Construction Of Efficient Electrocatalytic/photochemical System For Nitrate Reduction And Study On The Reduction Mechanism

Atmospheric nitrogen deposition and excessive use of nitrogen fertilizer are easy to cause the pollution of nitrate（NO₃^-）,posing a serious threat to the ecological environment and human health.The removal efficiency of NO₃^-is slow by the conventional water treatment methods due to its high solubility and ideal water stability,and harmful by-products such as nitrite（NO₂^-）and ammonia nitrogen（NH₃/NH₄⁺）are easily produced in the reduction process,resulting in the secondary pollution.Therefore,how to achieve efficient removal of NO₃^-while improving the selectivity of N₂ belong to a difficult problem to solve NO₃^-pollution.Electrocatalytic and photochemical technologies have the characteristics of high efficiency of water purification and mild operating conditions,and it can reduce NO₃^-to N₂ based on the role of electron（e^-）during the reaction process.There are usually two reduction mechanisms in the electrocatalytic reduction of NO₃^-,which are direct e^-reduction and indirect atomic hydrogen（H*）reduction.The reduction efficiency and selectivity are significantly affected by the reduction mechanisms,and the electrode materials act as the key factor to determine the dominant mechanism in the reduction process.Therefore,the NO₃^-reduction efficiency and N₂ selectivity can be improved by regulating the properties of the electrode materials.In addition to e^-and H*,other high-energy electrons,such as hydrated electron(e_aq^-),can also act on NO₃^-reduction theoretically.However,at present,the deep study and detailed discussion still remain devoid in relation to the NO₃^-reduction process,N₂ selectivity and mechanism under the action of e_aq^-.Focusing on the above key issues,this paper has carried out the following works.1.Single-atom Fe-supported nitrogen doped-ordered carbon spheres（Fe-NMCS）was designed using Fe as the active center,and applied to manage the N₂ selectivity in the electrochemical reduction of NO₃^-.And the reaction mechanism was also studied.The results showed that Fe_0.7%-NMCS electrode with Fe content of 0.7%exhibited the excellent efficiency of NO₃^-reduction,and the removal rate of 50 mg N·L^-1 NO₃^-reached 98%with the selectivity of N₂ of 80%.The effect of the Fe forms in Fe_x-NMCS on NO₃^-removal efficiency and N₂ selectivity were further compared and analyzed.The results showed that the Fe forms in Fe_x-NMCS gradually changes from single-atom Fe to nanoparticle with the increase of Fe incorporation,and single-atom Fe is more favorable to regulating NO₃^-reduction to N₂.Fe coordinated with N to form pyrrole-type Fe N₃ sites in Fe_0.7%-NMCS,and the Fe N₃ sites can conduct the reduction process of NO₃^-to N₂with NO₂^-,NO and N₂O as intermediate products.Investigating the free-energy diagram of the reaction pathways of NO₃^-to NH₃ and NO₃^-to N₂,the thermodynamically more favorable NO₃^-to N₂ process on Fe N₃ sites than NO₃^-to NH₃.In addition,the mesoporous channels in Fe_0.7%-NMCS can limiting the diffusion of products and providing favorable conditions for the selective control step of NO₃^-reduction to N₂.2.In order to further improve the electrocatalytic reduction efficiency of NO₃^-and to have a comprehensive understanding of the reduction mechanism with Fe as the active sites,the 3D ordered macroporous-microporous Fe-NC（3DOM Fe-NC）with large specific surface area was obtained by using 3D polystyrene spheres as the hard templates and 3D Fe-doped ZIF-8 as the precursor after calcination at high temperature.The experimental results indicated that the 3DOM Fe-NC electrode material presented a high NO₃^-reduction efficiency,and 50 mg N·L^-1 NO₃^-can be completely removed within 6 h at the reaction potential of-1.3 V vs.SCE.Besides,NO₃^-removal rate was not significantly affected and NH₃/NH₄⁺was no longer detected with adding 10mmol·L^-1 Cl^-in the solution.More importantly,the NO₃^-removal rate and N₂ selectivity remained stable after 6 times cycle experiments.The continuous flow device was applied to tackle NO₃^-in the actual wastewater via setting the hydraulic retention time of 12 h,and the effluent concentration of NO₃^-was always maintained below 10 mg N·L^-1.Integrating the experimental results with characterization results,it was proved that the pore structure of 3DOM Fe-NC endows with a large reaction surface,which promotes the electrolyte migration and the contact between reactants and active sites.Fe site reconstruction occurred during the reaction process,and the green rust generated as a new active site,which greatly improved the reduction efficiency of NO₃^-.The results of tert-butanol quenching experiments,electron spin resonance experiments and in situ Ramans tests showed that the process of electrocatalytic NO₃^-reduction is controlled by direct electron transfer,H*and active intermediate green rust.3.Based on the direct e^-transfer and H*mediated NO₃^-reduction processes mentioned in the previous work,a novel e_aq^- conducted NO₃^-reduction process was proposed in this work.Considering the advantage of the strong reducibility of e_aq^- and its high reaction rates with nitrogen-containing compounds,we found that the e_aq^-produced by UV irradiation sulfite could be used for the effective reduction of NO₃^-.In this section,NO₃^-removal rate and the N₂ selectivity were greatly elevated in the presence of e_aq^-.2 mmol·L^-1 NO₃^-was completely reduced to N₂ within 90 min,and no NH₃/NH₄⁺was detected during the reaction process.By laser flash technology and quenching tests,e_aq^- was identified as the mainly reactive species for NO₃^-reduction in the UV/sulfite system,and the NO₃^-reduction process is a single electron multi-step process.¹⁵N-isotopic experimental results confirmed that N₂ is the only end product of NO₃^-reduction.The high N₂ selectivity of UV/sulfite system benefited from the high dispersion of e_aq^- in water and the rapid reaction rate between e_aq^- and the N₂O.By means of theoretical calculation,we concluded that the process of NO₃^-reductio to N₂by e_aq^- is an exothermic process in thermodynamics,which is easy to occur.Finally,the UV/sulfite system showed excellent removal efficiency in the treatment of nickel-plating wastewater containing NO₃^-,and demonstrating the potential for practical application.