Ammonia removal in electrochemical oxidation: Mechanism and pseudo-kinetics

This paper investigated the mechanism and pseudo-kinetics for removal of ammonia by electrochemical oxidation with RuO2/Ti anode using batch tests. The results show that the ammonia oxidation rates resulted from direct oxidation at electrode-liquid interfaces of the anode by stepwise dehydrogenation, and from indirect oxidation by hydroxyl radicals were so slow that their contribution to ammonia removal was negligible under the condition with Cl−. The oxidation rates of ammonia ranged from 1.0 to 12.3 mg N L−1 h−1 and efficiency reached nearly 100%, primarily due to the indirect oxidation of HOCl, and followed pseudo zero-order kinetics in electrochemical oxidation with Cl−. About 88% ammonia was removed from the solution. The removed one was subsequently found in the form of N2 in the produced gas. The rate at which Cl− lost electrons at the anode was a major factor in the overall ammonia oxidation. Current density and Cl− concentration affected the constant of the pseudo zero-order kinetics, expressed by k = 0.0024[Cl−] × j. The ammonia was reduced to less than 0.5 mg N L−1 after 2 h of electrochemical oxidation for the effluent from aerobic or anaerobic reactors which treated municipal wastewater. This result was in line with the strict discharge requirements.