Reduction and dissolution of manganese(III) and manganese(IV) oxides by organics. 1. Reaction with hydroquinone

The chemical processes by which manganese oxides are solubilized by reduction in anoxic waters are poorly understood. A study of the reduction and dissolution of manganese oxide suspensions by hydroquinone was undertaken to determine the rate and mechanism of the solubilization reaction. Dissolution of the manganese-(III,IV) oxide suspension by hydroquinone in the pH range 6.5 < pH < 8.5 is initially described by the following empirical rate law: d[Mn²⁺]/dt = k₁[H⁺]^(0.46)[QH₂]^(1.0)([MnO_x]₀[Mn²⁺]) where [Mn²⁺] is the dissolved manganese concentration, [QH₂] is the hydroquinone concentration, and [MnO_x]₀ is the amount of manganese oxide added. The apparent activation energy was found to be at +37 kJ/mol. Calcium and phosphate inhibited the reaction, by adsorbing on the oxide surface. A model is proposed for the observed rate dependence, according to which complex formation between hydroquinone and manganese oxide surface sites occurs prior to electron transfer.