Environ. Sci. Technol. 2003, 37, 3435-3441
Complete Dechlorination of 1-Chloronaphthalene by Electrochemical Reduction with Naphthalene Radical Anion as Mediator ATSUSHI MATSUNAGA* AND AKIO YASUHARA The National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki, 305-0053 Japan
The possibility of electrochemical reduction for the complete dechlorination of toxic chlorinated aromatic compounds was investigated using 1-chloronaphthalene as a substrate. Sufficient current was obtained at the reduction potential of naphthalene rather than that of the substrate, and complete (>99.99%) dechlorination was achieved in tetra-n-butylammonium perchlorate/dimethylformamide solution in less than 1 h. The reaction was considered to proceed autocatalytically with the naphthalene radical anion, which was derived from the reduction of the initial dechlorination product (naphthalene) of the substrate, as mediator. The reaction rate was increased by the use of an excess of naphthalene at the start of the reaction, and complete dechlorination was possible without significant loss of current efficiency even if the water concentration was 0.3% v/v. The order of reaction of the mediated reduction was determined kinetically to be 0.5 for the substrate and 1 for naphthalene. Although the reaction of the naphthalene radical anion with 1-chloronaphthalene was very fast, it was considered to be the rate-determining step.
Introduction Electrochemical reduction is known to be one of the effective methods for the dechlorination of chlorinated organic compounds. Many studies of the electrochemical dechlorination of chlorinated aromatic compounds have been reported including those involving polarographic and voltammetric measurements (1, 2). However, it is not yet clear whether the method is practically useful. For toxic and persistent chlorinated compounds such as PCBs and PCDD/ Fs, complete dechlorination is necessary from the point of view of environmental protection. Usually, complete conversion is difficult in electrolytic reactions as the current or the reaction rate decreases with the decrease of substrate concentration. For example, Sugimoto et al. (3) reported the direct (i.e. without mediators) electrolytic reduction of Aroclor 1268. They used a gold-plated working electrode in tetraethylammonium perchlorate/dimethylformamide (DMF) solution. They showed no quantitative data, but it seems that small amounts of chlorinated biphenyls of low chlorine numbers were still present even after 5 h, as indicated by their included gas chromatogram. Connors et al. (4) used a * Corresponding author e-mail:
[email protected]. 10.1021/es026360z CCC: $25.00 Published on Web 06/21/2003
2003 American Chemical Society
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