Active and Passive Control of Nitrous and Nitric Acids Ratio in Plasma-Treated Water

A (re)growing interest in plasma-assisted nitrogen fixation is opening a new era in plasma science and technology, particularly in relation to plasma-water interaction and its impact on the chemical and physical properties of water. It is well known that the absorption of NO_x=1–3 gases in water provides industrially important products nitrous and nitric acids (HNO_y=2,3). Namely, plasma-treated water (PTW) is strongly affected by NO_x gases produced in plasma reactors. Therefore, to better control and utilize PTW, understanding the time-dependent production of NO_x and HNO_y is necessary, yet it is unclear. In this study, we conducted real-time observations of HNO_y/NO_y^- and NO_x formation in a surface dielectric barrier discharge reactor with deionized water. Two optical absorption spectroscopy systems simultaneously measured NO_x in gas and HNO_y/NO_y^- in PTW at different plasma driving frequencies (ranging from 30–50 kHz) and oxygen contents of 5–50%. Our findings reveal that reducing oxygen content in the reactor accelerates the chemical mode transition from NO₃ to NO_1,2, resulting in a predominance of nitrite in a PTW. Furthermore, as the driving frequency decreases, the NO₃-rich period becomes longer, leading to a nitrate-dominated PTW with low pH levels. These results demonstrate the selective production of HNO_y/NO_y^- in PTW through control of NO_x in a plasma reactor. We believe that these results hold promise for expanding our knowledge of PTW and thereby advancing plasma-assisted nitrogen fixation.