Investigation Of Electrochemical Reaction Characteristics And Mechanism Of Ammonia Nitrogen In Chloride-free Aqueous Media

Excessive ammonia nitrogen in water can cause serious harm to human body and ecological environment.Electrochemical method can effectively remove ammonia nitrogen in water.When chlorine ion is contained in water,a mmonia nitrogen removal is mainly attributed to indirect oxidation of available chlorine,while ammonia nitrogen is directly electrochemical oxidation which is often neglected.The main disadvantage of this process is the production of organic chlorides.Organic chloride by-products are produced in the chloride ion system.Therefore,the addition of chloride ions should be avoided during the electrochemical treatment of organic wastewater.At present,there is still a lack of understanding of the removal mechanism of ammonia nitrogen and electrochemical reaction products in electrolyte systems without chloride ions.Therefore,this paper built a two-dimensional electrode system without chlorine by inspecting different anode materials,different pH and gas-liquid phase product to investigate the mechanism of ammonia removal.In addition,the conversion of ammonia was further analyzed via cyclic voltammetry and Gibbs free energy.The main conclusions as below:（1）The polarity of the DC electric field plate affected the degradation rate of ammonia nitrogen in aqueous solution.Within 10 minutes,the degradation rate of the nearby anode was significantly larger than that the nearby cathode?s.As reaction time was prolonged,the ammonia nitrogen concentration in the system gradually decreased and closed to equilibrium after 40 minutes.（2）The degradation effect of anode material on ammonia nitrogen was affected by pH of the solution.The best initial pH of ammonia removal in the system with DSA,titanium mesh,lead dioxide as the anode were respectively 5,6,7.Regardless of different electrode,the ammonia removal rates of the different three anodes at the optimum initial pH were all higher than 80%after 40 minutes.（3）By analyzing the final conversion products of ammonia,we found that the final conversion products of ammonia were N₂,nitrate nitrogen and nitrite nitrogen in the DSA,titanium mesh and lead dioxide as anodes systems.There was also N ₂O in the final conversion product of ammonia in the DSA and titanium mesh as anodes system,but N₂O was not detected in the lead dioxide as anode system.Therefore,the same kind of wastewater was electrolyzed by different electrode materials,and the redox reactions occurred on the electrode surface and in the system were different.（4）By cyclic voltammetry we found that NH₄⁺and NH₃-N could not be directly oxidized on the surface of DSA electrode.NH₃-N could be directly oxidized on the surface of titanium mesh electrode,but NH₄⁺could not be directly oxidized on the surface of the titanium mesh electrode.NH₄⁺and NH₃-N could be directly oxidized on the surface of lead dioxide electrode.（5）Adding PCBA as a scavenger,the rate of ammonia removal was decreased significantly.It demonstrated that hydroxyl radicals played an important role in the electrochemical oxidation process.（6）According to the analysis of the Gibbs free energy of the compound in the process of ammonia nitrogen degradation,NH₂OH might be the intermediate product in the process of ammonia oxidation.