Study On The Application Of Coupled Ion Exchange And Electrolysis To Enhance Catalytic Reduction Of Low Concentration Nitrate And Recovery Of Ammonia

With the development of urbanization,nitrogen imbalance becomes more and more serious.As inorganic forms of nitrogen,ammonia nitrogen（NH₄⁺）and nitrate（NO₃^-）in water are the one of the main pollutants,which can cause eutrophication and harm human body,animals and plants.At the same time,a large amount of ammonia nitrogen is produced globally every year through the Haber Bosch process to meet the survival development needs of humans,consuming 1-2%of the global energy supply every year while causing 1.5%of the global carbon emissions.Compared with Haber-Bosch process,electrochemical process has the advantages of simple operation and less input of chemical reagents.Therefore,designing a process that can remove nitrogen pollutants and recover ammonia has attracted wide attention.However,the concentration of NO₃^--N and NH₄⁺-N in realistic N-containing wastewater tend to be low,and existing processes cannot achieve efficient operation.The purpose of this study is to develop a process with high efficiency and low energy consumption to efficient remove pollutants from low concentration N-containing wastewater and simultaneously recover ammonia as value-added products.An energy-efficient and reagent-free system（IE-BMED-MC system）consisting of ion exchange,bipolar membrane electrodialysis and membrane contactor was designed to recover ammonia from low NH₄⁺-N concentration wastewater.In this system,ion exchange resin was used to concentrate low concentration ammonia in wastewater,and the concentration of ammonia in treated water was less than 5 mg N·L^-1.Then the saturated ion exchange resin was regenerated by the acidic regenerant generated in the bipolar membrane electrodialysis unit.After the regenerant returned to the acid chamber,ammonia nitrogen migrated to the cathode chamber under the action of electric migration and concentration diffusion.In the alkaline cathode chamber,ammonia nitrogen was converted from an ionic state to a gaseous state and selectively recovered across the membrane in the membrane contactor.Under the optimal current density of2 m A·cm^-2,the removal efficiency,regeneration efficiency and recovery efficiency obtained from the IE-BMED-MC system were 97%,92%and 71%,respectively,and the nitrogen flux across the cation exchange membrane and energy consumption were177 g-N·m^-2·d^-1 and 8.5 k Wh·kg^-1-N,which were significantly better than those obtained from the ED-MC system without ion exchange process and the IE-ED-MC system without bipolar membrane.The energy consumption was further reduced to 6k Wh·kg^-1-N in continuous experiments with stable removal efficiency of about 90%over six cycles,and the NH₄⁺-N concentration of the ion exchange column effluent was below the national discharge standard.Compared with other electrochemical ammonia recovery systems,the IE-BMED-MC system achieved efficient recovery of ammonia from low NH₄⁺-N concentration wastewater with low energy consumption.The cost for recovering ammonia by IE-BMED-MC system was about$0.425 per kg-N,which was much lower than the cost of traditional biological nitrification and denitrification processes and even the Haber-Bosch process.An efficient and low energy consumption system（IE-EC-MC system）consisting of ion exchange process,electrochemical reduction unit and membrane contactor to efficient nitrate electro-catalytic reduction from low NO₃^--N concentration wastewater and synchronously recover ammonia product was developed.First,the Fe₂O_3-xelectrode was prepared by a simple heat-treat and electric reconstruction method.The results of LSV,CV and the electrocatalytic experiment showed that the Fe₂O_3-xelectrode had high activity.At the optimum current density of 20 m A·cm^-2,IE-EC-MC system achieved 98%removal efficiency,81%recovery efficiency,53%Faraday efficiency and 0.67 mg-N·cm^-2·h^-1 ammonia production rate.It is worth noting that the energy consumption of the IE-EC-MC system was only 19.78 k Wh·kg^-1（NH₄）₂SO₄,which was only one percent of the EC-MC system without ion exchange process.In five cycles,the removal efficiency and recovery efficiency maintained at over 95%,80%.Meanwhile,the NO₃^--N concentration of the ion exchange column effluent was below the national discharge standard.In summary,the IE-BMED-MC system was green,low energy consuming and efficient in recovering low concentration ammonia while guaranteeing good removal efficiency,showing obvious advantages over other techniques.The IE-EC-MC system lowered the catalyst threshold,enabled efficient electrocatalytic reduction of low concentrations of nitrate and recovery of the generated ammonia.