Comparative study of discharge schemes for production rates and ratios of reactive oxygen and nitrogen species in plasma activated water

Abstract

Plasma activated water (PAW) containing reactive oxygen and nitrogen species (RONS) is of great interest for applications in the agricultural and food industries, where processing methods with large capacities and high energy-yields are required. In this work, we studied the differences between seven types of discharge schemes in terms of the production rates and concentration ratios of RONS using deionized water and tap water in comparison. We used N 2 and air as the feed gas with a variable admixture of water vapour. When O 2 was incorporated into the reaction system, the major products in the PAW became NO − 2 and NO − 3 , while small amounts of H 2 O 2 and NH + 4 were detected only in O 2 poor situations with water vapour. Of the major products, the condition of whether NO − 2 or NO − 3 becomes predominant is dependent on the extent of successive oxidation reactions from NO to NO 2 and NO 3 and the competing rates between gas phase reactions and dissolutions into water. In our tested discharge schemes, those with volumetric glow-like discharge structures produced NO − 3 rich PAW, while those with spark discharge structures over the water surface or in water were favourable for the production of NO − 2 rich PAW. In particular, a discharge scheme with a wire-in-nozzle structure operated in a spark-mode with a bubbling gas flow yielded PAW with the highest NO − 2 concentration, more than 70%, at a high energy-yield of 2 g kWh −1 . In the storage period, NO − 2 was converted into NO − 3 due to ionic reactions in aqueous solution, but the buffer action of tap water was observed to suppress the conversion for a fairly long period.Keywords: plasma activated water, reactive oxygen and nitrogen species, NO − 2 , NO − 3 , H 2 O 2 , NH + 4 , energy-yield

Plasma‐activated water: Physicochemical properties, microbial inactivation mechanisms, factors influencing antimicrobial effectiveness, and applications in the food industry

“…However, after a certain treatment period, the pH reaches a steady state and remains constant. The decrease in the pH and formation of acidified compounds vary from plasma source (Lu et al., 2017), discharge schemes (Tachibana & Nakamura, 2019), and feeding gases used (Takamatsu et al., 2012). The change in pH has also found to be dependent on the polarity of the applied voltage, where negative discharge resulted in a greater decrease in the pH of the solution than positive discharge, indicating a significant role of ions produced (Lukes, Locke, & Brisset, 2012).…”

Plasma agriculture: Review from the perspective of the plant and its ecosystem

“…[48] Water is introduced in three ways: as part of the feed gas, in contact with the plasma, or exposed to an effluent. Tachibana et al [49] demonstrated preferential production of NO and reactive species are produced in direct contact with water. They showed that the production of NO x could be controlled by choosing between a glow or spark discharge.…”

“…When relatively neutral, the end product concentrations show initial decay, but remain stable over time. [49][50][51][52] Although tap water does act as a buffer, prolonged treatment may result in a decreased pH, similar to DI water. In cases of greater acidity (pH < 3.5), NO 2 − is not stable.…”

“…In cases of greater acidity (pH < 3.5), NO 2 − is not stable. [49,52,53] When NO 2 − concentration is on the same order of magnitude of NO 3 − , NO 3 − concentration increases rapidly by [49] :…”

“…When both NO 2 − and H 2 O 2 are present, peroxynitrite reactions must also be considered [49,51,53] NO…”