MINERALOGICAL CONTROLS ON REACTIVITY OF PERMANGANATE DURING IN SITU CHEMICAL OXIDATION IN FRACTURED SHALE

Permanganate is commonly used for treatment of chlorinated solvent contamination in unconsolidated sediments, and in fractured rock. In porous fractured rock where most of the contaminant mass may reside in the matrix, success of remediation will depend partly on the ability to enhance the rate of MnO₄^- diffusion into the matrix by delivering and maintaining high MnO₄^- concentrations in the fractures, and partly on the competition for MnO₄^- between the target contaminant and natural mineral and organic reductants.

We conducted experiments to measure the distance of MnO₄^- diffusive penetration into the matrix of shale over a period of 24 months, and to assess the mineralogical controls on MnO₄^- persistence. Five blocks of shale (1 x 2 x 3 cm) were immersed in a KMnO₄ solution (2 g/L). A single sample was removed at times of 2, 4, 6, 12 and 24 months following the immersion, and polished thin sections were prepared from each block. Profiles of relative Mn concentration versus distance from the block surface were measured in the directions parallel and normal to the bedding direction using SEM-EDS. The Mn profiles reflect the presence of solid Mn-oxide which is the reaction product of MnO₄^- reduction and they are used as an indicator of the depth of penetration into the shale. In addition, SEM-EDS, and STEM-EDS were used to investigate reaction processes that influence the transport of MnO₄^- in the shale matrix.

After 24 months, MnO₄^- had penetrated the shale matrix to a distance of approximately 120 to 150 µm. The short distance of penetration is attributable to the rapid reduction of MnO₄^- by reaction with minerals and organic carbon in the shale which results in precipitation of the Mn-oxide reaction product. This penetration distance corresponds to an apparent diffusion coefficient on the order of 10^-17 m²/s which is four to five orders of magnitude lower than might be expected for a conservative solute in this shale and suggests a retardation factor between 10⁴ and 10⁵. There is abundant evidence of reaction between MnO₄^- and pyrite, and the data suggest that this reaction is initially limited by diffusion. However, over time as a nano-scale network of Mn-oxide grows in the pore network, the reaction becomes limited by electronic conduction of electrons in Mn oxide between aqueous MnO₄^- at the sample surface and pyrite embedded in the matrix.