Combination of NO x mode and O3 mode air discharges for water activation to produce a potent disinfectant

Air plasma is commonly used to treat water for activation or purification, and plasma-activated water (PAW) is a promising green disinfectant that has attracted much attention in recent years. However, either the O3 discharge mode or the NO x discharge mode of air plasma lacks efficiency for water activation, mainly due to the low solubility of O3, NO and NO2. The transition process between those two modes could produce high-valence NO x such as N2O5 which should be more efficient for water activation, but this process is not easily controlled and water activation by N2O5 has rarely been reported before. In this letter, N2O5 is found to be produced effectively and stably by mixing the effluent gases of a NO x mode air plasma, produced by gliding arc discharge, and an O3 mode air plasma, produced by dielectric barrier discharge. The mixed gas rich in N2O5 is found to be very efficient for water activation. A colony-forming unit reduction of nearly 6 logs was achieved for a methicillin-resistant Staphylococcus aureus suspension by using PAW treated by the mixed gas, and the high bactericidal effect of the PAW could persist for more than 1 h after the plasma activation. The main reactive species for the bactericidal effect in the PAW are discussed, and the chemical pathways for N2O5 production and its usage for water activation are illustrated.