Kinetic Study of the Influence of Humic Acids on the Oxidation of As(III) by Acid Birnessite

Dissolved arsenic (As) in natural environments is controlled by sorption onto metal oxide surfaces; its speciation is principally influenced by biogenic manganese oxides and natural organic matter (NOM). Manganese (III/IV) oxides are strong oxidants that are ubiquitous in soils and sediments, and humic acids (HA) are typical substances found in the dissolved fraction of NOM. However, the mechanism of As(III) oxidation by MnO2 in the presence of NOM remains poorly understood. Here, we investigate how HA impact the oxidation of As(III) by a synthetic manganese oxide, i.e., acid birnessite (MnO2). We find that the reaction kinetics of As with HA and MnO2 are controlled by the formation of surface complexes. The main observed effect is a decrease of the overall oxidation rate in the presence of HA. In addition, pre-exposure of MnO2 to HA for 24 h further decreases the reaction rate, which is attributed to the occupation of MnO2 surface active sites, thus causing surface passivation. Our results demonstrate that HA influence the reaction mechanism of As(III) oxidation by MnO2 via sorption onto active surface sites and the formation of aqueous complexes.