Kinetic and mechanistic investigation of reductive dechlorination at iron surfaces

Abstract

The long-term performance of zero-valent iron for reductive dechlorination of trichloroethylene (TCE) and perchloroethylene (PCE) was investigated. The effects of elapsed time, mass transfer limitations, influent halocarbon concentration, and water chemistry on reductive dechlorination rates were studied in a series of column reactors. Dechlorination rates were pseudo-first order in reactant concentration for submillimolar halocarbon concentrations. With increasing elapsed time, reaction rates deviated from pseudo-first order behavior due to reactive site saturation, and increased iron surface passivation towards the influent end of each column. Corrosion current measurements indicated that halocarbon reduction on fresh iron surfaces was cathodically controlled, whereas on aged iron surfaces, iron corrosion was anodically controlled. The decrease in TCE reaction rates over time can be attributed to anodic control of iron corrosion, and not to increasing reactant mass transfer limitations associated with diffusion through porous corrosion products. The 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 absorbed on the iron surface. The kinetics, reaction mechanisms, and current efficiencies for electrochemical reduction of TCE and CT were investigated using flow-through, iron electrode reactors. Typical reduction half-lives for TCE and CT in the iron reactor were 9.4 and 3.7 minutes, respectively. Comparisons of amperometrically measured current efficiencies with those measured in the flow-through reactors, and the weak effect of electrode potential on TCE reaction rates, indicated that the primary pathway for TCE reduction by iron and palladized iron electrodes was indirect, and involves atomic hydrogen as the reducing agent. For CT, similar amperometric and analytically measured current efficiencies indicated that the primary mechanism for CT reduction is direct electron transfer. Chronoamperometry (CA) and chronopotentiometry (CP) analyses were used to determine the kinetics of CT and TCE reduction by a rotating disk electrode in solutions of constant halocarbon concentration. The transfer coefficient for CT was independent of temperature, while that for TCE was temperature dependent. This indicated that CT reduction was limited by the rate of electron transfer. The temperature dependent transfer coefficient for TCE indicated that its reduction was limited by chemical dependent factors. In accord with a rate limiting mechanism involving an electron transfer reaction, the apparent activation energy (Ea) for CT reduction was found to decrease with decreasing electrode potential. Conversely, the Ea for TCE reduction showed a slight increase with decreasing electrode potential, supporting the conclusion that its reaction rate was not limited by the rate of electron transfer.