Electroreductive Remediation of Halogenated Environmental Pollutants

Abstract

Electrochemical reduction of halogenated organic compounds is gaining increasing attention as a strategy for the remediation of environmental pollutants. We begin this review by discussing key components (cells, electrodes, solvents, and electrolytes) in the design of a procedure for degrading a targeted pollutant, and we describe and contrast some experimental techniques used to explore and characterize the electrochemical behavior of that pollutant. Then, we describe how to probe various mechanistic features of the pertinent electrochemistry (including stepwise versus concerted carbon-halogen bond cleavage, identification of reaction intermediates, and elucidation of mechanisms). Knowing this information is vital to the successful development of a remediation procedure. Next, we outline techniques, instrumentation, and cell designs involved in scaling up a benchtop experiment to an industrial-scale system. Finally, the last and major part of this review is directed toward surveying electrochemical studies of various categories of halogenated pollutants (chlorofluorocarbons; disinfection byproducts; pesticides, fungicides, and bactericides; and flame retardants) and looking forward to future developments.

Reductive dehalogenation of a chloroacetanilide herbicide in a flow electrochemical cell fitted with Ag‐modified Ni foams

“…Most chlorinated molecules can be directly reduced on cathodes but at very cathodic potentials close to the reduction of water . This results in hydrogen evolution, sometimes preventing efficient reduction of the compound.…”

“…20 Most chlorinated molecules can be directly reduced on cathodes but at very cathodic potentials close to the reduction of water. 21,22 This results in hydrogen evolution, sometimes preventing efficient reduction of the compound. Therefore, electrodes with catalytic properties allowing the reductive dechlorination at less cathodic potentials are required.…”

Cyclohexyl Bromide and Iodide: Direct Reduction at Vitreous Carbon Cathodes together with Nickel(I) Salen‐ and Cobalt(I) Salen‐Catalyzed Reductions in Dimethylformamide

“…Related work involving the reduction of 2‐bromo‐ and 2‐iodooctane in DMF containing tetramethylammonium perchlorate (TMAP) was performed in our laboratory; the first compound undergoes one‐step reduction, whereas the second species exhibits stepwise reduction, with mixtures of n ‐octane, 1‐octene, 2‐octene, 7,8‐dimethyltetradecane, and 2‐octanol arising from each starting material . Other secondary alkyl halides, including some monobromocycloalkanes, have been studied with the aid of polarography; an extensive list of references on these topics can be found in two recent reviews ,. In addition, summaries by Hawley, by Becker, and by Casanova and Reddy provide more information about the electrochemistry of monohalogenated cycloalkanes, and a two‐volume treatise by Torii includes descriptions of electroorganic syntheses based on the reduction of halogenated compounds.…”

“…A compilation of key references and researchers relevant to the use of these two catalysts for the reduction of halogenated organic compounds, together with applications in the arena of electrosynthesis, can be found in the previously cited reviews ,. In the context of catalytic processes, mention should be made of work by Sofranko and coworkers, that dealt with the reduction of cyclohexyl halides by electrogenerated zero‐valent rhodium[1, 2‐bis(diphenylphosphino)ethane] to afford cyclohexane, cyclohexene, and bicyclohexyl via the intermediacy of cyclohexyl radicals.…”

Insights into Electroreductive Dehalogenation Mechanisms of Chlorinated Environmental Pollutants

“…In addition, electroreductive dehalogenation offers another possibly as a preliminary treatment step for the completely removal of organic chlorides, which can be further combined with other approaches such as biodegradation and advanced oxidation processes for realizing the complete mineralization of these recalcitrant compounds . In this context, much effort has been devoted toward achieving the electroreductive dehalogenation of organic chlorides with high dechlorination performance, and importantly, some useful results on the dehalogenation reactivity and mechanisms have been obtained accordingly …”