Electrochemical hydrodehalogenation of chlorinated phenols in aqueous solutions - I. Material aspects

Pentachlorophenol (PCP) and 2,4-dichlorophenol (DCP) have been hydrodehalogenated by electrochemical reduction in aqueous solutions at ambient and elevated temperatures. Galvanostatic and/or potentiostatic hydrodehalogenation (HDH) was carried out in conventional H-cells or solid polymer electrolyte cells operated in batch or/and batch-recycle modes. The processes were monitored by both chloride ion analysis and high performance liquid chromotography product analysis. The effect of the cathode, separator, and cell type on the rate and efficiency of HDH of 1 mM DCP and 0.071 mM (saturated) PCP in 0.05 M Na2SO4 /H2SO4 (pH 3) and/or pure water are reported. Several types of cathodes, e.g., Fe gauze and foil, Pd/Fe gauze, Pd/Fe foil, carbon cloth, and Pd/carbon cloth, were tested. Palladized cathodes showed high catalytic activity and long term stability for the HDH processes. Both cation and anion exchange membranes were employed to separate the divided H-cells and solid polymer electrolyte cells. Complete HDH of 1 mM DCP with a current efficiency of 70% and an energy consumption below 20 kWh/kg DCP, was realized in an H-cell with a Pd cathode at ambient temperature. Similar results were obtained for HDH of 0.071 mM PCP but with lower current efficiency, e.g., 16% and higher energy consumption, e.g., 80 kWh/kg PCP. The cell gave a current efficiency of 15% and an energy consumption of 11.6 kWh/kg DCP for complete HDH of 1 mM DCP in pure water solution. For complete HDH of 0.071 mM PCP in the SPE cell, current efficiency and energy consumption were 10% and 90 kWh/kg PCP. (C) 2002 The Electrochemical Society.