A pulsed high-voltage electrical discharge can begin in gas phase close proximity to the liquid surface, liquid phase and gas-liquid phase. When in liquid, it produces spark, streamers and corona, which are called pulsed spark discharge (PSD), pulsed streamer corona discharge (PSCD), and pulseless corona discharge (PCD) respectively. When it discharges on surface of water (WSP), it can only produce corona, arc and so on [4].

Glow discharge plasma (GDP) is one of them where the plasma is sustained by glow discharges across a pointed electrode and the surface of the liquid electrolyte [21].

Gao Jinzhang et al. [21-23] made a series study of the oxidative degradation of phenolic compounds (o-chlorophenol; 2, 4-dichlorophenol; 4-chlorophenol (4-CP)); in aqueous solution induced by plasma with submersed glow discharge has been investigated. Various influencing factors such as the initial pH, the concentration of phenolic compounds and the catalytic action of Fe²⁺ were examined. It was showed that degradation of phenolic compounds can be efficiently increased in alkaline conditions with the addition of Fe²⁺. Liu, Y. J. et al. [24] also found that adding appropriate amounts of Fe²⁺ or Fe³⁺ to the solution was favorable for phenol degradation by plasma with submersed glow discharge. Furthermore, carbonate ions or n-butanol in the solution can decelerate the phenol removal. When the treatment time is increased and the pH value of the solution is decreased, leading to an increase in the phenol decomposition.

Wang L. et al. [25] investigated the degradation of bisphenol A (BPA) and simultaneous formation of hydrogen peroxide induced by glow discharge plasma in contact with a queous solution. Experimental results indicated that the BPA degradation rate was higher in sodium chloride solution than that in sodium sulfate or phosphate solutions. However, the formation rates of hydrogen peroxide were on the opposite case.

 The schematic diagram of the glow discharge set-up is shown in Figure 2 [22].

The gliding arc discharge is a simple and inexpensive way to generate non-thermal plasma. It has a dual character of thermal and non-thermal plasma, and can involve relatively high electric powers compared with the corona discharge [26].

Ch. M. Du et al. [21-24] has designed a gliding arc reactor in their laboratory with the aim to degrade aqueous phenol solutions and 4-CP solution at atmospheric pressure. They have already made a series study on the various factors affecting the degradation efficiency, including supply voltage, electrode gap distance, and gas–liquid flow properties. The efficiency, which steeply increases when increasing the supply voltage, can reach 96% when the minimum electrode distance is fixed at 3 mm. Experiments show that their degradation efficiency also depends on solution pH, Fe²⁺ addition, gas nature and gas flow rate. Furthermore the chemical oxygen demand (COD) abatement of 4-CP solution with stainless steel electrode is higher than that with aluminum or brass electrode .Increasing gas–liquid mixing rate could also increase the degradation of 4-CP.They also used this method to degrade highly concentrated phenol wastewater [31], about 1000mg•L^-1. It was found that the COD removal efficiency is very high, which can reached 88.82% after degradation.

The schematic diagram of the gliding arc discharge reactor is shown in Figure 3 [30].

Depending on the degradation region, discharge process can be conveniently grouped into two categories of effects: 1) localized and 2) extended. Localized effects occur in the immediate vicinity of the plasma channel and include thermal oxidation within the plasma channel, vacuum UV photolysis at the surface of the plasma channel, and supercritical oxidation within the subsequent steam bubbles. On the other hand, extended effects are due to radical reactions in which active species generated in the vicinity of the plasma channel expand outwardly from the circumference [33]. In different discharge types, the radical reactions in localized process are different, but in extended process are nearly the same as each other.

For the pulsed high-voltage electrical discharge in liquid, initial models of the bulk-phase corona-induced chemical reactions were reported by M. Dors et al [34]. In that study it was assumed that the pulsed corona discharge leads to the formation of hydrogen peroxide, hydroxyl radicals, and aqueous electrons through reactions (1-3) as shown in Table 1. The other major species produced by the corona reactor were assumed to be the same as those formed in radiation processes such as electron beam radiation and pulse radiolysis as indicated by the propagation and termination reactions given in Table 1. When degrading phenols, the reactions as reactions (57-68) shown in Table1.

For the pulsed high-voltage electrical discharge in gas and in gas–liquid phase [4,6,19,35], ozone was first produced by gas phase discharge, then went into liquid phase to induce the radical reaction and produced OH radicals (reactions 56-57). Furthermore, reactions between high energetic electrons in discharge streamer and the initial molecules in aqueous solution could form ·OH (as shown in reactions 4-16 given in Table 1).

In the glow discharge, the gaseous water molecules were dissociated into positive water ions and electrons as indicated in reaction [22] (2):

The positive water ions were accelerated in the cathode-fall of the GDP and entered the solution with enough kinetic energy to break up the liquid water molecules to form the following primary species both through energy transfer and through charge transfer (Equations (3)–(5)):

For gliding arc plasma in humid air , it is revealed that OH· and NO· radicals are simultaneously, present in the discharge [36]

In many studies [7,37,38], it was revealed by high performance liquid chromatography and ionic chromatography that the main intermediates of phenol decomposition are hydroquinone, pyrocatechol, p-benzoquinone and organic acids. The OH• radical reacts with phenol to form dihydroxycyclohexadienyl radical •C₆H₅(OH)₂ (DHCHD). Under the circumstance of oxygen, oxygen will attack these radicals to produce dihydroxycyclohexadienylperoxyl (DHCHDP) radicals. The DHCHDP radical may decay in a first-order process to form catechol by elimination of a hydroperoxyl radical. (HO₂•)4-CP [39] was mainly converted to 4-chlorocatechol, hydroquinone, 1, 4-benzoquinone, 2, 4-dichlorophenol, 5-chloro-3-nitropyrocatechol, and 4-chloro-2-nitrophenol, and then, they all degraded into organic acids, such as formic acid, acetic acid, oxalic acid, etc., which converted into CO₂ and H₂O finally.

The intermediate products are easily oxidized further under the action of the radicals and oxygen. The opening of the aromatic ring leads to the formation of low molecular weight compounds, mainly organic acids. DHCHDP form endoperoxides through repeated scavenges of oxygen. These very instable intermediates are decomposed by ring cleavage. The intermediate product or organic acids can also be oxidized through hydroxylation and hydration, finally forming carbon dioxide. Figure 1 shows the degradation pathway of phenol by pulsed electrical discharge in the presence of oxygen [38].

The degradation rates under oxygen gas and oxygencontaining ozone bubbling were greatly improved. When oxygen was bubbled to the aqueous solution in the discharge reactor through the discharge electrode [5], the phenolic compounds in the aqueous solution were degraded through the following steps (As shown by the following equations). First, ozone was formed due to the oxygen discharge in the strong electric field. And then ozone transfered from gas bubble to the aqueous solution

containing phenolic compounds, not only lots of •OH radicals were produced (reaction56, 57) to degrade phenol, but also other species such as ozone, or superoxide (O₂−) and singlet oxygen (¹O₂), participated in the chemical reactions (as shown in Table 1) [16]. The ozone and OH· radical in the aqueous solution could react with the phenols, which was shown as Figure 4.

Shinta et al. [6] studied the effect of added hydrogen peroxide on phenol removal efficiency. They found that the phenol degradation rate was higher in the presence of hydrogen peroxide. It’s rational to assume [10] the reaction scheme where hydroxyl radical would act as the most responsible key-species for the organic degradation in competition with the coupling to hydrogen peroxide. Anto et al. [40] found that the addition of a small amount of hydrogen peroxide greatly increased the degradation rate of phenol, in which the ultraviolet light from discharge plasma was considered to be a cause of effective degradation. However [16], since the reaction of phenol with hydrogen peroxide is very slow at the ambient temperature, the degradation process may be due to the reaction of hydroxyl radicals produced by photolysis of hydrogen peroxide, which was caused by UV radiation from electrical discharge plasma. The chemical effects of the electrical discharge plasma are due to direct photolysis (the electrical discharge plasma only), indirect photolysis (combination of chemical additives and the electrical discharge plasma) and pyrolysis destruction in plasma Channels and pyrolysis destruction in plasma Channels [41]. In conclution, it can be mainly explained by three equations (reaction 36,43,44) as shown in Table 1.

Grymonpre DR et al. [42] found that it was useful to add iron salts to enhance hydroxyl radical formation and increase the efficiency of phenol degradation through Fenton’s reaction (Equations (3)):

It can be seen that in the presence of Fe2+, more free radicals were produced due to the catalysis of Fe2+, which in turn, accelerated the reaction.

Neyens et al. [43] found that by adding Fe²⁺ highly influenced the phenol removal process. With the addition of Fe²⁺ ions the removal increased significantly from 15.45% to 94.94%, but decreased if continuing. Gao Jinzhang et al. also found that the removal rate cannot be further enhanced when the concentration of Fe2+ is over some value (80mg.l) [22]. However Lei Wang et al. [25] demonstrated that Fe³⁺ exhibits a better catalytic effect than Fe²⁺, which is contradictory to the principles of the Fenton reaction because the reaction rate between ferric ion and hydrogen peroxide is much lower than that ferrous ion with hydrogen peroxide. They thought this phenomenon may be attributed to the following. When the aqueous BPA was exposed to GDP, the BPA initially react with hydroxyl radicals to form the intermediate products through the hydrogen abstraction or electrophilic addition to benzene ring of BPA. Lei Wang et al. [44] newly reported that when degrading phenol, one most probable way was by dihydroxycyclohexadienyl radicals, which were resulted from the attack of the hydroxyl radicals on phenol. The dihydroxycyclohexadienyl radicals then can react with Fe³⁺ ions, so Fe³⁺ exhibits a better catalytic effect than Fe²⁺.

Activated carbon particles suspended in the liquid phase have been found to play a role in enhancing-phase corona reactions, in addition to their role as physical adsorbents [19].There exists also the possibility of reactions occurring on the surface of the activated carbon-induced by the electrical discharge, thus continually regenerating the activated carbon [45]. David R. et al. [46] also found that there was a strong possibility that the activated carbon participates in catalytic reactions with phenol and its primary byproducts. This occurs by adsorption of the organic molecules and possible reactions of the phenol on the carbon surface. After the carbon particles have been suspended in the solution treated by the pulsed corona, the physical properties of the carbon remain unchanged. An analysis of the carbon showed that little of the phenol initially adsorbed on the activated carbon was still present after treatment was completed.

Lukes et al. [47] and Hao, X L et al. [48] both studied the non-thermal plasma-induced degradation of phenols in the presence of photocatalytically active TiO₂. It was found that the phenols removal efficiency with TiO₂ was obviously increased. The main effect of TiO₂ addition was in the utilizing of ultraviolet radiation from the plasma resulting in the photocatalytical formation of OH radicals on the surface of TiO₂ particles and, thus, in the increase of the yield of OH radicals available for phenolic compounds degradation [49]. Farre et al. [50] suggested that the OH radical was produced by reaction of absorbed H₂O molecule with photogenerated holes at the illuminated TiO₂ particle. Equation (4).

Because when discharged， there will be much O₃ produced. It can also generate OH• radicals through the formation of an ozonide radical (O₃•−) in the adsorption layer. Equation (5) [51].

Hao, XL et al. compared the 4-CP removals between original TiO₂ and TiO₂ (used thrice), and found no obvious differences