# Plasma-Activated Tap Water Production and Its Application in Atomization Disinfection
## 4 Discussion  
The underwater DBD first produced UV, charged particles, and gaseous RONS. These substances interacted with water to produce aqueous RONS, such as peroxpiritrite, NO3, NO2, OH, hydrogen peroxide, ozone, etc. (the reactions pathways are summarized in Figure 13) [36; 37; 38; 39], while reducing the PAW's pH. [PERSON] et al. found that PAW maintained a long-term bactericidal effect, in which hydrogen peroxide in an acidic environment was the dominant substance because other  
Figure 12: Disinfection effect of atomized PATW on clothing surface. Figure 13: Generation pathways of major aqueous RONS in PATW. active ingredients (short lifetime RONS) decomposed in ms-s time range [20]. However, other scholars thought that hydrogen peroxide alone could not play a decisive role. The bactericidal effect of PAW is jointly determined by various long-lived and short-lived active substances [40; 41; 42]. Whether PAW has a strong bactericidal effect not only depends on the individual effects of various substances, but also depends on the complex reactions between various species. In addition to several long-lived substances, the substances with bactericidal effects in PAW also include short-lived substances such as peroxnitrite, singlet oxygen, hydroxyl radicals, and superoxide anion radicals. Among them, peroxnitrite is produced by the reaction of NO\({}_{2}^{-}\), H\({}_{2}\)O\({}_{2}\), and H\({}^{+}\). It is a strong oxidant, which can oxidize and thus denature the protein in the bacteria, thereby changing the bacterial morphology and thus inactivating the bacteria [43]. At low pH, peroxnitrite decomposes into hydroxyl radicals and nitrogen dioxide. Hydroxyl free radicals have strong oxidative properties and can attack bacterial cell membranes, increasing their permeability, inhibiting the regeneration ability of biofilms, and severely damaging bacteria [44]. Singlet oxygen is the excited state of oxygen molecules and has potent cytotoxicity. It can interact with biological macromolecules such as proteins and DNA in cells and cause damage to the cell membrane system by combining with molecules, causing oxidative stress in bacteria [45]. Figure 14 compares the sterilization effect of PATW and the chemical mixed solution. It can be seen that the chemical mixed solution had a killing effect of less than one logarithm on _E. coli_, which was far less than the five logarithm inactivation effect of PATW on _E. coli_. Therefore, long-lived RONS are not the most important factor for the bactericidal effect in PATW, and we should focus on the impact of other short-lived RONS. Figure 15 shows that these scavengers alone had no appreciable sterilization effect on _E. coli_. Figure 16 reflects the bactericidal effect of PATW after adding several scavengers. L-histidine almost completely eliminated the sterilizing effect of PATW, and after adding L-histidine to remove singlet oxygen, _E. coli_ survived almost completely. Adding SOD also greatly weakened the effect of PATW on E. coli, and the bacteria were only inactivated by about 1 order of magnitude. In contrast, the addition of mannitol did not significantly affect the sterilization effect of PATW. Therefore, singlet oxygen and superoxide anion radicals play a key role in the sterilization by PATW. Figure 14: Comparison of the sterilization effect of PATW and a chemical mixed solution of a slightly higher concentration. According to the Reactions (1)-(6), we speculate that the peroxynitrite produced by the plasma treatment further slowly reacts with hydrogen peroxide to form peroxynitrate, and the decomposition of peroxynitrate produces a singlet oxygen and superoxide anion radicals. These reactions allow the continuous production of important short-lived (ms-s range lifetime) reactive species after the discharge, and the strongly oxidative environment of the PATW slows down the decomposition of short-lived ROS [46] so that the sterilizing effect is maintained.