Aqueous-phase chemistry and bactericidal effects from an air discharge plasma in contact with water: evidence for the formation of peroxynitrite through a pseudo-second-order post-discharge reaction of H2O2 and HNO2

The formation of transient species (OH center dot, NO2 center dot, NO radicals) and long-lived chemical products (O-3, H2O2, NO3-, NO2-) produced by a gas discharge plasma at the gas-liquid interface and directly in the liquid was measured in dependence on the gas atmosphere (20% oxygen mixtures with nitrogen or with argon) and pH of plasma-treated water (controlled by buffers at pH 3.3, 6.9 or 10.1). The aqueous-phase chemistry and specific contributions of these species to the chemical and biocidal effects of air discharge plasma in water were evaluated using phenol as a chemical probe and bacteria Escherichia coli. The nitrated and nitrosylated products of phenol (4-nitrophenol, 2-nitrophenol, 4-nitrocatechol, 4-nitrosophenol) in addition to the hydroxylated products (catechol, hydroquinone, 1,4-benzoquinone, hydroxy-1,4-benzoquinone) evidenced formation of NO2 center dot, NO center dot and OH center dot radicals and NO+ ions directly by the air plasma at the gas-liquid interface and through post-discharge processes in plasma-activated water (PAW) mediated by peroxynitrite (ONOOH). Kinetic study of post-discharge evolution of H2O2 and NO2- in PAW has demonstrated excellent fit with the pseudo-second-order reaction between H2O2 and NO2-. The third-order rate constant k = 1.1 x 10(3)M(-2) s(-1) for the reaction NO2- + H2O2 + H+ -> ONOOH + H2O was determined in PAW at pH 3.3 with the rate of ONOOH formation in the range 10(-8)-10(-9) Ms-1. Peroxynitrite chemistry was shown to significantly participate in the antibacterial properties of PAW. Ozone presence in PAW was proved indirectly by pH-dependent degradation of phenol and detection of cis, cis-muconic acid, but contribution of ozone to the inactivation of bacteria by the air plasma was negligible.