Immobilization of Mn(II) via Homogeneous and Heterogeneous Oxidation

Abstract:

This study investigated the immobilization of Mn(II) via homogeneous and heterogeneous oxidation by air. A series of kinetic experiments were performed with stirred batch reactors equipped with air spargers. The reactions were initiated by adding Mn(II) stock solutions to pH-buffered solutions amended with dissolved Fe(II), Fe oxyhydroxides, or Mn oxides. Under experimental conditions, the homogeneous oxidation of Mn(II) itself was minimal over pH 6.5-8.5. However, when dissolved Fe(II) was present, the immobilization of Mn occurred, with the extent becoming greater at higher pH. By Mn-K edge XAS analysis, the Mn removal at pH 6.5 was due to the oxidation of labile Mn(II) into insoluble Mn(III) solids, which was catalyzed by the homogeneous oxidation of Fe(II). On the other hand, Mn(II) at pH 7.0-8.5 remained largely unoxidized; instead, it was immobilized by forming co-precipitates with Fe(III) oxyhydroxides. Goethite, HFO, Mn₂O₃, and MnO₂ were added to mediate the heterogeneous oxidation of Mn(II). In the presence of goethite and HFO, dissolved Mn was initially quickly decreased, and later gradually decreased. In both batches, the initial removal was due to the surface complexation of Mn(II) with Fe oxyhydroxides. On the other hand, the later removal in goethite-amend batches resulted from the formation of co-precipitates with Fe(III) oxyhydroxides, whereas the later removal in HFO-amended batches was due to the heterogeneous oxidation of Mn(II) into Mn(III) solids. When Mn₂O₃ and MnO₂ were used as heterogeneous catalysts, XAS analysis did not provide mechanistic insight into Mn removal. Nonetheless, Mn₂O₃ was found to immobilize Mn(II) under oxic conditions. Notably, MnO₂ was far more effectively immobilize Mn(II) under both oxic and anoxic conditions, pointing to its superior oxidative capability.

Acknowledgement: Financial support was provided by “R&D Project on Environmental Management of Geologic CO₂ Storage” from the KEITI (Project Number: 2014001810003).