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

Abstract

The rates and mechanisms of the reactions between \naqueous As(III) and synthetic birnessite (δ-MnO_2) \nparticles were studied. The experimental results at pH \n4 indicate that the depletion of As(III) from solution \nis rapid, with a time scale of minutes. The oxidation \nproduct As(V) is released almost as quickly, while \nthe release of the reduction product Mn(II) is slightly \nslower. The results also show that the concentration \nof dissolved oxygen has no effect on the rate of reaction. \nThese observations suggestthat (i) birnessite directly \noxidizes As(III) through a surface mechanism, (ii) the \nadsorption of As(III) is the slowest step in the \nproduction of As(V), and (iii) the reaction products As- \n(V) and Mn(II) are released via different mechanisms. \nThe time-dependent behavior of the aqueous reactants and products over a pH range from 4 to \n8.2, and a temperature range from 15 to 35 °C is also \ndiscussed. The rates of As(III) oxidation by inorganic \nredox reactions with manganese dioxides are \ncompared to observed As(III) oxidation rates in natural \naquatic systems.