Formation of reactive chlorine species in saline solution treated by non-equilibrium atmospheric pressure He/O2 plasma jet

Abstract

The aqueous-phase chemistry of reactive chlorine species initiated in saline solution by the gas phase discharge plasma above the liquid surface was investigated. A micro-atmospheric pressure 13.56 MHz plasma jet operating with a 0.6% O2 in He was used to promote reactions of plasma-generated oxygen atoms with chloride anions at the gas-liquid interface. Physiological NaCl solution and phosphate buffered saline (PBS) were used. Three chlorine oxoacids or their conjugate bases were detected in the plasma treated saline: hypochlorites HOCl/OCl−, chlorites ClO2−, and chlorates ClO3−, being formed in the concentration ratio 4.5:1:1, respectively. In addition, chlorine dioxide ClO2 was formed in minor amounts. The final pH value of the plasma treated saline was typically 10.2. The total hypochlorite production, [HOCl] + [OCl−], was directly proportional to the initial NaCl concentration, supporting the idea of a direct reaction of Cl− with O atoms. Subsequent post-discharge chemical processes in the bulk plasma treated saline solution were found to lead to the disproportionation of hypochlorite into different oxidation states of chlorine with formation of ClO3− as the final product. The reactivity of O atoms in NaCl solution was studied using taurine and phenol as chemical probes. Experiments with 50 mM taurine revealed a very high initial formation rate of OCl− in PBS (up to 4 μM s−1). Chlorinated products of phenol (2- and 4-chlorophenol) in addition to the hydroxylated products (hydroquinone and catechol) demonstrated chlorination of phenol by hypochlorite. The rate of Cl− oxidation by O atoms was estimated to be 2–3 orders slower than the reaction of O atoms with phenol.