Manganese (II) oxidation at mineral surfaces: A microscopic and spectroscopic study

Abstract

The products of the heterogeneous oxidation of Mn(II)aq at hematite, goethite, and albite surfaces were studied, using Scanning Force Microscopy (SFM), X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Auger Electron Spectroscopy (AES), Scanning Auger Microscopy (SAM), and X-ray Diffraction (XRD). Oxidation experiments were conducted at room temperature in aerated solutions containing 4.0–26.7 ppm Mn(II)aq at pHs ranging from 7.8 to 8.7. Exposure times ranged from a few minutes up to six months. Through this study, it was found that steps are the most reactive sites for initiating the adsorption-oxidation reaction, independent of the mineral surface used. After the initial oxidation of Mn(II) at or near these sites, the continued adsorption-oxidation process is mineral surface dependent resulting in two types of growth paths, substrate and precipitate controlled growth. Substrate controlled growth (hematite and goethite) is characterized by the formation of a thin coating of protocrystallites that grows across the mineral surface away from the initial adsorption-oxidation site. In contrast, precipitate controlled growth (albite) is characterized by the development of precipitate ridges along a step edge. These two surface-controlled growth processes influence the distribution of the precipitates found with SEM and in hand-samples. However, independent of the mineral surface used in the experiments, the resulting precipitates consisted of Mn(III) bearing oxyhydroxides, predominately β-MnOOH.