Inhibition of As(III) Oxidation by Manganese Oxides in the Presence of Fe(II)
High arsenic (As) concentrations can be found in drinking water around the world, the consumption of which can lead to increased incidences of skin cancers, arsenicosis, and cardiovascular disease. The source of arsenic in contaminated regions is geogenic, where its fate and transport is affected by other naturally-occurring mineral constituents within the soil solid phase. Manganese (Mn) oxides within soil profiles are strong oxidants of many metals and metalloids found in soil, including As, where the more mobile and more toxic arsenite [As(III)] form can be oxidized to the less mobile and less toxic arsenate [As(V)]. Within redox transition zones in soil profiles, including the diffusion-limited matrix of soil aggregates, Fe(II) can come into contact with Mn oxides, altering the reactivity of the Mn oxides. This change in reactivity of Mn oxides may therefore alter the impact of Mn oxides on the fate and transport of arsenic in these interfacial zones. In the current study, we quantitatively demonstrate the impact of the formation of a potentially passivating Fe(III)-oxide coat on Mn-oxide mediated contaminant oxidation. By simulating diffusion-limited transport using a Donnan cell reactor, we investigated the oxidation of As(III) by Mn-oxides after exposure to varying concentrations of Fe(II). Our findings show that Fe(II) treatment of birnessite, a poorly-crystalline Mn oxide, leads to the formation a combination of Fe(III) oxides with a range of crystallinities as determined by Fe EXAFS analysis. Concomitant reduction of birnessite leads to formation of hausmannite, a Mn(II, III) oxide. Reaction of the resultant Fe-Mn oxide with As(III) shows decreased rate of As oxidation as compared to systems without Fe. Additionally, increasing Fe(II) concentrations exposed to birnessite by two folds leads to a five fold decreases in As(III) oxidation rate. During As(III) oxidation, hausmannite is rapidly reductively dissolved, resulting in increased aqueous Mn concentrations; continued As(III) oxidation is then mediated by birnessite. Furthermore, our results show that a higher concentration of arsenic partitions into the solid phase at high Fe(II) treatment due to the tendency of arsenic to bind preferentially to Fe oxides as compared to Mn oxides.