Assessing arsenic redox state evolution in solution and solid phase during As(III) sorption onto chemically-treated sewage sludge digestate biochars

[Display omitted] •As redox species in biochar assessed by solid-liquid extraction and chromatography.•As(III) is predominantly (76–92%) preserved during its sorption onto biochars.•Oxidation of As(III) in solution induced by the solid-phase of the biochar.•As redox distribution onto biochar can differ from its distribution in solution.•As redox conversions (solution and biochar) depend on chemical treatment of biochar. This work aimed to determine the arsenic redox state distribution during As(III) sorption onto chemically-modified biochars. A solid–liquid extraction protocol using phosphoric (0.3 M) and ascorbic (0.5 M) acids at 80 °C for 20 min was established to ensure a quantitative recovery and stability of As(III) during the extraction. During sorption experiments, the redox conversions of As occurred and As(III) was either stable or partially oxidized in solution. The As distribution strongly varied depending on the biochar chemical treatment performed as well as the selected washing procedures (batch versus column washings). As(III) oxidation was favored with the KOH-modified biochar washed in batch mode. This oxidation was mostly induced by the biochar solid compounds rather than by soluble compounds released in solution. The As redox state distribution of As sorbed onto the biochars was successfully assessed using the extraction procedure. Arsenic was predominantly sorbed as As(III) (76–92%) onto the biochars.