Reductive dissolution of lead dioxide (PbO2) in acidic bromide solution

Reductive dissolution of lead dioxide (PbO(2)(s)) has been attributed to a major route leading to elevated lead concentrations in drinking water. However, surface processes involved in this heterogeneous reaction have not been elucidated. In this study, the kinetics and mechanism of reductive dissolution of PbO(2)(s) in acidic bromide solutions were investigated to reveal the detailed surface reactions. The reduction of PbO(2) by bromide can be expressed as PbO(2)(s) + 2Br(-) + 4H(+) --> Pb(2+) + Br(2) + 2H(2)O. The reaction kinetics was found to be proportional to the concentration of PbO(2)(s), and the reaction orders were 1.08 and 1.77 with respect to bromide and proton concentration, respectively. The observed kinetic data can be explained by the following reaction mechanism: adsorption of bromide on the PbO(2)(s) surface to form a precursor surface complex identical with Pb(IV)Br, and two separate one-electron transfers from adsorbed Br(-) to structural Pb(IV), followed by the release of Pb(2+) and Br(2) into water. The adsorption of bromide ion on the PbO(2) surface and the first one-electron transfer reaction were found to be important in regulating the overall rate of the reaction. It is expected that a similar reaction scheme can be applied to other reductive ions.