Transient Spark Discharge Generated in Various N2/O2 Gas Mixtures: Reactive Species in the Gas and Water and Their Antibacterial Effects

Reactive species generated in the gas and in water by cold air plasma of the transient spark discharge in various N2/O2 gas mixtures (including pure N2 and pure O2) have been examined. The discharge was operated without/with circulated water driven down the inclined grounded electrode. Without water, NO and NO2 are typically produced with maximum concentrations at 50% O2. N2O was also present for low O2 contents (up to 20%), while O3 was generated only in pure O2. With water, gaseous NO and NO2 concentrations were lower, N2O was completely suppressed and HNO2 increased; and O3 was lowered in O2 gas. All species production decreased with the gas flow rate increasing from 0.5 to 2.2 L/min. Liquid phase species (H2O2, NO2‾, NO3‾, ·OH) were detected in plasma treated water. H2O2 reached the highest concentrations in pure N2 and O2. On the other hand, nitrites NO2‾ and nitrates NO3‾ peaked between 20 and 80% O2 and were associated with pH reduction. The concentrations of all species increased with the plasma treatment time. Aqueous ·OH radicals were analyzed by terephthalic acid fluorescence and their concentration correlated with H2O2. The antibacterial efficacy of the transient spark on bacteria in water increased with water treatment time and was found the strongest in the air-like mixture thanks to the peroxynitrite formation. Yet, significant antibacterial effects were found even in pure N2 and in pure O2 most likely due to high ·OH radical concentrations. Controlling the N2/O2 ratio in the gas mixture, gas flow rate, and water treatment time enables tuning the antibacterial efficacy.