Atmospheric Pressure Humid Argon DBD Plasma for the Application of Sterilization - Measurement and Simulation of Hydrogen, Oxygen, and Hydrogen Peroxide Formation

Hydrogen, oxygen, and hydrogen peroxide have been measured downstream of an atmospheric pressure humid argon dielectric barrier discharge. The yield of the three species was studied as a function of the discharge power and gas flow rate. Hydrogen peroxide was measured after dissolution into water downstream of the discharge, while hydrogen and oxygen were measured in the gas phase. The production rates of both hydrogen and oxygen were found to be at least one order of magnitude greater than that of hydrogen peroxide. In all cases, the molar rate of molecular hydrogen production was more than twice that of molecular oxygen. At various total gas flow rate, and over a range of energy density of 0 to 1500 J/L, the concentration of hydrogen found at the reactor outlet varied from 0 to 1000 ppm, while oxygen reached a maximum concentration of about 400 ppm. The corresponding concentration of hydrogen peroxide that was calculated to have been in the reactor outlet gas did not exceed 11 ppm. Mechanisms for this system based on experiments and a numerical model are presented. The results of experiments on the use of post-discharge effluent gas of humid argon plasma for sterilization purposes (inactivation of E. Coli) bacteria are also presented and discussed in relation to the results of the chemical measurements and simulations. I. I NTRODUCTION Non-thermal plasma technology at atmospheric pressure using oxygen-nitrogen mixtures has been shown to be effective for sterilization purposes (1), and dielectric barrier discharge (DBD) in humid argon at atmospheric pressure has been shown to produce hydrogen peroxide (2). Here, the application of humid argon plasma effluent gas for sterilization is investigated. In order to better understand the chemical pathways involved not only in the production of hydrogen peroxide, but that lead to the inactivation of bacteria, measurements of two other stable species produced in humid argon plasma, hydrogen, and oxygen have been made and a mathematical simulation of the system has been developed. The application of atmospheric pressure non-thermal argon plasma produced by a variety of techniques has recently begun to receive attention for the application of sterilization of surfaces associated with the medical industry (1,3-7); investigation of argon plasma 'cleaning' of surfaces has also been made (8-9). In addition, there is considerable work on atmospheric pressure plasma sterilization processes using other working gases (10-13). A major advantage of non-thermal plasma technology is its ability to operate at or near room temperature, thereby allowing treatment of polymer surfaces which cannot be subjected to conventional dry or wet heat treatment. Here it is important to note that any use of plasma technology in the application of sterilization must consider the requirement of not doing major physical damage to the material(s) underlying the bacteria. In addition, since plasma treatment of polymer surfaces is well-known to increase surface hydrophilicity, thereby increasing the future capacity of surfaces to support bacterial growth (14), this issue should be carefully considered. One investigation of humid argon plasma sought to measure OH radical concentration and to develop numerical simulations of the chemical kinetics in such a system (15). Other work on electrical discharges in water has shown that that type of discharge leads to the formation of hydrogen, oxygen, and hydrogen peroxide in a roughly 4:2:1 ratio (16). The results of these two studies will be shown to have similarities with the results presented here.