## 3 Plasma Application to Eliminate Organic Pollutants in Water
### Azo Dyes  
According to research by [PERSON] et al. [108] and [PERSON] et al. [109], dyes, which constitute a significant portion of textile and paper wastewater, are known to be extremely dangerous, mutagenic, and even carcinogenic. Reports indicate that between 10 and 15 percent of the world's total dye production is released into various water bodies, resulting in adverse effects on aquatic ecosystems. This impact is seen through reduced dissolved oxygen levels, which hinder the processes of photosynthesis and respiration in aquatic organisms [110; 111; 112]. Among these, azo dyes are frequently utilized in the textile and food industries, and pose a particular challenge in wastewater treatment due to its complex chemical structure and low biodegradability. Traditional methods employed to remove these organic contaminants include biological, physicochemical (adsorption, coagulation/flocculation, reverse osmosis), or chemical treatment (chlorination and ozonation). However, all of these approaches have drawbacks. Biological therapy is unsuccessful for stable and azo-resistant dyes, while only a few unstable azo dyes may be oxidized in aerobic conditions. Additionally, the anaerobic decomposition of these dyes can result in the production of toxic and potentially cancer-causing aromatic amines [113]. Ar plasma treatment is a highly effective method for dye (reactive orange 16, azo dye) removal, even at a low flow rate of 1 slm. The addition of 10% O\({}_{2}\) to the Ar stream sped up dye oxidation and decreased the amount of time needed for total decolorization [49]. Since it happens faster in the 494 nm band than in the other bands, RO16 decolorization frequently manifests as a decrease in intensity in this band. The most vulnerable chromophore (-N=N) is the first to be attacked by these reactive species, which are the most significant ingredient in the discharge, causing dye degradation [114]. Reactive species, such as -OH, as a result, attack the chromophore moiety (-N=N-) of the dye molecule first, and then obliterate its aromatic benzene and naphthalene rings [47]. According to [PERSON] et al. [111], the hydrazinyl radical form of the chromogenic group (-N=N-) may be produced by adding -OH. This can cause color to be lost in the visible spectrum to the unaided eye and to eventually mineralize into perfectly innocuous gaseous nitrogen (N) [47]. [PERSON] et al. [115] evaluated the production of -OH in feed gases with different compositions, and discovered that feed gas mixes containing Ar and O\({}_{2}\) had the highest concentration of -OH. Additionally, atomic O can directly react with organic pollutants during the -OH generation process, resulting in the production of extremely active O\({}_{3}\) and O\({}_{2}\) as well. However, [PERSON] et al. [116] demonstrated that the formation of O\({}_{3}\) is minimal when the concentration of Ar in the mixture of Ar and O\({}_{2}\) is equal to or higher than 80%. Since [PERSON] et al. [49] utilized 90% Ar in their trials, they believed that O\({}_{3}\)'s effect was not particularly significant. [PERSON] et al. [117] employed an indigo carmine target that is commonly used in the food industry to evaluate the efficacy of plasma treatment. Due to the presence of an H-type chromophore group, indigo carmine absorbs certain visible light wavelengths, turning them blue [118]. Reactive species and free radicals produced by plasma break the H-type chromophore of indigo carmine, converting it into isatin sulfonic acid [119], which can be detected by measuring the absorbance. Meanwhile, reactive oxidative species (ROS) were believed to be the main factor causing indigo carmine to degrade [117]. In a recent publication [30], low-temperature plasma treatment of dye wastewater was combined with conventional biological treatment methods. Benzoic acid and its derivatives react with O\({}_{3}\) and -OH in dye wastewater under plasma degradation conditions, which affects the wastewater's efficacy in degrading dye to some extent.