Chemical species generated in water by an atmospheric-pressure air plasma jet

Summary form only given. Non-thermal plasma jets operated at atmospheric pressure in air are studied extensively for the inactivation of pathogens on the skin surface. Recent results show that efficacy and kinetics of the method are strongly affected by chemical reactions in the plasma when water is present in the microbial environment. We have studied the interaction of water with a microplasma jet that is operated with dry air at atmospheric pressure. The jet is expelled by a continuous gas flow from a microhollow cathode discharge geometry into a closed, but not sealed, glass reactor which could be partially filled with water. Observed reaction products of the afterglow plasma with air or water strongly depended on the distance of the jet from the liquid. In the absence of water, we measured primarily nitric oxide (NO), while the nitric dioxide (NO₂) concentration was insignificant. With water present, the NO concentration was lower, while the concentration of NO₂ slightly increased. In addition, we found that pH and liquid resistivity decreased dramatically with exposure times. Using a colorimetric detection method and a radical scavenger (p-nitrosodimethylaniline), we quantified hydrogen peroxide (H₂O₂) concentrations and hydroxyl radical (·OH) concentrations, respectively. The presence of hydroxyl radicals could suggest that atomic oxygen was transported from the plasma jet into the liquid. Our chemical analysis confirms that an aqueous environment can enhance sterilization efforts by non-thermal plasma technologies. It further suggests the possibility that plasma jets can be an effective method for treatment of wastewater and water purification.