Nitric and nitrous acid formation in plasma-treated water: Decisive role of nitrogen oxides (NO_x=1-3)

Nitrogen fixation using low-temperature plasma, particularly in relation to plasma-treated water (PTW) and its chemical and physical properties, has received a renewed research focus. Dissolving highly concentrated nitrogen oxides (NO_x = 1-3) generated by air discharge into water results in the formation of two aqueous oxiacids (nitrous and nitric acids; HNO_y = 2,3) and their conjugates (nitrate and nitrite ions; NO_y^-). Nonlinear formation of these species in PTW with respect to plasma conditions has been observed; however, the significance of the time-varying NO_x on this nonlinearity has not yet been thoroughly investigated. Here, we demonstrate real-time observations of HNO_y/NO_y^- as well as NO_x production in a surface dielectric barrier discharge reactor containing distilled water. Synchronized two optical absorption spectroscopy systems were employed to simultaneously measure gas-phase NO_x and liquid-phase HNO_y/NO_y^- in the plasma reactor operated under different oxygen contents of 5, 20, and 50%. Our results showed that reducing the oxygen content in the reactor accelerated the chemical transition from O₃ and NO₃ to NO_1,2, leading to a predominance of nitrite in PTW. Specifically, the NO₃-rich period was extended with increasing O₂ content, resulting in the production of nitrate-dominant PTW at low pH levels. Our findings highlight the potential for the selective generation of HNO_y/NO_y^- in PTW through the active and passive control of NO_x in a plasma reactor. The direct, real-time observation of NO_x-HNO_y/NO_y^- conversion presented here has potential for improving the control and optimization of PTW, thereby enhancing its applicability.