Degradation of Micropollutants and Formation of Oxidation By-Products during the Ozone/Peroxymonosulfate System: A Critical Review

: The O 3 /PMS system has appeared as an effective wastewater treatment method because of the simultaneous generation of hydroxyl radicals ( • OH) and sulfate radicals (SO 4 •− ). Many research achievements have been made on the degradation of micropollutants and the reaction mechanism of the O 3 /PMS system. However, an integral understanding of the O 3 /PMS system is lacking, which limits the development of safe and effective AOP-based water treatment schemes. Therefore, in this review, the degradation effects, toxicity changes, and reaction mechanisms of various micropollutants in the O 3 /PMS system are reviewed. The formation of oxidation by-products (OBPs) is an important issue that affects the practical application of O 3 /PMS systems. The formation mechanism and control methods of OBPs in the O 3 /PMS system are overviewed. In addition, the inﬂuence of different reaction conditions on the O 3 /PMS system are comprehensively evaluated. Finally, future research needs are proposed based on the limited understanding of O 3 /PMS systems in the degradation of micropollutants and formation of OBPs. Speciﬁcally, the formation rules of several kinds of OBPs during the O 3 /PMS system are not completely clear yet. Furthermore, pilot-scale research, the operational costs, sustainability, and general feasibility of the O 3 /PMS system also need to be studied. This review can offer a comprehensive assessment on the O 3 /PMS system to ﬁll the knowledge gap and provide guidance for the future research and engineering applications of the O 3 /PMS system. Through this effort, the O 3 /PMS system can be better developed and turned towards practical applications.


Introduction
At present, the emergence of some pollutants (such as drugs, personal care products, endocrine disruptors, and other refractory organics) pose a threat to water quality and safety, which has aroused widespread concern [1][2][3]. Sulfate radicals (SO 4 •− )-based advanced oxidation processes (AOPs) have received widespread attention owing to their strong oxidation ability, fast reaction rate, and wide applicability to contaminants in wastewater [4][5][6]. SO 4 •− can be obtained by activation of PMS through various methods, which include ultraviolet light (UV) irradiation, heating, or addition of transition metals, carbon materials and ozone (O 3 ) [4,[7][8][9]. As a strong oxidant, O 3 can activate PMS to produce SO 4 •− , while at the same time it will decompose to produce a large amount of hydroxyl radicals ( • OH) [10][11][12]. In addition, singlet oxygen ( 1 O 2 ) and superoxide radicals (O 2 •− ) also can be generated in the process of O 3 activating PMS [13,14]. Under the combined action of these reactive oxygen species (ROS), different types of micropollutants can be efficiently degraded. Specifically, the results of studies showed that O 3 /PMS achieved 81% removal of ATZ in 10 min [10], and PMT was eliminated by 99.27% approximately in O 3 /PMS system within 10 min [15], while pCBA was fully degraded by O 3 /PMS in less than 5 min [16]. Furthermore, Gholikandi et al. reported that in terms of sludge stabilization and dewatering, O 3 /PMS was a better choice than other processes (i.e., O 3 , O 3 /H 2 O 2 , O 3 /PS) [17]. The study of Andrés et al. indicated that the O 3 /PMS combination produced a synergistic effect in the inactivation of microorganisms [18]. All these studies have shown that the O 3 /PMS system has a very great application potential in water treatment.
In addition, the influence of different reaction parameters (e.g., concentration of reactive substances), reaction conditions (e.g., pH, temperature), and water quality (e.g., concentration of inorganic and organic substances) on the reaction system is also one of the key points that needs to be studied urgently in AOPs. The results of several studies have shown that the concentration of O 3 and PMS have an appropriate range. Excessive dosage of O 3 and PMS would have a negative impact on the degradation of micropollutants in the O 3 /PMS system [30][31][32]. Temperature affects the decomposition of O 3 and the activation of PMS [33][34][35]. In addition, pH also shows influence on the conversion of free radicals [30,31,36]. Inorganic ions (e.g., Cl − , NO 2 − , CO 3 2− , HCO 3 − , phosphate) usually inhibit the degradation of micropollutants in the O 3 /PMS system by scavenging free radicals [37][38][39][40]. Natural organic matter (NOM) in water acts as a promoter or inhibitor for the generation of free radicals [41][42][43]. Therefore, the influence of these external conditions on the O 3 /PMS system needs to be comprehensively considered in both the analysis of the degradation effect of O 3 /PMS system on micropollutants and the study of the generation and control of OBPs in O 3 /PMS system.
To the best of our knowledge, the current research on the O 3 /PMS system mainly focuses on the theoretical exploration of a single direction. There has been no specific review on the O 3 /PMS system so far, which stimulated us to write this review article on this fast-growing research area with emphasis on the introduction, influence factors, degradation of micropollutants and formation and control of OBPs of O 3 /PMS system. The aim of this work is to develop an integrated understanding of the O 3 /PMS system through a critical evaluation of the relevant publications. As a result, the knowledge gaps in related research and future research directions are explored, so that the O 3 /PMS system can be better developed and used for practical applications.

Proposal of the O 3 /PMS System
As a strong oxidant, O 3 can effectively degrade many organic substances which are refractory to traditional oxidation processes [44,45]. However, O 3 has strong selectivity and tends to attack the double bonds, activated aromatic groups and non-protonated amines of organic substances [21,41]. On the other hand, • OH produced in the process of O 3 decomposition is a non-selective strong oxidant (Equations (1) and (2)), which can rapidly react with various micropollutants at nearly diffusion-controlled rates, and the diffusion-controlled rate of • OH is~10 8 -10 10 M −1 s −1 [46,47]. Typically, the degradation of micropollutants by O 3 is achieved by the combined activities of molecular O 3 and • OH. However, the oxidation efficiency of O 3 alone is very low for the refractory micropollutants in water due to the smaller amount of • OH produced by O 3 decomposition and the selectivity of molecular O 3 .
The • OH-based AOPs have attracted widespread attention. The reaction between O 3 and hydrogen peroxide (H 2 O 2 ) is one of the most common AOPs to produce • OH for contaminant degradation [48]. The O 3 /H 2 O 2 system was firstly proposed in a study by Staehelin and Hoigne [49]. Subsequently, the underlying mechanism of the O 3 /H 2 O 2 system through quantum-chemical and thermokinetic analysis was revised [46,50]. O 3 and H 2 O 2 firstly react to form the adduct HO 5 − (Equation (3)), which subsequently decomposes in two ways (Equations (4) and (5)). Eventually, • OH is generated through Equations (6)- (8).
SO 4 •− , as a strong oxidant, has a higher selectivity and higher redox potential (E 0 = 2.5-3.1 V) than • OH, and can react with many micropollutants at nearly diffusioncontrolled rates [51]. Additionally, compared with • OH, the reactions between SO 4 •− and micropollutants are less affected by alkalinity and NOM [51][52][53]. In many studies, the formation of SO 4 •− was achieved through activating persulfate (i.e., peroxodisulfate (PDS) and peroxymonosulfate (PMS)). The activation strategies include ultraviolet light (UV) irradiation, heating, or addition of transition metals and carbon materials [54][55][56][57]. The structure of PDS and PMS are shown in Figure 1. PMS has an asymmetric structure and a parallel peroxy bond (O-O) with H 2 O 2 , indicating that it is likely to substitute H 2 O 2 by PMS in the O 3 /H 2 O 2 system to achieve a synergistic effect [58]. Wen et al. reported that O 3 -activated PMS enhanced the degradation of pCBA, proving that PMS had a similar effect as H 2 O 2 in promoting the generation of free radicals during ozonation [16]. Furthermore, the study of Li et al. theoretically demonstrated that high chemical reactivity and low kinetic stability of PMS prompted its reaction with O 3 [13]. However, according to Figure 1, PDS exists in the form of symmetric structure, where the peroxy group of it is stable and can hardly react with O 3 [46]. The research by Yuan et al. also indicated that no radical signal was detected in the O 3 /PDS system [36]. Wen et al. reported that O 3 decomposition was only slightly enhanced in the presence of PDS [16]. Therefore, extensive research has used O 3 to activate PMS to generate • OH and SO 4 •− simultaneously, which could quickly and effectively degrade a variety of micropollutants. activated PMS enhanced the degradation of pCBA, proving that PMS had a similar effect as H2O2 in promoting the generation of free radicals during ozonation [16]. Furthermore, the study of Li et al. theoretically demonstrated that high chemical reactivity and low kinetic stability of PMS prompted its reaction with O3 [13]. However, according to Figure 1, PDS exists in the form of symmetric structure, where the peroxy group of it is stable and can hardly react with O3 [46]. The research by Yuan et al. also indicated that no radical signal was detected in the O3/PDS system [36]. Wen et al. reported that O3 decomposition was only slightly enhanced in the presence of PDS [16]. Therefore, extensive research has used O3 to activate PMS to generate • OH and SO4 •− simultaneously, which could quickly and effectively degrade a variety of micropollutants.

O 2 and O 2
•− would be produced in ozonation system. The self-decomposition of PMS would also produce 1 O 2 according to Equation (23) [14].
The mechanism of the simultaneous production of • OH and SO 4 •− in the O 3 /PMS system.

Influencing Factors
The influence of reaction conditions on the O 3 /PMS system is shown in Table 1. Related research mainly focuses on the influence of pH, concentration and molar ratio of O 3 and PMS, temperature, inorganic ions, and NOM on the O 3 /PMS system. These studies have explored the internal mechanism by analyzing the impact of the changes in external conditions on the O 3 /PMS system. These factors mainly influence the O 3 /PMS system by affecting the decomposition of O 3 , the activation of PMS, and the generation and conversion of free radicals. − .
[ 10,14,15,32,70] pH is an important factor in the O 3 /PMS system because of its remarkable effect on the decomposition of O 3 , the speciation of PMS, and the conversation of free radicals. In acidic conditions, the presence of excessive proton (H + ) could scavenge • OH and SO 4 •− based on Equations (24) and (25) [31]. As the pH increases up to alkaline, the decomposition of O 3 accelerates, resulting in the formation of more • OH [71]. In addition, since pK a2 of PMS is 9.4, the dominant species of PMS would change from HSO 5 − to SO 5 2− under alkaline conditions, which could induce more SO 4 •− generation [36,72]. Besides, according to Equations (1), (2), (26), and (27), O 3 and PMS could react with OH − to produce HO 2 − , which then reacts with O 3 and PMS to generate • OH and SO 4 •− , respectively [32,41,73]. At the same time, the presence of OH − leads to the transformation of SO 4 •− to • OH based on Equation (21) [13,15,30].
The dosage and the molar ratio of O 3 and PMS are also very important influencing factors in the O 3 /PMS system. The increase of O 3 and PMS dosage leads to the generation of more free radicals in a proper range [10,36], while self-consumption between free radicals also occurs when there are too many • OH and SO 4 •− in the solution, according to Equations (28)- (30) [13,15,47,51,74]. On the other hand, excessive O 3 and PMS would exhibit an inhibitory effect on the reaction. Specifically, excessive O 3 could influence the amount of free radicals and act as scavenger based on Equations (31) and (32) [32,63,75]. Excessive PMS could act as a scavenger of • OH and SO 4 •− and facilitate the transformation of abundant SO 4 •− into SO 4 2− , as described in Equations (33) and (34)  In addition, the high concentration of PMS would reduce the pH value and excessive H + could scavenge free radicals [31,32]. When the molar ratio of PMS: O 3 was 1:1, the amount of PMS that could be activated by O 3 tended to stabilize [60]. By contrast, H 2 O 2 :O 3 = 0.5 was the optimal molar ratio for the O 3 /H 2 O 2 system [76,77].
Temperature is a very important influencing factor in all reaction systems. Although the study by Shao et al. indicated that the O 3 /PMS system was not controlled by thermodynamics in the temperature range of 5-40 • C [32], other studies have shown that the amount of free radicals in the O 3 /PMS system increased with the increase of temperature [33,78,79]. Specifically, the O-O bond of PMS was easily broken at a higher temperature while PMS activation was reduced at a lower temperature, leading to the reduction of SO 4 •− . Furthermore, the solubility and availability of O 3 to produce free radicals in aqueous solution were reduced at a higher temperature [80][81][82].
The presence of some kinds of inorganic ions has a significant impact on the O 3 /PMS system [14], while the impact of different ionic strengths on the O 3 /PMS system is very limited. According to Equations (35)-(37), Cl − had limited effect on • OH because the reaction between Cl − and • OH was reversible and the generation of Cl • occurred only at low pH conditions [21]. On the other hand, Cl − could scavenge SO 4 •− to produce less reactive Cl • (Equations (38) and (39)) [32,37,38]. The reaction between Cl − and SO 4 •− could lead to the generation of • OH [40,83]. Br − affected the O 3 /PMS system through rapid and irreversible reacting with • OH and SO 4 •− (Equations (40) and (41)) [84,85]. Equations (42)-(45) describe the reaction of free radicals with CO 3 2− and HCO 3 − [32]. CO 3 2− and HCO 3 − could quench the free radicals effectively to generate CO 3 •− , with lower redox potential (E 0 = 1.78 V) than • OH and SO 4 •− [14]. NO 2 − influenced oxidants and free radicals due to its reducibility (Equations (46) and (47)) [14,86]. Phosphate ions showed a strong inhibitory effect on O 3 decomposition [87]. Therefore, the use of phosphate buffer solution in the O 3 /PMS system should control the concentration of phosphate ions.
NOM plays a dual role in the O 3 /PMS system [41,42]. The low concentration of NOM enhanced the decomposition of O 3 to produce • OH [43]. However, NOM acted as a scavenger for • OH and SO 4 •− at relatively high concentrations [70]. HA, as an important component of NOM, also played an obvious dual role in the O 3 /PMS system [15].

Degradation Effect and Energy Efficiency
The O 3 /PMS system can quickly and effectively generate • OH and SO 4 •− , so it is widely used in the research of micropollutant degradation. As shown in Table 2, the O 3 /PMS system exhibits a good degradation effect when treating sewage-containing general chemicals, agricultural chemicals, and medical chemicals. The SO 4 •− formed by PMS activation exists in the system for a long time, so it can oxidize micropollutants more effectively. Specifically, the O 3 /PMS system has high efficiency in degrading typical micropollutants in agricultural and medical industries, so it can be used for soil remediation and medical wastewater treatment. There are many factors that affect the degradation effect of O 3 /PMS on micropollutants, such as the type and concentration of micropollutants, the concentration and molar ratio of O 3  The reaction rate constants between different micropollutants with O 3 and free radicals are shown in Table 3. The reaction rate constants determine which ROS plays a key role in the degradation of target micropollutants in the O 3 /PMS system. For example, when the solution pH shifted from neutral to alkaline, the proportion of O 3 that directly reacted with ACE decreased, resulting in an enhanced formation of SO 4 •− and suppressed formation of • OH. Considering that SO 4 •− degraded ACE more slowly than • OH did, the oxidation capacity of the system was weakened due to the decrease of • OH formation [32]. On the other hand, the synergy between the various ROS (i.e., O 3  •− -based AOPs [90]. This is mainly due to the selectivity of SO 4 •− , which will consume more energy when degrading the target micropollutants at a low reaction rate with SO 4 •− .

Toxicity Changes and Degradation Pathway
The O 3 /PMS system can significantly reduce the toxicity of micropollutants. Specifically, the biodegradability of activated sludge containing 2,4-D was increased from 8.3% to 58.9%, and the toxicity was reduced from 76.5% to 3.8% after treatment by the PMS/MCFNs/O 3 system [14]. With the oxidation of O 3 /PMS, the toxic equivalent (TE) and the relative inhibition light ratio (RILR) of BCPMW were significantly lowered from 0.08 mg/L to 0.02 mg/L and 36% to 9%, respectively [88]. Tan et al. studied the degradation effect of O 3 /PMS system on micropollutants containing a variety of anti-inflammatory drugs. Toxicity was calculated based on the toxicity parameter 50% lethal concentration (LC50) of each DBP. The results indicated that the toxicity of the system was decreased after O 3 /PMS pre-oxidation. Specifically, the toxicity of disinfection by-products (DBPs) reduced from 6.63 × 10 −2 min −1 to 5.27 × 10 −2 min −1 under neutral conditions [93].
Among the ROS generated in the O 3 /PMS system, • OH and SO 4 •− have the strongest oxidizing ability. Therefore, the priority attack sites of these two free radicals should be firstly considered when analyzing the degradation path of micropollutants. SO 4 •− has electrophilicity and tends to react with electron-donating groups such as hydroxyl (-OH), alkoxy (-RO) and amino (-NH 2 ) groups, but does not easily react with the nitro (-NO 2 ), carbonyl (C=O), or other electron-withdrawing groups [115,116]. On the other hand, • OH is nonselective toward organic pollutants in the oxidation reaction. For some examples, the aromatic ring or the side chains (isopropylamino and alkoxy) of PMT are likely to be attacked by • OH and SO 4 •− mainly through addition to unsaturated carbon, H-abstraction, and electron abstraction [15,[117][118][119]. In addition, • OH and SO 4 •− participated in the degradation of ACE and the attack sites were C=C, C-O, and C-N bonds [32].

Formation Pathway and Influencing Factors
The OBPs formed in the O 3 -based oxidation process are mainly low-molecular-weight carbonyls, organic halogenated OBPs, and inorganic OBPs. Among them, the inorganic OBPs generally includes chlorinated OBPs, brominated OBPs, and iodinated OBPs. Compared to brominated OBPs, the production of chlorinated and iodinated OBPs during the O 3 /PMS system is negligible [65]. On the other hand, Frederik et al. reported the formation rule of AOC in O 3 alone, but there is no relevant research on the O 3 /PMS system [120,121]. As typical brominated OBPs, the formation mechanism of bromate (BrO 3 − ) in the treatment of bromide-containing water by O 3 /PMS has been reported in detail by Wen et al., as shown in Figure 3 [64]. The interaction between bromide (Br − ) and molecular O 3 [64]. Compared with the BrO 3 − generation path of the traditional ozone oxidation process, the SO 4 •− path is added in the O 3 /PMS system. Therefore, the O 3 /PMS system will generate more BrO 3 − than O 3 alone. In addition, the research by Liu et al. indicated that some brominated OBPs including dibromoacetaldehyde and tribromoacetaldehyde may possess much higher cytotoxicity than BrO 3 − [65]. Thus, more attention should be paid to the formation and control of organic halogenated OBPs during O 3 -based processes [124]. The influence of reaction conditions on the OBP formation is shown in Table 4. The amount of BrO3 − produced increases with the increase of Br − concentration within a certain range. However, too much Br − exhibits an inhibition effect [64]. The pH value of the solution comprehensively affects the formation of OBPs in the O3/PMS system by affecting O3 decomposition, Rct,•OH and Rct,SO4• − , and PMS speciation [64,125]. According to the research results, BrO3 − formation would increase as O3 and PMS dosage increases [64,125,126]. However, according to the reaction mechanism of the O3/PMS system, this promotion effect may be reduced with the addition of excessive O3 and PMS. The HCO3 − in the inorganic ions inhibits the formation of BrO3 − by scavenging free radicals. On the other hand, NH4 + prevented the conversion of Br − into BrO3 − by masking important intermediate products (HOBr/OBr − ) [64]. HA, as an important constituent of NOM, could scavenge ROS and thus reduce the formation of BrO3 − [78,127]. In addition, HA could readily capture the intermediates, providing an additional inhibitory effect [122].  The influence of reaction conditions on the OBP formation is shown in Table 4. The amount of BrO 3 − produced increases with the increase of Br − concentration within a certain range. However, too much Br − exhibits an inhibition effect [64]. The pH value of the solution comprehensively affects the formation of OBPs in the O 3 /PMS system by affecting O 3 decomposition, R ct,•OH and R ct,SO4• − , and PMS speciation [64,125]. According to the research results, BrO 3 − formation would increase as O 3 and PMS dosage increases [64,125,126]. However, according to the reaction mechanism of the O 3 /PMS system, this promotion effect may be reduced with the addition of excessive O 3 and PMS. The  78,127]. In addition, HA could readily capture the intermediates, providing an additional inhibitory effect [122].

Control Strategy
The current research on the control methods of OBP formation in the O 3 /PMS system focuses on inhibiting the formation of BrO 3 − . Several methods were used to control the formation of BrO 3 − in O 3 alone: reducing pH [125], adding carbon materials [128,129], H 2 O 2 [130], and ammonia (NH 3 ) and chlorine (Cl 2 ) [123,131] [92]. The research results indicated that the addition of carbon materials significantly inhibited the BrO 3 − formation, and the order of the inhibition efficiency was as follows: graphene (GO) > carbon nano tube (CNT) > powdered activated carbon (PAC). According to the study, the carbon materials could block the BrO 3 − formation by reducing HOBr/OBr − in the reaction system [92]. Besides, Wen et al. synthesized a catalyst (CuCo 2 O 4 -GO), which could simultaneously inhibit the formation of BrO 3 − and enhance the degradation of micropollutants in the O 3 /PMS system. Specifically, when 100 mg/L CuCo 2 O 4 -GO was added, the BrO 3 − inhibition efficiency reached 96.17% and the degradation efficiency of SMX increased from 0.163 min −1 to 0.422 min −1 [133]. The pretreatment strategy (i.e., NH 3 , Cl 2 -NH 3 and NH 3 -Cl 2 ) was also used to inhibit BrO 3 − generated in the O 3 /PMS system. All the pretreatment strategies reduced 90% or more of the overall BrO 3 − formation, while the NH 3 -Cl 2 pretreatment strategy was prior to that of the NH 3 and Cl 2 -NH 3 [134].  [137][138][139][140]. These two kinds of BrO 3 − inhibition strategies may be able to inhibit the formation of BrO 3 − in the O 3 /PMS system through similar mechanisms. In general, as shown in Figure 4, the control strategies are used to inhibit the formation of BrO 3 − by affecting the initial Br − or HOBr/OBr − .

Recommendations and Future Prospects
In terms of micropollutant degradation, the research on the O3/PMS system is stil the laboratory level. The investigation using real water should be strengthened to ref the feasibility of O3/PMS system in practical applications, because many substances c tained in actual water will affect the O3/PMS system. Besides, the degradation efficien under different actual water conditions (i.e., surface water and groundwater) should studied and comparable to explore the water quality condition which is suitable for application of the O3/PMS system. At the same time, more pilot-scale research is need to promote the conversion of O3/PMS system to practical applications.
Due to the generation of SO4 •− , the O3/PMS system has higher selectivity than O3/H2O2 system. Therefore, the degradation rules of different types of micropollutant the O3/PMS system should be extensively researched. The toxicity changes of the trea micropollutants also need to be studied, which are important indicators for evaluating practical application potential of the O3/PMS system. In order to evaluate the advanta and disadvantages of the O3/PMS system and the suitable application conditions, O3/PMS system should be compared with the O3-alone and O3/H2O2 systems when c ducting the above research.
The formation rules of several kinds of OBPs under different conditions during O3/PMS system are not completely clear yet. Notable are the structure change of NO and the formation rule of small molecular organic matter after treatment by O3/PMS s tem. At the same time, the effectiveness of various OBP control methods in the O3/P system has not been widely studied. Since the O3/PMS system can generate several kin of ROS, the formation and control of the OBPs need to be compared with the O3 alone a

Recommendations and Future Prospects
In terms of micropollutant degradation, the research on the O 3 /PMS system is still at the laboratory level. The investigation using real water should be strengthened to reflect the feasibility of O 3 /PMS system in practical applications, because many substances contained in actual water will affect the O 3 /PMS system. Besides, the degradation efficiency under different actual water conditions (i.e., surface water and groundwater) should be studied and comparable to explore the water quality condition which is suitable for the application of the O 3 /PMS system. At the same time, more pilot-scale research is needed to promote the conversion of O 3 /PMS system to practical applications.
Due to the generation of SO 4 •− , the O 3 /PMS system has higher selectivity than the O 3 /H 2 O 2 system. Therefore, the degradation rules of different types of micropollutants in the O 3 /PMS system should be extensively researched. The toxicity changes of the treated micropollutants also need to be studied, which are important indicators for evaluating the practical application potential of the O 3 /PMS system. In order to evaluate the advantages and disadvantages of the O 3 /PMS system and the suitable application conditions, the O 3 /PMS system should be compared with the O 3 -alone and O 3 /H 2 O 2 systems when conducting the above research.
The formation rules of several kinds of OBPs under different conditions during the O 3 /PMS system are not completely clear yet. Notable are the structure change of NOM and the formation rule of small molecular organic matter after treatment by O 3 /PMS system. At the same time, the effectiveness of various OBP control methods in the O 3 /PMS system has not been widely studied. Since the O 3 /PMS system can generate several kinds of ROS, the formation and control of the OBPs need to be compared with the O 3 alone and O 3 /H 2 O 2 systems to explore the mechanism. In addition, micropollutants are not fully mineralized by the O 3 /PMS system but degraded into transformation products (TPs), which arouse a growing concern because of the unknown structures and potential biological effects. Therefore, more research needs to pay attention to the TPs formed during the degradation of micropollutants in the O 3 /PMS system.
The operational costs (e.g., energy consumption, chemical input), sustainability (e.g., resource use, carbon footprint), and general feasibility (e.g., physical footprint and oxidation by-product formation) of the O 3 /PMS system need to be studied to enable to compare their efficiency with other AOPs and alternative treatment processes (i.e., O 3 /H 2 O 2 , O 3 /UV). In addition, the combination of O 3 and biological activated carbon (BAC) is a very common water treatment process in practical applications, which can enhance the degradation efficiency of organic matter while reducing OBPs in the effluent. Therefore, the combined effect of O 3 /PMS and BAC is also worth studying.

Conclusions
As a new advanced oxidation process, O 3 /PMS degrades many refractory micropollutants rapidly and effectively by generating many strong oxidizing ROS simultaneously. Compared with the widely used O 3 and O 3 /H 2 O 2 systems, the O 3 /PMS system produces more types of free radicals and has higher selectivity. Based on the current research, the O 3 /PMS system has a good degradation efficiency on general chemicals, agrochemicals and medical chemicals, and the degradation effect is affected by a variety of influencing factors (e.g., pH, the concentration of O 3 and PMS, temperature, and inorganic ions). These factors mainly influence the O 3 /PMS system by affecting the decomposition of O 3 , the activation of PMS, and the generation and conversion of free radicals. The generation and control of OBPs during the degradation of micropollutants in the O 3 /PMS system is another current research focus. According to the research results, the BrO 3 − produced in the O 3 /PMS system is mainly due to the interaction between Br − and molecular O 3 , • OH and SO 4 •− , and the BrO 3 − formation can be effectively inhibited by addition of carbon materials, or NH 3 and Cl 2 combined pretreatment strategy. However, it is not practical enough to apply the O 3 /PMS system to actual water treatment processes, and there are still many key problems that need to be addressed. Specifically, the degradation rule and toxicity change of different types of micropollutants in the O 3 /PMS system should be extensively studied. The formation rules of several kinds of OBPs during the O 3 /PMS system are not completely clear yet. Furthermore, pilot-scale research, the operational costs, sustainability, and general feasibility of the O 3 /PMS system also need to be studied. Currently, there is no integrated understanding of the O 3 /PMS system. It is expected that the findings of this review may advance future research and application of O 3 /PMS system. Specifically, the continuous exploration in the research directions proposed by this article will not only make the O 3 /PMS system perform better in the degradation of micropollutants, but also enhance the potential of applications of the O 3 /PMS process in other areas such as sludge stabilization, dewatering, and inactivation of microorganisms.