Reactions at Oxide Surfaces. 2. Oxidation of Se(IV) by Synthetic Birnessite

The rates and mechanisms of the reactions between aqueous Se(IV) and synthetic birnessite (δ-MnO2) particles were studied. The experimental results at pH 4 and 25 °C indicate that the initial disappearance of Se(IV)(aq) from solution is rapid with a time scale of minutes. The oxidation product Se(VI)(aq) appears in detectable concentrations after 12 h of reaction and is slowly produced at a constant rate throughout the duration of the experiments (28 days). The extended reaction period results in most of the product Mn(II) being adsorbed by the oxide surface. The lack of oxidation of Se(IV)(aq) in an oxygenated homogeneous solution and the appearance of Se(VI)(aq) in an O_2-free birnessite suspension is evidence for the redox reaction between Se(IV) and Mn(IV). Increasing the pH of the particle suspension from 4 to 7 decreases the initial uptake of Se(IV) and the production of Se(VI). Increasing the temperature of the particle suspension from 25 to 35 °C has little effect on the rate of disappearance of aqueous Se(IV) during the initial 100 h, but after this time period, higher temperatures increase the rate of Se(IV) depletion. Increasing the temperature of the suspension does increase the rate and amount of Se(VI) that is released into solution. These observations suggest the following: (i) birnessite directly oxidizes Se(IV) through a surface mechanism, (ii) the rate-limiting step in the production of Se(VI) is most likely the electron transfer step, and (iii) the reaction products Se(VI) and Mn(II) are released by different steps. The rates and energetics of Se(IV) oxidation by inorganic redox reactions with manganese dioxide are compared to those previously reported for As(III) and Cr(III).