The role of O <sub>3</sub> on the selective formation of nitrate and nitrite in plasma-treated water
Yung-Hsin Teng, Yen‐Ting Lin, Wei-Hsiang Wang, Ying‐Hao Liao
Abstract
Abstract This study is to investigate the role of O 3 on NO and NO 2 oxidations to selective formations of NO 2 − and NO 3 − 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 N 2 and O 2 . Results of surface DBD show that O 3 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 O 3 decrease. Formations of NO 2 − and NO 3 − are associated with gas-phase nitrogen oxides while the presence of O 3 has a critical influence on a selective formation of NO 2 − and NO 3 − . The strong oxidation of O 3 favors the formation of NO 3 − while inhibits NO 2 − formation. O 3 serving as an inhibitor of NO 2 − is confirmed with the DBD jet experiment where O 3 is admixed in the downstream of the post discharge. O 3 enhancement on NO 3 − formation is demonstrated by exposing water to a synthetic NO gas with and without O 3 . In addition to O 3 , the presence of water vapor in the working gas, presumably facilitating formation of HNO 2 and HNO 3 , greatly promotes NO 2 − and NO 3 − concentrations in solution. This is thought to be due to the stronger dissolution susceptibility and the relatively larger rate constant of HNO 2 and HNO 3 than those of N 2 O 3 and N 2 O 5 . Finally, based on observations obtained here, a schematic roadmap of selective NO 2 − and NO 3 − formations is presented.