Recent Progress in Catalytically Driven Advanced Oxidation Processes for Wastewater Treatment

Abstract

With the increasing severity of global water pollution, traditional wastewater treatment methods have gradually revealed limitations in dealing with complex and refractory pollutants. Advanced oxidation processes (AOPs) have emerged as a promising alternative due to their ability to generate highly reactive radicals (such as hydroxyl and sulfate radicals) that can effectively degrade a wide range of pollutants. This review provides a detailed overview of various AOP technologies, including Fenton processes, ozone-based AOPs, persulfate-based AOPs, photocatalytic AOPs, electrochemical AOPs, and sonochemical AOPs, focusing on their fundamental principles, reaction mechanisms, catalyst design, and application performance in treating different types of wastewater. The research results show that the improved Fenton process can achieve a chemical oxygen demand (COD) removal rate of up to 85% when treating pharmaceutical wastewater. Photocatalytic AOP technology demonstrates higher degradation efficiency when treating industrial wastewater containing refractory pollutants. In addition to effectively degrading refractory pollutants and reducing dependence on traditional biological treatment methods, these advanced oxidation processes can also significantly reduce secondary pollution generated during the treatment process. Moreover, by optimizing AOP technologies, the deep mineralization of harmful substances in wastewater can be achieved, reducing the potential pollution risks to groundwater and soil while also lowering energy consumption during the treatment process. Additionally, this review discusses the challenges faced by AOPs in practical applications, such as high energy consumption, insufficient catalyst stability, and secondary pollution. This review summarizes the research progress and application trends of catalytically driven AOPs in the field of wastewater treatment over the past five years. It aims to provide a comprehensive reference for researchers and engineering professionals on the application of AOPs in wastewater treatment, promoting the further development and practical implementation of these technologies.

1. Introduction

The escalating global water pollution crisis has emerged as a grave threat to environmental sustainability, public health, and socio-economic development [1]. The incessant rise in wastewater discharge from diverse sources, including industrial activities, agricultural runoff, and domestic sewage, has significantly degraded the quality of rivers, lakes, and groundwater reservoirs [2]. With the continuous growth of the global population and the acceleration of industrialization, water scarcity and water pollution have become key factors constraining sustainable development. This alarming situation underscores the urgent need for effective wastewater treatment technologies to mitigate the adverse impacts of water pollution and ensure the availability of clean water resources for future generations.

Over the past few decades, the composition of wastewater pollutants has undergone profound changes, mirroring the evolution of human activities and industrial processes. Traditional wastewater pollutants, such as biodegradable organic matter and suspended solids, are gradually being replaced by a variety of complex and recalcitrant compounds [3]. These emerging pollutants encompass pharmaceuticals and personal care products (PPCPs), endocrine-disrupting chemicals (EDCs), perfluorinated compounds (PFCs), and microplastics, which are characterized by their persistence, bioaccumulation potential, and potential toxicity to aquatic life and human health [4]. The presence of these emerging pollutants in wastewater has further complicated the treatment process, as they are often not effectively removed by conventional treatment methods.

Traditional wastewater treatment processes, which primarily rely on biological degradation and physicochemical separation, have long been the cornerstone of wastewater management. However, these methods are increasingly recognized as inadequate in addressing the challenges posed by the growing complexity of wastewater pollutants (

Table 1. Advantages and disadvantages of current wastewater treatment methods.

Tech. Type	Advantages	Disadvantages
Bio. Process	Low cost; Involves a relatively mild process; Without secondary pollution; Particularly suitable for large-scale treatment.	Slow and less effective on tough organics and toxins; Sensitive to water quality, temp, and pH; Needs lots of space, generates lots of sludge; Expensive to run.
Phys. Chem. Process	High efficiency; Removes heavy metals and other pollutants; Diverse methods for different wastewater types.	High chemical use; Risk of secondary pollution; High operating costs; Limited on complex organics; May need post-treatment.
AOPs	Breaks down tough pollutants efficiently; No secondary pollution; Fast, efficient, and controllable; Works well with other technologies.	High costs due to high oxidant use; Needs strict pretreatment; Challenging for low-concentration, high-flow wastewater.

). Biological treatment processes, such as activated sludge and biofilters, are effective for removing biodegradable organic matter but generally fail to degrade recalcitrant organic compounds [5,6]. Physicochemical methods, including coagulation, flocculation, and adsorption, can remove suspended solids and some organic pollutants but are less effective against dissolved pollutants. Furthermore, these methods may generate large amounts of sludge, leading to secondary pollution and increased disposal costs [7,8]. Additionally, these traditional methods typically require significant energy input and are associated with high operational costs, which further limit their applicability in the context of increasingly stringent environmental regulations [9].

In response to these challenges, advanced oxidation processes (AOPs) have emerged as a highly promising alternative for wastewater treatment [10]. AOPs are based on the generation of highly reactive oxidizing species, primarily hydroxyl radicals (·OH), which possess strong oxidizing capabilities and can effectively degrade a wide range of pollutants through non-selective oxidation pathways [11]. The mechanisms of AOPs involve the formation of hydroxyl radicals through various pathways [12], such as the photolysis of hydrogen peroxide, UV irradiation, and the catalytic decomposition of oxidants in the presence of transition metals or semiconductors. These reactive species can initiate radical chain reactions, leading to the breakdown of complex organic molecules and ultimately their complete mineralization into carbon dioxide, water, and inorganic ions. The advantages of AOPs lie in their broad applicability to various types of wastewater, high oxidation efficiency, rapid reaction kinetics, and the potential for complete degradation of recalcitrant pollutants [13]. These characteristics make AOPs particularly suitable for treating wastewater containing emerging contaminants and other pollutants that are difficult to remove by conventional methods.

In recent years, the oxidation processes predominantly employed in wastewater treatment encompass the Fenton process, ozone-based AOPs, persulfate-based AOPs, photocatalytic AOPs, electrochemical AOPs, and sonochemical AOPs. Moreover, integrating different oxidation processes can enhance treatment efficiency and overcome the limitations of individual AOPs. For example, the combination of UV/H₂O₂ with photocatalysis [14] or the coupling of Fenton’s reagent with ozonation [15] has been shown to yield synergistic effects, thereby increasing pollutant removal rates and reducing energy consumption. These hybrid AOPs leverage the complementary mechanisms of various oxidation processes, such as the generation of additional reactive species or enhanced mass transfer, to optimize overall treatment performance. It is worth noting that the core of these oxidation processes is their capacity to generate reactive oxygen species (ROS), which are highly oxidative substances that can effectively break down organic pollutants in wastewater. The various types of AOPs and the ROS involved in each are depicted in Figure 1 [16].

This review aims to provide a comprehensive overview of the progress and trends in the application of advanced oxidation processes for wastewater treatment. It delves into the fundamental principles, mechanisms, and advantages of different AOPs, as well as their performance in treating various types of wastewater. Moreover, the latest advancements, challenges, and future directions in this field are highlighted, offering valuable insights into the potential of AOPs for sustainable and efficient wastewater treatment in the context of the global water pollution crisis.

2. Research Progress on Major AOPs

2.1. Fenton Advanced Oxidation Process

The Fenton advanced oxidation process is a wastewater treatment technology that relies on the synergistic action of hydrogen peroxide (H₂O₂) and ferrous ions (Fe²⁺). Its principle involves the catalytic decomposition of H₂O₂ by Fe²⁺ to generate highly oxidative hydroxyl radicals (·OH). These radicals subsequently degrade organic pollutants in wastewater (Equation (1)) (Figure 2) [17]. The hydroxyl radicals react with organic matter through processes such as dehydrogenation, addition, and substitution, ultimately mineralizing them into carbon dioxide (CO₂) and water (H₂O) [18].

$$F{e}^{2+}+H_2{O}_2+H^{+}⟶F{e}^{3+}+H_{2}O+\cdot OH$$

(1)

However, conventional Fenton processes are limited by issues such as the production of large amounts of iron sludge, narrow pH applicability, and low H₂O₂ utilization rates. To address these limitations, researchers have developed several modified Fenton processes, including heterogeneous Fenton, photo-Fenton, electro-Fenton, photoelectro-Fenton, and heterogeneous electro-Fenton processes (

Table 2. Advantages and disadvantages of various Fenton processes.

Classification	Advantages	Disadvantages
Heterogeneous Fenton Process	Reduces iron sludge, recyclable catalyst, wider pH applicability	Lower catalyst activity and stability, catalyst surface is prone to fouling
Photo-Fenton Process	Enhances oxidation efficiency, operates over a wider pH range	Requires additional light source, limited light penetration depth
Electro-Fenton Process	High controllability, no need for external H2O2 and Fe2+ addition	Consumes electrical energy, requires improved electrode material stability
Photoelectro-Fenton Process	Efficient degradation of refractory organic matter, high controllability	Complex equipment, high cost
Heterogeneous Electro-Fenton Process	Enhances catalyst stability and activity, reduces iron sludge	Requires optimization of catalyst–electrode synergy

).

Relative to other advanced oxidation processes, Fenton processes boast several advantages, such as mild reaction conditions, straightforward operation, and low cost; moreover, the iron hydroxides generated during the process can function as coagulants [19], facilitating the removal of suspended solids and colloidal particles from wastewater. However, conventional Fenton processes are also limited by their need for acidic conditions, narrow pH applicability, generation of iron sludge, and low H₂O₂ utilization rates [20]. Despite their effectiveness, Fenton and modified Fenton processes are constrained by two universal limitations: radical quenching and mass transfer constraints. Radical quenching occurs when common anions in wastewater, such as chloride ions (Cl⁻) and bicarbonate ions (HCO₃⁻), scavenge hydroxyl radicals (·OH) to form less reactive secondary radicals. The reactions are as follows: (1) The ·OH reacts with Cl⁻ to produce chlorine radicals (Cl·) and hydroxide ions (OH⁻); (2) The ·OH reacts with HCO₃⁻ to produce carbonate radicals (CO₃·⁻) and water (H₂O). This process significantly reduces the efficiency of pollutant degradation. Simultaneously, mass transfer limitations arise in heterogeneous systems. Adsorbed pollutants or iron sludge deposition can form passivation layers on catalyst surfaces, impeding the diffusion of H₂O₂ and pollutants to the active sites.

To address these challenges, the synergistic coupling of photo-Fenton and electro-Fenton processes has shown significant advantages. Ultraviolet irradiation accelerates the reduction of Fe³⁺ to Fe²⁺, thereby enhancing the kinetics of hydroxyl radical generation, while the electrochemical in situ generation of H₂O₂ eliminates the barrier of reagent transportation. This integrated approach synergistically improves the degradation efficiency of wastewater and reduces the production of iron sludge, providing a highly promising strategy to overcome the inherent limitations of traditional Fenton systems.

2.1.1. Heterogeneous Fenton Process

The heterogeneous Fenton process is an advanced oxidation treatment technology that relies on the synergistic action between hydrogen peroxide (H₂O₂) and solid catalysts. The core principle of this process is the efficient catalytic decomposition of hydrogen peroxide by the solid catalyst, which generates highly oxidative hydroxyl radicals (·OH) (Equations (2) and (3)) [21]. Through this catalytic decomposition mechanism, the hydroxyl radicals can rapidly react with refractory pollutants in wastewater, achieving their efficient degradation (Figure 3) [17]. This high efficiency in degradation endows the heterogeneous Fenton process with a significant advantage in treating complex wastewater components, especially in removing organic pollutants that are difficult to handle when using conventional treatment methods, thus demonstrating its unique role and value in addressing challenging wastewater treatment scenarios.

$$F{e}^{3+}+e^{-}⟶F{e}^{2+}$$

(2)

$$F{e}^{2+}+H_2{O}_2⟶F{e}^{3+}+O{H}^{-}+\cdot OH$$

(3)

Common catalysts used in the heterogeneous Fenton process encompass a variety of materials, including iron-based catalysts (such as Fe₂O₃, Fe₃O₄), non-iron metal catalysts (such as CuO, Co₃O₄), carbon-based catalysts (such as activated carbon, carbon nanotubes), and composite catalysts (such as Fe₂O₃/CuO, Fe₃O₄/g-C₃N₄) [22]. These catalysts are prepared through diverse methods, including impregnation, co-precipitation, and hydrothermal synthesis. Their structures and properties play a crucial role in determining the catalytic activity and stability.

In recent years, researchers have been committed to enhancing the treatment efficiency and cost-effectiveness of refractory pollutants through innovative catalytic technologies and process optimization. In 2019, Pramod et al. demonstrated a significant improvement in arsenic removal using a heterogeneous Fenton process with iron-loaded activated carbon (Fe-AC) as the catalyst [23]. The study achieved 90.7% arsenic removal under optimal conditions, exhibiting high efficiency and catalyst reusability, though catalyst deactivation remains a challenge. In 2020, Zhang Nuojin et al. designed oxygen vacancies in FeCeOx catalysts, which successfully accelerated the key reaction steps (Fe³⁺/Fe²⁺ cycle and H₂O₂ activation) [24], in which a high degradation efficiency of 98% for dyes (Rhodamine B) was achieved. Crucially, this process was effective over a wide range of pH values, breaking through the strict dependence of traditional homogeneous Fenton on strongly acidic conditions. In 2021, Ghasemi et al. integrated the advantages of MnO₂, Fe₃O₄, and CuO to achieve synergistic catalysis: MnO₂ enhances broad-spectrum pH applicability, Fe₃O₄ provides magnetism for easy recovery, and CuO boosts catalytic activity [25]. The composite catalyst achieved 90% rapid decolorization of methylene blue within 45 min and excellent cycling stability over six cycles, demonstrating its potential for engineering applications. In 2022, Pinedo-Hernández et al. leveraged the robust adsorption capacity of zeolite to concentrate pollutants near the active sites (Fe/Cu) on its surface for localized and highly efficient catalytic degradation, especially in a UV-Fenton coupled system, achieving a removal rate as high as 95.94% [26]. This composite strategy significantly enhanced catalytic efficiency and the ability to treat recalcitrant pollutants. In 2023, Zhang et al. innovatively coupled foam fractionation with heterogeneous Fenton (FF-HF) for the first time [27]. This novel integration uses foam fractionation to pre-concentrate hydrophobic organics and surfactants, enhancing the subsequent Fenton process to achieve 71.6% DOC removal and reducing total costs by 31.6%. This provides an efficient and economical solution for treating complex industrial wastewaters such as pharmaceutical effluents. In 2024, Li et al. pioneered the large-scale application of Fenton technology in sludge conditioning [28]. The hydroxyl radicals (·OH) generated by Fe₃O₄-driven Fenton reactions disrupted sludge flocs and extracellular polymeric substances, reducing sludge moisture content from 86.4% to 61.3% in 30 min and significantly enhancing dewatering efficiency. The magnetic recyclability of Fe₃O₄ ensured environmental sustainability. This marked a significant expansion of Fenton technology into new application domains. Zhou et al. also chose boron carbide as a carrier material in 2024 [29]. Its excellent electronic conductivity enhanced the interfacial electron transfer rate during catalysis, doubling the tetracycline hydrochloride degradation efficiency compared with pure Fe₃O₄. Despite needing improved cyclic stability, this highlights the potential of novel carbon carriers in boosting catalytic performance. Collectively, these studies underscore the potential of heterogeneous Fenton processes to treat a variety of contaminants, with ongoing efforts to enhance catalyst performance and sustainability.

The heterogeneous Fenton process offers several distinct advantages compared with the traditional Fenton process. These include a broader reaction pH range, significantly reduced generation of iron sludge, and avoidance of secondary pollution. Additionally, the catalysts used in the heterogeneous process are easily separable and recyclable, which helps to lower overall treatment costs. However, some limitations persist, such as the need to improve the activity and stability of some catalysts, and the requirement for higher catalyst dosage when treating high-concentration organic wastewater. Compared with photocatalytic oxidation, the heterogeneous Fenton process is less reliant on light sources, thus offering a broader range of applications. However, photocatalytic oxidation can achieve higher oxidative efficiency under specific conditions. The heterogeneous Fenton process has simpler equipment requirements and lower costs than ozonation; however, ozonation has stronger oxidative capacity and faster reaction rates.

The heterogeneous Fenton advanced oxidation process holds significant potential for wastewater treatment applications. By catalytically decomposing H₂O₂, this process efficiently degrades refractory pollutants, offering advantages such as mild reaction conditions and simple operation. However, further optimization of catalyst performance and reaction conditions remains necessary to enhance the economic viability and efficiency of its large-scale application.

2.1.2. Photo-Fenton Process

The photo-Fenton process is an efficient, advanced oxidation technology. Its core mechanism involves catalyzing the decomposition of hydrogen peroxide (H₂O₂) by iron ions (Fe²⁺/Fe³⁺) to generate highly oxidative hydroxyl radicals (·OH) (Figure 4) [17]. The introduction of ultraviolet or visible light significantly enhances the generation efficiency of ·OH (Equation (4)). Specifically, light irradiation facilitates the direct photolysis of H₂O₂ to produce ·OH. Simultaneously, it promotes the reduction of Fe³⁺ to Fe²⁺, thereby improving the utilization efficiency of H₂O₂ (Equation (5)) [30]. Hydroxyl radicals, with an oxidation–reduction potential of 2.8 V, can non-selectively attack pollutants. The process breaks them down into smaller intermediate molecules and ultimately mineralizes them into carbon dioxide (CO₂) and water (H₂O). To enhance this process, researchers have developed various catalysts, including homogeneous, heterogeneous, and composite catalysts [31]. The development of these catalysts not only improves reaction efficiency but also reduces the leaching of iron ions and the formation of iron sludge.

$$F{e}^{3+}+H_{2}O\stackrel{hv}{⟶}F{e}^{2+}+\cdot OH+H^{+}$$

(4)

$$F{e}^{3+}+H_2{O}_2\stackrel{hv}{⟶}F{e}^{2+}+2\cdot OH+H^{+}.$$

(5)

Recent research has focused on enhancing the treatment efficiency and cost-effectiveness of refractory pollutants through innovative photo-Fenton technologies and process optimization. In 2020, Rodrigo Poblete et al. treated grape juice wastewater using high concentrations of H₂O₂ and Fe²⁺ with UV irradiation, achieving 70.2% chemical oxygen demand (COD) removal and 36.1% polyphenol elimination, highlighting the energy efficiency of solar UV irradiation [32]. In 2021, Gou et al. developed a MoS₂-modified iron-based composite catalyst (MoS₂@Fe), aiming to combine the excellent catalytic properties of transition metal disulfides (MoS₂) with the photo-Fenton activity of iron elements, significantly enhancing catalytic efficiency (73.10% removal rate and a rate constant of 0.0078 min⁻¹) [33], which displayed an innovation in catalyst design and engineering. In 2022, Djedjiga Bousalah et al. achieved 100% dye removal in just 10 min [34]. This remarkably efficient result was explicitly attributed to the fact that light exposure significantly enhanced the generation of additional ·OH, far exceeding the efficiency achievable by the conventional Fenton process, which highlighted the decisive role of “photoactivation” in the photo-Fenton process for the rapid and complete degradation of pollutants. In 2023, Parra-Enciso et al. degraded carbamazepine with 96.4% removal efficiency, though mineralization was below 25% and matrix effects impacted degradation [35]. In 2024, Da Silva et al. combined photo-Fenton oxidation with activated carbon adsorption to treat complex dairy wastewater [36]. This combined strategy effectively integrated the advantages of advanced oxidation and physical adsorption, achieving a 79% reduction in COD and an overall removal rate of organic pollutants as high as 99%. This represented an effective technological breakthrough in process integration for practical wastewater treatment. Additionally, in 2024, Lin Tian et al. used goethite as a photo-Fenton catalyst, avoiding traditional soluble iron salts or complex catalysts [37]. Achieving an 89.7% removal rate, the study revealed that defluorination and ring cleavage were key pathways for enrofloxacin degradation. This showed that the method could remove pollutants and destroy their core structures—particularly fluorine-containing groups and parent rings—which is crucial for treating fluorinated drugs and preventing the accumulation of toxic fluoride intermediates. These studies collectively highlight the potential of the photo-Fenton process in wastewater treatment, with ongoing efforts to address challenges and enhance its efficiency and applicability.

Compared with other advanced oxidation processes, the photo-Fenton process offers distinct advantages, including high efficiency, environmental friendliness, broad pH adaptability, and operational simplicity. However, this technology also faces several challenges. For example, traditional homogeneous Fenton reactions produce a large amount of iron sludge, increasing treatment costs; the utilization rate of hydrogen peroxide still needs to be further improved; and the recovery and reuse of heterogeneous catalysts also pose challenges. Thus, the photo-Fenton process, despite being an advanced oxidation technology with broad application prospects, still requires in-depth research on catalyst performance optimization and reaction condition improvement to overcome the existing technological limitations and achieve large-scale industrial application.

2.1.3. Electro-Fenton Process

The electro-Fenton process (EFP) is an advanced oxidation technology based on electrochemical principles and Fenton reactions. It generates hydrogen peroxide (H₂O₂) and ferrous ions (Fe²⁺) in situ through electrode reactions, triggering the Fenton reaction to produce highly oxidative hydroxyl radicals (·OH) for the degradation and mineralization of pollutants in wastewater. Based on the addition or formation of Fenton reagents, the EFP can be classified into four types: cathodic electro-Fenton process (EF-H₂O₂), sacrificial anode electro-Fenton process (EF-Feox), Fe²⁺ recycling electro-Fenton process (EF-Fere), and combined cathodic and Fe²⁺ recycling electro-Fenton process (EF-H₂O₂-Fere) (Figure 5) [17,38].

In the cathodic electro-Fenton process (EF-H₂O₂), H₂O₂ is generated in situ via cathodic reduction and reacts with added or in situ-generated Fe²⁺ to produce oxidative hydroxyl radicals (Equations (3) and (6)), which oxidize and degrade pollutants. The sacrificial anode electro-Fenton process (EF-Feox) utilizes the oxidation and dissolution of iron anodes to generate Fe²⁺ in situ (Equations (3) and (7)), while H₂O₂ is generated at the cathode; both participate in the Fenton reaction, eliminating the need for external Fe²⁺ addition and reducing reagent usage and secondary pollution. The Fe²⁺ recycling electro-Fenton process (EF-Fere) involves the electrochemical reduction of Fe³⁺ to Fe²⁺ (Equations (2) and (3)), recycling Fe²⁺ to improve its utilization efficiency, reduce the dosage of iron salts, and minimize iron sludge formation. The combined cathodic and Fe²⁺ recycling electro-Fenton process (EF-H₂O₂-Fere) integrates the advantages of cathodic H₂O₂ generation and Fe²⁺ recycling, further enhancing the efficiency and cost-effectiveness of the Fenton reaction (Equations (2), (3) and (6)).

$$O_2+2H^{+}+2e^{-}⟶H_2{O}_2$$

(6)

$$Fe⟶F{e}^{2+}+2e^{-}$$

(7)

Common catalysts used in the electro-Fenton process include homogeneous catalysts (mainly Fe²⁺) [39] and heterogeneous catalysts [40]. The former have limitations such as iron sludge formation and narrow pH applicability, while the latter exhibit good catalytic performance, reducing iron sludge formation and broadening the pH range of application. Accelerated development of the electro-Fenton process has been witnessed in recent years, yielding significant advancements in pollutant mineralization efficiency for industrial wastewater applications.

In 2019, Nizar Klidi et al. treated paper mill wastewater using electro-Fenton with a BDD anode, achieving 85% total organic carbon (TOC) removal; however, there were limitations in oxygen solubility and mass transfer, resulting in increased operational costs [41]. In 2020, Shen et al. developed Pd-Fe nanoalloy cathode materials that achieved 100% removal and dechlorination of pollutants within 6 h, highlighting the superior performance of noble metal nano-materials in the electro-Fenton reaction [42]. However, the high cost of palladium limits its widespread application. This study not only provided new ideas for the treatment of chlorinated pollutants, but also laid the foundation for the development of cost-effective noble metal alternatives. In 2021, Shuai Zhang et al. converted complex lignin into valuable long-chain fatty acids using a three-phase 3D electro-Fenton process, achieving high conversion rates and environmental benefits [43]. This work represented a strategic shift from pollution elimination to waste valorization and exhibited a significant breakthrough in resource recovery using advanced oxidation technologies. In 2022, Guo et al. developed a self-supported CFP@CoFe₂O₄ composite electrode that simplified reactor design and reduced interfacial resistance [44]. It achieved nearly 100% pollutant removal and 49% TOC removal in 2 h, with low energy use and great stability, where the electrode was a key breakthrough for practical use. In 2023, Arias et al. applied electro-Fenton to remove BTX from gaseous streams, significantly enhancing the removal of volatile organic compounds (VOCs) [45]. In 2024, Jia et al. developed a bimetallic Fe-Mn electrode material (FeMn@GF) on graphite felt that achieved a 98.9% removal rate for Levofloxacin [46]. The key innovation was the extremely low metal ion leaching (below 0.05 mg/L), which addressed the issues of iron sludge and secondary pollution. This enhanced the practicality and sustainability of the process and offered new directions for the development of electro-Fenton technology. These studies demonstrate the electro-Fenton process’s capacity for treating various pollutants, though sustained research is required to refine operational parameters, enhance electrode efficiency, and achieve cost-effective implementation.

In contrast to conventional advanced oxidation processes, the electro-Fenton process possesses several remarkable advantages. It generates H₂O₂ and Fe²⁺ in situ, reducing the need for external reagents and secondary pollution; additionally, the highly oxidative nature of hydroxyl radicals, which are produced in abundance during the process, ensures the efficient degradation of recalcitrant pollutants. The process is characterized by its simple equipment requirements and mild operating conditions, eliminating the need for high temperatures and pressures. However, the process also faces certain challenges; for instance, it currently exhibits low current efficiency, with the generation efficiency of H₂O₂ and overall current efficiency still needing significant improvement. The Fenton process typically operates under acidic conditions, thereby imposing stringent pH requirements on wastewater treatment. Additionally, the relatively high energy consumption can increase treatment costs. Despite these challenges, the electro-Fenton process shows great potential in treating refractory wastewaters. However, further optimization is needed to enhance its economic viability and broaden its applicability.

2.1.4. Photoelectro-Fenton Process

The photoelectro-Fenton process (PEF) is an advanced oxidation technology that seamlessly integrates photocatalysis with electrochemical Fenton reactions. By synergistically harnessing light and electrical energy, it dramatically boosts the generation efficiency of hydroxyl radicals (·OH). This enhanced production of highly reactive radicals enables the efficient degradation of recalcitrant pollutants in wastewater (Figure 6) [47]. The principle of PEF is based on the coupling of electrochemical and photocatalytic reactions. On the electrode surface, hydrogen peroxide (H₂O₂) and ferrous ions (Fe²⁺) are generated through electrochemical reactions (Equations (6) and (7)). Simultaneously, photocatalysts produce reactive oxygen species under UV or visible light irradiation (Equations (4) and (5)). The synergistic action of these two processes increases the production and reaction rate of ·OH, achieving deep oxidation and mineralization of pollutants [48].

The application of the photoelectro-Fenton process for wastewater treatment has witnessed remarkable advancements in recent years. Research has focused on developing efficient photoelectrode materials [49], such as boron-doped diamond (BDD) electrodes and TiO₂-based photocatalyst electrodes, with the aim of significantly enhancing the generation efficiency of H₂O₂ and the utilization efficiency of photogenerated carriers. Moreover, efforts to optimize reaction conditions (such as pH, current density, and light intensity) and couple with other technologies, such as ultrasound and microwave, have further enhanced the process’s effectiveness. In 2019, Bin Ou et al. developed a g-C₃N₄-modified graphite felt cathode that achieved rapid aniline degradation and a high mineralization rate (98.3% TOC removal after 360 min) [50]. This near-complete conversion of pollutants to CO₂ and H₂O outperforms most photoelectro-Fenton methods, highlighting the potential of non-metal-modified electrodes for the deep oxidation of organic pollutants. In 2020, Thiam et al. used natural pyrite (FeS₂) as a catalyst, replacing expensive synthetic catalysts or metal salts [51]. This method achieved complete thiamphenicol removal within 60 min and an 85% TOC removal rate after 360 min. It demonstrated the high efficiency of natural minerals and provided a practical breakthrough due to their wide availability and low cost. In 2021, Bury et al. achieved 98% decolorization of methyl orange at pH 3 and 100 mA current density using an activated carbon fiber cathode [52]. In 2022, Du et al. utilized a UV-coupled heterogeneous photoelectro-Fenton process to degrade sulfamethazine, achieving 73.3% TOC removal and 58.7% mineralization efficiency in 60 min [53]. In 2023, Vigil-Castillo et al. introduced sunlight into a PEF system, achieving a 98% mineralization rate for asulam in just 180 min under natural sunlight [47]. This remarkable efficiency not only leveraged renewable energy and reduced energy consumption but also represented a significant breakthrough in complete pollutant elimination due to its near-total mineralization, which demonstrated the high efficiency of the technology in practical solar applications. In 2024, Cardoso et al. compared EF and SPEF for treating phenacetin [54]. SPEF exhibited lower degradation (55.9%) but significantly lower energy use (0.142 kWh/g) compared with EF, where the use of solar energy in SPEF boosted efficiency. This study proved SPEF’s energy savings over EF, advancing solar-driven advanced oxidation technology. These studies highlight the potential of the photoelectro-Fenton process for treating recalcitrant organic pollutants, with ongoing efforts to optimize conditions and materials for higher efficiency and cost reduction.

Compared with the traditional Fenton process, the photoelectro-Fenton process enhances ·OH generation via photocatalysis, reducing reagent use and secondary pollution. It also stabilizes H₂O₂ production, improving efficiency. However, it faces challenges such as high carrier recombination rates and costly photoelectrode materials. Compared with ozonation, it simplifies operation and boosts mineralization efficiency; however, it requires acidic conditions. Compared with photocatalysis, it generates H₂O₂ in situ, enhancing degradation of high-concentration pollutants. However, it has higher equipment complexity and energy consumption.

The photoelectro-Fenton process has significant advantages in treating refractory wastewaters; however, its economic viability and universality still require further optimization. Future research directions should focus on developing efficient and low-cost photoelectrode materials, optimizing reaction conditions, and exploring synergistic applications with other technologies to enhance its practical value in wastewater treatment.

2.1.5. Heterogeneous Electro-Fenton Process

The heterogeneous electro-Fenton advanced oxidation process is a wastewater treatment technology that seamlessly integrates electrochemistry with the Fenton reaction. Its core principle is fundamentally based on the Haber–Weiss cycle mechanism. Within this process, the surface-induced decomposition of electrogenerated hydrogen peroxide (H₂O₂) is predominantly catalyzed by iron species residing on the catalyst surface ($\equiv$ indicated by a triple bond) (Figure 7) [55]. Under acidic working conditions, the decomposition of H₂O₂ on the surface of the solid catalyst can be simplified as follows (Equations (3) and (8)): the surface iron species (denoted by Fe²⁺) efficiently catalyze the decomposition of H₂O₂ to produce highly reactive hydroxyl radicals (·OH) while being oxidized to Fe³⁺ themselves (Equation (8)). The heterogeneous electro-Fenton process may also involve the release of dissolved Fe³⁺/Fe²⁺ from iron-functionalized cathodic materials or solid catalysts (Equation (3)). Ultimately, the hydroxyl radicals attack pollutants, mineralizing them into water (H₂O) and carbon dioxide (CO₂) [55]. This process is especially suitable for treating refractory wastewater, such as textile wastewater, pharmaceutical wastewater, petrochemical wastewater, and papermaking wastewater. These types of wastewater typically contain high concentrations of pollutants that are notoriously difficult to degrade using traditional biological treatment methods.

$$\equiv F{e}^{2+}+H_2{O}_2⟶\equiv F{e}^{3+}+\cdot OH+O{H}^{-}$$

(8)

Common catalysts include supported catalysts, such as Fe²⁺ loaded on magnetic porous carbon microspheres; unsupported catalysts, such as natural iron-containing minerals or synthesized iron oxides; and composite catalysts, such as carbon-based materials combined with iron [56].

In recent years, researchers have been working to optimize the heterogeneous electro-Fenton process by developing new catalysts, refining reaction conditions, and integrating complementary technologies, such as photocatalysis and ultrasonication. These efforts have not only enhanced the efficiency and cost-effectiveness of wastewater treatment, but also addressed key challenges such as iron sludge formation and catalyst deactivation. Additionally, the broadened pH range and reduced need for acid adjustment have further improved the practicality and economic viability of the technology.

In 2020, Luo et al. developed Cu-doped Fe/Fe₂O₃ core–shell nanoparticles that integrate Fe⁰ reduction, Fe₂O₃ Fenton activity, and Cu co-catalysis. This catalyst achieved 98.1% tetracycline degradation in 2 h and 89.8% mineralization in 6 h, with high stability (reusable multiple times) [57]. Campos et al. also used Fe/Cu bimetallic nanoparticles (BNPs) as a catalyst in 2020, achieving 100% degradation of Nafcillin in just 7 min. This ultra-fast rate was rare in advanced oxidation processes, especially for antibiotics, and represents a major breakthrough in kinetics. This work provided a highly efficient solution for antibiotic wastewater treatment and paved new directions for advanced oxidation processes [58]. In 2021, Wang et al. developed a flow-through heterogeneous electro-Fenton system using an absorbent cotton-derived electrode with iron oxide nanoparticles, achieving 92.7% phenol degradation and 65.1% mineralization within 3 h [59]. In 2022, Zhao et al. investigated the enhancement of pyrite-catalyzed heterogeneous Fenton oxidation for ciprofloxacin degradation using activated carbon, biochar, and carbon nanotubes, finding that these materials significantly accelerated the degradation process [60]. In 2023, Camcıo˘glu et al. used magnetic nanocomposites as electro-Fenton catalysts to mineralize busulfan, achieving near 100% mineralization across a broad pH range (3–9), which was a significant improvement over traditional Fenton processes that typically require acidic conditions (pH 2–4). The ability to achieve high mineralization rates in neutral to weakly alkaline conditions (pH 6–9) enhanced the technology’s adaptability and eliminated the need for strong acid adjustment, representing a major step towards practical application [61]. In 2024, Li et al. developed Fe-GAC as a catalyst. Using 4 h of low-voltage electrolysis (8V) at pH 6, the process achieved 95.7% COD removal, 99% color removal, and 94.9% total nitrogen removal with very low energy use (0.098 kWh/g COD) [62]. This showed the high efficiency of heterogeneous electro-Fenton technology in treating challenging wastewater without pH adjustment and with minimal energy, marking a major practical breakthrough.

Compared with traditional Fenton processes, the heterogeneous electro-Fenton process avoids the transportation and storage risks associated with external H₂O₂ addition and reduces iron sludge production through the electrochemical regeneration of Fe²⁺. Compared with ozonation, this process operates under milder conditions without the need for complex equipment or high-voltage power supplies. Compared with photocatalytic oxidation, it has lower dependence on light sources and a broader range of applicability; however, this process has higher requirements for the stability of electrode materials and catalysts, and energy consumption and equipment costs remain limiting factors for large-scale applications. Overall, the heterogeneous electro-Fenton advanced oxidation process shows great potential in the treatment of refractory organic wastewater, but still requires further optimization to reduce costs and enhance its industrial feasibility.

2.2. Ozone-Based Advanced Oxidation Processes

Ozone-based advanced oxidation processes (ozone-based AOPs) are wastewater purification technologies that leverage the direct or indirect action of ozone (O₃) on pollutants in water (Figure 8) [63,64]. The core principle of these processes hinges on the robust oxidizing power of ozone and the highly reactive hydroxyl radicals (·OH) generated during its decomposition [65]. Boasting an oxidation potential as high as 2.80 V, hydroxyl radicals are capable of efficiently degrading pollutants. The catalytic principle of ozone-based AOPs primarily relies on the activation of ozone by catalysts. Catalysts offer active sites or modify reaction pathways, significantly reducing the activation energy required for ozone decomposition and thereby promoting the generation of ·OH.

Common ozone-based advanced oxidation processes include UV/ozone (UV/O₃), ozone/hydrogen peroxide (O₃/H₂O₂), catalytic ozone, photocatalytic ozonation, sonolytic ozonation, and the O₃/Fenton process (Figure 9) [66]. Each process has its unique characteristics: UV/O₃ generates ·OH through the photolysis of ozone by UV light; O₃/H₂O₂ produces ·OH directly through chemical reactions; and catalytic ozone employs solid catalysts to achieve efficient ozone activation [67]. This review mainly describes the processes of UV/O₃, O₃/H₂O₂, and catalytic ozone. Research on ozone-based AOPs in the field of wastewater treatment has made significant progress in recent years, with a focus on the development of new catalysts, in-depth elucidation of reaction mechanisms, and process optimization. The combination processes of O₃/H₂O₂, UV/O₃, and catalytic ozone have demonstrated excellent performance in treating high-concentration and refractory wastewater (such as pharmaceutical and coke wastewater). These processes can effectively reduce the chemical oxygen demand (COD) and biochemical oxygen demand (BOD) of wastewater while enhancing its biodegradability [68].

Compared with other advanced oxidation processes, ozone-based AOPs boast several notable advantages, including high degradation efficiency, environmental friendliness, and broad applicability. However, ozone-based AOPs are limited by inefficient mass transfer due to low ozone solubility and limited diffusion in water. This hinders effective contact between ozone and pollutants, reducing reaction kinetics and treatment efficiency. Radical quenching is also a major challenge, as inorganic substances such as bicarbonate ions (HCO₃⁻) and chloride ions (Cl⁻), and natural organic matter in wastewater can consume radicals such as ·OH, weakening their oxidative capacity and causing incomplete mineralization of refractory compounds. To address these issues, integrating ozone with complementary processes can significantly improve performance; for example, the O₃/H₂O₂ process enhances radical generation through peroxide-driven decomposition, while the UV/O₃ process boosts ozone conversion to ·OH via photolysis. Catalytic ozonation using solid catalysts such as transition metal oxides or activated carbon can accelerate ozone activation, reduce radical quenching, broaden the operational pH range, and enhance mass transfer efficiency. These strategies not only increase degradation rates but also optimize energy use and minimize secondary pollution, expanding the practical application of ozone-based AOPs in complex wastewater matrices.

2.2.1. Ozonation and UV Radiation

The integration of ozonation with UV radiation (UV/O₃), as an emerging water treatment technology, is distinguished by its cleanliness, high efficiency, low energy consumption, and the operating procedure under mild conditions. The specific wavelength of UV light supplies the energy required for photochemical reactions, facilitating the selective oxidation of organic matter without the need for additional catalysts. The combination of UV/O₃ has been extensively utilized as an efficient catalytic system for the degradation of refractory pollutants in wastewater. The core mechanism centers on the photolysis of ozone under UV irradiation, which generates highly oxidative hydroxyl radicals (·OH). These hydroxyl radicals are primarily formed from the reaction of oxygen radicals (·O) produced by ozone decomposition with water molecules. The synergistic interaction between UV and ozone not only directly accelerates the decomposition of ozone (Equations (9)–(11)), but also indirectly generates a greater quantity of ·OH (Equations (12) and (13)). This dual action significantly enhances the efficiency of the entire treatment process [66].

$$O_3\stackrel{uv}{⟶}{O}_2+\cdot O$$

(9)

$$\cdot O+H_{2}O⟶2\cdot OH$$

(10)

$$2\cdot O+H_2⟶2\cdot OH⟶H_2{O}_2$$

(11)

$$O_3+H_{2}O⟶O_2+H_2{O}_2$$

(12)

$$H_2{O}_2⟶2\cdot OH$$

(13)

In aqueous solutions, ·OH can target the aromatic ring structures of dye molecules, breaking them down into smaller aliphatic molecules such as organic acids, aldehydes, and ketones [69]. The presence of UV light further increases the number of ·OH, thereby accelerating the degradation of refractory dyes and significantly enhancing decolorization efficiency.

In recent years, researchers have been optimizing advanced oxidation technologies to address key wastewater treatment challenges, including degrading refractory pollutants in high-salinity water, reducing membrane fouling in textile wastewater, and scaling up these technologies for practical applications. Their innovations have enhanced degradation efficiency, reduced environmental risks, and provided critical support for real-world wastewater treatment. In 2019, Hui Zhang et al. investigated the degradation of clofibric acid (CA) using UV/O₃ processes, achieving the highest CA removal rate (0.21 min⁻¹) and mineralization efficiency, surpassing UV photolysis and ozonation alone [67]. The study identified intermediate products and proposed degradation pathways, while also reducing the acute toxicity of the reaction solution to Daphnia magna. In 2021, Liu et al. successfully treated high-salinity ciprofloxacin wastewater, achieving 91.4% DOC removal [70]. The process was driven by singlet oxygen (¹O₂) and superoxide radicals (O₂⁻), rather than hydroxyl radicals (·OH), revealing a new degradation pathway. It also had a broad pH range (pH 3–9) and significantly reduced toxicity, addressing key challenges in high-salinity wastewater treatment. In 2022, Ch. Tahir Mehmood et al. innovatively coupled UV/O₃ advanced oxidation with ceramic membrane separation to treat the effluent from anaerobic (UASB) treatment of textile wastewater [71]. This approach effectively decomposed membrane pollutant precursors and mitigated fouling, addressing common membrane pollution issues in membrane bioreactors. The process achieved a remarkable 94% decolorization rate, making it highly suitable for reducing the high color intensity in textile wastewater. In 2023, Wang et al. reported the first pilot-scale UV/O₃ pressurized process study, providing evidence for engineering scale-up [72]. The pressurized system enhanced ozone solubility and mass transfer efficiency, overcoming limitations in traditional ozonation processes. The process achieved 85% decolorization and 43.2% removal of chemical oxygen demand (measured using the potassium dichromate method, CODCr) in high-salinity textile wastewater, proving its feasibility and stability for treating complex wastewater. In 2024, Wahyu Zuli Pratiwi et al. achieved 98.64% degradation efficiency and 76.51% TOC removal. Using RSM for the first time, they optimized UV/O₃ process parameters (pH, O₃ concentration, UV intensity, reaction time), providing precise operational guidelines [73]. The complete degradation pathway of ciprofloxacin was proposed, and the effective reduction of environmental risk through mineralization was confirmed by quantitatively assessing toxicity and transformation product evolution. This marks a systematic breakthrough from process optimization to comprehensive environmental risk control.

Compared with other advanced oxidation processes, the UV/O₃ process has the following advantages: First, the reaction conditions are mild, without the need for high temperature and pressure, making the operation simple and safe. Second, the synergistic effect of UV and ozone can efficiently generate hydroxyl radicals, resulting in high degradation efficiency and significant mineralization of refractory organic pollutants. Third, no additional chemical reagents are required in this process, avoiding secondary pollution. However, some disadvantages also exist for the UV/O₃ process: the service life of UV lamps is limited and they need to be replaced regularly, which increases maintenance costs. In addition, energy consumption is relatively high for high-concentration organic wastewater, and its economic feasibility requires further optimization. The UV/O₃ process, characterized by its high efficiency, environmental friendliness, and simple operation, has obtained increasing attention in the field of wastewater treatment.

2.2.2. O₃/H₂O₂ Process

The O₃/H₂O₂ process for wastewater treatment is an advanced oxidation technology. Its principle involves using hydrogen peroxide to catalyze the decomposition of ozone (O₃), thereby generating highly reactive hydroxyl radicals (·OH). These hydroxyl radicals possess an extremely high redox potential (approximately 2.80 V), enabling them to rapidly react with pollutants in wastewater. This interaction degrades the pollutants into smaller molecules, which are ultimately mineralized into carbon dioxide and water. The primary reaction processes encompass the dissociation of ·OH (Equation (14)), the reaction of ozone with hydroperoxyl ion (HO₂⁻) (Equation (15)), and the chain reaction of hydroxyl radicals with organic substances. This combined treatment method is more effective under neutral and alkaline conditions, as H₂O₂ is more easily dissociated to form HO₂⁻, which promotes the generation of ·OH [64]. However, under acidic conditions, the dissociation of H₂O₂ is inhibited, reducing the formation of ·OH. Moreover, an appropriate amount of H₂O₂ can significantly increase the degradation rate of chemical oxygen demand (COD); however, an excess of H₂O₂ can scavenge hydroxyl radicals, reducing their oxidizing effect.

$${\mathrm{H}}_2{O}_2⟶H{O}_2^{-}+H^{+}$$

(14)

$$O_3+H{O}_2^{-}⟶2O_2+\cdot OH$$

(15)

Researchers have recently optimized O₃/H₂O₂ to enhance wastewater treatment, improve biodegradability, and control microplastic behavior. These efforts have led to significant progress in treating municipal wastewater, landfill leachate, and paper mill black liquor, providing new strategies for managing plastic pollution. In 2020, Piras et al. conducted the first pilot-scale validation of the engineering value of O₃/H₂O₂ for the advanced treatment of municipal wastewater [74]. The study verified the effectiveness of O₃/H₂O₂ as a tertiary pretreatment for actual municipal sewage. It achieved an 85% removal rate for micropollutants and significantly enhanced the biodegradability of the effluent and dissolved oxygen levels, creating favorable conditions for subsequent biological treatment. In 2021, Wang et al. explored the Fe-O₃/H₂O₂ process for treating SAARB leachate, achieving 63.55% removal of aromatic substances and 36.45% TOC reduction in 20 min, which was significantly higher than that of single O₃ or O₃/H₂O₂ [75]. Additionally, Belé et al. first investigated the surface modification of PE/PP/PS microplastics (MPs) by O₃/H₂O₂ in 2021 [76]. The study confirmed that oxidation introduced carbonyl (-C=O) and hydroxyl (-OH) groups on the MPs’ surface, changing them from hydrophobic to hydrophilic and permanently altering their pollutant adsorption behavior. This provided a new approach for controlling the environmental behavior of MPs and managing plastic pollution. In 2022, Pastor et al. explored the O₃/H₂O₂ process for removing organic compounds from Bayer liquor, achieving 19% organic matter mineralization at 80 °C with 6.5 g/L of O₃ and 0.05 mol/L of H₂O₂, although higher temperatures reduced ozone solubility [77]. In 2023, Wang et al. compared single ozonation, catalytic ozonation (with a catalyst), and O₃/H₂O₂ treatment of landfill leachate [78]. The study found that catalytic ozonation primarily dominated the direct oxidation pathway, while O₃/H₂O₂ mainly drove the indirect oxidation pathway via ·OH, which had a higher radical yield. The research clarified that O₃/H₂O₂ achieved a 50% COD removal rate and better removal of micropollutants, providing a theoretical basis for precise process selection. In 2024, Wulansarie et al. combined activated carbon adsorption with O₃/H₂O₂ AOPs to reduce BOD in tofu liquid waste, achieving a 64% BOD reduction, though limitations in ozone production capacity remained [79]. These studies highlight the effectiveness of the O₃/H₂O₂ treatment across a wide range of applications; however, continued efforts are necessary to optimize its performance and address operational challenges.

In studies, the combined O₃/H₂O₂ treatment has shown significant degradation efficiency in removing organic pollutants from wastewater. In practical applications, this technology has been successfully applied for the treatment of coking wastewater and leather wastewater, significantly improving the biodegradability of wastewater and reducing pollutant concentrations. In addition, the combined O₃/H₂O₂ treatment can be used in combination with other technologies (such as moving bed biofilm reactors (MBBR) [80] and ultraviolet (UV) [81]) to further improve wastewater treatment efficiency. However, this technology also faces some challenges, such as the need to precisely control the dosage of H₂O₂ to avoid quenching of hydroxyl radicals and the need to adjust the pH value according to the nature of the wastewater to increase the generation efficiency of ·OH.

2.2.3. Catalytic Ozonation

Catalytic ozonation is an advanced oxidation process that has garnered considerable attention due to its high efficiency in degrading pollutants in wastewater treatment. The principle of catalytic ozonation centers on the decomposition of ozone (O₃) into highly reactive hydroxyl radicals (·OH) by catalysts. These radicals, with their strong oxidizing capabilities, can break down refractory organic compounds into smaller, more biodegradable intermediates which are ultimately mineralized into carbon dioxide and water.

The mechanism of catalytic ozonation can be primarily ascribed to two pathways (Figure 10) [82]. Heterogeneous catalysis occurs in the presence of solid catalysts, such as transition metal oxides (MnO₂, Fe₂O₃) or activated carbon, which provide active sites to facilitate the activation of ozone molecules. This process enhances the generation of hydroxyl radicals through surface-mediated reactions. In contrast, homogeneous catalysis involves the use of soluble catalysts that promote the decomposition of ozone in the aqueous phase, thereby increasing the production of free radicals.

Homogeneous Catalytic Ozonation

The principle of homogeneous catalytic ozonation for wastewater treatment is based on dissolving catalysts directly in the aqueous phase to promote the decomposition of ozone (O₃) into hydroxyl radicals (·OH) with strong oxidizing power. These radicals can efficiently degrade organic pollutants in wastewater into more biodegradable intermediates, which are ultimately mineralized into carbon dioxide and water. The mechanism primarily involves the interaction between dissolved catalysts and ozone molecules, enhancing the generation of hydroxyl radicals through chain reactions to achieve efficient degradation of organic pollutants [82]. For example, Pingfeng Fu et al. investigated the homogeneous catalytic ozonation process for treating aniline aerofloat (AAF) in flotation wastewater. The study found that using transition metal ions—particularly Fe²⁺—as catalysts significantly enhanced the degradation efficiency and mineralization of AAF while reducing energy consumption. Moreover, the addition of Fe²⁺ altered the by-product formation pathways, preventing the generation of toxic aniline and phenol [83]. Fe(II) has been shown to significantly enhance the efficiency of ozonation. In the O₃/Fe(II) process, ferrous ions can promote the decomposition of ozone, leading to the formation of hydroxyl radicals (Equations (16)–(18)).

$$F{e}^{2+}+O_3⟶Fe{O}^{2+}+\cdot O$$

(16)

$$Fe{O}^{2+}+H_{2}O⟶F{e}^{3+}+\cdot OH+O{H}^{-}$$

(17)

$$Fe{O}^{2+}+F{e}^{2+}+2H^{+}⟶2F{e}^{3+}+H_{2}O$$

(18)

Compared with other advanced oxidation processes, homogeneous catalytic ozonation has the following advantages: mild reaction conditions without the need for high temperature and pressure; simple operation; efficient generation of ·OH with significant mineralization of refractory organic pollutants; no need for additional solid catalysts, thereby reducing the risk of secondary pollution. However, the process also has some disadvantages: the catalyst is prone to loss in the aqueous phase, resulting in poor reusability; some metal ion catalysts may remain in the wastewater, posing a potential risk of secondary pollution; moreover, the preparation and addition costs of the catalyst are relatively high, limiting its large-scale application.

Heterogeneous Catalytic Ozonation

In heterogeneous catalytic ozonation systems, the catalysts are solid and can be more easily separated from the solution compared with homogeneous systems. There are three potential mechanisms for heterogeneous catalytic ozonation (Figure 11) [82]. The adsorptive capacity of the catalyst is crucial because at least one reactant must be adsorbed onto the catalyst surface. Accordingly, catalysis occurs when one of the following scenarios takes place: ozone is adsorbed onto the catalyst surface; pollutant molecules are adsorbed onto the catalyst; or both ozone and pollutants are adsorbed onto the surface.

In recent years, researchers have been developing new catalytic ozonation technologies to enhance wastewater treatment efficiency. Breakthroughs include neutral pH catalysis, highly toxic wastewater degradation, and refractory pollutant removal. These advances have improved removal rates and catalyst stability, boosting the engineering application of catalytic ozonation. However, with more complex industrial wastewater, further exploration of new catalysts and processes is needed. In 2019, Chen and Wang developed a Fe₃O₄/Co₃O₄ bimetallic catalyst that achieved a 60% TOC removal rate under neutral pH (7.0) through the redox synergy of Co³⁺/Fe²⁺ and Fe³⁺/Fe²⁺, which was a 275% improvement over single ozonation [84]. This was the first time that deep catalysis was achieved under neutral pH, solving the industrial problem of traditional catalytic ozonation relying on a strongly acidic environment and promoting the engineering application of the technology. In 2020, Zhiyong Yang et al. explored the heterogeneous catalytic ozonation treatment of complex and highly toxic coal gasification wastewater (initial COD of 1057 mg/L) [85]. The process degraded key toxic substances within 90 min, reducing the COD to 362 mg/L (a 65.8% decrease). The biodegradability index also increased by 94%, reaching a value of 0.35. In 2021, Hai Chen et al. used a Co₃O₄/C composite for norfloxacin catalytic ozonation, achieving 48% TOC removal in 60 min under optimal conditions, with peak efficiency at pH 9.0, and maintaining stability over five cycles [86]. In 2023, Cruz’s group found that γ-MnO₂ and δ3-MnO₂ achieved TOC removal efficiencies of 51% and 46.7%, respectively, in bisphenol A (BPA) catalytic ozonation, compared with 10% using single ozonation, attributing enhanced activity to oxygen vacancies and surface hydroxyl groups [87]. In 2024, Jie Zhang et al. designed a Ce-doped Mn-based catalyst (MnCexOy) that leverages the dynamic oxygen vacancy enhancement effect of Ce⁴⁺/Ce³⁺ to achieve a quinoline removal rate of 93.73%, which is a 50% improvement over traditional α-MnO₂ [88]. The catalyst still maintained an activity of over 90% after five cycles, overcoming the dual bottlenecks of low removal rate of nitrogen-containing heterocyclic pollutants and catalyst deactivation. These studies highlight the potential of optimized catalysts in enhancing catalytic ozonation for treating complex pollutants in wastewater.

Heterogeneous catalytic ozonation for wastewater treatment features efficient degradation capabilities, generating hydroxyl radicals under ambient conditions to significantly mineralize refractory organic pollutants. It is particularly suitable for treating high-concentration and complex industrial wastewaters. The process is environmentally friendly, with catalysts that are easily separable and recyclable, reducing secondary pollution and resource waste. However, challenges still exist, such as high costs and poor stability of high-performance catalysts that are prone to deactivation during long-term operation. Ozone generation requires specialized equipment and energy consumption, increasing operational costs. Additionally, inorganic ions and organic constituents in wastewater may inhibit the catalytic oxidation process, affecting efficiency.

2.3. Persulfate-Based Advanced Oxidation Processes

Advanced oxidation processes (AOPs) based on persulfate (PS) have attracted widespread attention in recent years due to their high efficiency and adaptability in wastewater treatment. Persulfate, including peroxymonosulfate (PMS) and peroxydisulfate (PDS), can generate sulfate radicals (SO₄·⁻) and hydroxyl radicals (·OH) with strong oxidizing abilities upon activation. These radicals are capable of effectively degrading a variety of recalcitrant organic pollutants; therefore, it is of great significance to deeply investigate the activation mechanisms of persulfate and its applications in wastewater treatment.

The activation mechanisms of persulfate mainly include thermal activation, UV activation, ultrasonic activation, electrochemical activation, and alkaline activation. These activation methods promote the decomposition of persulfate through different mechanisms, thereby enhancing its oxidizing ability [89].

Despite the diverse activation methods available for persulfate-based AOPs, these technologies still face several common challenges. Common constituents in wastewater—such as chloride ions, bicarbonate, and natural organic matter—can quench free radicals, significantly weakening the efficiency of radical generation and severely compromising degradation performance. For example, during UV and alkaline activation processes, the presence of nitrates, humic acids, or bicarbonate can inhibit the formation and reactivity of sulfate radicals. In addition, mass transfer limitations are a key issue, especially in electrochemical and ultrasonic activation systems, where the diffusion efficiency of persulfate ions to active sites or the efficiency of cavitation effects may be restricted, thereby affecting overall reaction kinetics. To address these challenges, synergistic approaches that integrate multiple activation mechanisms have shown great potential. Future research should focus on optimizing these synergistic strategies to further advance the practical application of persulfate-based AOPs.

2.3.1. Thermal Activation

Thermal activation is a fundamental method for enhancing the reactivity of persulfate (PS) in advanced oxidation processes (AOPs). Upon heating (Figure 12) [90], persulfate undergoes thermolytic decomposition, generating highly reactive sulfate radicals (SO₄·⁻) and, to a lesser extent, hydroxyl radicals (·OH) [91]. These radicals possess strong oxidizing capabilities, facilitating the degradation of recalcitrant organic pollutants in wastewater. The principle of thermal activation relies on the provision of thermal energy to overcome the activation energy barrier of persulfate decomposition. The rate of persulfate dissociation into radicals increases with increasing temperature. The primary reaction pathways involve the homolytic cleavage of the peroxo bond in persulfate, yielding sulfate radicals (Equations (19) and (20)).

$$S_2{O}_8^{2-}\stackrel{heat}{⟶}2\cdot S{O}_4^{-}$$

(19)

$$\cdot S{O}_4^{-}+H_{2}O⟶\cdot OH+S{O}_4^{2-}+H^{+}$$

(20)

The generated sulfate radicals (SO₄·⁻) exhibit a higher oxidation potential (2.6–3.1 V) than hydroxyl radicals (·OH), enabling efficient oxidation of organic contaminants [92]. Additionally, thermal activation can induce non-radical pathways, such as direct thermal decomposition of pollutants, which may complement radical-based degradation mechanisms under high-temperature conditions.

Researchers have recently refined advanced oxidation technologies to boost pollutant degradation and deep detoxification. Innovations in oxidant combinations, reaction conditions, and new strategies have notably enhanced performance. In 2019, Wang et al. found that sulfate and hydroxyl radicals contribute to the degradation of phthalates, with complete removal achieved at 60 °C; however, this process is inhibited by bromide ions, resulting in the formation of toxic by-products [93]. In 2021, Ren et al. demonstrated that imidazolium-based ionic liquids could be completely degraded at 60 °C using thermally activated persulfate, although common water constituents—such as humic acid and chloride ions—inhibited the process [94]. In 2022, Qiu et al. introduced H₂O₂ into a thermally activated persulfate (PS) system, creating a dual oxidant synergistic system that increased degradation efficiency by 200–400% over a wide pH range (3–11) [95]. This innovation overcame the limitation of PS being efficient only under acidic conditions. The optimal H₂O₂ dosage (10 mM) was shown to prevent efficiency loss. This pH-insensitive oxidation paradigm has enabled multiple groundwater remediation projects and advanced the technology from the laboratory to engineering applications. In 2023, Liang et al. achieved 100% mineralization of BDE-47 within 180 min (TOC removal rate exceeding 95%). EPR and quenching experiments confirmed that sulfate radicals (SO₄·⁻) dominated the reaction (contribution rate exceeding 85%) [96]. The study established a degradation pathway from debromination to ring-opening and then to mineralization, eliminating brominated toxic by-products and addressing the deep detoxification challenge of persistent halogenated pollutants. In 2024, Lalas et al. developed a low-temperature ultra-rapid degradation technology for Acesulfame, achieving 100% removal in 45 min at 50 °C—five times faster than traditional persulfate (PS) processes [97]. This pulsed PS dosing strategy offsets inhibition by bicarbonate and humic acids, maintaining over 90% efficiency in real water and setting a new kinetic record for treating emerging contaminants. These studies highlight the potential of thermally activated persulfate processes for treating various contaminants, with careful consideration needed for water matrix components and potential by-products.

In summary, thermal activation of persulfate provides a straightforward and effective means of enhancing its oxidizing power in AOPs, with ongoing research exploring optimized conditions and synergistic strategies to maximize its application in wastewater treatment.

2.3.2. Ultraviolet Activation

The ultraviolet activation persulfate advanced oxidation process (UV/PS AOP) is based on the generation of highly reactive sulfate radicals (SO₄·⁻) from persulfate upon absorption of UV light. The energy provided by UV light (typically at 254 nm) cleaves the O-O bond in persulfate, producing sulfate radicals with strong oxidizing potential (E₀ = 2.6 V) and a relatively long lifetime. Sulfate radicals have a longer reaction time than hydroxyl radicals. This process can be represented by the reactions below (Equations (21) and (22)).

$$S_2{O}_8^{2-}\stackrel{uv}{⟶}2\cdot S{O}_4^{-}$$

(21)

$$\cdot S{O}_4^{-}+O{H}^{-}⟶\cdot OH+S{O}_4^{2-}$$

(22)

The primary mechanism of UV/PS AOP involves the reaction of sulfate radicals with organic pollutants through electron transfer, hydrogen abstraction, or addition reactions, leading to pollutant degradation. Additionally, the generated sulfate radicals can react to produce hydroxyl radicals (·OH), which also participate in the oxidation process. This dual-radical generation significantly enhances the overall oxidizing capacity and efficiency of the system. For example, in a water treatment project, the degradation rate of lindane increased to 93.2% due to the activation of persulfate (PDS) by UV light at 254 nm, and the degradation of lindane followed first-order kinetics (Figure 13) [91].

In recent years, researchers have advanced the application of AOPs through innovative catalyst design, optimization of reaction conditions, and the establishment of digital models. In 2019, Gu et al. achieved complete ketamine degradation within 30 min using UV-activated PS at pH 7 and 500 μM PS, with sulfate and hydroxyl radicals as the primary oxidants, although degradation was inhibited by nitrate and humic acid [98]. In 2020, Wang et al. developed a Cu-Cu₂O heterojunction catalyst from industrial solid waste, achieving 100% sulfadiazine removal and 30% TOC mineralization within 30 min [99]. The catalyst continuously activated persulfate via Cu⁺/Cu²⁺ cycling, overcoming issues of iron sludge deposition and deactivation. After five cycles, its activity declined by only 12.2%, accompanied by a threefold increase in efficiency and an 80% reduction in cost. It has been applied in the treatment of pharmaceutical wastewater. In 2021, Yan et al. used UV/PS to degrade the neurotoxin BMAA, with bicarbonate enhancing and natural organic matter inhibiting the process [100]. In 2023, Lai’s group established the world’s first linear prediction equation for PS dosage–degradation rate [101]. It achieved a 92.5% benzothiazole removal rate under neutral pH conditions. This equation guided the precise dosage of reagents, reducing the ineffective decomposition of PS by 42%. It overcomes the operational bottleneck of the traditional UV/PS system, which involves excessive dosage, radical quenching, and soaring costs. This model signifies the transition of the technology from an experience-driven approach to an era of digital precision, which enhances pollutant degradation efficiency and cost-effectiveness. Future efforts should focus on optimizing catalyst stability and efficiency in complex waters, advancing digital models for precise dosing and control, and exploring new oxidant combinations to address complex pollutants. In summary, UV/PS AOP is a promising technology for treating recalcitrant organic pollutants, offering high degradation efficiency and broad applicability.

2.3.3. Ultrasonic Activation

The ultrasound-activated persulfate advanced oxidation process (US/PS AOP) leverages the cavitation effects of ultrasound to activate persulfate (PS), generating highly reactive sulfate radicals (SO₄·⁻). The mechanical, thermal, and chemical effects induced by cavitation provide sufficient energy to cleave the O-O bond in persulfate, producing sulfate radicals with strong oxidizing potential (E₀ = 2.6 V) [89]. This activation process is represented by Equation (23).

$$S_2{O}_8^{2-}\stackrel{\mathrm{energy}(\mathrm{from} \mathrm{cavitation})}{⟶}2S{O}_4^{-}$$

(23)

The primary mechanism underlying US/PS AOP involves the formation of sulfate radicals that react with organic pollutants via electron transfer, hydrogen abstraction, or addition reactions, leading to the degradation of contaminants [102]. Additionally, the cavitation process generates localized high temperatures and pressures, which enhance the activation of persulfate and the subsequent oxidation reactions. This dual action of radical generation and physical enhancement significantly improves the degradation efficiency of organic pollutants.

Recent studies have focused on advancing the application of US/PS AOPs in treating complex pollutants through innovative catalyst design and optimization of reaction mechanisms. In 2019, Wang et al. achieved the first dynamic transformation of a dual-radical mechanism using zero-valent copper (ZVC) coupled with ultrasound to activate persulfate [103]. Sulfate radicals (SO₄·⁻) dominated under acidic conditions, while under neutral/alkaline conditions, SO₄·⁻ and ·OH worked synergistically. This approach successfully removed 97% of bisphenol AF across a wide pH range (3–10), overcoming the traditional limitation of ZVC being effective only in strongly acidic environments and reducing energy consumption to 4.2 kW·h/m³. This mechanistic innovation was recognized as a milestone discovery in the US-PS field. In 2021, Xu et al. developed a composite oxidant system of iron foam loaded with chlorite (US/PS-NaClO₂/ZVI foam), achieving 98% degradation of triphenylmethane pollutants under low-temperature conditions (5–15 °C) [104]. This system established a ternary free radical chain reaction involving chlorite radical (·ClO₂), SO₄·⁻, and ·OH, expanding the applicable pH range to 2–11. The technology has been applied in emergency treatment projects for dyeing wastewater in cold regions, solving a major challenge in low-temperature water treatment. In 2022, Pandya et al. optimized the use of ultrasound cavitation combined with persulfate oxidation for treating pharmaceutical wastewater, achieving 39.5% COD removal under specific conditions, highlighting the need for careful parameter optimization and higher energy input [105]. In 2024, Li et al. reviewed the application of ultrasound-assisted persulfate oxidation for degrading emerging contaminants, achieving high degradation efficiencies under optimal conditions; however, high energy consumption and reactor design optimization remain challenges [106]. These studies have potential drawbacks, including the release of copper ions from zero-valent copper, complex multi-radical systems, and stability/reusability issues associated with iron foam catalysts.

2.3.4. Electrochemical Activation

Advanced oxidation processes (AOPs) based on the electrochemical activation of persulfate (PS) have attracted widespread attention in recent years due to their high efficiency in degrading recalcitrant organic pollutants. This process promotes the decomposition of persulfate through the action of an electric field, generating sulfate radicals and hydroxyl radicals (·OH) with strong oxidizing abilities (Equations (24) and (25)), thereby achieving efficient degradation of organic pollutants [107]. The principle of electrochemical activation lies in the reduction of persulfate ions (S₂O₈²⁻) at the electrode surface to produce sulfate radicals, as represented by the following reaction:

$$S_2{O}_8^{2-}+e^{-}⟶\cdot S{O}_4^{-}+S{O}_4^{2-}$$

(24)

$$2\cdot S{O}_4^{-}⟶S_2{O}_8^{-}+2e^{-}$$

(25)

Sulfate radicals possess a high oxidation potential of up to 2.6 V, enabling the effective oxidation of a variety of organic pollutants. The electrochemical process may also generate other reactive oxygen species, further enhancing the oxidative capacity of the system. The mechanisms of action include both direct and indirect pathways. The direct mechanism involves the direct reduction of persulfate ions at the electrode surface to generate sulfate radicals, which is closely associated with the electrocatalytic properties of the electrode material. For example, BDD electrodes have been extensively studied due to their excellent electrocatalytic activity [108]. The indirect mechanism involves the generation of other reactive species through electrochemical reactions or the induction of high-energy electrons by the electric field, which further activate persulfate, producing synergistic effects and enhancing the generation efficiency of free radicals.

Recent studies have focused on improving the removal efficiency of refractory pollutants and reducing costs through innovative electrochemical activation of persulfate technology. In 2019, Malakootian and Ahmadian achieved over 94% removal of ciprofloxacin using iron electrodes, demonstrating high efficiency through continuous sulfate radical generation; however, they faced challenges of high energy consumption [109]. In 2020, Zhang et al. developed a microchannel flow-through cathode (with a 15-fold increase in specific surface area). By enhancing mass transfer through localized turbulence and regulating surface electron density, it achieved a 97.9% phenol removal rate and increased the decomposition efficiency of PMS to 71.9% [110]. This innovation addressed the industry’s pain points of low mass transfer efficiency and poor oxidant utilization in traditional electro-activation systems. In 2021, Gholamia et al. treated real oilfield-produced water using iron electrodes, achieving 94% COD removal and 37% ammonia removal, with thermal activation further enhancing ammonia removal to 69% [111]. In 2022, Babu and Nidheesh developed an iron electrode-activated PS process that achieves 100% oxidation and removal of As(III) within 30 min. The process uses sulfate radicals (SO₄·⁻) to oxidize As(III) to As(V) [112], which is then co-precipitated by Fe³⁺/Fe(OH)₃. This innovation reduces the treatment cost to USD 0.33 per ton of water (one-fifth of traditional coagulation), addressing the economic challenge of arsenic removal in developing countries. Li et al. also created a 3D electrode coupled with a PS activation system (80% particle electrode filling rate) in 2022 [113]. By enhancing free radical chain reactions via a spatial electric field (increasing ·OH yield by 2.8 times), it achieved 80% COD removal and 60% TOC mineralization of N-methyldiethanolamine. This addressed the issue of refractory organic amines in petrochemical wastewater, with system energy consumption at only 4.2 kW·h/m³ (60% less than traditional electro-Fenton processes). In 2023, Nidheesh et al. designed an electrocoagulation–persulfate activation coupled process targeting landfill leachate with a COD greater than 8000 mg/L, achieving an 88.67% reduction in COD while simultaneously removing heavy metals and color [114]. This process overcame the limitation of single technologies that cannot effectively remove both high organic content and heavy metals, providing a “one-stop” solution for landfill leachate treatment. In 2024, Yakamercan et al. optimized PS activation using a boron-doped diamond anode for CIP removal, achieving 94% removal with Fe²⁺ as a coactivator; however, operational costs and by-product formation need further optimization [115]. These studies highlight the potential of electrochemical PS activation for treating various pollutants, with efforts needed to address energy efficiency and operational challenges. AOPs on the electrochemical activation of PS hold great promise for the treatment of recalcitrant pollutants. Future research should focus on developing highly efficient and energy-saving electrode materials, optimizing reaction conditions, minimizing the formation of by-products, and enhancing the stability and cost-effectiveness of the system to facilitate the practical application of this technology.

2.3.5. Alkali Activation

The alkali-activated persulfate advanced oxidation process (AAPS-AOP) is a technology that enhances the oxidation capacity of persulfate (PS) by adjusting the pH of the solution. Under alkaline conditions, the persulfate anion (S₂O₈²⁻) undergoes a decomposition reaction to generate sulfate radicals (·SO₄⁻) and hydroxyl radicals (·OH) (Equations (26)–(28)), both of which possess extremely high oxidation potentials. These radicals can efficiently oxidize and degrade organic pollutants.

$$S_2{O}_8^{2-}+H_{2}O⟶2S{O}_4^{2-}+H{O}_2^{-}+H^{+}$$

(26)

$$S_2{O}_8^{2-}+H{O}_2^{-}⟶\cdot S{O}_4^{-}+\cdot S{O}_4^{-}+\cdot O_2^{-}+H^{+}$$

(27)

$$\cdot S{O}_4^{-}+O{H}^{-}⟶\cdot OH+\cdot S{O}_4^{-}$$

(28)

The mechanism of alkali-activated persulfate involves both radical and non-radical oxidation pathways (Figure 14) [91]. Sulfate radicals and hydroxyl radicals are the primary reactive species under alkaline conditions. These radicals oxidize organic pollutants through addition, substitution, or electron transfer reactions, breaking down the pollutants into smaller intermediate products and ultimately mineralizing them into carbon dioxide and water. Additionally, alkali activation can alter the ionic strength and pH of the solution, influencing the adsorption and desorption behavior of pollutants, thereby enhancing degradation efficiency [91].

The alkali-activated persulfate advanced oxidation process has been widely applied in the remediation of soils and water bodies contaminated with organic matter in recent years. In 2019, Nie et al. achieved 90.8% bisphenol A degradation and 98.9% phosphate removal within 80 min using Ca(OH)₂-activated peroxymonosulfate (PMS), driven by singlet oxygen and superoxide radicals, albeit with limited mineralization capacity (3.21% TOC removal) [116]. In 2020, Dominguez et al. used NaOH to activate sodium persulfate (PS), achieving 99% dichloromethane degradation in groundwater over 96 h without energy input [117]. This method reduced oxidant consumption to 0.8 g pollutant, with mineralization products of CO₂ and Cl⁻. The cost was only USD 0.08 per ton of water, one-twentieth of traditional methods. This technology was successfully applied to in situ remediation. In 2021, Bolade et al. achieved 96% degradation of total petroleum hydrocarbons in 8 days using NaOH-activated sodium persulfate, confirming the role of radicals and highlighting the need for dosage optimization [118]. In 2022, Garcia-Cervilla et al. found that surfactant-assisted persulfate activated by alkali significantly reduced the time required for dense non-aqueous phase liquid abatement; however, persulfate consumption increased [119]. In 2023, Chen et al. developed an alkali–thermal co-activation PS technology for soil containing tetrabromobisphenol A (TBBPA), achieving 88.99% debromination (bromide release over 85%) and reducing soil toxicity by 98% (earthworm survival from 0% to 92%) [120]. The debromination pathway involving ·OH and singlet oxygen (¹O₂) has been confirmed using electron paramagnetic resonance, addressing the potential issue of secondary pollution that may arise after remediation. These studies highlight the potential of alkali-activated persulfate processes for treating various contaminants, with efforts needed to address mineralization, secondary pollution, and operational costs.

However, the alkali-activated persulfate process also faces some challenges; for instance, excessive use of alkali may alter the properties of water and soil, leading to secondary pollution. Additionally, the generation and reaction pathways of radicals during alkali activation are relatively complex, and their mechanisms require further in-depth investigation. Future research directions should include optimizing alkali activation conditions, developing novel composite activation systems, and exploring the application potential of this process for different types of pollutants. In summary, the alkali-activated persulfate advanced oxidation process is feature by its high efficiency, cost-effectiveness, and environmental friendliness, exhibiting its significant utility in the field of organic pollution remediation.

2.4. Photocatalytic-Based Advanced Oxidation Process

Photocatalytic advanced oxidation processes (PAOPs) have garnered significant attention in the field of environmental remediation in recent years as an efficient technology for wastewater treatment. The core principle of PAOPs is based on the generation of highly oxidative reactive oxygen species (ROS) by semiconductor photocatalysts under light irradiation, thereby facilitating the degradation and mineralization of harmful pollutants in wastewater. When a photocatalyst absorbs photons with energy higher than its bandgap width, electrons in the valence band are excited and transition to the conduction band, forming electron–hole pairs. These photogenerated charge carriers subsequently react with water molecules and dissolved oxygen adsorbed on the surface of the catalyst to produce ROS, such as hydroxyl radicals (·OH) and superoxide radicals (O₂⁻·), as shown in Figure 15 [121]. Among these, hydroxyl radicals possess an extremely high oxidation–reduction potential and can effectively oxidize pollutants, breaking them down into smaller intermediate products and ultimately mineralizing them into carbon dioxide, water, and inorganic salts.

Common photocatalysts used in PAOPs include titanium dioxide (TiO₂) [122], zinc oxide (ZnO) [123], graphitic carbon nitride (g-C₃N₄) [124], and MXene-based catalysts [125]. TiO₂ is widely used due to its high photocatalytic activity, chemical stability, and non-toxicity. It exhibits high reliability in diverse wastewater matrices, with consistent performance across multiple experimental runs, demonstrating excellent repeatability in degradation efficiency for pollutants such as dyes and pharmaceuticals. However, its light absorption range is primarily in the ultraviolet region, limiting its application under visible light [126]. ZnO exhibits higher photocatalytic activity but is prone to dissolution under acidic conditions, restricting its application in wastewater treatment [127]. Despite its high initial activity, ZnO suffers from limited reliability in fluctuating pH environments, as dissolution compromises repeatability in repeated batches. g-C₃N₄, a novel non-metallic photocatalyst, has gained increasing attention due to its good visible light response and high stability. It offers superior reliability under visible light and varied pH conditions, with high repeatability in pollutant degradation. MXene-based catalysts, known for their high surface area, hydrophilic surface, and structural flexibility, have demonstrated excellent photocatalytic performance [128]. These catalysts show remarkable reliability in complex wastewater environments due to their exceptional chemical stability and resistance to photocorrosion. Their repeatability is evidenced by consistent degradation efficiencies across multiple trials for persistent pollutants such as antibiotics and dyes. By constructing heterojunctions and doping with metals or non-metals [129], MXene-based catalysts have shown significant performance improvements in photocatalysis and peroxysulfate advanced oxidation processes.

However, PAOPs face several critical limitations that impede their efficiency, particularly the rapid recombination of photogenerated charge carriers (similar to radical quenching in other AOPs) and the mass transfer resistance between catalysts and pollutants. The recombination of electrons and holes reduces the quantum yield, diminishing the generation of reactive oxygen species (ROS) and the efficiency of pollutant degradation. Meanwhile, inefficient diffusion of pollutants to the active sites on the catalyst surface, especially in high-viscosity or complex wastewater matrices, leads to incomplete mineralization and suboptimal treatment outcomes due to mass transfer barriers. To address these challenges, the construction of heterostructures has emerged as a promising strategy for synergistic enhancement. By integrating materials with staggered band structures, heterojunctions facilitate the spatial separation of charge carriers, suppress recombination, and enhance light absorption across a broader spectrum. This not only boosts photocatalytic activity but also improves mass transfer by providing a larger specific surface area and more accessible active sites, thereby accelerating the kinetics of pollutant degradation.

In recent years, researchers have committed to enhancing the efficiency and cost-effectiveness of photocatalytic technology in treating complex pollutants and microorganisms through innovative material design and process optimization. In 2019, Murugesan et al. reviewed the photocatalytic disinfection efficiency of graphitic carbon nitride (g-C₃N₄)-based nanocomposites, highlighting their effectiveness in deactivating microorganisms; however, challenges such as charge carrier recombination and limited visible light absorption remain [130]. In 2020, Fernandes et al. explored the use of TiO₂ photocatalytic advanced oxidation processes for treating refinery effluents. Through a synergistic TiO₂/UV/O₃/H₂O₂ process, a 38% reduction in chemical oxygen demand (COD) and an 84% degradation of volatile organic compounds (VOCs) were achieved; however, high energy costs remain a limitation [131]. In 2021, Yu et al. used ball milling to produce ZnO/biochar nanocomposites from industrial sludge, achieving a 95.19% methylene blue removal rate under visible light [132]. The biochar enhanced charge separation by 3.2 times and increased the specific surface area to 487 m²/g (six times that of pure ZnO). The treatment cost was only USD 0.18 per ton of water (90% cheaper than commercial TiO₂). In 2022, Amir et al. designed a biochar–ZnO hierarchical catalyst that achieved 98.5% degradation of ciprofloxacin (within 60 min under visible light) and 74% mineralization, breaking the bottleneck of less than 50% mineralization for antibiotics [133]. The technology utilizes biochar’s photothermal effect (a local temperature increase of 25 °C) and defect engineering (a 300% increase in oxygen vacancies), and has been applied in advanced pharmaceutical wastewater treatment. In 2023, Shahriyari Far et al. developed an MXene/Metal–Organic Framework composite for dye degradation, achieving 62% methylene blue and 35% Direct Red 31 degradation within 80 min, showcasing efficient photocatalytic performance [134]. In 2024, Yang et al. developed an MOF-derived Ni-Co bimetallic nitrogen-doped porous carbon, achieving a record of 100% methylene blue complete mineralization within 60 min (TOC removal rate exceeding 99%), zero activity loss over 20 cycles (breaking the lifespan limit of photocatalysts), and broad-spectrum degradation performance (tetracycline/sulfonamide antibiotics rate exceeding 95%) [135]. These studies highlight the potential of various photocatalytic materials and composites for treating diverse contaminants, with ongoing efforts needed to address efficiency, stability, and cost-effectiveness.

PAOPs have several advantages compared with other advanced oxidation processes, including being environmentally friendly, highly efficient in degradation, and relatively low cost. They primarily utilize solar energy and natural photocatalysts, eliminating the need for large amounts of chemical reagents and thereby reducing secondary pollution. The generation of highly reactive free radicals enables effective mineralization of recalcitrant organic pollutants. Moreover, photocatalysts such as TiO₂ are relatively inexpensive and can be reused. However, PAOPs face challenges such as low light utilization efficiency, difficulties in catalyst recovery, and restricted reaction conditions. Most photocatalysts are only sensitive to UV light, limiting the efficient use of solar energy. Additionally, photocatalytic reactions are sensitive to light intensity, wavelength, the initial pH of wastewater, and pollutant concentrations. The formation of reactive ROS can also lead to unwanted side reactions, generating toxic by-products. Furthermore, the scalability of photocatalytic reactors and the long-term stability of photocatalysts in real-world applications remain significant challenges. Future research should focus on enhancing visible light responsiveness of photocatalysts, optimizing reactor design, and reducing catalyst costs to facilitate the widespread application of this technology.

2.5. Electrochemical-Based Advanced Oxidation Process

Electrochemical advanced oxidation processes (EAOPs) are a type of advanced oxidation technology based on electrochemical principles. They generate highly oxidative radicals (such as hydroxyl radicals and superoxide radicals) through electrode reactions to degrade organic pollutants and achieve mineralization [4]. The principle of EAOPs is that the anode oxidizes organic matter through direct electron transfer or the generation of highly oxidative intermediates (such as ozone and hydrogen peroxide) (Figure 16) [4,136]. Here, M(·OH) indicates hydroxyl radicals adsorbed on the anode M or remaining near its surface (Equations (29) and (30)) [137].

$$M+H_{2}O⟶M(\cdot OH)+H^{+}+e^{-}$$

(29)

$$M(\cdot OH)+R⟶mC{O}_2+n{H}_{2}O+H^{+}+e^{-}$$

(30)

Despite the high controllability and environmental friendliness of EAOPs, they still face common inherent limitations, including significant mass transfer resistance at the electrode surface and electrolyte interference, which can lead to radical quenching. Mass transfer resistance arises from the diffusion-limited transport of pollutants and oxidants to the electrode interfaces, particularly in heterogeneous matrices, reducing reaction kinetics and overall efficiency. Concurrently, electrolyte components such as Cl⁻ or HCO₃⁻ can scavenge reactive radicals (for example, Cl⁻ reacts with ·OH to form less reactive chlorine species), diminishing oxidative capacity and generating unintended by-products. To address these challenges, synergistic strategies that combine electrode modification and process coupling have emerged as effective optimization methods. Moreover, coupling EAOPs with complementary processes, such as photoelectro-Fenton or ultrasound-assisted systems, can enhance radical generation through synergistic mechanisms. These integrated approaches not only mitigate mass transfer barriers and electrolyte interference but also reduce energy consumption and operating costs, highlighting their potential as scalable and efficient wastewater treatment solutions.

In terms of catalysts, common catalysts used in EAOPs include anode materials such as boron-doped diamond (BDD) [138], dimensionally stable anodes [139], and metal oxide electrodes (RuO₂/Ti [140]).

In recent years, researchers have focused on enhancing the removal efficiency and cost-effectiveness of refractory pollutants through innovative EAOPs. In 2019, Silva et al. used a Nb/BDD thin film anode for electrochemical oxidation (EO) of Atenolol, achieving 100% degradation and 65% mineralization to CO₂ across a wide pH range, albeit with low mineralization efficiency and high energy consumption at higher current densities [138]. In 2020, Peralta-Hernández’s group developed a system for the overproduction of ·OH through the synergistic action of UVA irradiation and electro-Fenton processes, achieving 97% decolorization and 95% COD mineralization of azo dyes within 60 min [141]. This breakthrough surpassed the efficiency limits of traditional electro-oxidation and electro-Fenton processes. It was demonstrated, for the first time, that the triple synergy of photo-electro-chemical processes can completely decompose the chromophoric groups and benzene ring structures of azo dyes. In 2021, Yu et al. developed a nitrogen-doped polyaniline-modified graphite felt cathode (N-PANI/GF) that achieved 93% efficient conversion of H₂O₂ to ·OH (compared to less than 40% for traditional cathodes), 97% bisphenol A degradation within 30 min, wide pH adaptability (3–9), and zero attenuation over 50 cycles. This innovation solved the long-standing issues of low radical production and short lifespan in non-precious metal cathodes [142]. In 2022, Feijoo et al. used in situ Raman spectroscopy to reveal the direct ·OH generation pathway on a BDD anode, achieving 100% degradation and 85% mineralization of carbamazepine under acidic conditions [143]. The study quantified the distribution of ·OH at over 91% and developed a predictive model for current density and mineralization efficiency, clarifying the reaction mechanism of BDD electrodes. In 2023, Fu et al. analyzed the direct and indirect oxidation mechanisms in EAOPs, achieving 81% TOC removal in florfenicol degradation using BDD electrodes, indicating strong oxidation capacity. However, EAOPs were associated with challenges such as electrode passivation and high energy demand [144]. In 2024, Mahmud et al. reviewed EAOPs for treating antibiotic-contaminated wastewater, highlighting the efficiency of EO with BDD electrodes, PEC with TiO₂ and UV light, and combined EF processes such as SEF and PEF; however, they noted challenges such as high costs and potential by-product formation [145]. These studies underscore the potential of EAOPs for treating various contaminants, with efforts needed to address operational challenges and cost-effectiveness. It is hoped that further optimization of electrode materials and reaction conditions will enhance mineralization efficiency, reduce energy consumption, and improve the stability and reusability of electrodes.

EAOPs have significant advantages and limitations compared with other advanced oxidation processes (such as the Fenton process, photocatalytic oxidation, and ozonation). The advantages include: (1) no need for external chemical reagents, avoiding secondary pollution and aligning with the principles of green chemistry; (2) the oxidation process can be precisely controlled by adjusting current density and electrode materials; (3) good adaptability to treating high-salinity and high-concentration organic wastewater, as well as the ability to operate over a wide pH range. However, EAOPs also have some disadvantages: (1) high cost of electrode materials, especially high-performance anode materials; (2) relatively high energy consumption, limiting large-scale application; (3) the stability of electrodes needs further improvement, especially during long-term operation. In summary, as an efficient and green water treatment technology, electrochemical advanced oxidation processes play an important role in treating refractory organic pollutants. However, issues associated with cost and energy consumption still need to be further addressed to achieve wider industrial applications.

2.6. Sonochemical-Based Advanced Oxidation Process

The sonochemical advanced oxidation process (SAOP) is a wastewater treatment technology based on the cavitation effect of ultrasound. Its principle lies in the generation of local high-temperature and high-pressure environments when ultrasound propagates through a liquid medium, causing water molecules to dissociate and form highly oxidative hydroxyl radicals (·OH) and hydrogen radicals (·H) [146]. These radicals can react with organic pollutants in wastewater through a chain reaction, gradually degrading them into smaller molecules and ultimately mineralizing them into carbon dioxide (Figure 17) [4,147]. Meanwhile, the mechanical shearing effect of ultrasound can break the long chains of large molecular organic compounds, enhancing the mass transfer efficiency between pollutants and radicals, thereby improving degradation efficiency. However, the primary limitations of SAOPs stem from mass transfer barriers at the cavitation bubble interfaces and radical quenching in the bulk solution, which restrict the contact between free radicals and pollutants, thereby reducing degradation efficiency. Integrating SAOPs with other complementary advanced oxidation processes (such as photo-Fenton, catalytic ozonation, or persulfate activation) can overcome these intrinsic limitations by generating additional reactive species (such as sulfate radicals, singlet oxygen) and enhancing interfacial reactions, thus achieving significant synergistic effects in terms of degradation efficiency and mineralization rates.

The use of catalysts in SAOPs can significantly enhance reaction efficiency. Common catalysts include transition metal catalysts (such as metals and their oxides like iron, cobalt, and manganese), carbon-based catalysts (such as activated carbon, carbon nanotubes, and graphene), and composite catalysts (such as composites of metals and metal oxides) [148]. These catalysts promote the generation and transfer of radicals, further increasing the reaction rate and degradation efficiency.

In recent years, researchers have focused on enhancing the treatment efficiency and cost-effectiveness of refractory pollutants through innovative sonochemical technologies. In 2020, Al-Bsoul et al. explored the combined ultrasound (US)/UV/TiO₂ process for treating olive mill wastewater, achieving 59% COD removal within 90 min [149]. During the process, it outperformed individual treatments through enhanced hydroxyl radical generation, although it faced challenges in scaling up and controlling operating conditions. Additionally, in 2020, Yadav et al. enhanced the ultrasonic cavitation effect using quartz sand, achieving 77% COD removal and 96% color removal without adding any chemicals [150]. The technology also completed seawater desalination, revealing the cavitation nuclei effect of sand particles. The cost of treating one ton of water was only USD 0.15 (a 70% reduction compared with traditional ultrasonic oxidation), pioneering an integrated mechanism of “cavitation–adsorption–desalination.” In 2022, Noor et al. optimized the sonication-assisted synthesis of magnetic Moringa oleifera for treating palm oil mill effluent, identifying optimal conditions for enhanced coagulation efficiency, although challenges in coagulant regeneration remained [151]. In 2023, Dey et al. synthesized a Ce-TiO₂ heterojunction catalyst via ultrasound (with a grain size of 8.2 nm) and combined it with H₂O₂ sono-photocatalysis, achieving 84.75% deep COD removal. The catalyst maintained over 95% activity after 10 cycles [152]. The technology created a cerium-induced oxygen vacancy gradient and used ultrasound to inhibit particle aggregation, addressing the issue of easy deactivation in nanocatalysts. In 2024, Lakshmi et al. developed an acoustic cavitation coupled with ozone process that achieved nearly 100% removal of ciprofloxacin (CIP) in 90 min with 82% mineralization, breaking the less than 50% mineralization bottleneck for antibiotics [153]. The process consumed only 1.8 kW·h per ton of water (one-fifth of UV/O₃). These studies highlight the potential of SAOPs for treating various wastewaters, with efforts needed to address operational challenges and optimize costs.

The SAOP has several advantages compared with other advanced oxidation processes (such as Fenton oxidation, ozonation, and photocatalytic oxidation), including simple operation, efficient degradation, and no secondary pollution. The equipment is easily accessible, and the operating conditions are mild, without the need for complex devices or harsh reaction conditions. SAOP can rapidly degrade recalcitrant organic compounds, transforming them into less toxic smaller molecules or even completely mineralizing them. Moreover, no harmful by-products are generated during the reaction, making it environmentally friendly. However, the process has some disadvantages; for example, it is associated with high energy consumption, as the generation of ultrasound requires a significant amount of energy, which limits its large-scale application. The industrialization of this process is challenging due to the insufficient research on the degradation mechanisms, reaction kinetics, and scaling up of the reactor design of the sonochemical reaction. Additionally, the requirements for catalysts are high, with some catalysts being associated with insufficient stability or high costs.

3. Challenges and Perspectives

AOPs have shown great promise in breaking down stubborn pollutants in wastewater. However, implementing these processes on a wide scale poses some persistent challenges that need solutions specific to each technology. This section carefully examines these challenges and the new approaches to addressing them, with reference to a summary of comparative performance parameters (

Table 3. Comparison of key parameters for major catalytically driven AOPs.

Parameter Type	Fenton-Based AOPs	Ozone-Based AOPs	Persulfate-Based AOPs	Photocatalytic AOPs	Electrochemical AOPs	Sonochemical AOPs
Cost	High	Medium to high	Medium	Low	High	Medium
Degradation Efficiency	50–90%	70–95%	60–90%	60–95%	60–90%	50–85%
TOC Removal	30–70%	50–85%	35–75%	40–80%	30–70%	20–60%
Secondary Pollution Risk	High	Low to medium	Low	Low	Low	Low
Energy Consumption (kWh/m3)	0.1–0.5	0.5–1.5	0.4–1.2	0.2–0.8	0.3–1.0	1.0–2.0
pH Range	2–4	6–9	3–10	4–10	3–11	4–10
Catalyst Stability	Medium	Medium	High	Low	Medium	Medium

) and a detailed strengths–weaknesses analysis (

Table 4. SWOT analysis of major advanced oxidation process (AOP) categories.

AOP Category	Strengths (S)	Weaknesses (W)	Opportunities (O)	Threats (T)
Fenton-based AOPs	• High oxidation capacity and destruction rates • Relative simplicity and low equipment cost • Effectiveness on diverse contaminants	• Strict acidic pH requirement (2–4) • Sludge generation and handling • Catalyst (Fe) loss/leaching • H2O2 consumption	• Development of heterogeneous catalysts (Fe@supports) • Integration with light/electro (Photo/Electro-Fenton) • Iron-free Fenton-like catalysts (e.g., using Cu, Mn)	• Regulatory pressure on sludge disposal • Competition from sulfate radical-based AOPs • Cost volatility of H2O2
Ozone-based AOPs	• Powerful direct oxidant (O3) • Disinfection capability • No chemical sludge/residuals (direct path)	• High energy input for O3 generation • Low O3 solubility and mass transfer limitations • Bromate (BrO3−) formation risk • Incomplete mineralization	• Catalytic ozonation (improving efficiency/selectivity) • Synergy with H2O2 (O3/H2O2–Peroxone) •Pre/post-treatment with biological processes	• Strict regulations on bromate • Operational complexity and safety concerns • Energy cost sensitivity
Persulfate-based AOPs	• SO4·− radicals potent and persist longer than HO· • Multiple activation methods (heat, UV, transition metals, carbon) • Flexible operation contaminants	• Residual persulfate/SO42− in effluent • Potential toxic by-product formation • Catalyst dependency/cost • Cl−/HCO3− quenching effects	• Exploiting non-radical pathways (catalysis, E-transfer) • Waste-derived catalysts (e.g., biochar) • Novel activation methods (e.g., ultrasound)	• Regulatory uncertainty on persulfate residuals/by-products • Chemical cost (PS/PMS) • Scalability of activation methods
Photocatalytic AOPs	• Potential for solar-driven operation • Ambient conditions • Minimal chemical consumption	• Low quantum yield/charge separation efficiency • Recombination losses and catalyst deactivation • Catalyst recovery/reuse challenge • Limited penetration depth	• Development of visible-light-responsive catalysts • Formation of heterojunctions/composites • Immobilization strategies (films, membranes) • Combined photoelectrocatalysis	• High cost of UV lamps (if used) • Weather dependency (solar) • Competition from PV-driven electrochemical AOPs
Electrochemical AOPs	• High degree of process control/tunability • No chemical additions required • Potential for automation • Direct and indirect oxidation pathways	• High capital cost (electrodes, e.g., BDD) • Electrode fouling/passivation • High energy consumption • Low current efficiency in complex matrices	• Development of novel, stable, cost-effective electrodes • Integration with renewable energy • Simultaneous resource recovery (e.g., H2, metals) • Hybrid systems (e.g., Electro-Fenton, Electro-Persulfate)	• Rising electricity costs • Complexity of treatment optimization •Disposal/recycling of spent electrodes
Sonochemical AOPs	• Synergistic effects with other AOPs • Enhanced mass transfer • No chemical additions required • Destroys volatile compounds	• Very high energy consumption (large volumes) • Limited reactor design/scalability • Inefficient radical yield • Noise pollution	• Optimization of reactor configuration/cavitation • Hybridization (e.g., Sono-Fenton, Sono-Persulfate) • Focused applications (sludge treatment, niche pollutants) consumption	• High operational costs • Scalability barriers • Noise and material erosion concerns • Limited commercial traction

).

Fenton-based AOPs, though characterized by high oxidation capacity and operational simplicity, are constrained by a narrow pH operating range (typically 2–4) and the generation of iron sludge, which increases disposal costs and poses risks of secondary contamination. The development of more stable heterogeneous catalysts, such as iron-based composites supported on carbon materials or metal–organic frameworks, can reduce sludge production and broaden pH applicability. However, long-term catalyst stability is still compromised by metal leaching, as evidenced by the medium stability rating in Table 3, due to the risk of catalyst deactivation. The integration of light or electrochemical systems can enhance the generation of hydroxyl radicals and the efficiency of H₂O₂ utilization.

Ozone-based AOPs face challenges, including high energy consumption for ozone generation (0.5–1.5 kWh/m³) and the potential formation of toxic by-products such as bromate in bromide-containing matrices. Designing efficient catalysts, such as transition metal oxides or nitrogen-doped carbon materials, to promote the decomposition of ozone into hydroxyl radicals aligns with the catalytic enhancement opportunities identified in Table 4. However, Table 3 highlights persistent mass transfer limitations, which can be addressed by optimizing reactor configurations (such as pressurized or membrane-integrated systems) to enhance mass transfer and reduce energy consumption. Combining ozone with H₂O₂ or UV can further reduce bromate formation by accelerating ozone consumption.

Photocatalytic AOPs, despite their promising prospects for solar energy-driven applications, face challenges such as rapid charge recombination and a lack of sensitivity to visible light. The SWOT analysis in Table 4 identifies low quantum yield as a critical weakness, which can be addressed by developing visible-light-responsive catalysts (such as g-C₃N₄ heterojunctions) to enhance solar energy utilization and extend the catalyst lifespan. However, as confirmed by Table 3, the difficulty in catalyst recovery remains a significant issue, which can be simplified by employing immobilization techniques (such as coating on porous membranes).

Electrochemical AOPs, despite their high controllability and the absence of chemical reagent addition, are hindered by the high costs of advanced electrode materials such as boron-doped diamond (BDD) and issues with electrode passivation. These challenges can be partially addressed by developing cost-effective electrode materials, such as titanium coated with metal oxides or carbon-based composites, which can enhance the yield of ·OH and operate across a broad pH range, leveraging the process control strength highlighted in Table 4. Nevertheless, the high energy consumption mentioned in Table 3 is being mitigated through the optimization of operating parameters, such as pulsed current, to reduce energy consumption and scaling. The integration of renewable energy sources can further enhance sustainability.

Sonochemical AOPs, which rely on ultrasonic cavitation, are limited by extremely high energy consumption, low radical generation efficiency, and difficulties in reactor scaling. The high operational costs threat mentioned in Table 4 can be countered by enhanced radical generation through hybrid configurations (such as sono-Fenton or sono-ozone) and innovative reactor designs (such as focused ultrasound) to concentrate cavitation effects and minimize energy waste. However, catalyst aggregation during prolonged operation remains unresolved (Table 3 stability rating: Medium), although nano-confinement in mesoporous silica shows preliminary success.

The future development of AOPs will depend on integrating these solutions into a comprehensive, system-wide approach. This includes the organic combination of AOPs with existing wastewater treatment processes, such as using AOPs as a pre-treatment to enhance the biodegradability of recalcitrant pollutants for subsequent biological treatment, or as a polishing step to remove trace pollutants, thereby achieving a balance between efficiency and cost. In-depth research into the reaction mechanisms of AOPs under complex real wastewater conditions is crucial for understanding the formation of by-products and optimizing the mineralization process. Conducting rigorous technical and economic analysis (TEA) and life cycle assessment (LCA) to compare the performance of AOPs with traditional methods in different application scenarios is expected to provide strong guidance for their practical application. Adhering to the principles of green chemistry, such as utilizing solar energy, developing reagent-free processes, and transforming waste into catalysts, will ensure that AOPs not only meet performance targets but also comply with the requirements of sustainable development. By addressing these challenges through interdisciplinary innovation, AOPs are expected to transform from laboratory achievements into scalable, cost-effective global solutions for water pollution, playing a key role in sustainable water resource management.

4. Conclusions

As global water pollution intensifies, the need for efficient and sustainable wastewater treatment technologies becomes increasingly urgent. Advanced oxidation processes (AOPs) have emerged as a promising solution in this context, due to their strong oxidation capabilities and ability to degrade refractory organic pollutants. This review provided an overview of various AOPs, including Fenton processes, ozone-based AOPs, persulfate-based AOPs, photocatalytic AOPs, electrochemical AOPs, and sonochemical AOPs, highlighting their principles, mechanisms, catalyst designs, and application challenges. Despite significant laboratory success, the practical application of AOPs poses challenges such as the requirement for complex equipment, high energy consumption, catalyst instability, and secondary pollution. Future research must focus on developing cost-effective and environmentally friendly catalysts, optimizing process integration, improving energy efficiency, and enhancing the adaptability of AOPs to real-world wastewater conditions.

With the deepening of the concepts of green chemistry and sustainable development, greater emphasis will be placed on energy efficiency and environmental friendliness in AOPs. For example, solar-driven photocatalytic AOPs and electrochemical AOPs coupled with renewable energy will become key directions for future research. By developing new catalysts and optimizing reaction conditions, AOPs are expected to achieve higher efficiency and lower costs in treating complex wastewater and emerging pollutants. This will not only help to address the current water pollution problems, but also provide strong support for the sustainable use of global water resources.