The electrochemical oxidation technology is not only easy to operate, but also has a high degradation rate. It has attracted more and more attention in environmental pollution control, especially in the removal of biodegradable organics from wastewater (Table 1). The electrochemical oxidation treatment of refractory wastewater is the direct electrochemical reaction of pollutants on electrodes or the indirect reaction of pollutants by using strong oxidizing active substances generated on the surface of electrodes to convert pollutants into harmless or less toxic substances through anodic oxidation. The electrochemical oxidation technology is effective in treating refractory and highly toxic wastewater. Common anode materials include Pt, PbO₂, SnO₂, RuO₂ electrodes, etc. The NCNS-CE-PBO₂ electrodes have denser surfaces, smaller grains, and many active sites [9]. Furthermore, Xia et al. studied that electrochemical oxidation of AO7 by Fe-doped PbO₂ electrode is a promising method, and proposed possible degradation pathways by identifying five kinds of aromatic intermediates for AO7 degradation during the electrochemical process [10]. It also compounded carbon nanotubes into PbO₂ electrodes by composite electrodeposition technology and obtained multi-layer CNT-PBO₂ electrodes. The PbO₂ electrode and CNT-PbO₂ electrode were analyzed by SEM and XRD. Considering comprehensively the effects of ISN removal efficiency, COD, and average current efficiency, the ISN degradation rate reached 99.4%, and the COD degradation rate reached 86.8% after the electrode was degraded by electrochemical oxidation for 120 min under the best conditions [11]. A new type of graphene oxide and Yb co-modified PbO₂ electrode (Yb GO PbO₂) prepared by electrodeposition has good stability, can effectively improve the degradation ability of lamivudine, and has feasibility and good application prospects for the treatment of pharmaceutical wastewater [12]. The decontamination performance of the filter membrane in electrochemical treatment technology is closely related to its electrochemical activity. A novel metal-free composite electrochemical filtration carbon membrane was prepared by deposition of polyaniline layer on CM surface via the facile electro-polymerization oxidation method [13]. At present, the use of three-dimensional electrode is a new electrochemical oxidation technology. Fe/C granules were prepared with iron filings as carrier and sodium alginate as hydrothermal carbonizing agent. Rice husk and iron filings are widely available and inexpensive raw materials for particle electrodes, which use Fe/C particles suspended between the cathode and anode to produce a three-dimensional electrode [14]. The Sn/FCC, Sb/FCC and Sb–SnO₂/FCC composites were fabricated via impregnation method [15]. This study provides a new idea for the preparation of electrodes and has a broad application prospect in the field of dye degradation. In addition, a new electrochemical switching ion exchange (ESIX) system is proposed for the treatment of high concentration sodium phenol salt wastewater. A FeHCF coated electrode with large interstitial sites in the rigid open framework was fabricated for the insertion/extraction of Na⁺ ions from aqueous sodium phenoxide solution by switching the redox state of FeHCF [16].

A key direction of future electrochemical oxidation technology research is to develop the combination of electrochemical oxidation and various water treatment processes, overcome the shortcomings of high energy consumption of electrochemical oxidation technology, and achieve the efficient treatment of a large amount of industrial wastewater or dispersed domestic wastewater at a lower cost and in a shorter time.

Ozonation is favored by researchers because of its strong oxidation capacity, rapid degradation efficiency, simple operation, and no secondary pollutants. The ozonation technology can be used for disinfection, improving color in dye wastewater, and solving taste and odor problems (Table 2). There are still many problems with a single ozonation system. On the one hand, the solubility and mass transfer efficiency of ozone in an aqueous solution is very poor, resulting in low practical utilization. On the other hand, the limited oxidation capacity (E₀ = 2.07 V) and the selective oxidation of molecular ozone lead to the degradation and mineralization are not ideal. Due to the selective oxidation of ozone and the low utilization efficiency of ozone alone, it is difficult to apply single ozonation technology to the actual high-concentration wastewater treatment project. If the treatment efficiency of ozone can be improved, its application in actual wastewater treatment will be significantly improved.

At present, the research focus in the field of ozonation water treatment is to develop efficient multiphase ozonation catalysts. A variety of metals (including Fe, Cu, Pt, Ce, Ru, Mn, etc.) and metal oxides (including Fe₂O₃, MnO₂, MgO, TiO₂, Al₂O₃, CeO₂, etc.) coated on porous supports (including activated carbon, alumina, titanium dioxide, silicon dioxide, etc.) have been proposed and studied for ozonation. Song et al. synthesized a magnetic Fe₃O₄@SiO₂@La₂O₃ nano-catalyst with a core-shell structure to improve the catalytic ozonation efficiency of cinnamyl alcohol. With the help of the catalyst, the degradation rate of cinnamyl alcohol could reach 99.8% at 30 min, and the COD could be removed at 28.5% at 60 min [17]. Its role in development has received more attention due to the inefficiency in the ozonation process and the generation of toxic by-products that limit it. When studying the removal of oxalic acid from water by MnO₂/O₃, Andreozzi et al. found that increasing the amount of MnO₂/O₃ catalyst could significantly improve the degradation rate of oxalic acid [18]. Zhu et al. prepared an ordered mesoporous Fe₃O₄ catalyst by nano casting method for catalytic ozonation degradation of atrazine. The results show that the mesoporous Fe₃O₄ catalyst has good catalytic activity for the ozonation degradation of atrazine [19]. A composite Mn-Cu-Ce tri-metal oxide supported on γ-Al₂O₃ (Mn-Cu-Ce/Al₂O₃) catalyst was prepared by an impregnation calcination method and the synergistic effect of Mn, Cu and Ce oxides greatly enriched the catalytic active center and improved the ozonation performance [20]. Fe-MCM-48 catalyst with three-dimensional cubic pore structure was directly synthesized by hydrothermal method. Compared with single ozonation process, it was found that the mineralization efficiency of the catalyst for diclofenac (DCF) during ozonation was higher [21]. The magnetic Fe₃O₄@SiO₂@CeO₂ catalyst, which can be easily separated by magnetic force, was used to catalyze the degradation of ASA in an aqueous solution by ozone. In addition, the Fe₃O₄@SiO₂@CeO₂ catalyst has magnetic recyclability and low metal leaching [22]. The catalytic materials of MnOx/γ-Al₂O₃, CeO₂/γ-Al₂O₃, and Mn_xCe_1-xO₂/γ-Al₂O₃ were synthesized and used in heterogeneous catalytic ozonation of the wastewater containing NFZ. Wherein, Mn_xCe_1-xO₂/γ-Al₂O₃ has excellent catalytic stability, is conducive to recycling, and maintains high activity [23].

As an environmentally friendly, powerful, and common oxidant, H₂O₂ does not produce toxic residues during decomposition [24]. Therefore, it is commonly used in wastewater treatment. With the addition of H₂O₂ as the core, a common advanced oxidation process was formed by introducing O₃, UV, Fe²⁺, and other reagents to activate the decomposition process of H₂O₂ to produce ^•OH [25]. The peroxone technology involves a series of chain reactions between O₃ and H₂O₂ to generate ^•OH with strong oxidizing properties. In O₃-related advanced oxidation water treatment technology, the consumption and conversion of O₃ molecules is a key step in the generation of ^•OH. The peroxone technology is considered to be a friendly and efficient advanced oxidation technology for wastewater treatment because H₂O₂ and O₃ are green oxidants that can be decomposed into H₂O and O₂ without producing any secondary by-products [26].

The peroxone technology is a typical advanced oxidation technology, in which additional H₂O₂ is added on top of ozonation, relying on the reaction of O₃ with H₂O₂ to generate highly reactive radicals ^•OH with high oxidation capacity. Although the peroxone process can provide a feasible way to treat wastewater, it requires the addition of external H₂O₂, which is an expensive chemical. Due to the nature of highly concentrated H₂O₂ solutions, their transportation, storage, and handling are hazardous. These problems greatly limit the practical application of the peroxide process in water treatment [27,28].

The electro-peroxone technology is a novel advanced oxidation process that outperforms ozonation or peroxone technology due to its superiority [27]. This technology, combining ozone and hydrogen peroxide generated electrochemically leads to the production of ^•OH, which is the strongest oxidizing agent [27,29]. The electro-peroxone technology overcomes the limitation of the conventional peroxone process [30]. The conventional peroxone process requires the addition of H₂O₂ in the O₃ oxidation process. In the electro-peroxone process, H₂O₂ can be generated on the cathode by O₂ in the O₃ oxidation process.

A pair of electrodes are installed in the O₃ reactor and converted to electro-peroxone equipment. In the electro-peroxone system, the O₃ produced by the O₃ generator passes through the pipeline to the bottom of the electrochemical device, and the microbubbles are emitted by the aeration head, and the bubbles dissolve the O₃ and O₂ mixture in the water during the rising process. The O₃ system is used to provide a mixture of O₂ and O₃ to enhance the mass transfer of gaseous O₃ to the liquid and the H₂O₂ aqueous solution generated at the cathode during the reaction, thus synergistically generating stronger ^•OH.

At present, there is also research on emitting penetrating electro-peroxone ozonation device system. The experimental device is made of plexiglas, mainly composed of an electrochemical workstation, magnetic mixer, calomel reference electrode, penetrating porous membrane electrode as anode, cathode, proton exchange membrane, and so on. In this kind of dual-function system, the cathode as the transmission of the porous composite electrode, effectively breaks traditional electrochemical oxidation fluid mass transfer limit operation system, wastewater in the inside of the membrane pore structure in streaming operation, effectively overcomes the problem of limited mass transfer by streaming operation. The degradation and even mineralization of pollutants can be realized in a very short hydraulic retention time, and a large amount of H₂O₂ can be produced on the cathode. The constructed dual-function electrolytic system achieves high efficiency in both wastewater oxidation and hydrogen production. This hybrid electrolytic system combines the cathode and anode co-production of H₂O₂, which has the advantages of wastewater treatment and high value-added chemicals production at the same time (Fig. 1) [31].

A DC power supply is used to apply an electric field to generate H₂O₂ in situ in electro-peroxone experiments. The generation of H₂O₂ and ^•OH is monitored for cathodes, including electrolytic peroxone and electrolysis processes, with oxygen lines connected directly to the bottom of the reaction vessel and ejected for in situ generations of H₂O₂. The aeration device is introduced from the bottom to produce H₂O₂ efficiently on the cathode surface, which is carried into the water by a large number of bubbles generated, thus greatly improving the efficiency of ^•OH production. The O₂ in the O₃ line is used as input for O₃ generation (Fig. 2) [32].

The plexiglas reactor is used for electrochemical ozonation processes and other electrochemical processes and has a size diameter of about 15 cm and a height of about 15 cm. The aeration device consists of an aerator head and a pipe extending into the bottom of the tank, which is a further improvement on the ozonation device and combines electrochemical techniques (Fig. 3) [33].

The reactor size of the electro-peroxone process is larger than conventional advanced oxidation technology. There will be a higher O₃ dose which can reduce the reaction time required to effectively degrade O₃-refractory pollutants. In an electro-peroxone system, O₃ is generated from O₂ using an O₃ generator. A gas mixture of O₂ and O₃ from the O₃ generator is injected into a reactor. The H₂O₂ in situ reacts with the sprayed O₃ to form a very powerful oxidant ^•OH, thus achieving the synergistic effect of O₃ and H₂O₂ on the degradation of organic pollutants (Eqs. 3–5) (Fig. 4).

Given the limitations of currently used oxidation technologies, the development of new advanced oxidation technologies has become a hot research topic in recent years. The advantages of the electro-peroxone technology can be shown through theoretical analysis of the degree of pollutant mineralization, kinetic analysis, and degradation efficiency (Table 3). The electro-peroxone technology can remove most refractory pollutants and typical emerging pollutants [34]. The use of O₃/H₂O₂ and electro-peroxone processes (by adding an external H₂O₂ reserve or in situ generations of H₂O₂ through cathodic O₂ reduction during ozonation) improved by about 5%-12% and 5%-7%, respectively, compared with conventional ozonation [35]. The electro-peroxone process may provide a simple and effective way to improve the existing ozonation system for pharmaceutical removal from secondary effluents [36]. The degradation of venlafaxine and the removal of total organic carbon (TOC) were greatly enhanced by electro-peroxone treatment due to significant ^•OH production compared with ozonation and electrolysis alone [37]. The H₂O₂ generated at the cathode can also effectively quench hypochlorite produced by the oxidation of chloride ions in surface water at the anode. Compared with conventional ozonation, the electro-peroxone process significantly accelerated the removal of refractory micropollutants from surface water [38]. The addition of MnO₂ and H₂O₂ or the H₂O₂ produced by electrochemistry can promote the conversion of O₃ to ^•OH to varying degrees [39]. The energy consumption in the electro-peroxone water treatment technology can be reduced by the use of circulation system, which can enhance the mass transfer of gasses and pollutants and improve the current efficiency due to the improvement of convection [40]. A flow-through electro-peroxone system consisting of electrolysis and ozonation was also developed for the disinfection of wastewater [41].

No chemicals and catalysts are required in the reaction process, and no pH adjustment is needed. It will readily decompose into H₂O₂ alone, resulting in rapid mineralization of refractory organic pollutants. In general, the reaction in the electro-peroxone system does not leave unwanted by-products (e.g., NH₃ and bromate) and catalysts that may require further treatment, separation, and regeneration. In addition, the whole process is electrically driven and therefore easy to automate and control. The H₂O₂ generated by the cathodic reduction in synergy with ozone can effectively generate ^•OH, a much more powerful oxidant than O₃ and H₂O₂ alone, resulting in rapid mineralization of refractory organic pollutants, while the process utilizes O₂ that would otherwise be wasted to generate H₂O₂ in situ, thus significantly improving TOC removal efficiency.

In the electro-peroxone process, the generation of H₂O₂ in situ is essential to achieve the synergistic effect of peroxide on the degradation of pollutants. Therefore, the selection of cathode materials that can efficiently produce H₂O₂ from oxygen is crucial for the success of the electro-peroxone process. Many carbon electrodes are available for H₂O₂ production, such as graphite, carbon PTFE, carbon felt, RVC, carbon nanotubes, and graphene electrodes. It showed that pulsed electrochemical oxidation using PTFE-Fe-PbO₂ electrodes may be a promising alternative for improving the practicality of electrochemical treatment of dye wastewater [44]. Among the three carbon-based cathodes (carbon-PTFE, carbon felt, and RVC), the carbon-PTFE cathode achieved the highest TOC removal yield, which can mainly be attributed to its highest electrocatalytic activity for H₂O₂ generation from O₂ reduction [45]. The performance of many other carbon-based cathodes (such as plain metal foam electrodes, carbon-coated metal foam electrodes, and sponge electrodes) is still unexplored for the application of electro-peroxone [46]. The Carbon-PTFE electrode has higher H₂O₂ yield and current efficiency than the general electrode, and the hydroxyl radical yield and current efficiency under alkaline conditions are better than that under acidic conditions. A vast number of studies have demonstrated that although many different carbon-based electrodes can be used to generate H₂O₂ from O₂ reduction, carbon-PTFE is usually the most effective electrode and can produce H₂O₂ with current efficiency approaching 100%.

Nickel foam usually has good structural characteristics, such as a three-dimensional porous structure, high specific surface area, good electrical conductivity, and low cost. The porous structure and high surface area lead to better mass transfer and active sites for oxygen reduction reactions (Eqs. 6 and 7)

The dissolved oxygen in the pollutant solution is reduced in situ at the cathode to generate H₂O₂, which is effectively decomposed by O₃ in the cathode catalyst layer. The ^•OH is produced to achieve efficient degradation of pollutants, and the penetration of cathode materials can effectively improve the mass transfer efficiency of pollutant molecules [47].

Based on anodic oxidation and cathodic reduction, the cathode-anode collaboration technology can also be developed, that is, the double-electrode reaction method can be adopted to further improve the current utilization rate, enhance the cathodic reaction, and improve the degradation rate of refractory organic pollutants. The most selected cathode materials are mainly graphite, mesh porous electrode, and Carbon-PTFE gas diffusion electrode. Studies have shown that the Carbon-PTFE gas diffusion electrode has higher H₂O₂ yield and current efficiency than the graphite electrode [43].

In the research and development of electro-peroxone technology, it is found that changing the carbon-PTFE gas diffusion cathode to a titanium sheet, boron-doped diamond film electrode, and stainless steel cathode in electro-peroxone technology formed another type of ozonation structure treatment technology, which is better than traditional ozonation and electrochemical oxidation technology. Also, the results show that the H₂O₂ concentration generated in the electrolysis system is notably increased in the presence of TiO₂-GAC (Fig. 5) [48]. And the actual current efficiency of cathodic generation of H₂O₂ is expected to be higher due to the higher hydrogen precipitation overpotential and lower H₂O₂ decomposition catalytic activity of carbon-based electrodes or autolytic decomposition in the native solution.

The ^•OH is generated by cathodic O₂ reduction using carbon-based electrodes due to their higher hydrogen precipitation overpotential and lower H₂O₂ decomposition catalytic activity (Eq. 6) or autolytic decomposition in the native solution (Eq. 7) [44,49].

The generation of H₂O₂ is an important factor affecting the treatment effect of the electro-peroxone technology [20]. The current efficiency of H₂O₂ is different under different currents. The concentration of H₂O₂ increases with the current, and there is a linear relationship between the concentration of H₂O₂ and the reaction time [43]. However, the mass transfer process of H₂O₂ is limited after some time, and the further upgrade of the current has little influence on H₂O₂. Due to the poor solubility of O₃ in water, the rate of transfer of O₃ from the gas phase to the liquid phase may eventually limit the rate of ^•OH production as the current gradually increases [50]. The efficiency of H₂O₂ generation at the electrode can be verified by Eq. 8 under different current conditions, thus investigating the effect of current density on the reaction in the electro-peroxone system [45]. The rate of H₂O₂ anodic decomposition increases with the concentration of H₂O₂ in the solution, and the apparent current efficiency for H₂O₂ generation decreased slightly as the process proceeds. Compared with the carbon-PTFE cathode, the carbon felt and RVC cathodes generated H₂O₂ at much lower apparent current efficiencies [51].

The generation of H₂O₂ in situ from a Carbon-PTFE gas diffusion electrode is a current-limited process. High current density does not significantly improve the removal efficiency, probably because of the increased side reaction and ohmic heat loss at higher current density. Therefore, it is necessary to select the best current density for the experiment.

The content of O₃ is one of the important factors of the electro-peroxone process. Different O₃ levels have different effects on the degradation of the target pollutants in wastewater. O₃ usually accounts for only a small fraction of the ozone generator gas effluent, since the ozone generator can only convert a small fraction of the feed O₂. In conventional ozonation treatment, O₂ (which is the major part of the injected gas) is wasted. The electro-peroxone technology, when the electrochemical oxidation and ozonation technology are simply combined to drive the electro-peroxone reaction to generate a large number of ^•OH.

In energy, only a small fraction of O₂ is converted to O₃, and a large fraction of O₂ and O₃ in the ozone generator is unconverted O₂. The gas mixture leaving the ozone generator still contains a large amount of O₂. When using pure O₂ as the feed gas, the volume ratio of the O₂/O₃ mixture is usually greater than 90% [52]. After the injection of O₃ into the reactor, a little O₂ is used to remove contaminants. This wastes large amounts of oxygen feed gas and energy. In the electro-peroxone process, the H₂O₂ generated by the cathodic reduction in synergy with O₃ effectively produces ^•OH, which are much more powerful oxidant than O₃ and H₂O₂ alone, resulting in rapid mineralization of refractory organic pollutants (Table 5) [40].

The electro-peroxone technology has made some progress in the wastewater treatment process with the advantages of high degradation efficiency, TOC mineralization, the short reaction time required for water treatment, and low energy consumption. Meanwhile, the research mechanism and result of the electro-peroxone technology are introduced from two aspects: cathodic reaction and bulk reaction. The influence of experimental parameters on the wastewater treatment effect is also discussed.

The electro-peroxone technology overcomes some disadvantages and inherent limitations of traditional advanced oxidation technology in sewage treatment, energy saving, and emission reduction. It has improved the performance of wastewater treatment in many aspects and achieved more satisfactory results. In general, the following advantages of the electro-peroxone technology make it a feasible new option for advanced oxidation technology wastewater treatment applications: (1) Simple modifications and upgrades of existing oxidation system equipment. (2) High O₃ utilization and low energy consumption. (3) High degradation efficiency mean that ^•OH is efficiently generated under the synergistic action of O₃ and H₂O₂. (4) Less by-product generation means less secondary pollution, thus protecting the environment. (5) Low risk, that is, it solves the safety problem of transporting H₂O₂ outward. (6) High pollutant degradation kinetics and energy efficiency.

So far, although the electro-peroxone technology has shown excellent performance in advanced oxidation technologies, there are still some aspects that need more attention in future research. Future research will focus on the following aspects.

(1) At present, many experimental studies on the wastewater treatment of refractory organic compounds have been carried out and achieved success, providing important information on reaction kinetics and mechanisms, water matrix effects, energy efficiency, etc., but most of them are still in the pilot stage and have not been put into practical application.

(2) Due to the different structures of the reaction equipment, the electro-peroxone technology needs to focus on the complex effects in water and the formation mechanisms and control strategies of reactions in the process, and conduct more detailed pilot evaluations so that it can be put into practical wastewater treatment.

(3) The decomposition of O₃ and H₂O₂ on the cathode into ^•OH in the electro-peroxone system, which may lead to the disinfection of the process that O₃ is both an oxidant and disinfectant in wastewater treatment, improves the oxidation capacity of the process compared to the conventional O₃ oxidation system, but the disinfection performance needs to be optimized and evaluated in future studies.

(4) The electro-peroxone technology can generate more ^•OH and shows an obvious synergistic effect of electrochemical oxidation and ozonation for wastewater treatment, which can be potentially widely used in wastewater treatment.