Application of O₃/PMS Advanced Oxidation Technology in the Treatment of Organic Pollutants in Highly Concentrated Organic Wastewater: A Review

Abstract

The ozone/peroxymonosulfate (O₃/PMS) system has attracted widespread attention from researchers owing to its ability to produce hydroxyl radicals (•OH) and sulfate radicals (SO₄^•−) simultaneously. The existing research has shown that the O₃/PMS system significantly degrades refinery trace organic compounds (TrOCs) in highly concentrated organic wastewater. However, there is still a lack of systematic understanding of the O₃/PMS system, which has created a significant loophole in its application in the treatment of highly concentrated organic wastewater. Hence, this paper reviewed the specific degradation effect, toxicity change, reaction mechanism, various influencing factors and the cause of oxidation byproducts (OBPs) of various TrOCs when the O₃/PMS system is applied to the degradation of highly concentrated organic wastewater. In addition, the effects of different reaction conditions on the O₃/PMS system were comprehensively evaluated. Furthermore, given the limited understanding of the O₃/PMS system in the degradation of TrOCs and the formation of OBPs, an outlook on potential future research was presented. Finally, this paper comprehensively evaluated the degradation of TrOCs in highly concentrated organic wastewater by the O₃/PMS system, filling the gaps in scale research, operation cost, sustainability and overall feasibility.

1. Introduction

With the industrial activities that humans operate, all kinds of organic wastewater are inevitably produced. Among them, the highest concentration of organic wastewater is produced in animal husbandry and medical industries. Studies have shown that such highly concentrated organic wastewater still shows high TrOC concentration and toxicity after being treated by a conventional wastewater treatment plant (WWTP) [1,2,3,4]. Discharging TrOCs into the natural water body leads to the eutrophication of the water body and a decrease in the oxygen content in the water, which in turn result in the formation of a black and odorous water body [5,6,7]. Therefore, treating highly concentrated organic wastewater is a significant challenge for traditional WWTP treatment [8,9]. Some researchers have solved this problem using a chemically enhanced primary treatment, reducing chlorination byproducts, and applying a machine learning model to pH and chemical dose control [10,11,12]. However, subsequent observation of the receiving waters revealed that the microbial community in the river was significantly affected [13]. This showed that compared with the activated sludge and chemical precipitation methods in the traditional WWTP, this new enhanced primary and secondary treatment scheme had improved effects; however, there are still limitations in treating TrOCs [14,15].

Several scholars are focusing on advanced oxidation processes (AOPs) due to the water quality problem in WWTPs. Traditional AOPs mainly degrade various TrOCs by producing •OH, and the treatment effect is not ideal. However, the O₃/PMS system can simultaneously produce •OH and SO₄^•−, which eliminates the limitation of traditional AOPs in terms of degradation effect [16,17,18,19,20,21,22,23,24]. Relevant experiments showed that [25], in the O₃/PMS system, the more types and contents of TrOCs present in wastewater, the higher the proportion of •OH is in the oxidation contribution. The experiment of tert-Butyl alcohol quenching •OH showed that [26] the O₃/PMS process demonstrated a higher removal efficiency than most AOPs dominated by •OH, especially at a higher pH. At that time, SO₄^•− played a significant role in the degradation of TrOCs, which is the advantage of the process that produced two kinds of active free radicals compared with the process that only produced a single free radical. However, at that time, the degradation efficiency of chemical oxygen demand (COD) and biochemical oxygen demand (BOD) in the system decreased, mainly because the selectivity of SO₄^•− to TrOCs was higher than that of •OH [20,27]. In addition, when PMS was used alone for degradation, the toxicity of the treated water was often much higher than that of the O₃/PMS process. Therefore, it was likely that O₃ and hydroxyl inhibited the generation of disinfection byproducts (DBPs) [28].

Given the advantages of the O₃/PMS process—environmental sustainability, high efficiency and strong anti-interference ability—the development of the O₃/PMS process has been paid more and more attention by scholars. However, there are still some gaps when the O₃/PMS system is applied to the degradation of highly concentrated organic wastewater, such as the specific degradation effect of various TrOCs, the toxicity changes in the degradation process, the reaction mechanism, various influencing factors and OBPs generated by them. Therefore, in this paper, the reaction mechanism of O₃/PMS was summarised, and various influencing factors of the system and a series of chemical changes in the treatment of highly concentrated organic wastewater were described in detail. Furthermore, it introduces the latest research progress on the O₃/PMS process in highly concentrated wastewater treatment, which provides a valuable reference for future studies to carry out in-depth research on the O₃/PMS process. In addition, future research directions were proposed, given the limited understanding of the O₃/PMS system in the degradation of TrOCs and the formation of OBPs.

2. Mechanism and Influence Factors of O₃/PMS Process

2.1. Reaction Mechanism

As shown in Figure 1, from a microscopic perspective, the structural formula of PMS (H₂SO₅) is such that a hydrogen atom in H₂O₂ is replaced by a sulfo group (–SO₃H) and HOO– is combined with –SO₃H. Therefore, some characteristics of PMS are similar to those of H₂O₂, but the difference is that the free radicals (SO₄^•− and •OH) produced by breaking the O-O bonds are different. There are many effective oxidants in the O₃/PMS system. As a strong oxidant, O₃ can effectively degrade many organic substances that are difficult to deal with in the traditional oxidation process [29,30]. Generally, O₃ attacks only the double bonds, activated aromatic groups and nonprotonated amines of TrOCs [31,32], but it has strong selectivity. PMS slowly hydrolyses sulfuric acid and H₂O₂ in water and reacts slowly with TrOCs, mainly acting as the primary source of SO₄^•− in the system. •OH is a strong nonselective oxidant, which can diffuse rapidly in the system and react with various TrOCs, with a reaction rate range of 1 × 10⁹–3 × 10¹⁰ M⁻¹·s⁻¹ [33].

In general, the degradation of micropollutants by O₃ is achieved through the joint activities of molecular O₃ and OH [34]. The degradation selectivity of SO₄^•− is higher than that of •OH, but its higher redox potential (E0 = 2.5~3.1 V) indicates that SO₄^•− has a more efficient degradation rate in the system. Further, the reaction between SO₄^•− and TrOCs has less alkali resistance and is less interfered with by natural organic matter (NOM) and inorganic nonmetallic ions [35,36,37]. Therefore, SO₄^•− produced by PMS activation can exist in the system for a long time and effectively and continuously oxidise TrOCs. In each reaction process in Figure 1, the direct oxidation and indirect oxidation of •OH in reaction processes 1, 2 and 3 are dominant. However, for some TrOCs, the indirect oxidation of SO₄^•− produced in process 4 plays an important role. On the other hand, the selectivity of •OH and SO₄^•− produced in the O₃/PMS system is lower than that of O₃, especially •OH. This means that the process has a broader application range [38,39,40,41].

As shown in

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

No.	Reaction	Reaction Rate Constant	References
1	${\mathrm{SO}}_5{}^{2-}{+\mathrm{O}}_3\to {\mathrm{SO}}_8{}^{2-}$	2.12 × 104	[17]
2	${\mathrm{SO}}_8{}^{2-}\to {\mathrm{SO}}_5{}^{•-}{+\mathrm{O}}_3{}^{• }$	none	[17]
3	${\mathrm{SO}}_8{}^{2-}\to {\mathrm{SO}}_4{}^{2-}{+2\mathrm{O}}_2$	none	[17]
4	${\mathrm{O}}_3{}^{•-}\rightleftharpoons {\mathrm{O}}_2{+\mathrm{O}}^{•-}$	none	[43]
5	${\mathrm{O}}^{•-}{+\mathrm{H}}_2\mathrm{O}\rightleftharpoons •\mathrm{OH}+{\mathrm{OH}}^{-}$	none	[43]
6	${\mathrm{SO}}_5{}^{•-}{+\mathrm{O}}_3\to {\mathrm{SO}}_4{}^{•-}{+2\mathrm{O}}_2$	1.6 × 105 M−1·s−1	[43,44]
7	${2\mathrm{SO}}_5{}^{•-}\to {2\mathrm{SO}}_4{}^{•-}{+\mathrm{O}}_2$	2.1 × 108 M−1·s−1	[43,44]
8	${2\mathrm{SO}}_5{}^{•-}\to {\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}{+\mathrm{O}}_2$	2.2 × 108 M−1·s−1	[43,44]
9	${\mathrm{SO}}_4{}^{•-}{+\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+{\mathrm{SO}}_4{}^{2-}+•\mathrm{OH}$	<3 × 103 M−1·s−1	[36,43]
10	${\mathrm{SO}}_4{}^{•-}{+\mathrm{OH}}^{-}\to {\mathrm{SO}}_4{}^{2-}+•\mathrm{OH}$	7.3 × 107 M−1·s−1	[36,43]

, deprotonated PMS first reacts with O₃ to generate SO₈²⁻ (Reaction 1). Then, this substance decays in the form of biomolecules for decomposition (Reactions 2 and 3). O₃^•− (superoxide radical) is converted into •OH (Reactions 4 and 5) in water. The SO₅^•− produced by Reaction 2 can not only react with ozone again to generate SO₄^•− (Reaction 6) but also self-decompose to generate a sulfate radical (Reaction 7) or decay to S₂O₈²⁻ (Reaction 8). At the same time, excessive SO₄^•− in the system further oxidises H₂O or OH⁻ to form •OH (Reaction 9, 10) [17,35,42,43].

In addition, When PMS in the system is excessive, under an alkaline environment, it hydrolyses to produce H₂O₂ (Reaction 11 and 12) [17,44], and then the H₂O₂ reacts with O₃ to produce •OH (Reaction 13) [45]. Excess SO₄^•− in the system hydrolyses to produce •OH or forms new •OH with OH⁻ (Reaction 14 and 15) [44,46].

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

No.	Reaction	Reaction Rate Constant	References
11	${\mathrm{HSO}}_5{}^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{HSO}}_4{}^{-}+{\mathrm{H}}_2{\mathrm{O}}_2$	none	[17]
12	${\mathrm{SO}}_5{}^{2-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{SO}}_4{}^{2-}+{\mathrm{H}}_2{\mathrm{O}}_2$	none	[44]
13	${\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{O}}_3\to {3\mathrm{O}}_2+2•\mathrm{OH}$	none	[45]
14	${\mathrm{SO}}_4{}^{•-}+{\mathrm{OH}}^{-}\to {\mathrm{SO}}_4{}^{2-}+•\mathrm{OH}$	(6.5 ± 1.0) × 107 M−1·s−1	[46]
15	${\mathrm{SO}}_4{}^{•-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+{\mathrm{SO}}_4{}^{2-}+•\mathrm{OH}$	<3 × 103 M−1·s−1	[44]

shows the reaction formulas and corresponding reaction rate constants of PMS and SO₄^•− promoting •OH formation [17,44,45,46].

2.2. Influence Factors

2.2.1. pH Value

pH value significantly influences the degradation efficiency of the O₃/PMS process. When the pH value rises from 6 to 7, the contribution rate of •OH to the degradation in the system is the highest, but it shows a trend of gradual decline. When the pH value rises from 7 to 9, the degradation rate in the system gradually increases. This may be attributed to two phenomena: (1) the increase in hydroxyl concentration further enhances the decomposition rate of O₃ to •OH. (2) The essence of PMS catalysing O₃ is the reaction with deprotonated PMS (SO₅²⁻), and their reaction rate constant is 2.12 × 10⁴ M⁻¹·s⁻¹ (Reaction 1). The secondary dissociation constant of PMS dissociation to SO₅²⁻ is 9.4. Therefore, within the studied pH range, the closer it is to 9.4, the higher the concentration of deprotonated PMS, and the faster the catalytic decomposition of O_3. When the pH increased to more than 9.4, the degradation rate decreased slightly or remained unchanged, and then the decline increased. The main reason is that PMS decomposes in a nonfree radical way, the production rate of SO₄^•− decreases, and a too-high pH concentration makes the free radicals in the system lose their reactivity [17,47,48,49,50,51].

2.2.2. O₃ Dosage

The concentration of O₃ is critical to the yield of two free radicals in the system, and a higher O₃ concentration can significantly increase the rate of free radicals, especially ·OH. However, when excessive O₃ is introduced, excessive molecular O₃ in the system can quench the free radicals, resulting in a decrease in the reaction rate [17,52,53,54].

2.2.3. PMS Dosage

The effect of the concentration of PMS is similar to that of O₃, which shows that a slight increase can improve the free radical generation rate. The only difference is that when PMS is slightly excessive, the growth rate of SO₄^•− generation rate is greater than that of •OH. Excessive PMS reduces the pH value of the solution, and undissolved PMS produces the phenomenon of quenching free radicals [42,55,56].

2.2.4. Ratio of O₃ to PMS

The system’s molar mass ratio of O₃ to PMS is also critical to the degradation process. Research has shown that [23,24] the ratio of O₃ to PMS of 1:1 is the optimal ratio, at which point the degradation rate in the system is the fastest, indicating that sufficient O₃ is the key to producing free radicals efficiently. However, when further analysing the influence of the concentration of O₃ and PMS on the degradation rate, it was found that when the degradation rate reached it is maximum, further increasing the concentration of O₃ and PMS reduced the removal efficiency. This may be due to the quenching effect of excessive O₃ and PMS on the two free radicals [57,58,59].

2.2.5. Temperature

A significant number of studies have confirmed that, in the range of 5–40 °C, with the increase in temperature, the degradation rate increases in the experiment. This may be that the O-O bond in PMS breaks at high temperatures, which increases the conversion of SO₄^•− [60,61]. However, when the temperature is higher than 40 °C, increasing the temperature causes the reaction rate of the system to decrease. This is probably because the temperature is too high at this time, so O₃ escapes from the solution into the air before it decomposes.

2.2.6. Inorganic Anions

The influence of inorganic ions on the O₃/PMS system is summarised in Table 3, which is described as follows.

Common inorganic ions in solution, such as chloride ions, can react with •OH and SO₄^•− to generate chlorine-containing free radicals (Cl^•, Cl₂^•−, etc.) with strong selectivity. It can be seen from Reactions 16 and 17 that chloride ions and •OH can easily generate ClOH^•− and Cl⁻ under acidic conditions. When the concentration of H⁺ is higher (i.e., the pH value is lower), ClOH^•− then reacts with H⁺ to generate Cl^• (Reaction 22). However, when in a neutral or alkaline environment, the hydrogen ion concentration is low, so the capture effect of chloride ions on •OH is weak, and the degradation reaction dominated by •OH is less affected. The reaction of chloride ions with SO₄^•− (Reaction 18) is independent of pH, so it has a noticeable capture effect on SO₄^•− under experimental conditions and generates Cl^• and Cl₂^•− through Reactions 18–21 [62,63].

Carbonate is also the capture agent of •OH and SO₄^•−, and its reaction formula for capturing free radicals is shown in Reactions 24–27. Carbonate reduces the decomposition rate of ozone and captures •OH and SO₄^•− to generate a carbonate radical (CO₃^•−), a more selective free radical. This enhances the selectivity of the reaction in the system and reduces the applicability of the O₃/PMS process [62,64,65].

Nitrite and phosphate ions can also capture free radicals; see Reactions 26–35. However, the generated nitrate radical (NO₂^•) and the phosphate radical (HPO₄^•−) have higher reaction selectivities and lower oxidation capacities, negatively affecting the system. Therefore, when a phosphate buffer solution is used in the O₃/PMS system, the concentration of phosphate ions should be controlled [62,66,67].

Table 3. The reaction formula of each inorganic ion in the O₃/PMS system.

Inorganic Anions	No.	Reaction	Reaction Rate Constant (L·M−1·s−1)	References
Cl−	16	${•\mathrm{OH}+\mathrm{Cl}}^{-}\to {\mathrm{ClOH}}^{•-}$	4.3 × 109	[68]
	17	${\mathrm{ClOH}}^{•-}\to {•\mathrm{OH}+\mathrm{Cl}}^{-}$	6.1 × 109	[68]
	18	${\mathrm{SO}}_4{}^{•-}+{\mathrm{Cl}}^{-}\to {\mathrm{SO}}_4{}^{2-}{+\mathrm{Cl}}^{•}$	3.0 × 108	[69]
	19	${\mathrm{SO}}_4{}^{2-}{+\mathrm{Cl}}^{•}\to {\mathrm{SO}}_4{}^{•-}+{\mathrm{Cl}}^{-}$	2.5 × 108	[69]
	20	${\mathrm{Cl}}^{-}{+\mathrm{Cl}}^{•}\to {\mathrm{Cl}}_2^{•-}$	8.5 × 109	[70]
	21	${\mathrm{Cl}}_2^{•-}\to {\mathrm{Cl}}^{-}{+\mathrm{Cl}}^{•}$	6.0 × 104	[71]
	22	${\mathrm{ClOH}}^{•-}{+\mathrm{H}}^{+}\to {\mathrm{Cl}}^{•}{+\mathrm{H}}_2\mathrm{O}$	2.1 × 1010	[68]
	23	${\mathrm{Cl}}_2^{•-}+•\mathrm{OH}\to {\mathrm{HOCl}+\mathrm{Cl}}^{-}$	1.0 × 109	[72]
HCO3−	24	${\mathrm{HCO}}_3^{-}+•\mathrm{OH}\to {\mathrm{CO}}_3^{•-}{+\mathrm{H}}_2\mathrm{O}$	8.6 × 106	[33]
	25	${\mathrm{HCO}}_3^{-}+{\mathrm{SO}}_4{}^{•-}\to {\mathrm{CO}}_3^{•-}+{\mathrm{SO}}_4{}^{2-}+{\mathrm{H}}^{+}$	3.9 × 108	[73]
CO32−	26	${\mathrm{CO}}_3^{2-}+•\mathrm{OH}\to {\mathrm{CO}}_3^{•-}$	2.8 × 106	[33]
	27	${\mathrm{CO}}_3^{2-}+{\mathrm{SO}}_4{}^{•-}\to {\mathrm{CO}}_3^{•-}+{\mathrm{SO}}_4{}^{2-}$	6.1 × 106	[74]
NO2−	28	${\mathrm{NO}}_2^{-}+•\mathrm{OH}\to {\mathrm{NO}}_2^{ •}{+\mathrm{OH}}^{-}$	None	[75]
	29	${\mathrm{NO}}_2^{-}+{\mathrm{SO}}_4{}^{•-}\to {\mathrm{NO}}_2^{ •}{+\mathrm{SO}}_4{}^{2-}$	None	[75]
	30	${\mathrm{NO}}_2^{-}{+\mathrm{HSO}}_5^{-}\to {\mathrm{NO}}_3^{-}{+\mathrm{HSO}}_4^{-}$	None	[76]
	31	${\mathrm{NO}}_2^{-}{+\mathrm{O}}_3\to {\mathrm{NO}}_3^{-}{+\mathrm{O}}_2$	None	[76]
HPO42−	32	${•\mathrm{OH}+\mathrm{HPO}}_4^{2-}\to {\mathrm{HPO}}_4^{•-}{+\mathrm{OH}}^{-}$	1.5 × 105	[35]
	33	${•\mathrm{OH}+\mathrm{H}}_2{\mathrm{PO}}_4^{2-}\to {\mathrm{HPO}}_4^{•-}{+\mathrm{H}}_2\mathrm{O}$	2.0 × 104	[35]
	34	${\mathrm{SO}}_4{}^{•-}{+\mathrm{HPO}}_4^{2-}\to {\mathrm{HPO}}_4^{•-}+{ \mathrm{SO}}_4{}^{2-}$	1.2 × 106	[77]
	35	${\mathrm{SO}}_4{}^{•-}{+\mathrm{H}}_2{\mathrm{PO}}_4^{2-}\to {\mathrm{HPO}}_4^{•-}+{ \mathrm{HSO}}_4{}^{2-}$	5.0 × 104	[77]

Table 3. The reaction formula of each inorganic ion in the O₃/PMS system.

Inorganic Anions	No.	Reaction	Reaction Rate Constant (L·M−1·s−1)	References
Cl−	16	${•\mathrm{OH}+\mathrm{Cl}}^{-}\to {\mathrm{ClOH}}^{•-}$	4.3 × 109	[68]
	17	${\mathrm{ClOH}}^{•-}\to {•\mathrm{OH}+\mathrm{Cl}}^{-}$	6.1 × 109	[68]
	18	${\mathrm{SO}}_4{}^{•-}+{\mathrm{Cl}}^{-}\to {\mathrm{SO}}_4{}^{2-}{+\mathrm{Cl}}^{•}$	3.0 × 108	[69]
	19	${\mathrm{SO}}_4{}^{2-}{+\mathrm{Cl}}^{•}\to {\mathrm{SO}}_4{}^{•-}+{\mathrm{Cl}}^{-}$	2.5 × 108	[69]
	20	${\mathrm{Cl}}^{-}{+\mathrm{Cl}}^{•}\to {\mathrm{Cl}}_2^{•-}$	8.5 × 109	[70]
	21	${\mathrm{Cl}}_2^{•-}\to {\mathrm{Cl}}^{-}{+\mathrm{Cl}}^{•}$	6.0 × 104	[71]
	22	${\mathrm{ClOH}}^{•-}{+\mathrm{H}}^{+}\to {\mathrm{Cl}}^{•}{+\mathrm{H}}_2\mathrm{O}$	2.1 × 1010	[68]
	23	${\mathrm{Cl}}_2^{•-}+•\mathrm{OH}\to {\mathrm{HOCl}+\mathrm{Cl}}^{-}$	1.0 × 109	[72]
HCO3−	24	${\mathrm{HCO}}_3^{-}+•\mathrm{OH}\to {\mathrm{CO}}_3^{•-}{+\mathrm{H}}_2\mathrm{O}$	8.6 × 106	[33]
	25	${\mathrm{HCO}}_3^{-}+{\mathrm{SO}}_4{}^{•-}\to {\mathrm{CO}}_3^{•-}+{\mathrm{SO}}_4{}^{2-}+{\mathrm{H}}^{+}$	3.9 × 108	[73]
CO32−	26	${\mathrm{CO}}_3^{2-}+•\mathrm{OH}\to {\mathrm{CO}}_3^{•-}$	2.8 × 106	[33]
	27	${\mathrm{CO}}_3^{2-}+{\mathrm{SO}}_4{}^{•-}\to {\mathrm{CO}}_3^{•-}+{\mathrm{SO}}_4{}^{2-}$	6.1 × 106	[74]
NO2−	28	${\mathrm{NO}}_2^{-}+•\mathrm{OH}\to {\mathrm{NO}}_2^{ •}{+\mathrm{OH}}^{-}$	None	[75]
	29	${\mathrm{NO}}_2^{-}+{\mathrm{SO}}_4{}^{•-}\to {\mathrm{NO}}_2^{ •}{+\mathrm{SO}}_4{}^{2-}$	None	[75]
	30	${\mathrm{NO}}_2^{-}{+\mathrm{HSO}}_5^{-}\to {\mathrm{NO}}_3^{-}{+\mathrm{HSO}}_4^{-}$	None	[76]
	31	${\mathrm{NO}}_2^{-}{+\mathrm{O}}_3\to {\mathrm{NO}}_3^{-}{+\mathrm{O}}_2$	None	[76]
HPO42−	32	${•\mathrm{OH}+\mathrm{HPO}}_4^{2-}\to {\mathrm{HPO}}_4^{•-}{+\mathrm{OH}}^{-}$	1.5 × 105	[35]
	33	${•\mathrm{OH}+\mathrm{H}}_2{\mathrm{PO}}_4^{2-}\to {\mathrm{HPO}}_4^{•-}{+\mathrm{H}}_2\mathrm{O}$	2.0 × 104	[35]
	34	${\mathrm{SO}}_4{}^{•-}{+\mathrm{HPO}}_4^{2-}\to {\mathrm{HPO}}_4^{•-}+{ \mathrm{SO}}_4{}^{2-}$	1.2 × 106	[77]
	35	${\mathrm{SO}}_4{}^{•-}{+\mathrm{H}}_2{\mathrm{PO}}_4^{2-}\to {\mathrm{HPO}}_4^{•-}+{ \mathrm{HSO}}_4{}^{2-}$	5.0 × 104	[77]

2.2.7. NOM

NOM widely exists in actual water bodies, and some functional groups (such as phenols and amines) can react with ozone to form •OH; further, their reaction with •OH can also promote the reaction process, thus promoting the decomposition of O₃ and the generation of free radicals [78,79,80]. At the same time, NOM competes with the target for oxidants such as O₃, •OH and SO₄^•−, thus reducing the removal rate of the target. Therefore, a low concentration of NOM promotes ozone decomposition to produce •OH, but when the concentration of NOM is too high, it competes with TrOCs in the system for two free radicals, inhibiting the degradation efficiency. NOM accelerates the formation of •OH from O₃, resulting in the concentration of •OH being greater than that of SO₄^•− [69,81,82,83,84].

To sum up, the main influencing factors in the O₃/PMS system are presented in

Table 4. The main influencing factors in the O₃/PMS system.

Influence Factors	Performance			References
pH	pH = 6→7	pH = 7→9.4	pH > 9.4	[17,47,48,49,50,51]
	The contribution rate of •OH is greater than that of SO4•−, but it first increases and then decreases with the increase in pH value.	The decomposition rate of O3 and PMS is improved. The contribution rate of SO4•− degradation gradually increases. SO4•− increases the generation rate of •OH.	The degradation rate decreases gradually. PMS decomposes with nonfree radicals, and SO4•− generation rate decreases gradually.
O3 dosage	Higher O3 concentration can improve the yield of free radicals and degradation efficiency. Excessive O3 concentration quenches free radicals.			[17,52,53,54]
PMS dosage	Higher PMS concentration increases the decomposition rate of O3. Excessive PMS causes the pH value of the solution to drop, and the undecomposed PMS quenches free radicals.			[42,55,56]
O3: PMS	When the ratio of O3 to PMS is 1:1, the decomposition rate of O3 is the highest. When the ratio of O3 to PMS is 1:2 or 2:1, the reaction rate has no significant change.			[57,58,59]
Temperature	The decomposition of O3 is accelerated with the increase in temperature. Higher temperature promotes the formation of free radicals and improves degradation efficiency. There is no obvious thermodynamic control effect within 5–40 °C. When the temperature is higher than 40 °C, the formation rate of free radicals decreases.			[60,61]
Inorganic anions	Chloride ions: when the concentration is low, the degradation of TrOCs is promoted, the degradation effect gradually decreases as the concentration increases until the effect is the same as that of no addition, and then if the chloride ions are further increased, the degradation is inhibited. Bicarbonate ions and carbonate ions: reduce the decomposition rate of ozone, reduce the reaction rate and improve the selectivity of the reaction, reducing the applicability of the O3/PMS process. Nitrite ions and phosphate ions: have a strong inhibitory effect.			[62,63,64,65,66,67]
NOM	Low concentration of NOM promotes ozone decomposition to generate •OH, but when the concentration of NOM is too high, it competes with TrOCs in the system for two free radicals and inhibits the degradation efficiency. NOM accelerates O3 to generate •OH, resulting in •OH concentration being greater than that of SO4•−.			[69,79,80,81,82,83,84]

.

3. Research Status of the O₃/PMS Process on Highly Concentrated Wastewater

The essence of the advanced oxidation process is to produce enough free radicals to oxidise and degrade organics in water [85]. The advanced oxidation system is dominated by •OH. It has a broader scope of application as it 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 a 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 [86,87]. As described below, extensive research has been carried out on the O₃/PMS process in treating highly concentrated organic wastewater.

TrOCs in high-concentration organic wastewater are challenging to degrade efficiently in sewage treatment plants, resulting in pollution of receiving water bodies [17,88,89]. This usually requires pretreatment or advanced treatment of this organic wastewater [90]. When the O₃/PMS process is used for pretreatment, O₃ can decompose macromolecular organic pollutants into small molecular organic pollutants with low toxicity and easy biochemical degradation, and then SO₄^•− and •OH degrade a part of TrOCs to improve the biochemical index of the wastewater. On the other hand, when the O₃/PMS process is used for advanced treatment, TrOCs are directly oxidised by O₃ or •OH or co-oxidised by SO₄^•− and •OH radicals and are eventually mineralised directly into CO₂ and H₂O, which significantly reduces the COD and BOD in the water [91,92,93,94].

In terms of pretreatment, Ghanbari F. et al. [95] added ultraviolet (UV) to treat polluted water with landfill leachate (PWLL) based on the O₃/PMS system. In the O₃/PMS/UV process under the optimum conditions of pH 7, PMS = 5.7 g/L and O₃ = 1.7 mg·min⁻¹, after 75 min of reaction, the total organic carbon (TOC) in the system and chromaticity, ammonia nitrogen, COD and BOD removal rates reached 74%, 98%, 93%, 81% and 69%, respectively. The results showed that the generation rates of SO₄^•− and •OH active radicals were increased under the action of UV, and the removal ability of TOC in PWLL was significantly improved. At the same time, adding ferrous ions could further increase the TOC removal rate to 77%. The removal of TOC by O₃/PMS/UV conformed to the quasi-first-order kinetic model, and the pseudo-first-order rate constants (K_obs) were 0.0203 min⁻¹. Compared with H₂O₂/UV, O₃/PMS/UV had higher removal rates of TOC, COD, chromaticity and ammonia nitrogen. Wang H.W. et al. [96] added polymerised ferric sulfate (PFS) and nanoscale zero-valent iron (NZVI) to the O₃/PMS system to form a NZVI/PMS/O₃ process to treat the reverse osmosis concentrated leachate (ROCL). Under the optimum conditions of PFS concentration of 8 g·L⁻¹, O₃ dosage of 100 mg·min⁻¹, PMS dosage of 1.71 g/L and NZVI dosage of 5.58 g/L, the maximum removal rates of COD and TOC were 89.1% and 83.2%, respectively, and the biochemical index (BOD₅/COD) was increased from 0.02 to 0.32. The analysis of excitation–emission matrix (EEM) fluorescence spectroscopy showed that humus-like and fulvic-acid-like substances in ROCL were effectively removed, indicating that most of the macromolecular organics in the system can be transformed into small molecule organics, and the biodegradability was significantly improved after treatment. Jaafarzadeh N. et al. [75] used the magnetic copper ferrite nanoparticle (MCFN) CuFe₂O₄ as a catalyst to treat 2,4-dichlorophenoxyacetic acid (2,4-D) based on the O₃/PMS system to reduce the toxicity of highly concentrated organic wastewater. After treatment with the O₃/PMS/MCFNs system, its biochemical index increased from 8.3% to 58.9%, toxicity decreased from 76.5% to 3.8%, TOC removal rate was 67.3% and 2,4-D removal rate was 42.7%. Based on the breath measurement test, the O₃/MCFNs/PMS process significantly degraded the 2,4-D solution and improved the biodegradability of wastewater. Tan C.Q. et al. [97] studied the degradation of aspirin (ASA) and phenacetin (PNT) by the O₃/PMS system. After O₃ preoxidation, the toxicity of DBPs in the system decreased significantly. It was found that the increase in pH value significantly promoted the degradation effect of the above two TrOCs, and the K_obs values of ASA and PNT increased by 3.3 × 10⁻² min⁻¹ and 8.3 × 10⁻² min⁻¹, respectively. The degradation rates of SO₄^•−, •OH and O₃ for ASA were 44.5%, 32.9% and 22.6%, respectively, which were relatively balanced, while •OH had the best degradation effect for PNT, which was 71.4%, followed by SO₄^•− and O₃ with 16.7% and 11.9%, respectively.

In terms of advanced treatment, Zheng Z.W. et al. [98] studied the synergistic effect of the O₃/PMS process on the degradation of isothiazolinone fungicide. It was found that at pH 7, the degradation efficiency of methylisothia-zolinone (MIT) and chloro-methyl-isothiazolinone (CMIT) increased to 91.0% and 81.8%, respectively, within 90 s. The total radical formation value (R_ct, •R) of the O₃/PMS process was 24.6 times that of the O₃ process. Because O₃/PMS has a low selectivity for macromolecular organic pollutants in the system and a good activation effect for some inorganic anions (such as HCO₃⁻) precipitated by PMS, O₃/PMS shows better adaptability to actual wastewater. Therefore, the O₃/PMS process has a good application prospect for the degradation of TrOCs in highly concentrated organic wastewater. Tang G.M. et al. [99] combined the O₃/PMS system with upflow biological aerated filter (UBAF) for the biological treatment of biotreated Chinese patent medicine wastewater (BCPMW). Under the conditions of an initial pH of 7.4–8.9, O₃ concentration of 20 mg·L⁻¹, potassium bisulfate dosage of 50 mg·L⁻¹, UBAF gas–liquid ratio of 4:1, UBAF hydraulic retention time of 4 h, UBAF backwash time of 7 d and temperature of 16–28 °C, the O₃/PMS/UBAF combined process could effectively degrade organic pollutants in the waste liquid. The removal rates of dissolved organic carbon (DOC), COD and chromaticity were 77.60%, 85.68% and 81.79%, respectively. After O₃/PMS/UBAF treatment, the COD load was less than 48 mg·L⁻¹, and the BCPMW chromaticity grade was less than 25, meeting the requirements of the emission standard (GB21906-2008). In addition, O₃/PMS oxidation improved the overall removal rate and biodegradation rate of dissolved organic matter (DOM) in BCPMW. Ghanbari F. et al. [48] investigated the effects of different operating parameters on the degradation of benzotriazole (BTA) by ultrasonic (US) combined with the O₃/PMS process. The results showed that the O₃/PMS/US process was the best under the conditions of pH 7, O₃ = 6.8 mg·L⁻¹, PMS = 1.71 g/L and US power = 200 W. After 60 min of reaction, BTA with a concentration of 40 mg·L⁻¹ was completely degraded, with a COD removal rate of about 85%, a TOC removal rate of 75% and a total organic nitrogen (TON) removal rate of 73.3%. The removal test of BTA found that •OH was the main oxidant for the oxidation of BTA in the O₃/PMS/US collaborative process. The inhibition of anions on BTA removal was NO₂⁻ > HCO₃⁻ > Cl⁻ > NO₃⁻ > SO₄²⁻. The degradation intermediates of BTA were identified, and the oxidation pathway of BTA was proposed, as shown in Figure 2.

The above studies showed that the O₃/PMS process, as a sustainable and efficient AOP, is feasible in treating highly concentrated organic wastewater, but there are still some challenges. Firstly, the optimal reaction conditions of the O₃/PMS process are neutral and weakly alkaline. However, it is often difficult to control the pH value of the reaction due to the complexity of various organic substances in the actual wastewater owing to complex organic substances in real wastewater. Secondly, for most TrOCs, it is often •OH that plays the main role in oxidation. Nevertheless, for the degradation of some TrOCs, O₃ and SO₄^•− both play a role that cannot be ignored. Therefore, in practical application, it is necessary to summarise and form a systematic quantitative dosing standard to realise the optimisation of the effectiveness and economy of the O₃/PMS process. The applications of the O₃/PMS method in highly concentrated organic wastewater treatment are summarised briefly in

Table 5. O₃/PMS method applications in organic degradation in highly concentrated organic wastewater.

Application Scenario	Process Category	Research Findings	Ref.
High-concentration organic wastewater	O3/PMS	At pH = 7.0, the degradation efficiency of MIT and CMIT increased to 91.0% and 81.8%, respectively, within 90 s. Rct, •R of the O3/PMS process was 24.6 times that of ozonation alone.	[63]
		After O3 preoxidation, the toxicity of DBPs in the system reduced from 6.63 × 10−2 min−1 to 5.27 × 10−2 min−1. However, the increase in pH value could significantly promote the degradation of the two TrOCs, and the kobs values of ASA and PNT increased by 3.3 × 10−2 min−1 and 8.3 × 10−2 min−1, respectively.	[62]
	PMS/O3/UV	Under the optimal conditions of pH = 7, PMS = 5.7 g/L and O3 = 1.7 mg/min, the removal rates of TOC, chromaticity, ammonia, COD and BOD by PMS/O3/UV at 75 min were 74%, 98%, 93% and 69%, respectively.	[59]
	NZVI/PMS/O3	Under the optimal conditions of 8 g/L FPS, 100 mg/min O3, 1.71 mg/L PMS and 5.58 g/L NZVI, the maximum removal rates of COD and TOC were 89.1% and 83.2%, respectively, and the biodegradability index (BOD5/COD) was increased from 0.02 to 0.32.	[60]
	PMS/MCFNs/O3	The biodegradability of activated sludge containing 2,4-D increased from 8.3% to 58.9%, the toxicity decreased from 76.5% to 3.8%, the removal rate of TOC was 67.3% and the removal rate of 2,4-D was 42.7%.	[61]
	O3/PMS-UBAF	After O3/PMS/UBAF treatment, the COD load was less than 48 mg·L−1, and the BCPMW chromaticity grade was lower than 25. In addition, the overall removal rate and biodegradation rate of DOM in BCPMW were improved by O3/PMS oxidation.	[64]
	O3/PMS/US	Hydroxyl radical was the main oxidant of BTA oxidation by the O3/PMS/US combined process. The inhibition of anions on BTA removal was NO2− > HCO3− > Cl− > NO3− > SO42−.	[36]

.

4. Evaluation of O₃/PMS Process Operation Characteristics

Currently, the O₃/PMS process research is still in the laboratory stage, and the relevant economic evaluation is still relatively lacking. Therefore, researchers should optimise the treatment parameters in their different environments to achieve maximum effectiveness and economic benefits.

In the previous combined process based on O₃, the reaction between O₃ and OH⁻ was often mistaken as the main source of •OH. As a result, the rate constant of this reaction was very low, and a considerable reaction rate could be achieved only at a very high pH, while other competitive reactions are usually much faster, such as the ozone reaction with DOM [100,101]. However, in this process, some O₃ enters the system and reacts with TrOCs and inorganic ions in the system too early, leading to the premature consumption of O₃ and the generation of various OBPs [59,102]. Researchers have gradually solved this defect; that is, by adding other catalysts and optimising the reaction device, O₃ is preferentially consumed to produce two free radicals [103,104]. Subsequently, H₂O₂ generated by PMS hydrolysis is also converted into •OH. At the same time, excess SO₄^•− in the system also reacts with H₂O or hydroxide ions to form •OH, reaching dynamic equilibrium [34,35]. During the degradation process, the concentration of O₃, the concentration of PMS and the molar mass ratio of O₃ to PMS are the keys to an efficient reaction. Sufficient O₃ and PMS can keep the reaction at a high level. See Figure 3 for details.

Therefore, to ensure the production rate of •OH in a neutral environment, it usually needs to be achieved by adding a catalyst or by electrolysis [105]. In addition, the decomposition of O₃ is very slow when •OH is used as the main active oxidation free radical under acidic conditions in an OH⁻-deficient system. This can affect the formation of •OH and SO₄^•− active free radicals [106]. At this time, TrOCs were directly oxidised in the form of O₃ molecules in the system. Neutral and alkaline environments can often improve the degradation efficiency of TrOCs. However, in a system where SO₄^•− is the main active oxidation radical or •OH does not occupy the main degradation position, the increase in pH has a minor impact on the degradation efficiency of TrOCs. The main reason is that increasing pH can increase the O₃ consumption rate constant, OH⁻ triggers the decomposition of ozone to generate •OH, and •OH can accelerate the decomposition of PMS [107]. O₃/PMS usually shows the strongest oxidation performance when the pH is about 9. More free radicals are produced in the system with increasing pH, and these free radicals may quench each other, resulting in a decline in the degradation effect, which may be the reason for the decline in the oxidation effect of the O₃/PMS process in a high-pH environment.

Generally, •OH and SO₄^•− are the main reactive oxygen species (ROS) in O₃/PMS. Researchers deduced the degradation path of TrOCs in the system by considering the preferential attack sites of these two free radicals. On the one hand, SO₄^•− was electrophilic and was easy to react with electron donor groups such as hydroxyl (•OH), alkoxy (^•RO) and amino (^•NH₂) but not easy to react with nitro (^•NO₂), carbonyl (C=O), or other electron-withdrawing groups [23,108]. On the other hand, •OH had a low selectivity for most TrOCs in the system. Therefore, the two active radicals had good synergy under general conditions. For example, prometon (PMT)’s aromatic ring or side chain (isopropylamino and alkoxy) may be attacked by •OH and SO₄^•−, mainly through addition to unsaturated carbon, H-abstraction and electron abstraction [109,110,111]. SO₄^•− mainly degrades organics through electron transfer, while •OH degrades TrOCs through its addition. This also reflects how selective SO₄^•− is in the degradation of organic matter.

Generally speaking, the greater the amount of O₃ added, the higher the degradation rate of TrOCs. In this process, O₃ can directly oxidise TrOCs or react with H₂O to generate •OH to further improve the oxidation capacity of the system. The higher the concentration of O₃, the faster the rate of •OH formation. However, there is an optimal value for the additional amount of PMS. If the addition exceeds this optimal value, the reaction rate remains unchanged, and the degradation rate does not increase significantly. This phenomenon may be because excessive HSO₅⁻ can react with •OH and SO₄^•−, so only a few oxide species can be generated. Another reason may be that excessive PMS quenches free radicals. It can be seen that excessive PMS can promote not only the degradation of pollutants but also increase the operation cost [47,112].

Additionally, the impact of other reaction parameters (such as reactant concentration), reaction conditions (such as pH and temperature), water quality (such as the concentration of inorganic substances (Cl⁻, Br⁻ and I³⁻) and organic compounds) on the reaction system is also one of the urgent key points in the O₃/PMS process to explore. The SO₄^•−-based traditional advanced oxidation technology can be used for degrading organic pollutants in the O₃/PMS system to realise technical iteration in the future.

5. Outlook

The above analysis concludes that the following improvements can be made in the future use of the O₃/PMS process to obtain a broader application scenario. See

Table 6. Recommendations for improvement of the O₃/PMS process.

Scheme	Advantages	Disadvantages
Enhancing reactor [113]	It can enable a rapid consumption of O3 to produce free radicals. On the one hand, it can reduce the production of OBPs in the system and the toxicity of secondary effluent. On the other hand, it can reduce the consumption of O3, thereby reducing energy consumption and cost.	It increases equipment costs.
Adding catalyst [114]	The addition of a catalyst can increase the formation rate of two free radicals by adding metal ions such as Fe2+, M2+ or other catalysts.	The catalyst can react with other inorganic ions or NOM in the system to form DBPs.
Integrating with other processes [115]	Integration with UV, US and other processes can increase the generation rate of the two free radicals.	Currently, this scheme does not qualitatively improve the degradation efficiency of the O3/PMS process but increases the cost.
Preremoval of inorganic ions and NOM [116]	Inorganic ions with adverse effects in the system can be removed in advance, or the concentration of NOM can be controlled by biosorption or sludge method.	The cost increases, and the degradation process becomes complex and lengthy.

for details.

At present, there is still a lack of a comprehensive understanding of the O₃/PMS system, including specific degradation effects and mechanisms of various TrOCs in high-concentration organic wastewater, OBPs generated in the degradation process, toxicity changes in the degradation process, residual toxicity of the secondary effluent and reliable economic research of this process. Therefore, the above problems limit the application of this process to specific water treatment schemes.

6. Conclusions

The O₃/PMS process has the characteristics of high efficiency. When used in the pretreatment of highly concentrated organic wastewater, O₃/PMS can decompose macromolecular organic pollutants into small molecular organic pollutants with low toxicity and easy biochemical degradation to improve the efficiency of subsequent biochemical degradation. When used as an advanced treatment, TrOCs are directly oxidised by O₃ or •OH or co-oxidised by SO₄^•− and •OH. In the future, the traditional advanced oxidation technology based on SO₄^•− can be used for degrading organic pollutants in the O₃/PMS system. The O₃/PMS process has high degradation efficiency for TrOCs, but there are differences in the treatment effects of different organics under different pH environments, so the optimal reaction conditions still need to be further studied.

Finally, it is proposed that further research should explore the relevant reaction mechanism further; that is, the concentration of a single oxidant is relatively accurately detected in the oxidation system containing many oxidising substances, and the system displays relatively accurate identification of reactive oxygen species. When used for pretreatment, it is essential to distinguish the individual effects of each reactive oxygen species relatively and accurately on subsequent biological treatment. When used for advanced treatment, identifying the type or structure of organic matter in nonbiodegradable or biodegradable parts is crucial. In addition, at present, the economy of the O₃/PMS process is relatively poor. Therefore, researchers should also focus on sustainable economic development and develop more cost-effective advanced oxidation water treatment schemes.