Oxidation of volatile organic compound vapours by potassium permanganate in a horizontal permeable reactive barrier under unsaturated conditions: experiments and modeling

Abstract

In this research we evaluated the potential of using solid potassium permanganate to create a horizontal permeable reactive barrier (HPRB) for oxidizing VOC vapours in the unsaturated zone. We have performed batch experiments, short column, and long column experiments, and have fully analyzed the data. In the batch experiments, we investigated the ability of potassium permanganate to fully oxidize three selected target compounds, namely, trichloroethylene (TCE), toluene, and ethanol. We also determined kinetic parameters for their oxidation reactions in both gas and water phases. Results for both phases showed that oxidation rate decrease in the order TCE>ethanol>toluene. In addition, the determined reaction rate constants for target compounds in the vapour phase were found to be much smaller than for aqueous phase. In short column experiments, VOC vapours arising from a liquid pool of a target compound were allowed to diffuse through a one-centimeter thick layer of mixed sand and potassium permanganate, representing a horizontal permeable reactive barrier (HPRB) and become accumulated in the headspace above the layer. Through analyzing the headspace concentration, we determined the capability of the layer to oxidize VOC vapours. We observed a high removal efficiency and reactivity of the HPRB for all target compounds at the highest water saturation (Sw=0.6). The change in pH was found to be a main reason for the reduction in the oxidation rate VOC in the HPRB.Results showed that adding carbonate minerals into initial HPRB increases the longevity of potassium permanganate during the oxidation of TCE vapour by buffering the pH. Furthermore, we found that the negative effect of low pH on the reactivity of the HPRB was mitigated by adding water during the TCE experiment.A developed model for reactive vapour transport, which included pH-dependent oxidation rates, was able to simulate the experimental data for TCE, toluene, and ethanol. With increasing water contents, an increasing discrepancy between the simulation results and experimental data was found for ethanol. The total oxidized mass of ethanol vapour was found to be higher than TCE and toluene for identical water saturations, despite a larger oxidation rate constant for TCE than for ethanol and toluene. To test the effectiveness of the HPRB under situations similar to field conditions, we performed larger column experiments. In this setup, we investigated the effects of the thickness of HPRB and the elevation of the HPRB layer above the groundwater table on the reactivity of HPRB. Results suggest that the thickness of the HPRB was a very important factor in oxidizing VOC vapours. We further found that the location of the HPRB relative to the water table, and consequently its background water saturation, had a strong effect on the removal capacity of the HPRB. A rough estimate of the longevity shows that HPRB could be a viable option for preventing VOC vapours to reach indoor spaces.