Reductive dechlorination of trichloroethene and carbon tetrachloride using iron and palladized-iron cathodes

This research investigated the effectiveness of electrochemical reduction for removing trichloroethylene (TCE) and carbon tetrachloride (CT) from dilute aqueous solutions. The kinetics, reaction mechanisms, and current efficiencies for TCE and CT reduction were investigated using flow-through, iron electrode reactors and with amperometric measurements of reduction rates. The electrode reactors were operated over a range of flow rates, pH, ionic strength, dissolved oxygen concentration, and working electrode potentials. Typical reduction half-lives for TCE and CT in the iron reactor were 9.4 and 3.7 min, respectively. The addition of palladium as an electrocatalyst at a level of 1 mg of Pd per m² of electrode surface area increased the reaction rates by a factor of 3. When operated continuously, reaction rates in the palladized-iron reactor were stable over a 9-month period of operation, indicating that there was no loss of palladium from the electrode. In both the iron and Pd-iron reactors, TCE was reduced primarily to ethane and ethene, while CT was reduced almost exclusively to methane. Under all operating conditions, chlorinated compounds accounted for less than 2% of the total reaction byproducts. 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 is indirect and involves atomic hydrogen as the reducing agent. Direct reduction of TCE appeared to be inhibited by the preferential reduction of water. The finding that electrodes coated with a hydrophobic polymer to inhibit water reduction showed current efficiencies greater than 90% for direct TCE reduction supports this hypothesis. For CT, similar ampermetric and analytically measured current efficiencies indicated that the primary mechanism for CT reduction is direct electron transfer. Carbon dioxide and bisulfide, which have been found to foul palladium in other catalytic systems, did not deactivate the catalyst. The fast reaction kinetics and electrode stability indicate that electrochemical reduction may be feasible for treating waters contaminated with chlorinated organic compounds.