Review on reactive species in water treatment using electrical discharge plasma: formation, measurement, mechanisms and mass transfer

The generation of physical processes involves bubble formation and development [5–7], UV radiation [8–10], cavitation effects and supercritical oxidation [11, 12], high temperature pyrolysis [12], shock waves [13, 14], and electric field effects [15–17]. Bubble formation is mainly involved in the liquid–gas phase discharge plasma process, in which bubbles form when the carrier gas passes through the hollow needle of high-voltage electrodes (the tip of the sharp is 0.2–0.3 mm, and the diameter of the body is 0.7–1.2 mm). Bubbles can also form based on the theory of thermal bubble formation; in this thermal theory, the heat generated by an electric field can produce bubbles, and then the electric field can propagate through the bubbles in a manner similar to that observed in the gas phase [21]. In the presence of gas bubbles, preexisting current resulting from the electron avalanche can heat the surrounding water, and then low density regions can be formed for discharge plasma initiation and propagation. Therefore, discharge plasma can be triggered much more easily in the presence of gas bubbles because a much lower initiation voltage is needed compared with direct liquid phase discharge [6]. Generally, a strong electric field (approximately 10⁹ V m⁻¹) is necessary to initiate liquid phase discharge, while about 10⁶ V m⁻¹ is enough to realize gas phase discharge [22]. Discharge in liquid in the presence of bubbles is an effective pathway for reactive species formation, which can enhance the diffusion of reactive species into the solution, benefiting contaminant degradation.

UV radiation is one of the main forms of discharge power dissipated from electrical discharge in water [8–10]. Generally, the UV radiation from atmospheric pressure gas phase discharges is weaker than that from liquid phase discharges. The types of UV radiation from atmospheric pressure gas phase discharges usually fall within the UVA region with wavelength of 320–400 nm and UVB with wavelength of 280–320 nm; whereas vacuum ultraviolet (VUV, 100–280 nm) can be formed in discharge plasmas in direct contact with water [23]. VUV irradiation efficiently induces chemical reactions because the absorption of VUV by water molecules is large. For example, the absorption of VUV by water containing dissolved oxygen can result in the dissociation of water and O₂, and then ·OH, hydrated electrons, and ·O can be generated. UV radiation can induce chemical reactions with lots of organic pollutants, nitrates and nitrites after penetrating into aqueous solution, as shown in reactions (1)–(5) [24]. In addition, UV radiation can activate some contaminants, resulting in the breakage of the contaminants' molecular bonds, and finally lead to their degradation. Sun [18] reported that there existed an optimum peak pulsed discharge voltage to achieve the strongest UV radiation, and approximately 3.2% of the input energy was released as UV radiation. It was reported that UV radiation was able to contribute to more than 10% of phenol decomposition in a pulsed discharge plasma system [19]. Mok [20] found that the contribution of the UV radiation generated in a dielectric barrier discharge plasma system applied to a wastewater decoloration process was approximately 42%. The UV radiation effect during a discharge plasma process was investigated by Robinson [25], who reported that approximately 28% of the discharge energy was transformed into UV radiation; in that study, the stored capacitor energy was 1500 J, and 420 J was converted to UV radiation with a peak radiant power of 200 MW. Lukes and co-workers [26] found that increasing the electroconductivity of the solution could increase the flux of UV radiation. Matsumoto et al [27] reported that UV radiation played significant roles in bacteria inactivation. Moreover, UV radiation was also generated in the gas phase discharge plasma process (above the water surface) [28]. In our study, we also found that UV radiation contributed to methyl red decoloration in a gas phase surface discharge plasma system [29].

$$\begin{eqnarray}&&{hv}+{{\rm{H}}}_{{\rm{2}}}{\rm{O}}\to \cdot {\rm{H}}+\cdot {\rm{OH}},\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{hv}+{{\rm{H}}}_{{\rm{2}}}{\rm{O}}\to {{\rm{H}}}_{2}+{\rm{O}},\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{hv}+{{\rm{NO}}}_{2}^{-}\to \cdot {\rm{NO}}+\cdot {{\rm{O}}}^{-},\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&hv+{{\rm{NO}}}_{3}^{-}\to \cdot {{\rm{NO}}}_{2}+\cdot {{\rm{O}}}^{-},\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&hv+{{\rm{NO}}}_{3}^{-}\to {{\rm{NO}}}_{2}^{-}+\cdot {\rm{O}}.\end{eqnarray} \tag{ 5 }$$

Supercritical oxidation and cavitation effects are both important physical processes during aqueous discharge plasma, and they can participate in the organic contaminants degradation process. Zhang et al [11] reported that supercritical oxidation was one of the significant effects in the aqueous discharge plasma process. Shi et al [12] found that approximately 68% of p-nitrophenol degradation efficiency could be attributed to the high temperature pyrolysis initiated by supersonic cavitation effects. One important mechanism derived from high temperature pyrolysis is the generation of ·OH in a core high temperature region, as shown in reaction (6), after which a series of other chemical reactions are triggered [30].

$$\begin{eqnarray}&&{\rm{M}}+{{\rm{H}}}_{{\rm{2}}}{\rm{O}}\mathop{\longrightarrow }\limits^{{\rm{thermal}}}\cdot {\rm{H}}+\cdot {\rm{OH}}+{\rm{M}}.\end{eqnarray} \tag{ 6 }$$

In the aqueous discharge plasma process, shock waves with intensity of several MPa occur because of the rapid extension of discharge plasma channels, after which pyrolytic and radical reactions can be initiated [31]. Martin reported that strong shock waves with intensity of 5–20 kbar or 10–20 MPa were generated in a pulsed arc discharge system due to the rapidly expanding plasma channel (1–2 ms) [32]. The detailed characteristics of the shock waves during discharge plasma process were studied by Lu [33], who reported that increasing the discharge voltage could strengthen the shock waves. In addition, the velocity of the shock waves could reach approximately 1.5 km s⁻¹ at a discharge voltage of 10 kV, capacitance of 4.1 μF, and electrode distance of 7 mm; and the shock wave pressure in plasma channels was approximately 49 MPa [34]. The shock waves in electrical discharge plasma are mainly utilized in bacteria inactivation and algae removal. Sunka et al [35] found that red blood cells could be effectively damaged by shock waves. They also found that the internal structure of a potato (6 cm thickness) was destroyed by shock waves, whereas no injury was observed at the outer surface of the potato [35]. A much greater disinfection efficacy was observed for Pseudomonas putida bacteria treated by shock waves than by traditional UV radiation [36].

When an electrical discharge does not form, pulsed high-voltage electric field is an effective method for biological cell disruption and food purification. Electromechanical compression is the main principle driving biological membrane disruption by a pulsed electric field [37]. Lots of transmembrane pores may be formed at a high electric field; the membrane can not repair perturbations when the ratio of total pore area to total membrane area is unfavorable, and then irreversible disruption occurs [38]. Previous research reported that a strong electric field can inactivate microorganisms and remove biofilms on the wall of cooling and drinking water pipes [15–17]. Mizuno and Hori [39] reported that a pulsed high-voltage electric field could rapidly inactivate living cells. In addition, high electric field can also affect chemical reactions in the liquid phase. Hong and Noolandi developed a time-dependent Smoluchowski equation with an electric field, which was helpful for the analysis of the recombination, neutralization, and scavenging processes that occur in the liquid phase [40]. Kuskova described the reaction rate constant in highly polar liquids using the following equation (equation (7)); where E is the electric field, k is Boltzmann's constant, ε is the dielectric permittivity, and T is the absolute temperature. This equation can be used to consider the influences of an electric field on chemical reactions in a liquid phase corona discharge system [41].

$$\begin{eqnarray}&&k({E})=K(0)\exp \left(\displaystyle \frac{2\sqrt{{{e}}^{3}{E}/\varepsilon }}{{kT}}\right).\end{eqnarray} \tag{ 7 }$$

Chemical processes in electrical discharge plasma mainly involve chemical reactions of various reactive species (H₂O₂, O₃, ·OH, and ·O, etc). These chemically reactive species are primarily initiated by the attack of high energy electrons. The detailed reaction processes of these reactive species are discussed as follows.

2.2.1. Reactions of high energy electrons

Electrons are generated in the electrical discharge plasma process; the energy of such electrons is able to surpass the dissociation or ionization energy of water molecules [42, 43]. Consequently, the dissociation or ionization of water molecules can occur via electron collisions, resulting in the formation of ·H, ·OH and other hydrated cations [42–44]:

$$\begin{eqnarray}&&{{\rm{e}}}^{-* }+{{\rm{H}}}_{{\rm{2}}}{\rm{O}}\to \cdot {\rm{OH}}+\cdot {\rm{H}}+{{\rm{e}}}^{-},\end{eqnarray} \tag{ 8 }$$

$$\begin{eqnarray}&&{{\rm{e}}}^{-* }+{{\rm{H}}}_{{\rm{2}}}{\rm{O}}\to {{\rm{H}}}_{2}{{\rm{O}}}^{+}+2{{\rm{e}}}^{-},\end{eqnarray} \tag{ 9 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{2}{{\rm{O}}}^{+}+{{\rm{H}}}_{{\rm{2}}}{\rm{O}}\to {{\rm{H}}}_{3}{{\rm{O}}}^{+}+\cdot {\rm{OH}},\end{eqnarray} \tag{ 10 }$$

where * indicates a high-energy electron state. These generated radicals can then react with each other to form other reactive species, as presented in the following reactions [43, 44]:

$$\begin{eqnarray}&&{\rm{H}}+{\rm{H}}\to {{\rm{H}}}_{2},\end{eqnarray} \tag{ 11 }$$

$$\begin{eqnarray}&&2\cdot {\rm{OH}}\to {{\rm{H}}}_{2}{{\rm{O}}}_{2},\end{eqnarray} \tag{ 12 }$$

$$\begin{eqnarray}&&\cdot {\rm{OH}}+\cdot {\rm{H}}\to {{\rm{H}}}_{2}{\rm{O}},\end{eqnarray} \tag{ 13 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{{\rm{2}}}+{\rm{e}}\to {\rm{e}}+2\cdot {\rm{O}},\end{eqnarray} \tag{ 14 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{{\rm{2}}}+{{\rm{e}}}^{* }+{\rm{M}}\to {{\rm{O}}}_{2}^{-}+{\rm{M}},\end{eqnarray} \tag{ 15 }$$

$$\begin{eqnarray}&&{\rm{e}}+{{\rm{H}}}_{{\rm{2}}}{{\rm{O}}}_{2}\to {{\rm{OH}}}^{-}+\cdot {\rm{OH}}.\end{eqnarray} \tag{ 16 }$$

It should be noted that the reaction of electrons in water may be different from that in moist air. In moist air, electrons have a very high kinetic energy and can react with water vapor to generate reactive species. By contrast, in water, some electrons can become solvated electrons, which then participate in chemical reactions as a strong reductant [45].

2.2.2. OH reactions

·OH is the key oxidant in AOPs. It can react with most organic and several inorganic compounds. The reactions of ·OH with various compounds mainly involve three different mechanisms [46, 47].

2.2.2.1. Abstraction of hydrogen atoms

The abstraction of hydrogen atoms mainly occurs when ·OH reacts with saturated aliphatic hydrocarbons or alcohols (see reaction (17)). This reaction yields H₂O and an organic radical (·M), and then ·M reacts with dissolved oxygen to generate the peroxy radical ·MOO (see reaction (18)); the peroxy radical is a strong oxidative species, and it can react with organic compounds via the hydrogen abstraction process (see reaction (19)) [46, 48]

$$\begin{eqnarray}&&\cdot {\rm{OH}}+{\rm{M}}-{\rm{H}}\to \cdot {\rm{M}}+{{\rm{H}}}_{2}{\rm{O}},\end{eqnarray} \tag{ 17 }$$

$$\begin{eqnarray}&&\cdot {\rm{M}}+{{\rm{O}}}_{2}\to \cdot {\rm{MOO}},\end{eqnarray} \tag{ 18 }$$

$$\begin{eqnarray}&&\cdot {\rm{MOO}}+{\rm{M}}^{\prime} {\rm{H}}\to {\rm{MOOH}}+\cdot {\rm{M}}^{\prime} .\end{eqnarray} \tag{ 19 }$$

2.2.2.2. Electrophilic addition to double (triplet) bonds

The reactions of ·OH with unsaturated hydrocarbons are mainly attributed to this mechanism, by which ·OH can attack the C=C positions of unsaturated hydrocarbons, and then a C-centered radical (·M₂ (OH)C–CM₂) can be produced via the following reaction [46].

$$\begin{eqnarray}&&\cdot {\rm{OH}}+{{\rm{M}}}_{2}{\rm{C}}\text{}\text{}=\text{}{{\rm{\text{}}}{\rm{CM}}}_{{\rm{2}}}\to \cdot {{\rm{M}}}_{2}({\rm{OH}}){\rm{C}}\mbox{--}{{\rm{CM}}}_{2}.\end{eqnarray} \tag{ 20 }$$

2.2.2.3. Electron migration

Electron migration mainly occurs when ·OH reacts with halogen-substituted compounds (see reaction (21)); in this case, the other two ·OH reaction mechanisms might not be favored.

$$\begin{eqnarray}&&\cdot {\rm{OH}}+{\rm{MX}}\to \cdot {{\rm{XM}}}^{+}+{{\rm{OH}}}^{-}.\end{eqnarray} \tag{ 21 }$$

Due to these different reaction mechanisms, the speeds of the chemical reactions of ·OH with other contaminants are quite different. The second-order reaction rate constants of ·OH reacting with some selected substances are listed in table 1 [49–51]. Reaction rate constants can be used to determine the principal reaction when many compounds are present in one system, then the ·OH capture agent can be selected appropriately. For example, salicylic acid is usually selected as ·OH scavenger.

Table 1.  The second-order reaction rate constants of ·OH with some selected substances.

Compounds	Rate constants (107 l mol−1 s−1)
Fe2+	43
Cl−	430
Br−	1100
${{\rm{ClO}}}_{4}^{-}$	880
${{\rm{SO}}}_{4}^{2-}$	0.16
${{\rm{CO}}}_{3}^{2-}$	39
OH−	1200
H2O2	2.7
Phenol	660
2-chlorophenol	1200
3-chlorophenol	720
4-chlorophenol	930
Bisphenol A	1000
1,1,2-trichloroethane	13
Acetic acid	1.6
Oxalic acid	0.14
4-nitrophenol	380
Pentachlorophenol	400
Salicylic acid	2200
t-butanol	420
Chlorobenzene	550
Xylene	670–750

2.2.3. O₃ reactions

In the electrical discharge plasma process, ·O is easily generated after the excitation and dissociation of oxygen molecules via high energy electron attack, and O₃ would be generated by the reaction of ·O radical with O₂, as shown in reaction (22).

$$\begin{eqnarray}&&\cdot {\rm{O}}+{{\rm{O}}}_{2}+{\rm{M}}\to {{\rm{O}}}_{3}+{\rm{M}}.\end{eqnarray} \tag{ 22 }$$

O₃ can react with various compounds via two different reaction processes: direct O₃ oxidation and indirect O₃ oxidation (O₃ decomposition). The detailed reaction processes are discussed in the following sections.

2.2.3.1. Direct O₃ oxidation

O₃ molecules are characterized by dipolarity, nucleophilicity and eletrophilicity, and these properties contribute to three different reaction mechanisms for direct O₃ oxidation, which are Kerry's reaction, the electrophilic reaction, and the nucleophilic reaction [52–54]. Kerry's reaction is presented in equation (23), in which O₃ is added to C=C double bonds due to its dipolarity and generates carbonyl compounds and H₂O₂. The electrophilic reaction mainly occurs at positions with a high electron-cloud density in aromatic compounds. Usually, the ortho- and para- positions of the electron-donating groups have a high electron-cloud density, allowing electrophilic reactions to occur rapidly; whereas the ortho- and para- positions of the electron-withdrawing groups have a quite low electron-cloud density, in which case electrophilic reactions would mainly occur at the meta- position. Therefore, O₃ can easily react with aromatic compounds those have electron-donating groups (such as phenols and aniline) via electrophilic reactions, from which some ortho- and para- substitution byproducts would be generated, as shown in equations (24) and (25). The nucleophilic reaction is similar to the electrophilic reactions, in which an oxygen atom with negative charge attacks a carbon atom with electron-withdrawing groups.

2.2.3.2. Indirect O₃ oxidation

The pathways of indirect O₃ oxidation are radical reactions. O₃ can undergo decomposition in basic solutions to generate ·OH and other reactive species, which have high oxidation potentials and thus can react with various compounds. The detailed reactions are as follows [55–57]:

$$\begin{eqnarray}&&{{\rm{O}}}_{{\rm{3}}}+{{\rm{OH}}}^{-}\to {{\rm{HO}}}_{2}^{-}+\cdot {\rm{O}},\end{eqnarray} \tag{ 26 }$$

$$\begin{eqnarray}&&{{\rm{H}}}^{+}+{{\rm{HO}}}_{2}^{-}\to {{\rm{H}}}_{2}{{\rm{O}}}_{2},\end{eqnarray} \tag{ 27 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{2}{{\rm{O}}}_{2}+{{\rm{O}}}_{3}\to {{\rm{HO}}}_{2}^{-}+{{\rm{O}}}_{2}+\cdot {\rm{OH}},\end{eqnarray} \tag{ 28 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{{\rm{3}}}+{{\rm{H}}}_{2}{{\rm{O}}}^{-}\to {{\rm{O}}}_{2}+\cdot {{\rm{O}}}_{2}^{-}+\cdot {\rm{OH}},\end{eqnarray} \tag{ 29 }$$

$$\begin{eqnarray}&&{{\rm{H}}}^{+}+\cdot {{\rm{O}}}_{2}^{-}\to {{\rm{HO}}}_{2}\cdot ,\end{eqnarray} \tag{ 30 }$$

$$\begin{eqnarray}&&\cdot {{\rm{O}}}_{2}^{-}+{{\rm{O}}}_{{\rm{3}}}\to {{\rm{O}}}_{2}+\cdot {{\rm{O}}}_{3}^{-},\end{eqnarray} \tag{ 31 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{2}{\rm{O}}+\cdot {{\rm{O}}}_{3}^{-}\to \cdot {\rm{OH}}+{{\rm{OH}}}^{-}+{{\rm{O}}}_{2},\end{eqnarray} \tag{ 32 }$$

$$\begin{eqnarray}&&\cdot {\rm{H}}+{{\rm{O}}}_{3}\to \cdot {{\rm{HO}}}_{3},\end{eqnarray} \tag{ 33 }$$

$$\begin{eqnarray}&&\cdot {{\rm{O}}}_{3}^{-}+{{\rm{H}}}^{+}\to \cdot {{\rm{HO}}}_{3},\end{eqnarray} \tag{ 34 }$$

$$\begin{eqnarray}&&\cdot {{\rm{HO}}}_{3}\to \cdot {\rm{OH}}+{{\rm{O}}}_{2}.\end{eqnarray} \tag{ 35 }$$

O₃ can also undergo catalytic decomposition by UV radiation and pyrolysis to generate atomic oxygen radicals, and then ·OH can be produced, as shown in equations (36)–(38) [58–60].

$$\begin{eqnarray}&&{{\rm{O}}}_{3}\mathop{\longrightarrow }\limits^{h\nu }\cdot {\rm{O}}+{{\rm{O}}}_{2},\end{eqnarray} \tag{ 36 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{3}\mathop{\longrightarrow }\limits^{{\rm{pyrolysis}}}\cdot {\rm{O}}+{{\rm{O}}}_{2},\end{eqnarray} \tag{ 37 }$$

$$\begin{eqnarray}&&\cdot {\rm{O}}+{{\rm{H}}}_{2}{\rm{O}}\to \cdot {\rm{OH}}.\end{eqnarray} \tag{ 38 }$$

In addition, some carbon materials can also catalytically decompose O₃ to form other reactive species, such as ·OH (reactions (39)–(42)), which can enhance pollutant degradation efficiency [5]. Our previous research revealed that O₃ yield decreased in the presence of charcoal in a pulsed discharge plasma system, whereas the decoloration efficiency of dye-containing wastewater was enhanced [5].

$$\begin{eqnarray}&&{{\rm{O}}}_{3}+{\rm{H}}\mbox{--}{\rm{carbon}}\mbox{--}{\rm{H}}\to {\rm{carbon}}\mbox{--}{\rm{O}}\cdot +{{\rm{H}}}_{2}{{\rm{O}}}_{2},\end{eqnarray} \tag{ 39 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{3}+{\rm{carbon}}\mbox{--}{\rm{OH}}\to {\rm{carbon}}\mbox{--}{{\rm{O}}}_{{\rm{3}}}\cdot +\cdot {\rm{OH}},\end{eqnarray} \tag{ 40 }$$

$$\begin{eqnarray}&&{\rm{carbon}}\mbox{--}{{\rm{O}}}_{{\rm{3}}}\cdot \to {{\rm{O}}}_{2}+{\rm{carbon}}\mbox{--}{\rm{O}}\cdot ,\end{eqnarray} \tag{ 41 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{{\rm{3}}}+{\rm{carbon}}\mbox{--}{\rm{O}}\cdot \to \cdot {{\rm{O}}}_{2}+{\rm{carbon}}+{{\rm{O}}}_{{\rm{2}}}.\end{eqnarray} \tag{ 42 }$$

In summary, direct O₃ oxidation and indirect O₃ oxidation occur simultaneously in the aqueous discharge plasma process, and their roles are dependent on solution pH to a certain extent. The speciation of organic contaminants is influenced by the solution pH. Organic contaminants are primarily in the molecular state when the solution pH value is lower than the contaminant pK_a, and primarily in the ionic form when the pH is higher than the pK_a. The reaction rate constants of O₃ with the ionic forms of organic compounds are much higher than those of O₃ with the molecular forms. For example, the reaction rate constants of O₃ with ionic p-nitrophenol and molecular p-nitrophenol are approximately 1.5 × 10⁷ l mol⁻¹ s⁻¹ and 50 l mol⁻¹ s⁻¹, respectively. Similarly, the reaction rate constant of O₃ with dissociated pentachlorophenol is approximately 3.1 × 10⁷ l mol⁻¹ s⁻¹, while that of O₃ with the pentachlorophenol molecule is 6.7 × 10⁴ l mol⁻¹ s⁻¹ [61].

2.2.4. H₂O₂ reactions

H₂O₂ can directly or indirectly participate in the contaminant removal process. The oxidation potential of H₂O₂ is not very high, and thus the contribution of direct H₂O₂ oxidation to the degradation of contaminants is often quite weak. Usually, H₂O₂ can be decomposed into ·OH in electrical discharge plasma process via several pathways, such as electron collisions (equation (16)), reaction with O₃ (equation (24)), and UV photolysis (equation (43)) [4, 61].

$$\begin{eqnarray}&&{{\rm{H}}}_{2}{{\rm{O}}}_{{\rm{2}}}+hv\to \cdot {\rm{OH}}+\cdot {\rm{OH}}.\end{eqnarray} \tag{ 43 }$$

To enhance the pollutant degradation efficiency, several metallic cations are usually selected to catalytically decompose H₂O₂ to generate ·OH in electrical discharge plasma (see reactions (44)–(48)) [62, 63]. Furthermore, nano-TiO₂ is another common catalyst widely used in electrical discharge plasma process because it can be activated by the discharge plasma to generate holes and electrons, which then reacts with H₂O₂ to produce oxidative species, as shown in equations (49)–(51) [64, 65].

$$\begin{eqnarray}&&{{\rm{H}}}_{2}{{\rm{O}}}_{2}+{{\rm{Fe}}}^{2+}\to \cdot {\rm{OH}}+{{\rm{Fe}}}^{3+}+{{\rm{OH}}}^{-},\end{eqnarray} \tag{ 44 }$$

$$\begin{eqnarray}&&\cdot {\rm{OH}}+{{\rm{Fe}}}^{2+}\to {{\rm{OH}}}^{-}+{{\rm{Fe}}}^{3+},\end{eqnarray} \tag{ 45 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{2}{{\rm{O}}}_{2}+\cdot {\rm{OH}}\to {{\rm{HO}}}_{2}\cdot +{{\rm{H}}}_{2}{\rm{O}},\end{eqnarray} \tag{ 46 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{2}{{\rm{O}}}_{2}+{{\rm{Cu}}}^{2+}\to {{\rm{H}}}^{+}+{({{\rm{Cu}}}^{2+}{{\rm{OOH}}}^{-})}^{+},\end{eqnarray} \tag{ 47 }$$

$$\begin{eqnarray}&&{({{\rm{Cu}}}^{2+}{{\rm{OOH}}}^{-})}^{+}\to 1/2{{\rm{O}}}_{2}+\cdot {\rm{OH}}+{{\rm{Cu}}}^{+},\end{eqnarray} \tag{ 48 }$$

$$\begin{eqnarray}&&{{\rm{TiO}}}_{2}+hv\to {{\rm{e}}}_{{\rm{cb}}}^{-}+{h}_{{\rm{v}}{\rm{b}}}^{+},\end{eqnarray} \tag{ 49 }$$

$$\begin{eqnarray}&&{{\rm{e}}}_{{\rm{cb}}}^{-}+{{\rm{H}}}_{2}{{\rm{O}}}_{2}\to {{\rm{OH}}}^{-}+\cdot {\rm{OH}},\end{eqnarray} \tag{ 50 }$$

$$\begin{eqnarray}&&{h}_{{\rm{vb}}}^{+}+{\rm{H}}{}_{2}{\rm{O}}_{2}\to {{\rm{H}}}^{+}+{{\rm{HO}}}_{2}\cdot .\end{eqnarray} \tag{ 51 }$$

In our research, Cu²⁺ and Fe²⁺ were selected to improve polyvinyl alcohol (PVA) degradation in a hybrid gas–liquid pulsed discharge plasma system, and we observed that the presence of both Cu²⁺ and Fe²⁺ significantly enhanced PVA degradation during the pulsed discharge plasma process, whereas the detected H₂O₂ concentration was much lower in the presence of Cu²⁺ or Fe²⁺ [66]; these phenomena suggested that some amounts of H₂O₂ were catalytically decomposed by Cu²⁺ and Fe²⁺ in electrical discharge plasma.

In addition, some carbon materials can catalytically decompose H₂O₂ to form ·OH and HO₂· (reactions (52) and (53)), which then enhance the pollutant degradation efficiency [60, 67]. Our previous research illustrated that the H₂O₂ yield decreased in the presence of charcoal in a pulsed discharge plasma system, whereas the decoloration efficiency of dye-containing wastewater was enhanced [5]

$$\begin{eqnarray}&&{\rm{carbon}}+{{\rm{H}}}_{2}{{\rm{O}}}_{2}\to {{\rm{carbon}}}^{+}+\cdot {\rm{OH}}+{{\rm{OH}}}^{-},\end{eqnarray} \tag{ 52 }$$

$$\begin{eqnarray}&&{{\rm{carbon}}}^{+}+{{\rm{H}}}_{2}{{\rm{O}}}_{2}\to {\rm{carbon}}+{{\rm{HO}}}_{2}\cdot +{{\rm{H}}}^{+}.\end{eqnarray} \tag{ 53 }$$

2.2.5. Reactions of other active radicals

Some short-lived species (·H, ·NO and ·O, etc) are produced in the discharge plasma process, and these reactive species also play quite significant roles in wastewater treatment.

·H is a reductant, and it can react with some metal ions via reduction–oxidation reactions. This mechanism is usually utilized in the removal of heavy metals via the glow discharge plasma process. For example, Cr(VI) can be reduced rapidly by ·H to produce Cr(V) and Cr(III), according to reactions (54) and (55) [68, 69]. Therefore, ·H can be used to transform highly charged metal ions into low-charged metal ions, and thus decrease the toxicity of some metal ions.

$$\begin{eqnarray}&&\cdot {\rm{H}}+{\rm{Cr}}({\rm{VI}})\to {{\rm{H}}}^{+}+{\rm{Cr}}({\rm{V}}),\end{eqnarray} \tag{ 54 }$$

$$\begin{eqnarray}&&\cdot {\rm{H}}+{\rm{Cr}}({\rm{VI}})\to {{\rm{H}}}^{+}+{\rm{Cr}}({\rm{III}}).\end{eqnarray} \tag{ 55 }$$

·NO is mainly generated in the arc discharge plasma process, and its role is mainly to acidify solutions via the generation of N-containing acids such as ${{\rm{NO}}}_{3}^{-}$ y ${{\rm{NO}}}_{2}^{-}.$ The detailed reactions are presented in equations (56)–(60):

$$\begin{eqnarray}&&{{\rm{N}}}_{2}+2{\rm{e}}\to 2\cdot {\rm{N}},\end{eqnarray} \tag{ 56 }$$

$$\begin{eqnarray}&&\cdot {\rm{N}}+\cdot {\rm{O}}\to {{\rm{NO}}}_{2},\end{eqnarray} \tag{ 57 }$$

$$\begin{eqnarray}&&\cdot {\rm{N}}+{{\rm{O}}}_{2}\to {\rm{NO}}+\cdot {\rm{O}},\end{eqnarray} \tag{ 58 }$$

$$\begin{eqnarray}&&\cdot {\rm{OH}}+{{\rm{NO}}}_{2}\to {{\rm{HNO}}}_{3},\end{eqnarray} \tag{ 59 }$$

$$\begin{eqnarray}&&\cdot {\rm{OH}}+{\rm{NO}}\to {{\rm{HNO}}}_{2}.\end{eqnarray} \tag{ 60 }$$

·O is a quite strong oxidant, and it plays very significant roles in the degradation of contaminants. ·O can react with O₂ and H₂O molecules to produce O₃ and ·OH (see equations (22) and (38)), and it can also react with hydrocarbons, as shown in equations (61)–(63) [70]:

$$\begin{eqnarray}&&{{\rm{C}}}_{2}{{\rm{H}}}_{3}+4\cdot {\rm{O}}\to 2{{\rm{CO}}}_{2}+3\cdot {\rm{H}},\end{eqnarray} \tag{ 61 }$$

$$\begin{eqnarray}&&{{\rm{C}}}_{2}{{\rm{H}}}_{2}+5\cdot {\rm{O}}\to 2{{\rm{CO}}}_{2}+{{\rm{H}}}_{2}{\rm{O}},\end{eqnarray} \tag{ 62 }$$

$$\begin{eqnarray}&&{{\rm{C}}}_{2}{{\rm{H}}}_{2}{\rm{O}}+4\cdot {\rm{O}}\to 2{{\rm{CO}}}_{2}+{{\rm{H}}}_{2}{\rm{O}}.\end{eqnarray} \tag{ 63 }$$

In addition, active ions are quite important components formed in the electrical discharge plasma process. However, many research studies have been conducted to investigate the roles of active radicals and molecules during wastewater treatment by electrical discharge plasma; little attention has been paid to the active ions. In fact, some active ions, such as N⁺, ${{\rm{N}}}_{2}^{+},$ ·${{\rm{O}}}_{2}^{-},$ O⁻·, H₂O⁺, ${{\rm{O}}}_{3}^{-},$ and ·${{\rm{O}}}_{2}^{-},$ can also be generated and play certain roles in contaminant degradation.

Some N-containing ions, such as N⁺ y ${{\rm{N}}}_{2}^{+},$ can be generated when N₂ molecules are dissociated by electron attack in electrical discharge plasma process, and the possible reactions are shown in the following equations (64)–(68) [71, 72]. N⁺ y ${{\rm{N}}}_{2}^{+}$ can contribute to the formation of more ·OH. Zhang et al [72] discussed the roles of N-containing ions such as N⁺ y ${{\rm{N}}}_{2}^{+}$ in chlorophenol removal from wastewater in a pulsed discharge plasma system.

$$\begin{eqnarray}&&{{\rm{N}}}_{2}+{\rm{e}}\to {\rm{N}}+{{\rm{N}}}^{+}+2{\rm{e}},\end{eqnarray} \tag{ 64 }$$

$$\begin{eqnarray}&&{{\rm{N}}}_{2}+{\rm{e}}\to {{\rm{N}}}_{2}^{+}+2{\rm{e}},\end{eqnarray} \tag{ 65 }$$

$$\begin{eqnarray}&&{{\rm{N}}}_{2}^{+}+{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{N}}}_{2}^{+}({{\rm{H}}}_{2}{\rm{O}}),\end{eqnarray} \tag{ 66 }$$

$$\begin{eqnarray}&&{{\rm{N}}}_{2}^{+}+{{\rm{H}}}_{2}{\rm{O}}\to \cdot {\rm{NH}}+\cdot {\rm{OH}},\end{eqnarray} \tag{ 67 }$$

$$\begin{eqnarray}&&{{\rm{N}}}_{2}^{+}({{\rm{H}}}_{2}{\rm{O}})+{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{H}}}_{3}{{\rm{O}}}^{+}+{{\rm{N}}}_{2}+\cdot {\rm{OH}}.\end{eqnarray} \tag{ 68 }$$

In the presence of nano-TiO₂, electrical discharge plasma can excite the nano-TiO₂ to generate electrons (e⁻) and holes (h+ vb). It has been suggested that the electrons can attack oxygen molecules to form single atomic oxygen radical anions (·O⁻) and superoxide radical anions $(\cdot {{\rm{O}}}_{2}^{-});$ subsequently, $\cdot {{\rm{O}}}_{2}^{-}$ and ·O⁻ can be transformed into ·O₂ radical and ·O via reactions with h+ vb on the surface of the nano-TiO₂, leading to increased O₃ generation via the following possible reactions (reactions (69)−(73)) [73–75]. In an appropriate alkali environment, HO₂· can also be converted to $\cdot {{\rm{O}}}_{2}^{-}$ (reaction (30)), and the presence of $\cdot {{\rm{O}}}_{2}^{-}$ in the solution is essential for bacterial inactivation by the plasma [76].

$\cdot {{\rm{O}}}_{2}^{-}$ have a relatively long lifetime in solution (5 s at 1.0 × 10⁻⁶ M), which is much higher than that of ·OH (200 μs at 1.0 × 10⁻⁶ M) [77, 78]. Perni et al [79] reported that plasma-generated O atoms and $\cdot {{\rm{O}}}_{2}^{-}$ played the dominant roles in Escherichia coli inactivation. In addition, it must be noted that if the solution pH value is sufficiently low, $\cdot {{\rm{O}}}_{2}^{-}$ will be converted to HO₂·, which can also participate in bacteria inactivation via penetrating the cell membrane and damaging intercellular components [80].

$$\begin{eqnarray}&&{{\rm{O}}}_{{\rm{2}}}+{{\rm{e}}}^{-}\to 2\cdot {{\rm{O}}}^{-},\end{eqnarray} \tag{ 69 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{{\rm{2}}}+{{\rm{e}}}^{-}\to \cdot {{\rm{O}}}_{2}^{-},\end{eqnarray} \tag{ 70 }$$

$$\begin{eqnarray}&&\cdot {{\rm{O}}}_{2}^{-}+{h}_{{\rm{vb}}}^{+}\to \cdot {{\rm{O}}}_{{\rm{2}}},\end{eqnarray} \tag{ 71 }$$

$$\begin{eqnarray}&&\cdot {{\rm{O}}}^{-}+{h}_{{\rm{v}}{\rm{b}}}^{+}\to \cdot {\rm{O}},\end{eqnarray} \tag{ 72 }$$

$$\begin{eqnarray}&&\cdot {\rm{O}}+{{\rm{O}}}_{{\rm{2}}}\to {{\rm{O}}}_{3}.\end{eqnarray} \tag{ 73 }$$

In the glow discharge plasma process, the positive gaseous ion H₂O⁺ can react with H₂O to produce ·OH after it diffuses into the liquid phase via the driving force of ion wind (see equation (10)) [81]. The energy of H₂O⁺ is very high, and thus the ·OH yield is high, and then large amounts of H₂O₂ can be formed via ·OH recombination. It has been suggested that the yield of H₂O₂ per mole of electrons in glow discharge plasma can surpass the yield limit of Faraday's law [81].

Negative ions such as ${{\rm{O}}}_{3}^{-}$ y ${{\rm{O}}}_{2}^{-}$ are formed in corona discharge in oxygen above the water surface, and these species can lead to the formation of O, O₃, and ·OH (see reactions (74)–(77) and (38)) [82, 83]. In addition, the combination of ions and neutral species generated in gas phase discharge plasma is reported to be responsible for solution pH changes [83].

$$\begin{eqnarray}&&{\rm{e}}+{{\rm{O}}}_{2}\to {\rm{O}}+{{\rm{O}}}^{-},\end{eqnarray} \tag{ 74 }$$

$$\begin{eqnarray}&&{{\rm{O}}}^{-}+{{\rm{O}}}_{2}\to {\rm{O}}+{{\rm{O}}}_{2}^{-},\end{eqnarray} \tag{ 75 }$$

$$\begin{eqnarray}&&{{\rm{O}}}^{-}+2{{\rm{O}}}_{2}\to {{\rm{O}}}_{2}+{{\rm{O}}}_{3}^{-},\end{eqnarray} \tag{ 76 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{3}^{-}+{{\rm{O}}}_{2}\to {{\rm{O}}}_{2}+{{\rm{O}}}_{3}+{\rm{e}}.\end{eqnarray} \tag{ 77 }$$

In summary, the chemical effects of discharge plasma are quite complex, including radicals, ions, and molecular reactions; many parameters can affect these reactions. For instance, a high solution conductivity can lead to relatively lower formation rates of reactive species [84, 85]. Some species may be able to enter the liquid phase from the discharge electrodes and thus influence the chemical properties of solutions. The material quality of the discharge electrodes can also affect the chemical reactions [86]. Moreover, the type of carrier gas between the electrodes, such as O₂, N₂, or air, can significantly influence the variety and yield of oxidative species [71, 87, 88]. It is important to note that post-discharge reactions deriving from long-lived reactive species can also occur in the discharge plasma process, which can further enhance the removal efficiency of contaminants [89].

H₂O₂ is a relatively long-lived species, and it is usually detected directly using traditional chemical methods, such as iodometry method [43, 90], potassium titanium colorimetric method [91, 92], and direct instrumental analysis [93]. The potassium titanium colorimetric method has received great attention. Wang et al [92] employed the potassium titanium colorimetric method to measure H₂O₂ concentration during the gas–liquid pulsed discharge plasma process. They also evaluated the effects of radical scavengers on H₂O₂ formation. They observed that the presence of n-butanol significantly inhibited H₂O₂ formation, whereas adding Na₂CO₃ increased the H₂O₂ concentration. It is known that n-butanol and Na₂CO₃ are both ·OH scavengers, and the ·OH can recombine to form H₂O₂. Therefore, Wang et al [92] deduced that ·OH recombination is one mechanism for H₂O₂ formation, and that other reactions also participate in its formation. Under alkaline conditions (Na₂CO₃ addition), dissolved oxygen can undergo electrolytic dissociation on the surface of electrodes, which would contribute to the formation of some additional amounts of H₂O₂.

The H₂O₂ yield is affected by the carrier gas in the discharge plasma process. We evaluated the H₂O₂ formation in a hybrid gas–liquid pulsed discharge plasma system with various carrier gases, and the highest H₂O₂ yield was observed for bubbling O₂, followed in descend for bubbling air, while the lowest H₂O₂ yield was observed for bubbling N₂ [94]. When N₂ is selected as the carrier gas, ·OH is generated via the reactions of N-containing species with water molecules (see reactions (60) and (61)), which then recombine to form H₂O₂; however, O-containing species react with water molecules directly to form H₂O₂ when O₂ is selected as the carrier gas, in addition to generating ·OH. Therefore, a relatively high H₂O₂ yield is obtained when bubbling O₂ compared with that obtained when bubbling N₂.

The H₂O₂ yield is also affected by the solution pH and conductivity. Hsieh reported that the greatest H₂O₂ concentration was observed in solution pH = 4 and decreased as the pH value increased in alkaline conditions; an increase in solution conductivity also led to a decrease in H₂O₂ concentration in a gas–liquid film plasma system [95]. Increasing the solution pH or the amount of solute introduces more ions, which can influence discharge plasma occurrence due to the changes in solution conductivity [96]. A higher solution conductivity can lead to a lower H₂O₂ yield due to UV and thermal effects; this is because a larger discharge current, shorter discharge streamer length, higher power density, higher plasma temperature and UV radiation occur [26].

O₃ is a relatively long-lived species, and it can be easily detected quantitatively. The iodometry method and indigo method are two of the main techniques used to measure both gaseous and dissolved O₃ concentrations [97, 98]. Some oxidative species, including NO_x, H₂O₂ and ·O, can oxidize iodide and indigo as well, and therefore there exist some errors in O₃ concentration measurement; however, it is very difficult to distinguish the contributions of various active species to 'O₃ concentration'. As a result, the concept of 'O₃ equivalent concentration' has been proposed to characterize the oxidation potential of a discharge plasma system, which is calculated as the 'O₃ concentration' [99, 100]. In addition, the gaseous O₃ concentration can also be measured directly by an O₃ monitor [101]. Although the chemical methods reviewed above can measure the O₃ concentration quantitatively, they are unable to analyze the O₃ production process, O₃ spatial distribution, and O₃ dynamics.

Recently, laser absorption methods have been proposed to analyze O₃ generation behaviors in gaseous discharge plasma process [102–104]. Ono and Oda [102] investigated O₃ generation and distribution in discharge plasma via a two-dimensional laser absorption method, they found that the O₃ density increased continually when the post-discharge time was lower than 100 μs and then gradually migrated to the ground electrode when the post-discharge time was higher than 1 ms. The effect of humid air on O₃ distribution was also evaluated in the pulsed corona discharge plasma process, and the O₃ yield and formation time decreased when water vapor was added to the system [103].

In addition, the O₃ yield in aqueous solution is affected by the aqueous surface tension in discharge plasma process [105]. Our research found that the addition of small amounts of nonoic acid (NA), lactic acid (LA), and acetic acid (AA) to solutions decreased the aqueous surface tension in a surface discharge plasma system, as a result of which the O₃ equivalent concentration in solution was enhanced [105].

As a quite short-lived species, ·OH is more difficult to detect than O₃ and H₂O₂. Usually, it is measured using indirect methods such as radical scavengers [92, 106–108], paramagnetic resonance [109], spin trapping [110], chromatography [111], and absorption [112]; among these, the radical scavenger method is widely employed, in while salicylic acid, 4-hydroxybenzoate, n-butanol, Na₂CO₃, isopropanol, and dimethylsulfoxide are generally selected as ·OH scavengers. Bian et al [107] investigated the ·OH generation rate using 4-hydroxybenzoate as the ·OH scavenger during the pulsed discharge plasma process. They found that the ·OH formation rate was approximately 3.49 × 10⁻⁷, 3.56 × 10⁻⁷, 3.21 × 10⁻⁷ and 1.94 × 10⁻⁷ mol l⁻¹ s⁻¹ when N₂, argon (Ar), air, and O₂ was selected as the carrier gas, respectively. Wang et al [92] selected n-butanol and Na₂CO₃ to scavenge ·OH and found that the relative concentration of ·OH decreased after n-butanol and Na₂CO₃ addition. Fluorescence spectroscopy is another frequently-used method for ·OH measurement, and terephthalic acid is usually selected to scavenge ·OH in this case because terephthalic acid can react with ·OH to produce hydroxyterephthalic acid. The latter can be analyzed via fluorescence spectroscopy, and then the ·OH concentration can be calculated based on the spectral intensity of hydroxyterephthalic acid [111].

The above methods for ·OH measurement are mainly based on chemical probes, some of which cannot quantitatively measure the ·OH concentration. More importantly, such methods are unable to analyze the ·OH generation process, spatial distribution, and dynamics. Recently, optical emission spectroscopy (OES) has received great attention in detecting ·OH in the discharge plasma process because it can detect ·OH in situ. In such case, the ·OH generation process and spatial distribution can also be obtained [44, 86, 113–115]. The main principle for ·OH detection by OES is that some amounts of ·OH are in an excited state (upper state) due to the attack of high-energy electrons; these excited ·OH would transit spontaneously from the upper state to the lower state, accompanied by the liberation of photons of different wavelengths. Specific photons are captured by the detector, and their intensity is proportional to the ·OH concentration. A typical optical emission spectrum from the pulsed streamer discharge plasma process was obtained in our research, as shown in figure 1. Sun et al [113] studied gaseous ·OH formation and distribution in a negative pulsed discharge plasma system via OE, and reported that a higher pulse repetition rate, lower air flow rate, and smaller nozzle electrode diameter were all beneficial to obtain relatively higher ·OH concentrations; ·OH mainly appeared around the high voltage electrode. Liu et al [114] reported that increasing the He and Ar content in mixed N₂ + He and N₂ + Ar gases increased the intensity of ·OH in a pulse discharge plasma system via OES analysis, whereas increasing O₂ contents in O₂ + N₂ mixture gases decrease ·OH yield. In addition to ·OH detection in gas phase, OES could also analyze ·OH in aqueous solutions. Sun et al [87] studied the influences of discharge pattern, discharge polarity, and O₂ flow rate on ·OH generation in solution by OES, and found that the highest ·OH yield was observed in spark discharge; positive discharge was more beneficial for ·OH formation than negative discharge; increasing O₂ flow rate benefited ·OH generation to a certain extent. Our previous research evaluated ·OH generation via OES analysis in a gas–liquid pulsed discharge plasma combined with nano-TiO₂ system, and found that appropriate levels of nano-TiO₂ addition could increase the relative intensity of ·OH [44]. In addition, the highest ·OH concentration was observed in Ar atmosphere, followed by air atmosphere, and the lowest was observed in oxygen [116, 117]. It is known that ${{\rm{N}}}_{2}^{+}$ can be generated in the discharge plasma process, which can result in the generation of ·OH via reaction with H₂O (see equation (67)). Ar can also be excited by high-energy electrons to form Ar⁺, which then reacts with H₂O molecules to form ·OH radicals (equations (78) and (79)) [116, 118].

$$\begin{eqnarray}&&{\rm{Ar}}\mathop{\longrightarrow }\limits^{{\rm{e}}}{{\rm{Ar}}}^{+}+{\rm{e}},\end{eqnarray} \tag{ 78 }$$

$$\begin{eqnarray}&&{{\rm{Ar}}}^{+}({{\rm{H}}}_{2}{\rm{O}})+{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{H}}}_{3}{{\rm{O}}}^{+}+{\rm{Ar}}+\cdot {\rm{OH}}.\end{eqnarray} \tag{ 79 }$$

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Although OES diagnosis can be used to analyze the ·OH formation process and spatial distribution, it is unable to actually calculate the absolute ·OH concentration. Furthermore, it can only passively capture photons released from the ·OH spontaneous transition, and therefore, the measurement accuracy and temporal-spatial resolution are both low.

Another method for ·OH radical diagnosis is laser-induced fluorescence (LIF) [119–121]. A significant number of ·OH generated in the discharge plasma are in a relatively lower state. These can be excited by laser to transit to the upper state; and then photons can be liberated when the excited ·OH migrate to the lower state. These photons are captured and their intensity is proportional to the ·OH concentration. Therefore, compared with OES, LIF is an active method for ·OH detection, and its measurement accuracy and temporal-spatial resolution are both higher. More importantly, the two-dimensional distribution and density of ·OH can be obtained by LIF analysis combined with an ICCD camera. Ono and Oda [119] reported that ·OH was mainly produced in the discharge plasma area, and the highest ·OH density was observed at a post-discharge time of 40 μs. They also reported that the absolute density of ·OH in humid air with 3% H₂O at 50 μs post-discharge time was approximately 1 × 10¹⁵ cm⁻³. ·OH formation and dynamics on the water surface were also evaluated in the discharge plasma process [111, 122–124]. Kanazawa et al [111] studied ·OH formation on the liquid surface in the pulsed discharge plasma process by LIF and found that some gaseous ·OH could migrate and dissolve into the water.

Although LIF can provide a high measurement accuracy, high temporal-spatial resolution, and quantitative determination for ·OH diagnosis, at present, it can only detect ·OH in the gas phase.

In addition to H₂O₂, O₃, and ·OH, the identification and quantification of other reactive species such as ·H, $\cdot {{\rm{O}}}_{2}^{-},$ aqueous electron, N*, ·O, and singlet oxygen (¹O₂) have also received great attention due to their significant roles in the discharge plasma process. Chemical probes and spectrum diagnosis are two main methods for their measurement.

·H is generated when water molecules are dissociated by electron attack (equation (4)), and then HO₂· is formed via the reaction of ·H with O₂; while $\cdot {{\rm{O}}}_{2}^{-}$ and HO₂· speciation is a function of solution pH (see equations (80) and (81)) [125]. $\cdot {{\rm{O}}}_{2}^{-}$ has a relatively high oxidation potential and can participate in the decomposition of contaminants, and it can also affect the formation of other species (see equations (30), (82) and (83)). Sahni and Locke [126] quantitatively calculated the concentrations of $\cdot {{\rm{O}}}_{2}^{-}$ and ·H by tetranitromethane and nitroblue tetrazolium chloride scavenging probes. ·H radicals can also be detected by OES analysis. Sato et al [127] detected ·H radicals in solution during the pulsed discharge plasma process by OES and found that increasing the solution conductivity was not beneficial for ·H radical formation.

$$\begin{eqnarray}&&\cdot {\rm{H}}+{{\rm{O}}}_{{\rm{2}}}\to {{\rm{HO}}}_{2}\cdot ,\end{eqnarray} \tag{ 80 }$$

$$\begin{eqnarray}&&{{\rm{HO}}}_{2}\cdot \mathop{\longleftrightarrow }\limits^{{\rm{p}}Ka=4.8}\cdot {{\rm{O}}}_{2}^{-}+{{\rm{H}}}^{+},\end{eqnarray} \tag{ 81 }$$

$$\begin{eqnarray}&&{{\rm{HO}}}_{2}\cdot +\,\cdot {{\rm{O}}}_{2}^{-}+{{\rm{H}}}^{+}\to {{\rm{H}}}_{2}{{\rm{O}}}_{2}+{{\rm{O}}}_{2},\end{eqnarray} \tag{ 82 }$$

$$\begin{eqnarray}&&{{\rm{HO}}}_{2}\cdot +{{\rm{e}}}^{-}+{{\rm{H}}}^{+}\to {\rm{H}}{}_{2}{\rm{O}}_{2}.\end{eqnarray} \tag{ 83 }$$

The aqueous electron is a strong reductant in the electrical discharge plasma process, and it can be generated via subexcitation electrons (energy <7.4 eV) captured by surrounding water molecules, as shown in reaction (84) [128]. The presence of a reductant might benefit the removal of some contaminants by reduction mechanisms. Joshi et al [43] reported that the initiation rate constant of aqueous electron formation was independent of time and that its yield increased with increasing applied discharge voltage. The aqueous electron plays significant roles in the discharge plasma process. Phosphate could rapidly react with the aqueous electron with a reaction rate constant of more than 10⁷ M⁻¹ s⁻¹, and it is usually selected as the aqueous electron scavenger [129–131]. Leitner observed that there was an obvious decrease in atrazine degradation efficiency in an electro-hydraulic discharge plasma system when small amounts of H₂${{\rm{PO}}}_{4}^{-}$ were added [129]. Zhang et al [130] reported that the microcystin-LR degradation efficiency decreased in the presence of H₂${{\rm{PO}}}_{4}^{-}$ in a glow discharge plasma system. In our research, to evaluate the role of $\cdot {{\rm{O}}}_{2}^{-}$ and the aqueous electron in p-nitrophenol degradation in a pulsed discharge plasma system, 1,4-benzoquinone and H₂${{\rm{PO}}}_{4}^{-}$ were also selected to capture $\cdot {{\rm{O}}}_{2}^{-}$ and aqueous electrons, respectively; we found that the p-nitrophenol degradation efficiency significantly decreased after adding small amounts of 1,4-benzoquinone and H₂${{\rm{PO}}}_{4}^{-}$ [131].

$$\begin{eqnarray}&&{{\rm{H}}}_{2}{\rm{O}}+{{\rm{e}}}^{-}\to {{\rm{e}}}_{{\rm{aq}}}^{-}.\end{eqnarray} \tag{ 84 }$$

The typical optical emission spectrum obtained in a dielectric barrier discharge plasma system under N₂ atmosphere and O₂ atmosphere is shown in figure 2, where N-containing species (figure 2(a)) and O-containing species (figure 2(b)) can be observed. Singlet oxygen is unstably excited oxygen, and it is characterized as strong oxidation potential and short life [132]. Singlet oxygen can be detected via chemical probe or spectroscopic diagnosis. NaN₃ is an effective quencher of singlet oxygen, and the addition of NaN₃ to wastewater containing phenol can cause an approximately 40% decrease in phenol degradation efficiency [133]. Ono and Oda measured the excited oxygen in a pulsed discharge plasma system by LIF and reported that the excited oxygen was mainly generated in the secondary streamer channels, and its density could reach approximately 10¹⁴ cm⁻³ around the high voltage electrode [134]. The addition of water vapor in the discharge plasma process could enhance the excited oxygen content [135]. Hong et al [136] observed excited oxygen using OES in an argon–oxygen plasma jet system.

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