Preparation Of Co₃O₄ Catalytic Electrode And Its Electrochemical Reduction Of Nitrate Nitrogen

With the rapid development of social economy,nitrogen pollutants discharged into water bodies from agricultural,industrial and domestic water are on the rise,and the effluent of water treatment process often faces the problem of nitrate nitrogen(NO₃^--N)and total nitrogen(TN)exceeding the standard.Compared with other nitrate nitrogen(NO₃^--N)treatment technologies,electrochemical nitrogen removal technology has the characteristics of no need of external reagent,simple operation and environmental friendliness.However,the electrochemical denitrification technology also has the problems of short electrode service life,low removal efficiency and high energy consumption,which is difficult to meet the practical engineering application.Therefore,it is of great significance to develop electrode materials with high efficiency,stable performance and high economic benefit for electrochemical denitrification.Among the existing catalytic materials,cobalt trioxide(Co₃O₄)has been proved to have good electrocatalytic activity,and is widely used in electrocatalytic reduction reactions for electrochemical hydrogen production,carbon dioxide and nitrogen oxides,but there is little research on electrocatalytic reduction of nitrate nitrogen.In this study,Co₃O₄ was prepared by deposition on different substrates and used as electrodes in the electrochemical nitrate-nitrogen removal process.The denitrification performance was investigated and the process parameters were optimized to provide technical support for the practical application of electrochemical denitrification process.In this study,Co₃O₄ catalytic electrode was prepared by electroplating method as cathode,and IrO₂-RuO₂/Ti electrode was prepared by sol-gel method as anode to construct an electrochemical denitrification system.The materials were characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD)characterization and polarization curve test.The process conditions were studied from the nitrate nitrogen removal rate,reduction product analysis and total nitrogen removal rate,and the current efficiency and energy consumption were analyzed to evaluate the overall process.The main research contents are as follows:(1)Co₃O₄/CF material was prepared with carbon felt(CF)as substrate by electrodeposition and calcination method for cathode reduction of nitrate nitrogen.SEM and EDS analysis show that Co₃O₄/CF has a three-dimensional structure,and the Co₃O₄ deposited on the carbon blanket is distributed on the surface of the carbon blanket in granular form.The best removal performance of Co₃O₄/CF was obtained by electrodeposition at A current of 0.25A and calcination at 500?for 2h.Using Co₃O₄/CF as cathode and Pt as anode,under the conditions of p H 7.0,NO₃^--N concentration 100 mg/L and current density 15 m A/cm² for 2h,87.3%of NO₃^--N was removed and 62.53%of NH₃-N was generated.At the same time,nitrite nitrogen was detected to be generated during the reaction.It is mainly due to the carbon felt substrate participating in the reduction of nitrate nitrogen.Co₃O₄/Ti material was prepared by electrodeposition and calcination using titanium mesh as substrate for cathode reduction of nitrate nitrogen.The best properties of Co₃O₄/Ti were obtained by electrodeposition at 0.25A current for 5 min,deposition for 4 times,and calcination at 500?for 2 h.SEM and EDS analysis show that the Co₃O₄ deposited on the titanium mesh is spherically distributed on the surface of the titanium mesh.Using Co₃O₄/Ti as cathode,Pt as anode,p H 7.0,NO₃^--N concentration 100 mg/L,current density 15 m A/cm2 for 2 h,the NO₃^--N removal rate of 97.89%and NH₃-N generation rate of 60.30%can be obtained.The formation of nitrite nitrogen was not detected during the reaction.(2)IrO₂-RuO₂/Ti was prepared by sol-gel method,and the removal of ammonia nitrogen by anodic oxidation was investigated.The IrO₂-RuO₂/Ti electrode prepared with the mole ratio of Ir/Ru=2:1 has the best electrochemical performance.The surface of the prepared IrO₂-RuO₂/Ti electrode presents a homolytic structure,which increases the specific surface area and is beneficial to increase the number of active sites.The removal of ammonia nitrogen by anodic oxidation of IrO₂-RuO₂/Ti mainly depends on the indirect oxidation process.IrO₂-RuO₂/Ti is taken as the anode and Ti plate as the cathode.Under the conditions of p H 10.0,chloride concentration 2000 mg/L and current density 15 m A/cm² for 2 h,the removal rate of ammonia nitrogen and total nitrogen reached 69.16%and 66.39%under the conditions of p H 10.0,chloride concentration 2000 mg/L and current density 15 m A/cm².(3)The electrochemical denitrification process system was constructed with the self-made Co₃O₄ catalytic electrode cathode and IrO₂-RuO₂/Ti electrode anode,and the electrode performance was evaluated and the process parameters were optimized.Co₃O₄/CF as cathode,IrO₂-RuO₂/Ti as anode,p H 7.0,NO₃^--N concentration of 100 mg/L,Cl^-concentration of 2000mg/L,current density of 15 m A/cm² for 2h,The removal rate of NO₃^--N was 76.3%,the removal rate of TN was 56.65%,the energy consumption was 0.12(k W·h/g NO₃^--N),and the current efficiency was 33.0%.The electrode stability was found to be good after 10 cycles.Using Co₃O₄/Ti as cathode under the same conditions for 2h,the NO₃^--N removal rate of 91.12%and TN removal rate of 60.25%,the energy consumption of 0.10(k W·h/g NO₃^--N),the current efficiency of 40.31%,The removal rate of nitrate nitrogen was about 90%and the total nitrogen removal rate was over 60%after 10 times of recycling.Compared with Co₃O₄/CF electrode,the cathode of Co₃O₄/Ti has better denitrification performance,lower energy consumption and higher current efficiency,and the removal of nitrate nitrogen and total nitrogen can be achieved after 4h of reaction.