Reduction Performance And Mechanism Of 2,4,6-trichlorophenol By MnO₂/Pd Modified Foam-Ni Electrode

Due to the high toxicity,persistence and hard-biodegradability of 2,4,6-trichlorophenol（2,4,6-TCP）,it causes great damage to the ecosystem,which triggers series challenge in the context of water pollution threatening the public health.Among numerous chlorinated phenols removal techniques,the electrochemical dechlorination process has been reported to be the most promising approach for effective removal of the target contaminants due to its advantages,such as high reactivity,mild reaction conditions and green nature.Palladium（Pd）is the most effective and widely used catalyst.Manganese oxide（MnO₂）,due to its low cost,high catalytic activity,and avirulence,has been widely applied in the fields of catalysis,supercapacitors.In this study,a MnO₂/Pd-Ni composite electrode was prepared for rapid reduction of trichlorophenol with characterizing the physicochemical properties and stability of the composite electrode.We analyzed the external factors affecting the electrochemical dechlorination system,and concluded the possible degradation mechanism and pathway of TCP by electrocatalysis.In this work,manganese dioxide and palladium modified foam nickel electrode were prepared by a two-step method,denoted as MnO₂/Pd-Ni.The electrochemical characteristics of the surface were characterized by scanning electron microscopy,X-ray diffraction and other methods.MnO₂ was uniform Ly deposited on the surface of nickel and formed fluffy flakes.Palladium particles were dispersed on MnO₂.The introduction of MnO₂ and Pd enhanced the generation of atomic hydrogen,increased the reactive sites on the electrode surface.Then,we analyzed the influence of MnO₂/Pd loading,cathode potential,initial TCP concentration and other factors on the efficiency of reductive dechlorination.We found that the electrode prepared with 1 m M of Pd deposition liquid and 300 s MnO₂depostion time had the best dechlorination efficiency and electrochemical performanc with the efficiency increasing to 89.21%.The optimal reaction system conditions were cathodic potential-0.9 V,Na₂SO₄ 0.2 M,electrolyte pH=7,and TCP removal rate could reach 92.59%.The composite electrode can handle relatively high concentrations of 2,4,6-TCP（in a system with an initial concentration of 150μM/L,the removal rate can reach 86.53%in 3 hours）.However,the increase of pollutant concentration will decrease the efficiency of dechlorination.Exploring the influence of conventional inorganic anions,it was found that Cl^-ions（5 m M/L）promoted the dechlorination process,while S^2-,SO₃^2-,NO₂^-,NO₃^-retarded that.Particularly,the presence of S^2-and SO₃^2-completely inhibited dechlorination.Then,the reusability of electrode was evaluated.After 5 consecutive dechlorination reaction cycles,the reductive dechlorination efficiency of 2,4,6-TCP could still reached 79.70%,showing that the composite electrode had good stability and reusability.In addition to 2,4,6-TCP,the electrode had a good removal effect on many other halogenated organic compounds（TBP,TBBPA,etc.）.MnO₂/Pd-Ni electrode exhibited excellent dechlorination efficiency under anaerobic,micro-aerobic or aerobic conditions.In three conditions,there were direct dechlorination processes,while the mechanism of indirect dechlorination was different.In the reduction system,atomic hydrogen attacks the C-Cl bond to complete reductive dechlorination.In the micro-oxygen system,reducing and oxidizing free radicals act simultaneously to realize electrochemical reduction and oxidative dechlorination processes.The MnO₂/Pd-Ni electrode prepared in this study has a good electrochemical dechlorination effect on trichlorophenol,and offers avenues to achieve the effective degradation of persistent halogenated organic pollutants in water.