Efficiency And Mechanism Of Reductive Denitrification Of Nitrate By The UV/Sulfite Process

Nitrate can cause eutrophication in aquatic environment and also endanger public health, necessitating its efficient and advanced removal. Arising from its stable reactivity, nitrate cannot efficiently treated by conventional methods, while the current reduction treatments are still challenged by the low denitrification efficiency and significant formation of toxic ammonia.. In order to overcome the drawbacks of current denitrification techniques this study proposed a novel reduction technology, UV/sulfite process, which is based on the highly reducing reactivity of hydrated electron (eaq-).Using nitrate as the target compound, its degradation efficiency and kinetics were studied, and various factors including pH, temperature, sulfite concentration, light intensity, and water matrix, were investigated in UV/sulfite process. The degradation kinetics of nitrate were enhanced by increasing pH, and were also enhanced proportionally by increasing sulfite concentration and light intensity. Temperature showed positive effect on the kinetics, with apparent activation energy of about 31.5 kJ·mol-1. Typical ions carbonate/bicarbonate, chloride, and phosphate affected the kinetics only slightly, but ferrous ion and humic acid showed significant inhibitory effect.The denitrification mechanism was explored by qualitative identification of reactive radicals involved and the generation and transformation of the intermediate products. The almost complete inhabitation of nitrate removal by nitrous oxide indeed supports that the reductive radicals (eaq-、H) were responsible for the denitrification in the system. The dominant role of eaq- in reduction of nitrate was further proved by the competition kinetic study using MCAA. The pH dependence of reactive radicals transformation could also confirm that eaq’generated during the photolysis of SO32-induced the reduction of nitrate. Products analysis indicates that no ammonia was formed.The degradation kinetic model of nitrate was developed using the steady-state assumption for reactive species involved in the system. It was built based on the generation and transformation of reactive radicals. The models predicted well the degradation process of nitrate under various conditions, including pH and surface water matrix components. For practical water like surface water and electroplating water, UV/sulfite process also showed high efficiency in nitrate removal.In summary, UV/sulfite process shows the high efficiency for nitrite denitrification but without formation of ammonia, and this process will provide a promising alternative to the harmless denitrification methods.