Reductive dissolution of microparticulate manganese oxides

Electrochemical dissolution of immobilised microparticulate Mn(III,IV) oxides in slightly acidic solution (pH 4.4) was found to be a very general reaction, which is responsible for well-defined voltammetric peaks. Dissolution of six Mn(III,IV) oxides is initiated by the reduction of Mn(IV) to Mn(III) in the solid phase, which is followed by a massive dissolution via further reduction of Mn(III) to Mn(II), which finally yields soluble Mn2+. The reactivity of manganese oxides depends on their structure: the most reactive are amorphous (δ-MnO2) and layered structures (birnessite); more resistant toward reductive dissolution are α- and λ-MnO2 and electrochemical manganese dioxide; and least reactive is β-MnO2. Reductive dissolution of LiMn2O4 resembles that of λ-MnO2, whereas CaMnO3 dissolves via a different reaction mechanism.