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

This research investigated the long-term performance of zero-valent iron aggregates for reductive dechlorination of trichloroethylene (TCE) and perchloroethylene (PCE). The effects of elapsed time, mass transfer limitations, and influent halocarbon concentration on reductive dechlorination rates were investigated using groundwater obtained from a field site contaminated with chlorinated organic compounds. Over the first 300 days of operation, reaction 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 dechlorination. Dechlorination rates were pseudo-first-order in reactant concentration for submillimolar halocarbon concentrations. TCE concentrations near aqueous saturation resulted in passivation of the iron surfaces and deviation from first-order reaction kinetics. However, this passivation was slowly reversible upon lowering the influent TCE concentration. Tafel polarization diagrams for an electrode constructed from the iron aggregates indicated that corrosion of the aggregates was anodically controlled. At all halocarbon concentrations, aggregate oxidation by water accounted for more than 80% of the corrosion. Throughout the course of the 3-yr column investigation, reaction rates for TCE were 2-3 times faster than those for PCE. However, current measurements with the aggregate electrode indicated that direct PCE reduction was faster than that for TCE. This disparity between amperometrically measured reaction rates and those measured in the column reactor indicated that halocarbon reduction may occur via direct electron transfer or may occur indirectly through reaction with atomic hydrogen adsorbed to the iron. Comparison of aggregate corrosion rates with those of fresh iron suggested that anodic control of corrosion leads to predominance of the indirect reduction mechanism. The faster reaction rate for TCE under anodically controlled conditions can therefore be attributed to its faster rate of indirect reduction as compared to PCE.