Study On Removal Of Nitrate Ions In Water By Electrochemical Method

In recent years,with the rapid development of industry and agriculture,nitrate pollution in water has become more and more serious.In order to solve this problem,several strategies have been proposed to remove nitrate from the contaminated water,including biological denitrification,physical-chemical method and catalytic reduction method.Among them,electrochemical reduction is considered as the most promising denitration method owing to its convenience,environmental friendliness and high efficiency.In this thesis,electrochemical reduction was used to remove nitrate from water.Firstly,the effect of cathode?Fe?Cu and Ti?and anode?Ti/IrO₂ and Ti/RuO₂?materials on the electrolysis process was studied.On this basis,the removal of high concentration of nitrate was investigated.At the same time,the electrochemical reduction of nitrate in simulated groundwater was also studied.In order to improve the stability of cathode,porous Fe-Si electrodes were introduced into the experiment.The following main research results were obtained:?1?Compared to Cu and Ti cathodes,Fe cathode exhibited much higher NO₃^--N and total nitrogen?TN?removal efficiency.At the same time,increasing the electrolysis current density and introduction of appropriate amount of NaCl into the system were beneficial to improving TN removal efficiency.When Ti/IrO₂ and Ti/RuO₂ were used as anodes in the system with initial NO₃^--N concentration of 100mg L^-1 and presence of 1.0 g L^-1 NaCl,the nitrate removal efficiency were 80.5%and74.8%,respectively.Compared to Ti/IrO₂,Ti/RuO₂ was more superior to the evolution of chlorine,and the hypochlorite would be produced after chlorine gas was dissolved in water,which helped to oxidize ammonia into nitrogen and improved the TN removal efficiency.So,when removing low concentration nitrate,the NO₃^--N removal efficiency was lower on Ti/RuO₂ anode than that on Ti/IrO₂ anode,but the concentration of byproduct ammonia was much higher than that on Ti/IrO₂ anode.In the study of nitrate removal in simulated groundwater,it was found that after electrolysis for 3.5 h,only 7.2 mg L^-11 NO₃^--N was left,which was lower than the drinking water standard??10 mg L^-11 NO₃^--N?and no other by-products were generated.?2?Based on the above results,we designed a stepwise anode:Ti/IrO₂?0-3.5h?-Ti/RuO₂?3.5-7h?to efficiently remove high concentration of nitrate.Ti/IrO₂ was selected as the anode in the early stage of electrolysis,which owned higher conversion efficiency of nitrate and Ti/RuO₂ was selected as the anode in the later stage,which possessed high chlorine evolution activity.It was shown that with initial NO₃^--N concentration of 500 mg L^-1,the TN removal efficiency was improved and reached 85%when Ti/IrO₂?0-3.5h?-Ti/RuO₂?3.5-7h?was used as a stepwise anode.To improve the removal efficiency of nitrate ions per unit time,the volume of the solution increased by 4 times,i.e.,800 mL.Results showed that increasing the number of electrode groups could efficiently improve the nitrate reduction efficiency.When the number of electrode groups increased to 4 and the initial NO₃^--N concentration was 100 mg L^-1,the TN removal efficiency was 80%.?3?Furthermore,we proposed to remove nitrate efficiently by porous Fe cathode.Results showed that increasing the current density is beneficial to improving the NO₃^--N and TN removal efficiencies.When porous Fe was used as the cathode and electrolysis at 40 mA cm^-2,both of the NO₃^--N and TN removal efficiencies were94.3%and no by-product was generated.However,the porous Fe was corroded seriously and the Fe concentration in the resulted solution was as high as 1418 mg L^-1.In order to improve the stability of the cathode,a porous Fe-Si alloy with good stability in solution was used as the cathode.Result showed that both of the NO₃^--N and TN removal efficiencies reduced with the increasing of silicon content in the porous Fe-Si alloy.But the stability of the electrode was dramatically enhanced.Specifically,both of the NO₃^--N and TN removal efficiencies were 78.8%when Fe-Si50 was used as the cathode.At this time,the Fe concentration in the solution was only 41 mg L^-1.