## 2 NTAPP Sources and Plasma Chemistry
### Reactive Species Formed in NTAPP Discharge
\(\mathrm{H}_{2}\mathrm{O}_{2}\) are produced in plasma from the recombination of two hydroxyl radicals in the liquid or plasma phase [73]. In the plasma phase, \(\mathrm{H}_{2}\mathrm{O}_{2}\) can also be generated by the interaction between excited water molecules and hydroxyl radicals as shown in Equations (7)-(9) [68; 74]:  
\[\mathrm{OH}.+\mathrm{OH}.\rightarrow\mathrm{H}_{2}\mathrm{O}_{2} \tag{7}\]  
\[\mathrm{OH}.+\mathrm{H}_{2}\mathrm{O}^{\ast}\rightarrow\mathrm{H}_{2}\mathrm{O }_{2}+\mathrm{H} \tag{8}\]  
\[\mathrm{HO}_{2}+\mathrm{HO}_{2}\rightarrow\mathrm{H}_{2}\mathrm{O}_{2}+ \mathrm{O}_{2} \tag{9}\]  
Similarly, ozone is another plasma product to give the oxidative stress. Plasma electron can disassociate \(\mathrm{O}_{2}\) into atomic \(\mathrm{O}\). This mechanism are the main conditions for the occurrence of tree reaction bodies in the formation of \(\mathrm{O}_{3}\) via Equation (10) [64]. In this reaction, \(\mathrm{M}\) represents other molecules inside the reactor, such as \(\mathrm{N}_{2}\), \(\mathrm{O}_{2}\), \(\mathrm{Ar}\), \(\mathrm{He}\), and \(\mathrm{Ni}\). \[\mathrm{O}_{2}+\mathrm{O}+\mathrm{M}\rightarrow\mathrm{O}_{3}+\mathrm{M} \tag{10}\]  
Energy consumption estimation is another crucial step before real-life application of any technology. [PERSON] et al. [66] studied the \(\mathrm{H}_{2}\mathrm{O}_{2}\) concentration according to the total energy for various applied voltages and frequencies in underwater plasma discharge [66]. They found that the frequency source has vital effect on the total energy consumption. According to their estimation, underwater plasma discharge consumes a few tens of kilojoules per millimole to 100 kJ/mM H\({}_{2}\)O\({}_{2}\). The production rate versus energy consumption graph is almost linear, and the coefficient is approximately 0.12 mM/kj for different cases, as mentioned in [66]. Figure 4 depicts the variation of the \(\mathrm{O}_{3}\) synthesis efficiency with the electric field strength (E/n) in DBD from [75]. Where solid line represents the trend of scatter plot (*) of energy efficiency against the electric field. The \(\mathrm{O}_{3}\) synthesis efficiency depends on the electric field strength, and the optimum efficiency is approximately 225 g/kWh at 200 Td. Plasma chemistry can be used to decompose the various chemical and biological pollutants. Recently [PERSON] et al. [76] summarize the plasma enhanced water treatment technology to decompose the organic compound present in water. A graphical representation of breakdown of total organic compounds (TOC), denoted by R, with oxygen plasma at plasma-liquid interface is shown in Figure 5a [76]. The breakdown of TOC is only possible about the plasma-liquid interface, where OH radicals, \(\mathrm{H}_{2}\mathrm{O}_{2}\), \(\mathrm{O}_{3}\) is abundantly available. In their report, energy efficiency for breakdown of TOC in wastewater was about 0.22-0.29 g kWh\({}^{-1}\) and breakdown rate were about 6.0-7.3 mg h\({}^{-1}\)[76]. The TOC decomposition efficiency rate achieved by several methods including chemical, plasma, and synergistic effect of plasma and ozonizer are compared in Figure 5b [76]. A synergistic effect of plasma and ozonizer has reasonably high efficiency as compared to conventional AOPs with high organic compound breakdown rates [76]. Among the different plasma sources nanosecond-pulsed discharge plasma with water droplets reaches an elevated TOC breakdown rate and efficiency [77]. \[\mathrm{e}^{-}+\mathrm{H}_{2}\mathrm{O}\rightarrow\mathrm{H}+\mathrm{OH}^{+}+ \mathrm{e}^{-}\left(\mathrm{direct\ dissociation,\,4.4\;eV}\right) \tag{2}\]0 Variation of energy efficiency for ozone synthesis with electric field strength in DBD (Reprinted from [75]). \[\mathrm{e}^{-}+\mathrm{H}_{2}\mathrm{O}\rightarrow\mathrm{H}+\mathrm{OH}^{+}+ \mathrm{e}^{-}\left(\mathrm{direct\ dissociation,\,4.4\;eV}\right) \tag{2}\]1 (**a**) Graphical representation of main reactions involves in the breakdown of organic compound using oxygen plasma in plasma–liquid interface.