1. Introduction
UF is a highly effective method for advanced water purification [1], serving as an efficient barrier against suspended particles, colloids and high molecular weight natural organic matter (NOM) based on size exclusion principles [2]. Nonetheless, membrane fouling frequently occurs due to the build-up of waterborne pollutants within the pores or on the surfaces of the membrane, markedly degrading its permeability, functionality, and lifespan [3]. This fouling is a consequence of processes including pore adsorption, pore blocking, and filter cake layer contamination, and it is affected by complex physical and chemical interactions between pollutants and the membrane materials, as well as among the pollutants themselves. Among various membrane pollutants, organic matter is one of the most problematic pollutants because it is ubiquitous in natural water [4] and may lead to severe and irreversible pollution [5]. Waterborne inorganic ions, like calcium, can speed up membrane fouling caused by organic substances [6].
Membrane fouling is categorized into reversible and irreversible fouling. Reversible fouling is induced by contaminants that deposit on the membrane surface, temporarily blocking the pores or physically adsorbing onto the membrane [7]. This type of fouling typically does not lead to significant scaling and can be removed through physical methods like backwashing, without lasting effects on the membrane’s performance. Conversely, irreversible fouling results from the penetration of NOM, particularly colloidal substances, humic substances, and biopolymers in the 3–20 nm range. This leads to changes in the membrane’s pore structure, causing severe hydraulic scaling, and potentially resulting in chemical irreversible scaling [5]. Physical methods are usually insufficient to remove this type of fouling. More aggressive cleaning methods, such as chemical cleaning, are required, which may potentially damage the membrane material [8]. Membrane fouling diminishes both the performance and water quality stability of filters [6]. To maintain the required flux level, adjustments to operating conditions, including pressure adjustments, more frequent backwashing, and chemical cleaning, are essential. Such measures can lead to higher energy use and operational expenses in the long run. Severe fouling necessitates frequent membrane cleaning, which may easily cause damage to the membrane material and reduce its lifespan [9], thereby increasing maintenance and replacement costs for equipment. It is essential to effectively alleviate membrane fouling issues to improve efficiency and stability of the filter, as well as for lowering costs and complexity. Pretreatment, membrane modification, and regular cleaning can mitigate membrane pollution. This paper concentrates on the most extensively studied pretreatment methods currently available.
The method for mitigating organic-induced membrane fouling is to adopt pretreatment, including coagulation, adsorption and oxidation [10]. Among them, coagulation selectively removes high molecular weight biopolymers and larger substances but is less effective for small hydrophilic and neutral organics [11–13]. This can result in cake buildup, enhanced membrane fouling, and solid waste generation. Furthermore, the requirement for precise dosing and the potential for exacerbating fouling makes the application of coagulation challenging [14]. The effect of adsorption on removing colloidal substances is limited. As for the mitigating effect of adsorption pretreatment on membrane contamination, it is still widely debated [15]. Organic fouling is affected by the amount and properties of organics, attributable to their varied chemical makeup and structural heterogeneity [16]. Under conventional chemical oxidation, the mineralisation rate of organics is relatively low, usually less than 20%, at dosages and contact times commonly used in water treatment [17, 18]. Compared with conventional chemical oxidation, AOPs have a relatively high mineralization rate due to the in-situ generation of free radicals with high redox potential and strong activity. AOPs can achieve significant micropollutant degradation and organic mineralization, not to mention better mitigation of organic pollution [19, 20].
Persulfate-based Advanced Oxidation Processes (PS-AOPs) entail the production of sulfate radicals (
) by triggering peroxymonosulfate (PMS) or perodisulfate (PDS) via catalysts—such as transition metal ions or heterogeneous catalysts—or through exposure to radiation, including heat or light, as shown in Fig. S1. Compared to hydroxyl radical AOPs, PS-AOPs mainly acts as
. They exhibit greater stability, simpler storage requirements, and a more potent oxidation ability. However, they are still in the research stage and have limited practical applications, lacking the maturity of ·OH-AOPs. The formation of
can trigger a series of reactions that include the creation of additional reactive species, notably ·OH.
is recognized as a potent oxidizing agent due to its substantial standard redox potential (E0 =2.5–3.1 eV), which is similar in magnitude to that of ·OH (E0 =1.9–2.7 eV) [21]. Owing to its electron transfer capabilities,
possesses a longer half-life and exhibits greater reactivity selectivity towards organic compounds compared to ·OH [22]. Consequently, PS-AOPs are capable of efficiently decomposing a wide array of organic pollutants with less dependency on the water matrix composition. Given the strong oxidative nature of
, PS-AOPs, as a cutting-edge class of AOPs, may potentially enhance membrane functionality by mitigating the concentration of contaminants that afflict membranes. The integration of PS-AOPs pretreatment with UF membranes to simultaneously control membrane contamination and produced water quality has been widely investigated in recent years.
This paper offers a comprehensive overview of PS-AOPs and their activation mechanisms. It covers the diverse methods of PS activation via both homogeneous and heterogeneous routes, encompassing techniques like heat, ultraviolet radiation, transition metal ions and compounds, carbon-based materials, as well as photocatalysis, among others. It also systematically describes the advancements of membrane fouling mitigation and organic pollutant removal in UF process by coupling PS-AOPs. Additionally, it concludes with insights into future directions.
2. PS Activation Methods
In this paper, the two major systems of homogenous and heterogeneous catalysis activating PS are introduced. To quantitatively evaluate the efficacy of various activation techniques, a specific organic compound was chosen for analysis, focusing on its removal rate. By exposing the same pollutants to different activation methods, variations in efficiency were observed. Sulfamethoxazole (SMX) has been extensively studied due to its prevalence in surface waters, wastewater effluents, and drinking water sources, posing challenges for traditional treatment approaches. SMX was selected as the target compound for PS degradation activation. Table 1 offered a comprehensive summary of PS activation methods employed in SMX degradation, detailing their corresponding reaction conditions. Through quantitative assessment of SMX removal efficiency, insights into the performance and effectiveness of different activation methods can be obtained. The principle, benefits, and drawbacks of each activation method will be elucidated in conjunction with Table 1.
2.1. Homogeneous Activated PS
In homogeneous systems, PS is activated to produce
. The activation efficiency is enhanced in homogeneous systems compared to heterogeneous ones, which is attributable to the more complete contact between PS and the activator in the former. Common activation methods fall into three types: UV, heat and transition metal ion activation.
2.1.1. UV/PS
UV serves as a clean alternative for activating PS without chemical release and sludge increase. The UV/PS process has demonstrated its efficacy in breaking down a wide range of novel organic pollutants present in water. UV radiation directly breaks the chemical bonds of contaminants, decomposes NOM and inactivates microorganisms. Consequently, it is frequently employed in pretreatment to reduce membrane fouling [23]. Generally, a wavelength of 254 nm is utilized for UV activation. UV activates PS by providing energy to break the O-O bond, thereby producing by the mechanism as shown in Eq. (1), (2).
The O-O bond energy in PDS is 144 kJ/mol, and the bond energy in PMS is 370 kJ/mol. The PDS bond energy is lower, making it more readily activated by UV [24]. UV/PDS has been demonstrated to be more effective, as shown in Table 1. UV/PS is an effective technology for pollutant degradation. It works by creating free radicals that act on organic compounds, and also directly breaks down the structure of organic substances via UV.
Being environmentally friendly, UV/PS does not produce waste. During operation, just switching on the UV lamp is sufficient without any operational complexity. However, this activation method requires ensuring the presence of additional UV equipment, necessitates continuous application of UV, and takes a longer time to achieve a good degradation effect, leading to high energy consumption. In addition, UV/PS is insufficient for highly chromatic raw water. This is because suspended matter in the water absorbs or scatters UV light, making it difficult to penetrate.
2.1.2. Heat/PS
Direct activation of PS by heating is an early and simple method. 140 – 213 kJ/mol of thermal energy breaks the O-O bond of PS to produce
. The mechanism is depicted in Eq. (3), (4).
High temperatures speed up the activation of PS, which leads to an increase in the active substance and a faster reaction rate. As observed in Table 1, rising temperatures result in significantly reduced reaction times and a marked improvement in degradation efficiency.
Heating is a proven method for activating PS, offering simplicity and absence of secondary pollution. However, it comes with the drawback of requiring energy-intensive heating equipment, which leads to high operational costs. Not to mention, the reactor and materials must be capable of withstanding high temperatures. Recent studies have focused more on harnessing waste heat from industrial operations to activate PS, thereby addressing the energy consumption challenge [25].
2.1.3. Transition metal ion/PS
Transition metal elements have a valence electron layer structure characterized by (n-1)d1–10 ns1–2. Their central atoms have both filled and vacant d orbitals, which confer both electrophilic and nucleophilic properties to these metal centers. This dual nature significantly reduces the activation energy for the reaction between PS and pollutants and acts as a catalyst [26]. The activation of PS using transition metal ions (such as Fe2+, Ag+, Mn2+, Co2+, Cu2+, etc.) represents a practical approach for the field application of PS oxidation technology [27]. This paper focuses on ferrous iron (Fe(II)) as a representative transition metal ion. Ferrous iron’s (Fe(II)) widespread use as a metal activator in PS can be attributed to its natural abundance, high reactivity, and eco-friendliness [28]. It is a safer and more cost-effective option compared to cobalt (Co2+), which is carcinogenic, and copper (Cu2+), which is pricier. The Fe(II)/PS reagent comprises a transition metal and an oxidizing agent, thus sharing similarities with the Fenton system. The electron transfer-mediated activation of PS by Fe(II) results in the production of
, and the underlying mechanism is illustrated in Eq. (5), (6). As suggested by Eq. (1), (3), (5), the heating and UV activation of PDS yields two sulfate radicals, whereas only one radical is typically obtained through transition metal ion activation. Fe(II) activation is not as effective as heat and UV.
Homogeneous catalysis based on transition metal ions can typically be carried out at ambient temperatures and pressures without the need for external energy inputs. Transition metal ions exist in a simple form and directly activate the PS. They are also easily synthesized and relatively low in cost. However, there are still three key challenges for the Fe(II)/PS system: 1. It is significantly influenced by pH, necessitating the use of strongly acidic conditions for the activation of Fe(II). Table 1 showed that to achieve higher SMX degradation rates, the controlled pH is very low. Fe(II) is more stable and effective as an activator under acidic conditions. 2. Additionally, the control of metal ions in the solution can be challenging, leading to secondary pollution. 3. The recycling of Fe(III) back to Fe(II) is also sluggish, and the accumulation of iron oxide sludge due to hydrolytic precipitation can pose further issues. To mitigate these challenges, it’s crucial to employ chelating agents, reductants, and methods like UV radiation and electrochemistry to speed up the production of active substance and the conversion of Fe(III) to Fe(II) [28]. These enhancements help to optimize the reactivity and effectiveness of the Fe(II)/PS system, thereby diminishing the operational and environmental impacts associated with its use.
2.2. Heterogeneous Activated PS
In contrast to homogeneous systems, the activation of PS in heterogeneous systems results in the production of a more diverse spectrum of free radicals, including, ·OH, ·O2−, and 1O2 [29]. In homogeneous systems, UV and heat activation require external energy inputs, and the activation of transition metal ions is often constrained by pH conditions. In contrast, heterogeneous catalysts offer advantages such as stability, reusability, and a broader pH range of applicability [30]. Current heterogeneous PS activation methods primarily include activation by transition metal compounds, carbon materials, and photocatalysis.
2.2.1. Transition metal compounds/PS
Oxides of Co, Fe, Mn, and Ti, which are part of the d-block of transition metals, along with Cu oxides from the ds-block, exhibit PS catalytic activity. The interaction between multiple transition metals can enhance the Mn+ to M(n+1)+ redox cycle and expedite the PS activation process. The mechanism by which heterogeneous transition metal compounds activate PS is analogous to that of homogeneous transition metal ions. They activate PS to generate free radicals through electron transfer, and the activation mechanism is shown in Fig. S2.
The development of transition metal compound catalysts and the evaluation of their effectiveness in catalyzing PS degradation of organic pollutants and emerging contaminants have been the recent research focus. Various transition metal oxide materials have been developed for the activation of PMS and PDS, including Co3O4 [31, 32], Fe2O3 [33], MnO2 [34], and CuO [35], and so on. Among them, cobalt oxides show the strongest activation ability for PMS [36]. Wang et al. [37] prepared Co3O4 nanorods by hydrothermal method to activate PMS to degrade phenol. The degradation of phenol with a concentration of 20 mg/L reached over 90% after 90 min. It should be noted that in homogeneous catalytic systems, reactants and catalyst molecules are uniformly mixed within the solution, facilitating close interaction between the reactant molecules and the active sites. However, in heterogeneous catalytic systems, the active sites are often masked by the internal structure of the catalyst, leading to that not all particles can effectively participate in the reaction. Moreover, the different physical forms of the catalyst and reactants introduce mass transfer limitations. As a result, heterogeneous catalysis typically exhibits a lower number of active sites and lower catalytic efficiency compared to homogeneous catalysis. These monometallic heterogeneous catalysts require increased dosage, higher oxidant concentration, and prolonged reaction time to achieve effects comparable to homogeneous catalysis [26]. Binary composite catalyst makes up for the shortcomings of traditional monobasic catalyst, such as low catalytic efficiency and difficult recovery. They consist of two transition metals at the core and exhibit significantly higher catalytic effects than single-element catalysts. This was confirmed by the research in Table 1. Feng et al. [38] used nano CuFeO2 prepared by modified hydrothermal method to catalyze PMS, and sulfadiazine could be completely removed in 20 min. The preparation process of binary catalysts is more complicated than that of monobasic catalyst. Binary catalysts with a spinel structure, such as CuFe2O4 and CoMn2O4, need to provide higher energy to combine the two metal elements into bonds when forming crystal nuclei [26]. Therefore, it needs to be carried out at high temperature and high pressure. This significantly raises the use cost of the catalyst.
Transition metal compounds have a broader pH range of applications. Monobasic heterogeneous catalysts, despite their low catalytic efficiency and the same difficulty in recovery as transition metal ions, can have their drawbacks mitigated by binary catalysts. In the study of binary catalysts, the first step is to identify two or more metals that can act synergistically, such as Cu with Fe, Cu with Co, Fe with Co, etc., to enhance the rate of free radical production. Next, the optimal ratio of metals is determined based on the specific amount of PS and catalyst loading. Lastly, it is necessary to verify the reusability of metal catalysts. The stability of catalysts is contingent upon preventing metal leaching. Feng et al. [39] revealed that after four cycles, the performance of α-Fe2O3/Cu2O significantly degrades due to Cu(I) leaching. Robustness is a critical consideration for the broad adoption of binary catalysts in the future. However, these catalysts require careful consideration due to their complex synthesis and high costs.
2.2.2. Carbon materials/PS
Carbon materials exhibit distinctive characteristics such as a large specific surface area, well-structured pores, superior adsorption capabilities and electrical conductivity, along with a diverse spectrum of surface functional groups (e.g., -OH, -C=O, and -COOH) [40]. They also boast significant mechanical robustness and cost-efficiency, positioning them as attractive candidates for the development of PS activation [41]. Ye et al. [42] demonstrated that hybridization within the carbon backbone and the use of sp2 carbon materials can boost the catalytic activity of PS activation. Carbon materials can donate electrons to PS, resulting in the formation of reactive radicals as described in Eq. (7), (8). The specific pathway for reactive oxygen species (ROS) generation is depicted in Fig. S3.
Carbon materials degrade pollutants through two mechanisms: radicals such as
and ·OH, and non-radicals like 1O2. Table 1 illustrated the degradation efficiency of unmodified carbon materials and upgraded carbon materials, which incorporate heteroatoms, against SMX. Preliminarily, it can be observed that the unmodified carbon materials exhibit the least effective degradation. The limitation is primarily a result of the decreased number of reactive sites within carbon materials, their vulnerability to deactivation following PS activation, and the propensity of carbon materials to agglomerate, collectively affecting mass transfer efficiency [43]. Researchers have primarily focused on upgrading carbon materials by integrating heteroatoms, such as nitrogen or metal oxides, into their structures [44]. This modification of the carbon’s electronic framework through doping procedures serves to augment PS activation, resulting in the generation of highly oxidative active species and significantly boosting the catalytic performance of carbon materials. Each type of doping exerts distinct catalytic effects. Ye et al. [42] developed biochar-derived catalysts via a process encompassing graphitization and nitrogen doping, which triggered a multiphase catalytic reaction with PMS, resulting in a sevenfold increase in degradation rate compared to unaltered biochar. Sun et al. [45] synthesized nitrogen-modified carbon nanotubes (N-CNTs), which exhibited a 7.8-fold enhancement in catalytic activity for PMS phenol oxidation compared to unmodified carbon nanotubes. Wang et al. [44] demonstrated that nitrogen-doped porous carbon outperformed nitrogen-free porous carbon and even surpassed the efficiency of the most effective homogeneous Co2+/PMS activator. The nitrogen-doped porous carbon/PMS system exhibited excellent performance across a wide pH range (3.3–9.6) with notable reusability. Liu et al. [43] developed Co3O4@CNTs nanocomposites, achieving a 94.8% degradation efficiency of norfloxacin within 60 min with a catalyst dosage of 120 mg/L and PMS addition of 0.5 mM, surpassing the removal rate of CNTs alone on norfloxacin, which stood at 41.7%.
In metal-doped carbon materials, the embedding of metals enhances carbon stability. Additionally, the carbon matrix acts as a carrier to minimize metal leaching. The combination of metal and carbon materials functions synergistically. Yan et al. [46] prepared Fe0/Fe3O4@porous carbon as an activator of PMS. After five times of recycling, the Fe content still remained 99.73%. Similarly, Nguyen et al. [47] developed a hybrid CoO-3D ordered mesoporous carbon nitride catalyst. It effectively activated PMS, and no Co2+ leaching was observed over five cycles. Additionally, the data from Table 1 indicated that the efficiency of heterogeneous catalysts decreases in the order of: metal-doped carbon materials, binary catalysts, and monobasic catalysts. Similarly, when comparing carbon materials, the order is: metal-doped carbon materials, nitrogen- doped carbon materials, and unmodified carbon materials.
Carbon materials provide several benefits, such as cost-effectiveness [48], absence of environmental concerns related to toxic metal leaching [49], and unique catalytic capabilities for PS activation when compared to heterogeneous metal compound catalysts. While carbon materials can serve as both adsorbents and catalysts, their catalytic stability is often insufficient, and the active sites are susceptible to deactivation. Enhancements in the structure, composition, and functional groups of carbon-based materials are necessary to address these limitations. Doping serves as an efficient way to compensate for their defects in carbon materials. Carbon materials, when doped with metals, are widely studied and can be used to synthesize various catalysts, leveraging the advantages of both components. Additionally, the complexity of the carbon structure, combined with the functionalization of heteroatom-doped groups, leads to a more intricate activation mechanism of PS, necessitating further investigation.
2.2.3. Photocatalysis/PS
Photocatalysis has emerged as a sustainable and economical strategy for activating PS, presenting a viable path for the generation of
[50]. Solar energy, especially visible light—comprising approximately half of the solar spectrum—acts as the principal source of energy for photocatalytic processes [51, 52]. Visible light waves are longer and carry less energy than UV rays, making them too weak to directly activate PS. To overcome this limitation, the catalyst that can harness visible light energy and effectively convey it to PS is necessary. Under sunlight exposure, the photocatalysts absorb light, promoting electrons to its conduction band while leaving holes in its valence band, thus generating electron-hole pairs [53]. The generation and isolation of electron-hole pairs are the basis of photocatalytic activity, facilitating the transformation of light energy into chemical energy. However, when electrons and holes recombine, they reduce the effectiveness of photocatalysis. Electrons (e−) and holes (h+), along with PS, further react to generate
, ·OH. This process not only avoids the recombination of electrons and holes but also raises the quantity of reactive free radicals. The mechanism is described in Eq. (7), (8), (9), (10).
In recent years, traditional semiconductor materials, graphite carbon nitride and Fenton-like catalysts have been widely used as photocatalysts. Many advanced visible light photocatalysts are synthesized by modifying titanium dioxide and carbon nitride doped with metal oxides such as Fe, Bi, Co and Cu. For instance, reduced graphene oxide/TiO2 [54], TiO2@CuFe2O4 [55], Co-TiO2 [56] and Co-C3N5 [57] have been developed and utilized as activators for PS. The combination of PS and photocatalysis has demonstrated significant synergistic effects [58]. Zhang et al. [59] demonstrated that the hollow-CuWO4/PDS system was highly effective in decomposing 98% of SMX under visible light irradiation. In the absence of light, however, only 15% of SMX was degraded. Golshan et al. [55] found that the TiO2@CuFe2O4/PMS system had achieved a good degradation rate of 2,4-dichlorophenoxyacetic acid, which was 88.9%, but it was further improved to 94.9% under the action of UV. Zhang et al. [60] developed a CN/BiOBr heterojunction. Under the illumination of 420nm, CN/BiOBr/PMS removed 98% of tetracycline (TC) in 10 minutes. After 8 cycles, the activity kept good, with a final TC removal rate of 77.7%. In contrast, without light, the TC removal rate after the same number of cycles was only 5.9%. This highlighted the significant role of visible light in the self-cleaning efficacy of the system. Tian et al. [61] meticulously reviewed the photocatalytic activity of Ti, Fe, Bi, Cu, Co, Mn, Mo, MOF, and carbon nitride-based catalysts in the activation of PS. This review did not duplicate their findings. In addition, Table 1 summarizes the research on photocatalytic activation and degradation of SMX by PS. Compared with other PS-AOPs, it proves that the system is unexpectedly efficient.
Photocatalytic technology is extensively used for pollutant degradation because of its mild and environmentally friendly conditions. It combines the dual advantages of thermal catalysis and catalysts. By utilizing ultraviolet or visible light as a driving force, photocatalytic technology not only expands the range of light sources but also enriches the variety and quantity of active substances, significantly enhancing catalytic efficiency. Additionally, photocatalytic materials demonstrate good stability over a wide pH range (3.01 to 9.03) [60], making them more adaptable in practical applications. However, to further enhance the performance of photocatalytic PS activation, current research is gradually transitioning from single transition metal oxides and carbon materials to multi-metal oxides and metal-doped carbon materials. This developmental trend not only increases the complexity of catalysts but also raises requirements for material selection and design. The design of photocatalysts needs to consider their sensitivity to light, light absorption capacity, and synergistic interaction with light to achieve optimal photocatalytic performance. Despite these advancements, photocatalytic technology still faces some challenges. For example, metal-based catalysts may experience metal leaching issues, while carbon materials generally have weak absorption capabilities for visible light, limiting their application in photocatalysis. Moreover, the process of modifying catalysts is intricate and expensive, and the recovery and regeneration of these catalysts are urgent issues that need resolution. These challenges emphasize that in developing new photocatalysts, besides considering their performance, attention should also be given to their environmental friendliness, cost-effectiveness, and sustainability.
Based on the introduction and discussion of PS-AOPs in this section, we summarized the advantages and disadvantages of homogeneous and heterogeneous catalytic PS activation methods, as shown in Table 2.
3. Effect of PS-AOPs on UF Membrane Fouling
Membrane separation technology is a physical separation method. Organic pollutants are not directly degraded by the membrane but instead accumulate on its surface, leading to membrane fouling that may not be reversed through conventional cleaning methods. This significantly shortens the lifespan of the membrane. To address this issue, PS-AOPs have demonstrated unique advantages in effectively removing organic pollutants, especially in treating complex organic mixtures and water rich in algae. UF membranes are widely used in such water treatment scenarios, and the integration of PS-AOPs enhances their efficiency as an auxiliary method. Recently, researchers have been eager to explore the combination of PS-AOPs with membrane technology to develop novel self-cleaning UF membranes. This integration is expected to improve the membrane’s processing efficiency, reduce fouling, and extend its lifespan.
The second part of the paper discussed the advantages and limitations of PS-AOPs. This section delved into the real-world application of the combined PS-AOPs and UF membrane system in water treatment. We anticipate gaining new insights from this integration, which could lead to advancements in UF technology and improved water treatment outcomes.
In the review, we quantitatively analyze the performance of PS-AOPs in controlling membrane fouling. On one hand, the study focuses on the effects of PS-AOPs pre-oxidation on water quality parameters, including dissolved organic carbon (DOC), ultraviolet absorbance (UV254), recalcitrant organic compounds, and molecular weight distribution. On the other hand, we assess fouling control performance through normalized flux (J/J0), transmembrane pressure (TMP), and fouling resistance analysis. DOC and UV254 are used to represent the levels of total dissolved organic material and those with aromatic structures or unsaturated bonds (mainly humic), respectively [62]. The decrease of DOC and UV254 absorbance indirectly indicates that the concentration of organic compounds decreases. This decrease is considered an indicator of organic matter being removed. The normalized flux (J/J0) is the ratio of the final flux to the initial water flux [63]. This ratio serves as an assessment of the flux recovery, which reflects how well the membrane’s flux is preserved over time in the presence of fouling. A higher J/J0 suggests better flux recovery and less fouling accumulation.
3.1. Homogeneous PS Catalytic UF Membrane
The application of homogeneous PS catalysis in conjunction with UF membranes primarily addresses the issue of membrane fouling. Although the integration offers benefits, it does not entirely overcome the drawbacks associated with the homogeneous PS-AOPs process.
3.1.1. UV/PS coupled with UF
Table 3 summarized the outcomes of several investigations into the fouling mitigation capabilities of UV-activated PS membranes. The data revealed that UV/PS treatment is capable of significantly breaking down and partially mineralizing organic material. It can also be seen that the effectiveness of UV/PS to antifouling is optimized with longer exposure to UV light and higher concentrations of PS. Various studies have consistently shown that fouling caused by NOM on UF membranes cannot be effectively mitigated through UV irradiation or PS oxidation alone [20]. However, the combination of UV and PS in pretreatment has been shown to significantly reduce fouling. The mechanisms behind UV/PS fouling control are twofold, as depicted in Fig. S4. First, organic matter is degraded into smaller molecules, such as DOC, which can pass through the membranes, and some components are mineralized, leading to a reduction in retained organic matter and the formation of a thinner fouling layer [64]. Second, the intense oxidation by free radicals causes the conversion of organic constituents into less adsorbable organic acids, thereby increasing the hydrophilicity of these components, and consequently, improving the reversibility or washability of the fouling [65]. It is noteworthy that some research suggests UV/PS primarily reduces reversible fouling and has little impact on irreversible fouling [66, 67]. The increased production of low-molecular-weight hydrophilic compounds, resulting from UV/PS pretreatment, is likely responsible for the accumulation within membrane pores, contributing to the observed effect. Concurrently, the pretreatment significantly reduces the filter cake layer, which is unable to retain hydrophilic components, thereby enabling more hydrophilic components to permeate into the membrane pores. Additional research indicates that this issue can be improved by increasing the PS concentration. Furthermore, numerous studies have focused on comparing the efficacy of UV/PS and UV/H2O2 in managing membrane fouling. The degradation efficiency of organic matter in UV/PS is superior because of the own advantages of
. Therefore, UV/PS is more effective than UV/H2O2 at equimolar oxidant doses [66, 68, 69].
The aforementioned research progress indicates that UV radiation can effectively activate PS to generate free radicals with high oxidation capacity, thereby reducing UF membrane fouling. However, this method comes with some drawbacks:
1. Although it offers a high degradation rate, the mineralization rate is relatively low. 2. The energy infrastructure, electricity consumption and high costs are significant considerations. 3. UV radiation diminishes in intensity as it travels, and its efficiency is greatly influenced by temperature, which limits its application effectiveness. Nevertheless, continuous direct UV irradiation can also damage UF membranes. In practical application, the UF process is separated from UV independently, and the pre-UV design is more sensible. This allows UV to activate PS for reactions while the rapid decay of UV radiation prevents damage to the UF membranes. Further research is needed to address the shortcomings of UV.
3.1.2. Heat/PS coupled with UF
Asif et al. [70] initially demonstrated the performance of heat-activated PS oxidation process for the removal of micropollutants and control of membrane fouling. The results showed that at a temperature of 40 °C, PDS achieved over 90% effective degradation of micropollutants containing strong electron-donating functional groups in the molecules. It induced a moderate breakdown of 60–80% for micropollutants having both electron-absorbing and electron-donating groups. Notably, heat/PDS significantly reduced the total organic carbon (TOC) by 70% and total nitrogen by 40%, thereby helping to lessen the fouling layer on the membrane’s surface. Table 4 presented research from other researchers on controlling membrane fouling by heat-activated PS. Among them, Ding et al. [71] evaluated the effect of factors such as solution pH and temperature on the heat/PDS fouling removal rate. pH is the critical factor for the conversion of to ·OH. At pH < 7, the dominant radical in the reaction is
. At 7 < pH < 9, both and ·OH coexist. At pH > 9, it is mainly ·OH that plays a role [30]. The simultaneous presence and co-action of both radicals provide better restoration of permeate flux and removal of fouling than when either radical is present alone. Biased alkaline condition can damage the membrane structure. Therefore, neutral condition is a good choice for controlling membrane fouling by heat/PS processes. High temperatures favor PS activation and diffusive mass transfer rates. An increasing trend of permeate flux recovery and fouling removal with increasing temperature was found within 40–80°C [71]. The process of organic matter degradation is less demanding than mineralization. Heat/PS intensifies free radical generation and boosts chemical reaction rates, thereby swiftly targeting the unsaturated constituents within NOM. By destroying their structures, their combination with high-valence cations, such as Ca2+, is hindered [26]. Thus, the formation of macromolecular deposits is reduced, and the membrane contamination is effectively reduced. Whereas the temperature of 60 – 80°C had little effect on DOC removal over a short period of time (10 min), suggesting that NOM was only structurally disrupted and not mineralized [72]. The compatibility of heating with the membrane distillation process has led to a focus on the synergistic effects of heat and PS in enhancing membrane distillation. As a result, the application of thermal activation to combat the fouling of UF membranes has been less widely investigated. Heat/PS/UF technology does have some challenges. First of all, the disadvantages of this technology are brought by Heat/PS itself, including the high cost of heat activation. Moreover, to ensure the effective operation of the system, the material of UF membrane must be able to withstand high temperature environment. The working temperature of UF membrane is usually between room temperature and 50°C, as this range preserves the membrane’s filtration efficiency and structural integrity. However, the effective temperature for heat activation needs to reach 80°C. Therefore, the research and development of UF membrane materials that can work stably at high temperatures is a key step to promote the combination of Heat/PS and UF system.
In addition, solar energy emerges as a financially viable option for heating in practical applications. By integrating solar energy with UF systems, the UV band of the solar spectrum can activate PS to generate
, which in turn degrades NOM and micropollutants before UF. The application of far-infrared radiation promotes photothermal effect, raising the solution’s temperature, thereby improving activation and speeding up the chemical reaction rate. In a study by Guo et al [73], the solar-activated PDS pretreatment significantly mitigated membrane fouling. The solar light intensity was kept at 1 kW/m2, and the temperature was elevated to 70°C. Under solar light exposure, the greatest reductions in TMP were approximately 69.6%, UV254 absorbance by 60.40%, DOC by 47.04%, in reversible fouling by 87.6%, and in irreversible fouling by 87.1%. Notably, the micropollutant atrazine (ATZ) was completely degraded. In this study, the solar simulator is used to simulate sunlight conditions, crucial for understanding reactions under real solar exposure. Although there are some technical obstacles in direct heat activation using solar energy, which leads to less relevant research so far, this method shows great potential in reducing the cost of thermal activation.
3.1.3. Transition metal ion/PS coupled with UF
The application of Fe(II)/PS was investigated for its efficacy in controlling membrane fouling, as illustrated in Table 5. Commonly, UF and coagulation-UF processes fall short in removing certain micropollutants, such as ATZ [74], sulfamethazine (SMT) [75], and p-chloronitrobenzene (p-CNB) [67]. However, the incorporation of Fe(II)/PMS into UF significantly enhances removal rates. The study highlighted that the molar ratio of Fe(II) to PMS plays a pivotal role in membrane contamination. An excessive amount of either Fe(II) or PMS would result in unnecessary reagent consumption. The optimum molar ratio of Fe(II) to PMS, which shown to be 1:1, was identified as the most effective for managing membrane fouling and removing organic matter. Fe(II)/PMS serves as a powerful pretreatment to enhance the performance of UF membranes. It markedly diminishes both reversible and irreversible membrane fouling, emerging as a reliable method for managing membrane fouling and eliminating micropollutants. The versatile Fe(II)/PMS functions as both an oxidizing and flocculating agent in the prevention and control of membrane fouling. The activation of PS by Fe(II) not only generates Fe(III) but also
. In neutral conditions, iron hydroxide is formed through hydrolysis, while the facilitates the production of reactive species such as ·OH. The in situ formed Fe(III) functions as a flocculant [76], and the along with ·OH oxides the organic matter adsorbed on particle surfaces. This process alters the zeta potential of the particles, thereby bolstering coagulation efficiency [77]. Consequently, the shift in the membrane contamination mechanism from typical film buildup to cake filtration is prolonged [78], thereby delaying the onset of membrane fouling.
The Fe(II)/PS/UF system operates effectively at a neutral pH, unlike the Fe(II)/PS system, which requires more acidic conditions. This system exhibits a unique in-situ coagulation function. However, it faces a significant challenge in the low recoverability of Fe(II), which tends to leak out with the effluent and requires separate collection and separation processes. Moreover, the conversion of Fe(III) to Fe(II) is comparatively difficult, thus affecting the system’s overall performance. Researches have shown that UV radiation is commonly employed to expedite the regeneration of Fe(II). The synergistic interaction between UV and Fe(II) during the activation of PS significantly enhances the system’s ability to mitigate membrane fouling. Cheng et al.’ research [67] showcased the advantages of UV irradiation for enhancing the Fe(II)/PMS system’s effectiveness. Treatment with 720 mJ/cm2 of UV irradiation, when applied to a solution containing 0.1 mM Fe(II) and 0.2 mM PMS, resulted in the most significant increase in membrane flux. The normalized flux, denoted by J/J0, rose to 0.81, a figure that is higher than the Fe(II)/PMS system of 0.70.
3.2. Heterogeneous PS Catalytic UF Membrane
In heterogeneous systems, membrane technology combined with PS-AOPs primarily focuses on immobilizing catalysts on membranes through membrane modification methods, such as blending, in-situ growth, vacuum-assisted filtration [79]. This approach enables the development of novel catalytic membranes featuring two-fold use: filtration and advanced oxidation. UF membrane can act as both a filter medium and a carrier for the catalyst. This setup facilitates the recovery and reuse of catalysts, enhances mass transfer efficiency, and optimizes catalytic activity. It also improves the membrane’s hydrophilicity and other properties [79]. Meanwhile, the reactive species generated by PS-AOPs on the membrane’s surface are effective in breaking down pollutants and reducing membrane fouling.
3.2.1. Transition metal compounds/PS coupled with UF
As the carrier of the system, the introduction of membrane can increase the interaction between the functional groups on the transition metal catalyst and the carrier, which is beneficial to the dispersion of the catalyst and the exposure of more active sites, thus enhancing the catalytic activity. Wang et al. [35] successfully developed CuO-coated ceramic hollow fiber membranes with dual functionalities of membrane filtration and PMS activation. The catalytic activity remained stable after 5 cycles, with the Bisphenol A (BPA) degradation rate and TOC removal rate maintained at 90% and 50%, respectively, surpassing the efficiency of homogeneous catalysis membrane. Guo et al. [80] demonstrated that a composite membrane decorated with CoCu layered double hydroxide nanoparticles could effectively eliminate a variety of emerging organic compounds in water. The mineralization efficiencies for SMX, sulfacetamide, lomefloxacin, and carbamazepine were 72.5%, 65.3%, 69.3%, and 56.0%, respectively.
The development and deployment of heterogeneous metal-based catalytic membranes have garnered significant interest in current research. However, there is a lack of comprehensive investigation into membrane fouling control strategies. Despite this, the potential value and future development prospects of these membranes are highly appealing and warrant further in-depth exploration and study. Table 6 summarized the scholarly efforts on the activation of PS using transition metal compounds for membrane fouling control. The data indicate that heterogeneous catalysis combined with membranes exhibits superior PS activation capability, stability, and effectiveness in preventing and controlling membrane fouling. Numerous studies have developed diverse catalytic membranes incorporating transition metal compounds, such as Co@GAC/ceramic membrane (CM) [81], MeOx/polyvinylidene fluoride (PVDF) (Me representing Mn, Cu, and Co) membranes [82], CoFe2O4/polyethersulfone (PES) membranes [83], CuO@CuS/PVDF membranes [84], Mn2O3/Al2O3 membranes [85], and Co3O4-Bi2O3-Ti membranes [86]. The activation properties of these catalytic membranes vary significantly based on the material composition. Among these, Cheng et al. [82] compared the effectiveness of PMS and MnO2/PMS alone with CuO/PMS and Co3O4/PMS, revealing that the latter two significantly reduced both reversible and irreversible fouling compared to the former two, which were only effective in mitigating reversible fouling. It was also observed that the oxidation capacity of MeOx/PMS was influenced by the type of MeOx, with Co3O4>CuO>MnO2 in terms of effectiveness. MeOx/PMS delayed the transition of fouling mechanisms from standard clogging to cake filtration to varying degrees, showing promise in controlling membrane fouling in practical applications.
The mechanisms of membrane fouling control through PS activation by monolithic and binary metal compounds are illustrated in Fig. S5. The reaction activated by transition metal compounds mainly occurs on the catalytic membrane surface, generating free radicals that oxidatively degrade pollutants through the valency alteration of metal ions on the membrane surface. Fig. S5(a, b) demonstrates that transition metal compounds remove pollutants through two pathways: attacking pollutants in non-radical form (1O2) with high selectivity and low oxidative capacity [35] and attacking pollutants with
, ·OH -based radicals with high oxidative capacity and low selectivity [87]. Fig. S5(c) depicts the construction of catalytic membranes by coating different binary layered metal compounds. Surface-bound radicals and 1O2 are the primary ROS in the system. NOM undergoes rapid degradation upon contact with ROS, diminishing its molecular weight. This augments the repulsive forces and boosts the normalized flux, thereby significantly mitigating membrane fouling. The water quality of permeate water from catalytic ceramic membranes is also enhanced due to efficient in situ oxidation [88]. The fouling mechanism throughout the filtration process is predominantly characterized by standard and complete blockage without cake filtration [89].
Compared with uncoupled membrane and homogeneous metal ion catalytic membrane, heterogeneous metal-complexed catalytic membrane shows better stability and reusability. The metal catalyst is fixed on the carrier, which effectively avoids the loss of metal ions in the reaction process. However, it also has disadvantages. Transition metal oxide-activated PS have already been discussed in terms of their defects. When integrated with the membrane, the following issues predominate:
The carcinogenicity of Co2+ to the human body is a significant concern. The use of an UF membrane as a carrier significantly diminishes cobalt leaching. Nevertheless, the intricate nature of water composition introduces potential safety hazards when employing cobalt-based catalysts.
The high cost associated with the synthesis of the catalyst is still a drawback.
The system is not stable and reusable enough. Some transition metal compounds may undergo structural changes or become inactive during the course of the reaction [80] and need to be replaced periodically.
The mechanism of activation of PS by transition metal compounds is more complex, which increases the difficulty of understanding and optimising the reaction process.
3.2.2. Carbon materials/PS coupled with UF
Table 7 collated research efforts focused on the activation of PS with carbon materials for the control of UF membrane fouling. The comparison highlights that the efficacy of unmodified carbon materials/PS in addressing membrane fouling is limited. Modifying carbon materials substantially boosts their catalytic activity. On this basis, the catalyst membranes show a significant improvement in performance and possess relatively excellent reusability. Carbon materials and membrane filtration show synergistic and complementary effects on the activation of PS. According to Ma et al.’s [90] research, the removal rates of 2,4-dichlorophenol in nitrogen-doped carbon (NC)/PMS and Fe@NC/PMS systems were 22.54% and 96.15% respectively within 17 min. Notably, their performance significantly improved after being fabricated into catalytic membranes. The NC/PVDF/PMS system removed 84.54% of 2,4-dichlorophenol in 2.297 s, whereas the Fe@NC/PVDF/PMS system achieved 99.74% removal in only 0.867 s. The Fe@NC/PVDF/PMS system also demonstrated a higher TOC removal efficiency of 67.55% compared to the Fe@NC/PMS system’s 55.52%. The developed Fe@NC exhibited excellent PMS catalytic activity, and the catalytic membrane configuration maximized its catalytic potential. Additionally, TMP barely increased in the Fe@NC/PVDF/PMS system within 10 minutes, whereas it significantly rose in the pure PVDF membrane, indicating that the catalytic membrane effectively controlled membrane fouling. Cheng et al. [89] observed that pretreatment with carbon nanofibers (CNFs)/PMS led to a slight enhancement in permeate flux and a minor addition in irreversible fouling. In contrast, pretreatment with CuO@CNFs/PMS effectively slowed the decline in flux, with J/J0 reaching 0.61 (versus 0.25 without pretreatment), and markedly lessened both reversible and irreversible fouling. Liu et al. [91] developed a catalytic membrane composed of nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs). The N-MWCNTs/PVDF membrane removed phenol at a rate of 83.67% within 2 minutes, which was significantly higher than that of MWCNTs/PVDF (41%) and N-MWCNT powders (41.42%). The reusability of the membrane was assessed, with the degradation rate of phenol maintaining 100% in the first three cycles. However, the degradation rate decreased gradually in cycles 4, 5, and 6, reaching 90%, 75%, and 68% respectively. The amassing of pollutants and intermediate products on the membrane surface covered the active sites on the catalytic membrane, hindering the reaction between PMS and N-MWCNTs, thereby reducing the removal efficiency of pollutants. Ma et al. [92] developed a CNT@NC/Al2O3 membrane that, when combined with PMS activation, forms a MFPA system. The MFPA system demonstrated a higher J/J0 value of 0.75 compared to 0.36 without PMS. After three cycles, the SMX removal rate of the MFPA system decreased to 40.0%. However, after heat treatment, the CNT@NC/Al2O3 membrane regained its initial removal rate of 65.0%, indicating excellent regenerative properties of the membrane.
The catalytic efficiency of a catalyst-loaded membrane far exceeds that of an equivalent amount of powdered catalyst [92]. One reason for this is that the carrier membrane provides a more consistent distribution of active sites for the adsorption and degradation of pollutants. Additionally, membrane filtration, aided by applied pressure, accelerates the accumulation of pollutants and PS on the membrane surface, thereby enhancing their mass transfer to the catalyst’s surface. Both of these factors potentiate the activation of PS, leading to more effective improvement of catalytic efficiency and the production of reactive species with strong oxidative capabilities. Meanwhile, the enhanced interactions of the hybridized carbon materials led to the development of catalysts less prone to agglomeration, featuring larger active centers and enhanced adsorption capacity. The hybridized carbon-based PS catalytic membrane effectively reduces the organic zeta potential, facilitating increased opportunities for particle collision and agglomeration. This enrichment of organic matter on the membrane surface forms a porous fouling layer that is more easily degraded through radical and non-radical oxidation pathways, aiding in fouling reduction [93].
The integration of carbon materials with UF membranes constitutes a novel class of membranes. They play a synergistic and complementary role in activating PS. The catalysts remove pollutants from the membrane surface in situ. The membranes enable carbon material catalysts to reach their full potential. Carbon materials/PS coupled with UF membranes exhibit enhanced performance, addressing the limitations of carbon materials/PS in terms of catalytic stability and the number of active sites. Research on carbon-based catalytic membranes remains relatively scarce, with their performance and mechanism yet to be fully developed and explored. Nitrogen-doped carbon-based catalytic membranes offer a metal-free alternative, although the reaction time for organic removal is longer. The introduction of metals can expedite the reaction time, but this must be balanced with the risk of metal leaching.
3.2.3. Photocatalysis/PS coupled with UF
Challenges exist in the photocatalytic activation of PS, including the tendency for undesirable agglomeration among nanoscale catalysts, leading to limited active sites and reduced mass transfer efficiency [94]. The reunification of photo-induced electrons and holes on the interfaces of solitary-component catalysts leads to a reduction of charge carrier density, which subsequently diminishes the photocatalytic efficacy [95]. Additionally, the recyclability and reusability of most catalysts are problematic [96]. To address these challenges, researchers have developed catalytic membranes by immobilizing photocatalysts on membrane surfaces [97]. The self-cleaning properties of these catalytic membranes significantly alleviate membrane fouling [98]. The stable activity and large accessible surface area of photocatalysts on the membrane surface help overcome issues of agglomeration and recycling difficulties [99]. In recent years, photocatalytic membranes have garnered significant attention in the field of water treatment, owing to their distinct characteristics. Studies on these membranes have yielded rich results in terms of fabrication, characterization, and photocatalytic performance. Kusworo et al. [100] provided a comprehensive overview of membranes equipped with various photocatalysts such as semiconductor metal oxides, multi-hybrid metal oxides, and doped modified carbon materials and so on. Additionally, the integration of membrane separation and PS-AOPs enhances the mass transfer efficiency of catalytic membranes and the removal rate of pollutants [101]. As a result, the coupling of photocatalytic activated PS technology and UF membrane separation is a novel, robust, and environmentally friendly water treatment method that has recently emerged.
Wang et al. [102] developed a novel class of metal-free perylene imine carbon nitride (PI-g-C3N4) membrane. Irradiating with visible light with wavelength greater than 420nm, the photocatalytic activity of PMS was enhanced, manifesting as remarkable degradation efficiency against organic pollutants. The study revealed that the system achieved complete removal of BPA within 30 min. At the end of the third cycle can still degrade 98% BPA. Reduction-capable electrons can activate PS to generate ·O2−, 1O2, and ·OH, while holes and radicals can participate in pollutant degradation, as illustrated in Fig. S6(a). Subsequently, Lumbaque et al. [103] explored the utilization of UF tubular ceramic membrane as a carrier, in conjunction with UV and TiO2-P25, to enhance the efficiency of PDS activation, as depicted in Fig. S6 (b). The UVC/membrane coated with TiO2-P25/PDS system effectively eliminated model drugs from wastewater, obliterating 42.6% of paracetamol. In contrast, the non-catalytic membrane system (UVC/PDS) removed 32.4% whereas the TiO2/PDS and UVC/TiO2 systems achieved removals of only 6% and 11.1%, respectively. Leichtweis et al. [104] incorporated NiFe2O4/carbon composite particles into PES membrane, creating the M–5NFO/C membrane. This was operated under the specified conditions: 50 ppm TC, pH 5, 5 mmol/L PDS, < 1 W visible light, and 3 cm2 active area. The removal efficiencies for TC were 5% in the vis/membrane system, 30% in the membrane/PDS system, and 45% in the vis/membrane/PDS system. Photocatalysis/PDS combined with membrane showed the best performance. NiFe2O4 served as the active site. However, due to continuous Ni leaching over four cycles, the removal efficiency for organic matter progressively decreased. Li et al. [105] synthesized biomimetic polydopamine and zeolitic imidazolite framework-67 decorated polypropylene (PDA/ZIF-67@PP) membrane that markedly increased hydrophilicity by 47.3% and permeability by 150.0%. In the visible light PDA/ZIF-67@PP/PMS system, the dyes methyl blue and methyl orange achieved degradation rates of 92.3% and 99.5%, respectively, surpassing the removal efficiencies of PMS or visible light alone. The main active species involved in the reaction were determined to be and ·O2−. Moreover, the membrane fouling was successfully reduced by photocatalytic PMS activation. The recovery of J/J0 exceeded 0.935, compared to the original J/J0 of 0.55, showcasing the outstanding self-cleaning performance of the PDA/ZIF-67@PP membrane. This approach significantly alleviated membrane contamination, offering superior advantages over the hydrophilic modified membrane’s anti-sorption properties. Despite the burgeoning interest in photocatalytic activation of PS-catalyzed membranes for combating fouling, the corpus of research in this area remains limited, necessitating immediate and substantial expansion.
Photocatalysis/PS-UF membrane presents vast potentials for water treatment applications, but it also has its shortcomings.
The efficacy of these membranes heavily relies on the presence of highly active photocatalysts. Therefore, the practical implementation of nanomaterials in photocatalysts may be hindered by their demanding, costly, and intricate fabrication processes [106].
The distribution of most catalysts on the membrane surface is typically achieved through surface modification, leaving many pore walls unoccupied [107]. Particularly, colored substances within the solution often impede the passage of light [108], failing to fully utilize the active sites’ potential [105]. The integration of photocatalysis with transition metal compounds and carbon materials can significantly enhance the utilization of active sites on membranes, which further raises the high requirements for photocatalyst.
In practical applications, UF membrane modules are typically housed within membrane shells. Because of their structural design, light has a hard time passing through the membrane, so it is difficult for the photocatalyst fixed on the UF membrane to play its role effectively. To address this, photocatalysts are commonly secured on suitable carriers, which are then positioned within photocatalytic reactors preceding UF systems.
When choosing the driving mode of photocatalytic reaction, the photocatalytic membranes activated by visible light or sunlight are generally considered as an ideal choice because it is more environmentally friendly and sustainable. However, compared with the driving mode relying on UV, the efficacy of photocatalytic membranes driven by visible light is often limited. Therefore, to widely implement this technology, it’s essential to create efficient photocatalytic UF membranes that can be effectively driven by visible light.
Current research focuses extensively on photocatalytic membranes and the study of photocatalytic activation of PS. However, integrated research that combines all three aspects—photocatalysis, membrane technology, and PS activation—is relatively scarce. Furthermore, studies dedicated to controlling membrane fouling, a critical issue in membrane applications, are even less common. The scarcity of such research can be attributed to several factors: The interplay between photocatalysis, membrane separation, and PS activation is complex. This complexity makes research design and execution more challenging. There was less commercial interest in this integrated system compared to other systems, which reduces investment in research and development.
3.3. Other Methods
In addition to the aforementioned PS activation methods, recent studies have delved into electrochemical activation for controlling membrane fouling. It has become a particularly compelling approach. With the aid of electricity, membrane flux experiences a substantial increase, and selectivity is notably enhanced, even when the contaminant pore size is smaller than that of the membrane pores. This enhancement can be attributed to electrochemically enhanced adsorption, electrostatic repulsion, and electrochemical degradation [109]. Liu et al. [110] introduced a PS system for membrane cathodic filtration, which effectively mitigated membrane fouling. When PMS and PDS were utilized as electrolytes, the effluent J/J0 decreased to 0.93 and 0.84 within 60 min, respectively. The decrease in humic acid (HA) deposition can be attributed to the oxidation of HA by the applied electric field and the modification of pollutant interactions at the membrane surface due to the presence of active substances. The mitigation of membrane fouling stemmed from the efficient oxidation of ·OH,
, and 1O2 generated by the activation of the membrane cathode by PS in the system.
In addition, single-atom catalysts are emerging as a novel force in the realm of electrochemical catalysis. Single-atom catalysts harness a single atom as their active site, with nearly all atoms participating in the catalytic reaction. This unique design integrates the benefits of both homogeneous and heterogeneous catalysis, resulting in enhanced catalytic efficiency, selectivity, and stability [111]. The catalyst structures are distinguished by their homogeneity and simplicity, which are pivotal in elucidating the mechanisms that boost reactivity and mitigate membrane fouling [112]. Nonetheless, there is a pressing need for ongoing research and advancement to facilitate the broad implementation of single-atom catalysts. This involves refining synthetic methodologies, delving into novel catalytic reactions, and tackling the issue of long-term catalyst stability.
4. Conclusion and Outlook
The coupling of PS-AOPs pretreatment with UF membrane technology provides a dual benefit of improving water quality and managing membrane fouling. We begin by reviewing the homogeneous and heterogeneous activation methods of PS-AOPs, discussing their mechanisms and comparing their advantages and disadvantages. We then summarize recent research on the degradation of SMX by PS-AOPs, meticulously listing operational conditions, dosages of pollutants and oxidants, pH, reaction times, and removal efficiencies. Quantitative assessments of these data offer a comprehensive view of the different activation methods’ performance and efficacy. Homogeneous activation, involving UV, heat, and transition metal ions, is effective in PS activation, yet it demands substantial investment. UV and heat activation break bonds, while transition metal ions activate PS through electron transfer. This process generates ROS, such as
, ·OH, which react with pollutants. In contrast, heterogeneous activation, using transition metal compounds, carbon materials, and photocatalysis, faces mass transfer limitations and insufficient active sites. However, it offers greater stability and recyclability. Heterogeneous activation of PS involves the transfer of electrons, leading to the generation of a variety of active substances. This includes ROS, non-free radicals, and surface-bound reactive species. Additionally, photocatalysis/PS can generate h+.
Secondly, the impact of PS-AOPs on UF membrane fouling is a key focus. The study examines changes in water quality parameters under various PS activation mechanisms, including DOC, UV254 absorbance, and the removal efficiency of refractory organic compounds. In addition, a detailed analysis of a range of parameters related to membrane fouling is conducted, including normalized flux, TMP, and membrane fouling resistance. Synthesizing these findings, it is abundantly clear that pretreatment with PS-AOPs can significantly slow the pace of membrane fouling, thereby substantially extending the service life of UF membranes. Nevertheless, despite the theoretical and technological appeal of PS-AOPs-UF, practical implementation is still challenged by several obstacles.
PS-AOPs, having been developed over an extended period, have yielded abundant research results for the removal of organic matter. These systems, when combined with UF membranes, predominantly demonstrate their advantages in the field of water treatment. They are particularly effective in the elimination of high concentrations of organic compounds from wastewater, the removal of micropollution, odorous pollutants, and algae from drinking water, thereby enhancing the quality of the produced water. Simultaneously, they effectively control membrane fouling, reduce the intensity of backwashing and chemical cleaning, mitigate damage to the membranes, and extend their lifespan. As such, they have garnered interest and widespread attention from researchers. However, the studies on how the integration of PS-AOPs with UF functions to control membrane fouling are still in their infancy, representing an emerging field of research.
The combination of PS-AOPs with UF membrane in a homogeneous system has addressed the challenge of membrane fouling. However, it has not significantly improved the performance of homogeneous PS-AOPs, leaving the inherent drawbacks intact. The operational costs of UV and heat are high, and there’s a risk of metal leakage from transition metal ions. Nonetheless, the usage conditions have become more relaxed after integration with the membrane, without strict pH limitations. The integration of heterogeneous PS-AOPs with UF membranes offers a complementary synergy, creating a novel catalytic membrane with dual functions of filtration and advanced oxidation. The UF membrane, as a carrier, boasts a large specific surface area, well-developed pore structures, and excellent adsorption capabilities. Catalysts are uniformly dispersed on the membrane surface, which increases the number of active sites and boosting catalytic potential. This addresses the mass transfer limitations and insufficient active sites in heterogeneous systems. However, it also raises the complexity of the system, making the preparation more intricate and expensive. Metal-based catalytic membranes are prone to metal leaching, while carbon-based ones eliminate this risk but are more prone to deactivation and require longer reaction times. Metal-carbon catalytic membranes mitigate the drawbacks of both, proving to be superior. Photocatalytic membranes can be developed based on metal-carbon catalytic membranes. Currently, research on activating PS with photocatalytic membranes is limited, imposing various restrictions. Future efforts should focus on synthesizing metal-carbon catalytic membranes to create novel self-cleaning membranes with advanced oxidation capabilities. An immediate priority is to delve into the optimal reaction conditions and parameter configurations for the PS-AOPs-UF system to maximize its efficiency and effectiveness.
Heterogeneous PS catalyst membranes offer distinct advantages over the combination of homogeneous PS and UF membranes. In these heterogeneous systems, the catalyst is synthesized within the membrane material, creating a monolithic catalytic membrane that prevents catalyst loss and facilitates easy separation and retrieve. In contrast, homogeneous systems involve the simple addition of activating agents to the membrane, resulting in a simpler two-part assembly. Ideally, catalytic membranes should be reusable with stable activity. However, despite the ability to degrade organic matter, complete mineralization of pollutants is challenging. Oxidative intermediate products have a tendency to build up on the membrane surface, resulting in fouling and a decrease in catalytic activity. Research into the stability and regenerability of catalytic membranes is necessary. The goal is to develop membranes that can be recycled multiple times or operate for extended periods without any treatment; and can be regenerated through simple processes such as solvent washing (with H2O2, NaClO, etc.) or other methods to restore their initial performance and be reused. The development of novel materials is essential to fabricate catalytic membranes that showcase high efficiency, eco-friendliness, stability, and ease of recycling.
Current research indicates that the performance of PS-AOPs-UF systems in natural water bodies is noticeably inferior to that in pure water conditions. This is attributed to the complexity of natural water matrices, which contain a variety of inorganic anions and other water quality constituents. These components compete with the active species, thereby diminishing the efficacy of PS-AOPs-UF. Concomitant with these developments, practical application tests in real water treatment settings and protracted operational trials are indispensable to ascertain the efficacy and viability of the coupled process under authentic conditions.
PS is stable chemically, making it easy to store. Its dosage and reaction conditions are relatively simple to control. PS/AOPs are effective in a broad pH range for degrading recalcitrant organic matter, micro-pollutants, and high concentrations of NOM. However, the need for continuous addition of PS can impact the economic viability of the system. Additionally, the introduction of PS raises the potential for secondary pollution. Due to the strong oxidizing nature of sulfate ions ( ), they can readily react with halide anions (such as Cl−, Br−) in water to produce reactive halogens (like HClO, HBrO). These reactive species can then interact with natural organic matters in surface waters, producing halogenated organic by-products, including trihalomethanes [113]. This process can result in secondary pollution, posing risks to human health [114]. Subsequent research should focus on the by-products formed by PS in catalytic membrane system. Optimize membrane materials and reaction conditions to minimize the production of by-products.
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