Transportation behaviour of OH and H2O2 in plasma-treated water

The transportation of plasma-generated species through a liquid environment is a key step within the plasma-driven biocatalysis process, but is also of great importance for other systems with plasma-liquid interfaces. The aim of this study is to explore the transportation processes and lifetime of plasma-generated species in an aqueous solution. Therefore, a combination of experimental methods, reactive molecular dynamics (MD) simulations, and reaction-diffusion modelling was used. Experimentally, an atmospheric pressure plasma jet was used to treat an aqueous sample. Convective transport was visualized by particle image velocimetry in the plasma-treated water. Chemiluminescence measurements of OH were conducted by the use of luminol and 2D-UV-absorption spectroscopy was used to detect H2O2 in the plasma-treated water. The strength of convective transport was found to decrease with the gas flow rate through the jet, and at low gas flows, an effective diffusion coefficient for H2O2 could be calculated. OH was mainly present at the liquid surface under all treatments investigated. The reactive MD simulations form the basic model of an ideal system, where all transportation is purely diffusion-driven, and molecular diffusion coefficients can be calculated. The results of the MD simulations were compared with the experimental studies to gain a deeper understanding of the differences between the ideal and the real system. To bridge the gap between the time scales of the MD simulations and the experiments, a kinetic model was used to understand the spatio-temporal changes and the influence of transport mechanisms and reaction chemistry. For low flow rate cases good agreement between experimental measurements and kinetic modelling could be obtained when the experimentally measured effective diffusion coefficient was used as input to the model. The differences in the H2O2 concentration profiles in the liquid when using the molecular diffusion coefficient derived from MD and the effective diffusion coefficient from the experimental measurements are highlighted.