Reactive oxygen and nitrogen species (RONS) solubility controlled activation of water by atmospheric pressure air spark discharge

Abstract The activation of water by the atmospheric pressure air plasma is involved in the diffusion of reactive oxygen and nitrogen species (RONS) in air and water, their gas-phase and liquid-phase reactions, and their dissolution and evaporation. In this study, by generating the air spark discharge over the surface of water, we have evaluated the chemical and biological reactivities of direct–plasma treatment (DPT) and remote–plasma treatment (RPT) plasma-activated water (PAW) at different water temperatures. We have found that DPT-PAW is much more effective in increasing both the chemical and biological reactivities of PAW than RPT-PAW, and decreasing the water temperature from 40°C to 6°C leads to rapid activation of water. Our analysis shows that when the water temperature varies from 6°C to 40°C, the activation of water by the air discharge is RONS solubility controlled, and the gas-phase and liquid-phase RONS diffusion and chemical reactions are not the controlling steps during the activation of water. The direct plasma treatment of water at a relatively low temperature contributes to an obvious increase in the RONS solubility, thus a rapid activation of DPT-PAW.