| Seven investigations have been carried out to explore electroreductive remediation of several significant halogenated organic pollutants via cyclic voltammetry, controlled-potential electrolysis, gas chromatography--mass spectrometry, and high-performance liquid chromatography--electrospray ionization--mass spectrometry. Previous work has established that homogenous-phase electron-transfer mediators (such as nickel(I) salen) and external catalysts (such as silver cathodes) can serve to promote electrochemical dehalogenation at more positive potentials than direct reduction at inert cathodes, thereby decreasing the energy requirements of remediation. Therefore, this work focuses on the application of these catalysts for the reduction of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113), decabromodiphenyl ether (DBDE), 1,2,5,6,9,10-hexabromocyclododecane (HBCD), and 4,4&feet;-(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT). First, a comparison of the direct and nickel(I) salen-catalyzed reduction of CFC-113 revealed that the nickel catalyst shifts the two observed cathodic peaks to more positive potentials, but complete conversion to 1-chloro-1,2,2-trifluoroethene and 1,2,2-trifluoroethene, respectively, is inhibited due to modification of the catalyst species. Then a study of the same systems in the presence of carbon dioxide was carried out; although no carboxylic acid species were detected, a higher concentration of the less halogenated compounds (such as tri- and difluoroethene) was found along with carbon dioxide reduction products. To avoid concomitant side reactions, the third study employed silver cathodes for the reduction of CFC-113 in various organic and organic-aqueous solvents. Addition of water resulted in very positive reduction potentials, but incomplete dechlorination; therefore, an organic solvent, namely acetonitrile, served as the optimal medium for remediation at silver electrodes. Fourth, the electrochemical degradation of DBDE was investigated at carbon and silver cathodes in dimethylformamide and dimethyl sulfoxide; complete debromination was best achieved in dimethylformamide to form diphenyl ether, dibenzofuran, phenol, and benzene, as well as a mixture of isomers of dibromodiphenyl ether. Fifth, the reduction of HBCD at silver and carbon cathodes demonstrated that predominately 1,5,9-cyclododecatriene (CDT) is formed; silver cathodes lead to more positive peak potentials, but less efficient reduction at substrate concentrations higher than 2.0 mM in comparison to carbon. Next, catalytic reduction of HBCD was achieved at carbon cathodes in the presence of nickel(I) salen to afford CDT, but insufficient debromination occurs at substrate concentrations of 20.0 mM or greater. Finally, a complex mechanism was determined for the nickel(I) salen-catalyzed reduction of DDT at carbon cathodes to form less harmful compounds at more positive potentials than those recorded in the absence of the catalyst. |
