Electrocatalytic Reductive Dechlorination Of Chlorinated Volatile Organic Compounds (Cl-VOCs):Effects Of Molecular Structure And Indication Of Linear Free Energy Relationships

Chlorinatedvolatileorganiccompounds?Cl-VOCs?,includ ing polychloromethanes?PCMs?,polyc hloroethanes?PCAs?and polychloroethyle nes?PCEs?,belo ng to a group of ub iquitous contaminants that are widely found in all kinds of polluted media.The remo val o f these compounds has thus attracted considerable attention from wor ldwide.Electrochemical reductive dehalogenation is among one of the most promis ing approaches for the abatement of Cl-VOCs.Whereas a wealth of electrochemica l data has been obtained over the past years by extens ive investigations for these compounds,For exa mple,electron transfer mechanis m,dechlor ination kinetics and the reduction route etc,an integrated knowledge of ind ivid ual researches is still missing.This review considers polychloro methanes?PCMs?,polychloroethanes?PCAs?and polychloroethylenes?PCEs?as a whole and,systematically investigates the mo lecular structure effects on the reductive deha logenation of Cl-VOCs,in order to highlight their commo n and specific properties of dehalogenation reactivity and reduction mechanis ms and further,try to reveal their intrins ic relationships.Results show that mo lecular structure is a key factor in determining the dehalogenation reactivity and dehalo genatio n mechanis ms of Cl-VOCs.The dechlor ination reaction is found to become more thermodyna mically fa vored with the degree of chlorination,while an overall promotion effect of chemica l groups in Cl-VOCs toward dehalogenation reactivity follows the order:alkyl group<<?-chlorine<double bond<?-chlorine.Moreover,based on the electron transfer mechanisms,Cl-VOCs are classified as saturated or un-saturated,where concerted and stepwise dissociative electron transfer mechanis ms take place,respective ly.Based on the dehalogenation mechanis ms,these compounds can be categorized as gemina l or vic inal,which princ ipally undergo the hydrodehalogenation pathway and reductive?-elimination route,correspondingly,where gemina l Cl-VOCs mainly undergo hydrodehalogenation pathway by obtaining two electrons in the simultaneous remova l of a chlor ide ion at each step,and vic ina l Cl-VOCs principa lly follows reductive?-elimination route by expelling two adjacent chloride ions with double or trip le bond product for mation.Electrochemical deha logenation of Cl-VOCs?norma lly in no n-aqueous solvent?has long been studied for decades,but despite all the works that have been done on the kinetics or the dehalogenation mechanis ms,few studies on the linear free energy relations hips?LFERs?for exp laining or predicting the electrochemical dehalogenation reactivity and dehalo genatio n mechanis ms of Cl-VOCs have been conducted.According to the precious research,the deha logenation reactivity and dehalogenation mechanis ms of Cl-VOCs is closely related to its structure.In this study,we successfully developed several LFERs based on the electroche mical parameter?electrochemica l reduction potentia l,E_p?with the fo ur ther modyna mic parameters associated with mo lecular structure?lowest un-occupied mo lecular orbital?LUMO?energy,C-Cl bond dissociation e nergy?BDE?,one-?E₁?and two-?E₂?electron reduction potentials?to describe the deha logenation reactivity and dehalogenation mechanis ms of Cl-VOCs.Results clearly demonstrate that a good linear correlation of E_p with LUMO energy is obtained regardless the c lasses of Cl-VOCs,while separate correlations o f E_p against BDE or E₂ can give r ise to three satis factory linear relations hips according to the dehalogenation mechanis ms.In order to confir med the established LFERs relationship can well predict the dehalogenation reactivity and dehalo genatio n mechanisms for PCA,as well as a predictor of PCA reductio n peak potentia l va lue,this study finally choose pentachlorocyc lohexane?PCA?as the test compound.Hence,the developed LFERs in this study ma y provide a new ins ight in predicting the deha logenation reactivity and deha logenation mechanis ms for Cl-VOCs and other halocarbons.