Time-varying contributions of Fe and Fe to As immobilization under anoxic/oxic conditions: The impacts of biochar and dissolved organic carbon.

Engineering black carbon (e.g. biochar) has been widely found in natural environments due to natural processes and extensive applications in engineering systems, and could influence the geochemical processes of coexisting arsenic (As) and Fe, especially when they are exposed to oxic conditions. Here, we studied time-varying kinetics and efficiencies of As immobilization by solid-phase Fe (Fe) and Fe (Fe) in Fe-As-biochar systems under both anoxic and oxic conditions at pH 7.0, with focuses on the effects of biochar surface and biochar-derived dissolved organic carbon (DOC). Under anoxic conditions, Fe could rapidly immobilize As via co-adsorption onto biochar surfaces, which also serves as the dominant pathway of As immobilization at the initial stage of reaction (0-5 min) under oxic conditions at high biochar concentrations. Subsequently, with increasing biochar concentrations, Fe precipitation from aqueous Fe (Fe) oxidation (5-60 min) starts to play an important role in As immobilization but in decreased efficiencies of As immobilization per unit iron. In the following stage (60-300 min), Fe oxidation is suppressed and leads to As release into solutions at >1.0 g·L biochar. The decreasing efficiency of As immobilization over time is attributed to the gradual release of DOC into solution from biochar particles, which significantly inhibit As immobilization when Fe is generated from Fe oxidation in the vicinity of biochar surfaces. Specifically, 4.06 mg·L of biochar-derived DOC can completely inhibit the immobilization of As in the 100 μM Fe system under oxic conditions. The findings are crucial to comprehensively understand and predict the behavior of Fe and As with coexisting engineering black carbon in natural environments.