Roles of the reaction boundary layer and long diffusion of stable reactive nitrogen species (RNS) in plasma-irradiated water as an oxidizing media — numerical simulation study

Abstract

The concentration distributions of chemically reactive species in water exposed to low-temperature atmospheric-pressure plasma (APP) have been studied with one-dimensional numerical simulations. Highly reactive species supplied from an APP to the water surface all react in the “reaction boundary layer,” i.e. a thin layer with a thickness of about 100 nm on the solution side of the interface, and are converted to stable species. This study quantitatively shows that, in the case of pure water irradiated by an APP, the simultaneous presence of H2O2, NO2 −, and O3 in the solution is the only cause to make it an oxidizing medium as they continuously produce ONOOH and HO3, which then decay to generate OH radicals in the solution bulk. ONOOH and its accompanying HO2NO2 have much longer diffusion lengths than HO3 and therefore their diffusion can also contribute to the oxidizing capability of the solution.