Kinetic and mechanistic investigation of halocarbon reduction at iron cathodes

摘要

This research investigated the kinetics and mechanisms of reductive dechlorination of trichloroethylene (TCE) and carbon tetrachloride (CT) at iron cathodes. Reaction rates for TCE and CT were measured as a function of electrode potential, water chemistry, and influent reactant concentration in flow-through, porous iron electrode reactors, with and without the addition of palladium as an electrocatalyst. Rates of direct halocarbon reduction were also measured amperometrically using potential step chronoamperometry. Reaction rates for TCE and CT were first order in reactant concentration over the concentration range investigated. Typical reduction half-lives for TCE and CT in the iron reactor at neutral pH were 9.4 and 3.7 minutes, respectively. Palladium treatment at a level of 1 mg-Pd per square meter of electrode surface area increased reaction rates by a factor of three. When operated continuously, reaction rates in the palladized iron reactor were stable over a 9 month period of continuous operation. Amperometrically measured reaction rates for CT were the same as those measured analytically. This indicated that CT reduction occurred via direct electron transfer. In contrast, TCE reduction rates could not be measured amperometrically. However, the high current efficiencies in the electrode reactor indicated that the primary mechanism for TCE reduction was indirect, and occurred via reaction with atomic hydrogen produced from water reduction.