Diffusive Transport of Permanganate during In Situ Oxidation

The transport of permanganate in low permeability media (LPM) and its ability to degrade trichloroethylene (TCE) in situ were studied through diffusive transport experiments with intact soil cores. A transport cell was developed to measure the effective diffusion coefficient $\left(D_{\mathrm{eff}}\right)$ of a ${\mathrm{Br}}^{-}$ tracer through intact cores of silty clay LPM obtained from a field site and enable calculation of the apparent tortuosity $\left({\tau }_a\right)$ of the medium. Then, 5000 mg L⁻¹ of ${\mathrm{KMnO}}_4$ was added to the cell and diffusive transport and soil matrix interactions were observed. After three months, the soil cores were dissected for morphologic examination and characterization of matrix ions, total organic carbon, ${\mathrm{MnO}}_4^{-}\text{,}$ and manganese oxides $\left({\mathrm{MnO}}_2\right)\text{.}$ The experiment was then repeated after 2 μL of pure phase TCE were delivered into the center of each of two intact cores. Permanganate transport was observed for one month and then an extraction of the entire soil core was made to determine the extent of TCE degradation. This research demonstrated that permanganate can migrate by diffusion and yield reactive zones that can be predicted based on the properties of the LPM and the oxidant source. Under the experimental conditions examined, permanganate had little effect on the LPM’s pore structure or continuity, and appreciable soil organic matter remained even after 40–60 days of exposure to the oxidant. ${\mathrm{MnO}}_2$ solids, an oxidation by-product, were observed in the LPM, but not at levels sufficient to cause pore filling or alter the apparent matrix tortuosity, even when TCE was present. During diffusive transport of permanganate, TCE in the silty clay LPM was degraded by 97%.