Mechanisms controlling chlorocarbon reduction at iron surfaces

This research investigated whether rates of carbon tetrachloride (CT) and trichloroethylene (TCE) dechlorination on iron surfaces are limited by rates of electron transfer. The contributions of direct electron transfer and indirect reduction via atomic hydrogen to the overall dechlorination rates were also investigated. Electron transfer coefficients for CT and TCE were determined from measurements of dechlorination rates over a potential range from -600 to -1200 mV (SHE), and a temperature range of 2 to 42 °C. The transfer coefficient for CT was found to be independent of temperature, and the apparent activation energy was found to decrease with increasingly negative electrode potentials. These observations indicate that the rate of electron transfer controlled the observed rate of CT dechlorination. In contrast, the transfer coefficient for TCE was temperature dependent, and increased with increasingly negative electrode potentials. This indicated that TCE dechlorination was not controlled by an electron transfer step. Comparison of analytically and amperometrically measured reaction rates showed that CT reduction occurred primarily via direct electron transfer, while TCE reduction involved both direct electron transfer, and an indirect mechanism involving atomic hydrogen. Comparison of amperometrically and analytically measured reaction rates for TCE and perchloroethylene (PCE) also supports an indirect mechanism for chloroethene reduction.