Oxidation processes of NO for production of reactive nitrogen species in plasma activated water

To understand the mechanisms for selective productions of NO(2)(-)and NO(3)(-)as reactive nitrogen species (RNS) in plasma activated water, we studied the oxidation reactions of NO as the primary species supposing O, O-2, O-3, OH, and H(2)O(2)as possible oxidants. A coaxial-type dielectric barrier discharge reactor was employed in cascade connection to a reservoir and/or a water bubbler. First, the densities of NO and NO(2)in the gas phase were measured with gas sensors in two gas systems: N(2)admixed with water vapor and N(2)admixed with O-2, for comparison. It was found that while the density of NO in the latter case was one third of that in the former case owing to the difference in the first-step oxidation to NO(2)or HNO2, the total yields of RNS in both cases were comparable. To investigate the oxidation pathways, admixing NO and NO(2)in the downflow of the reactor was attempted to estimate their reactions with those oxidants and accomodation behaviours on the water surface. Injecting the effluent gas from discharge plasma into solutions of O-2, O-3, and H2O2, and injecting O(3)into PAW, vice versa, were attempted to inspect the reactions in the liquid phase or at the gas-water interface. Based on the obtained results, it was found that, in the N-2-H2O gas system, OH functions as the oxidant of NO to HNO2, exclusively yielding NO(2)(-)in solution with no effective second-step oxidation to NO(3)or HNO3. On the other hand, in the N-2-O(2)gas system, O(3)functions as the major oxidant in the successive oxidation from NO to NO(2)to NO3, producing NO(3)(-)predominant PAW; however, the second-step oxidation in the gas phase is competing with the oxidation reactions in the liquid phase.