Electrochemical reductive dechlorination of chlorinated volatile organic compounds (Cl-VOCs): Effects of molecular structure on the dehalogenation reactivity and mechanisms

Chlorinated volatile organic compounds (Cl-VOCs) are a group of common contaminants that are widely detected in various polluted media (e.g., water, soil and atmospheric environment). The removal of these compounds has thus attracted considerable worldwide attention. Electrochemical reductive dehalogenation is among one of the most prospective approaches for the treatment of Cl-VOCs. Whereas a wealth of electrochemical data has been obtained over the past years by extensive investigations for these compounds, there is still missing a combined knowledge from individual researches. This review considers polychloromethanes (PCMs), polychloroethanes (PCAs) and polychloroethylenes (PCEs) as a whole and, systematically investigates the molecular structure effects on the reductive dehalogenation of Cl-VOCs, in order to highlight their common and specific properties of dehalogenation reactivity and reduction mechanisms and further, try to reveal their intrinsic relationships. Results show that molecular structure plays a vital role in determining the dehalogenation reactivity and dehalogenation mechanisms of Cl-VOCs. The dechlorination reaction is found to become more thermodynamically favored with the degree of chlorination, while an overall promotion effect of chemical groups in Cl-VOCs toward dehalogenation reactivity follows the order: alkyl group << β-chlorine < double bond < α-chlorine. Moreover, based on the electron transfer mechanisms, Cl-VOCs are classified as saturated or un-saturated, where concerted and stepwise dissociative electron transfer mechanisms take place, respectively. Based on the dehalogenation mechanisms, these compounds can be categorized as geminal or vicinal, which principally undergo the hydrodehalogenation pathway and reductive β-elimination route, correspondingly. The recent development of reductive dehalogenation issues for Cl-VOCs and the possible directions are discussed in the last section.