Electrochemical Reduction Of Nitrate With Modified Co₃O₄/carbon Cloth Self-supporting Electrodes

In recent years,with the continuous advancement of industrialization,water pollution has become more serious,especially in the problem of eutrophic pollution caused by nitrate which has caused increasing concern and has become a hot spot in the field of water pollution prevention.Electrochemical technology for nitrate removal has been widely studied by domestic and foreign scholars for its advantages such as simple operation,environmental friendliness,and no secondary pollution.The type of electrode materials have a decisive effect on the electrochemical reduction efficiency of nitrate.Therefore,the preparation of high-performance electrode material is the key to the electrochemical removal of nitrate.In order to improve the removal efficiency of electrochemical reduction of nitrate and strive to achieve the harmless conversion of nitrate,the transition metal cobalt oxide and its modified electrodes with high catalytic performance were prepared and modified in this dissertation.,and the research on electrochemical reduction of nitrate was carried out.First,a carbon cloth-based nano-Co₃O₄/CC electrode was prepared by hydrothermal synthesis and high-temperature calcination.The microstructure of the electrode was characterized.The process parameters on nitrate removal such as reaction time,current density,initial NO₃^-concentration,and pH were studied which could obtain the optimal process conditions.Under the optimal experimental conditions,the removal efficiency of NO₃^-was 84%,and the generation efficiency of NH₄⁺-N was more than 50%.It could be considered that ammonia was the main product of electrochemical reduction of nitrate at the Co₃O₄/CC electrode.On this basis,in order to further improvethe performance of electrochemical reduction of nitrate,a low valence state Co₃O₄/CuO/CC-after-reduction electrode was prepared by doping CuO into nano-Co₃O₄ through the same preparation method,and assisting sodium borohydride reduction modification treatment.By adjusting the initial molar ratio of Co/Cu,a series of low valence composite electrodes were obtained.The crystal phase,morphology,chemical composition,valence state,specific surface area,and electrochemically active surface area of electrodes were characterized by SEM,XRD,XPS,nitrogen adsorption/desorption,and CV tests.During the electrochemical reaction,the NO₃^-removal efficiency,NO₂^-,NH₄⁺,N₂H₄ conversion efficiency and TN removal efficiency were measured,and the nitrate reduction performance of the prepared electrode was evaluated.Among the prepared electrodes,the low valence state Co₃O₄/CuO/CC-after-reduction electrode with cobalt/copper molar ratio of 1:1 had the best removal efficiency of NO₃^-at a current density of 20 mA cm^-2.The Co₃O₄/CuO/CC-after-reduction electrode electrochemical reduction experiments showed that the reduction of nitrate conforms to the first-order kinetic equation.The conversion products of nitrate in acidic environment were mainly organic nitrogen represented by hydrazine（N₂H₄）,while the reduction products of nitrate in alkaline environment were mainly ammonia.Analysis of the conversion mechanism of nitrate showed that the Co element and active hydrogen atoms could break the N-O bond,and the stepwise hydrogenation reaction reduced the nitrate to ammonia,while the Cu element only promoted the conversion of nitrate to nitrite.Removal of ammonia and nitrite from the reduction product mainly relied on active chloride produced by anodized Cl^-to oxidize ammonia to nitrogen,thereby achieving efficient nitrate removal.