The role of O3 on the selective formation of nitrate and nitrite in plasma-treated water

Abstract

This study is to investigate the role of O3 on NO and NO2 oxidations to selective formations of NO2 − and NO3 − in plasma-treated water. Two plasma reactors, a surface dielectric-barrier discharge (DBD) and a coaxial DBD jet, are employed to water treatment with a working gas primarily consisting of N2 and O2. Results of surface DBD show that O3 first is increased with plasma treatment time, reaching a maximum, and then decreased. NO takes time to grow and the grow of NO is accompanied by O3 decrease. Formations of NO2 − and NO3 − are associated with gas-phase nitrogen oxides while the presence of O3 has a critical influence on a selective formation of NO2 − and NO3 −. The strong oxidation of O3 favors the formation of NO3 − while inhibits NO2 − formation. O3 serving as an inhibitor of NO2 − is confirmed with the DBD jet experiment where O3 is admixed in the downstream of the post discharge. O3 enhancement on NO3 − formation is demonstrated by exposing water to a synthetic NO gas with and without O3. In addition to O3, the presence of water vapor in the working gas, presumably facilitating formation of HNO2 and HNO3, greatly promotes NO2 − and NO3 − concentrations in solution. This is thought to be due to the stronger dissolution susceptibility and the relatively larger rate constant of HNO2 and HNO3 than those of N2O3 and N2O5. Finally, based on observations obtained here, a schematic roadmap of selective NO2 − and NO3 − formations is presented.