# Synergistic Effect of Nonthermal Plasma and ZnO Nanoparticles on Organic Dye Degradation
## 1 Introduction  
The widespread use of organic dyes in various industries, such as textiles, cosmetics, and printing, has led to significant environmental problems due to the discharge of wastewater-containing dyes [1]. These dyes can be toxic and persistent, pose a threat to aquatic ecosystems, and potentially affect human health if they enter the water supplies [2; 3; 4]. Methylene blue (MB), C\({}_{16}\)H\({}_{18}\)N\({}_{3}\)SC\({}_{l}\), is a synthetic dye used in the textile industry to dye fabrics. Its discharge has effects on aquatic life and human health [5]. Therefore, the development of efficient and environmentally friendly techniques for dye degradation and the removal of dyes is indeed a challenging task [6; 7; 8; 9; 10]. A variety of dye degradation methods have been developed, such as biodegradation and biotransformation, which utilize microorganisms to degrade organic dyes [11; 12], an adsorption technique using natural or synthetic materials that adsorb and remove organic dyes from wastewater [13], and reverse osmosis and an ultra-filtration membrane [14; 15]. Though the biological method is low-cost, it is doubtful whether it degrades MB completely in water [16] and it generates a large amount of sludge, affecting the ecological system [12]. Meanwhile, adsorption and ultra-filtration approaches face challenges such as the regeneration of the adsorbent for re-usability and membrane fouling and high energy consumption, respectively. Other approaches for wastewater treatment are based on the generation and usage of oxidants, such as photocatalysis, in which a semiconductor nanostructure is illuminated with UV-Vis and, hence, generates oxidants that interact with the dye molecule [17]. The Fenton reaction involves Fe\({}^{2+}\) ions interacting with H\({}_{2}\)O andproducing hydroxyl radicals, OH [18]. The advanced oxidation process employes H\({}_{2}\)O\({}_{2}\), ozone (O\({}_{3}\)), and/or UV light to enhance hydroxyl radicals' production [19]. Plasma is a partially ionized gas containing various reactive species, including ions, radicals, and excited molecules. Its versatile chemical compositions enable its strong oxidizing and/or reducing properties, making them effective tools for organic dye degradation. The atmospheric-pressure plasma jet (APP) generates a plasma plume in open air. The plasma plume is composed of various reactive species and emits photons with a wide spectral range. The APPJ has received increasing attention due to its great potential for a variety of applications in nanotechnology, surface engineering, and material processing [20; 21; 22; 23]. Plasma technology has been shown to be a promising approach for the degradation of organic dyes [24], which has the advantages of having a short treatment time, being chemical-free, and leaving no residuals behind. However, there are several limitations associated with the plasma treatment of dyes, such as the lack of selectivity, higher energy consumption, limited penetration depth, and scalability. One possible strategy to improve plasma performance in dye degradation is to combine plasma with metal oxide nanomaterials. Photocatalytic studies of various photocatalysts combined with plasma have been investigated. [PERSON] et al. synthesized a novel photocatalyst and combined it with dielectric barrier discharge (DBD) plasma to remove isopropanol (IPA) [25]. [PERSON] et al. studied the mineralization of toluene using nonthermal plasma combined with CeO\({}_{2}\) as an adsorbent and found that the amount of ozone produced by the discharge plays a key role in the mineralization process [26]. The combination of plasma with nanomaterials could improve the generation of reactive species and the reaction area and increase the selectivity of the degradation process [27; 28; 29; 30]. Zinc oxide nanoparticles ZnO have attracted much attention in environmental remediation due to their unique properties, such as large surface area, photocatalytic activity, and stability [31].