Phase Transformations and Proton Promoted Dissolution of Hydrous Manganite (γ-MnOOH)

Abstract

The objective of this study was to describe the proton promoted disproportion of synthetic manganite (γ-MnOOH) and to characterise the resulting phase transformations. The solution and remaining solid phase after disproportionation was analysed by techniques including atomic absorbance spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). In suspensions with pH between 5 and 7, −log[H⁺] was monitored for 17 months and equilibrium constants were determined at 9, 12 and 17 months of reaction time for the following reaction (25 °C, 0.1 M (Na)NO₃):

$$2\gamma \hbox{-MnOOH} + 2\hbox{H}^{+} \rightleftharpoons\hbox{MnO}_{2} + \hbox{Mn}^{2+} +2\hbox{H}_{2}\hbox{O}$$

The formed MnO₂ ages with time and the equilibrium constant for a metastable phase (ramsdellite or nsutite) as well as the most stable phase, pyrolusite (β-MnO₂), was determined. Furthermore, combined pH and pe (Eh) measurements were performed to study the equilibrium;

$$\gamma \hbox{-MnOOH(s)} \rightleftharpoons\beta\hbox{-MnO}_{2}(\hbox{s}) + \hbox{H}^{+} + \hbox{e}^{-}$$

Real-time AFM measurements of the dissolution showed shrinkage of the length of the manganite needles with time (2 hours). After 1 week SEM images showed that this decreased length also was followed by a reduced thickness of the manganite needles. From the SEM images the morphology of the formed Mn(IV) oxides was studied. At pH 2.6, pyrolusite (β-MnO₂) and MnCl₂ were found in the XRD patterns. Throughout the pH range there were indications of ramsdellite (MnO_1.97) in the XRD patterns, which coincided with the existence of a fraction of needle shaped crystals with smaller dimensions (compared to manganite) in the SEM images. These observations together with the long term dissolution experiments suggest that the dissolution of manganite initially forms a ramsdellite or nsutite phase that over time rearranges to form pyrolusite.