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 H₂O₂and HNO₂

TLDR

The study evaluated how species produced by air‑discharge plasma in water contribute to chemical transformations and antibacterial effects, using phenol as a probe and Escherichia coli as a model organism. Transient radicals (OH·, NO₂·, NO·) and long‑lived species (O₃, H₂O₂) were generated by air‑discharge plasma at the gas–liquid interface and in the liquid, with production varying by gas mixture (20 % O₂ with N₂ or Ar) and buffer pH (3.3, 6.9, 10.1). Phenol oxidation products and kinetic analysis confirm that air‑discharge plasma generates NO₂·, NO·, OH· radicals and peroxynitrite in plasma‑activated water, with a pseudo‑second‑order H₂O₂ reaction (k = 1.1 × 10³ M⁻² s⁻¹ at pH 3.3) that drives antibacterial activity, while ozone plays a minor role.

Abstract

The formation of transient species (OH·, NO2·, NO radicals) and long-lived chemical products (O3, H2O2, , ) 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·, NO· and OH· 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 in PAW has demonstrated excellent fit with the pseudo-second-order reaction between H2O2 and . The third-order rate constant k = 1.1 × 103 M−2 s−1 for the reaction was determined in PAW at pH 3.3 with the rate of ONOOH formation in the range 10−8–10−9 M s−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.