Rate-limiting mechanisms for carbon tetrachloride and trichloroethylene reactions at iron surfaces

Permeable reactive barriers containing zerovalent iron are becoming increasingly popular for in situ remediation of groundwater contaminated with chlorinated organic compounds, e.g., trichloroethylene (TCE). A study was carried out to determine if the rate limiting mechanisms for reductive dechlorination of carbon tetrachloride (CT) and TCE involve electron transfer. The reaction rate of TCE might not be limited by an electron step. The transfer coefficient for CT reduction was independent of temperature, while that for TCE reduction was temperature dependent, indicating that the rate of CT reduction was limited by an electron transfer step, while that for TCE was limited by chemical dependent factors. The apparent activation energy (E_a) for CT reduction decreased with increasingly negative electrode potentials. This suggested that the rate of CT dechlorination was controlled by an electron transfer step. In an outer sphere reaction, a fraction of the polarization energy went towards changing the chemical activation energy. Cathodic polarizations resulted in negative overpotentials, and thus resulted in decreasing E_a values with decreasing electrode potential. In contrast to CT, the weak interdependence of E_a for TCE reduction and E further supported the conclusion that the rate-limiting step for TCE dechlorination does not involve electron transfer.