Post-discharge evolution of reactive species in the water activated by a surface air plasma: a modeling study

Plasma-activated water (PAW) has been reported to sustain a bactericidal ability for months. However, many reactive species regarded as the main antibacterial agents in PAW have short lifetimes of less than one second. In order to explain the prolonged antibacterial ability of PAW and predict how to extend its effective time, we studied the post-discharge evolution of reactive species in PAW based on a system-level model reported previously. Three common storage conditions for PAW were considered within the post-discharge time of 14 d: (I) leaving the residual gas and PAW in the sealed reactor; (II) leaving PAW in the open air; (III) sealing the container of PAW. In comparison, storage condition III was the best condition to preserve the long-lived species including H2O2 and HNO2/NO2-, whereas storage condition I was the best method to preserve the short-lived species including OH, HO2 and ONOOH/ONOO-. It suggests that the gas-liquid mass transfer plays an important role in the evolution of reactive species. We also found that O2NOOH/O2NOO- had an almost one order of magnitude higher concentration and a longer residue time than those of ONOOH/ONOO-. This distinction suggests that the biological effect of O2NOOH/O2NOO- may be important.