Estimation of Possible Mechanisms of Escherichia coli Inactivation by Plasma Treated Sodium Chloride Solution

Treatment of aqueous liquids by surface-DBD in atmospheric air resulted in bactericidal activity of the liquid itself. A 7 min treatment of sodium chloride (NaCl) solution and its immediate addition to Escherichia coli resulted in a complete bacteria inactivation (>= 7 log) after 15 min exposure time. With a 30 min delay between plasma treatment of liquid and its addition to the bacteria, bactericidal effect was reduced but still detectable. Nitrate (NO2-), nitrite (NO2-), and hydrogen peroxide (H2O2), respectively, as well as strong acidification are detected in plasma treated liquids and can explain this bactericidal activity partially. Combination of 1.5 mg . L-1 NO2- and 2.5 mg . L-1 H2O2 at pH 3 results in maximum 3.5 log E. coli reduction within 60 min. Plasma diagnostics and liquid analytics are combined with theoretical considerations to focus possible reaction channels of plasma-water interactions. Using FT-IR, stable molecules like nitrous oxide (N2O), ozone (O-3), carbon dioxide (CO2), and traces of nitric acid (HNO3) and/or peroxynitrous acid (ONOOH) were measured. Reactions of these molecules from the plasma/gas phase with the aqueous liquid can result in acidification and generation of H2O2, NO2-, and NO3- or peroxynitrite (ONOO-), respectively, via reactions which are associated with the occurrence of several more or less stable but biologically active chemical intermediates like NO center dot or nitrogen dioxide (NO2 center dot). On the other hand, H2O2, NO2-, and NO3-/ONOO- could serve as starting reaction partners to generate NO center dot, HO center dot, NO2 center dot, or hydroxyl radicals (HOO center dot) in the liquid.