Reductive dissolution of manganese(III, IV) (hydr)oxides by oxalate: the effect of pH and light

The dissolution of γ-MnOOH and β-MnO2 in the presence of oxalate was studied at various pH values, in the dark and under the influence of light. Unlike iron(III) (hydr)oxides, γ-MnOOH and β-MnO2 are reductively dissolved on a time scale of hours in the dark at low pH. A strong pH dependence of the rate of reductive dissolution of both manganese (hydr)oxides is observed; the rate of reductive dissolution decreases with increasing pH. This behavior may be interpreted, in part, in terms of the pH dependence of oxalate adsorption on the surface of the manganese(III,IV) (hydr)oxides. A plateau in the dissolution rate of γ-MnOOH is observed at high (total) oxalate concentrations, consistent with direct dependence of the rate of the reductive dissolution of the manganese(III,IV) (hydr)oxides on the concentration of surface-bound oxalate. At any given pH, the reductive dissolution rate per unit surface area is higher for γ-MnOOH than for β-MnO2. Light has only a weak catalytic effect on the dissolution kinetics of the two manganese (hydr)oxides studied, in contrast to iron(III) (hydr)oxides.