The persistent and photo-induced free radicals of biochar play significant roles in the transformation or degradation of inorganic and organic pollutants. However, the redox capacity of biochar for arsenite (As(III)) photochemistry under different pH conditions remains unclear. In this study, we discovered that solar radiation primarily expedited the oxidation of As(III) by biochar by augmenting the production of reactive oxygen species (ROS). Biochar demonstrated a strong pH reliance on the photooxidation of As(III). Under acidic and neutral conditions, solar radiation amplified the generation of center dot OH (hydroxyl radicals) by BC-P (phenolic -OH of biochar) and semiquinone-type BC-PFRs (persistent free radicals of biochar) by 4.9 and 2.0 times, respectively, resulting in enhanced As(III) oxidation. Under alkaline conditions, BC-P and BC-Q (quinoid C–O of biochar) facilitated the production of H2O2 (hydrogen peroxide) by 2.1 times through the spontaneous formation of semiquinone-type BC-PFRs via an anti-disproportionation reaction, promoting approximately 88.2% of As(III) photooxidation. Furthermore, solar radiation elevated around 11.8% As(III) oxidation driven by BC-Q and semiquinone-type BC-PFRs. This study provides a crucial theoretical foundation for using biochar to treat arsenic pollution in aquatic systems and understanding the migration and transformation of arsenic in different environments.
