Oxidation of Mn(II): Initial mineralogy,oxidation state and ageing

The initial solid phase oxidation products formed during the oxidation of aqueous Mn(II) at 25°C were studied as a function of time. The analyses included morphology (TEM), mineralogy (x-ray diffraction), $\text{O}\text{Mn}$ ratio (iodometric method), oxidation state of manganese (XPS), and dissolved manganese. The initial solid formed under our conditions was Mn₃O₄ (hausmannite) which converted completely to γMnOOH (manganite) after eight months. βMnOOH (feitknechtite) appeared to be an intermediate in this transformation. The $\text{O}\text{Mn}$ ratio was initially 1.37 and increased to 1.49 over the same time span. Throughout the course of this study the XPS analyses showed that the surface of the solids (<50 Å) was dominated by Mn(III). The solution pH and dissolved manganese concentrations were consistent with disproportionation and oxidation reactions that favor the transformation of Mn₃O₄ to γMnOOH but not to γMnO₂.