## 3 Results and Discussions
The concentration of nitrites and nitrates in the liquid medium after the plasma treatment is given in Figure 11b. It can be seen that the \(\text{NO}_{2}^{-}\) concentration was enhanced from 0.13 mM to 1.67 mM with the increase in the voltage from 90 V to 250 V. The concentrations of \(\text{NO}_{3}^{-}\) were significantly higher compared to the concentrations of \(\text{NO}_{2}^{-}\) at similar voltages. However, a slight increase in the nitrates' concentrations from 23.3 mM to 28.9 mM was observed with the enhancement of the voltage. In addition to N radicals, the plasma treatment also induced the formation of hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), which was indirectly detected using the iodometric method. The concentration of hydrogen peroxide in the BG-11 medium after the plasma treatment with the lowest output voltage was \(\sim\)5.9 mM. The concentration of H\({}_{2}\)O\({}_{2}\) increased almost three times with an increase in the voltage to 250 V, resulting in about 17.4 mM (Figure 11b). The composition and abundances of the radicals produced by the plasma treatment in the BG-11 media are also confirmed by OES spectra, which showed an increase in the intensity of the N\({}_{2}\), N\({}_{2}^{+}\), O, and \(\text{NO}_{\gamma}\) lines with an increase in the discharge voltage (Figures 8 and 9). However, it is necessary to highlight that the levels of radicals detected, especially those of the N species, showed variability between replicates and fluctuated with time after the treatment (Figure 11b). Our analysis of the plasma's ability to generate radicals showed an increase of more than 50% in the nitrate concentration and a 4-fold increase in the nitrite concentration at the voltage of 250 V (Figure 11b). [PERSON] et al. [6] observed that the conductivity of water increased, and the pH decreased, with the increase in the discharge power of the plasma. This effect was explained by the formation of higher concentrations of \(\text{NO}_{2}^{-}\) and \(\text{NO}_{3}^{-}\) ions in solutions or water when air plasma was used [6; 43; 44; 45]. [PERSON] et al. [21] observed that the concentration of nitrites and nitrates in water and deionized water were the highest when a mixture of \(\text{N}_{2}^{+}\) 21% \(\text{O}_{2}\) gases was used. Also, the conductivities of water and deionized water after their treatment with N\({}_{2}\), O\({}_{2}\), and N\({}_{2}\)-O\({}_{2}\) plasma was increased. [PERSON] et al. [45] observed that the nitrate concentration in water increased from 30 mg/L to -70 mg/L after increasing the plasma treatment time from 2 min to 6 min. However, a further increase  
Figure 5.2 The variation of the conductivity (**a**) and concentration of nitrites, nitrates, and hydrogen peroxide (**b**) in liquid medium as a function of the output voltage. in the plasma treatment time up to 16 min reduced the nitrate concentration down to 20 mg/L. This reduction in the NO3\({}^{-}\) concentration was attributed to the formation of HNO3. The conductivity of tap water increased from 70 to 190 uS/cm and NO3\({}^{-}\) from 0.002 to 0.156 mmol/L after a 5 min air plasma treatment [43]. It was demonstrated that the pH values of water polluted with various bacteria decreased with an increase in the treatment time when using GAD plasma. The decrease in pH values was attributed to the formation of nitrite and nitrate ions, which, in water, are the acids HNO3 and HNO2, after plasma treatment [5]. It should be noted that the main species produced in various liquids, media, or water after plasma treatment are nitrites, nitrates, nitrogen dioxide, and hydrogen peroxide [4; 5; 6; 8]. However, the concentrations of various species strongly depend on the type of water or media, type of plasma discharge, type of gases used, plasma treatment time, etc. [4; 5; 6; 8; 21; 43; 44; 45].