Recent Progress in Applications of Atmospheric Pressure Plasma for Water Organic Contaminants’ Degradation

Abstract

Owing to current global water scarcity, there is an urgent need for advanced water treatment technologies to be invested in wastewater treatment processes. Additionally, there is growing concern that some anthropogenic contaminants have been detected in finished drinking water and wastewater slated for reuse, such as organic chemicals, pharmaceuticals, industrial dyes and even viruses, and their health effects are poorly understood at low concentrations. Atmospheric pressure plasma (APP) is a kind of advanced oxidation technology with high efficiency, low energy consumption, and little environmental impact. In recent decades, as a new method of environmental pollution abatement, APP has proven able to decompose and even completely eliminate stubborn organic contaminants. This paper focuses on the application of different types of plasma in the wastewater purification, such as water containing perfluorinated compounds, pesticides, pharmaceuticals, dyes, phenols, and viruses. Then, the effects of discharge parameters (discharge power, electrode distance, gas flow rate and working gas composition) on degradation efficiency were summarized. Finally, the existing challenges and future prospects of plasma-based wastewater purification are outlined.

1. Introduction

As a vital resource, water is the basis for the survival of everything on earth. The availability and quality of water have become some of the most important priorities in today’s society. Nowadays, water pollution in manufacturing and agricultural processes is becoming increasingly serious, which directly affects ecosystems and human health. Among various forms of water pollution, organic contaminants are the most serious problem.

Organic pollutants are a diverse range of chemical compounds that can be detected in municipal, industrial, or agricultural wastewater, including but not limited to medications, insecticides, personal care products, and numerous home and industrial chemicals. Organic contaminants in water bodies can affect aquatic creatures and eventually pose a hazard to humans [1]. The commonly used technologies for wastewater purification include chlorination [2,3,4], ozonation [2,5], ultraviolet (UV) radiation [2], filtration [6], boiling [7], and conventional water filtration technologies combined with advanced oxidation processes (AOP) [8,9,10,11]. However, most of the traditional approaches have drawbacks. The chlorination of water with organic compounds can result in the formation of carcinogenic byproducts [2,3,4]. The ozonation treatments require expensive equipment [2,5]. The penetrating strength of UV radiation is modest, and its power consumption is substantial [2]. Filtration can immediately produce drinking water without adding unpleasant odors, but it is costly and not suitable for removing viral particles [6]. Boiling treatments are time-consuming and expensive [7].

AOP is one of the most frequently and widely utilized methods in wastewater treatment [12]. The most common AOPs include heterogeneous photocatalytic oxidation [13,14], ultrasounds [15], photo-Fenton oxidation [16,17], etc. For example, photocatalysis is a clean AOP technique that converts water contaminants into CO₂ and H₂O [18]. However, due to the additional equipment, traditional advanced oxidation is not one of the best or most cost-effective technologies available. To overcome these issues, extensive research has recently been carried out to find a cost-effective and suitable method for the one-step production of reactive oxygen and nitrogen species (RONS) [19,20,21,22].

Recently, atmospheric pressure plasma (APP) as an efficient, eco-friendly and eco-nomical technology has shown great potential in wastewater treatment. As shown in

Table 1. Comparison of water treatment technologies.

Water Treatment Technology	Advantages	Disadvantages	Refs.
Conventional methods
Biodegradation	Eco-friendly and economical	Unrestricted breakdown of products, low biodegradability of some pollutants, requirement of maintenance and management of microorganisms.	[28]
Flocculation, coagulation, ion ex-change, filtration	Simple, no chemical reagents and no secondary pollution	High consumption of energy and reagents, low selectivity with high investment and operational costs.	[6,28]
Thermal oxidation, boiling	High-efficiency	High running costs, emission of various dioxins and other pollutants into the environment.	[7]
Modern oxidation processes
Photo-Fenton, photocatalysis, ozonation, ultrasonication	Simple, eco-friendly and economical	Lack of a complete oxidation process.	[2,5,14,17]
Atmospheric pressure plasma	Does not rely on UV lamps and ex-pensive chemicals to produce reactive substances, emits light and shockwaves; high-efficiency, simple, eco-friendly, economical and easy-to-use technology	Energy output is unclear, small application scale.	[29,30]

, APP has superior application value relative to conventional AOP technologies: (1) No consumables are required, and plasma can be generated in either regular air or water. (2) Since no consumables are required, the cost of associated consumables and infra-structure is negligible. (3) During the advanced oxidation process of plasma, both contaminants disinfect as well as degrade, and the decomposition efficiency is superior to most chemical methods. In this regard, it is advantageous to consider plasma for the treatment of super-toxic wastes via the purification of water contaminated with bacteria and chemicals. APP is a neutral ionized gas containing electrons, UV photons, ions, electric fields, and various radicals (Figure 1) [23]. As a novel oxidation method for wastewater treatment, the APP process involves the interaction of free oxide species with UV light and organic contaminants. The main forms of APP discharge in the treatment of wastewater are classified as gas-phase discharge, liquid-phase discharge, and gas–liquid-phase discharge. Plasma interactions with liquid water involve complex physical and chemical processes [24]. Plasma discharge above or beneath the liquid water provides opportunities for the injection of gaseous RONS into water for sterilization or chemical treatment [18,25,26,27]. The degradation of organic and inorganic pollutants will be massively advanced by plasma-generated RONS. Among these RONS, hydroxyl radical (∙OH) has the ability to decompose complex toxin structures due to its high chemical reactivity. ∙OH is essentially a molecule with one or more unpaired electrons, which makes it particularly reactive [24]. ∙OH participates in the chemical process of decomposing a wide range of organic compounds through electron transport, electrophilic addition, dehydrogenation and other reactions, converting organic matter into easily biodegradable intermediates or directly mineralizing it into simple and harmless inorganic substances such as CO₂ and H₂O.

At present, the issue of protecting water resources from various types of pollution has become increasingly urgent. Additionally, there are growing organic contaminants of emerging concern including pharmaceuticals and personal care products, which have been detected in trace amounts in drinking water [24]. Additionally, the health impact of these organic contaminants in low concentration is not well understood. Various investigations on the use of APP technology for organic wastewater purification have already been performed now, and advancements are continuing to be made in this area. The application of plasma for water treatment can not be regarded as a new topic considering the intense research work carried out in the past few decades. In addition, the rapid growth of research on the use of plasma systems in wastewater treatment has prompted researchers to focus on the technical characteristics of these systems. As the number of available APP experimental studies increases, specialized thematic reviews need to be elucidated and summarized to facilitate the development of better technologies, and this fact has become evident in the use of plasma in water treatment.

This article provides a detailed discussion of the plasma treatment of wastewater, and special attention was paid to the degradation of organic pollutants in wastewater treatment by plasma active ingredients. The current state of research on plasma degradation of organic pollutants is first described. Secondly, the application of various plasma-generating devices in wastewater treatment is discussed. The removal and degradation of various organic pollutants (pharmaceuticals, chemical reagents, pesticides, industrial dyes, viruses, etc.) from water by plasma are then discussed, along with the impact of various plasma discharge parameters on the effectiveness of wastewater treatment. Finally, the existing benefits and drawbacks of the plasma wastewater treatment process are summarized and forecasted. These findings presented herein lay the foundation for future improvements that can be implemented in the application of APP in water treatment.

2. Types of Plasma Discharge for Water Treatment

In recent times, attention has been paid to atmospheric pressure non-thermal plasma technology due to its low energy requirements. Discharge methods under normal pressure include pulsed corona discharge, dielectric barrier discharge (DBD), plasma jets, gliding arc discharge, and glow discharge, etc. [31], which can be processed according to different processing requirements. Figure 2 illustrates the different types of plasma generator, the plasma generation process and the wastewater treatment process.

DBD is the workhorse of plasma technology. This approach is based on reducing the power usage of plasma devices. An electrical breakdown occurs when a sufficiently high voltage is delivered between two electrodes under air pressure [32]. It comprises two electrodes, at least one of which is coated with a dielectric barrier material such as glass, ceramic, quartz, and so on (Figure 2A). When the potential across the gap hits the breakdown voltage, the dielectric can cause a high number of micro-discharges and reduce the likelihood of electrode etching and spark creation [29,33]. Therefore, DBD plasma can maintain uniformity and stability when decontaminating samples with a large area [34,35,36].

As shown in Figure 2B, the plasma jet is another type of cold plasma discharge. The plasma jet has a structure made up of two concentric electrodes within a nozzle through which the carrier gas travels, the outer of which is grounded and the inner of which is connected to an external power supply. The inner electrode is frequently shocked with a high voltage at a high frequency, inducing ionization of carrier gases such as helium, oxygen (O₂), and other gases or gas combinations [37,38]. These gases are also helpful in pushing the stream carrying active species out of the electrodes [39]. In addition, the simple structure and easy operation of plasma jets seem to have a wider range of applications in wastewater treatment.

The gliding arc is a common generator of thermal plasma, but it may also create non-thermal plasma under certain conditions [39]. In comparison to other kinds of discharges, gliding arcs combine the advantages of thermal and non-thermal plasmas, which have higher operating pressure, power and plasma density [39,40]. Figure 2C depicts one kind of gliding arc discharge, which is extensively employed for liquid treatment. The arc is pushed away by the injected stream along the two divergent electrodes, pushing away from the ignition point, and moving along the electrode gap to their tips, where it forms a large plume of plasma. The plasma plume is placed close enough to the aqueous dye solution, so it licks the surface of the liquid and allows chemical reactions to occur at the plasma–solution interface, resulting in highly reactive species for the purpose of degrading organic contaminants [41]. This technique is widely recognized for its ability to degrade numerous organic components found in water at atmospheric pressure [41,42].

Corona discharge frequently occurs through the imposition of high voltage on sharp electrodes (Figure 2D), such as tips, pinpoints, or thin wires [39]. Generally, a high voltage is connected to the pin electrode, and the plate electrode is connected to the ground [43]. The gas-phase pulsed corona discharge has proven the most energy-efficient plasma for water treatment, and achieves the highest degree of decontamination efficiency [29,44]. It is worth mentioning that all these techniques undergo a mineralization process that leads to the disintegration of different organic contaminants, i.e., the transformation of compounds into inorganic intermediates, H₂O and CO₂. Many studies have demonstrated the potential of the different plasma to degrade toxic pollutants and stable substances [45].

Figure 2. Schematic of the (A) dielectric barrier discharge, (B) plasma jet, (C) gliding arcs, and (D) corona plasma reactor, adapted from [43,46,47]. (E) The processes of plasma generation and wastewater treatment processes.

Figure 2. Schematic of the (A) dielectric barrier discharge, (B) plasma jet, (C) gliding arcs, and (D) corona plasma reactor, adapted from [43,46,47]. (E) The processes of plasma generation and wastewater treatment processes.

3. Plasma Application to Eliminate Organic Pollutants in Water

The problem of surface and groundwater contamination by toxic chemicals is a major global challenge [48]. The chemistry of plasma and plasma–liquid interactions has received much attention over the past 10 years. Different types of plasma generators are used for the treatment of organic contaminants, as shown in

Table 2. Different types of plasma generators are used for the treatment of organic contaminants.

Plasma Source	Organic Contaminants	Operation Conditions	Degradation	Refs.
DBD reactor	Dimethoate	85 W, 7 min, air	>96%	[51]
DBD reactor	Methyl orange, methylene blue	4.5 W, 10.6 kHz, 10 min, O2	~98%	[52]
DBD reactor	Phenol	16 kV, 50 s	99%	[53]
DBD reactor	Dichlorvos	80 kV, 8 min	78.9%	[54]
DBD reactor	Ceftriaxone	5 min, O2	100%	[55]
DBD reactor	Caffeine	10 min, O2	80%	[55]
DBD reactor	Tobacco mosaic virus	20 kV, 10 min, air	Complete inactivation	[56]
Plasma jet	PFCs	300 W, 3–5 min, air	64–90%	[57]
Plasma jet	Ciprofloxacin	24 min, air	84.1%	[58]
Plasma jet	SARS-CoV-2	10.89 W, 300 s, argon (99.999%)	99.94%	[59]
Plasma jet	Swine coronavirus PEDV and SADS-CoV	10.89 W, 300 s, argon (99.999%)	99.86%, 99.74%	[59]
Plasma jet	Norovirus (feline calicivirus)	2.5 W, 15 s	6.0 log inactivation	[60]
Plasma jet	Potato virus Y	3 W, 1 min, Ar/O2 (99:1)	Complete inactivate	[61]
Corona plasma reactor	Bromate	20 kV, 60 min	95%	[62]
Corona plasma reactor	Seven recalcitrant pharmaceuticals	15–66 min	45–99%	[63]
Corona plasma reactor	Paracetamol	30 min, air/O2	100%	[64]
Corona plasma reactor	Carbofuran	24 min, air	91%	[65]
Arc plasma reactor	Perfluoroalkyl carboxylates	150 W, 60 min	98%	[66]
Arc plasma reactor	Anthraquinonic acid green 25	9 kV, 60 min	50%	[47]
Glow discharge	Pentachlorophenol	30 min, air/O2	<0.01 ppm	[67]
Glow discharge	Methyl orange	15 min, air	93%	[68]
Needle-plate reactor	Phenol	20 kV, 30–60 min	>99%	[69]

. The reactive radical production in plasma is the primary source for water purification [49]. Many harmful substances, including organic compounds, phenols, organic dyes, and pesticides, can be removed with the help of plasma-generated RONS. The effect of plasma treatment on different types of organic and inorganic compounds in drinking water or wastewater has been studied. The degradation pathways of some organic pollutants have also been predicted (

Table 3. Degradation pathways of organic contaminants degraded by APP.

Organic Contaminants	Degradation Pathway/Intermediates	Products	Refs.
Azophloxine		CO2, H2O	[70]
Dimethoate		PO43−, SO42−, NO3−, CO2, H2O	[71]
Nicotine		Nicotinic acid, CO2, H2O	[72]
PFOS		SO42−, H2O, F−, CO2	[73]
2-4-D		CO2, H2O	[74]
Phenol	(Hydroxylation) (Nitration/nitrosation)	CO2, H2O	[75]
Methyl orange		SO42−, NO3−, CO2, H2O, R-COOH	[76]

). The outcomes of the study by Judée et al. [50] showed that the concentration of some chemicals (e.g., ammonia (NH₃), ammonium ions (NH₄⁺), carbonate ion, bicarbonate and carbonic acid, etc.) in tap water changed after plasma treatment. Notably, the interaction of chemicals in tap water during plasma treatment is to blame for these modifications. Plasma can significantly improve the sensory properties of water, while raising the concentration of soluble oxygen and decreasing the concentration of unfavorable factors such as odor. On the other hand, plasma also negatively affects turbidity.

3.1. Pharmaceuticals

Another important form of micro-pollutant in water are pharmaceuticals, and even small amounts can accumulate in living organisms and cause serious health problems. Numerous studies have demonstrated that plasma therapy can break down medicines more efficiently than typical AOPs, and with less energy construction than ozone (O₃) treatment [63,77,78]. One of the most important benefits of plasma is its minimal environmental impact. Plasma generates reactive substances, such as ∙OH, hydrogen peroxide (H₂O₂) and O₃, which not only react with the contaminants on the surface but also diffuse into the interior of contaminants [79,80,81].

3.1.1. Antibiotics

Antibiotics are widely studied because bacteria develop resistance to them. Reports on the removal of antibiotics such as atenolol [82], verapamil [83], and enalapril [84] by plasma have been published. Plasma was used both above and below the water surface to break down oxytetracycline hydrochloride (OTC) and doxycycline hyclate (DXC) for removing antibiotics from water. This allows researchers to determine which plasma discharge method eliminates antibiotics from water the fastest. Due to their wide range of antibacterial activity, accessibility, and affordability, those two pharmaceutical drugs are mostly commonly used in veterinary care. Due to their poor biodegradability, they can be found in high concentrations in groundwater, surface water, and soil [85]. According to the study results of El Shaer et al. [86], the type of plasma produced below the water’s surface in a discharge vessel with air bubbles is more effective in breaking down antibiotics than plasma produced in air above the water. Additionally, antibiotic residues in water are also decreased with longer plasma treatment times. OTC was reduced by 30% of its initial value in plasma generated above the surface of water after 90 min of exposure, whereas DXC was reduced by just 3%. The period needed for plasma below the water surface was much less; after exposure to the plasma for less than 20 min, both antibiotics practically vanished. As a result of the degradation of the antibiotics, DXC displays one degradation product along with plasma discharge, whereas OTC displays two. Recently, a setup that incorporates a cavitation tube and plasma discharge in the air has been used to treat two common antibiotics (sulfathiazole and norfloxacin) and an azo dye (methylene blue). The setup improves the barrier to gas–liquid mass transfer by increasing the mass transfer of active species generated from cold plasma discharge from gas to liquid [87]. Results show that when operating at a flow rate of 5 L/min and a volume of 500 mL for 30 min, all three pollutants with an initial concentration of 8 mg/L or lower can be degraded to >80%.

3.1.2. Recalcitrant Pharmaceuticals

Pharmaceuticals are increasingly present in greater amounts, which burdens the ecosystem and might endanger drinking water sources. Banaschik et al. [63] developed a plasma reactor with a coaxial geometry to generate high-volume corona discharges directly in water. They also looked into the breakdown of seven resistant medications (carbamazepine, diatrizoate, diazepam, diclofenac, ibuprofen, 17a-ethinylestradiol, and trimethoprim). According to the authors, the efficient destruction of persistent chemical compounds is the main technical advantage of corona discharge generated directly in water. For procedures lasting 15 to 66 min, the majority of the examined medications were efficiently degraded by 45% to 99%. Diclofenac and ethinylestradiol were specifically and quickly destroyed. In recent research [88], in order to improve the treatment of wastewater contaminated with diclofenac, microbubble formation was coupled with cold plasma technology. The discharge voltage, gas flow, starting concentration, and pH value all had an impact on the degradation efficiency. The best degradation efficiency was 90.9% after a 45 min plasma bubble treatment under the optimum process parameters. Another study used pulsed corona discharge on a fluid produced in O₂ to degrade ibuprofen in water. Interestingly, it was observed that ibuprofen disappeared entirely after 20 min of plasma exposure, and mineralization rose to 76% after one hour [89]. It is important to note that mineralization is a crucial step in the transformation of toxic drugs into non-toxic drugs. The effectiveness of drug degradation is directly correlated with the rate of mineralization. Another compound called carbamazepine, known to be harmful to aquatic species, is the most prevalent environmental drug identified, and has endocrine-disrupting activity [90]. To deal with this issue, Liu et al. [91] devised a low-power in situ plasma treatment technique that demonstrated high carbamazepine breakdown efficiency when compared to DBD plasma.

3.1.3. Pesticides

The use of organophosphorus insecticides has increased agricultural output significantly. However, the accumulation of organophosphorus pesticide residues in the air, soil, food, surface water, and wastewater has become a substantial environmental hazard due to their widespread usage and moderate persistence [92,93]. Hu et al. [51] investigated the destruction efficiency of DBD plasma devices with different structures and discharge powers for pesticides (dimethoate and dichlorvos). The results showed that a smaller gap distance (5 mm) and a larger discharge power (85 W) could obtain a higher destruction efficiency. The authors also demonstrated the DBD reactor had a significant effect on pesticide degradation. More precisely, it was observed that when the initial concentration of organophosphorus pesticides rose, the rate of pesticide degradation fell. In another study by Hu et al. [71], the identification of dimethoate intermediates and products was explored using GC–MS, UV–VIS spectrometry, and FTIR techniques. The corresponding DBD plasma degradation pathways of dimethoate were also proposed (Table 3). Additionally, different 2,4 dichlorophenoxyacetic acid (2,4-D) intermediates were detected (Table 3), and the degradation pathway of 2,4-D in plasma treatment process was suggested in a study by Singh et al. [74]. These observations have important implications for the development of wastewater treatment, particularly the use of organophosphorus pesticides. In addition, it is impossible to overlook the negative effects other pesticides have on the environment and human health. Pesticides may contain the carcinogenic ingredient pentachlorophenol (PCP), which is difficult to break down naturally [94]. Sharma et al. [67] used glow discharge electrolysis to show how PCP can be reduced to a low level in water. For process periods under 0.5 h, PCP reduction was achieved below detection limits (0.01 ppm). The highest rates of breakdown occurred when using air or O₂, indicating that O₃ chemistry was crucial. The study also discovered that this method’s power cost was competitive with that of other traditional advanced oxidation techniques, such as UV/peroxide or UV/O₃.

3.2. Organic Chemical Reagents

Phenol is a widespread pollutant in wastewater; it has teratogenic and carcinogenic activity. Lukes et al. [69] looked into the plasma treatment’s ability to degrade phenol. When phenol liquids are subjected to plasma, reaction products can be identified using high-performance liquid chromatography (HPLC). Further research has shown that hydroxylated degradation products of phenol (catechol, hydroquinone, 1,4-benzoquinone and hydroxy-1,4-benzoquinone, see Table 3) were detected for APP. In this case, however, nitrated phenol degradation by-products (4-nitrophenol, 2-nitrophenol, 4-nitrocatechol and 4-nitrosophenol, see Table 3) were detected in addition to the hydroxylated products. Another earlier study found that the degree of phenol decomposition increased as solution thickness decreased under the impact of pulsed corona discharge in O₂ [95], and an identical effect was seen during glow discharge electrolysis [96]. Additionally, DBD discharge showed a decline in breakdown efficiency as phenol content rose [97]. In this case, most research, at least for the acid media wherein it occurs in our instance, presume that the principal events leading to the destruction are interactions of phenol with ∙OH [96,98] (reactions (1) and (2)). This points out the significance of plasma acids in the purification of effluents.

C₆H₅OH + ∙OH → H₂O + ∙C₆H₄OH

(1)

C₆H₄OH + ∙OH → C₆H₄(OH)₂

(2)

According to earlier research [99], carboxylic acids are converted to aldehydes under discharge conditions. In particular, it has been mentioned that hydroxylated phenols and carboxylic acids are the breakdown byproducts [100]. One can assume that the main byproduct of the interaction between phenols and free radicals is the formation of acids from hydroxylated phenols. A recent study has confirmed that the products of plasma degradation of phenol are carboxylic acids; these acids are further transformed into aldehydes, which are then further degraded to CO₂ and H₂O [101]. In addition, the degradation of a mixture consisting of phenol and other organic substances by plasma has been developed now. Each kind of runoff water contains chloride anions [102], so the authors performed a set of experiments with model solutions containing phenol and KCl to evaluate the overall degradation efficiency [103]. The results show it is evident that the part of chloride ions is converted into an available chlorine species via the plasma treatment of these model solutions. The available chlorine is able to increase the overall purification efficiency. Therefore, another pathway of organic pollutant degradation exists in this process, which involves available chlorine, in addition to the reaction of the organic matter with the active species of the O₃ and O₂ plasma.

The major component of the acid in the water generated by oil and gas extraction is acetic acid [104]. The results demonstrated that adding O₃ significantly increased the degradation of the highly concentrated acetic acid solution [105]. Researchers have developed a simple multiple plasma generation system to achieve efficient decomposition of persistent organic compounds in water [106]. By altering the capacitance of the ballast capacitor, which establishes the plasma input power for each plasma treatment orifice, it is possible to regulate the rate of reaction of H₂O₂ and O₃. The total organic carbon (TOC) reduction rate and efficiency increased significantly upon adding plasma-treated gas into the solution, which enhanced the reactivity of ∙OH produced by H₂O₂ and O₃. Despite the plasma approach’s slower rate of decomposition being a disadvantage in comparison to these AOPs [106], the key benefit of the plasma technique over AOPs that use chemicals is that no chemical transportation or storage is required. This feature offers a substantial advantage over chemical-based AOPs when it comes to treating wastewater in factories far from the land.

Dimethyl phthalate (DMP) is a chemical that is frequently used to create polymers and insect repellents. DMP was eliminated from liquids by Qi et al. [107] using an AOP method based on micro-plasma. It has been demonstrated that plasma-generated O₃ and ∙OH are the most active species in the degradation of DMP liquids, interacting with a sizable number of DMP functional groups for degradation. According to this study’s findings [107], plasma exposure may be a useful method for the cleaning of DMP wastewater.

3.3. Azo Dyes

According to research by Rodrigo et al. [108] and Gao 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% O₂ 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 Biljana 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]. Gumuchian et al. [115] evaluated the production of ∙OH in feed gases with different compositions, and discovered that feed gas mixes containing Ar and O₂ 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 O₃ and O₂ as well. However, Yasushi et al. [116] demonstrated that the formation of O₃ is minimal when the concentration of Ar in the mixture of Ar and O₂ is equal to or higher than 80%. Since Mitrovi et al. [49] utilized 90% Ar in their trials, they believed that O₃’s effect was not particularly significant.

Shimizu 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 O₃ and ·OH in dye wastewater under plasma degradation conditions, which affects the wastewater’s efficacy in degrading dye to some extent. The BOD₅/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 Attri et al. [52], O₂ 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 O₂ gas plasma included higher quantities of H₂O₂, O₃, and ∙OH than other feeding gases. Furthermore, the O₂ gas plasma shows strong methylene blue decolorization, achieving nearly 98% decolorization after 10 min of treatment. H₂O₂ 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 O₃. 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% N₂ 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, NO₂, and NH. According to Grabowski et al. [122], pH has a major effect on the oxidation of methylene blue because a higher pH increases the amount of O₃ that permeates the liquid phase, while a lower pH decreases the amount of O₃ that is absorbed. This could explain why there was more decolorization in the O₂ plasma reactor than in the air plasma, as the pH in the O₂ 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, Huang 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. Xu 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.

3.4. Common Industrial Pollutants

Perfluorinated compounds (PFCs) are utilized in numerous industrial sectors, including nonstick cookware, packaging, textiles, and more. As bio-recalcitrant anthropogenic compounds, these substances represent one of the most degradation tolerance types of growing concerns in chemical composition. According to Parsons et al. [127], these molecules are durable in the environment because of their distinctive C-F bond, which is recognized as one of the most robust organic bonds in the field of chemistry [127]. In recent years, the efficient degradation method of perfluorinated and polyfluoroalkyl substances (PFAS) has garnered more attention due to the ineffectiveness and high cost of existing degradation processes, particularly in wastewater. The efficiency of eliminating three particular PFAS from water, namely perfluorooctanoic acid (PFOA), perfluorohexanoic acid (PFHxA), and pefluorooctanesulfonic acid (PFOS), was investigated by Davide et al. [128] using a specially designed plasma generator. PFOS (1 ppb) demonstrated the greatest disintegration after plasma treatment, fully disappearing after 30 min, but PFOA and PFHxA decayed more slowly, at roughly 65% and 83%, respectively. Another study [57] investigated the plasma breakdown of PFCs (PFOA and PFOS) in wastewater created during the removal of PFCs from contaminated soil. Studies showed that the original PFC levels in water samples can be reduced by more than 50% in under 200 s with the use of plasma treatment, which demonstrates its reliability as a replacement approach. The assessed variables are significantly correlated with the final PFC concentration. Additionally, a notable non-linear relationship between time and the treatment impact was noticeable. These results suggest that plasma may be a useful tool for decreasing the strong C-F bonds found in PFCs. According to Blotevogel et al. [129], with the ability to destroy PFAS down to very low parts-per-trillion levels in various water matrices; these advanced water treatment technologies offer solutions to tap into the great wealth of unconventional water resources for beneficial use. Ammonia, chlorine, food additives, chemical disinfectants, cleansers, and aids, polychlorinated biphenyls, or pathogenic microorganisms, including fecal coliforms, are typical contaminants in effluent from the fish sector [130]. Previous studies have demonstrated that plasma can effectively degrade these pollutants from the seafood industry, thus enabling the reuse of treated fisheries’ wastewater [131,132]. Plasma treatment can replace more traditional wastewater treatment methods like O₃ or chlorine, since it generates significant levels of ROS in the solution. Although Patange et al.’s [133] research shows a marginal rise in the ecotoxicity of plasma-treated wastewater, this technology is still more environmentally benign than chlorination [132]. Recently, Naicker et al. [134] noted that with the envisioned success of plasma technology at the industrial level, a plasma treatment unit should be incorporated following chlorination to deal with the recalcitrant pollutants.

Heavy metal salts are commonly found in the wastewater of various industries, including chemical, textile, electroplating, and others. According to Wang et al. [135] and Khlyustova et al. [136], these salts often contain components of lead, iron, nickel, chromium, iron, cuprum, zinc, and arsenic. The removal of heavy metals from industrial effluent is a crucial aspect of wastewater purification technology. However, current degradation methods rely on the high consumption of chemical reagents and extreme conditions [137,138], which makes it difficult to reduce the environmental risks posed by heavy metals in industrial wastewater. Pervez et al. [139] used a high-voltage corona plasma to treat wastewater containing lead, cadmium, chromium, and nickel. The outcomes have shown that considerable reductions in lead, chromium, and nickel were accomplished utilizing high-voltage corona discharges, while virtually little decrease was obtained in the case of cadmium (Cd). It is evident that plasma discharge was a successful method for removing heavy metals from wastewater. According to another study [140], it is possible to simultaneously decrease Cr(VI) to Cr(III) and oxidize As(III) to As(V) using glow discharge plasma. By increasing the voltage input from 530 V to 600 V, the conversion of As(III) and Cr(VI) can also be effectively increased from 96% to 100% and 53% to 77%, respectively. Additionally, the authors pointed out that H₂O₂ produced in the glow discharge plasma can reduce Cr(VI) and produce highly oxidizing ∙OH, which is accountable for the oxidation of As(III). In turn, the reaction between As(III) and ∙OH prevented Cr(III) from being re-oxidized to Cr(VI) by ∙OH, favoring the net conversion of Cr(VI). Recently, Munnaf et al. [141] pointed out that plasma-generated reactive species, which are O, ∙OH, H₂O₂, and NOx, were promoted from As(III) to As(V) and Fe₃O₄ to Fe₂O₃. The plasma catalytic removal of As(III) was confirmed by the high eradication %, when compared without plasma. These investigations show that plasma may co-convert Cr(VI) and As(III) in aqueous solutions, which has great promise for global wastewater remediation.

3.5. Viruses

The most prevalent and diverse group of microbes on Earth are viruses, which have the potential to infect every cell-based organism, including bacteria, people, animals, and plants [142]. It is possible to spread viruses directly from one sick individual to another or indirectly through polluted intermediaries (such as contaminated surfaces, objects, air, food, and water) [29]. Each year, harmful viruses infect tens to hundreds of millions of plants, animals, and people, causing huge losses in agriculture and widespread deaths. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-related COVID-19 pandemic has demonstrated that transmission via contaminated surfaces and aerosols is extremely important [143]. To stop virus transmission in different matrices, a variety of virus inactivation techniques have been attempted, but regrettably, the most successful strategy has not yet been discovered [144]. In addition, viral transmission through wastewater is also a major concern, particularly in places with inadequate water and sanitation systems [143,145]. The implementation of suitable wastewater treatment technology is urgently needed in light of reports of SARS-CoV-2 transmission or coronavirus transmission through wastewater [146]. Considering the aforementioned issues, the adoption of appropriate wastewater treatment technologies is imperative to protect the environment and human health. According to recent studies, plasma is a safe, simple, and effective treatment procedure that effectively inactivates viruses while producing no waste or harmful byproducts. Plasma has been used to inactivate viruses in different matrices (Figure 3).

Today, HIV/AIDS is still arguably the biggest public health issue, especially in low and middle- income countries. Currently, antiretroviral therapy is currently being used to treat HIV infection, and has been demonstrated to prolong and improve the health of HIV-positive individuals (https://www.who.int/news-room/facts-in-pictures/detail/hiv-aids, accessed on 25 September 2023). Plasma was tested for its effects on HIV-1 replication in monocyte-derived macrophages (MDM) by Volotskova et al. in 2016 [147]. Before exposure to HIV, cells were given plasma therapy utilizing three injections (45 s each at 4.5 kV) per well in a 24-well plate. The study demonstrated that pre-treating MDM with plasma decreased viral reverse transcriptase activity by more than two-thirds, and inhibited the other steps necessary for a successful virus infection (i.e., by preventing virus-cell fusion, inhibiting viral reverse transcription, and impeding viral integration), without having any cytotoxic effects on the macrophages. The possible immunomodulatory mechanisms of plasma that may be used to induce antiretroviral therapy-free control of HIV-1 recrudescent infection have been reported for the first time by Mohamed et al. [148]. The fact that plasma can be employed to improve latently infected cells’ capacity to trigger an immunological response to HIV-1 serves as the foundation for the authors’ suggested immunotherapeutic strategy for treating HIV infection. Another example stems from the report by Sutter et al. [149]; herpes simplex virus type 1 (HSV-1) is an infectious agent, and the capacity of HSV-1 to control the oxidative stress response and create a cellular milieu that supports HSV-1 reproduction is crucial to the pathogenesis of the virus. In recent years, plasma has gained attention as a potential treatment for HSV-1 infection due to of its capacity to produce or release RONS, which can alter the redox balance of infected cells. The capacity of plasma to create and transport these RONS under control makes it a potential antiviral treatment that may be utilized to treat infection w viruses like HSV-1. During plasma therapy, plasma provides an extra layer of oxidative stress to limit HSV-1 infection. Thus, plasma has also proved to be a powerful disinfection method for eliminating viruses from hospital equipment and dinnerware (e.g., plastics and fabrics), such as norovirus (NoV), SARS-CoV-2 and Omicron variants, porcine respiratory coronavirus (PRCV), and coxsackievirus B₃ (CVB₃) [59,150,151,152,153].

Furthermore, treatment using plasma has been employed to combat three significant animal pathogens: avian influenza virus (AIV), Newcastle disease virus (NDV), and porcine reproductive and respiratory syndrome virus (PRRSv). These viruses cause a major threat to global food security and economic stability, and some strains of NDV can even result in mortality of up to 100% in some birds [154]. Currently, vaccination is the most efficient strategy to stop their spread. Because it effectively inactivates viruses with quick treatment times, plasma has been considered a potential inactivation method in the creation of vaccines. According to prior research, both NDV and AIV may be completely inactivated after 2 min of plasma treatment [155]. This is consistent with the specifications for vaccine preparation that guarantee total virus destruction without altering the antigens necessary to elicit an immune response and lower the likelihood of illness development [156,157]. Moreover, a sufficient treatment period can be used to produce inactivated AIV and NDV vaccines and cause equal or higher specific antibody titers when compared to the conventional formaldehyde inactivation method [155]. The treatment of feline calicivirus with a plasma jet for 15 s has proven to be one of the most successful liquid medium inactivation events [158,159]. This short-duration treatment suggests that plasma holds promise as an important means of inactivating enteroviruses in liquids, but is now only effective against small droplets of potentially contagious material.

Plant viruses are among the earliest known viruses and the majority of virus-to-plant transmission is facilitated by insects [160]. However, the high levels of water pollution and modifications to irrigation systems have recently led to water contamination, which now plays a role in virus transmission. Tobamovirus particles, including tobacco mosaic virus (TMV), exhibit exceptional resilient to physical and chemical threats [161], and they can survive in soil, water and agricultural equipment where they can infect commercially important crops [162]. Tobacco plants were inoculated with DBD plasma-irradiated and non-irradiated TMV solution in Hanbal et al.’s [56] investigation. The results showed that leaves inoculated with non-irradiated TMV displayed localized necrotic lesions, while those inoculated with irradiated (20 kV, 10 min) TMV did not. Additionally, it was discovered that plasma irradiation can damage TMV RNA and collapse viral particles to the sub-unit level, causing them to lose their infectivity. This was determined by analyzing viral infectivity, particle structure, coat protein sub-units, and genomic RNA of TMV solution after plasma irradiation. Another study found that even in water samples with either a high or low organic background, just one minute of plasma jet treatment was sufficient to inactivate the potato virus Y (PVY) successfully [61]. ROS was identified as the primary mechanism of viral inactivation through plasma-mediated inactivation, which was efficient even at virus concentrations significantly higher than expected in irrigation water.

4. Effect of Different Discharge Parameters on Wastewater Purification Efficiency

Many studies have reported the effect of discharge mode, reactor type, configuration/geometry on wastewater purification efficiency. The study by Iervolino et al. [55] pointed out that the degradation of pollutants in water and the avoidance of the formation and accumulation of intermediates in the treatment system can be achieved under suitable operating conditions. Based on the works of literature on the decolorization of industrial dyes [163,164,165,166,167,168,169], the present decolorization rate depends upon the gas flow rate, working gas composition, duration of the voltage pulses, gap distance, injected power, barrier thickness and types of electrodes materials, etc. Therefore, the process parameters and treatment conditions must be optimized in the treatment process to improve the decolorization rate [170].

Recent studies point out that the plasma treatment process is more significantly influenced by the quality and quantity of the input gas [171]. Reddy et al. [172] reported that an increase in the gas flow rate improved the removal efficiency of pollutants by increasing the amounts of reactive species and O₃ production. However, beyond the optimal value of the gas flow rate, the increase in degradation efficiency was not significant. Jovicic et al. [57] evaluated the impact of various plasma gases (air, O₂, and N₂) on the PFC reduction in water. It has been shown that the amount of PFCs in the treated samples decreased overall, and the concentration of H₄PFOS increased from <0.01 μg/L to >0.7 μg/L when O₂ was utilized as the working gas. A possible route to produce this compound starts from ROS (such as O₃) initiating a breaking up of the C-F bond in a PFC chain, while the fluorine atoms can be replaced by hydrogen atoms from water. The N molecules that had been excited in the N₂ plasma were the dominating radicals, and because of their far lower reduction potential than O₃ radicals, they were unable to significantly break down PFC. Therefore, of all the analyzed gases, the working gas air produces the best results.

In another study, Kim et al. [173] investigated the impact of O₂ concentration on the breakdown of antibiotics and discovered that O₂ outperformed dry air. Furthermore, DBD plasma produced in O₂, N₂, Ar and air gaseous environments was employed by Attri et al. [52]. This study demonstrates that while N₂ and O₂ gas plasma discharge powers are equivalent, pure air plasma discharge powers in their system are incredibly high. Compared to other feeding gases, the DBD plasma with O₂ as the working gas leads to higher decolorization in a shorter amount of time. which is explicable by the fact that more ROS may be transported to the solution, increasing the decolorization efficiency [52].

The discharge power of the plasma has an equally significant influence on the decline in the mineralization efficiency of the contaminants. Among other things, an increase in input power points out to a significant increase in pollutant removal rates [172]. Chen et al. [174] found that increasing the supply voltage during the degradation of phenol in aqueous media with pulsed high-voltage discharge plasma increased the removal efficiency from 55% to 86%; similar results were found by Kim et al. [173]. These results were primarily caused by the intensification of the discharge, which increased the number of active species during the discharge process, the increase in electron energy in the electric field, and the further ionization of the medium (O₂ and H₂O molecules) during plasma generation.

In the same study by Jovicic et al. [57], it was shown experimentally that the final concentration level of PFCs is highly dependent on the reactor nozzle geometry, that is, whether the size of the plasma jet is adequately matched with the size of the reactor zone, which has a significant impact on the final concentration level of PFCs. The best results were observed when small reactors were paired with big nozzles, showing the strongest PFC concentration reduction levels, indicating that under these circumstances, it is possible to obtain high-plasma-radical concentrations, which are beneficial for the breakdown of C-F bonds. Another study demonstrated that the discharge gap impacts the strength of the electric field, and the distance between electrodes is essential for the efficiency of hazardous chemical degradation [171]. The time needed to obtain a specific pollutant removal efficiency will unavoidably grow as the electrode distance increases. Furthermore, as reported by Sano et al. [175], according to their research on the degradation of organic compounds in water, the cathode gap must be set at a specific value for a particular voltage input to maximize rhodamine B degradation. The properties of the plasma may change over time due to variations in the amount of H₂O molecules in the gas during the treatment, according to Bobkova et al.’s [96] research on the plasma degradation of phenol solutions. The degradation rate constant will be impacted by the solution’s rising temperature during the treatment process.

5. Conclusions and Future Perspectives

In recent decades, due to the dramatic rise in water contaminants, plasma technology for the treatment of raw and industrial wastewater has drawn the attention of academics and technologists. This review lists various plasma discharge techniques (e.g., DBD, plasma jets, gliding arc discharge, and glow discharge, etc.), and illustrates the impact of various plasma parameters (e.g., input gas, discharge power, discharge gap, cathode gap, etc.) on effluent purification effectiveness. A thorough description of the degradation of a range of typical organic pollutants by plasma is also included in this manuscript. These pollutants include pharmaceuticals (antibiotics, recalcitrant pharmaceuticals and pesticides), organic chemical reagents, azo dyes, common industrial pollutants, viruses and more. In comparison to other conventional approaches and AOPs, plasma has also demonstrated excellent efficiency in eliminating numerous aqueous contaminants, including SARS-CoV-2, a lethal and contagious virus in wastewater. These studies have demonstrated the potential of plasma–liquid interaction as a novel water management strategy for the removal of pollutants from drinking water.

However, looking ahead, in order to advance plasma research and development for commercial use and industrial application, there are still numerous challenges to be overcome. (1) The performance of plasma-based water purification in small-scale experiments clearly illustrates the promise of this technology. Scaling up may still be the biggest challenge. (2) Given the diversity and uniqueness of plasma generators, the “plasma dose,” as the fundamental of plasma application, is still inconclusive. (3) Better understanding of the physical processes occurring at the interface, and how best to control them, is key to further improving reactor efficiency. (4) The existing literature has generally considered the degradation mechanisms and performance of APPs, while little research has been done on the energy yield of APPs. In order to bring APP as close to the field as possible, more comparative examinations will be needed, using APP energy yield as a key factor in implementation. (5) The fate of the intermediates created during the treatment process of plasma-driven water purification remains unknown. (6) Most of the current studies involve single-component treatment, and there is a lack of research on the treatment of complex organic molecules. The exclusion efficacies of the considered compounds may not be transferable to actual situations when applied to organic molecule mixtures, and re-active species interactions among pollutants may appear. Plasma technology faces issues more generally, but this is one that requires additional exploration. More detailed quantitative data and parameters from experimental studies are still required to advance this topic.