Material Design and Operation Strategy of the Electro-Fenton System for the Treatment of High Pollutant Load Wastewater

Abstract

Electro-Fenton (EF) technology holds significant promise for degrading recalcitrant organic pollutants. Still, it faces distinct challenges in high-pollutant-load wastewater, including insufficient radical generation, electrode passivation, and mass transfer limitations. This review systematically organizes recent advances in material design and operational strategies to address these issues. We highlight innovative cathode materials (e.g., graphene-based structures, carbon nanotubes, and metal–organic frameworks), stable anodes such as boron-doped diamond, and catalysts tailored for harsh conditions. Key operational improvements are discussed, including pH adaptability, current density optimization, and oxygen supply enhancement. The integration of hybrid systems, such as bio-electro-Fenton and photo-electro-Fenton, is also examined. Looking forward, future research for treating high-pollutant load wastewater should focus on: (1) Developing electrodes and catalysts with superior antifouling properties and long-term stability in high-strength, complex wastewaters; (2) Constructing intelligent control systems capable of real-time response to water quality fluctuations for adaptive parameter optimization; (3) Exploring energy-efficient, self-sustaining EF systems coupled with renewable energy sources or incorporating energy recovery units. This review aims to provide a comprehensive reference for subsequent research endeavors and practical applications related to the treatment technology of EF systems in high-pollutant-load wastewater contexts.

1. Introduction

Wastewater with a high pollutant load has become one of the most serious environmental challenges in today’s industrial development, primarily originating from chemical manufacturing processes, textile dyeing operations, pharmaceutical production facilities, and municipal landfill leachate treatment systems [1,2]. This type of wastewater is not only high in load and complex in organic content, but also causes environmental problems such as soil salinization and eutrophication of water bodies if not treated properly [3,4]. In the face of this growing environmental problem, there is an urgent need to find effective management solutions to achieve sustainable development.

However, the treatment of such wastewater faces many technical difficulties. High organic loading exerts a strong inhibitory effect, significantly reducing the treatment efficiency of conventional biological technologies and hindering their effective application [5,6]. Although physical and chemical treatment methods can remove some pollutants, they are generally characterized by high cost, complicated operation, and easy to produce secondary pollution. Compared with the conventional methods, AOPs, including Fenton oxidation [7], electrochemical oxidation technology [8], ozonation [9], photocatalysis [10] and so on, has been widely studied in the field of high-pollutant wastewater treatment in recent years because of its characteristics such as a rapid reaction, a thorough reaction, and high oxidation efficacy [11,12].

Among the various AOPs available, the EF process is suitable for complex scenarios of treating high pollutant load wastewater, this is because high-salt organic wastewater generally has a high electrical conductivity, providing a good development space for the application of electrochemical methods in the treatment of high-salt organic wastewater [13,14], and EF systems can solve the problem of high and fast consumption of hydrogen peroxide (H₂O₂) and iron by accelerating the circulation of iron ions, thus further reducing sludge production and iron loss, which ultimately achieves partial mineralization of organic pollutants and sludge formation [15]. The EF process begins with the in situ generation of H₂O₂ at the cathode through the two-electron reduction in dissolved oxygen, as shown in the reaction (Equation (1)) [16]. This electrochemically produced H₂O₂ subsequently participates in the classical Fenton reaction with Fe²⁺ (Equation (2)) [17], generating highly reactive •OH that possesses exceptional oxidative power with a standard reduction potential of 2.8 V [9,18]. The system maintains its catalytic efficiency through the continuous electrochemical regeneration of Fe²⁺ from Fe³⁺ at the cathode (Equation (3)) [16], creating a self-sustaining cycle that minimizes iron sludge formation while maximizing the utilization of the iron catalyst.

O₂ + 2H⁺ + 2e⁻ → H₂O₂

(1)

Fe²⁺ + H₂O₂ → Fe³⁺ + •OH + OH⁻

(2)

Fe³⁺ + e⁻ → Fe²⁺

(3)

Beyond its technical mechanisms, the implementation of EF technology at an industrial scale presents significant economic and environmental implications. From an economic perspective, while the initial capital investment in electrodes and electrical infrastructure can be substantial, EF systems offer potential operational cost savings compared to conventional Fenton processes [19]. These savings primarily stem from the in situ generation of H₂O₂, which eliminates the need for continuous purchase, transport, and storage of this unstable and hazardous chemical. Furthermore, the dramatic reduction in iron sludge production directly translates to lower sludge handling and disposal costs, a major economic burden in traditional wastewater treatment [20]. Environmentally, EF represents a greener alternative. It minimizes the carbon footprint associated with the industrial synthesis and logistics of H₂O₂. More importantly, by enabling effective degradation of recalcitrant organic pollutants into harmless end-products like CO₂ and H₂O, EF technology prevents the release of persistent and potentially toxic compounds into ecosystems, thereby protecting aquatic life and preserving water quality [21]. This synergy of potential cost-effectiveness and a reduced environmental footprint positions EF as a key technology for advancing sustainable industrial wastewater management.

Despite its theoretical advantages, the practical application of EF technology under high pollutant load faces substantial challenges that significantly impact system performance and economic viability. Simultaneously, the challenge of high organic loading presents additional complexities that significantly impact the performance of the electro-Fenton (EF) system. Elevated concentrations of organic matter can lead to severe mass transfer limitations by fouling the electrode surfaces, which impedes electron transfer and the diffusion of reactants and products [22]. Afolabi et al. [23] found that high organic load in treating brewery wastewater by EF method increases the operating cost of the system, which needs to be further evaluated and optimized its operating parameters, and ultimately a chemical oxygen demand (COD) removal rate of 93.15% was achieved at pH 3, catalyst ferrous sulfate dosage of 0.002 M, voltage of 20 V, and electrode spacing of 6 cm.

The discussion in this review will focus on the design strategies of heterogeneous EF catalysts, highlighting milestone research from the past decade that has significantly advanced the field, with particular emphasis on studies that reveal novel catalytic mechanisms or address the critical activity–stability trade-off. So far, many EF-related studies have been conducted [24,25]. However, few papers have focused on the optimization and improvement of EF systems under high salt conditions. Therefore, this paper will focus on two critical and interrelated areas: (1) rational material design of core components, including design improvements for cathode and anode as well as catalyst materials; (2) optimization of operation strategies, covering control of key process parameters and development of innovative coupling processes as shown in Figure 1. Through a systematic analysis of recent research advances, identification of key performance limitations, and evaluation of emerging solutions, this review aims to provide valuable insights for researchers and practitioners working to advance the practical application of EF technology in industrial wastewater treatment.

2. Material Design for Electro-Fenton Systems

The choice of electrode materials is critical for ensuring high efficiency and durability in EF systems under high-salt conditions. In these challenging environments, electrodes need to possess exceptional corrosion resistance to withstand aggressive chloride attack and maintain structural integrity over extended operating periods.

2.1. Cathode Materials

Generally, the development of suitable cathode materials with excellent properties is crucial for enhancing the yield of H₂O₂ in multiphase electrolysis processes. The cathode materials should have high catalytic activity, good stability, high electrical conductivity, and high specific surface area. In recent years, many scholars have worked to develop cathode materials with high stability and conductivity [26,27].

At present, the widely used cathode materials include metal-based and carbon-based materials. The use of precious metals as cathode materials can increase the probability of dual-electron oxygen reduction reaction (ORR) and the selectivity of H₂O₂ [28], but their application is limited by cost considerations [29]. Compared with metal-based materials, carbon-based cathodes such as graphite [30], activated carbon (AC) fibers [31], gas-diffusion electrodes [32], carbon felts [33], and carbon sponges have been widely adopted due to their high surface area, favorable porosity, and robustness under corrosive conditions [34]. Among these electrodes, gas-diffusion electrodes can enhance oxygen mass transfer and improve H₂O₂ production efficiency through hydrophobic carbon materials such as carbon fiber and PTFE-modified carbon [35].

However, traditional carbon-based materials exhibit low electrocatalytic activity for H₂O₂ production, and their chemical structures need to be modified to increase the active sites and improve the selectivity of HOO• to produce H₂O₂. These modifications include defect engineering (Figure 2a) [36], surface doping (Figure 2b,c) [37,38,39], interface engineering (Figure 2d) [40], and so on. Wang et al. [41] modified carbonaceous electrode materials by direct calcination in air. The results showed that EF with a modified carbon cathode showed a high kinetic removal of levofloxacin in a wider pH range of 3.0–6.6 compared with homogeneous Fenton. Liu et al. [42] developed nitrogen-doped carbon aerogel (NDCA) electrodes to enhance Fe²⁺ regeneration in EF processes. Pyrrolic nitrogen provides unpaired electrons for Fe³⁺ reduction, while graphitic and pyridinic nitrogen coordinate with Fe³⁺ to form C–O–Fe–N₂ bonds that facilitate electron transfer. The Fe²⁺/NDCA-EF system achieved 5.8 ± 0.3 fold higher •OH generation than conventional Fenton systems and demonstrated complete removal of organic pollutants with low energy consumption. Treatment of domestic sewage resulted in 90.1 ± 0.6% dissolved organic carbon removal and 83.3 ± 0.9% NH₃-N removal. Liu et al. [43] found that nano porous carbon (NPC) derived from multi-walled carbon nanotube and zeolite imidazole framework-8 composites was employed to modify graphite felt (GF) electrodes for EF applications. The hierarchical porous structure facilitated two-electron oxygen reduction, enhancing H₂O₂ generation. NPC/GF electrodes exhibited superior current response and reduced charge transfer resistance compared to pristine GF, achieving 17-fold improvement in H₂O₂ yield. The recent studies on the modification of cathode materials in EF systems as shown in

Table 1. Recent studies on the modification of cathode materials in EF systems.

Cathode Material	Operating Conditions	Target Pollutant	Removal Efficiency	Reference
Carbon cloth gas-diffusion electrode	pH 3.0, 30 mA cm−2, 0.5 mM Fe2+	Fipronil	100% (20 mg L−1 fipronil in 60 min)	[45]
CNT/MoS2/FeCo-LDH membrane	pH 3–9, 1.5 mA cm−2	Phenol	98.8% (60 min)	[46]
N-doped AC	pH 3–5	Phenol	93.7%	[47]
Carbon nanotube fiber (CNTF)	Constant current	Acid Orange 7	TOC removal 73.9% (2 h)	[48]
Magnetite nanoparticles on carbon felt	-	Aspirin	Enhanced aspirin degradation	[49]
AC/stainless steel mesh (ACSS)	Neutral pH, 100 mA	Reactive Blue 19	H2O2: 8.9 mg L−1, RB19 removal: 61.5% (90 min)	[50]
Fe3+-loaded N-doped carbon nanotubes	-	Multiple organics	Accelerated Fe2+ regeneration, high degradation rate	[42]

.

2.2. Anode Materials

Appropriate anode materials are essential for the generation of •OH and other oxidants because the anode affects the electron transfer between the cross-interfaces and biofilms in the system, significantly influencing the power generation performance and treatment capacity of the overall system [51]. High-performance electrode materials can enhance power generation efficiency, improve electron transfer, and promote bacterial adherence. Traditional anode materials include Pt, dimensionally stable anodes (DSAs), graphite, and others [52]. For example, Pt has good electrical conductivity, high catalytic activity, and good stability [53]. However, due to the high price of Pt, the cost-effectiveness is not ideal. In order to solve this problem, the platinum-plated anode is a technique for coating or electroplating a certain amount of platinum on a suitable metal [54]. It has been used in many EF processes and can achieve the same effect as a platinum anode.

In addition, DSAs are also commonly used as anodes [55] based on titanium (Ti) and coated with metal oxides such as RuO₂, IrO₂, PbO₂, or their mixture [56], which are particularly effective in promoting the precipitation of chlorine and oxygen. The electrode has excellent electrical conductivity and chemical stability, high oxygen evolution potential, and is very popular in the chlor-alkali industry and water electrolysis [57]. Among them, Ti/Sb-SnO₂ exhibits a high organic oxidation rate and improved mass transfer performance on a mesh substrate. For example, Tao et al. [58] employed electrochemical oxidation enhanced by persulfate activation on Ti/BDD and Ti/SnO₂-Nb₂O₅ anodes for treating biologically pre-treated textile wastewater. Optimal conditions for Ti/BDD achieved 94.78% color removal, 64.57% COD reduction, and 41.57% TOC elimination, while Ti/SnO₂-Nb₂O₅ attained 73.04%, 41.32%, and 39.22%, respectively. In order to further improve the oxidation performance of anode materials, researchers have developed many methods to modify DSAs. For example, Rajoria et al. [55] used a graphene oxide (GO) coated TiO₂ nanotube (GO/TiO₂NTs) electrode to study the treatment of simulated electroplating wastewater by the EF method. Compared with the traditional commercial electrodes IrO₂, RuO₂, PbO₂, SnO_2, and BDD, the electrode has a lower cost, suitable conductivity, and simple production. Luo et al. [59] prepared a TiO₂/Ru-IrO₂ double-sided electrode. Using this electrode, the degradation efficiency of 2,4-DCP was 3.6 times and 5.7 times higher than that of TiO₂ and Ru-IrO₂, respectively.

In addition, the BDD electrode [60] is highly praised for its wide potential window, low background current, high stability, chemical inertness, and excellent corrosion resistance [61]. These characteristics are conducive to the direct generation of •OH, making it the preferred anode for the EF system. The BDD electrode can overcome the limitations of traditional AOPs by regenerating ferrous ions and generating H₂O₂ to prevent secondary pollution, and the BDD electrode is particularly effective in complex and highly polluted wastewater [62]. The higher the salt content in high-pollutant load wastewater, the higher the conductivity and the lower the energy consumption.

2.3. Catalysts

In the EF system, catalysts play a critical role in decomposing H₂O₂ to generate •OH, which is essential for degrading organic pollutants. Under high pollutant conditions, catalysts should maintain high activity, resist deactivation, and mitigate side reactions. This section provides a comprehensive overview of homogeneous and heterogeneous catalysts.

In homogeneous EF systems, soluble ferrous salts such as FeSO₄ are commonly used as catalysts. The core reaction is the Fe²⁺ in the solution and the H₂O₂ generated in situ at the cathode [25] (Equation (2)).

Although the reaction rate of a homogeneous system is fast, there are still some problems in homogeneous EF technology, which hinder its large-scale application. For example, the addition of iron salt in the solution will lead to a high concentration of Fe²⁺/Fe³⁺ in the final waste liquid of the reaction, and the used catalyst cannot be effectively recovered [63,64], which is not conducive to the reuse of the catalyst; furthermore, under typical acidic conditions (pH > 3.5), Fe³⁺ are rapidly hydrolyzed to form iron hydroxide (Fe(OH)₃) precipitates. This precipitation not only causes the loss of the catalyst and reduces the reaction efficiency, but also the formation of iron sludge requires secondary treatment, which increases the additional operating cost and environmental burden [65]. Furthermore, the reaction is limited by strong acidic conditions (pH 2.8~3.5), and the treated wastewater needs to be neutralized [42].

To overcome the above shortcomings of homogeneous systems, heterogeneous catalysts have emerged. This kind of catalyst exists in solid form and immobilizes the active site, thus broadening the working pH range and avoiding the iron sludge problem [7]. It is easy to recycle and reuse and is a hot spot in the current research of EF technology.

2.3.1. Heterogeneous Iron-Based Catalysts

Iron minerals are widely used in EF systems due to their low cost, environmental friendliness, and natural abundance. As the source of Fe²⁺, pyrite (FeS₂) can adjust the pH of the solution through surface reaction to maintain the acidic environment required for the Fenton reaction (Figure 3a) [66,67,68]. Its low toxicity and high catalytic activity make it excellent in a high-pollutant environment. Jiang et al. [69] developed an EF system based on natural pyrite (com-FeS₂), which almost completely degraded 0.2 mM sulfamethazine within 1.5 h under high chloride and high antibiotic concentrations. The system is effective in the range of pH 3.0–10.0, showing wide pH applicability.

Magnetite (Fe₃O₄) and goethite (α-FeOOH) are the most used iron oxide catalysts. Fe₃O₄ exhibits excellent Fenton-like activity due to its structure containing both Fe²⁺ and Fe³⁺. They retain their activity over a wider pH range (up to 5–9) and are easily separated and recovered due to their magnetic properties. Görmez et al. [73] reported that Fe₃O₄ nanoparticles can effectively degrade dyes and drugs such as chloramphenicol in an EF system, showing high catalytic efficiency under high organic loading. In addition, Fe₃O₄ nanoparticles can be modified through doping transition metals (Co, Mn) or surface functionalization (carbon coating), improving pollutant degradation efficiency and reducing iron sludge formation. In the study by Liu et al. [70], it was demonstrated that the peroxidase-like activity of Fe₃O₄ can be significantly enhanced through the use of acetylated chitosan-based hydrogels (Figure 3b). This modification allows for efficient degradation of organic pollutants, such as ciprofloxacin and bisphenol A, with notable degradation efficiencies observed. However, Fe₃O₄ catalysts also have the disadvantages of easy agglomeration and easy decomposition under acidic conditions. Therefore, to address this problem, Fe₃O₄@C composite was further synthesized as a heterogeneous catalyst [74]. Fe₃O₄@C is composed of a Fe₃O₄ core and carbon shell, which prevents iron dissolution and enhances the stability in high-pollutant environments. Its high surface area and functional groups (such as C=O, C-N) promote the adsorption of pollutants and H₂O₂. Beyond conventional core–shell structures, the one-step ionothermal carbonization of biomass to construct multi-element doped biochar offers a green alternative for designing efficient cathodic heterogeneous EF catalysts. Li et al. [71] synthesized Fe, N-doped biochar (Fe/N/biochar) using the ionic liquid [Bmim] [FeCl₄] as both Fe and N sources, coupled with sawdust biomass. The optimized Fe/N/biochar-600 exhibited a hierarchical porous structure and uniformly distributed active sites, featuring metallic Fe, Fe₃C, and Fe₃O₄ phases alongside pyridinic and pyrrolic N species, as shown in Figure 3c. It demonstrated remarkable catalytic activity in sulfadiazine degradation (rate constant: 0.024 min⁻¹) and mineralization (57% TOC removal). Mechanistic studies revealed that N sites facilitated the two-electron oxygen reduction to H₂O₂, while surface Fe (II) species activated H₂O₂ to generate ·OH as the primary radical. Notably, the catalyst exhibited progressively enhanced activity over 10 consecutive cycles. The high-resolution transmission electron microscopy analysis indicated that an interfacial crystalline-phase transformation formed an amorphous Fe^(II)O-Fe^(III)OOH surface layer, exposing more active sites and promoting ·O₂⁻ generation, thereby synergistically improving performance. This work provides a sustainable strategy and mechanistic insight for fabricating highly efficient and stable EF catalysts from renewable biomass.

MOFs are a prominent class of heterogeneous catalysts widely researched for their application in Fenton and EF processes, as shown in Figure 3d, particularly for wastewater treatment and pollutant degradation [72,75]. The application of MOFs as catalysts for EF systems is generally categorized into modified cathode catalysts and suspended particle catalysts. However, the current research mainly focuses on modified cathode catalysts. Fe-MOFs exhibit exceptional catalytic performance due to their ultra-high specific surface areas, tunable pore structures, and uniformly distributed iron active sites [76]. Najafzadeh et al. [77] enhanced the EF process using Fe-MIL-88B nano-catalyst with simultaneous cathode-anode rotation for Acid Blue 25 removal. Under optimal conditions (0.3 g/L Fe-MIL-88B, pH 3, current 0.228 A, rotation 100 rpm), the system achieved 92.3% dye removal in 90 min, representing 14.5% improvement over conventional processes, with COD and TOC removal efficiencies of 77.08% and 63.63%, respectively.

2.3.2. Other Heterogeneous Catalysts

Generally, most iron-based catalysts face challenges of active site loss caused by shedding and agglomeration during prolonged operation. This leads to excessive Fe²⁺/Fe³⁺ leaching and consequently lowers the pollutant removal efficiency in electro-Fenton processes [78,79]. Furthermore, these materials typically exhibit optimal catalytic activity only within a narrow pH range [80]. Therefore, researchers began to try to find other, more efficient and stable heterogeneous catalysts.

Cobalt, manganese, and copper compounds have all been demonstrated to possess Fenton-like catalytic activity, effectively activating H₂O₂ to generate active free radicals [81]. For instance, materials like Co-MCM-41, CuO nanoparticles, and MnO₂ have shown superior performance to iron-based catalysts in specific systems, particularly at neutral pH. Xu et al. [82] demonstrated that CoFe₂O₄ magnetic catalyst achieved 92.01% polyacrylamide removal in heterogeneous EF systems within 120 min at pH 3. Xue et al. [83] developed a trimetallic CuCoFe-LDH catalyst for heterogeneous EF degradation of acetaminophen, achieving 100% removal efficiency within 60 min and >80% mineralization across pH 3–9.

In addition, carbon-based materials are a good choice for catalysts due to their low cost, high electronic conductivity, abundance, chemical stability, and tunable surface, as well as their structural and chemical properties. Recent research focused on both metal-free carbon catalysts [84] and carbon-supported single-atom [85] or carbon-based metal nanocomposites [86]. Metal-free carbon catalysts include heteroatom-doped carbons (e.g., nitrogen-doped graphene, carbon nanotubes) and graphitic carbon nitride, which show high activity and stability for reactions of oxygen reduction and complex pollutant degradation. For instance, Qin et al. [87] doped oxygen into commercial carbon nanotubes as a bifunctional catalyst by acidification treatment with concentrated nitric and sulfuric acid solutions, which significantly facilitated the production and activation of H₂O₂ to •OH for phenol degradation. Single-atom catalysts (SACs) refer to isolated metallic atoms anchored to carbon carriers due to the high surface free energy of single atoms [88], which maximizes atomic efficiency and provides unique electronic properties. Hai et al. [89] fabricated a series of single Co-N_x sites from zeolitic imidazolate frameworks (ZIFs) via thermal pyrolysis, and also designed a core–shell-structured Co-N₃C₁@GC catalyst, which was synthesized by heterostructure Co/Zn-ZIF@ZIF-67 crystals through the direct carbonization, integrating a high density of locally optimized single Co active sites (Co-N₃C₁) as shown in Figure 4a. Like Co-SACs, other atomically dispersed metals (e.g., Ni, Fe) can also be fabricated by direct pyrolysis of ZIF incorporating metal. In addition, carbon materials combined with metal nanoparticles or nanostructures (e.g., cobalt- and porous carbon) can enhance catalytic activity. For instance, Wang et al. [90] synthesized NiCo alloy nanoparticles embedded in sulfur (S) and nitrogen (N) co-doped one-dimensional carbon nanofibers by combining a simple electrostatic spinning technique with a pyrolysis process, this combination not only provided a large number of active sites but also facilitated the rapid transport of both electrons and matter as shown in Figure 4b. Moreover, the NiCo₂-A@S-NCNFs catalysts exhibited remarkable durability.

3. Operation Strategy and Process Optimization for Electro-Fenton Systems

3.1. Regulation of Key Operating Parameters

3.1.1. pH

pH is one of the most critical parameters affecting the EF process. The optimal pH for conventional homogeneous EF systems typically ranges from 2.8 to 3.5, where Fe²⁺ solubility and •OH production is maximized [91,92]. However, this narrow pH range presents challenges for high-pollutant wastewater. High chloride concentrations (>0.5 M NaCl) can alter the solution pH through side reactions, thereby reducing the •OH yield [93]. Complex organic pollutants may act as buffers and complicate pH control. In addition, Fe³⁺ precipitates as iron sludge (Fe(OH)₃), decreasing the catalytic efficiency at pH > 4 [94]. At very low pH (<2.5), the formation of [Fe(H₂O)₆]²⁺ can slow down the reaction with H₂O₂, and the hydrogen evolution reaction becomes more pronounced, decreasing current efficiency for H₂O₂ production [95]. Additionally, H₂O₂ can be scavenged by H⁺ to form H₃O₂⁺, which is less reactive.

To overcome these pH constraints, particularly for industrial wastewaters that are often neutral or alkaline, several strategies have been explored [96]. Recent developments in heterogeneous EF systems have expanded the operable pH range to neutral (pH 6–7) and even slightly alkaline (pH 9) conditions through employing heterogeneous catalysts, as shown in Figure 5a, enhancing the applicability of EF technology for real wastewater treatment [37,97,98]. These catalysts can immobilize iron or other transition metals, preventing precipitation and maintaining catalytic activity outside the conventional acidic window [99]. For instance, Yu et al. [100] observed complete degradation and 82% mineralization of diclofenac sodium at neutral pH using hydrothermally synthesized pyrite. Wang et al. [101] used AC to load Fe₃O₄ to remove diuron (10 mg L⁻¹) from aqueous solution at pH 6.7. In another study, Pan et al. [102] explored the use of Cu-doped g-C₃N₄ matrix as a heterogeneous catalyst in the EF process using a one-pot pyrolysis method for the degradation of amoxicillin (100 mg L⁻¹) at neutral pH. The degradation of amoxicillin at neutral pH was more favorable than that at acidic pH due to the stronger scavenging ability of H⁺ to •OH. Zhu et al. [37] successfully developed a heterogeneous EF system with high efficiency and energy-saving properties within a wide pH range (1–9) by introducing electron-deficient boron at the single-atom Cu-N4 site. The energy consumption for treating actual coking wastewater was as low as 19.0 kWh kgCOD⁻¹, demonstrating excellent engineering application potential.

Another approach for successfully extending the working pH range of EF is the use of chelating agents or ligands. These materials form soluble substances by coordination with Fe²⁺/Fe³⁺ and prevent the precipitation of iron in the form of iron hydroxide at neutral and near-neutral pH values [94,103]. Varindani et al. [104] obtained a COD removal rate of 67% for mixed industrial wastewater using 80 mg L⁻¹ Fe²⁺ and EDTA at a near-neutral pH value, which is comparable to the COD removal rate (66%) at a pH value of 2.5. Zhang et al. [105] also used EDTA to eliminate rhodamine B (0.08 mM) in the UV-assisted flow-through EF system, resulting in a degradation of 90% at pH 7.0. According to the free radical scavenging experiment, •OH plays a leading role in the removal of pollutants, while the UV photolysis of Fe³⁺-EDTA complex leads to EDTA degradation.

Figure 5. (a) The effect of pH on the degradation performance. Reprinted with permission from Ref. [97]. Copyright 2025 John Wiley and Sons. (b) The effect of current density on the degradation performance. Reprinted with permission from Ref. [37]. Copyright 2024 American Chemical Society. (c) The schematic diagram illustrates energy conservation in the local O₂ concentration method. Reprinted from Ref. [106].

Figure 5. (a) The effect of pH on the degradation performance. Reprinted with permission from Ref. [97]. Copyright 2025 John Wiley and Sons. (b) The effect of current density on the degradation performance. Reprinted with permission from Ref. [37]. Copyright 2024 American Chemical Society. (c) The schematic diagram illustrates energy conservation in the local O₂ concentration method. Reprinted from Ref. [106].

3.1.2. Current Density and Voltage

Current density directly influences the rate of H₂O₂ generation and Fe³⁺ reduction at the cathode via the ORR and influences overall energy consumption [107]. Proper current density balances pollutant degradation efficiency and energy costs. While higher current densities generally lead to increased degradation rates, excessive current can result in parasitic reactions such as hydrogen evolution and direct reduction of H₂O₂, reducing process efficiency and increasing energy consumption [108]. For example, in the study of Olvera-Vargas et al. [20], the EF process was innovatively used to selectively synthesize oxalic acid or oxamic acid from acetaminophen. The results show that the product type was controlled by the current density: low current density is beneficial to the selective formation of oxalic acid, while high current density promotes the formation of oxamic acid.

Therefore, the increase in current density brings the benefits of accelerated degradation rate and shortened removal time but also brings the disadvantages of increased energy consumption and reduced current efficiency, as shown in Figure 5b. To achieve a balance between the desired efficiency and energy cost, the applied current density needs to be adjusted. Jiang et al. [109] demonstrated that a current density of approximately 2.5 mA cm⁻² provides optimal conditions for nanofiltration concentrate treatment in EF systems. Deng et al. [110] developed a dual-cathode pulsed current EF system, achieving 10% higher H₂O₂ accumulation and 93.8% Fe³⁺ reduction rate compared to conventional single-cathode systems (71%). The optimized system operated at 10 mA cm⁻² on a gas diffusion cathode for H₂O₂ generation and 2.5 mA cm⁻² on a carbon felt for Fe³⁺ reduction with 20s switching intervals, resulting in 98.5% sulfamerazine removal and 85.6% mineralization efficiency. The optimum applied current and the corresponding removal efficiency of the EF process from various studies as shown in

Table 2. Optimum current density values of the EF process in various studies.

Target Pollutant	Operating Conditions	Current Density	Removal Efficiency	Reference
Fipronil	pH 3.0, 0.50 mM Fe2+, 60 min	30 mA cm−2	~100% removal (20 mg L−1)	[45]
Benzophenone-4	pH 3.0, 0.75 mM Fe2+, 2–7 min	20 mA cm−2	100% mineralization (1–40 mg L−1)	[111]
p-Nitrophenol	pH regulated via secondary cell	2 mA cm−2	Near-complete degradation	[112]
Acid Red G dye	pH 3.0, 80 min	20 mA cm−2	94.05% removal (300 mg L−1)	[113]

.

3.1.3. Aeration Rate

Since oxygen is a key reactant for H₂O₂ generation, maintaining adequate oxygen supply is essential for efficient EF operation, which can raise the dissolved oxygen level and mass transfer rate in the system, and therefore promote H₂O₂ yield and system efficiency [114]. The study of Xia et al. [115] in the range of O₂ addition rate of 0–0.28 L min⁻¹ showed that when the O₂ addition rate exceeded 0.28 L min⁻¹, the concentration of H₂O₂ produced was the highest, which was 0.21 L min⁻¹. This is because when the O₂ addition rate exceeded 0.28 L min⁻¹, excessive bubbles were produced and the regeneration of H₂O₂ was reduced.

Insufficient O₂ supply leads to low H₂O₂ production rates, limiting the overall EF process efficiency. Conversely, excessive aeration might not always be beneficial and can increase operational costs. Therefore, the flow rate of air or pure O₂ sparged into the reactor needs to be carefully controlled. An optimal aeration rate ensures sufficient dissolved O₂ concentration at the cathode surface to sustain H₂O₂ production at the desired current density without causing excessive turbulence [91]. Chai et al. [116] investigated EF treatment of acid mine wastewater containing Fe²⁺ and Mn²⁺. Under optimal conditions (200 mA current, pH 4.0, 1.5 cm electrode spacing, 100 mL min⁻¹ aeration), Fe²⁺ and Mn²⁺ removal efficiencies reached 99.2% and 90.5%, respectively. Box–Behnken design optimization further improved the removal to 99.6% and 92.9%, respectively, at a current of 210 mA, spacing of 1.6 cm, pH of 4.0, and aeration rate of 110.4 mL min⁻¹, with an operational cost of only 0.56 $/m³.

However, due to the low solubility (~8 mg L⁻¹) and slow diffusion (~1.96 × 10⁻⁹ m²s⁻¹) of O₂ in water at room temperature and pressure, high-energy aeration is usually used to ensure sufficient oxygen supply [117,118], but this method consumes high energy (more than 75%) [119] and the O₂ utilization efficiency (OUE) is less than 1% [120]. Xu et al. [106] proposed a local O₂ concentration method. Compared with the traditional simple O₂ diffusion path, the oxygen utilization efficiency increased by more than 11 times, and the energy saving effect was achieved by 65.56% as shown in Figure 5c.

3.1.4. Catalyst Dosage

The concentration of the catalyst plays a vital role in the rate of •OH production. Initially, increasing the catalyst dosage generally enhances the degradation rate of organic pollutants due to a higher concentration of active sites for H₂O₂ [121]. However, the concentration of Fe²⁺ as a catalyst needs to be kept within a moderate range, and insufficient or excessive Fe²⁺ concentration will harm the EF process. Insufficient Fe²⁺ leads to underutilization of H₂O₂, while excess Fe²⁺ not only increases the operating cost and iron mud production but also results in scavenging of •OH through the reaction (Equation (4)) [122]:

Fe²⁺ + •OH → Fe³⁺ + OH⁻

(4)

The optimal Fe²⁺ concentration depends on the pollutant concentration and the specific EF system configuration. For example, Dominguez et al. [123] pointed out that the concentration of Fe²⁺ at 0.05 mM can significantly improve the efficiency of •OH generation. In addition, when the applied current intensity was at 400 mA, the complete degradation of 10 mg L⁻¹ lindane solution and 80% of TOC removal were achieved at 15 min and 4 h, respectively. Moreover, for some non-homogeneous catalysts (e.g., Fe₃O₄, FeOOH), the dosage was positively correlated with the removal of organic matter within a certain range, but excessively high dosages might inhibit the reaction [91]. For example, Shokri et al. [124] used Fe₃O₄/SiO₂ as a catalyst for the removal of chlorophenol from wastewater, and the optimal catalyst dosage (4.5 g L⁻¹) achieved nearly 99% removal of chlorophenol. The optimum catalyst dosage and removal efficiencies from the EF process as shown in

Table 3. The optimum catalyst dosage and removal efficiencies from the EF process from various studies.

Pollutants	Electrode Material	Operational Conditions	Catalyst Dosage	Removal Efficiency	Reference
Polyacrylamide	GF+ Ru-Ir/Ti electrodes	pH = 3, [Na2SO4] = 0.1 M, air flow rate = 1.0 L min−1, current density = 5 mA cm−2	CoFe2O4, 0.3 g L−1	92.01%	[82]
Fipronil	BDD anode + carbon cloth air-diffusion cathode	pH = 3.0, current density = 30 mA cm−2	Fe2+, 0.50 mM	85%	[45]
Benzophenone-4	BDD anode + air- diffusion cathode (PTFE-treated carbon cloth)	pH = 3.0, [Na2SO4] = 0.050 M, current density = 20 mA cm−2, [pollutant] = 40 mg L−1	Fe2+, 0.75 mM	88%	[111]
Coal gasification wastewater	IrO2-RuO2 electrode anode + carbon felt cathode	current density = 82.4 mA cm−2, electrode gap = 1 cm	Fe-based tourmaline, 7.57 g L−1	88.25%	[125]
Synthetic pharmaceutical wastewater	graphite plate anode + Fe@Fe2O3/GF cathode	pH = 3, air flow rate = 10 mL min−1	iron, 18.56%	89.9%	[126]
Pesticide wastewater	SS 316 anode + graphite cathode	time = 125 min, current intensity = 272 mA, [H2O2] = 8.102 × 10−3 M	zeolite Y-nZVI, 1.78 g	83.69%	[127]
Membrane-concentrated landfill leachate	DSA + needle coke electrode cathode	pH = 6, applied voltage = 8 V, the reaction time = 4 h	granular AC loaded with iron oxides, 16.67 g L−1	95.7%	[128]
Antipyrine	BDD anode + carbon-felt cathode	current density = 10 mA cm−2, [Na2SO4] = 0.05 M, air flow rate = 1 L min−1, electrode gap = 2.5 cm	AC -NZVI, 1.4 g L−1	97%	[21]
Ofloxacin	Pt anode + carbon felt cathode.	[Na2SO4] = 0.05 M, [pollutant] = 10 mg L−1, current density = 4 mA cm−2	CuFeO2@polyvinylpyrrolidone, (PVP), 0.4 g L−1	94.3%	[129]
Surrogate naphthenic acids	Ti/IrO2 DSA + GF cathode	pH = 8.6, air flow rate = 1 L min−1, current density = 6.25 mA cm−2	Fe-modified biochar, 0.5 mg L−1	70%	[130]
Acid Blue 25	graphite electrode (cathode and anode both made of graphite plates)	pH = 3, air flow rate = 4 L min−1, current density = 0.228 A, [pollutant] = 75 mg·L−1	Fe-MIL-88B nanocatalyst, 0.3 g L−1	92.3%	[77]
Nitrobenzene	titanium-based ruthenium dioxide anode + titanium mesh cathode	iron: carbon = 3:1, current density = 30 mA cm−2, [H2O2] = 50 mM, cathodic aeration = 0.8 L min−1	iron-carbon particle electrode, 100 g L−1	67.38%	[131]
Pesticide wastewater	graphite modified with industrial carbon black (anode) and iron plate (cathode)	current intensity = 253 mA, air flow rate = 1.56 L min−1, hydraulic retention time = 126 min	Fe2+, 0.63 g	77.1%	[132]
Reactive Yellow 186 azo dye	Ti anode + stainless steel cathode	pH = 3, [pollutant]= 0.15 g L−1, current density = 0.1 mA cm−2, [H2O2] = 0.2 g L−1	Fe, 0.015 g L−1	99%	[133]

.

3.2. Coupling of Electro-Fenton with Other Processes

Although EF technology demonstrates significant potential in degrading recalcitrant organic pollutants, treating industrial wastewater with high concentrations, high salinity, or complex compositions solely with EF often faces challenges such as high energy consumption, incomplete mineralization, or elevated operational costs. Therefore, coupling the EF process with other treatment technologies, such as biological methods, coagulation/precipitation, membrane separation, and other advanced oxidation processes, to form synergistic hybrid systems represents an important developmental direction for enhancing treatment efficiency, reducing costs, and achieving wastewater valorization.

3.2.1. Electro-Fenton Coupled with Photocatalysis/Ultrasound

The integration of photocatalytic processes with EF technology leverages light energy to activate catalysts, thereby generating electron-hole pairs and enhancing the efficiency of •OH production. Photocatalysis-EF utilizes ultraviolet radiation (including solar energy) to improve EF efficiency by promoting the reduction of Fe³⁺ to Fe²⁺, destroying Fe³⁺ complexes, and degrading by-products such as oxalic acid, which leads to higher degradation efficiency and faster Fe²⁺ regeneration [24,134].

In recent years, many studies have focused on the addition of photocatalysts (e.g., TiO₂) to implement the photocatalytic EF process [135,136]. For example, Jamal et al. [137] used Mn-doped Bi₂O₃ nanoparticles (4%) exposed to UV light, degrading 93.16% of methylene blue at 3.77 eV in 150 min. Chen et al. [138] found that the removal rate of tetracyclines by porous TiO₂/BiOI adsorbent was as high as 99.12% within 30 min, and it could be easily regenerated under light (60 min), and excellent recyclability (>98%) of up to 10 cycles could be obtained. Jiad et al. [139] used a new composite system combining photocatalysis (UV/ZnO) and EF to treat refinery wastewater, demonstrating that 90.15% of COD was removed in 40 min.

Ultrasonic radiation is another way to improve the efficiency of the EF process, which is due to the formation of extra •OH from water by cavitation in case of ultrasonic radiation, as well as the enhancement of mass transfer, accelerating the diffusion of reactants to the electrode and catalyst surfaces; this, in turn, cleans the electrode and catalyst surfaces, reducing passivation and maintaining their activity [92,140]. Ahmadi et al. [141] explored the treatment of saline petrochemical wastewater by the acoustic-electric Fenton method. It was found that 80.2% of COD could be removed after 120 min of treatment at 50 kHz and 1.2 V. Recently, Ghjair et al. [142] investigated ultrasonic-assisted EF process for the treatment of hospital wastewater using response surface methodology. The COD removal was 88.31% at optimum operating conditions of current density 10 mA cm⁻², H₂O₂/COD ratio of 0.2, and treatment time of 46.67 min. It was reported that compared with EF, ultrasound EF enhanced the degradation of 4-chlorophenol and achieved a removal rate of >99.9%, while the removal rates of EF and ultrasonic decomposition alone were 83% and 1.85%, respectively [143].

3.2.2. Electro-Fenton Coupled with Biological Treatment

Bio-electro-Fenton (BEF) processes integrate bio-electrochemical systems with Fenton reactions, utilizing biological electron generation for wastewater treatment with enhanced efficiency and sustainability [144]. The BEF technology, driven by microbial fuel cells (MFC) or microbial electrolysis cells, achieves pollutant elimination through dual-chamber microbial fuel cells while reducing energy consumption and operational costs [145,146]. There are two BEF configurations: one is MFC-in situ EF, where EF operates in the cathode chamber of MFC [147], and the other is MFC-ex situ EF, where the external EF reactor is powered by MFC [148]. It was reported that the continuous flow BEF equipped with Fe@ Fe₂O₃/graphite plate cathode enhanced the remediation of herbal wastewater and achieved a COD removal rate of up to 93% [149]. Sathe et al. [150] developed a BEF-MFC to degrade sodium dodecyl sulfate in wastewater. The degradation of sodium dodecyl sulfate in BEF-MFC reached 70.8 ± 6.4% within 120 min at neutral pH.

3.2.3. Electro-Fenton Coupled with Membrane Technology

While EF suffers from mass transfer limitations and inefficient utilization of •OH in practical applications [151], EF membrane technology, as a hybrid electrochemical advanced oxidation process with flow-through reactors, combines EF chemistry with membrane technology. The use of flow-through membrane electrodes significantly improves the kinetics and efficiency of the pollutant removal process compared to conventional batch electrochemistry, which can be attributed to the enhanced mass transfer of the target substance to the electrode surface and the elimination of byproducts from the catalyst surface due to convective flow [152,153].

Compared to other electrochemical advanced oxidation processes, EF membrane systems have considerable advantages and potential due to their high electroactive area, efficient mass transfer, and nano-structuring. Therefore, research on the design of smart membrane materials with on-demand nanostructures and filled active sites has attracted the attention of many scholars. Zhang et al. [19] constructed a multi-layer CNT/MoS₂/Fe₃O₄ composite heterostructure through a multi-layer film structure. Under optimized operating conditions, the specific energy consumption was 0.60 ± 0.01 kWh m⁻³, demonstrating extremely low energy consumption characteristics. Yin et al. [154] constructed a Fe/N co-doped biochar electrochemical membrane system to degrade tetracycline. This cathode material was rich in oxygen functional groups and nitrogen species, which promoted H₂O₂ production and accelerated Fe³⁺/Fe²⁺ cycling. The electrochemical membrane system exhibited excellent self-cleaning and tetracycline removal performance at neutral pH, and the porous biochar membrane had a higher electroactive area compared to the bulk electrode. Guo et al. [155] reported a nanoconfined flow-through EF system with MnO_x-in-CNTs for highly efficient degradation of aqueous micropollutants. MnO_x nanoparticles were encapsulated in a carbon nanotube cathode filter. Under nanoconfinement, the pathway of the EF process was transformed from classical radical oxidation to a Mn (IV)-mediated non-radical pathway, resulting in high pollutant removal efficiency over a wide pH range (3.7–11.2).

4. Challenges and Perspectives

4.1. Limitations of Existing Treatment Facilities and Opportunities

Existing mainstream technologies face significant limitations when treating high-pollutant load wastewater, which creates specific application opportunities for EF technology. Conventional biological treatment is often ineffective against highly toxic, recalcitrant organic compounds, since it suffers from a slow start-up and its microbial communities are vulnerable to shock loads. Common physicochemical processes (e.g., coagulation, sedimentation) primarily transfer pollutants rather than degrade them, potentially generating substantial chemical sludge and causing secondary pollution. Other AOPs (e.g., ozonation, UV/H₂O₂) are often limited when treating high-strength wastewater due to the massive dosage requirements of oxidants, high operational costs, or sensitivity to background water matrix components (e.g., turbidity, alkalinity). In contrast, the core advantage of EF technology lies in its ability to generate highly active ·OH in situ under ambient conditions, enabling deep mineralization of organics within a relatively compact system. Therefore, EF technology is not intended to completely replace existing processes but to play a key role in specific scenarios: serving as a pretreatment unit to detoxify wastewater and enhance its biodegradability for subsequent biological treatment or acting as a polishing step to thoroughly remove trace, bio-recalcitrant toxic pollutants that survive biological processes.

4.2. Material and Process Challenges

At first, cathode materials exhibit trade-offs, with graphene-based electrodes providing excellent conductivity but facing structural degradation and poor wettability; carbon nanotubes offer high surface areas but struggle with aggregation and quality issues; metal–organic frameworks allow for precise pore tuning yet lack electrical conductivity and hydrolytic stability in acidic conditions. Anode materials similarly present contradictions, as BDD electrodes exhibit superior corrosion resistance and extensive electrochemical windows but are economically unfeasible due to complex synthesis. Catalyst selection highlights the dilemma between iron-based systems that demonstrate exceptional Fenton activity but are prone to leaching and narrow pH tolerance, versus carbon-based alternatives that ensure better stability and pH flexibility at the expense of reduced catalytic efficiency and increased loading demands. The primary conclusion from this materials analysis is that no single material can concurrently resolve all EF system challenges, indicating that future advancements will likely stem from intelligent hybrid material combinations that exploit the strengths of diverse material classes while alleviating their respective shortcomings, rather than seeking universal material solutions.

In addition, treating wastewater with high COD requires a substantial input of electrical energy to generate sufficient •OH or other oxidizing species. The high pollutant load means that a large amount of H₂O₂ must be electrochemically produced in situ, which directly translates to elevated energy consumption per unit of pollutant removed. This not only increases operational costs but also limits the economic competitiveness of EF systems compared to alternative treatment technologies such as advanced biological processes or combined physicochemical methods. The excessive energy demand is further exacerbated by mass transfer limitations, low current efficiency under high substrate concentrations, and overpotential losses at the electrodes. In addition, fluctuations in influent wastewater composition may require higher energy input to maintain treatment efficiency, further reducing overall process sustainability.

4.3. Prospective Solutions

To tackle these material and operational limitations, several forward-looking strategies are recommended:

(1)

Developing Intelligent Hybrid Materials: Design composite cathodes, anodes, and catalysts that integrate the strengths of multiple material classes while mitigating their weaknesses. For instance, constructing core–shell structures where conductive carbon matrices stably encapsulate highly active iron sites can simultaneously address activity, stability, and conductivity issues.

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Optimizing Reactor Design and System Integration: Increase electrode surface area, improve flow dynamics, and enhance oxygen transfer to improve the efficiency of oxidant generation. More critically and efficiently integrate EF as an intensification unit—coupling it with anaerobic (pre-treatment) or aerobic (post-treatment) biological processes to create synergistic treatment trains that balance efficiency and operational costs.

(3)

Implementing Intelligent Process Control: Develop machine learning and AI-driven systems to establish dynamic models correlating operational parameters (current density, aeration intensity, pH adjustment, H₂O₂ dosage) with treatment efficiency. Through real-time data analysis and predictive control, the system can automatically adjust parameters to adapt to influent quality variations, minimizing energy and chemical consumption while ensuring effluent compliance.

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Conducting Life-Cycle and Techno-Economic Assessments: Embed Life Cycle Assessment (LCA) and Techno-Economic Analysis (TEA) into the EF technology development and evaluation framework. This ensures its role as a reliable and sustainable solution for high-pollutant-load wastewater treatment and clarifies its competitiveness and directions for improvement in practical applications.

5. Conclusions

In summary, the EF process is a highly promising and effective technology for treating high-pollutant load wastewater, yet its viability is constrained by practical challenges related to cost, stability, energy consumption, and byproducts. The advancement of this technology is driven by the parallel progress of material innovation and process optimization. This particularly includes developing efficient heterogeneous catalysts, integrating intelligent controls, and coupled processes to enhance overall efficiency. For future research, specific directions should include:

(1)

Designing Robust Materials for High-Load Environments: Developing low-cost, highly durable electrodes and poisoning-resistant catalysts suitable for continuous operation in high-strength, complex wastewaters, with a focus on antifouling properties and self-regeneration capabilities.

(2)

Optimizing Reactor Configuration to Overcome Mass Transfer Limitations: Developing scalable, efficient reactor configurations (e.g., three-dimensional electrodes, fluidized-bed reactors) and hybrid EF systems (e.g., EF-membrane filtration) specifically designed to enhance mass transfer and degradation efficiency under high pollutant concentrations.

(3)

Implementing Intelligent, Real-Time Process Control: Implementing machine learning and AI-driven systems to dynamically adjust key operational parameters (e.g., current density, pH, aeration) in real-time response to fluctuations in complex wastewater composition, ensuring robust treatment performance and energy efficiency.

(4)

Advancing System Sustainability and Practical Application Assessment: Conducting comprehensive assessments of the environmental footprint (e.g., carbon footprint) and techno-economic feasibility of EF systems treating high-strength wastewaters and developing sustainable materials like biomass/waste-derived catalysts to ensure environmental friendliness and scalability.

Through addressing these aspects, EF technology can evolve into a more cost-effective, stable, and environmentally friendly solution, becoming a core strategy for the treatment of complex industrial wastewater.