Manganese oxides formed during in situ chemical oxidation: The potential for manganese and trace metal mobilization under reducing conditions

In situ chemical oxidation (ISCO) by permanganate is one method that is used for the remediation of chlorinated-solvent contamination in aquifers. The reaction causes the precipitation of Mn oxide (MnOx ), which forms coatings on sand grains and also acts as an effective adsorption substrate for aqueous metals that enter the groundwater with the permanganate solution or via reactions with naturally-occurring aquifer minerals. If the redox state of the groundwater becomes reducing following treatment, it is possible that the MnOx will dissolve, resulting in the release of Mn and trace metals to the groundwater. This study assessed the long-term stability of the MnOx and the possibility for trace-metal release by considering two factors: the valence of Mn in the MnOx formed during the ISCO reaction, and the potential for dissolution of the MnO x under reducing conditions.;Microbially-mediated reducing conditions were initiated in a column containing MnOx-coated sand and the geochemical controls on the pore water composition were assessed using aqueous-geochemical and mineralogical investigations. The MnOx was susceptible to microbially-mediated reductive dissolution, as indicated by elevated Mn concentrations, consumption of acetate, and production of bicarbonate in the column. Trace-metal (Cr, Cu, Ni, Pb, and Zn) concentrations were low or below detection throughout the experiment, indicating that the reductive dissolution of the MnOx had little effect on their mobility, while in contrast, Mo was mobile under near-neutral pH conditions. The precipitation of rhodochrosite (MnCO3) acted as a limitation on the aqueous concentrations of Mn and trace metals in the water eluted from the column.;A method was developed to quantify the average Mn valence in minerals at the nanometer scale using electron energy-loss spectroscopy with scanning transmission electron microscopy. The method is based on the change in Mn L3/L2 intensity ratios as Mn valence changes. The measured valence of Mn in the MnOx formed under simulated-ISCO conditions was 2.2 ± 0.5. Variation in pH, within the range of 2 to 7, had no effect on the Mn valence. Prior to this work, researchers have generally assumed that the product of ISCO reactions is MnO2 (i.e., Mn IV oxide). An important implication of these findings is that the residual oxidizing potential in an aquifer after ISCO treatment may be considerably lower than would be the case if MnO2 was the reaction product.