Investigation Of The Factors Influencing Performance Of (Bio)electrochemically Catalytic Reduction Of Nitrate

Considerable amounts of nitrogen（particularly existing in the form of nitrate）in the groundwater will cause eutrophication;when the groundwater is used as the drinking water,it will cause a serious health threat.Therefore,it is of great necessity to remove nitrogen from the groundwater.The electrochemical method adopted for nitrate removal has attracted much attention,because it overcomes the limitations of many physical and chemical methods,showing the potential to be utilized in the remediation of the real groundwater.The electrochemical denitrification mainly includes electrochemically catalytic denitrification and bioelectrochemically denitrification,with difference in the cathode catalyst（i.e.,electrocatalyst for the fomer and the bacteria for the latter）.Here,we focus on the investigation of some key factors affecting performance of（bio）electrochemically denitrication systems.These include the impact of electrode surface modification on the bacterial adhesion and thus on the efficiency of bioreduction of nitrate using the cathode as the electron donor,and the influence of the coexisting pollutant（e.g.,As）in the groundwater on the electrochemical reduction of nitrate and the exploration of feasibility of simultaneous removal of nitrate and As in one electrochemical reactor.Carbon-based cathodes are widely used in the biofilm-electrode reactor（BER）favoring a direct bioelectrochemical denitrification.However,little is known about how the surface chemistry of carbon affects the formation of bacterial biofilms on the electrode surface,which is fundamental to the functioning of BER for the nitrate removal.In the present study,graphite granules used in the three-dimension BER were oxidized by different methods,such as treatment with HCl,H₂O₂,HNO₃ and HNO₃+heat,providing the surface oxygen content as1.98,4.10,7.70 and 11.54 at%,respectively.Likewise,the denitrifying BER equipped with the HNO3+heat-treated cathode exhibited a reaction rate of 27.36 mg NO₃^--N L^-1 d^-1,exceeding 19.96,14.82 and 13.93 mg NO₃^--N L^-1 d^-1 obtained from the BERs with HNO₃,H₂O₂-and HCl-treated cathodes,respectively.It was revealed that the oxidation treatment led to the increase in the amounts of oxygen functional groups（largely in the form of carboxyl）,which were beneficial for the growth of metabolically active bacteria and thus the increased bioelectrochemical reaction kinetics.Bacterial analysis showed that the cathode-driven autotrophic denitrifiers were dominantly represented by Thiobacillus and Sulfurimonas,which displayed the highest abundance on the cathode pretreated with a hot and refluxed HNO₃solution.The coexisted pollutants in the groundwater should significantly affect the electrochemical denitrification,which deserves investigation.Another work was then conducted to study the presence of As on the removal efficiency of nitrate electroreduction,and the possibility of the simultaneous removal of nitrate and As in one electrochemical reactor.The comparisons of the nitrate reduction performance of several cathode materials indicate the optimal one is Cu-Zn electrode,which enabled a 90%removal efficiency after 5 h reaction.The addition of NaCl to the electrolyte can selectively transform nitrate to nitrogen,with the best performance achieved at 500 mg L^-1 NaCl.To alleviate the possible impact of the coexistence of As on electrochemical reduction of nitrate,a dual-anode one-cathode electrochemical reactor was established,in which nitrate electroreduction is coupled to As（Ⅲ）electrooxidation and electrocoagulation,and simultaneous removal of nitrate and As（Ⅲ）is realized.The experimental results showed that 50 mg L^-1 NO₃^--N and 1000μg L^-1 As（III）was almost completely disappeared after 150 min reaction.It was referred from the physiochemical characterizations that the main components of the solid precipitate are iron hydroxide and magnetite.These iron-containing minerals function as the adsorbents that remove As（III）and As（V）（product of As（III）electrooxidation）.The above findings may provide insights into the application of（bio）electrochemical methods for removing nitrate in the groundwater.