Reductive Dechlorination Of 2,4-dichlorophenoxyacetic Acid With Ni?OH?₂ Or Co₃O₄ Modified Pd/Ni Foam Electrode

In the 20th century,chlorinated organic compounds?COCs?were produced in large quantities and widely used in agriculture,chemical industry,biomedicine and electronics.2,4-Dichlorophenoxyacetic acid?2,4-D?is a typical chlorine-containing organic matter,which was often used as a farmland herbicide to control the growth of broad-leaved weeds such as cereal crops,lawns,and pastures with strong biological activity.However,the residue in the process of use is difficult to degrade,resulting in serious damage to the soil and water ecosystem.Therefore,the study of 2,4-D dechlorination has become very important in the environmental protection industry.The electrochemical reduction dechlorination technology can remove the chlorine from the pollutants in the form of Cl^-,which has the characteristics of low toxicity,high selectivity of the generated intermediates and environmental friendliness,and is effective for removing chlorine-containing organic pollutants.However,the electrode used in the conventional electrochemical dechlorination method required a large Pd load,and the dechlorination efficiency is not high.In this paper,a bifunctional Pd-Ni?OH?₂/Ni foam electrode was prepared in one step using a simple,green chemical deposition method,and various material characterization methods such as X-ray diffraction?XRD?,field emission scanning electron microscopy?FE-SEM?,transmission electrons microscope?TEM?,infrared spectroscopy?ATR-FTIR?,raman spectroscopy?Raman?,X-ray photoelectron spectroscopy?XPS?,inductively coupled plasma emission spectroscopy?ICP-OES?and electrochemical methods were used to analyze physical and chemical properties of the prepared electrodes.The electrode preparated by displacement reaction as the molar ratio of PdCl₂ and NaCl with 1:30,amorphous Ni?OH?₂ with appropriate amount can be situ formed on the surface of the Pd/Ni foam electrode,which named as Pd₁Cl₃₂ electrode.The electrode with the Pd loading<0.2 mg/cm² was applied to the dechlorination of2,4-D with the removal efficiencies of 81.5%at the cathode potential of-0.65 V after electrolysis for 120 min.As the PdCl₂ precursor solution prepared by replacing the NaCl with hydrochloric acid,no Ni?OH?₂ formed on the surface of the Pd/Ni foam electrode named as Pd₁HCl₃₂.This electrode has almost no removal ability on 2,4-D under the same conditions.The results showed that the Pd₁Cl₃₂ electrode had the highest 2,4-D removal efficiency,and its maximum current efficiency?CE?is 19.3%,and the average CE is 16.0%.The electrochemical experiments of Pd₁Cl₃₂ electrode showed that amorphous Ni?OH?₂ played a dual role on the electrode surface,it could promote the dissociation of water,accelerated the production of atomic hydrogen H*and formed Pd-anchored Ni?OH?₂ to shorten the transfer path of H*on the electrode surface for providing dechlorination efficiency.The Pd₁Cl₃₂ electrode was applied to dechlorination of 2,4-D in the flowing cathode solution.By studying the effect of operating conditions of 2,4-D dechlorination such as Pd loading,cathode solution flow rate,2,4-D initial concentration,current density,reaction temperature and external anion on the electrode.As catholyte flow rate of 28.5 mL/min,2,4-D initial concentration of 50mg/L,Na₂SO₄ concentration of 17 mmol/L,the cathode potential of-1.25 V,the Pd loading of 0.195 mg/cm²,and the reaction temperature of 298 K were the best dechlorination conditions.When the reaction time is 300 min,the removal rate of2,4-D can reach 100%.The Pd₁Cl₃₂ electrode was found in 5 consecutive cycles,and the electrode showed excellent electrode stability.In this paper,Co₃O₄ and Pd/Ni foam composite electrode named as Pd-Co₃O₄/Ni foam was synthesized by two-step method with Pd loading of 0.47 mg/cm² and Co₃O₄loading of 0.51 mg/cm².The existence of Co₃O₄ on the electrode was determined by XRD,FE-SEM,TEM,ICP-OES and XPS.95%2,4-D was removed under current density of 1.5 mA/cm² by the Pd-Co₃O₄/Ni foam electrode after reaction 60 min.The electrode with the same Pd loading prepared by chemical deposition method and potentiostatic precipitation method named as Pd?CD?/Ni foam and Pd?CPD?/Ni foam,used to dechlorination under the same experimental condition,and the removal rate of2,4-D was only 61.5%and 66.4%after 120 min reaction.It is worthwhile noting that four times the amount of palladium?1.88 mg/cm²?in the 4Pd?CD?/Ni foam was required to achieve similar removal efficiency to the Pd-Co₃O₄/Ni foam.The kinetics,selectivity and mechanism of dechlorination for 2,4-D with Pd-Co₃O₄/Ni foam electrode were also researched.In addition,the operating conditions of the dechlorination process such as the Pd and Co₃O₄ loading on the electrode surface,the initial concentration of 2,4-D,the current density and the reaction temperature were also studied.The optimal reaction conditions were found as follows:the Pd and Co₃O₄ loading as 0.47 mg/cm² and 0.51 mg/cm²,respectively,the current density as1.5 mA/cm²,the initial concentration of 2,4-D as 50 mg/L,the reaction temperature at298 K,and the apparent activation energy as 42.33?10³ J/mol.Under the optimal reaction conditions,the effects of coexistence of other anions on the dechlorination of the cathode solution,the dechlorination of other chlorine-containing organic compounds and the dechlorination of 2,4-D in the actual water were investigated.The results showed that added 5 mmol/L Cl^-,5 mmol/L HCO₃^-,respectively,in the catholyte showed no effect on 2,4-D dechlorination.1 mmol/L SO₃^2-and 1 mmol/L S^2-seriously reduced the dechlorination activity of the electrode.The main reason for the improvement of dechlorination performance of Pd-Co₃O₄/Ni foam electrode was that the introduction of Co₃O₄ can change the dispersion of Pd on the electrode surface and increase the active site on the electrode surface.In addition,Co₃O₄ also can reduce the electrochemical impedance of the electrode surface and accelerates electron transport,thereby accelerating the dissociation of water and producing more adsorbed atomic hydrogen(H*_ads)on the surface of electrode.Because adsorbed atomic H*_ads is the main active species for dechlorination,more H*_ads can accelerate the dechlorination reaction,resulting in higher dechlorination removal efficiency.The application experiment of the Pd-Co₃O₄/Ni foam electrode further indicated that the Pd-Co₃O₄/Ni foam electrode studied in this paper can achieve high dechlorination removal efficiency and has a broad practical application prospects.The stability experiment of dechlorination of Pd-Co₃O₄/Ni foam electrode for 5 cycles provided practical support for the electrode.In summary,the Pd₁Cl₃₂ and Pd-Co₃O₄/Ni foam electrodes prepared in this paper have high dechlorination efficiency,low Pd loading,good electrode stability,and further provide the possibility of electrochemical reduction treatment of wastewater.