Investigation of Reactive Species in NaCl Solution by One-Dimensional Reaction-Diffusion Numerical Simulation*

In applications of low-temperature gas plasmas in medicine and biology, various chemical species are generated by atmospheric pressure plasma (APP) in air. The chemical species generated in the gas phase such as H₂O₂, O, O₂, and O₃ interact with a liquid solution before reaching the tissue cells. Some of them are highly reactive and may be converted to other species in the liquid. This study focuses on the reactions and transport of plasma-generated species in liquid and examines their dynamics in NaCl solution with pH buffer. The numerical simulations were performed based on the one-dimensional (1-D) reaction-diffusion-advection equation.[1,2] Our goal is to find the dynamics of depth profiles of plasma-generated reactive species in NaCl solution and understand the validity of the corresponding zero-dimensional (global) simulations. The species of our interest were H₂O₂ (1.49x10¹² cm^-3) and O (0 - 1.49x10¹³ cm^-3) in the gas phase.[3] The plasma irradiation time was 5 seconds. The 1-D simulation results showed that the concentration of H₂O₂ in the NaCl solution decreased when the amount of O from the plasma increased, as in the global simulation. O supplied from the gas phase reacts with Cl^- in the solution forming ClO^-species, which then reacts with H₂O₂ to produce H₂O, O₂, Cl^- by-products. In the 1-D simulation, the decomposition of H₂O₂ by ClO^- species mostly occurs near the gas-water boundary,  typically within the depth of 250mm under the conditions examined here.