Changes of sewage sludge digestate-derived biochar properties after chemical treatments and influence on As(III and V) and Cd(II) sorption

This work seeks to extend the knowledge on the effect of chemical treatment of sewage sludge digestate (SSD)-derived biochar for the As(III and V) and Cd(II) sorption ability using potassium hydroxide (KOH) or hydrogen peroxide (H2O2). Results showed the increases of the pH of point of zero charge, the Brunauer-Emmett-Teller (BET) surface area and cation exchange capacity (CEC) after chemical treatment of biochar. The sorption ability was enhanced from 1.6 μmol g−1 (As(V)) and 16.1 μmol g−1 (Cd(II)) on raw biochar to 8.5 μmol g−1 (As(V)) and 318.5 μmol g−1 (Cd(II)) on KOH-modified biochar. Furthermore, arsenic redox distribution showed a large oxidation (70%) of As(III) to As(V) in KOH-biochar with batch washing, while a partial oxidation (7%) was observed in KOH-biochar with batch and subsequent column washing. The washing procedures after KOH treatment play an important role on arsenic sorption, due to the release of phosphate (PO43−) as well as organic matter from the biochar that may subsequently lead to the oxidation of As(III) to As(V). Our findings highlight the potential influence of biochar on the redox transformation of As(III) to As(V) and therefore requires a careful assessment while investigating the fate of As in aquatic environments. [Display omitted] •Chemical treatment of biochar enhanced both anion (As) and cation (Cd) sorption.•Arsenic sorption is affected by its redox speciation.•Biochar can induce arsenic oxidation in solution.•Incomplete washing of the biochar strongly affects arsenic speciation and sorption.