Mechanisms controlling halocarbon reduction rates by zerovalent iron

Metallic iron filings could be used in passive groundwater remediation schemes. A study was carried out to investigate the long-term effectiveness of three types of zerovalent iron material for reductive dechlorination of trichloroethylene (TCE) and perchloroethylene (PCE). The effects of water chemistry, elapsed time, and influent halocarbon concentration on reaction rates and rate limiting mechanisms were studied. In the presence of nitrate, the iron corrodes at a slower rate due to anodic inhibition. Reaction rates were initially first order in reactant concentration, but showed increasing deviation from first order behavior with increasing elapsed time. Deviation from first order behavior was attributed to iron surface passivation leading to anodic control of iron corrosion. TCE concentrations near aqueous saturation rapidly passivated the iron. On fresh iron surfaces, the corrosion rate increased with TCE concentration, up to a concentration of 6 mM. However, increasing the TCE concentration above 6 mM resulted in a decreased rate of iron corrosion. Although corrosion rates in the blank electrolyte solutions were similar for the fresh and magnetite coated iron, increasing TCE concentrations had little effect on the corrosion rate of the magnetite coated iron. There were two mechanisms involved in halocarbon reduction, i.e., the chronoamperometry and corrosion current measurements detected the rate of direct electron transfer, while the analytical measurements detect both the direct and indirect mechanisms. The rate of indirect reduction was faster for TCE, while the rate of direct reduction was faster for PCE.