Development of a rate law for arsenite oxidation by manganese oxides

Abstract

Abstract Arsenite (As(III)), may be rapidly oxidized to arsenate (As(V)) by manganese oxides over a wide range of pH. Interestingly, pseudo-first order rate constants for Mn(IV) oxides (MnO2) reported in the literature differ by orders of magnitude, mainly due to the fact that experimental conditions have not been normalized and surface passivation may hinder the reaction rate. As a result, a formal kinetic rate law for the oxidation of As(III) by MnO2 does not exist. In this study, a comprehensive rate law that describes the abiotic oxidation of As(III) by vernadite (δ-MnO2) was developed by systematically varying pH and the concentrations of δ-MnO2 and As(III). Experiments were performed with excess δ-MnO2 compared to As(III) in buffered solutions with an ionic strength typical of freshwater environments to reflect the geochemical conditions of most environments. The half-life of the pseudo-first order reaction is less than three minutes in all experiments. Results show the initial reaction rate is proportional to the concentrations of δ-MnO2 and As(III) and independent of pH in the pH range 4–9. However, below pH 6 and above pH 8 the pseudo-first order rate constant is slightly lower due to the protonation of the surface δ-MnO2 and the deprotonation of As(III). Overall, the reaction follows a first order rate law with respect to As(III) and effective surface sites of δ-MnO2 and zeroth order with respect to pH. The overall second order rate constant, k, was calculated to be 0.36 ± 0.11 L h−1 m−2. This rate constant is comparable to the vast majority of rate constants derived from previously published data when using the effective surface sites of MnO2 as reactive species only, indicating the most influential factors of As(III) oxidation by MnO2 are constrained in the developed rate law. Based on the present experiments, the oxidation of As(III) by MnO2 is proposed to proceed as one two-electron transfer reaction between As(III) and Mn(IV) to eventually form As(V) and Mn2+ species. Two steps of one-electron transfer, however, may be possible though it is likely that both Mn(IV) and Mn(III) species are ultimately reduced to Mn2+. Comproportionation of Mn2+ and Mn(IV) to Mn(III) does not seem to be important on the time scale of these experiments.