Electrochemical Hydrodehalogenation of Chlorinated Phenols in Aqueous Solutions

Abstract

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 Na 2 SO 4 /H 2 SO 4 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. Halogenated liquid wastes are routinely produced in industrial processes, e.g., approximately 1 million tonnes of such wastes are generated annually in the U.K. 1 Until very recently, the disposal practice for these wastes was landfill for short-chain chlorinated organics, incineration for resistant and intractable compounds such as polychlorobiphenyls PCBs and pesticides, and use as a fuel supplement in cement kilns where a high calorific value of the organic made this a practical proposition. Disposal to landfill is now virtually precluded by the Environment Agency. Incineration has a number of serious drawbacks, such as high capital costs of plant, processing, and transport, production of harmful substances, e.g., dioxin and adverse public reaction. Therefore, other routes have been explored, such as bioremediation, 2 chemical and electrochemi-cal dehalogenation. 2-4 Bioremediation has been applied to dehalogenate a wide variety of halogenated compounds using the metabolism of microorganisms. 2 Bioremediation greatly depends on the ability of microorganisms to survive in an environment containing haloge-nated compounds. A more challenging issue is that the products of bioremediation are often toxic and, in some cases, may be more harmful to human health than the parent compounds. 4 Microorganisms can evolve relatively quickly to develop biochemical traits but in some cases, long-term operation is necessary, e.g., months for the bioremediation of PCBs. 5 Hydrodehalogenation HDH was considered as a low-waste technology for detoxifying organic halogenated waste and regenera-tion of the initial raw materials. 6 Chemical dehalogenation has been developed as an alternative to incineration for disposal of haloge-nated organic compounds, which can proceed several ways, e.g., catalytic and reactive procedures. The reactive dehalogenation involves the use of relatively expensive chemicals, such as LiAlH 4 or NaBH 4 , as hydrogen donors, and is therefore considered only for preparative synthesis. 7 Catalytic HDH of organic halogenated compounds is accepted as a practical choice at the moment and can be carried out in the gas 8 or liquid phase. 9 As there are harsh conditions in catalytic HDH, e.g., high temperature above 400°C in most cases, 10 there are requirements for thermal, mechanical, and chemical stability of catalytic reactor components. More severely, catalytic HDH often takes place at a gradually decreasing rate through progressive poisoning of the catalyst in some cases. This is accompanied by a rapid deactivation of the catalyst. Low-temperature catalytic HDH of bromobenzene was performed, e.g., at 40°C, but the results and other reaction conditions are unacceptable for industry. 7 More competitive ways of chemical HDH have been developed based on zero-valent metals such as iron, known as dissolving metal reductions. 2,3 Over the last few years, it has been shown that these metals can effect the HDH of a range of chlorinated organic compounds in contaminated groundwater. 2,3,11,12 Chemical HDH using iron alone suffered from slow reaction rates, particularly for aromatic halogenated compounds, under ambient temperature and pressure. 5 Partial HDH of PCBs by iron was only achieved under high-temperature and high-pressures, e.g., 250°C, 10 MPa. 13 Interestingly, the deposition of small amounts, ca. 0.05 wt %, of Ni or Pd onto the iron has been found to significantly enhance the rate of HDH, e.g., one to two orders of magnitude for trichloroeth-ylene TCE, compared to the rate at Fe alone which extends the range of halogenated organic compounds amenable to treatment from TCE to polychlorinated biphenyls. 14,15 Recently, the electrochemical HDH of chlorinated organic compounds has been explored as another means to replace incineration for disposal of halogenated organic wastes. 16-21 The cathode material has been found to have a major effect on the efficiency of the electrochemical HDH of organic halogenated compounds. A typical example is the HDH of 1,2,3,5-tetrachlorobenzene TCB and chlo-robenzene CB in methanol or dimethylsulfoxide and acetonitrile with 0.25 M tetraethylammonium bromide at a cathode potential of 3.3 V vs. Ag/AgCl. 17 More than 95% conversion of 12 mM CB was achieved with a current efficiency of 15-20% on the carbon cloth or Pb cathodes. On the other hand, Pt, Ti, and Ni cathodes only gave current efficiencies of ca. 5% and lower conversions. The electrochemical dechlorination of 153 ppm 4-chlorophenol to phenol in 0.05 M sodium acetate-acetic acid solution has been reported. A conversion, up to 100% dechlorination, of 153 ppm 4-chlorophenol was achieved by using palladized carbon cloth or graphite cathodes after 15 h electrolysis. 18 The majority of work examining the effect of cathode materials, even using Fe as a cathode, concentrated on mechanistic analysis rather than practical applications. Moreover, environmentally unac