CARBON ISOTOPE FRACTIONATION DURING OXIDATION OF CHLORINATED ETHYLENES BY POTASSIUM PERMANGANATE

Chlorinated ethylenes, especially trichloroethylene (TCE) and tetrachloroethylene (PCE), are some of the most frequently detected groundwater contaminants in the US, and have been identified as priority pollutants by the EPA. Oxidation of chlorinated ethylenes using potassium permanganate has been demonstrated to be a rapid and effective technique for remediation. As with other degradation processes affecting these contaminants, stable isotope analysis has the potential to qualify and/or quantify the efficacy of permanganate oxidation as a remediation technique.

Oxidation experiments have been conducted with TCE, PCE and cis-dichloroethylene (cDCE) concentrations between 30 and 60 mg/l, and potassium permanganate concentrations between 50 and 1460 mg/l. TCE, PCE and cDCE concentrations and d13C values were monitored vs. time using headspace GC-C-IRMS. As demonstrated previously, the reaction is first-order with respect to permanganate concentration, and oxidation rates are very rapid, e.g. 95% oxidation is effected in approximately 40-80 minutes for TCE, 200-1200 minutes for PCE, and 15 minutes for cDCE. The d13C-concentration data follows Rayleigh fractionation trends, and yields average fractionation factors of aTCE=0.9786, aPCE=0.9830, and acDCE=0.9789. The magnitude of the fractionation factors for the individual compounds do not appear to show any correlation vs. oxidation rate. Fractionation factors during incomplete oxidation (TCE) and for mixtures (TCE and PCE) were the same as for single compounds with complete oxidation. The fractionation factors during oxidation of TCE in the presence of 5M NaCl was significantly smaller (a=0.9786) than in the absence of NaCl.

Fractionation factors are surprisingly large in light of the rapid reaction rates, and show a relatively small range for the three compounds studied, despite a total variation of reaction rate of almost two orders of magnitude, and is probably caused by a common rate-determining step. The large fractionation factors indicate that monitoring d13C values of chlorinated ethylenes during oxidation with permanganate may be a sensitive technique to quantify the extent of remediation.