Insights into Electroreductive Dehalogenation Mechanisms of Chlorinated Environmental Pollutants

The electroreductive cleavage of carbon-chloro bonds has attracted increasing attention from both environmental and mechanistic points of view over the past decades, taking into consideration a large variety of chlorinated organic compounds. Important discoveries have been made on the underlying electron/hydrogen transfer mechanisms and the enhancement of dechlorination performance for electrocatalysis purposes over the last years. However, there is still missing a combined knowledge from individual researches to specially elucidate the electroreductive dehalogenation mechanisms of organic chlorides. Herein, we summarize the diverse electrochemical strategies applied to reductive dehalogenation of chlorinated environmental pollutants, including direct electrochemical reduction, electrocatalytic hydro-dechlorination and electrochemically mediated reduction methods, and, especially, highlight the differences in reductive dehalogenation mechanisms. Specifically, direct electron transfer is the most common dechlorination mechanism involved in the direct electrochemical reduction of organic chlorides, in which the electron transfer to C-Cl bonds can be classified as concerted or stepwise dissociative electron transfer mechanism. Indirect atomic hydrogen reduction is the primary dechlorination mechanism in the electrocatalytic hydro-dechlorination process, which often competes with the direct electrochemical reduction route. Electrochemically mediated reduction, as another indirect route, uses a redox-active mediator to indirectly transfer electrons from cathode to organic chlorides, resulting in reductive dehalogenation reaction. Moreover, the effects of critical factors on the dehalogenation reactivity and mechanisms are also discussed to better demonstrate the electroreductive dehalogenation of organic chlorides. Lastly, this contribution highlights recent relevant advances in electroreductive dehalogenation issues, as well as the challenges and potentials of this process.