The Role of HNO₂ in the Generation of Plasma-Activated Water by Air Transient Spark Discharge

• Main Authors: Mário Janda, Karol Hensel, Peter Tóth, Mostafa E. Hassan, Zdenko Machala
• Format: Article
• Language: English
• Published: MDPI AG 2021-07-01
• Series: Applied Sciences
• Subjects: non-thermal plasma; transient spark; electrospray; plasma-activated water; nitrous acid; nitrites
• Online Access: https://www.mdpi.com/2076-3417/11/15/7053

Transient spark (TS), a DC-driven self-pulsing discharge generating a highly reactive atmospheric pressure air plasma, was employed as a rich source of NOx. In dry air, TS generates high concentrations of NO and NO₂, increasing approximately linearly with increasing input energy density (<i>E_d</i>), reaching 1200 and 180 ppm of NO and NO₂, at <i>E_d</i> = 400 J/L, respectively. In humid air, the concentration of NO₂ decreased down to 120 ppm in favor of HNO₂ that reached approximately 100 ppm at <i>E_d</i> = 400 J/L. The advantage of TS is its capability of simultaneous generation of the plasma and the formation of microdroplets by the electrospray (ES) of water directly inside the discharge zone. The TS discharge can thus efficiently generate plasma-activated water (PAW) with high concentration of H₂O₂⁻_(aq), NO₂⁻_(aq) and NO₃⁻_(aq), because water microdroplets significantly increase the plasma-liquid interaction interface. This enables a fast transfer of species such as NO, NO₂, HNO₂ from the gas into water. In this study, we compare TS with water ES in a one stage system and TS operated in dry or humid air followed by water ES in a two-stage system, and show that gaseous HNO_2, rather than NO or NO₂, plays a major role in the formation of NO₂⁻_(aq) in PAW that reached the concentration up to 2.7 mM.