Tetrachloroethene Release and Degradation During Combined ERH and Sodium Persulfate Oxidation

Electrical resistance heating (ERH) is a thermal treatment technology that involves passing electrical current through soil to increase subsurface temperatures. In addition to volatizing and recovering contaminant mass in the gas phase, heating the subsurface has the potential to decompose contaminants by increasing the rate of degradation reactions. Prior laboratory studies using convective heating demonstrated that the rate of tetrachloroethene (PCE) degradation was not sufficient to cause substantial in situ PCE mass destruction. However, similar experiments have not been performed using ERH, which has the potential to degrade PCE in reaction with the heating electrodes and electrochemically. Thus, the objective of this study was to determine the extent of PCE degradation during thermal treatment of PCE-contaminated soil using a bench-scale ERH system. The contaminated soil, a silty clay loam, was collected from a single borehole at a former dry cleaning facility prior to undergoing ERH treatment. After 30days of ERH, 52% of the initial PCE mass was recovered, potentially indicating that 48% of the PCE was degraded during ERH. Although potential degradation products such as carbon dioxide were observed, their presence was attributed to the degradation of soil organic carbon and carbonates rather than PCE destruction. A second ERH experiment was conducted to assess the potential benefit of adding the heat-activated oxidant, sodium-persulfate, during treatment. After 19days of ERH and three persulfate injections, 93% of the initial PCE was recovered, with 3% PCE destruction based on chloride evolution. However, the difference in mass recovery between the first and second experiments could have been due to differences in the initial mass of PCE, even though soil from the same core was used in both experiments. The results of this work suggest that the majority of mass recovered during ERH of the PCE-contaminated soil at the former dry cleaning facility will be due to volatilization and gas phase extraction rather than abiotic degradation, even with the addition of sodium persulfate. Article Impact Statement: Mass recovery during ERH of PCE-contaminated soil will be due to gas phase extraction rather than abiotic or persulfate degradation.