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

Abstract

Organic pollution is a significant challenge in environmental protection, especially the discharge of refractory organic pollutants in chemical production and domestic use. The biological treatment method of traditional sewage treatment plants cannot degrade such pollutants, which leads to the continuous transfer of these pollutants into the water environment. Therefore, it is necessary to study clean and efficient advanced treatment technologies to degrade organic pollutants. The ozone/UV advanced oxidation process (O₃/UV) has attracted extensive attention. This paper summarizes the reaction mechanism of O₃/UV and analyzes its application progress in industrial wastewater, trace polluted organic matter and drinking water. The existing research results show that this technology has an excellent performance in the degradation of organic pollutants and has the characteristics of clean and environmental protection. In addition, the control of bromate formation and its economy is evaluated, and its operating characteristics and current application scope are summarized, which has a practical reference value for the follow-up in-depth study of the O₃/UV process.

1. Introduction

The ozone/ultraviolet (O₃/UV) combined advanced oxidation process (AOPs) was first developed by American scientists in the 1970s to treat high-concentration mixed cyanide, and the amount of oxidized cyanide could quickly fall below the detection limit [1]. Later, Fochtman and E.G. [2] used this process to treat trinitrotoluene (TNT) wastewater and found that the O₃/UV process can completely decompose TNT, and 85% of carbon was oxidized to release CO₂. In further research, the O₃/UV process was specified as “the best practicable control technology for polychlorinated biphenyl (PCB) wastewater treatment at this stage” [3]. As one of the advanced oxidation processes, O₃/UV is considered by the U.S. National EPA (Environmental Protection Agency) as the process with the most development potential in AOPs. The process can degrade organic matter that cannot be degraded by ozone or ultraviolet radiation alone. The reason is that hydroxyl radicals (·OH) with strong oxidation are produced in the reaction process. The redox potential of ·OH was 2.80 V, which can degrade organic pollutants without selectivity and has a high degree of mineralization [4,5,6,7]. O₃/UV can also directly kill pathogens and bacteria in the water during the reaction, so the disinfection step is omitted [8].

In recent years, refractory organics such as personal care products, drugs, pesticides, flame retardants, artificial sweeteners and industrial chemicals have been widely detected in different water bodies, such as industrial wastewater, groundwater, surface water and drinking water [9,10,11,12,13,14]. Urban sewage treatment plants have been recognized as the vital source for transferring these refractory organic pollutants to the water environment [15,16], mainly because the traditional sewage treatment plants usually only use primary and secondary treatment, rather than a specially designed treatment to remove such organic pollutants. So, the removal efficiency of these substances by traditional biological methods may be very low [17,18,19,20,21,22], thus leading to the discharge of incompletely degraded organic matter and its incompletely degraded intermediate products into the water body. In addition, numerous studies have shown that certain organic pollutants released into the water at certain concentrations can dramatically affect aquatic life and pose a severe threat to human health [23]. Therefore, improving or strengthening the traditional biological treatment process through three-stage treatment methods such as O₃, O₃/H₂O₂ and O₃/UV is necessary. Some AOPs involving ultraviolet radiation and ozone have been used in drinking water and water recycling facilities [24,25].

This paper summarizes the reaction mechanism of the O₃/UV process in the oxidation process. Introducing the research progress of the O₃/UV process in the treatment of industrial wastewater, trace organic pollutants and drinking water provides theoretical support for the practical application and prospects for applying the green advanced oxidation process in water treatment.

2. Reaction Mechanism of O₃/UV Process

The reaction mechanism of the O₃/UV process to degrade organic pollutants in the environment is mainly divided into three categories: first, the O₃ molecule directly oxidizes and degrades pollutants; secondly, UV directs the photodegradation of pollutants; and finally, UV guides O₃ molecules to generate ·OH for the indirect oxidation of pollutants.

Of these categories, the dominant one is the indirect oxidation of pollutants by ·OH. The formation process of ·OH in O₃/UV system is as follows [26]:

$${\mathrm{O}}_3+\mathrm{hv}\to {\mathrm{O}}_2+{\mathrm{O}}^{·}$$

(1)

$${\mathrm{O}}^{·}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(2)

$${\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{hv}\to 2·\mathrm{OH}$$

(3)

$${\mathrm{H}}_2{\mathrm{O}}_2\leftrightarrow {\mathrm{HO}}_2^{-}+{\mathrm{H}}^{+}$$

(4)

$${\mathrm{HO}}_2^{-}+{\mathrm{O}}_3\to {\mathrm{O}}_3^{·}{}^{-}+{\mathrm{HO}}_2^{·}$$

(5)

$${\mathrm{HO}}_2^{·}\leftrightarrow {\mathrm{H}}^{+}+{\mathrm{O}}_2^{·}{}^{-}$$

(6)

$${\mathrm{O}}_2^{·}{}^{-}+{\mathrm{O}}_3\to {\mathrm{O}}_3^{·}{}^{-}+{\mathrm{O}}_2$$

(7)

$${\mathrm{O}}_3^{·}{}^{-}+{\mathrm{H}}^{+}\to ·\mathrm{OH}+{\mathrm{O}}_2$$

(8)

$$·\mathrm{OH}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{HO}}_2^{·}+{\mathrm{H}}_2\mathrm{O}$$

(9)

$$·\mathrm{OH}+{\mathrm{HO}}_2^{-}\to {\mathrm{HO}}_2^{·}+{\mathrm{OH}}^{-}$$

(10)

$$·\mathrm{OH}+{\mathrm{O}}_3\to {\mathrm{HO}}_2^{·}+{\mathrm{O}}_2$$

(11)

The reaction rate constants of the above formulas are shown in

Table 1. ·OH formation reaction and reaction rate constant.

No.	Reaction	Reaction Rate Constant (L·mol−1·s−1)
1	${\mathrm{O}}^{·}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{O}}_2$	1.8 × 1010
2	${\mathrm{HO}}_2^{-}+{\mathrm{O}}_3\to {\mathrm{O}}_3^{·}{}^{-}+{\mathrm{HO}}_2^{·}$	2.8 × 106
3	${\mathrm{O}}_2^{·}{}^{-}+{\mathrm{O}}_3\to {\mathrm{O}}_3^{·}{}^{-}+{\mathrm{O}}_2$	1.6 × 109
4	${\mathrm{O}}_3^{·}{}^{-}+{\mathrm{H}}^{+}\to ·\mathrm{OH}+{\mathrm{O}}_2$	5.2 × 1010
5	$·\mathrm{OH}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{HO}}_2^{·}+{\mathrm{H}}_2\mathrm{O}$	2.7 × 107
6	$·\mathrm{OH}+{\mathrm{HO}}_2^{-}\to {\mathrm{HO}}_2^{·}+{\mathrm{OH}}^{-}$	7.5 × 109
7	$·\mathrm{OH}+{\mathrm{O}}_3\to {\mathrm{HO}}_2^{·}+{\mathrm{O}}_2$	3.0 × 109

.

It can be seen from the above reaction formula that O₃ molecules dissolved in water split out active oxygen radicals (O·) under UV irradiation (λ = 254 nm), and O· reacted with H₂O to generate H₂O₂. There are two ways of decomposing H₂O₂: one is direct photolysis, to produce strong oxidizing ·OH; the other is to dissociate the molecule into hydrogen peroxide anion (HO₂⁻) and a hydrogen ion (H⁺). The dissociated HO₂⁻ reacts with O₃ to produce an ozone anion radical (O₃·⁻) and hydrogen peroxide radical (HO₂·), in which HO₂· is dissociated to produce superoxide radicals (O₂·⁻), and then ·OH. In the reaction, the organic matter is finally oxidized by the ·OH generated in the system and degraded into small molecules until direct mineralization. Therefore, the O₃/UV system includes UV/H₂O₂ and O₃/H₂O₂ processes, and has a stronger oxidation ability than other advanced oxidation technologies.

3. Research Status of O₃/UV Process

As a new water treatment process, O₃/UV has the characteristics of clean, efficient and low energy consumption compared with other AOPs. The process has mild operating conditions. The specific wavelength of ultraviolet light provides energy for photochemistry’s start-up and continuous operation. The oxidation selectivity of organic matter is controllable, and no additional catalyst is required. The synergistic effect of ozone and ultraviolet radiation can realize the efficient and stable degradation of organic pollutants in complex water bodies, and the treatment capacity is greatly improved compared with the single ozone oxidation process [27]. Nowadays, the O₃/UV process is still widely studied, proving that it has a good performance in treating industrial wastewater, antibiotics, natural organic matter, drinking water and micro-polluted organic matter. However, it is too expensive to treat the secondary sewage of sewage plants. Therefore, it has not been widely used in the conventional treatment process of water plants up until now. However, the process can generally treat specific pollutants or further improve the water quality after reaching the Class A standard in the discharge standard of pollutants for urban sewage treatment plants (GB18918-2002).

3.1. Treatment of Industrial Wastewater by O₃/UV Process

Industrial wastewater discharge is mainly concentrated in papermaking, the petrochemical industry, chemical manufacturing, printing and dyeing, the textile industry, slaughtering and the pharmaceutical industry [28]. The quality of industrial wastewater is complex, the discharge is large, and the drainage is disordered. Most industrial water contains refractory organics and a large concentration of toxic substances. The biochemical process is the first to be considered when treating such wastewater, but usually the biochemical process cannot effectively degrade toxic substances and specific organic substances and is affected by temperature, alkalinity, the oxygen content, the salt content and other factors. Therefore, cooperating with different physical and chemical methods for pretreatment or subsequent deep treatment is often necessary. When the O₃/UV process is used for pretreatment, macromolecular refractory organics can be degraded, and the biodegradability of wastewater can be increased, which is conducive to the subsequent biodegradation process [29]. In terms of advanced treatment, conventional advanced wastewater treatment processes such as coagulation sedimentation, air flotation and membrane process have the disadvantages of iron bed waste accumulation and blockage and low treatment efficiency. In contrast, the O₃/UV process can degrade difficult-to-degrade toxic substances and remove macromolecular organic matter. When degraded into small molecular organic matter that is less toxic and easy to be biochemically degraded [30] or directly mineralized into CO₂ and H₂O to remove organic matter, the effect is significant.

Zhang J.C. et al. [31] found that O₃/UV has an ideal effect on tailwater treatment with weak pH or neutral pH when studying the O₃/UV system’s deep synergistic removal of organic matter in the biochemical tailwater of a chemical industry park. The best treatment conditions of O₃/UV were a UV lamp power of 16 W, an O₃ dosage of 0.6 mL/min, pH = 7.1 and an irradiation time of 120 min, at which the chemical oxygen demand (COD) could be reduced from 82.3 mg/L to 49.5 mg/L, reaching the first-class A standard. Meanwhile, the research showed that the O₃/UV process was an effective method for the advanced treatment of biochemical tailwater in chemical sewage treatment plants. Its ability to degrade COD in tailwater was greater than that of O₃ and UV alone, and the effect was the sum of the two, indicating that O₃/UV had a significant synergistic effect. When Fu P.F. et al. [32] studied the collector sodium butyl xanthate (SBX) in the flotation of sulfide minerals degraded by O₃ alone, vacuum ultraviolet (VUV) alone and VUV/O₃, they found that both O₃ alone and VUV/O₃ could completely degrade SBX within 5 min. After 40 min, the removal rates of COD by the three processes were 38.3%, 40.5% and 64.9%, respectively. It was observed that the removal rate of COD was lower than that of SBX. In addition, compared with O₃ alone, the COD removal rate and sulfur mineralization rate of the VUV/O₃ process increased by 30.4-41.6% and 16.2-23.3%, respectively. When the dosage of O₃ increased from 14.7 mg/min to 125.9 mg/min, the removal rate of COD increased from 13.8% to 99.2%, and the removal rate of SBX increased from 36.4% to 99.3%. Although a hefty dose of O₃ can increase the removal rate of COD and enhance the mineralization of SBX, it reduces the utilization rate of O₃ from 96.8% to 24.5%. The above study also detected the concentration of sulfur by-products (CS₂ and SO₄²⁻) in the degradation process. It was observed that CS₂ was rapidly converted to SO₄²⁻, indicating that the sulfur by-products were effectively mineralized in the VUV/O₃ process. The difference between VUV and UV irradiation to generate ·OH was that 185 nm VUV could photolyze water to generate a hydrogen radical and a hydrated electron, which reacted with the dissolved oxygen to generate HO₂·. It then generated ·OH through a series of complex reactions. The comparison of the ·OH generation pathways in the O₃, VUV and VUV/O₃ processes and the proposed decomposition pathway of SBX in the VUV/O₃ process are shown in Figure 1.

Song Y.P. et al. [33] experimented with integrated equipment to treat printing and dyeing biochemical tailwater. The equipment included tubular O₃/UV, an ozone generation system, a gas-liquid separation system, an ozone destruction system and a biological aerated filter (BAF). In this experiment, the O₃/UV process was used as the pretreatment process of biodegradation. The experimental results showed that after pretreatment with this integrated equipment, the five-day biochemical oxygen demand (BOD₅) concentration of effluent increased by more than three times, and the biodegradability of the sewage was significantly improved. The comparative experiment showed that the COD removal rate of the single BAF treatment was only 22.5%, while the COD removal rate of the O₃/UV-BAF collaborative process was 46.93%. At the same time, the removal rate of UV₂₅₄ was 39.73%. In addition, the removal rates of chromaticity, total nitrogen (TN) and total phosphorus (TP) were 54%, 34.52% and 53.81%, respectively, which can meet the emission standard. Rajabizadeh K. et al. [34] used O₃/UV to degrade p-nitroaniline (PNA) in petrochemical industrial wastewater. Their results showed that under an initial PNA concentration of 10 mg/L, an ozone input and supply rate of 0.9 g/h, a contact time of 40 min and a pH value of 9.0, the PNA removal rates of the synthetic solution and the real sample reach 94% and 81, respectively. Therefore, O₃/UV was considered to be a suitable method for removing PNA from industrial wastewater. Kaplan A. et al. [35] used the UV/O₃/H₂O₂ combined process to treat the reverse osmosis concentrate (WWROC) rich in metal ions and refractory trace organic compounds (TOrC). It was found that although the concentration of WWROC used in the experiment was several times higher than usual, the O₃/UV process after adding H₂O₂ produced a more robust oxidation, more ·OH was generated, making the reaction more active, and the TOrC removal rate was higher than 90%. Although the treated matrix was still rich in metals and nutrients, the process can degrade refractory organics into more unstable compounds for subsequent biochemical degradation. Jing L. et al. [36] degraded atrazine (ATZ) production wastewater by the O₃/UV process and found that UV photolysis had a significant contribution to the degradation of ATZ. Regardless of the UV intensity, the removal rate of ATZ exceeded 95% after a 180-min treatment. The removal of COD and ammonia nitrogen (NH₃-N) was mainly guided by ·OH, especially in an alkaline environment, and the optimal pH value was 12.

There is a problem of low utilization of O₃ in homogeneous solutions, which is well solved by heterogeneous catalysis technology. Zhu et al. [37] studied the degradation of 2,4,6-trichlorophenol (2,4,6-TCP) in pesticide production by a O₃/UV/TiO₂ coupling process. The results showed that the utilization rate of ozone in the O₃/UV/TiO₂ coupling process was 11.7% higher than that in the O₃/UV process. In this coupling process, the mineralization rate of 2,4,6-TCP first increased and then decreased with the increase of the initial 2,4,6-TCP concentration. Even at the concentration of 30 mg/L, the mineralization rate still reached 83.31%. Moreover, this coupling process had a strong adaptability to pH, and maintained a high mineralization rate, at pH 3-11. Compared with traditional AOPs, the introduction of a catalyst in the reaction system was conducive to the re-capture of electrons by oxygen, the recycling of residual oxygen and the utilization of the catalyst and ozone.

Sun et al. [38] found that the maximum adsorption capacity of Brilliant Red X-3B(X3B) on the surface of TiO₂ nanotubes (TNTs) was three times that of P25TiO₂ when TNTs and P25TiO₂ were used as photocatalysts to degrade X3B, respectively, in the O₃/UV/TiO₂ process, and the photocatalytic activity of TNTs was better than that of P25TiO₂ under the same conditions. In the O_2/UV/TiO₂ system, X3B was degraded only by photocatalytic oxidation but not by chemical oxidation (ozonation). Quenching experiments showed that the photocatalytic degradation of this system occurred through direct hole oxidation, which was the first example of direct hole oxidation of organic pollutants in the O₃/UV/TiO₂ system. As shown in Figure 2, TiO₂ nanoparticles are excited under UV irradiation, resulting in the separation of electrons and holes. These carriers can quickly migrate to the surface, where the electrons are finally captured by O₂ (or O₃) to form O₂^·− (or O₃^·−), and the holes are captured by X3B adsorbed on the surface to form X3B^·+ radical. Under the action of O₂/O₂^·− (or O₃/O₃^·−), the X3B^·+ radical can be further degraded.

From the above research, as a green and efficient water treatment process, the O₃/UV process was feasible in treating industrial wastewater, but there were still many problems. Firstly, the best reaction conditions of the O₃/UV process were weak acidity and neutrality. At the same time, industrial wastewater often could not meet the requirements due to the complexity of the organic matter, which required water quality regulation and increased the additional cost. Secondly, the dosage of O₃ for treating different industrial wastewater was different. An excess would reduce the utilization rate of O₃, and too little would not meet the treatment requirements. Therefore, much research is still needed to form a systematic quantitative delivery standard to optimize the operating parameters to realize the maximum economy and effectiveness of the practical application of the O₃/UV process.

3.2. Treatment of Trace Polluted Organic Matter by O₃/UV Process

Organic pollution is presently an essential feature of micro-polluted water. These organic compounds have many kinds, low concentrations (ng·L⁻¹-μg·L⁻¹) and a complex nature. The water quality is characterized by eutrophication, dissolved organic matter and pathogenic microorganisms, excessive TN, COD and BOD, and sometimes colour, smell and taste. At present, trace organic pollutants are frequently detected in municipal sewage treatment plant effluent, surface water and drinking water. Many of these substances have been proved to cause significant harm to the human body, and some of them are determined to be carcinogenic, teratogenic and mutagenic. They are highly stable and difficult to degrade, and the traditional biological treatment process cannot guarantee the removal rate. As a known strong oxidant, ·OH has a redox potential (·OH, E⁰ = 2.80 V) second only to fluorine (F₂, E⁰ = 3.06 V). Therefore, the AOPs with ·OH as the main reaction group are believed to be the effective control of the micro-pollution of organic compounds [39].

When Rao Y.F. et al. [40] studied the degradation of linuron treated with O₃/UV, O₃ alone and UV alone, they found that the rate of flaxseed pyrimidine treated with O₃/UV was about 3.5 times and 2.5 times faster than that of UV alone and O₃ alone, respectively. The experimental results showed that the removal of total organic carbon (TOC) was not observed in the process of UV reaction alone. In the process of O₃ reaction alone, TOC reached about 15% mineralization after 100 min, and the effect was not pronounced. However, nearly 80% mineralization was achieved after 100 min using O₃/UV. Considering the influencing factors, it was found that when the pH value was more significant than 9.0, the three process rate constants increased exponentially, while when the pH value was less than 9.0, the rate constants in the O₃ system did not increase. To study the effect of the O₃/UV degradation of sulfamethoxazole (SMX), Liu X.W. et al. [41] investigated the effects of different pollutant concentrations and found that when the SMX concentrations were 0.5 mg/L, 1.0 mg/L and 4.5 mg/L, the pseudo-first-order reaction rate constants were respectively 3.8 × 10⁻³ s⁻¹, 2.9 × 10⁻³ s⁻¹ and 1.9 × 10⁻³ s⁻¹. Their experimental results showed that the degradation rate of pollutants decreased with the increase of the initial concentration due to the insufficient amount of ·OH caused by the increase of SMX concentration, increasing competition, which reduced the degradation rate [42]. In a study on the degradation effect of O₃/UV on ibuprofen in water, Wang Y.B. et al. [43] deduced that O₃/UV had a nearly 82% mineralization effect on ibuprofen when the reaction lasted 130 min. The mineralization process was per pseudo-zero reaction kinetics model, which was helpful to reduce the secondary pollution of intermediate products to the environment. The comparative experiment showed that when ibuprofen was treated with O₃ and UV alone, there was no mineralization effect when the reaction lasted 200 min. The degradation rate of ibuprofen by the above three methods was inversely proportional to the initial concentration, and the alkaline condition was conducive to the degradation of the O₃/UV system. Paucar N.E. et al. [44] treated personal care products (PPCPs) with the O₃/UV process. The results showed that when the treatment time was 10 min and the O₃ concentration was 6 mg/L, among the 38 PPCPs detected in secondary sewage, 31 degraded to or below its detection limit. Furthermore, 8 PPCPs, including dipyridamole, diclofenac, mefenamic acid and diltiazem, decreased to 1 mg/L within 5 min. It was concluded that by establishing the classification of O₃/UV sensitivity to PPCPs this process could remove a large amount of PPCPs from the effluent of the secondary sewage plant. Wang W.J. [45] compared the degradation of five fluoroquinolone antibiotics widely used in China by three processes: O₃ alone, UV alone and O₃/UV. Using the response surface optimization method, it was obtained that the optimal reaction conditions of the O₃/UV process were an ozone dosage of 5 mg/L, a UV intensity of 4.16 W/L, an initial pH of 9.41, a reaction time of 154 min and an average removal rate of fluoroquinolone antibiotics that could reach 68.81%.

As for the applications of O₃/UV in a heterogeneous solution, Hashemi et al. [46] obtained the following conclusions by optimizing the heterogeneous catalytic oxidation degradation of ceftriaxone (CTX) in water by O₃/UV/Fe₃O₄ & TiO₂. With a reaction time of 30 min, a photocatalyst dosage of 2 g/L, pH = 9, an initial CTX concentration of 10 mg/L and an ozone dosage 0.2 g/h, the removal rate of CTX and TOC were 92.4% and 72.5%, respectively. At the same time, after six continuous uses of the catalyst, the TOC removal rate was still high, proving that Fe₃O₄ & TiO₂ has a good reutilization potential. Asgari et al. [47] found that O₃/UV and O₃/UV/ZnO can degrade methyl-, ethyl-, propyl-, buthyl and benzyparabens in a group of common p-hydroxybenzoates (Parabens). The highest degradation rate of O₃/UV reached 60% at 60 min, while the highest degradation rate of O₃/UV/ZnO reached 93% at 20 min. The results showed that the O₃/UV/ZnO process combined with nano-ZnO was an effective method for the treatment of water containing p-hydroxybenzoate, the mineralization rate reached 42%, and the value of BOD₅/COD after photocatalytic ozonation increased, which improved the biodegradability and can be used as the pretreatment of the bioreactor.

The O₃/UV process has a significant effect on the degradation of trace polluted organics, which is attributed to the fact that most trace polluted organics contain unsaturated functional groups, phenolic hydroxyl groups and amino groups. These electron-rich structures can be attacked by ozone and then degraded. However, two issues still need attention: first, the concentration of micro-polluted organic matter is low, and the utilization rate of oxidant O₃ and H₂O₂ generated by UV irradiation is also low, resulting in an incomplete reaction. Therefore, subsequent treatment processes such as activated carbon adsorption need to be added to prevent secondary pollution. Secondly, the organic matter in water has different dissociation states under different pH conditions due to its different ionizable acidic or basic functional groups, which ultimately affects the photolysis process of organic matter.

3.3. Treatment of Drinking Water by O₃/UV Process

With the development of industry, more and more refractory organic pollutants have been detected in water sources, and the preparation process of drinking water is facing severe challenges. Furthermore, the most widely used ozone-biological activated carbon process also has unsolvable problems. For example, the biological reaction speed is limited, and the organic pollutants cannot be wholly degraded within the residence time. Moreover, incompletely degraded organics produce toxic organic pollutants in the disinfection process and accumulate in the human body, threatening life safety, such as bromate, haloacetic acid, haloacetonitriles, chloroform, etc. In contrast, as a typical AOP, the O₃/UV process can degrade organics efficiently. Therefore, several scholars tend to use the O₃/UV process as an advanced treatment of drinking water [48].

Yang Z.Z. [49] compared the efficiency of sulfadiazine degradation in water by O₃ alone, UV alone and O₃/UV alone and its influence factors and concluded that the degradation rate of the O₃/UV process was higher than the sum of O₃ and UV. Studying the comparative experimental data of TOC removal, this author found that ·OH played an essential role in making the degradation process more thorough. At the same time, the configuration of sulfadiazine was optimized by the DFT theoretical calculation method, and the charge distribution and Fukui function value of each atom were obtained. The active sites that may react with ·OH in the sulfadiazine molecule were found to infer the degradation products. The results showed that, compared with the single process, the oxidation products of sulfadiazine degraded by the O₃/UV process were more abundant, and the degradation of sulfadiazine and its intermediate products was more thorough (Figure 3). This showed that the O₃/UV process had the potential to remove antibiotics efficiently.

Ding C.S. et al. [50] compared the experimental results of O₃, UV, H₂O₂ and their combined technologies on trichloronitromethane (TCNM), a by-product of chlorine disinfection in drinking water, and showed that the treatment effect of TCNM by a single process was not ideal, but it could be effectively removed by a UV/O₃/H₂O₂ combined process. When the initial concentration of TCNM was 20 μg/L, the UV intensity was 31 μW/cm², the H₂O₂ dosage was 15 mg/L, and the ozone dosage was 10 mg/L, the removal rate of the process reached 97.82% after 150 min. Lau T.K. et al. [51] studied O₃/UV degradation of butylated hydroxyanisole (BHA), a chemical that destroyed endocrine substances in water, and found that BHA can form precipitation and be easily removed during ozonation. When the reaction time was 180 min, O₃/UV could achieve a 90% mineralization effect. The intermediate products such as hydroquinone and 1, 4-benzoquinone detected in the experiment were entirely removed after the reaction.

Dustschke et al. [52] developed a set of AOP devices for the application of groundwater quality to fully mineralize the organic trace pollutants contained in groundwater. The device combined an O₃ bubbling tower reactor with a TiO₂/UV system to carry out photocatalytic degradation, studied the degradation of several chloroethylene and methane derivatives common in the groundwater contaminated by chloro-volatile organic compounds, and used a Box-Behnken experimental design to carry out parameter optimization. The key to their success was the fact that a bubbling tower transferred the chloro-volatile organic carbon to the gas phase. The experimental results showed that the degradation rate of trichloromethane reached more than 85%, and the degradation rate of CIS dichloroethylene, trichloroethylene and tetrachloroethylene reached more than 98%. Moreover, no oxidation by-product or toxic conversion product increase was observed, so it was considered that the pollutants were fully mineralized. Lin et al. [53] studied the removal effect and mechanism of the O₃/UV/TiO₂ process on three pesticides (cypermethrin, malathion and dichlorvos) in fresh tea and water. The results showed that the removal effect of this process on malathion and dichlorvos in water under alkaline conditions reached 100%, while under acidic and neutral conditions the removal rate of cypermethrin in water was close to 100%. The degradation of three pollutants in tea by O₃/UV/TiO₂ was not affected by the pH. Simple water flushing was effective for dichlorvos, but not for insoluble cypermethrin and malathion. The removal rates of the above two pollutants in 30 min reached 80% and 78%, respectively.

It can be seen from the above that the O₃/UV process has a significant effect on the degradation of organic matter in drinking water, but the bromate problem is inevitable in all ozone processes. When there is a specific concentration of bromine ion in the water, the excellent oxidation capacity of the process may lead to the excessive concentration of bromine ion after treatment. This problem has been addressed by scholars worldwide in the application of the drinking water treatment process. Reducing the ozone dosage is a measure to control bromate concentration, but it still needs in-depth research and process optimization.

For an easy reading, the applications of the O₃/UV method in organics degradation in industrial wastewater, trace polluted organic matter and drinking water are summarized briefly in

Table 2. Applications of the O₃/UV method in organics degradation in industrial wastewater, trace polluted organic matter and drinking water.

Application Occasions	Process Types	Research Results	Data Sources
Industrial wastewater	O3/UV process	The best treatment conditions of O3/UV were a UV lamp power of 16 W, O3 dosage of 0.6 mL/min, pH = 7.1 and irradiation time of 120 min, at which the chemical oxygen demand could be reduced from 82.3 mg/L to 49.5 mg/L, reaching the first-class A standard.	[31]
		Under an initial PNA concentration of 10 mg/L, ozone input and supply rate of 0.9 g/h, contact time of 40 min and pH value of 9.0, the PNA removal rates of synthetic solution and real sample reach 94% and 81%, respectively.	[34]
		UV photolysis made a significant contribution to the degradation of ATZ. The removal rate of ATZ exceeded 95% after a 180-min treatment. The removal of COD and ammonia nitrogen was mainly guided by ·OH, especially in an alkaline environment, and the optimal pH value was 12.	[36]
	O3/VUV process	Degraded by O3 alone, vacuum ultraviolet alone and VUV/O3, they found that both O3 alone and VUV/O3 could completely degrade SBX within 5 min. After 40 min, the removal rates of COD by the three processes were 38.3%, 40.5% and 64.9%, respectively.	[32]
	O3/UV-BAF process	The COD removal rate of the single BAF treatment was only 22.5%, while the COD removal rate of the O3/UV-BAF collaborative process was 46.93%. In addition, the removal rate of UV254 was 39.73%, and the removal rates of chromaticity, total nitrogen and total phosphorus were 54%, 34.52% and 53.81%, respectively.	[33]
	UV/O3/H2O2 process	The UV/O3/H2O2 combined process to treat the reverse osmosis concentrate rich in metal ions and refractory trace organic compounds. After adding H2O2, the O3/UV process produced a more robust oxidation, and more ·OH was generated, making the reaction more active, and the TOrC removal rate was higher than 90%.	[35]
	O3/UV/TiO2 process	The O3/UV/TiO2 process increased the ozone utilization rate by 11.7% compared with the O3/UV process. In the coupled process, the mineralization rate of 2,4,6-TCP increased first and then decreased with the increase of the initial concentration. The mineralization rate still reached 83.31% even at the 2,4,6-TCP concentration of 30 mg/L. Moreover, this coupling process had a strong adaptability to pH. Sun et al. found that the maximum adsorption capacity of Brilliant Red X-3B(X3B) on the surface of TiO2 nanotubes (TNTs) was three times that of P25TiO2 when TNTs and P25TiO2 were used as photocatalysts to degrade X3B, respectively, in the O3/UV/TiO2 process, and the photocatalytic activity of TNTs was better than P25TiO2 under the same conditions. In the O2/UV/TiO2 system, X3B was degraded only by photocatalytic oxidation but not by chemical oxidation (ozonation).	[37,38]
Trace polluted organic matter	O3/UV process	The rates of flaxseed pyrimidine treated by O3/UV were about 3.5 times and 2.5 times faster than those treated by UV alone and O3 alone, respectively. No removal of total organic carbon was observed during the process of UV reaction alone. TOC mineralization reached about 15% after 100 min in the process of O3 reaction alone, and the effect was not obvious. However, nearly 80% mineralization can be achieved after 100 min using O3/UV.	[40]
		The pseudo-first-order reaction rate constants were 3.8 × 10−3 s−1, 2.9 × 10−3 s−1 and 1.9 × 10−3 s−1 when the SMX concentration was 0.5 mg/L, 1.0 mg/L and 4.5 mg/L, respectively. The experimental results show that the degradation rate of pollutants decreases with the increase of the initial concentration.	[41,42]
		The O3/UV process had a nearly 82% mineralization effect on ibuprofen when the reaction lasted 130 min. The mineralization process was per pseudo-zero reaction kinetics model, which was helpful to reduce the secondary pollution of intermediate products to the environment.	[43]
		When the treatment time was 10 min and the O3 concentration was 6 mg/L, among the 38 PPCPs detected in secondary sewage, 31 degraded to or below its detection limit.	[44]
		Using the response surface optimization method, it was obtained that the optimal reaction conditions of the O3/UV process were ozone dosage of 5 mg/L, UV intensity of 4.16 W/L, initial pH of 9.41, a reaction time of 154 min, and the average removal rate of fluoroquinolone antibiotics could reach 68.81%.	[45]
	O3/UV/Fe3O4 & TiO2 process	Under a reaction time of 30 min, photocatalyst dosage 2 g/L, pH 9, initial CTX concentration 10 mg/L and ozone dosage 0.2 g/h, the removal rate of CTX and TOC were 92.4% and 72.5%, respectively. At the same time, after six continuous uses of the catalyst, the TOC removal rate was still high, proving that Fe3O4 & TiO2 has a good reutilization potential.	[46]
	O3/UV/ZnO process	The O3/UV and O3/UV/ZnO can degrade methyl-, ethyl-, propyl-, buthyl and benzyparabens in a group of common p-hydroxybenzoates. The highest degradation rate of O3/UV was 60% at 60 min, while the highest degradation rate of O3/UV/ZnO was 93% at 20 min. The results showed that the value of BOD5/COD after photocatalytic ozonation increased, which improved the biodegradability and can be used as the pretreatment of bioreactor.	[47]
Drinking water	O3/UV process	The degradation rate of the O3/UV process was higher than the sum of O3 and UV rates alone. By studying the comparative experiment of TOC removal, it was found that ·OH played an essential role in making the degradation process more thorough and reducing the formation of intermediate products.	[49]
		BHA can form precipitation and be easily removed during ozonation. When the reaction time was 180 min, O3/UV could achieve a 90% mineralization effect. The intermediate products such as hydroquinone and 1, 4-benzoquinone detected in the experiment were entirely removed after the reaction.	[51]
	UV/O3/H2O2 process	When the initial concentration of TCNM was 20 μg/L, the UV intensity was 31 μW/cm2, the H2O2 dosage was 15 mg/L and the ozone dosage was 10 mg/L, and the removal rate of the process reached 97.82% after 150 min.	[50]
	O3/UV/TiO2 process	The degradation rate of trichloromethane was higher than 85% and the degradation rate of cis-dichloroethylene, trichloroethylene and tetrachloroethylene were higher than 98%. No oxidation by-product or toxic conversion product increase was observed, so it was considered that the pollutants were fully mineralized. The removal effect on malathion and dichlorvos in water under alkaline conditions reached 100%, while under acidic and neutral conditions the removal rate of cypermethrin in water was close to 100%. The degradation of three pollutants in tea by O3/UV/TiO2 was not affected by pH. Simple water flushing was effective for dichlorvos, but not for insoluble cypermethrin and malathion. The removal rates of the above two pollutants in 30 min were 80% and 78%, respectively.	[52,53]

.

4. O₃/UV Process Evaluation

The O₃/UV process has a good oxidation capacity, cleanness and environmental protection. However, if it is used as the conventional treatment process of the water plant, the bromate formation control and economy problems need to be evaluated.

4.1. Evaluation of Bromate Formation Control

Bromate is a common by-product of ozone oxidation and must be considered a pollutant in drinking water. According to the research, bromate has been found to induce kidney tumors in laboratory animals. The International Agency for Research on Cancer (IARC) has classified bromate as a potential carcinogen of grade 2B. Although the World Health Organization, EPA and China’s newly revised Hygienic Standard for Drinking Water (GB5749-2006) set the maximum bromate concentration at 10 μg/L, the ideal value is not detected in the water. Therefore, when the concentration of bromine ion in raw water is below 20 μg/L, the ozone oxidation technique does not need to consider the problem of bromine ion concentration exceeding the standard. However, when the concentration exceeds 50 μg/L, the problem of bromate formation should be considered. The oxidation process of bromine ions by ozone and ·OH is shown in Figure 4 [54].

It can be seen from Figure 4 that O₃ and ·OH were both used as oxidants of bromine ions, and bromate was mainly formed in three ways: (1) ozone oxidation; (2) ozone-·OH oxidation; (3) ·OH ozonation. The reactions and reaction rate constants involved in Figure 4 are shown in

Table 3. Bromate formation reactions and reaction rate constants.

No.	Reaction	Reaction Rate Constant (L·mol−1·s−1)
1	${\mathrm{Br}}^{-}+{\mathrm{O}}_3\to {\mathrm{OBr}}^{-}+{\mathrm{O}}_2$	160
2	${\mathrm{OBr}}^{-}+{\mathrm{O}}_3\to {\mathrm{BrO}}_2^{-}+{\mathrm{O}}_2+{\mathrm{H}}^{+}$	100
3	${\mathrm{HOBr}}^{-}+{\mathrm{O}}_3\to {\mathrm{BrO}}_2^{-}+{\mathrm{O}}_2+{\mathrm{H}}^{+}$	≤0.013
4	${\mathrm{BrO}}_2^{-}+{\mathrm{O}}_3\to {\mathrm{BrO}}_3^{-}+{\mathrm{O}}_2$	>105
5	${\mathrm{OBr}}^{-}+·\mathrm{OH}\to \mathrm{BrO}·+{\mathrm{OH}}^{-}$	4.5 × 109
6	$\mathrm{HOBr}+·\mathrm{OH}\to \mathrm{BrO}·+{\mathrm{H}}_2\mathrm{O}$	2.0 × 109

[55].

It can be seen from Table 3 that BrO⁻ is an essential intermediate product for bromate formation. The reaction rate of ·OH was 10⁷ times faster than that of O₃. UV can stimulate O₃ to generate ·OH to react with organic pollutants itself can itself react with BrO⁻ to form bromate. Ratpukdi T. et al. [56] showed that UV could reduce BrO₃⁻ to BrO₂⁻ and finally produce Br⁻. The reactions are as follows:

$$2{\mathrm{BrO}}_3^{-}+\mathrm{hv}\to 2{\mathrm{BrO}}_2^{-}+{\mathrm{O}}_2$$

(12)

$$2{\mathrm{BrO}}_2^{-}+\mathrm{hv}\to 2{\mathrm{BrO}}^{-}+{\mathrm{O}}_2$$

(13)

$$2{\mathrm{BrO}}^{-}+\mathrm{hv}\to 2{\mathrm{Br}}^{-}+{\mathrm{O}}_2$$

(14)

This experiment found that the main factor affecting the reduction efficiency of BrO₃⁻ was the UV wavelength. The UV lamp that can emit the wavelength below 200 nm was more effective in reducing bromate than the UV lamp that only emitted the wavelength of 254 nm. This was because when the ultraviolet wavelength was higher than 250 nm, the ultraviolet absorption of BrO₃⁻ was relatively low. Therefore, given this phenomenon, they proposed the use of ozone/vacuum ultraviolet radiation (VUV, 185–254 nm). The experimental results showed that the bromate concentration produced by the O₃/VUV process was 1/4 and 1/6 times that of the O₃ and O₃/UV process alone. The advantage of the O₃/VUV process was that O₃ oxidation and VUV irradiation were carried out simultaneously in the same reactor, providing sufficient contact time for the reduction of bromate by VUV at 185 nm. Using the O₃/VUV process to reduce bromate concentration, bromate formation can be controlled by adjusting O₃ dose, UV power and pH, and dissolved organic carbon (DOC) can be removed to the greatest extent to realize a detailed optimization. Before studying bromate, T. Ratpukdi also studied the removal of natural organic matter (NOM) by the O₃/VUV process. The experimental results showed that the O₃/VUV process had a better performance in DOC mineralization, UV₂₅₄ reduction and biodegradability than O₃ and O₃/UV alone. The best treatment effect was achieved at pH 7, and the synergistic effect was achieved [56]. Zhao G.Y. [55] studied the treatment of emerging pollutants in drinking water by ultraviolet/micro-ozone process (UV-micro O₃) and controlled bromate formation. It was found that UV-micro O₃ had a slightly lower organic matter removal level than O₃/UV but had significant advantages in controlling bromate formation. The NOM in water treated by the O₃/UV process can compete with ·OH in the bromate system to inhibit bromate formation to a certain extent. When the concentration of NOM in raw water was low, but the concentration of bromine ion was high, the UV-micro O₃ process was a very suitable bromate control technology. When Zhao Guangyu studied the treatment of humic acid (HA) and bromate by the O₃/UV process, he found [57] that HA showed different inhibitory effects on bromate formation in the O₃/UV process. When the HA concentration reached 2 mg/L, the bromate concentration formed in the O₃/UV process was lower than that formed by O₃. However, when a high dose of O₃ was used to treat water containing low concentrations of HA and high concentrations of bromate ions, the bromate concentration in the system still exceeded the limit of 10 μg/L. It can be seen that a low dose of O₃ was a promising method to adjust HA removal and bromate control in the O₃/UV process.

4.2. Economic Evaluation

Electric energy consumption is the main index of economic evaluation of the O₃/UV process. Kim I.H. et al. [58] studied the wastewater reuse technology using the UV/H₂O₂, O₃ and O₃/UV processes to remove PPCPs. Based on the energy consumed by each process to achieve a 90% removal rate in the operation process, it was calculated that the power consumption of the UV lamp and the power consumption required for 1 kg ozone generation were 72 W and 15 kWh, respectively. The results showed that 0.54 kWh/m³, 0.09 kWh/m³ and 1.09 kWh/m³ were consumed during the UV/H₂O₂, O₃ and O₃/UV treatment, respectively, to achieve an effective PPCP removal. Based on this result, it was found that the UV, UV/H₂O₂ and O₃/UV processes required a considerably higher power consumption than the O₃ process alone. Mascolo G. et al. [59] conducted a preliminary operation cost assessment on the UV/H₂O₂ process using a batch treatment experiment to repair groundwater polluted by methyl tert-butyl ether (MTBE) benzene, toluene, p-xylene, styrene and ethylbenzene. The study found that the power required to remove one order of magnitude of the studied pollutants was 2.8 kWh/m³, more than twice the power required by Kim I.H. et al. to remove 90% PPCPs (1.09 kW/m³) by the O₃/UV process.

Yang S.P. et al. [60] evaluated the chemical and key reactor parameters of the O₃/UV process in the application of industrial wastewater treatment. Considering the cost of wastewater treatment, they pointed out that the combination of advanced oxidation and biochemical treatment is the most economical way. Their studies showed the effect of removing COD_Cr by adding different O₃ doses when advanced oxidation was combined with a biofilter. When the dose range was 0 to 15 mg/L, the COD_Cr removal rate increased significantly with the O₃ dose, the highest removal rate reached 50.25%, and the effluent COD_Cr concentration was 39.8 mg/L, which met the discharge requirements. When the dose was greater than 15 mg/L, the removal rate of COD_Cr changed slightly without significant improvement. The experimental results showed that the effluent of the O₃/UV reactor met the requirements of biochemical degradation at the O₃ dose of 15 mg/L. Therefore, combining the O₃/UV process with the O₃ dosage of 15 mg/L and the biochemical process was the most economical method.

Ultraviolet radiation intensity was another critical factor affecting the economy of the O₃/UV process. In the study, Yang S.P. et al. still adopted the process of advanced oxidation and biochemical treatment. The O₃ dose was 15 mg/L, and the water inflow was 4 L/min. Different numbers of light sources were used in the experiment. The output powers of ultraviolet lamps were 0 W, 105 W, 210 W, 315 W and 420 W, respectively. The results showed that increasing the UV lamp power can significantly improve the COD_Cr removal rate, but when the UV lamp output power is more significant than 315 W, the sewage COD_Cr treatment effect improvement is not apparent. It can be seen that there was a critical point for the demand for light intensity at a specific O₃ concentration, but the effect of increasing the light intensity was not evident when the light intensity was higher than the critical point. Therefore, the most economical UV output power was 315 W (the UV irradiation intensity was about 822.88 W/m²).

5. Conclusions

O₃/UV, as a clean, efficient and energy-saving water treatment technology, has mild reaction conditions, a fast rate, a controllable organic oxidation selectivity and no additional catalyst. The synergistic effect of ozone and UV can realize organic pollutants’ efficient and stable degradation in complex water bodies.

For the pretreatment process, the O₃/UV treatment of industrial wastewater can effectively enhance the biodegradability of effluent and provide an excellent operating environment for subsequent biological treatment. At the same time, when used for advanced treatment, the process has a different degradation effect on the organic matter in the tailwater after biochemical treatment. Because of the high degradation efficiency of micro-polluted organic substances (such as antibiotics and pesticides), the treatment effects of different organic substances vary in different pH environments, and the reaction conditions still need to be further studied. Furthermore, the concentration of bromine ions in the water body needs to be investigated when applied to drinking water treatment. The presence of NOM can inhibit bromate formation to a certain extent, but when the concentration of the bromine ion is too high, the treated effluent still cannot meet the discharge requirements. On the other hand, the UV-micro O₃ process can significantly control bromate formation, and the dosage of O₃ needs to be controlled. Therefore, the application of the O₃/UV process in drinking water treatment needs a careful evaluation.

At present, the O₃/UV process is mainly in the laboratory research stage. Due to the relatively high cost, it has not been widely used in practical engineering except for a few occasions. Future research should focus on the technical direction of the highly effective degradation of organic pollutants and sustainable economic development. In addition, at present, the mineralization capacity of the existing processes is insufficient. Therefore, the goal of the researchers should be to develop water treatment technologies that can completely mineralize organic pollutants and avoid the generation of conversion substances with higher toxicity than the parent compound. At the same time, we should also actively explore the improved process of using material adsorption or biofiltration to eliminate toxic intermediates and conversions.