Review of plasma-based water treatment technologies for the decomposition of persistent organic compounds

The establishment of economical and eco-friendly technologies for water treatment is a crucial issue for the realization of a sustainable society. Plasma-based treatments are promising methods for the decomposition of persistent organic compounds. This progress report summarizes recent improvements to plasma-based water treatment technologies by focusing on two types of contaminated solutions: solutions containing high concentrations of acetic acid and solutions containing surfactants, such as perfluorooctane sulfonic acid. Decomposition processes were analyzed based on chemical and physical characteristics, such as chemical reactions, the transportation of reactive species, and characteristics of target compounds. When treating solutions containing acetic acid, the optimization of bulk reactions involving ozone, which regenerates OH radicals from hydrogen peroxide, is a key factor for achieving high decomposition rates and energy efficiency. In contrast, the treatment of surfactants involves interfacial reactions at the plasma–liquid interface, where the accumulation of surfactants is a major concern.


Introduction
Significant air pollution caused by economic growth poses a major threat to human health. Environmental technologies, including electrostatic precipitators, catalysts, and nonthermal plasma techniques, 1) have been developed to prevent air pollution. Based on growth in population, industry, and agriculture, water pollution has become another global issue of significant concern. Conventional water treatment technologies include coagulation, sedimentation, filtration, ozonation, and disinfection using chlorine. 2,3) However, persistent organic compounds that are soluble in water cannot be treated using these conventional technologies. Although the use of membranes, such as reverse osmosis membranes, is effective for the removal of organic compounds, this method has some disadvantages, such as fouling, which necessitate the maintenance or replacement of membranes. The use of OH radicals (•OH), which have high oxidation potential, as shown in Table I, 4,5) is another promising option for the treatment of persistent organic compounds. The aim of such treatment is to decompose organic compounds into CO 2 gas. This process is also referred to as mineralization. Water treatment processes using •OH are called advanced oxidation processes (AOPs). Various types of AOPs have been studied extensively, 6,7) including the H 2 O 2 /O 3 process, 6,8) ultraviolet (UV)/H 2 O 2 process, [6][7][8] UV/O 3 process, 6,8) photo-Fenton reaction, 6,9) and titanium dioxide (TiO 2 )-assisted photocatalytic process. 6,7,10) The use of plasma in contact with water is an AOP that has been studied as a promising technology for water treatment for several decades [11][12][13][14] by many researchers and scientific committees, such as the Investigating R&D Committee on Plasma with Liquid of The Institute of Electrical Engineers of Japan, 15,16) Committee on Research in Plasma Water Treatment of The Institute of Electrostatics Japan, and European Cooperation in Science and Technology Action TD1208 (Electrical discharges with liquids for future applications). 17) When plasma comes in contact with water, •OH is generated from water molecules via electron-impact dissociation and reactions with radicals, such as O radicals (•O), and metastable atoms, such as Ar metastable atoms (Ar*). Although the generation of excited •OH in the gas phase can be confirmed using optical emission spectroscopy (OES), OES is not applicable to the detection of ground-state •OH, which does not emit light. For the detection of both ground-state and excited •OH, the laser-induced fluorescence (LIF) method is preferable. [18][19][20] A portion of the gas-phase •OH diffuses into the water and decomposes organic compounds. In addition to the diffusion of gas-phase •OH, direct generation mechanisms for •OH in water via the photodissociation of water with vacuum UV (VUV) irradiation 21) and charge exchange with positive ion irradiation 22) have been proposed. The concentrations of liquid-phase •OH have been measured using chemical probe methods based on terephthalic acid 23) and disodium terephthalate. 24) In addition to •OH, other reactive species, such as O 3, and energetic species, such as electrons and ions, contribute to the decomposition of various organic compounds. Several types of plasma reactors have been proposed, including pulsed corona discharge in water, 25) DC or pulsed corona discharge over water, [26][27][28] pulsed discharge over water, [29][30][31][32] DC or pulsed discharge in bubbles, [33][34][35][36][37] dielectric-barrier discharge (DBD) in a gas-liquid two-phase flow, 19,38) microwave plasma in water, 39) DBD in a mist flow, 40) DBD with a falling water film, 41,42) and pulsed plasma with droplets. 43,44) Because these plasma methods can operate without the addition of chemicals, they have significant advantages over AOPs that use chemicals for the treatment of wastewater at offshore plants and environmental water in remote areas.
A wide range of decomposition rates and energy efficiencies have been observed for the treatment of organic compounds with different types of plasma reactors and experimental conditions. For example, differences in energy efficiency greater than four orders of magnitude have been observed for the decomposition of dyes in water. 45) From an economic perspective, an energy-efficient process should be developed to reduce operating costs. Additionally, a high decomposition rate and large scalability in terms of treatment capacity must be achieved for practical use to treat large amounts of wastewater. This progress report discusses how plasma-based water treatment technologies have been improved in terms of their decomposition rate, energy efficiency, and scalability. We focus on two types of contaminated solutions, namely "produced water", which is generated during oil and gas extraction and contains high concentrations of acetic acid, 46) and solutions containing surfactants, such as perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA).

Treatment of acetic acid solutions and byproducts
2.1. Characteristics of acetic acid Acetic acid (CH 3 COOH) is a popular organic compound that is used and discharged in homes and industrial plants. It constitutes the bulk of the acid mass in produced water. As restrictions on the emission of organic compounds into the environment become more strict, 47,48) there is an urgent need to establish an economical and eco-friendly treatment method for soluble organic compounds, such as acetic acid. Such compounds are frequently discharged in remote areas, such as offshore oil plants. Here, the goal of treatment is the decomposition of organic compounds into CO 2 gas. This process is referred to as mineralization.
Acetic acid is a persistent organic compound that cannot be decomposed using conventional water treatment methods using chlorine and/or O 3 based on the presence of strong C-C bonds. However, •OH can decompose acetic acid based on its high oxidation potential. Therefore, AOPs, including plasma in contact with water, have been used to treat solutions containing acetic acid. The decomposition pathways of acetic acid, which proceed via the generation of short-lived radicals, have been reported. 36,49) Formic acid and oxalic acid have been observed as byproducts in solution during treatment, while CO 2 gas is the main byproduct emitted into the atmosphere. The reaction rates of acetic acid, formic acid, and oxalic acid with •OH and O 3 have been summarized in the literature. 50) These acids in treated solutions can be quantified using ion chromatography (IC). To evaluate the decomposition of organic compounds, including byproducts, the concentration of total organic carbon (TOC) in a solution should be considered. Such concentrations can be measured using a TOC meter. Gaseous byproducts, such as CO 2 and CO, can be quantified using gas chromatography (GC) or Fourier transform infrared spectroscopy (FT-IR). The mass balance of carbon has been maintained at nearly 100% during the treatment of acetic acid using an AOP (H 2 O 2 /O 3 method) 51) and several types of plasma methods 29,52) with consideration for acetic acid, formic acid, and oxalic acid in the treated solutions and gaseous CO 2 .
We conducted several trials to achieve the effective treatment of acetic acid using plasma technologies and related knowledge. The efficiency of treatment was evaluated based on TOC concentrations. In some trials, a phosphate buffer was added to an acetic acid solution to simulate typical produced water, which is almost neutral.
2.2. Treatment using plasma generated over a solution 29) The experimental setup presented in Fig. 1 was used to treat acetic acid solutions. 29) A metal disk with 84 attached needles is fixed over the solution (20 ml). There is a gap of 1 mm between the tips of the needles and the surface of the solution. The initial TOC concentration was 10.7 mg TOC l −1 . A pulsed voltage is applied to the high-voltage (H.V.) electrode through a 109 Ω resistor with Ar gas to generate filamentary plasma channels over the solution surface. It was confirmed that acetic acid was decomposed into CO 2 gas via the generation of formic acid and oxalic acid as byproducts in the solution. Experimental results with varying voltage amplitudes V, repetition rates f, and pulse durations d were evaluated based on TOC reduction after 30 min of treatment. Table II lists the input power P, peak current I pc , TOC decomposition rate r TOC , amount of H 2 O 2 produced in the solution m H 2 O 2 , and TOC decomposition energy efficiency η TOC . Conditions with a low peak current, low repetition rate, and short pulse duration are desirable for achieving high energy efficiency in the reduction of TOC. However, there is a tradeoff between the decomposition rate and energy efficiency. It can be confirmed that conditions with a large amount of H 2 O 2 yield low efficiency. This tradeoff was also confirmed by Miichi et al. 32) An axisymmetric two-dimensional numerical simulation of plasma was conducted to understand the dominant reactions during the process of acetic acid decomposition by Ar plasma (V = 3 kV, f = 1 kHz, d = 200 ns). 29) The mass transfer of •OH, H 2 O 2 , and HO 2 • through the gas-liquid interface was considered by assuming that a gas-liquid equilibrium was established in accordance with Henry's law and that the fluxes of these species were continuous. A strong electric field in the cathode-fall region generated a high electron energy of approximately 10 eV and a high number density of The fluxes of the reactive species at the gas-liquid interface on the axisymmetric axis are presented in Fig. 2(a). The duration of the •OH flux is approximately 10 μs, whereas the H 2 O 2 flux can be observed continuously. The flux of HO 2 • is significantly less than those of the other species. Figure 2(b) presents the liquid-phase concentration of •OH along the axisymmetric axis. The concentration near the plasma-solution interface peaks approximately 1 μs after the voltage increase and then decreases to 2% of its maximum value at 10 μs based on loss reactions between •OH and acetic acid, H 2 O 2 , HO 2 •, and •OH. In the simulated TOC concentration (10 mg TOC l −1 ), the reaction rate of •OH with acetic acid is one to two orders of magnitude smaller than those with •OH, H 2 O 2 , or HO 2 •. Even at a depth of 0.2 μm, the concentration of •OH is less than 1% of the maximum value near the interface. Therefore, the •OH penetration depth is considered to be less than 0.2 μm and the decomposition reactions of acetic acid are similar to interfacial reactions at the plasma-solution interface. Tachibana et al. discussed the possibility that short-lived active species generated by plasma can react with solutes only in the topmost layer of the water surface, which is an atomic monolayer in the liquid in contact with the gas phase. 53) According to the literature, this is because these active species prefer remaining near the gasliquid interface, rather than moving into the bulk liquid, based on their surface activity. 54,55) 2.3. Treatment using plasma generated within gas bubbles 34,35) A cross-sectional view of a typical configuration of a reactor for generating plasma within gas bubbles is presented in Fig. 3. An H.V. electrode is placed under a hole (diameter of several hundred micrometers) in a ceramic plate. The H.V. electrode can be a metal needle, mesh, or plate. The solution is grounded by an inserted electrode. A discharge gas, such as O 2 , is injected into the solution through the hole, resulting in continuous bubble formation. By applying a strong electric field inside the bubble that attaches to the ceramic plate, plasma is generated between the H.V. electrode and bubble surface. The appearance of the plasma strongly depends on the applied voltage waveform, gas composition, and solution conductivity, as shown in Fig. 4. Images of the bubbles are captured by a high-speed camera (upper right), pulsed plasma is captured by a digital camera over several pulses (left), and typical plasma is captured by an intensified CCD camera over one pulse (lower right).
In addition to the plasma generated over a solution surface, a tradeoff between the TOC decomposition rate and efficiency was observed in the treatment of an acetic acid solution using plasma generated within gas bubbles. The typical relationship between the decomposition rate and efficiency when the input power is increased by increasing the amplitude of the pulsed voltage is presented in Fig. 5.  Although high input power is required to achieve large TOC reduction, a high concentration of •OH results in low efficiency based on self-quenching reactions that generate H 2 O 2 and loss reactions with H 2 O 2 and HO 2 •. O 2 plasma achieves a higher efficiency than Ar plasma in this type of plasma treatment. However, Ar plasma is more efficient in certain situations, such as phenol decomposition using plasma over a solution 31) and acetic acid decomposition using gas-liquid two-phase flow plasma. 19,38) It is possible that the generation and loss mechanisms of •OH, contributions of O 3 generated in O 2 plasma, and mass-transfer coefficients of reactive species cause these differences in decomposition characteristics. When plasma is generated in O 2 bubbles, O 3 is generated in addition to •OH, H 2 O 2 , and HO 2 •. Figure 6 presents the concentrations of H 2 O 2 and O 3 in water treated by plasma driven by a DC constant current. 56) When the input power, which is proportional to the discharge current, increases, the amount of H 2 O 2 produced increases and the amount of O 3 decreases. A zero-dimensional simulation (global simulation) considering mass transfer between the gas and liquid phases revealed that the •O in the plasma generated •OH or O 3 in bubbles (gas phase) via a reaction with H 2 O or O 2 , respectively, as follows: 57,58) By assuming an increase in the water vapor concentration in bubbles with increasing discharge current, the simulated The use of ballast capacitors facilitates the generation of multiple plasmas in parallel holes, which can increase the solution volume from a range of several tens of milliliters to a range of liters. One liter of acetic acid solution with an initial concentration of 30 mg TOC l −1 was treated with 21 plasmas generated within O 2 or Ar bubbles under the various conditions listed in Table III. This table also     corresponding to their production rates by plasma under condition (1). The decomposition rate of the AOP was almost the same as that of the plasma. 52) Therefore, •OH directly generated from plasma provides a negligible contribution to TOC reduction under low-power conditions. In contrast, in the case of Ar plasma, the decomposition process is likely initiated by •OH directly generated in the plasma. Because a large amount of O 3 was exhausted from the plasma reactor without dissolving based on its low absorption rate, treatments with an O 3 absorber were conducted to enhance O 3 absorption. Exhaust gas containing O 3 was injected into the solution through a diffuser in the absorber while the solution was circulated between the plasma reactor and absorber. The decomposition efficiency under condition (1) was 0.19 g TOC kWh −1 without the absorber and was improved to 0.43 g TOC kWh −1 with a 400 mm tall absorber, representing a 126% improvement in efficiency. 52) A similar enhancement was observed by Bradu et al. for the decomposition of organic compounds using plasma. 60) 2.5. Plasma/ozone combination processes [61][62][63][64] Because high humidity decreases the production efficiency of O 3 for plasma methods, plasma in contact with water is unsuitable for O 3 production. Therefore, O 3 must be produced by an energy-efficient ozonizer utilizing DBD and supplied to a solution containing H 2 O 2 produced by a plasma reactor.
An example plasma/ozone combination process using plasma generated within O 2 bubbles and an ozonizer is presented in Fig. 8. 61) The O 3 produced by the ozonizer is supplied to a solution containing H 2 O 2 produced by the plasma. The production rate of H 2 O 2 and supply rate of O 3 are varied by changing the input power for the plasma and ozonizer, respectively. An acetic acid solution (50 ml, 108 mg TOC l −1 , pH 7.5, phosphate buffer) was considered for this process. In Fig. 9, the time variation of the TOC concentration is presented under various conditions, where the H 2 O 2 and O 3 production rates g and efficiencies η are changed by the input powers for the plasma P p and ozonizer P o . As shown in Fig. 9     It is known that the production energy efficiency of H 2 O 2 using plasma heavily depends on the electrode configuration, gas composition, and discharge parameters. [65][66][67] If the supply ratio of O 3 to H 2 O 2 is maintained at a suitable value, 51) then the improvement of H 2 O 2 production efficiency directly leads to the improvement of TOC decomposition efficiency. Therefore, a diaphragm discharge plasma, which can achieve a higher production rate of H 2 O 2 at a higher energy efficiency compared to the plasma generated within gas bubbles, was combined with an ozonizer. [62][63][64] Figure 10 presents a diaphragm discharge plasma reactor used with an O 3 supply. 63,64) This reactor consists of two solution containers separated by a ceramic plate with ten small holes (0.3 mm in diameter), and an electrode in each container. An AC rectangular voltage is applied between the electrodes to generate diaphragm discharge plasma in the holes. The H 2 O 2 production efficiency reaches as high as 2.93 g kWh −1 . 62) An acetic acid solution (1 l, 477 mg TOC l −1 , pH 7.1, phosphate buffer) was treated using the plasmas generated in parallel at an input power of 579 W with a supply of O 3 at a concentration of 180 g m −3 and flow rate of 1 l min −1 , resulting in a decomposition rate of 0.48 g TOC h −1 and efficiency of 0.59 g TOC kWh −1 . 63) Decomposition efficiency can be improved by enhancing O 3 absorption. The use of a microbubble generator (Nitta Corp., BL12PP-12-SC4) for the O 3 supply in a solution treated by diaphragm discharge plasma with one hole resulted in an efficiency of 1.7 g TOC kWh −1 at a decomposition rate of 0.18 g TOC h −1 , while the efficiency when using a diffuser was 0.68 g TOC kWh −1 at a rate of 0.13 g TOC h −1 . 64) Other types of plasma, such as corona discharge over a solution 68) and plasma generated in a solution with a fine bubble supply, 69) were combined with an ozonizer and synergistic effects were confirmed. In these systems, O 3 was directly supplied to the plasma region.
A schematic of the most important reactions during the decomposition of organic compounds (represented by R) using O 2 plasma in contact with a liquid is presented in Fig. 11. In the case of the treatment of an acetic acid solution, R is either acetic acid, formic acid, oxalic acid, or their ions. 51,59) The evaporation of water plays a pivotal role in the generation of •OH and H 2 O 2 in the gas phase. 56) The decomposition of organic compounds occurs in the vicinity of the plasma-solution interface, where •OH is directly generated by the reaction defined in Eq. (1) within the plasma. When O 3 generated by an ozonizer is supplied to the solution, •OH is generated through the reactions summarized in Eq. (3). The decomposition of organic compounds with •OH through bulk reactions can be leveraged in addition to reactions in the vicinity of the plasma-solution interface. The effectiveness of adding O 3 is demonstrated in Fig. 9.

Comparison of various processes
Comparisons of the TOC decomposition rates and efficiencies obtained by various processes, including plasma methods, plasma/ozone combination processes, and other AOPs, are presented in Table IV and Fig. 12. A combination of plasma methods and an ozonizer solves the issue of the decomposition rate versus efficiency tradeoff, resulting in a relatively high efficiency comparable to that of conventional AOPs with a higher decomposition rate. It should be noted that nanosecond-pulsed plasma with droplets achieves a very high decomposition rate and efficiency. 44   as surfactants and high stability. However, their particularly high toxicity 70) and resistance to environmental 71,72) and biological 73,74) degradation (PFOS and its salts have been listed as persistent organic pollutants by the Stockholm Convention) limit the production and use of these compounds.
The chemical stability of PFOA and PFOS is a result of strong C-F bonds that cannot be decomposed, even by •OH. 75) Some methods have been reported for the decomposition of PFOA, including UV-induced photochemical reactions, 76) sonochemical reactions, 77) VUV irradiation, 78) persulfate-added hot water treatment, 79) and microwavehydrothermal methods. 80) Methods for the decomposition of PFOS include zerovalent iron in subcritical water, 81) photodegradation in the 2-propanol solution method, 82) aquated electrons generated from iodide photolysis, 83) and sonochemical methods. 77,84,85) A surfactant molecule contains a hydrophobic component, which typically consists of carbon chain(s), and a hydrophilic component, such as a sulfate group or carboxylic group. Some surfactant molecules dissolved in water accumulate on the water surface, which is considered as adsorption. When a new gas-water interface is formed in water containing a surfactant, the surface tension at the interface gradually decreases based on surfactant adsorption, and then reaches    29) 10.7 0.02 -7.0 × 10 -4 4.3 × 10 -1 Plasma with droplets 43) 4.6 7 -3.6 × 10 -4 3.5 × 10 -1 Plasma with droplets 44) 1000 1 -4.7 × 10 -1 6.5 Microwave plasma 39)  a stable value. The nonstationary surface tension is called dynamic surface tension and can be measured using a dynamic surface tensiometer. Figure 13 presents the dynamic surface tension of a PFOS solution at a bulk concentration of 50 mg l −1 (1 × 10 −4 mol l −1 ). 86) The surface tension decreases logarithmically with time and eventually reaches a static surface tension of 67 mN m −1 on the order of tens of seconds. The surface concentration of a surfactant at a gaswater interface can be calculated from the surface tension using the Gibbs adsorption isotherm equation. 86) The decrease in surface tension in Fig. 13 corresponds to an increase in the surface concentration of PFOS, which is also shown in Fig. 13. The static surface tension and surface concentration are functions of the bulk concentration, as shown in Fig. 14.
The interfacial activity of perfluorocarboxylic acids (PFCAs: C n F 2n+1 COOH) heavily depends on the lengths of their carbon chains (number of carbon atoms n), as shown in Fig. 15. As the length of the carbon chains increases, the surface concentration increases and the surface tension decreases at the same bulk concentration. 87,88) 3.2. Basic characteristics of the decomposition of PFCs using plasma methods Various plasma methods, including glow-like discharge plasma over a solution, [86][87][88] streamer-like discharge plasma over a solution with Ar bubbling, 89,90) plasma generated within gas bubbles, [91][92][93][94][95][96] nano-pulsed corona discharge plasma, 97) DBD plasma, 97) non-thermal plasma jet irradiation, 98) radio-frequency plasma, 99) and gliding arc plasma, 100) have been applied to the decomposition of PFOS, PFOA, and other PFCs in water. When PFOA and shorter PFCAs are decomposed, fluorine atoms are detached to the solution in the form of fluoride ions, while carbon atoms are converted into CO 2 gas with the generation of PFCAs with shorter carbon chains in the solution, as well as a small amount of fluorocarbon gases, such as CHF 3 , C 2 F 6 , or C 2 HF 5 . 91) The concentrations of PFCs in water can be quantified using liquid chromatography mass spectrometry while those of ions can be quantified using IC. The concentrations of gaseous components can be quantified using GC or FT-IR. The concentrations of both target compounds and fluoride ions can be used to evaluate decomposition rates and efficiencies. Additionally, the mass balances of carbon and fluorine can be analyzed to understand the decomposition process.
The time variations in the concentrations of PFOA and short PFCAs during the treatment of a PFOA solution by plasma generated within O 2 bubbles are presented in Figs. 16(a) and 16(b), respectively. 91) The decomposition reaction of PFOA is a first-order reaction and a very small amount of PFCAs are generated during treatment as byproducts. The mass balances of carbon and fluorine are maintained at high values (90%-100%) when the amounts of PFCAs, fluoride ions, and CO 2 gas are considered. 91,94,95) When a solution containing PFCAs is treated using plasma methods at a consistent bulk concentration, the decomposition rate is higher for PFCAs with longer carbon    86) chains. 87,88,91,101) This is because longer PFCAs accumulate at the gas-water interface at higher surface concentrations where direct contact with plasma and decomposition reactions occur. In contrast, in persulfate-added hot water treatments for PFCAs based on bulk reactions with SO 4 radical anions, a consistent decomposition rate is obtained, regardless of the length of the carbon chains, as shown in Table V. 101) Therefore, the decomposition of surfactants using plasma methods proceeds via interfacial reactions that occur at the plasma-liquid interface, where surfactants accumulate.
In the case of PFOS decomposition, sulfate ions are generated in the solution in addition to PFCAs and fluoride ions. In contrast to PFOA decomposition, the mass balance of fluorine decreases to approximately 70% during treatment when the amounts of PFOS, PFCAs, and fluoride ions are considered. 96,102,103) This is because a relatively large amount of C 8 HF 16 SO 3 H, which can be analyzed qualitatively using liquid chromatography-mass spectrometry, is generated in the solution. As treatment proceeds, the mass balance of fluorine increases to over 90% when 96% of the PFOS is decomposed, as shown in Fig. 17, indicating that C 8 HF 16 SO 3 H can be decomposed by plasma with fluoride ion generation. 103) For large-capacity treatment, the parallel generation of plasma within gas bubbles has been conducted using ballast resistors, 94,95) inductors, 104) or capacitors, 102,103) facilitating the treatment of solution volumes as high as 1 l.

Decomposition processes of PFOA and PFOS
It is known that the numbers of carbon atoms in the carbon chains of PFCAs decrease one by one, as shown in Fig. 18(a), during treatments using bulk reactions, such as persulfate-added hot water treatments. 79) A non-dimensional simulation model was developed to investigate the decomposition pathways of PFOA in plasma treatments. The experimentally obtained decomposition rate constants for PFOA and short-carbon-chain PFCAs were used for simulation and the decomposition pathways were obtained by fitting the experimental results. 91) During plasma treatment, the    91) dominant reaction is the direct decomposition of PFCAs into gaseous products, such as CO 2 , with a little generation of shorter PFCAs, as shown in Fig. 18(b). 91) We have suggested the decomposition process of PFOA during plasma treatment presented in Fig. 19. 91) In the first step, PFOA molecules adsorb onto the gas-liquid interface and contact the plasma, which has a high temperature and contains various energetic species, such as electrons, ions, and radicals. The gas temperature in the plasma is approximately 2000 K based on calculations of the rotational temperature of •OH using a spectroscopic method. Because the thermal decomposition of PFOA at 1473 K is possible, 78) successive thermal cleavages of the C-C bonds in PFOA molecules can occur if PFOA molecules are released into the plasma via water vaporization or sputtering based on ion irradiation. Stratton et al. suggested that aqueous electrons account for a sizable fraction of PFOA decomposition and that collisions with electrons and positive ions are the other main candidates. 90) Thermal cleavages generate gaseous CF m (m = 1-3) radicals in plasma. Based on the reduction and oxidation of CF m radicals by •H and •OH, respectively, HF, CO 2 , and CO gases are generated. The HF gas dissolves in the solution, whereas the CO 2 and CO gases are emitted as final products. This suggested pathway is similar to that for the sonochemical decomposition of PFOA and PFOS. 84) For short-carbon-chain PFCAs, such as trifluoroacetic acid (TFA), thermal cleavage is negligible based on low surfactant activity and the decomposition reaction rate constants are small compared to those for long-carbon-chain PFCAs. We have also suggested a decomposition process for PFOS in which the generation of C 8 HF 16 SO 3 H is considered. 96)

Comparison of various processes
The energy efficiency of PFOS decomposition using 21 plasmas generated within Ar gas bubbles 103) is compared to those of other methods in Fig. 20. Here, energy efficiency is calculated by dividing the solution volume by the energy required to reduce the PFOS concentration to 50% to eliminate the effects of the initial concentration of PFOS. The plasma method achieves more energy-efficient decomposition compared to the other methods and provides a large treatment capacity. This is because there is a positive correlation between the decomposition rate and surface concentration of the target surfactant, as described above. Effective interactions between plasma and surfactant molecules accumulating at the plasma-liquid interface makes energy-efficient treatment possible. Thagard et al. developed a decomposition model for organic compounds using plasma with three decomposition mechanisms occurring at the "subsurface", "surface", and "above-surface" levels. They concluded that above-surface reactions take place in the plasma interior between highly energetic plasma species and exposed portions of compounds that extend out of the interface, which is largely responsible for the decomposition of surfactant-like contaminants. 89) To evaluate mineralization efficiency, the generation efficiency of fluoride ions η F is a more suitable metric than PFOS decomposition efficiency because fluoride ions are generated via the decomposition of PFOS and other PFCs, which are generated as byproducts. The 21 plasmas generated within Ar gas bubbles achieved a η F value of 12.3 mg kWh −1 when 96% of the PFOS was decomposed, 103) which is much greater than the values of other methods, such as treatment using aquated electrons generated from iodide photolysis (1.6 mg kWh −1 at 32% PFOS decomposition), 83) sonochemical methods (4.3 mg kWh −1 at 100% PFOS decomposition), 84) and photodegradation in a 2-propanol solution (0.6 mg kWh −1 at 75% PFOS decomposition). 105)

Conclusions
In this paper, we reported the progress of research from the past 15 years on plasma-based water treatment technologies that decompose persistent organic compounds. A summary of important achievements, including several topics that were not detailed above, is provided in Table VI. The final goal of this research is to establish a scalable, energy-efficient process for the high-speed decomposition of persistent organic compounds in water. We considered two types of organic compounds, namely non-surfactants, such as acetic acid, and surfactants, such as PFOA and PFOS. The differences in the properties of these target compounds resulted in different decomposition processes.
Non-surfactants are almost uniformly distributed in a solution. However, the •OH directly generated by plasma  can react with such compounds only near the plasma-liquid interface. This results in a tradeoff between decomposition rate and efficiency in conventional plasma treatments. To overcome this issue, we developed plasma/ozone combination processes in which •OH is regenerated from H 2 O 2 by reacting with O 3 .
In contrast to non-surfactants, high concentrations of surfactants accumulate at gas-liquid interfaces, where effective contact with plasma is possible. Surfactant molecules can be decomposed by various energetic species in plasma, such as electrons, ions, radicals, and other neutral molecules. This can occur after they undergo irradiation by plasma via adsorption and the subsequent vaporization of water and/or sputtering. This results in more energy-efficient and rapid decomposition of PFOS and PFOA compared to other methods.
Because the operation of plasma-based water treatment technologies is possible without the addition of chemicals, such treatments are more advantageous than conventional methods that use chemicals, particularly for wastewater treatment at offshore plants. We are confident that plasmabased water treatment technologies will play an important role in the realization of a sustainable society.