Formation of Nitric and Nitrous Acids in Plasma-Treated Water: Effects of Process Parameters

A renewed interest in plasma-assisted nitrogen fixation has highlighted the importance of plasma-treated water (PTW) as a tunable source of reactive nitrogen species. In particular, the formation of nitrous and nitric acids (HNO_y=2,3, respectively) in PTW depends largely on the behavior of gaseous nitrogen oxides (NO_x=1–3) produced in plasma. However, despite its importance, the time-dependent correlation among NO_x and HNO_y in plasma-water reactors remain elusive partly due to the lack of reliable measurements. Here, we present how water temperature and stirring speeds influence the formation of HNO_y=2,3 and their conjugate bases (NO_y⁻) in PTW generated by a surface dielectric barrier discharge. Using synchronized dual optical absorption spectroscopy, we monitored both gas and liquid species in real time and identified certain correlations between O₃/NO_x dynamics and the formation of NO₂⁻, HNO₂, and NO₃⁻ in water. Our results revealed that increasing water temperature raised humidity, which delayed the chemical transition from O₃- to NO_x-dominated plasma, resulting in greater NO₃⁻ accumulation and reduced formation of NO₂⁻/HNO₂. Optimized stirring improved gas–liquid transfer and reactive species dissolution. Nitrate concentration increased steadily with stirring speed, while nitrite peaked at 600 rpm, showing a 38% increase compared to static conditions. These findings offer insights into how process parameters can be adjusted to tailor PTW composition for specific applications.