Proton-Promoted and Electron Transfer Dissolution of Nano-Hausmannite: Implications for Cr and Mn Cycling in Surficial Environments

Abstract

In natural waters, dissolution of manganese (Mn) oxides occurs through proton-promotion, reduction and synergistic pathways and can affect the solubility, mobility, bioavailability, and toxicity of metalloids in the environment through these pathways. Thus, the present work intends to elucidate the complex nature of Mn oxide dissolution in the presence or absence of a redox-active metalloid by examining hausmannite (Mn3O4) and chromium (Cr) as model compounds. Hausmannite is the fifth most common Mn oxide and contains both reduced Mn(II) and oxidized Mn(III) in its structure. Cr(VI) is a well-known environmental pollutant and is used for quantifying the reductive (electron transfer) dissolution of hausmannite in this study. To examine the specificities involved in hausmannite dissolution, batch reactions were performed with varying solution pH (4, 5.5, and 7) and Cr(III) and Cr(VI) concentrations (0, 5.23, and 52.3 ppm). Solution samples were collected at selected times (T = 0, 0.5, 1, 2, 4, and 8 hours) and analyzed for total Cr and Mn as well as oxidized Cr(VI). To gain insights about the Cr interaction on the hausmannite surface, the solid materials were characterized by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy before and after 8 hours of the reaction. The results of this work demonstrate that the release of Mn(II) through hausmannite dissolution was greatest when both proton-promotion (pH 4) and electron transfer dissolution (High Cr(III)) pathways are allowed. Nearly all the Cr(III) added underwent oxidation to Cr(VI) (94.9% or greater) by hausmannite at this pH. Conversely, Mn(II) release at pH 5.5 and 7 was comparatively insensitive to both the initial Cr concentration and its oxidation state (Cr(III) or Cr(VI)). Oxidation of Cr(III) to Cr(VI) was also significantly reduced at these pHs due to a loss of initial Cr(III) through adsorption and precipitation on the hausmannite surface. This observation is supported by both Cr2p XPS and Cr K-edge x-ray absorption near-edge spectroscopy (XANES) analyses, indicating that in the presence of hausmannite, Cr(III) is present either as adsorbed Cr(III) ions or as precipitated Cr(OH)3, although Cr(III) sorption is preferred given sufficient surface area. Regardless of the solution pH, Cr(VI) accounts for the majority of the total dissolved Cr, but no Cr(VI) was detected in recovered solids. Furthermore, at pH 4, electron transfer dissolution of hausmannite by Cr(III) produced higher average oxidation state (AOS) of Mn (>3.39) than that of the Cr(VI) and unreacted treatments. These findings indicate that Cr(III) may facilitate Mn disproportionation in reacted hausmannite, and cause synergism in hausmannite dissolution. However, at neutral pH (pH 5.5 or higher) Cr(III) adsorption and precipitation reactions cause difficulties in quantification. Thus, the studied hausmannite dissolution reactions play a critical role in Cr and Mn cycling in surficial environments, and the degree of contribution and the type of dissolution pathway are primarily controlled by solution pH and the concentration and species of a reducing agent.