiently reduce over 95% into Cr(III). The findings provide a novel pathway for developing hydrochar-based advanced oxidation processes and highlight the potential of hydrochar in pollutant transformation mediated by ROS in dark and anaerobic water environments.