Phenol-quinone redox cycling enables oxygen-dependent H2O2-assisted As(III) oxidation by pine-sawdust biochar

Biochar (BC)-mediated aerobic oxidation of arsenite (As(III)) has been reported, but the coupling among phenol-quinone redox cycling, O 2 activation, and pH-regulated H 2 O 2 formation remains insufficiently resolved. In this study, pine-sawdust-derived BC and molecular analogues of phenolic/quinoid moieties, hydroquinone (HQ) and 1,4-benzoquinone (BQ), were used to examine As(III) oxidation at pH 3.0, 7.0, and 9.5 under oxic and oxygen (O 2 )-limited conditions. Under oxic conditions, BC oxidized 42.4%, 21.6%, and 74.7% of As(III) after 24 h at pH 3.0, 7.0, and 9.5, respectively, whereas arsenate (As(V)) formation was negligible under O 2 -limited conditions. BC generated hydrogen peroxide (H 2 O 2 ) in air-equilibrated suspensions, with maximum concentrations of 88.5, 38.8, and 204.2 μM at pH 3.0, 7.0, and 9.5, respectively·H 2 O 2 -associated pathways contributed an estimated 69.2%, 77.2%, and 67.4% to As(III) oxidation at pH 3.0, 7.0, and 9.5, respectively. Superoxide dismutase suppressed O 2 •- generation without affecting H 2 O 2 accumulation or As(III) oxidation, indicating that O 2 •- primarily served as an intermediate in H 2 O 2 formation. Tert-butanol negligibly altered As(V) formation, consistent with the absence of detectable •OH. The BQ/HQ model system confirmed the chemical feasibility of quinone reduction, HQ re-oxidation by O 2 , and associated H 2 O 2 generation, but it should be interpreted as a molecular analogue rather than direct proof of the heterogeneous BC surface mechanism. Overall, BC mediates As(III) oxidation mainly via O 2 -dependent H 2 O 2 production promoted by phenol-quinone-like redox cycling, while the non-monotonic pH trend (pH 9.5 > pH 3.0 > pH 7.0) reflecting the combined effects of As(III) speciation, surface redox behavior, and H 2 O 2 availability.