Study On Effective Reduction Of Nitrate To Nitrogen By Electrochemical Corrosion Cooperated With Photo Catalytic Of Iron/copper-based Materials

Nitrate（NO₃^--N）pollution is a fairly common and important global environmental problem which can cause serious effect on human health and ecosystems.Among the many denitrification technologies,Fe~0 is widely favored due to its wide sources,high removal efficiency and fast reaction rate of NO₃^--N.However,there are two main problems of this technology:one is that Fe oxide passivation layer will be formed during the reaction process and will reduce the reaction rate;the other is that the final reduction product is mainly NH₄⁺-N,which is secondary pollutant to the environment.To solve the above problems,this paper proposes a new method for the electrochemical corrosion coupling with photocatalysis of iron-copper composites to efficiently and selectively reduce NO₃^--N to N₂:firstly,by regulating the combination of Fe~0 and Cuto couple with photogenerated electrons,NO₃^--N can be efficiently reduced to NO₂^--N,and then the generated Fe³⁺is used to promote the photolysis of small molecular organic acids to generate CO₂^·-,which has strong reducibility,and the formed NO₂^--N intermediate product is further efficiently reduced to N₂.In this method,suppressing the overreduction of NO₂^--N and improving the yield of CO₂^·-is the most important.For this reason,two iron-copper composites,Cu~0@Fe~0-CuFe₂O_4-x with oxygen vacancies and Fe~0-Cu~0-CuFe₂O₄ grown in situ,are designed in this paper.By inhibiting the adsorption and reduction of NO₂^--N by Fe~0,and h⁺_vb inhibit the reduction of NO₂^--N by Fe²⁺,to realize the efficient reduction of NO₃^--N to NO₂^--N;the Fe³⁺produced by the corrosion of Fe~0 is complexed with small molecular carboxylic acids,and the ligand-to-metal charge transfer（LMCT）increases the yield of CO₂^·-.The structure of the materials is characterized by means of SEM,TEM,XRD,XPS,etc.,and the optical properties of the materials are tested by UV-Vis DRS,Mott-Schottky,EIS and transient photocurrent.The effects of material preparation conditions and reaction conditions on NO₃^--N reduction are investigated,and the reaction mechanism of iron-copper composites for NO₃^--N reduction is discussed through control experiments,free radical tests and free radical quenching experiments.Finally,two novel NO₃^--N reduction systems(Cu~0@Fe~0-CuFe₂O_4-x/H₂O₂/OA/UV and Fe~0-Cu~0-CuFe₂O₄/CA/UV)are successfully constructed,achieving high efficiently and high selectivity reduce NO₃^--N to N₂.The conclusions of the study are as follows:（1）The photolysis experiments of NO₃^--N and NO₂^--N in the presence of oxalic acid（OA）show that CO₂^·-reduces NO₂^--N with high efficiency and N₂ selectivity.The Fe~0-Cu~0 bimetallic combine with CuFe₂O₄ photocatalysis achieves 89.2%removal of NO₃^--N,and accumulative concentration of NH₄⁺-N and NO₂^--N are 17.3 mg N/L and1.9 mg N/L,respectively,and the N₂ selectivity is only 28.2%.（2）The Cu~0@Fe~0-CuFe₂O_4-x with oxygen vacancies is prepared by modification of CuFe₂O₄ by KBH₄ and CuSO₄.A series of characterizations and tests prove that it has good structure and photocatalytic performance.Cu~0@Fe~0-CuFe₂O_4-x coupled with CO₂^·-generated by photocatalytic with OA and H₂O₂ is used to reduce NO₃^--N.The reaction process is as follows:NO₃^--N is first reduced by the Cu~0@Fe~0-CuFe₂O_4-x/UV system,and the products are mainly NO₂^--N and N₂,the effluent mainly contains NO₂^--N and Fe³⁺ions.In the second step,the effluent/OA/H₂O₂/UV system is constructed by adding OA and H₂O₂.The·OH produced by the Fenton reaction and the photochemical process are used to decompose OA to generate CO₂^·-,the intermediate NO₂^--N is further selectively reduced to N₂.At the conditions of initial pH of 6,Cu~0@Fe~0-CuFe₂O_4-x dosage of 4 g/L in the first step,and OA dosage of 4 mmol/L,initial pH of 2.7 in the second step,Cu~0@Fe~0-CuFe₂O_4-x/H₂O₂/OA/UV system achieves 90.7%NO₃^--N removal and 83.0%N₂ selectivity.（3）Fe~0-Cu~0-CuFe₂O₄ material was prepared by in situ growth method.A series of characterizations and tests prove that Fe~0-Cu~0-CuFe₂O₄ has a unique and excellent structure,good energy band structure and photocatalytic performance.Fe~0-Cu~0-CuFe₂O₄is used to reduce NO₃^--N in coupling with citric acid（CA）and UV to generate CO₂^·-.The reaction process is as follows:NO₃^--N is first reduced to NO₂^--N and N₂ by Fe~0-Cu~0-CuFe₂O₄/UV system,the effluent contains NO₂^--N and Fe³⁺ions.In the second step,the Fe³⁺/CA/UV system is constructed by adding CA,and the CO₂^·-generated by the photochemical process is used to further efficiently and selectively reduce NO₂^--N to N₂.At the conditions of initial pH of 4,Fe~0-Cu~0-CuFe₂O₄ dosage of 5 g/L in the first step,and CA dosage of 2 mmol/L,initial pH of 2.8 in the second step,Fe~0-Cu~0-CuFe₂O₄/CA/UV system achieves 100%NO₃^--N removal and 96.3%N₂ selectivity.The system is applied to the treatment of NO₃^--N in actual wastewater.In the polluted river water and the secondary effluent of municipal wastewater,NO₃^--N is completely reduced in the first step,and NO₂^--N was removed in the second step,with the efficiency of 100%and 80.2%,respectively.This paper innovatively proposes a new method for the efficient and selective reduction of NO₃^--N in water to N₂ by electrochemical corrosion coupling with photocatalysis of iron-copper composite.It has the advantages of a wide range of material sources and a simple material preparation process.It solves the problems of traditional chemical denitrification methods,such as low N₂ selectivity,deactivation of reducing agents or catalysts,and secondary pollution,etc.,and achieves the goal of efficiently and selectively reducing NO₃^--N in water to N₂.The research results provide a new idea for the removal of NO₃^--N in water.