Dechlorination of Monochlorobenzene Using Organic Mediators

Abstract

Dechlorination of monochlorobenzene using organic mediators ABSTRACTIn the presence of an organic mediator such as dibenzofuran, the reduction of chlorobenzene occurs indirectly and at substantially less negative potentials compared to its direct reduction at a glassy carbon cathode in acetonitrile. By using the indirect, mediator approach to reduction of chlorobenzene, constant current electrolysis at carbon plate cathodes can give complete dechlorination with high current efficiency Both divided and undivided cells were used, each having their own advantages. Besides dibenzofuran, naphthalene and biphenyl were successfully tested as organic mediators for chlorobenzene reduction. During the entire electrolysis, the mediator concentration remained practically constant so that substantially less mediator was required in comparison to the substrate, i.e., chlorobenzene. Higher concentrations of mediator can be beneficial, e.g., a dibenzofuran:chlorobenzene ratio of 2.5:5 as compared to 1:5, if the electrolysis is to be conducted at higher current density. For electrolysis of large amounts of chlorobenzene, an approach where the substrate is added periodically to a solution of the (reduced) mediator is recommended. A comparison of the results from direct vs. indirect dechlorination of chlorobenzene clearly demonstrates the substantial superiority of the latter approach.

Mediated Approach for the Electrochemical Reduction of Chlorobenzenes in Nonaqueous Media

“…14 The reduction of trichlorobenzene with dibenzofuran as mediator gives very similar results to those obtained with biphenyl as mediator. Here again, the trichlorobenzene concentration decreases rapidly and a large portion of the benzenes disappears.…”

“…This method has already been described in an earlier paper. 14 The result thus obtained for trichlorobenzene with phenanthridine as mediator is log k 3r2 ϭ 4.3. This rate of reaction is calculated assuming that trichlorobenzene is reduced via a two-electron transfer to dichlorobenzene and that the further reduction of dichlorobenzene can be neglected.…”

“…The selected mediators were then tested for the dechlorination of different polychlorinated benzenes. The rate of electron transfer was measured by voltammetry (as described previously 14 ). The successful mediators had to show a good ability in reducing the polychlorobenzene (log k > 3) but at the same time had to show a slow rate of reaction with the initial dechlorination products so that the reaction would not proceed beyond removal of the first chlorine.…”

Chemical Degradation of Chlorinated Organic Pollutants for In Situ Remediation and Evaluation of Natural Attenuation

“…The major remediation method for more the accumulation of chlorobenzenes with a higher chlorination level in soils is incineration at high temperatures for digging out of soils. In the previous reports of electrochemical dehalogenation of chlorinated benzenes (Miyoshi et al, 2004;Mohammad and Dennis, 1997;Farwell et al, 1975;Kargina et al, 1997;Guena et al, 2000), the chlorine is eliminated step by step from the highly chlorinated benzenes to yield less-chlorinated benzenes and finally transform to benzene. Farwell et al (1975) reported on chlorobenzenes and the main cathodic reaction pathway for hexachlorobenzenes as follows: hexachlorobenzene → pentachlorobenzene → 1,2,3,5-tetrachlorobenzene → 1,2,4-trichlorobenzene → 1,4-dichlorobenzene → monochlorobenzene → benzene.…”

Characterization of Kaolin Glassy Carbon Modified Electrodes: Preconcentration of 2‐Chlorophenol

“…However, such compounds are known to oxidize with concomitant fouling of the electrode surface. This leads to a rapid drop of the relevant electrochemical signal, which loses any proper analytical usefulness [4,5]. Thus a wide range of surface treatment and modification approaches have been used for the prevention of surface fouling and restoration of electrodes during detection of phenolic compounds [6 -8].…”