Electrochemical and column investigation of iron-mediated reductive dechlorination of trichloroethylene and perchloroethylene

This research investigated the long-term performance of zero-valent iron ag gregates for reductive dechlorination of trichloroethylene (TCE) and perchl oroethylene (PCE). The effects of elapsed time, mass transfer limitations, and influent halocarbon concentration on reductive dechlorination rates wer e investigated using groundwater obtained from a field site contaminated wi th chlorinated organic compounds. Over the first 300 days of operation, rea ction rates for TCE and PCE gradually increased due to increasing porosity of the iron aggregates. Although there was microbial growth in the column, biological activity did not measurably contribute to reductive dechlorinati on. Dechlorination rates were pseudo-first-order in reactant concentration for submillimolar halocarbon concentrations. TCE concentrations near aqueou s saturation resulted in passivation of the iron surfaces and deviation fro m first-order reaction kinetics. However, this passivation was slowly rever sible upon lowering the influent TCE concentration. Tafel polarization diag rams for an electrode constructed from the iron aggregates indicated that c orrosion of the aggregates was anodically controlled. At all halocarbon con centrations, aggregate oxidation by water accounted for more than 80% of th e corrosion. Throughout the course of the 3-yr column investigation, reacti on rates for TCE were 2-3 times faster than those for PCE. However, current measurements with the aggregate electrode indicated that direct PCE reduct ion was faster than that for TCE. This disparity between amperometrically m easured reaction rates and those measured in the column reactor indicated t hat halocarbon reduction may occur via direct electron transfer or may occu r indirectly through reaction with atomic hydrogen adsorbed to the iron. Co mparison of aggregate corrosion rates with those of fresh iron suggested th at anodic control of corrosion leads to predominance of the indirect reduct ion mechanism. The faster reaction rate for TCE under anodically controlled conditions can therefore be attributed to its faster rate of indirect redu ction as compared to PCE.