Nitric and nitrous acid formation in plasma-treated water: Decisive role of nitrogen oxides (NO=1–3)
Jin Hee Bae, Hyungyu Lee, Seong-Cheol Huh, Sanghoo Park
Abstract
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 O 3 and NO 3 to NO 1,2 , leading to a predominance of nitrite in PTW. Specifically, the NO 3 -rich period was extended with increasing O 2 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. • Direct observation of the correlation between NO x = 1–3 and aqueous HNO y = 2,3 . • Dual time-synchronized optical absorption spectroscopy was suitable for this work. • Time-varying characteristics of the plasma reactor and plasma-treated water. • We demonstrate that ratio of HNO y = 2,3 can be controlled by changing O 2 content.