PROPERTIES AND EFFECTS OF MANGANESE OXIDES PARTICLES GENERATED DURING IN SITU PERMANGANATE OXIDATION

In situ chemical oxidation, specifically permanganate (MnO₄^-) oxidation, is receiving increased attention and application as a method of organic contaminant degradation at hazardous waste sites. Permanganate has been successful in destroying targeted organic contaminants such as trichloroethylene (TCE), as demonstrated in both laboratory studies and field applications. However, many questions remain regarding the fate of the manganese introduced into the subsurface. A specific area of interest is the genesis of manganese oxides particles as a product of the reaction of MnO₄^- with organic contaminants (e.g., eqn. 1, reaction of permanganate with TCE), as well as with natural aquifer materials, due to their potential to impact hydraulic conductivity if deposited in the subsurface. This presentation will highlight laboratory and field investigations that have examined manganese oxides particle genesis and resulting impacts following permanganate oxidation.

2MnO₄^- + C₂HCl₃=2MnO₂(s) + 2CO₂ + 3Cl^- + H⁺ (1)

Experiments conducted at the Colorado School of Mines (CSM) to examine these manganese oxides particles will be the focus of the presentation. These laboratory-scale experiments examine the properties of manganese oxides particles resulting from permanganate reactions under varied potential site conditions. Conditions investigated include MnO₄^- and organic contaminant concentrations, pH, and groundwater composition. Results of these studies indicate that these particular reaction conditions can have important influences on the characteristics of generated manganese oxides, including their size and morphology. For example, the presence of calcium in the reaction matrix results in the genesis of larger-sized particles than without calcium present, and these particles destabilize in solution and can potentially deposit in the subsurface. On the other hand, a lower pH (i.e., 3 vs. 7) results in small particles that are stable suspended in solution, thus are less likely to exert a negative impact on subsurface permeability. These experimental studies provide insight for predicting the fate and transport of manganese oxides resulting from in situ permanganate oxidation, and examine the ability to control conditions for a desired outcome such as avoiding particle deposition.