| Chlorinated organic compounds are used widely in chemical,pesticide,paint,construction and other industries.Many chlorinated organic pollutants in the wastewater can cause serious water pollution;their significant toxicity and persistence pose a serious threat to human health and to the ecological environment.Traditional methods for treating chlorinated aromatic organic compounds,such as physical methods,biochemical methods and chemical oxidation are inefficient.In recent years,hydrodechlorination has been studied widely because of the efficient removal of chlorine,such as by using zero-valent iron,catalyst hydrogen,and electrochemical methods.Among them,electrocatalytic hydrodechlorination is a promising method because of its high efficiency,low consumption,ease of regulation and no secondary pollution,and its selectivity for removing chlorine without producing more toxic intermediates.The reactivity of the electrode materials depends on the role of the electrode material in different reduction mechanisms:direct and indirect reduction.Pd has been studied widely for the catalytic reduction of contaminants with its ability to retain the highly activated intermediate,atomic H*,which has strong reduction-potential energy,so palladium(Pd)-modified electrodes show excellent activities for electrocatalytic hydrodechlorination.The enhanced catalytic performance of Pd could be obtained after the addition of a second metal element.In this paper,an electrode with good electrocatalytic reduction activity and stability was prepared for chlorophenols.In the electrocatalytic experiments of 4-CP and 2,4-DCP,the reduction effects of this electrode and other electrodes were compared.The influence of different reaction conditions was explored,and the degradation mechanism was analyzed,then the stability of the electrode was verified in multiple batch experiments.The main conclusions of this paper are as follows:(1)GF with a three-dimensional structure was used as the electrode substrate,and the multi-walled carbon nanotubes were doped into the pretreated electrodes,to improve the GF conductivity and to serve as a metal-loaded skeleton.Then Ni and Pd were deposited on the electrode surface stepwise to obtain a well-aligned,highly active and stable Pd-Ni/MWCNTs/GF electrode.SEM shows that the electrode has a high specific surface area and water permeability,the nickel-palladium coating is uniformly deposited,the surface is smooth,and the metal has a good dispersion,and TEM can be seen that the metal particles are evenly dispersed on the carbon fiber,and their diameter is 10-20 nm.EDS,XPS and XRD reflect that Pd exists mainly in the zero-valence state on the electrode and the ratio of surface Ni to Pd is 2.56 to 0.91.In the cyclic voltammetry experiments,the Pd-Ni/MWCNTs/GF electrode exhibited lower reduction potential,higher current response,and better electrocatalytic reduction activity than the single metal electrode.(2)In the experiment of electrocatalytic reduction of 4-CP on Pd-Ni/MWCNTs/GF electrode,the main products of the reaction were phenol and chloride with very few byproducts.The removal rate of 4-CP was 100%,the reaction rate was 0.162 min-1 at 60 mA within 30 min,and the reactions all met the pseudo first-order reaction kinetics.Compared with other electrodes,bimetallic Pd-Ni/MWCNTs/GF electrodes have higher reactivity for 4-CP.Electrocatalytic reduction of 4-CP by Pd-Ni/MWCNTs/GF electrode has the best reaction effect at 60 mA.When the current is too high,the hydrogen evolution reaction will reduce the rate of electrocatalytic reduction reaction.And the current efficiency of the reaction is to increase first and then decrease.At different initial pH,acidic conditions favor the reduction of 4-CP and do not cause metal loss on the electrodes.Through radical screening experiments,the mechanism analysis is revealed that the indirect reduction of hydrogen radicals played nearly two-thirds of the role in the reaction.In a multi-batch experiment,it was confirmed that the bimetallic electrode can maintain a longer reaction activity and stability compared to a single palladium metal.(3)In the experiment of electrocatalytic reduction of 2,4-DCP,it is found that the para-chloro group is more easily removed and 2,4-DCP is reduced to phenol after 2-chlorophenol formation.The reaction rate is 0.0443 min-1.In comparison with different electrodes,Pd-Ni/MWCNTs/GF electrodes still exhibits the highest reactivity.In experiments where different currents affect the reaction,it is shown that electrocatalytic reduction of 2,4-DCP requires a higher current than 4-CP and has the best reaction effect at 100 mA.The pH of the initial solution is the same as that of 4-CP,and the reaction rate of 2,4-DCP is linear,and the reaction rate of 2,4-DCP decrease with the initial pH of the solution.Indirect reduction plays a dominant role in the electrocatalytic reduction of 2,4-DCP,and the presence of two chlorine groups makes the electrocatalytic reaction more difficult to occur,while the indirect reduction of hydrogen radicals with a stronger reduction effect plays a greater role.The role.The SEM image shows that the metal agglomerates on the electrode surface after multiple reactions.These particles will reduce the catalytic activity of the electrode in the electrochemical reaction.This problem will limit the long-term use of the material.(4)In the treatment of chlorophenol wastewater,electrocatalytic reduction-oxidation has the best effect on the degradation of organic matter.The reduction process is beneficial to reduce the toxicity of wastewater and the oxidation process generates byproducts to increase the toxicity of wastewater.So electrocatalytic reduction-oxidation is most suitable for treatment of Chlorophenol wastewater.The effect of Pd-Ni/MWCNTs/GF cathode is better than titanium plate and graphite felt electrode.Considering the degradation effect of pollutants and electric energy consumption,the electrocatalytic reduction time of chlorophenol wastewater treatment was 1 h,and the current density was 10 mA/cm2. |
