Application Progress of O₃/PMS Advanced Oxidation Technology in the Treatment of Organic Pollutants in Drinking Water

Abstract

In recent years, due to the abuse of pharmaceuticals and personal care products (PPCPs), many refractory trace organic compounds (TrOCs) have been transferred into natural water bodies, posing significant challenges to the water environment. On the other hand, advanced oxidation processes (AOPs) are cleaner and more efficient than traditional biochemical degradation processes. Among them, the combined ozone/persulfate advanced oxidation process (O₃/PMS) based on sulfate radicals (SO₄^•−) and hydroxyl radicals (•OH) has developed rapidly in recent years. Thus, this paper summarised the reaction mechanism of O₃/PMS and analysed its research and application progress in drinking water treatment. In addition, the process’s operation characteristics and current application scope were discussed, and the generation ways and inhibition methods of bromate and halogenates, by-products in the oxidation process, were summarised, which had a specific reference value for further research on O₃/PMS process.

1. Introduction

In recent years, TrOCs have been detected in different water bodies, mainly from the disorderly discharge of PPCPs. PPCPs contain a variety of organic groups. Typical PPCPs such as antibiotics, antibiotics, hormones, etc. [1,2,3] are widely found in natural water bodies and drinking water [4,5,6,7]. For the degradation of TrOCs, sewage treatment plants are the most critical line of defence. The current activated sludge method commonly used in sewage treatment plants cannot precisely remove all kinds of TrOCs in water. Therefore, after TrOCs are discharged into natural water, it consumes much oxygen due to microbial degradation, which reduces dissolved oxygen concentration in water leading to black and smelly water, a surge of anaerobic bacteria and various problems [8,9]. In addition, extensive research shows that [5,6,7,8,9], when such organic pollutants reach a specific concentration in water, they will severely impact aquatic organisms and endanger human health. Hence, AOPs are widely used in the degradation of TrOCs in water because of their high-efficiency oxidation [10,11], which not only makes up for the shortcomings of traditional biological treatment processes but also promotes benign development of the water treatment industry.

The O₃/PMS process is a new sulfate radical-based advanced oxidation process (SR-AOPs) proposed by Ma Jun, a professor at Harbin Institute of Technology in China, in 2015; it is mainly used to treat atrazine (ATZ). After oxidation, the concentration of ATZ can be reduced below the detection limit [12]; this process can produce •OH and SO₄^•− simultaneously, showing the advantages of rapidly degrading organic pollutants and mineralising them into CO₂ and H₂O, which has significant advantages over the traditional advanced oxidation process. SO₄^•− in the O₃/PMS process is mainly provided by persulfuric acid (PMS). SO₄^•− has a high redox potential, which is equivalent to •OH (2.5–3.1 eV) under neutral conditions, but slightly higher than •OH (1.9 eV) under acidic conditions [13,14], and has a long half-life (30–40 μs), and a wide range of pH application [15,16]. Under a conventional environment, the decomposition rate of persulfate anion (S₂O₈²⁻) to produce SO₄^•− was slow, and the effect of using PMS alone was moderate [17,18]. Therefore, various methods for activating PMS have been derived, including thermal decomposition, ultrasonic, ultraviolet irradiation, alkali activation, laser flash activation and transition metal catalysis. Cong J. et al. [19] showed that ozone could activate PMS and simultaneously produce SO₄^•− and •OH. In a weak alkaline environment, SO₄^•− continued to react with OH⁻ or H₂O to produce •OH [20], further improving the degradation efficiency. Extensive experiments showed that [12,15,16,20], although SO₄^•− and •OH were both detected in the system, •OH was the prominent active oxidation radical.

This paper summarises the reaction mechanism of O₃/PMS. Furthermore, it introduces the latest research progress on the O₃/PMS process in drinking water treatment, which provides a valuable reference for future studies to carry out in-depth research on the O₃/PMS process.

2. Reaction Mechanism of O₃/PMS Process

O₃/PMS process can simultaneously produce two kinds of strong oxidising active free radicals, •OH and SO₄^•−. The O₃/PMS process’s reaction mechanism is divided into four categories:

(1)

O₃ molecule directly oxidises and degrades pollutants;

(2)

Persulfate (S₂O₈²⁻/SO₅²⁻) directly oxidises pollutants;

(3)

O₃ reacts with H₂O to produce •OH, •OH indirectly oxidises pollutants;

(4)

O₃ guides PMS to generate SO₄^•−, and SO₄^•− and •OH work together to oxidise pollutants indirectly. At the same time, SO₄^•− can promote •OH generation in turn.

Among them, the direct oxidation in processes 1, 2 and 3 and the indirect oxidation of •OH are dominant, but for some TrOCs, the indirect oxidation of SO₄^•− produced in process 4 plays an important role.

From the microscopic point of view, the structural formula of PMS (H₂SO₅) is that a hydrogen atom in H₂O₂ is replaced by a Sulfo group (⁻SO₃H); that is, HOO⁻ is combined with –SO₃H. Consequently, some properties of PMS are similar to those of H₂O₂, with the difference that the free radicals (SO₄^•− and •OH) produced after disconnecting the O-O bonds are different, as shown in Figure 1.

The formation process of •OH and SO₄^•− in O₃/PMS system, the reaction equations and reaction rate constants are shown in

Table 1. The reaction equation for the formation of •OH and SO₄^•− and the corresponding reaction rate constants.

No.	Reaction	Reaction Rate Constant (L·M−1·s−1)	References
1	${\mathrm{SO}}_5^{2-}{+\mathrm{O}}_3\to {\mathrm{SO}}_8^{2-}$	2.12 × 104	[12,21]
2	${\mathrm{SO}}_8^{2-}\to {\mathrm{SO}}_5^{•-}{+\mathrm{O}}_3^{• }$	none	[12]
3	${\mathrm{SO}}_8^{2-}\to {\mathrm{SO}}_4^{2-}{+2\mathrm{O}}_2$	none	[12]
4	${\mathrm{O}}_3^{•-}\rightleftharpoons {\mathrm{O}}_2{+\mathrm{O}}^{•-}$	none	[22]
5	${\mathrm{O}}^{•-}{+\mathrm{H}}_2\mathrm{O}\rightleftharpoons •\mathrm{OH}+{\mathrm{OH}}^{-}$	none	[22]
6	${\mathrm{SO}}_5^{•-}{+\mathrm{O}}_3\to {\mathrm{SO}}_4^{•-}{+2\mathrm{O}}_2$	1.6 × 105	[22,23]
7	${2\mathrm{SO}}_5^{•-}\to {2\mathrm{SO}}_4^{•-}{+\mathrm{O}}_2$	2.1 × 108	[22,23]
8	${2\mathrm{SO}}_5^{•-}\to {\mathrm{S}}_2{\mathrm{O}}_8^{2-}{+\mathrm{O}}_2$	2.2 × 108	[22,23]
9	${\mathrm{SO}}_4^{•-}{+\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+{\mathrm{SO}}_4^{2-}+•\mathrm{OH}$	<3 × 103	[22,24]
10	${\mathrm{SO}}_4^{•-}{+\mathrm{OH}}^{-}\to {\mathrm{SO}}_4^{2-}+•\mathrm{OH}$	7.3 × 107	[22,24]

[12,21,22,23,24]; it can be seen from the reaction Reactions (1)–(10) that in the O₃/PMS combined system, PMS reacted with O₃ in the form of divalent anion SO₅²⁻ to produce SO₈²⁻ (Reaction (1)) [12,21], and then SO₈²⁻ decay in two different forms to produce SO₅^•− and SO₄²⁻ (Reactions (2) and (3)) [12]. O₃^•− is converted into •OH in water (Reactions (4) and (5)) [22], and SO₅^•− reacts with O₃ or generates SO₄^•− through bimolecular decay (Reactions (6)–(8)) [22,23]. At the same time, SO₄^•−, which was more oxidising, further oxidised H₂O or OH⁻ to produce •OH (Reactions (9) and (10)) [22,24].

In addition, the monoanionic HSO₅⁻ and divalent anion SO₅²⁻ in PMS react with H₂O to generate H₂O₂ (Reactions (11) and (12)) [12,25], and O₃ oxidises H₂O₂ to generate •OH, which further improves the •OH generation rate in the O₃/PMS system Reaction (13)) [25]. Recent studies have found that SO₄^•− reacts with H₂O or OH⁻ to form •OH (Reactions (14) and (15)) [22,26].

Table 2. Reaction equations and corresponding reaction rate constants for PMS and SO₄^•− promoted •OH formation.

No.	Reaction	Reaction Rate Constant (L·M−1·s−1)	References
11	${\mathrm{HSO}}_5^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{HSO}}_4^{-}+{\mathrm{H}}_2{\mathrm{O}}_2$	none	[12]
12	${\mathrm{SO}}_5^{2-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{SO}}_4^{2-}+{\mathrm{H}}_2{\mathrm{O}}_2$	none	[25]
13	${\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{O}}_3\to {3\mathrm{O}}_2+2•\mathrm{OH}$	none	[25]
14	${\mathrm{SO}}_4^{•-}+{\mathrm{OH}}^{-}\to {\mathrm{SO}}_4^{2-}+•\mathrm{OH}$	(6.5 ± 1.0) × 107 M−1·s−1	[26]
15	${\mathrm{SO}}_4^{•-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+{\mathrm{SO}}_4^{2-}+•\mathrm{OH}$	<3 × 103 M−1·s−1	[22]

shows the reaction formulas and corresponding reaction rate constants of PMS and SO₄^•− promoting •OH formation [12,22,25,26]. The reaction formulas in Table 1 and Table 2 correspond to processes (a) and (b) in Figure 2, respectively.

3. Status of O₃/PMS Process Research

The essence of the AOPs is to produce enough free radicals to oxidise and degrade organics in water [27]. Currently, most AOPs are catalysed to generate •OH [28]. The advantages and disadvantages of several mainstream AOPs are compared in

Table 3. Comparison of advantages and disadvantages of several mainstream AOPs.

Process	Advantages	Disadvantages	Ref.
UV	(a) Green and environmentally friendly, with little impact on water quality; (b) Stable maturity and sustainable output of light.	(a) It will lead to the phenomenon of photoreactivation and dark repair. (b) The effect is average when used alone and is usually used in combination with other processes, such as UV/H2O2, UV/O3, UV/NH2Cl, etc.	[29,30,31]
Fenton method	(a) The equipment is simple, the reaction conditions are mild, and it can be operated under normal temperature and pressure; (b) Fast oxidation speed and high efficiency; (c) Green and environmentally friendly, low environmental pressure.	(a) H2O2 is unstable, and FeSO4 is added to generate Fe2+, which is difficult to operate; (b) It needs to operate under strong acid conditions and is highly corrosive; (c) High cost and a lot of sludge.	[32,33]
O3/H2O2	(a) Fast oxidation speed and high efficiency; (b) Less secondary pollution; (c) It has excellent colour removal performance.	(a) H2O2 is unstable; (b) The effective reaction time is short; (c) Complex equipment, high energy consumption and high cost.	[34,35]
O3/PMS	(a) The equipment is simple, the reaction conditions are relatively mild, and it can be operated under normal temperature and pressure; (b) Fast oxidation speed and high efficiency; (c) Green and environmentally friendly, with less toxic by-products; (d) Two active free radicals with stronger applicability.	(a) The mineralisation ability is average; (b) It generally needs to operate in a weak alkaline environment.	[12,25]

.

Current research shows that the use of combined processes can significantly improve the oxidation rate and treatment effect of organics, shorten the reaction time, and reduce the limitations of a single process. All the main oxidative radicals of the Fenton method, O₃/H₂O₂, and O₃/PMS mentioned above are •OH. In the advanced oxidation system dominated by •OH, •OH has a broader scope of application, does not produce secondary pollution, and the cost of equipment and consumables is low, but it still faces problems such as insufficient mineralisation capacity. While the advanced oxidation system based on SO₄^•− shows the advantages of fast reaction and short cycle, compared with the system based on •OH, SO₄^•− has a more substantial selectivity for the reaction environment and smaller scope of application [36,37]. As described below, extensive research has been carried out on the O3/PMS process in drinking water treatment.

3.1. Using the O₃/PMS Process for the Pretreatment of Drinking Water

Recently, some natural water bodies have been found to contain many dissolved organic matter (DOM) and pathogenic microorganisms. At the same time, excessive total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD), biochemical oxygen demand (BOD) and other adverse pollutants cause colour, taste and odour problems in water. The coagulation-sedimentation-filtration-disinfection process is the most widely used in traditional water treatment. However, for the above water quality problems, excessive coagulants and disinfectants will increase sludge and produce harmful disinfection by-products (DBPs). Thus, in order to find the pretreatment method of conventional water treatment process suitable for the above water quality, more and more researchers turn their attention to the SR-AOPs process, especially O₃/PMS process.

Cao Y. et al. [38] used O₃/PMS process to degrade chloramphen-icol (CAP). The experiment showed that the degradation efficiency of CAP by the O₃/PMS process was extremely high, and CAP could be reduced below the detection concentration in only 5 min under optimal conditions. After pre-oxidation, it was easy for CAP to form the by-product Dichlo-roace-Tamide (DCAcAm), which has strong biological toxicity and cytotoxicity. Compared with other AOPs processes, O₃/PMS produced the least DCAcAm because SO₄^•− reduced the attack of the oxidant on the DCAcAm side chain in CAP. The degradation efficiency of CAP increased with the increase in NOM concentration. At lower NOM concentrations, a higher concentration of free radicals is generated due to the accelerated ozone decomposition by NOM, thus promoting CAP degradation. Nevertheless, when the concentration of NOM was high, the degradation of CAP was significantly inhibited because the competition between NOM and CAP consumed active free radicals. In addition, HCO₃⁻ and Cl^– concentrations also impact the degradation efficiency of the O₃/PMS process, but only about 10%. Du X. et al. [39] used ozone/persulfate/coagulation (O₃/PMS/(O/C)) combined with a ceramic membrane process to simultaneously remove iron (Fe²⁺: 2.0~4.2 mg·L⁻¹), manganese (Mn²⁺: 0.99~4.12 mg·L⁻¹) and sulfadimidine ((SMZ) = 400~800 µg·L⁻¹). The results showed that Fe²⁺ and Mn²⁺ in groundwater played an in-situ catalytic role in the system, which accelerated the yields of •OH and SO₄^•− and further oxidised SMZ or NOM. However, NOM and SMZ had a competitive effect. NOM in groundwater promotes the formation of by-products such as ferric hydroxide crystallisation or manganese precipitation, which reduces ceramic membranes’ filtration efficiency and requires regular precipitation removal. The highlight of this method was that Fe²⁺ in groundwater could be used to activate PMS in situ. During the oxidation process, NOM in water was also degraded. The order of dependence of the three main NOM components in sewage on indirect oxidation was: humic substances > yellow humic substances > protein substances [40]. O₃/PMS process is vulnerable to various factors in the treatment of natural water, such as inorganic ions such as Cl⁻, Br^−, I³⁻ and NOM [41].

Yuan Z. et al. [25] tested the degradation effect of O₃/PMS on ibuprofen (IBP). Under the optimal conditions, within 20 min, the degradation efficiency of O₃/PMS for IBP was 72.0%, which was 22.9% higher than that of O₃/H₂O₂, indicating the high-efficiency synergistic effect of PMS and O₃ led to the high degradation efficiency of O₃/PMS for IBP. Compared with other processes, O₃/PMS has a highly efficient degradation ability for IBP. The control experiment showed that with the increase in pH value, the higher the concentration of the two active radicals, the higher the efficiency of oxidation reaction and the degradation rate of IBP reached the highest at pH = 9. By simulating the natural water matrix with humic acid (HA), in the O₃ system, a low concentration of HA can promote the decomposition of IBP, while a high concentration of HA can inhibit it. Yet, in O₃/PMS system, any concentration of HA can inhibit the degradation of IBP, but there is no negative correlation. Liu X.Y. et al. [42] disinfected and degraded ribavirin through the O₃/PMS process during the COVID-2019 pandemic. The second-order reaction rate constants of ribavirin with •OH, SO₄^•− and O₃ were 1.9 × 10⁹, 7.9 × 10⁷ and 9.8 M⁻¹·s⁻¹, respectively. In this system, PMS did not directly degrade ribavirin but indirectly affected ribavirin by promoting the production of •OH, which was more apparent when the pH value increased. The comparison results show that the O₃ process also had an effective degradation effect on ribavirin. However, the decreased dissolved organic carbon (DOC) value was slight, indicating that only limited mineralisation occurred in the oxidation process, and some intermediate products were produced. In contrast, the removal rate of ribavirin in surface water and groundwater by the O₃/PMS process increased by 35% and 43%, respectively.

Deniere E. et al. [43] conducted free radical scavenging experiments and hydroxyl probe experiments on p-nitrobenzoic acid (pNBA) to compare the performance of the two free radicals in a real water matrix. The experimental results showed that the fewer the species and the lower the content of TrOCs in the system, the higher the contribution of SO₄^•− to its degradation. The main reason may be that the selectivity of SO₄^•− to TrOCs was higher than •OH. In addition, Cl⁻ generated more selective chlorine radicals through reaction in the system, which can convert SO₄^•− into •OH, further improving the degradation capacity of the system. When testing the effects of different oxidising species in the O₃/PMS system in the presence of hydroxyl scavengers, the O₃/PMS process showed higher removal efficiency than the O₃/H₂O₂ process, especially at higher pH values. During this process, SO₄^•− played a significant role in the degradation of TrOCs [44], which was the advantage of the process that produced two kinds of active free radicals compared with the process that only produced a single free radical. In addition, the presence of PMS in the system reduced total organic carbon (TOC) and restricted the growth of bacteria. Though, in the case of using PMS alone for disinfection and sterilisation, the toxicity in the treated natural water is often much higher than that of using the O₃/PMS process. O₃ and hydroxyl groups likely inhibit the production of DBPs [45].

The O₃/PMS process for the treatment of natural water is mainly for the purpose of pretreatment, which has the advantage of high efficiency and controllability. The main active radical in the reaction is •OH. For most TrOCs in a natural water body, O₃/PMS process shows significant degradation ability. However, at the same time, the influence of various factors such as NOM, pH, chloride ions and ferrous ions in water on the process must be considered to minimise secondary pollution and avoid severe damage to a natural water body. More importantly, when applied to real scenes, O₃/PMS process needs to build a reaction device, which is not as convenient as traditional biological treatment (such as adding cyanobacteria, fungus, etc.).

3.2. Using the O₃/PMS Process for the Advanced Treatment of Drinking Water

With industry development, more and more TrOCs are detected in drinking water. Especially for developing countries, the preparation process for drinking water faces severe challenges [46]. The traditional ozone/biological activated carbon process has limited treatment capacity for emerging organic pollutants. As a result, organic substances are incompletely degraded, and various intermediates accumulate in the human body, threatening life safety. Research shows that O₃/PMS process, as a new SR-AOPs process, can effectively degrade many kinds of organic pollutants. Nevertheless, due to ozone limitations, toxic substances such as bromate, halogenate, and chloroform will inevitably be produced, but their harm can be minimised [47]. Therefore, some scholars prefer to use O₃/PMS process as the advanced treatment process for drinking water.

Advanced drinking water treatment is similar to pretreatment, but compared with the latter, the former needs to control various parameters more strictly. For example, not only should all kinds of inorganic ions and NOM in water be considered, but factors such as pH and temperature should also be considered to avoid producing toxic intermediates or control them within the specification.

Liu Z. et al. [48] added catalysts CuCo₂O₄ and CuCo₂O₄-GO to the O₃/PMS process to degrade sulfame-thoxazol (SMX). In the O₃/PMS/CuCo₂O₄-GO combined process, the addition of catalyst considerably shortened the complete degradation time of SMX. After 10 min of reaction, the removal rate of SMX was almost 100%. CuCo₂O₄ was unevenly dispersed and attached to the surface of GO. Compared with CuCo₂O₄ alone, its reactive surface area was significantly increased, thereby improving its catalytic activity. However, GO material had a negative effect on the oxidation performance of O₃, which was reflected in that after 30 min of reaction, the degradation rates of TOC of O₃/PMS, O₃/PMS/CuCo₂O₄ and O₃/PMS/CuCo₂O₄-GO were 47.66%, 72.68% and 68.34%, respectively, and the k_obs values were 0.163, 0.522 and 0.422 min⁻¹, respectively. The comparison of TOC removal rate and k_obs value between CuCo₂O₄ and CuCo₂O₄-GO showed that CuCo₂O₄-GO sacrifices a small part of catalytic performance compared with CuCo₂O₄ but actually brings the control ability of composite carbon materials to bromate at the same time. Shao Y. et al. [49] used O₃/PMS process to degrade acesulfame (ACE), a common sweetener in drinking water. The results showed that when the dosage of O₃ was 60 µg·min⁻¹ and 0.4 mM PMS, 90.4% of ACE in the system was degraded after 15 min of reaction. The contribution ratios of O₃, •OH and SO₄^•− degradation rates were 27.1%, 25.4% and 47.5%, respectively. SO₄^•− was the main active oxidation free radical in the system. Components such as chloride and NOM greatly influence the degradation of ACE in the system. In addition, the degradation performance of ACE in the system is the best at neutral pH. Wu G.Y. et al. [50] studied the degradation of prometon (PMT) by O₃/PMS process. The results showed that when the pH value was 9, 7.5 mg·min⁻¹ O₃ and 100 mg·L⁻¹ PMS was injected, and the reaction time was 7 min, the removal rate of PMT was 97.34%, and K_obs = 0.6095 min⁻¹. After 15 min, the TOC removal rates of O₃ and O₃/PMS processes were 20.97% and 80.30%, respectively. Seventeen kinds of transformation by-products (TBPs) were produced during the whole reaction. After dealkylation, dealkylation-hydrogenation, alkylation, alkyl oxidation and deamino-hydroxylation, PMT is finally mineralised into inorganic ions, CO₂ and H₂O. At the same time, HA showed obvious inhibition of the degradation of PMT in the system. With the increase in HA concentration from 1 mg·L⁻¹ to 10 mg·L⁻¹, the k_obs value gradually decreased from 0.8644 min⁻¹ to 0.6890 min⁻¹, and the TOC removal rate decreased from 98.24% to 63.12%.

Iopamidol (IPM) is the primary source of iodinated by-products (I-DBPs) produced in the water disinfection process. Mao Y.X. et al. [22] degraded IPM through O₃/PMS process and evaluated its effect on forming iodinated tri-halomethanes (I-THMS) during chlorination treatment. When the ambient temperature was 20 °C, pH was 7, 41.7 µmM O_3, and 10 µmM PMS, the degradation rate of IPM reached 99%, and the k_obs value was 4.5 min⁻¹ after 4 min of reaction. At the same time, the oxidation of IPM by O₃/PMS also reduced its potential to form I-THMS. After the oxidation of IPM, the I-THMS generated by 5 µmM IPM in the chlorination process decreased from 14.7 µg·L⁻¹ to 3.3µg·L⁻¹. The •OH and SO₄^•− rapidly oxidised the I⁻ in the system to iodate, which rapidly reduced the concentration of COD and BOD in the system, providing a new way to accelerate the degradation of IPM and control the formation of I-DBPs. The above research showed that if the O₃/PMS system has the maximum degradation rate in the non-alkaline environment, the main contributor of degradation in the system was often not •OH; this was because excessive H⁺ ions reduce •OH to H₂O under acidic conditions. At this time, SO₄^•− is combined with H⁺ and becomes HSO₄⁻. Excessive H⁺ removed two kinds of free radicals, but the reaction of •OH is often faster, so when the reaction tends to be stable, the main contributor of observed degradation is often not •OH.

N-Dimethylnitrosamine (NDMA) is a new DBPs produced in the process of drinking water treatment, which has strong carcinogenicity [51,52]. Research shows that unsymmetrical dimethylhydrazine (UDMH) is the ozone-reactive nitrosamine precursor of NDMA and the main source of NDMA [53]. Huang Y.J. et al. [54] used O₃/PMS process to inhibit the generation of NDMA in the UDMH ozonation process. Within the pH value from 5 to 9, with the increase in pH value, O₃/PMS process achieved a higher NDMA degradation rate, reaching the maximum value of 69.9% at pH 9. The increase in PMS dose also positively affected the inhibition of NDMA, reaching an efficiency of 81% at 80 μM PMS and a slight increase in the degradation efficiency from 66.3% to 70.6%. For inhibiting NDMA production, SO₄^•− was more efficient than •OH, and the inhibition rates were 58.6% and 41.4%, respectively. However, the promoting effect of both on the conversion of UDMH to NDMA was lower than that of O₃. Hence, in the O₃/PMS process, the ozone oxidation process should be further optimised, and the rapid depletion of ozone should be used to produce active free radicals to reduce the generation of NDMA. Merle T. et al. proposed a catalytic ozonation process based on membrane pore aeration (MEMBRO₃X) -a new combination of membrane contactor and AOPs (O₃/H₂O₂ or O₃/PMS), which can effectively control the formation of NDMA [55,56]. MEMBRO₃X rapidly fused the incoming ozone with H₂O₂/PMS in the front end of the reactor through many fine membrane holes. In this case, O₃ was quickly oxidised to form •OH or SO₄^•−, thus avoiding the direct contact of O₃ with UDMH to form NDMA. The combined process reduced the probability of inducing the precursor to form NDMA, accelerated the formation of •OH or SO₄^•− and accelerated the degradation of NDMA. Through this method, UDMH and NDMA were rapidly reduced below the detection concentration, and the reaction was more efficient. Figure 3 shows the control strategy of MEMBRO₃X for NDMA.

The above research showed that O₃/PMS process had a significant degradation capacity for various organic substances in drinking water. The current ozone-combined process still cannot avoid the problem of bromate formation. Yet, this negative effect can still be minimised by adding catalysts or improving the process, and all research at this stage is also devoted to this direction.

For easy reading, the applications of the O₃/PMS method in drinking water are summarised briefly in

Table 4. Applications of O₃/PMS method in drinking water.

Application Occasions	Process Types	Research Results	References
Pretreatment	O3/PMS	Under optimal conditions, the O3/PMS process can reduce the CAP to below the detection concentration within 5 min. Furthermore, compared with other AOPs processes, O3/PMS produced the least DCAcAm because SO4•− reduced the attack of oxidant on the DCAcAm side chain in CAP.	[38]
		The O3/PMS process can degrade IBP by 72% within 20 min. Using HA to simulate the natural water matrix, any concentration of HA can inhibit the degradation of IBP, but there was no negative correlation.	[25]
		The PMS in the system did not directly degrade ribavirin but indirectly acted on it by promoting the generation of •OH, and the effect was more evident when the pH value increased.	[42]
		In natural water, although SO4•− has a great contribution to the removal of TrOCs, •OH is still the main oxidant for the removal of TrOCs. SO4•− also promotes the generation of •OH.	[43,44]
	O3/PMS/(O/C)	Fe2+ and Mn2+ in groundwater can activate PMS in situ and promote the generation of •OH and SO4•−. However, NOM promotes the formation of by-products such as ferric hydroxide crystallisation or manganese precipitation, which reduces the filtration efficiency of the ceramic membrane, and the precipitation needs to be removed regularly.	[39]
Advanced treatment	O3/PMS	SO4•− is the primary oxidative radical in the system, and the degradation contribution rate is 47.5%. Components such as chloride and NOM greatly affect the degradation of ACE in the system. Furthermore, the degradation performance of ACE in the system was the best at neutral pH.	[49]
		Under the condition of pH 6.5, 7.5 mg·min−1 O3 and 100 mg·L−1 PMS was introduced, and after 7 min of reaction, the removal rate of PMT was up to 97.34%; this process produced 17 TBPs, which were finally mineralised into inorganic ions, CO2 and H2O.	[50]
		•OH and SO4•− rapidly oxidise I− in the system to iodate, which rapidly reduces COD and BOD concentrations in the system, accelerates the degradation of IPM and effectively controls the formation of I-DBPs.	[22]
		O3 oxidises UDMH to NDMA, a strong carcinogen, before reacting with PMS to generate free radicals. Excessive PMS inhibits the formation of NDMA.	[54]
	O3/PMS/CuCo2O4-GO	After the O3/PMS/CuCo2O4-GO process was reacted for 10 min, the removal rate of SMX was almost 100%. Compared with CuCo2O4, CuCo2O4-GO sacrifices a small part of the catalytic performance but brings the control ability of the composite carbon material to bromate. Furthermore, the GO material has a negative effect on the oxidation properties of O3.	[48]
	O3/PMS/MEMBRO3X	Using MEMBRO3X equipment, O3 was quickly oxidised to form •OH or SO4•−, which can quickly reduce UDMH below the detection concentration and effectively control the generation of disinfection by-product NDMA.	[55,56]

.

4. Evaluation of Bromate in the O₃/PMS Process

The O₃/PMS process has high efficiency and clean and low secondary pollution characteristics. However, when the ozone oxidation process is used in drinking water treatment, the formation of the by-product bromate is of critical concern. Many studies have shown that bromate can increase the burden on animals’ kidneys and even induce tumours. Consequently, the International Agency for Research on Cancer (IARC) has listed it as a potential carcinogen of grade 2B [57]. However, the World Health Organization (WHO), the United States Environmental Protection Agency (EPA) and China’s newly revised “Drinking Water Sanitation Standards” (GB5749-2006) set the maximum concentration of bromate at 10 μg/L, and the test results in specific water matrix are generally more significant than this ideal value. Hence, when the concentration of bromine ions in raw water is lower than 20 μg/L, the ozone oxidation technology does not need to consider the problem of excessive bromine ion concentration. Bromate formation should be considered when its concentration exceeds 50 μg/L. Figure 4 and Figure 5 show the formation mechanism of bromate [58].

The formation mechanism of BrO₃⁻ in the treatment of brominated drinking water by the O₃/PMS process is mainly caused by the interaction between bromine ions and molecular ozone. The specific process is as follows: the free radicals produced in O₃/PMS system will oxidise Br⁻ to Br•, Br• will then react with O₃ to form BrO•, and BrO• will finally be converted to BrO₃⁻ under the further oxidation of •OH and SO₄^•−. In addition, Br⁻ can also be oxidised by O₃ to form hypobromic acid (HOBr/OBr⁻), and hypobromic acid will be further oxidised to BrO₃⁻ [59]. A large amount of •OH and SO₄^•− accelerates the formation of BrO₃⁻, which reacts with various metal cations to form bromate. At present, the formation control of BrO₃⁻ in the O₃/PMS process mainly includes the following methods: directly removing bromide (Br⁻) and carbon materials, optimising ozone dosage and contact time, adding chlorine, adding ammonia, reducing the pH value of raw water, and adding H₂O₂. Among them, the control strategy of reducing the pH value of raw water and adding H₂O₂ is still controversial.

The above methods have been applied in practice. Regarding direct bromide (Br−) removal in raw water, Soyluoglu, M. et al. [60] used new polymer carbon materials Purolite-BR and MIEX-BR resins to remove Br− in raw materials water by anion exchange directly. Under the condition of uniform generation, the subsequent generation of brominated disinfection by-products (Br-DBPs) and total organic halogens (TOX) decreased by 90%. In terms of adding composite carbon materials, Huang X. et al. [61,62] used reduced graphene oxide (RGO) to control bromate and inhibited the formation of BrO₃⁻ by moving the balance of HOBr/OBr⁻ to the OBr⁻ side. Similarly, Liu Z. et al. [48] also adopted this idea to synthesise a new catalyst, CuCo₂O₄-GO, which can not only inhibit the formation of BrO₃⁻ but also retain the degradation ability of TrOCs in CuCo₂O₄ catalysed O₃/PMS system. When 100 mg/l CuCo₂O₄-GO was added, the inhibition rate of BrO₃⁻ reached 96.17%, and the degradation rate of micropollutants increased from 0.163 min⁻¹ to 0.422 min⁻¹. The above research showed that adding new carbon composites can significantly inhibit the formation of BrO₃⁻, and the order of inhibition effect was graphene > carbon nanotubes > powdered activated carbon. By adding chlorine and ammonia, bromate is usually inhibited in the form of pretreatment. The research showed that when NH₃, Cl₂-NH₃ and NH₃-Cl₂ strategies were adopted, the productivity of BrO₃⁻ was reduced by more than 90% by the three pretreatment strategies, while NH₃-Cl₂ was slightly better than NH₃ and Cl₂-NH₃ [63]; this method consumed the content of bromine ions through NH₃-Cl₂ and Cl₂-NH₃, so bromide mainly exists in the form of brominated haloamines (NH₂Br, NHBr₂ and NHBrCl) in the reaction process, indirectly inhibiting the formation of BrO₃⁻ [64]. In terms of reducing the pH value of raw water and adding H₂O₂, Yang J.X. et al. [65] reduced the formation rate of •OH and SO₄^•− by reducing the pH value, thereby inhibiting the formation of BrO₃⁻; this is mainly based on the following principles: HOBr/OBr⁻ are competitive products in O₃/PMS system. The higher the pH value, the more the equilibrium moves to the end of HOBr to form BrO₃⁻ with O₃. When the pH decreases, the equilibrium moves to OBr⁻ and is reduced to Br⁻. In addition, excess H₂O₂ can inhibit the formation of BrO₃⁻ in the separate O₃ system by reducing HOBr/OBr⁻ to Br⁻. The above two BrO₃⁻ inhibition methods may inhibit the formation of BrO₃⁻ in O₃/PMS system through similar mechanisms [66]. As shown in Figure 4, in general, control strategies are mostly used to inhibit the formation of BrO₃− by affecting the initial Br⁻ or changing the balance of HOBr/OBr⁻. The inhibition mechanism of bromate formation is shown in Figure 6 [58,63].

5. Conclusions

The main feature of the O₃/PMS process is that it can generate two active radicals, hydroxyl radicals and sulfate radicals, at the same time, which makes it possible to apply it in dealing with the complex water environment of drinking water sources (including pretreatment and advanced treatment). It was deduced in this study that when the O₃/PMS process is used to treat drinking water, more consideration should be given to the concentration of inorganic anions and organic compounds in the water body to avoid the generation of conversion substances that are more toxic than the parent compounds. Meanwhile, it is also necessary to explore the use of material adsorption or biological filtration to reduce the content of toxic intermediate products and conversion substances. In addition, future studies on the O₃/PMS process also need to consider the influence of DOM and Br⁻ in water to comprehensively evaluate the possibility of the practical application of the process.

In the future, the critical content of further research is to improve the mineralisation ability of the O₃/PMS process, which is relatively common at present; this requires an in-depth exploration of the relevant reaction mechanism. Better results may be achieved by setting up better reaction devices or adopting different catalysts based on O₃/PMS process. In addition, the economy of the O₃/PMS process is relatively blank. Researchers should also focus on sustainable economic development and develop advanced oxidation water treatment schemes with more economic benefits.