Reactions at Oxide Surfaces. 1. Oxidation of As(III) by Synthetic Birnessite

The rates and mechanisms of the reactions between aqueous As(III) and synthetic birnessite (δ-MnO_2) particles were studied. The experimental results at pH 4 indicate that the depletion of As(III) from solution is rapid, with a time scale of minutes. The oxidation product As(V) is released almost as quickly, while the release of the reduction product Mn(II) is slightly slower. The results also show that the concentration of dissolved oxygen has no effect on the rate of reaction. These observations suggestthat (i) birnessite directly oxidizes As(III) through a surface mechanism, (ii) the adsorption of As(III) is the slowest step in the production of As(V), and (iii) the reaction products As- (V) and Mn(II) are released via different mechanisms. The time-dependent behavior of the aqueous reactants and products over a pH range from 4 to 8.2, and a temperature range from 15 to 35 °C is also discussed. The rates of As(III) oxidation by inorganic redox reactions with manganese dioxides are compared to observed As(III) oxidation rates in natural aquatic systems