The Electrochemical Reduction Efficiency And Mechanism Of Typical Chlorinated Organophosphorus Flame Retardants And Chlorophenol Pollutants

In recent years,chlorinated organic pollutants in water has caused domestic and foreign concern.The chlorine-containing unit is easy to be replaced by reduction method.As an environmentally friendly technology,electrochemical reduction has gradually become an important control technology for chlorinated organic compounds removal in water.This study mainly selected chlorinated organophosphorus flame retardants（Cl-OPFRs）and chlorophenol organics（CPs）as typical alkyl chlorinated organic and aromatic hydrocarbons chlorinated organic.Both of them have been widely detected in the water environment,which have potential environmental risks.For two types of pollutants,this study constructed an electrochemical reduction system to explore the reduction efficiency on the electrochemical reduction of two pollutants,influence factors,reduction pathway,reduction mechanisms and electrode reusability.Firstly,tris（2-chloroethyl）phosphate（TCEP）as a typical Cl-OPFRs was selected to explore the the efficiency and mechanism of Cl-OPFRs removal by electrochemical reduction system with commercial electrode materials as the cathode.Compared to other metal cathodes,Cu foam exhibited the best electrochemical reduction and dechlorination performance.When the TCEP concentration is low(1mg·L^-1),the removal rate of TCEP by Cu foam could reach more than 95%within 20 minutes,and the reaction rate constant was 0.127 min^-1.With assistance of the product analysis and Fukui function calculations,we inferred that TCEP molecular reduction is a gradual fracture process,and the three oxygen-ethyl-chlorine chains are split one by one.Five intermediate products were detected during the entire 60 min reaction process,and the reduction pathway was further speculated,which mainly contains hydroxylation,dechlorination,dealkylation and other steps.The toxicity of these intermediate products was significantly reduced.Secondly,in order to solve the problem of low cathode reduction efficiency for CPs under the traditional electrode system,a new type of Pd/MnO₂ catalyst was prepared and dispersed in the electrochemical system with Cu foam as the cathode.The reduction efficiency and mechanism of typical CPs under electrochemical system with Pd/MnO₂ catalyst were explored.A series of Pd/MnO₂ catalyst materials with different Pd loading ratios were prepared by chemical reduction deposition method.Results showed that the reduction rate of p-chlorophenol（4-CP）in the electrochemical system was the fastest when the Pd loading of the particle electrode was 5%,and the removal efficiency could reach 100%within 45 min reaction.With the analysis of changes in electrochemical impedance spectrosco（EIS）and cyclic voltammetry（CV）,Pd/MnO₂ catalyst had no active hydrogen（H*）reduction peak on the cyclic voltammogram of MnO₂ alone in the Pd/MnO₂ catalyst system.In contrast,obvious oxidation peaks of H atom and reduction peaks of H*_ads were both found in the Pd/MnO₂ catalyst system,indicating that the catalyst had excellent electrochemical performance.Quenching experiments and product analysis clarified that the indirect reduction mechanism of 4-CP was:the adsorbed H*generated by water molecules under the catalysis of Pd/MnO₂ attacks the C-Cl bond of 4-CP,resulting in dechlorination of4-CP which was finally transformed to phenol.In the recyle experiment,the reduction performance of 4-CP dropped from 100%to about 75%after the electrode material reused 5 times,indicating the material stability was better than other metal catalyst.Two different cathode electrochemical reduction systems were conducted for the two types of chlorinated organics removal to explore and analyze the reduction efficiency and mechanism of these two types of pollutants.This electrode catalytic materials can promote the reduction of chlorinated organics,which provide a theoretical basis for the removal of different types of chlorinated pollutants in water bodies.