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 N2 and air as the feed gas with a variable admixture of water vapour. When O2 was incorporated into the reaction system, the major products in the PAW became and , while small amounts of H2O2 and were detected only in O2 poor situations with water vapour. Of the major products, the condition of whether or becomes predominant is dependent on the extent of successive oxidation reactions from NO to NO2 and NO3 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 rich PAW, while those with spark discharge structures over the water surface or in water were favourable for the production of 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 concentration, more than 70%, at a high energy-yield of 2 g kWh−1. In the storage period, was converted into 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.