## 3 Plasma Application to Eliminate Organic Pollutants in Water
### Azo Dyes
The BOD\({}_{5}\)/CODcr ratio raised from the initial 0.17 to 0.33, and the biochemical characteristics of the wastewater treated using low-temperature plasma technology were significantly enhanced. Methylene blue is commonly used as a model organic contaminant. According to a study by [PERSON] et al. [52], O\({}_{2}\) gas DBD plasma demonstrated better methylene blue degradation than the other input gases. After analyzing the reactive species generated by various gases plasma, the authors concluded that the O\({}_{2}\) gas plasma included higher quantities of H\({}_{2}\)O\({}_{2}\), O\({}_{3}\), and -OH than other feeding gases. Furthermore, the O\({}_{2}\) gas plasma shows strong methylene blue decolorization, achieving nearly 98% decolorization after 10 min of treatment. H\({}_{2}\)O\({}_{2}\) by itself is unable to remove the color from the methylene blue dye, according to another study by the scientists [120]. Therefore, the dominant species responsible for the degradation are -OH and O\({}_{3}\). The primary effects of microwave plasma injection on methylene blue decolorization were discussed in another work, and the authors discovered a considerable increase in decolorization efficiency with the addition of 1-3% N\({}_{2}\) to the Ar plasma [121]. They asserted that the acceleration of methylene blue decolorization was caused by the combined actions of the highly reactive radicals NO, NO\({}_{2}\), and NH. According to [PERSON] et al. [122], pH has a major effect on the oxidation of methylene blue because a higher pH increases the amount of O\({}_{3}\) that permeates the liquid phase, while a lower pH decreases the amount of O\({}_{3}\) that is absorbed. This could explain why there was more decolorization in the O\({}_{2}\) plasma reactor than in the air plasma, as the pH in the O\({}_{2}\) plasma decreased to 5.1, while it decreased in the air plasma to 2.5. It is well-known that the pH of plasma treatment decreases over time, and, moreover, that this pH decline (acidification) is followed by an increase in the solution's conductivity [121]. Recent studies on the breakdown of methylene blue revealed that it was removed most efficiently from N-containing plasma at mildly acidic, low conductivities (approximately 10 S/cm), but removal was reduced at high conductivities (100-10,000 S/cm) [121; 123; 124]. In addition, [PERSON] et al. [125] also discovered that after 40 min in a DBD reactor, methylene blue decomposed by 99% in an acidic solution, 91% in an alkaline solution, and 75% in a neutral solution. Saline organic wastewater is a prominent challenge within the wastewater treatment industry. Acid Red 73 (AR 73) is a chemically synthesized red azo dye which is frequently utilized as a benchmark to evaluate the degradation of salt-based dyes in aqueous solutions. [PERSON] et al. [126] reported the effects of plasma discharge on a number of saline and azo dye degradation parameters in wastewater. The findings demonstrated that the electrolyte, initial concentration of AR 73, and initial pH all had a substantial impact on the degradation of AR 73. Lowering these parameters resulted in improved degradation outcomes for AR 73. This suggests that plasma can be effectively applied to the treatment process of saline dye wastewater.