Membrane-Based Persulfate Activation for Wastewater Treatment: A Critical Review of Materials, Mechanisms and Expectation

Abstract

Membrane-based persulfate catalysis technology offers a dual approach to wastewater treatment by facilitating both physical separation and chemical oxidation. This innovative method significantly enhances pollutant removal efficiency while mitigating membrane fouling, positioning it as a promising advanced oxidation technology for wastewater management. This review comprehensively examines the critical aspects of material design, activation mechanisms, and technological challenges. Membrane materials and structures are crucial for enhancing the overall efficiency of the technology. By analyzing various catalytic materials and modification strategies, the study reveals the intricate interactions between membrane structures, catalytic performance, and pollutant degradation. The clear mechanism of pollutant degradation is the key to achieve accurate degradation. The research highlights three primary activation pathways: free radical, non-radical, and hybrid mechanisms, each offering unique advantages in addressing complex water contamination. Finally, the future challenges and research directions are put forward. Despite remarkable progress, challenges remain in membrane stability, economic feasibility, and large-scale implementation. Therefore, this study outlines the latest materials, mechanisms, and prospects of membrane-based persulfate technology, which are expected to promote its widespread application in environmental governance.

1. Introduction

With the rapid advancement of industrialization, the composition of wastewater from industries such as printing and dyeing, healthcare, and chemical manufacturing has grown more complex [1,2,3,4]. The rising prevalence of pollutants such as antibiotics, azo dyes, and polycyclic aromatic hydrocarbons, which are characterized by high toxicity, environmental persistence, and bioaccumulation potential, poses a significant challenge [5,6]. These pollutants cannot be completely mineralized by traditional methods like physical adsorption and biodegradation. The residual pollutants left in the environment have far-reaching impacts, threatening ecological systems and human health in multiple ways [7,8]. Thus, there is an urgent need to develop new water treatment technologies that are efficient, cost-effective, and environmentally sustainable.

Persulfate advanced oxidation processes (PS-AOPs) offer unique advantages due to their broad pH adaptability, cost-effectiveness of persulfate, and the generation of highly oxidative sulfate radicals (SO₄·⁻, E₀ = 2.5–3.1 V) and hydroxyl radicals (·OH, E₀ = 2.8 V), which enable non-selective degradation of pollutants. These features make PS-AOPs promising for advanced water treatment applications [9,10,11,12,13,14]. However, traditional PS-AOPs face practical limitations. Homogeneous catalysts risk secondary pollution and are difficult to recover, while heterogeneous catalysts are constrained by slow solid–liquid interfacial reaction rates and metal ion leaching. Additionally, activation methods relying on light or heat require significant energy inputs [15,16].

As a well-established separation technology, membrane technology offers distinct advantages in water treatment, including high separation efficiency, environmental sustainability, and ease of operation [17]. However, traditional membrane technologies primarily rely on physical screening mechanisms, which limits their ability to achieve deep degradation of pollutants simultaneously [18]. In recent years, there has been an increasing interest in integrating membrane technology with persulfate-based advanced oxidation processes (PS-AOPs) to address the limitations inherent in both approaches and to enhance the separation and degradation of pollutants.

To attain dual functionality characterized by efficient catalytic separation, researchers began modifying membrane materials early in the 21st century [19]. Initially, efforts concentrated on the straightforward combination of homogeneous catalysts with ultrafiltration membranes; for instance, metal ions supported by polymer membranes were explored to investigate the preliminary coupling between catalytic oxidation and membrane separation [20]. Subsequently, composites comprising ceramic films and carbon-based materials, such as graphene and carbon nanotubes, emerged as focal points of research due to their enhanced stability and regeneration properties for catalysts [21]. With advancements in nanotechnology and materials science, innovative designs such as smart responsive membranes and biomimetic catalytic membranes have surfaced. Additionally, the utilization of porous framework materials like covalent organic frameworks (COFs) and metal-organic frameworks (MOFs), along with various two-dimensional materials, has significantly enhanced both catalytic efficiency and selectivity [22,23,24].

Table 1. Comparison between membrane-based persulfate technology and other processes.

Treatment Technology	Advantages	Limitations	Cost
Membrane-based Persulfate Activation	Physical separation and chemical oxidation High pollutant removal efficiency Reduced membrane fouling Broad pH adaptability Continuous operation	Initial investment costs Membrane stability concerns	Moderate to high
Traditional Membrane Filtration	Physical separation Stable operation Mature technology	Severe membrane fouling No degradation capability Regular cleaning required	Moderate
Conventional Persulfate Oxidation	Strong oxidation capability High mineralization rate	No separation function Secondary pollution risk Poor selectivity	Low to moderate
Biological Treatment	Low operating cost Environmentally friendly Good nutrient removal	Limited for refractory pollutants Long retention time Sensitive to toxic substances	Low

compares membrane-based persulfate technology with other processes.

In this paper, the research progress, challenges, and future directions of membrane-based persulfate activation technology in wastewater treatment were reviewed, focusing on the following contents: (1) The selection of membrane materials and modification strategies; (2) selection of catalytic materials and optimization of membrane structure; and (3) the catalytic separation coordination mechanism and its practical application in different scenarios are introduced in detail; followed by (4) an analysis of current challenges and future prospects, research needs, and opportunities in the field.

2. Material Selection and Modification Strategy

2.1. Basic Membrane Materials and Main Performance Parameters

The selection and performance parameters of membrane materials have a decisive impact on the wastewater treatment effect [25]. According to material types and structural characteristics, membrane materials can be mainly divided into polymer membranes [26], ceramic membranes [27], and composite membranes [28]. Reasonable selection of membrane materials and optimization of their performance parameters are very important to improve the activation efficiency of persulfate and the removal rate of pollutants.

Polymer membranes are prevalent in water treatment due to their low cost and favorable processing properties [29]. Among them, polyvinylidene fluoride (PVDF) membranes stand out for their excellent mechanical strength, chemical stability, and oxidation resistance. The unique molecular structure of PVDF membranes enables them to endure oxidation treatment and facilitates combination with catalysts, making them an ideal substrate for preparing catalytic composite membranes [30]. It has been reported that PVDF-based composite membranes can be preserved in hydrochloric acid and sulfuric acid (90%) at 50 °C for at least 30 days [31]. PES membrane has good thermal stability and oxidation resistance. A thermal performance test by Yue et al. [32] showed that the glass transition temperature decreased from 184 °C to 157 °C with the increase in aliphatic chain size. However, its hydrophilicity is poor [33], and surface modification is needed to improve its performance. Polyacrylonitrile (PAN) membrane has excellent chemical stability, can withstand strong acids and bases, has good hydrophilicity and retention rate, and is widely used in ultrafiltration applications, but has low mechanical strength [34]. Polyamide (PA) membrane has excellent separation performance, but it is prone to chlorine oxidation damage in wastewater containing Cl⁻, so it needs to be modified by various means to improve its antioxidant capacity and durability [35].

Ceramic membranes have experienced rapid development in recent years, offering significant advantages over polymer membranes [36]. Research indicates that ceramic films exhibit excellent corrosion resistance across a wide pH range, can withstand temperatures up to 700 °C, and possess high mechanical strength with a service life of 5–10 years [37]. Among these materials, Al₂O₃ ceramic membranes are notable for their adjustable porosity and compressive strength [38], making them suitable for treating high-salinity wastewater [39]; thus, they have become the most commonly utilized ceramic membrane material. TiO₂ ceramic membranes demonstrate photocatalytic activity and can generate active free radicals under UV light; however, practical applications necessitate modifications for visible light responsiveness. For instance, nitrogen doping extends the light absorption range to 500 nm, facilitating in situ degradation of organic pollutants [40]. SiO₂ ceramic films are characterized by good hydrophilicity and exceptional anti-fouling performance [41]. ZrO₂ ceramic films exhibit even higher hydrophilicity and remarkable chemical stability within a pH range of 1–14 [42], although their preparation involves large energy consumption due to high sintering temperature [43].

Composite films can leverage synergistic performance by integrating complementary material advantages; however, challenges related to interface compatibility and long-term stability must be addressed [44]. Experimental findings indicate that organic–inorganic composite membranes can concurrently achieve the high flux characteristic of organic membranes and the superior stability of inorganic materials [45]. Through rational design of the functional layer, the retention rate of multilayer composite membranes can be enhanced by 20–30%. Surface-modified composite membranes significantly improve anti-pollution performance and increase the reversibility of membrane fouling by 40–60% [46]. While material type directly influences membrane properties, practical applications also necessitate consideration of key parameters such as catalytic activity, stability, and anti-pollution characteristics.

In the persulfate catalytic membrane system, the performance evaluation framework for membrane materials encompasses several critical aspects: catalytic activity [47], stability and durability, fouling resistance, and separation efficiency [48]. The collaborative optimization of these parameters is essential for enhancing overall treatment efficiency. Comprehensive evaluation of membrane material properties should consider the following parameters (

Table 2. Important parameters of membrane materials.

Performance Parameter	Representative Index	Optimization Strategy
Catalytic activity	Free radical yield	Catalyst loading
Stability and durability	Mechanical strength, chemical tolerance	Crosslinked modified, protective coating
Antifouling property	Pollution index	Hydrophilic modification, surface charge regulation
Separation efficiency	Flux and retention	Gradient hole structure and functional layer design

). During modification, it is crucial to balance trade-offs between performance parameters, particularly the inherent conflict between catalytic activity and stability. For instance, highly active catalysts tend to dissolve easily, but encapsulating the catalytic entities within the membrane can mitigate this issue and balance activity with stability [48]. Additionally, there is a trade-off between high throughput and high retention [49], as well as cost-performance considerations, such as ceramic membranes that offer long lifespans but entail high initial investment costs. Modular designs, like short-channel structures, can reduce unit processing costs [50]. Further research is necessary to investigate the interactions between different parameters and their mechanisms of influence on overall performance.

2.2. Selection of Catalytic Materials

The selection of catalytic materials plays a pivotal role in determining the performance of membrane-based persulfate activation systems. Research has demonstrated that the structural characteristics, surface electronic states, and interfacial reactivity of these materials directly influence the activation efficiency and pathways of free radical generation from persulfate [51]. Currently, mainstream catalytic materials can be categorized into three types: nanomaterials, porous materials, and novel catalytic materials. Optimizing their performance requires consideration of catalytic activity, stability, and compatibility with the membrane matrix [52]. The composition, application scenarios, and treatment effects are summarized in

Table 3. Nanometer materials.

Catalytic Material	Mechanisms	Application Scenarios	Treatment Effect	References
Fe3O4 nanoparticle	Efficient activation of Peroxymonosulfate (PMS) to produce reactive oxygen species (ROS)	Antibiotic wastewater	Short hydraulic retention time achieved 99% Atrazine removal in 5.7 s	Liangdy et al. [53]
Nitrogen-doped reduced graphene oxide (rGO)	N-doped active sites activate PS	Organic micropollutant wastewater	After 24 h of continuous operation of three fluoroquinolone antibiotics, the removal rate of pollutants was as high as 91%	Vieira et al. [54]
Polyvinyl alcohol (PVA) nanofiber membrane	SO4·− and ·OH are generated. The hydrophilic surface is rich in hydroxyl (-OH) groups and adsorbs methyl blue (MB) dyes	Dye wastewater	The degradation rate of MB reached 94%. The removal rates of MB and methyl orange (MO) double dye systems reached 76% and 68%, respectively	Pervez et al. [55]
CuO/Cu2(V2O7)/V2O5	The synergistic effect between Cu and V facilitates the regeneration of the catalyst and the generation of the active species SO4·−, ·OH, and singlet oxygen (1O2)	Antibiotic wastewater	The degradation rate of ciprofloxacin reached 90%	Xue et al. [56]
CuO@CuS	CuO can produce Cu (III), which collaborates with CuS to activate the PS reaction to produce SO4·− and 1O2	Antibiotic wastewater	Under the condition of pH 3, the degradation rate of tetracycline reached 87.4% within 3 h	Liu et al. [57]
Iron-supported nitrogen-doped carbon nanotubes (Fe-NCNT-W)	Electron transfer pathway, non-radical pathway of hypervalent ferrite species, and radical pathway of ·OH and SO4·−	Complex matrix wastewater	Fe-NCNT-W/PMS system showed high degradation efficiency for acidic orange 7 (AO7) and phenol mixed wastewater, and the Kobs of AO7 was 0.452 min−1, with little interference from solution pH or background matter	Wang et al. [58]
Mn3O4 nanodot-g-C3N4 nanosheet (Mn3O4/CNNS) composites	The surface-OH group of Mn (IV/III) reacts with HSO5− to form 1O2, Mn (III), and Mn (IV), yielding ·OH and SO4·−	Highly toxic and refractory wastewater	Within 60 min, the degradation rate of 4-chlorophenol was more than 90%, and the removal efficiency of TOC was more than 80%	Chen et al. [59]
NiCo@NCNT	NiCo alloy nanoparticles can effectively facilitate the transfer of electrons from contaminants (electron donors) to PMS	Complex matrix wastewater	100% Ibuprofen degradation was achieved, and the Kobs = 0.31 min−1. The degradation efficiency of MB, MO, naproxen, sulfamicloropyridine, and phenol reached 99%, 100%, 89%, 85%, and 78%	Kang et al. [60]
Zero-valent iron (ZVI)	The activation of PMS on the membrane surface generates SO4·− and Fe3+. Fe3+ has flocculation and can reduce membrane contamination	Algal wastewater	The flux reached 387.9 L·m−2·h−1 in the stationary phase, and no severe cell rupture was observed	Huang et al. [20]
MnO2	MnO2 activates PMS on the surface of the ceramic membrane to produce SO4·−, ·OH and 1O2	Organic refractory wastewater	The catalytic membrane with 1.67% MnO2 load also achieved 98.9% degradation of 4-hydroxybenzoic acid within 30 min	Wu et al. [61]
NiO/C	NiO activates PS to form SO4·−, and the reduced Ni (0) directly transfers electrons with PS through a non-radical pathway	Organic refractory wastewater	The removal rate of diclofenac by NiO/C membrane in the presence of PS was more than 97%, and showed good stability in the presence of HCO3− and Cl−.	Hesaraki et al. [62]
Co nanoparticle-modified N-doped carbon nanosheet array (Co-N-C)	Through plasma effect and molecular thermal vibration effect of N-doped carbon, Co NPs promote the decomposition of PS, generate 1O2, and degrade organic pollutants through non-free radical pathway	Solar powered interfacial water evaporation and treatment of organic refractory wastewater	Under sunlight, the water evaporation rate reaches 1.88 kg m−2 h−1, the solar-steam efficiency is about 87%, and the phenolic pollutants can be effectively removed	Cui et al. [63]
MnCo2O4	The spinel structure provides a variety of active sites that promote electron transfer through valence changes (Mn3+/Mn4+ and Co2+/Co3+), thereby activating PMS	Dye wastewater	The rhodamine B (RhB) degradation rate of 0.04 g L−1 by MnCo2O4/PMS system reached 99.92%	Shi et al. [64]

,

Table 4. Porous material.

Catalytic Material	Mechanisms	Application Scenarios	Treatment Effect	References
Cu@C/SiO2	Zero-valent copper generates Cu+ and Cu2+ by electron transfer, activates PMS to generate SO4·− and ·OH radicals, and degrades organic pollutants by 1O2 and other non-radical pathways	Antibiotic wastewater	95% tetracycline hydrochloride (TCH) can be degraded within 40 min at a reaction rate of 0.054 min−1	Shan et al. [65]
Mil-53 (Fe)	Under visible light irradiation, Mil-53(Fe) produces photoelectron (e−) and hole (h+) pairs. At the same time, the coordination-unsaturated metal sites (CUS) can activate PS to generate free radicals	Oil–water separation and dye wastewater	For different concentrations of water-in-oil emulsions (1 ppm, 5 ppm, 10 ppm), the separation efficiency remained above 90%, and the degradation efficiency of RhB, methylene blue, and methyl violet reached 100% within 60 min	Xiang et al. [66]
Cobalt-coated nitrogen-doped porous carbon materials (Co/CoOx@NC)	The extraction of hydrophobic organic pollutants simultaneously rejects the passage of natural organic matter (NOM) and water	Organic refractory wastewater	It can achieve a phenol removal rate of 80%, while the amount of PMS and catalyst are reduced by 40% and 97.8%, and can be used for soil remediation	Qiu et al. [67]
ZIF-67	The oxygen vacancy increased the active site of the catalyst, promoted the adsorption and activation of PMS, and the electrophilic attack characteristic of 1O2 selectively attacked SMX	Antibiotic wastewater	The removal rate of SMX was 96.3%, much higher than with membrane filtration alone (0.4%) and PMS oxidation alone (25.4%)	Ma et al. [68]

and

Table 5. Novel catalytic material.

Catalytic Material	Mechanisms	Application Scenarios	Treatment Effect	References
CC/FeOCl	FeOCl promotes the mutual conversion of Fe (II) and Fe (III) through internal charge transfer and activates persulfate to produce SO4·− and ·OH	Antibiotic wastewater	Under the action of electric field, the degradation rate of TC reached 93%, and the composite membrane showed excellent separation performance and anti-pollution performance	Wang et al. [69]
PLA-CNF@ZIF-8	ZIF-8 produces SO4·− and ·OH by activating persulfate	Dye wastewater	The degradation efficiency of RhB reached more than 90% and remained above 75% after five cycles of use	Moyo et al. [70]
B-NiFe2Ox	B doping in B-Nife2Ox forms oxygen vacancies (OVs), promotes electron transfer, and generates SO4·− and ·OH	Secondary sewage treatment	The removal rates of COD and UV254 in secondary wastewater were significantly increased, reaching 67.14% and 92.16%	Mao et al. [71]
NG/rGO/CNTs	Free radical and non-free radical synergism	Antibiotic wastewater	The SMX removal rate of NG/rGO/CNTs composite membrane reached 94.3%, and the corresponding SMX removal rate was 21.7 mg m−2·h−1, which was about 17% higher than that achieved with rGO/CNT composite membrane	Qian et al. [72]
Prussian blue analogue-modified Mg-Al hydrotalcite PBA−LDH	The metal site Co (II) rapidly activates the PMS to form SO4·− by single electron transfer. The resulting Co (III) is subsequently reduced by Fe (II), promoting the REDOX cycle of the Co species. Reactive species with oxidation potential are formed on the surface through non-radical pathways	Antibiotic wastewater	The degradation efficiency of sulfadiazine (SDZ) was increased to 92.8% and showed excellent anti-fouling performance	Liu et al. [73]
Iron embedded with S and N co-doped carbon (NSC-Fe)	The S and N doped carbon matrix improved electron transfer efficiency. Iron nanoparticles (Fe NPs) activated the PMS by single electron transfer in the carbon matrix, forming SO4·−. The Fe2+ can further generate SO4·−and ·OH by reacting with the PMS	Dye wastewater	The degradation efficiency of Orange II reached 97.7%, and the pH value in the range of 2.05–10.85 had little effect on the degradation efficiency	Yao et al. [74]
MIL-88A(Fe)/g-C3N4	The Fe2+ site activates PDS to form SO4·− by single electron transfer, simultaneously promoting the Fe3+/Fe2+ cycle and enhancing catalytic activity. The introduction of g-C3N4 increases the active Fe2+ content of MIL-88A(Fe) surface, promoting the adsorption and activation of PDS	Antibiotic wastewater	TC removal rate of 95.71% was achieved in a short time (<10 min)	Wang et al. [75]
Iron and nitrogen co-doped biochar membrane (Fe/N/BC membrane)	It has high electron transport efficiency and promotes the activation of PDS and the degradation of SMX	Antibiotic wastewater	The SMX removal rate of Fe/N/BC membrane reached 99.4%	Liu et al. [76]
Nitrogen-doped biochar supported monoatomic cobalt (CoNBC)	The PMS is activated by an internal electron transfer process to form a surface-bound active complex (CoNBC600-PMS*), which then degrades APAP through the electron transfer process	Organic refractory wastewater	APAP was completely degraded within 11 min and kobs = 0.46 min−1. The organic pollutants with low half-wave potential and high occupied molecular orbitals (EHOMO) were selectively degraded	Meng et al. [77]

.

2.3. Modification Methods

Membrane material modification techniques significantly enhance catalytic activity and separation performance by regulating surface properties, internal structure, and functional component distribution. Current mainstream modification methods include surface modification, grafting modification, material compounding, and novel functionalization strategies, with each method requiring targeted selection based on material characteristics and application scenarios [78,79,80].

Surface modification methods optimize membrane surface characteristics through physical or chemical means. Physical coating methods such as impregnation, spraying, and spin-coating can quickly construct catalytic layers, but the coating adhesion is weak and requires chemical crosslinking to enhance interface bonding [81]. Layer-by-layer (LBL) self-assembly utilizes electrostatic adsorption or hydrogen bonding to alternately deposit polyelectrolytes and nanomaterials, forming functionally controllable layers (10–100 nm) thick, though interlayer stacking may block pores and reduce flux [82]. Chemical oxidation–reduction methods generate active groups (such as -OH or carboxyl groups) through plasma treatment or UV light initiation, improving surface hydrophilicity and providing anchoring sites for catalysts [78].

Surface grafting modification fixes functional molecules to the membrane surface through covalent or non-covalent interactions. Free radical polymerization can introduce hydrophilic groups under mild conditions, but grafting rates are limited by monomer diffusion rates (typically < 10%) [81]. Click chemistry offers near 100% reaction efficiency and can precisely graft thiol or amino functional groups, though residual copper catalysts may cause secondary pollution [83]. Non-covalent modifications are simple to operate but prone to functional molecule detachment under long-term external forces, resulting in poor stability [84].

Material composite modification enhances membrane performance through multi-component synergy. In situ growth methods can achieve uniform loading within membrane pores, but high-temperature conditions may damage polymer matrices [85]. Mixed composite methods improve filler dispersion by optimizing dispersant ratios, but excessive addition can lead to membrane brittleness, requiring careful parameter design [86]. Chemical crosslinking enhances interface bonding by sacrificing porosity.

Other novel modification technologies provide innovative directions for functional design, with current challenges focusing on balancing modification effects and scalability. Future research should emphasize directional design of modification strategies.

2.4. Membrane Structure Optimization and Performance Enhancement

Precise regulation of membrane structure is the core strategy for improving catalytic-separation synergistic efficiency, with the key lying in balancing the competitive relationships between mass transfer dynamics, catalytic activity, and mechanical stability [87]. Through multi-scale structural design (from nanoscale pores to microscale surface morphology), the coupling process of pollutant interception and degradation can be significantly optimized.

Pore structure optimization focuses on the coordinated design of pore size distribution and porosity. Designing composite membranes with gradient pore sizes can simultaneously address selectivity and flux while providing hierarchical space for catalyst loading [88,89]. Porosity regulation requires balancing permeability and mechanical strength, but excessively high porosity may lead to structural collapse risks, which need to be maintained through methods such as crosslinking agents or nanofiber reinforcement [90,91]. Additionally, catalytic membranes with nanoscale pores in ceramic substrates or 2D spacing formed by assembling porous carbon materials (such as graphene oxide) combined with functional nanoparticles can enhance free radical-based oxidation processes [92]. Precise control of pore size or spacing improves reaction efficiency within these confined spaces.

Mass transfer channel design using biomimetic principles, such as leaf vein fractal structures, can reduce mass transfer resistance, shortening pollutant diffusion paths by 30–50% and improving contact efficiency [93]. Synergistic optimization of surface roughness and hydrophilicity can form a stable hydration layer, reducing pollutant adsorption [94]. However, excessive roughness may trigger local turbulence, leading to catalyst detachment and reduced service life [95]. Loading multi-layer catalytic membranes can also address limitations such as short retention time within the catalytic layer and interference from co-existing organic pollutants in actual water [96].

Interface structure engineering aims to solve compatibility issues between catalytic components and membrane matrix, with the core strategy being efficient anchoring and stable operation of catalysts through chemical modification, physical confinement, or dynamic bonding. Silane coupling agents (such as APTES) can increase interface bonding strength by 3–5 times (peeling force increasing from 0.5 N/m to 2.5 N/m). Li et al. used in situ growth strategies to fix ZnCo-ZIF nanoparticles on PAN fibers, achieving chemical bonding that prevents catalyst loss while demonstrating higher PMS activation efficiency [97]. Constructing core-shell structures can protect loaded catalysts to enhance dynamic interface stability while also strengthening synergistic interactions between components [98].

However, further research is still needed to explore the structure–performance relationship of membranes and develop novel structural design strategies to achieve continuous membrane performance improvement.

3. Activation Mechanism

3.1. Free Radical Pathway

The free radical activation mechanism is the most extensively researched and applied reaction pathway in PS processing technology. Through specific activation methods, the O-O bond in PS is cleaved to generate core active oxidative species SO₄·⁻ and ·OH, whose strong oxidative properties are key to treating complex organic pollutants [99]. Existing research further reveals that in some systems, multiple free radical species can be produced, including superoxide radicals (O₂·⁻). The advantages of the free radical mechanism lie in its exceptional reaction rate and powerful oxidative capacity, enabling the decomposition of various hard-to-degrade organic compounds. However, its inherent limitations cannot be overlooked: free radicals can be quenched by NOM or inorganic ions (such as Cl⁻, HCO₃⁻) in water, significantly reducing oxidation efficiency [100]. Moreover, when local free radical concentrations are too high, complex self-quenching reactions between free radicals will further decrease oxidation efficiency, and most free radicals lack selectivity, making it difficult to perform directional oxidation of specific pollutants [101]. More critically, some activators are prone to oxidative deactivation and may potentially cause secondary pollution [102].

To enhance the scientific effectiveness of the free radical mechanism, nanomaterials (such as ZVI, Fe₃O₄, Co₃O₄) or carbon-based materials (like graphene, carbon nanotubes) are often used as catalysts. These advanced materials not only significantly enhance catalytic activity but also effectively reduce metal ion leaching [103]. Based on the free radical characteristics shown in

Table 6. Advantages and disadvantages of common free radicals, reinforcement materials, and application scenarios.

Free Radical	Advantages	Disadvantages	Reinforcement Materials	Application Scenarios
SO4·−	High oxidation potential, suitable for degradation of refractory pollutants Long half-life period and can be diffused to the depth of membrane pores Strong pH adaptability (more stable in acidic conditions)	Some reactions depend on transition metal activation, which may lead to metal dissolution. In the presence of high concentration Cl−, it is easy to form chlorine byproducts	Transition metal oxides; Carbon-based material	High salt wastewater, wastewater containing perfluorinated compounds
·OH	The oxidizing capability is exceptionally strong, allowing it to indiscriminately attack most organic matter	It possesses a very short half-life and is readily quenched. Its effectiveness is significantly higher under neutral to acidic conditions, while its activity diminishes in alkaline environments	Ultraviolet photocatalyst; Bimetallic catalyst	Medical wastewater, dye wastewater
O2−·	It can act as an intermediate in chain reactions. Under alkaline conditions, it readily forms and exhibits a reducing effect on certain pollutants, such as Cr(VI)	The capacity for direct oxidation is limited and relies on the synergistic action of other free radicals. This process is prone to disproportionation, leading to energy loss	Materials rich in oxygen vacancies; Graphene quantum dots	Alkaline wastewater treatment, heavy metal reduction
Cl·/ClO·	Chloride ions (Cl−) can serve as a source of free radicals in high-salinity wastewater	There is a tendency to generate chlorine byproducts, which may elevate ecological risks	SO4·− and ·OH transformation	High salt and high Cl− wastewater

, researchers can more precisely understand and regulate the free radical activation mechanism, providing a crucial scientific basis for innovative wastewater treatment technologies.

3.2. Non-Radical Pathway

In recent years, non-radical pathways that have attracted significant academic attention primarily include mechanisms such as ¹O₂, surface-bound oxides, electron transfer, and high-valence metal species (such as Fe (IV)) [104]. Distinctly different from traditional radical oxidation mechanisms, non-radical mechanisms do not rely on generating highly active free radicals. Instead, they achieve degradation through direct interactions between catalyst surfaces and pollutants or indirect oxidation via intermediate products [104]. Some novel functional materials, such as oxygen vacancy-rich metal oxides, graphene nanodiamond (G-ND), and metal-organic frameworks (MOFs), can promote non-radical pathways by fine-tuning surface chemical properties and porous structures [105]. The advantages of non-radical mechanisms lie in their unique environmental adaptability: lower sensitivity to water background interference, relatively low energy consumption, and more precise and controllable degradation capabilities for specific pollutant types [104,106]. Compared to radical oxidation, the reaction process is milder, applicable across a wider pH range, and effectively avoids potentially harmful byproducts generated in radical mechanisms. However, their oxidation capacity is typically weaker, reaction rates are slower, and they have higher requirements for catalyst surface properties [106,107,108].

3.3. Radical-Non-Radical Hybrid Mechanism

The radical-non-radical hybrid mechanism combines the advantages of both mechanisms, achieving rapid oxidation through radical pathways while simultaneously improving reaction selectivity and stability via non-radical pathways through synergistic effects [106]. For instance, a series of advanced composite materials demonstrate exceptional multi-oxidation mechanisms when activating persulfate: generating SO₄·⁻ and ·OH while also enabling pollutant degradation through surface-mediated electron transfer or singlet oxygen generation pathways [109]. The advantages of the hybrid mechanism lie in its unique adaptability: it can accommodate complex water environments and exhibit significantly enhanced degradation efficiency in the presence of multiple pollutants. More critically, the hybrid mechanism can effectively reduce radical quenching effects and substantially improve catalyst utilization [110]. However, this innovative approach is not without drawbacks: its primary limitation is the increased complexity of the reaction process, which places more stringent requirements on catalyst design and preparation [111].

3.4. Process Integration and Optimization

Membrane-based persulfate technology can construct a multi-technology coupled system through synergistic integration with other water treatment processes, significantly enhancing the comprehensive treatment efficiency of complex wastewater. Specifically, when combined with biological treatment technologies (such as membrane bioreactors, MBR), persulfate oxidation precisely breaks down recalcitrant organic compounds (like antibiotics and dyes) into biodegradable small molecules [112]. The subsequent biological unit further mineralizes and removes nitrogen and phosphorus, while the membrane components effectively retain microorganisms to maintain system stability. However, careful handling of oxidant residuals that may potentially inhibit microbial activity is necessary [113].

In the field of electrochemical collaborative treatment, when coupled with electrochemical oxidation, electrical energy drives the anode to directly degrade pollutants or generate active oxygen, while the cathode simultaneously activates persulfate to generate free radicals [114]. This approach is particularly suitable for high-salinity wastewater treatment, but simultaneously faces severe challenges of high electrode costs and membrane material corrosion. Photocatalytic synergistic technology, as an emerging direction, utilizes photocatalysts loaded on membrane surfaces to activate persulfate under light irradiation, successfully achieving solar-driven efficient degradation, though still limited by membrane material light transmittance and light energy utilization efficiency [115].

Additionally, membranes modified with adsorption materials (such as MOFs and activated carbon) can enhance pollutant removal through a “concentration-oxidation” cycle, while coagulation pretreatment can significantly reduce membrane fouling and utilize residual metal ions to activate oxidants. For heavy metal wastewater, reduction–oxidation sequential processes (such as zero-valent iron reducing Cr (VI) followed by persulfate oxidation of organic compounds) demonstrate unique synergistic advantages [116].

4. Applications in Seawater Desalination

In desalination systems, the integration of membrane-based persulfate activation has revolutionized pretreatment and membrane performance enhancement strategies [117]. As a pretreatment stage, ceramic membranes functionalized with persulfate-activating catalysts (β-PDI/MIL-101 (Fe)) effectively reduce organic fouling potential by degrading NOM and algal organic matter (AOM) through radical-mediated oxidation (·OH and SO₄·⁻) and direct hole (h+) transfer mechanisms. The membrane properties are improved [118].

Beyond pretreatment, the technology directly enhances desalination membrane performance through three key pathways: persulfate-derived reactive species modify CaCO₃ and CaSO₄ crystallization kinetics, reducing scaling propensity compared to conventional antiscalants [119]; continuous in situ oxidation of adsorbed foulants maintains RO membrane flux with little deviation over long-term operation [120]; sustained generation of ¹O₂ and O₂·⁻ radicals achieves reduction of marine microorganism, extending membrane cleaning intervals [121]. It is worth noting that this technology has a unique advantage in the degradation of emerging pollutants in seawater, so that seawater with complex substrates can be processed through integrated treatment [122].

5. Current Technical Challenges

As a critical element of persulfate activation technology, the stability and longevity of the membrane directly influence the efficacy and economic viability of the entire treatment system. In practical operations, membrane materials are often exposed to extreme high-concentration pollutants, strong oxidants, and complex and variable water environments, which may lead to accelerated chemical degradation, mechanical damage, or significant declines in catalytic activity. Although ceramic membranes and composite membranes have better chemical resistance, their preparation costs are high and their complexity increases [123].

The economic viability of persulfate activation technology represents a critical factor constraining its widespread adoption. The substantial costs associated with catalyst preparation, intricate reactor design, and high operational energy consumption pose significant economic challenges for commercial applications. Compared to polymer membranes, the complex manufacturing process for these materials results in a cost increase of 30–50%, or even more. The preparation costs for nano-catalysts can reach several hundred dollars per kilogram, while the installation of high-performance membrane components constitutes 40–60% of the initial investment, significantly elevating the overall expenses associated with water treatment processes. Although advanced membranes initially demonstrate an impressive contaminant rejection rate of 80–90%, prolonged oxidative stress from persulfate radicals leads to irreversible alterations in pore structure. Consequently, regular maintenance and replacement of the diaphragm are necessary. Furthermore, energy consumption during persulfate activation and the costs associated with reaction reagents exacerbate the economic burden. Therefore, reducing technology costs while preserving processing efficiency has emerged as a critical challenge that researchers and engineers must collaboratively address [104,106,124].

From laboratory-scale to industrial application scaling, membrane-based persulfate activation technology confronts numerous critical challenges. Specifically, process parameters optimized under laboratory conditions may be difficult to fully reproduce in industrial applications, and the preparation and installation of large-scale membrane components involve complex technical difficulties. Even more troublesome is that the extremely high complexity and variability of industrial wastewater pose unprecedentedly higher requirements for the technology’s stability and adaptability [120,125].

6. Conclusions and Outlook

Membrane-based persulfate activation technology shows immense potential for advanced wastewater treatment. Future developments should focus on novel material design, deeper mechanism understanding, and process optimization. Interdisciplinary approaches combining advanced characterization techniques, theoretical modeling, and innovative material synthesis will be crucial in overcoming current technological limitations and realizing the full environmental governance potential of this promising technology. The following aspects should be further explored.

(1)

Targeting technological breakthroughs, developing efficient, stable, and low-cost novel membrane materials and catalysts is the core research demand for membrane-based persulfate activation technology [126]. Simultaneously, active exploration of bio-based materials and renewable resources in membrane material preparation should be pursued to promote technological greening and sustainability [127].

(2)

Deeply understanding the reaction mechanism of membrane-based persulfate activation is a critical pathway for enhancing technological performance. Current research requires significant advancements in the following areas: elucidating the precise kinetic mechanisms of both free radical and non-free radical reactions [128], establishing a comprehensive molecular mechanism model for pollutant degradation, and developing in situ characterization and real-time monitoring techniques to uncover the dynamic changes at the reaction interface. These mechanistic studies will provide a robust theoretical foundation for optimizing technologies [129].

(3)

At the practical level, optimizing the operational parameters and process design of membrane-based persulfate activation is a key approach to improving technological efficiency and economic viability [130]. Additionally, active exploration of deep integration with other wastewater treatment technologies (such as biological treatment and adsorption) should be pursued to achieve collaborative removal of multiple pollutants and resource recovery. Finally, intelligent process control strategy should be developed, and process optimization models based on big data and artificial intelligence established. The processing efficiency and economy can be significantly improved through refined process design [131].

Overall, membrane-based persulfate activation technology demonstrates significant advantages in wastewater treatment but still faces complex technical challenges. Through new material development, mechanism understanding, and process optimization, technological performance and economic feasibility can be significantly improved. In the future, intelligent reaction membranes, green and sustainable technologies, and industrial applications will become important development directions for this technology, promising widespread application in environmental governance.