Engineering Photocatalytic Membrane Reactors for Sustainable Energy and Environmental Applications

Abstract

Photocatalytic membrane reactors (PMRs), which combine photocatalysis with membrane separation, represent a pivotal technology for sustainable water treatment and resource recovery. Although extensive research has documented various configurations of photocatalytic-membrane hybrid processes and their potential in water treatment applications, a comprehensive analysis of the interrelationships among reactor architectures, intrinsic physicochemical mechanisms, and overall process efficiency remains inadequately explored. This knowledge gap hinders the rational design of highly efficient and stable reactor systems—a shortcoming that this review seeks to remedy. Here, we critically examine the connections between reactor configurations, design principles, and cutting-edge applications to outline future research directions. We analyze the evolution of reactor architectures, relevant reaction kinetics, and key operational parameters that inform rational design, linking these fundamentals to recent advances in solar-driven hydrogen production, CO₂ conversion, and industrial scaling. Our analysis reveals a significant disconnect between the mechanistic understanding of reactor operation and the system-level performance required for innovative applications. This gap between theory and practice is particularly evident in efforts to translate laboratory success into robust and economically feasible industrial-scale operations. We believe that PMRs will realize their transformative potential in sustainable energy and environmental applications in future.

1. Introduction

The merging of rapid urbanization and industrialization has led to increased environmental pollution and energy insecurity, presenting major global challenges. A key sign of these pressures is the rising demand for clean water, which has attracted widespread international concern. Solar-driven photocatalytic processes have been recognized as a highly promising technological solution for addressing these interconnected issues [1,2]. The capacity of photocatalytic processes to be driven by either solar or artificial light positions them as a paradigm of sustainable technology. By leveraging renewable solar energy, photocatalysis facilitates the complete mineralization of recalcitrant pollutants and the effective inactivation of viruses [3,4,5,6]. Consequently, owing to its inherent environmentally benign nature and demonstrated efficacy, this technology has garnered substantial interest and is recognized as one of the most promising solutions for environmental remediation. It also allows for splitting water into hydrogen and oxygen, highlighting its potential in sustainable energy applications [7,8,9,10]. From a semiconductor photochemistry perspective, photocatalysis works by initiating or speeding up specific redox reactions through the irradiation of semiconductor materials. The core mechanism begins when a photocatalyst absorbs light energy equal to or exceeding its band gap. By utilizing free sunlight to drive strong redox reactions, photocatalytic technology has demonstrated unique advantages and broad prospects in the field of environmental remediation, in terms of both pollution control and energy generation. The mechanistic pathway of photoreduction is contingent upon its specific application, predominantly proceeding via a single-electron transfer process in domains such as organic synthesis, environmental remediation, and solar energy conversion [11,12,13]. Alternatively, it can proceed through more complex multi-electron transfers, such as in the reduction of water (H₂O) or protons (H⁺) to produce hydrogen (H₂) fuel [14]. This process relies on exciting semiconductor catalysts with photons whose energies match or surpass their band gaps, causing electrons to move from the valence band to the conduction band, leaving behind holes. These photogenerated holes are strongly oxidizing, capable of withdrawing electrons from nearby molecules. Upon migration to the catalyst surface, interfacial redox reactions are initiated; the photo-generated electrons, possessing reduction potentials between +0.5 and −1.5 V vs. NHE, mediate the reduction of electron acceptors, while the holes, with oxidation potentials ranging from +1.0 to +3.5 V vs. NHE, drive the oxidation of electron donors [15]. The consequent generation of highly reactive oxygen species (ROS), notably hydroxyl and superoxide radicals, possessing potent oxidizing capabilities, is instrumental in the degradation of a wide spectrum of aqueous contaminants. The photoreduction process can happen through various pathways, which might involve single-electron transfer, common in organic synthesis, environmental cleanup, and solar energy conversion. For applications such as producing hydrogen (H₂) fuel from water (H₂O) or protons (H⁺), the process can proceed via more complex multi-electron transfers. One major advantage of photocatalytic technology is its natural sustainability. It operates under mild conditions and does not require chemical additives. By effectively utilizing sunlight as a renewable energy source, photocatalysis stands out as a key technology for sustainable environmental management.

Photocatalytic technology is increasingly acknowledged for its profound potential in water treatment, primarily owing to its capacity for the comprehensive degradation of persistent pollutants [4,5,16]. The efficacy of this photoelectron-driven process is underscored by a compelling suite of advantages, including its potent oxidizing capabilities, facile operational control under ambient temperature conditions, extended operational stability, and the negligible formation of secondary pollutants [12]. These attributes, combined with its cost-effectiveness, culminate in superior water purification efficacy [13]. However, widespread industrial use of this technology faces several ongoing challenges [14]. A primary issue is the rapid recombination of photogenerated electrons and holes, which significantly reduces quantum efficiency. Additionally, the small size of photocatalysts makes their separation from treated water difficult, hindering reuse. The nanoscale dimensions of these particles further complicate recovery and increase operational costs [15]. Conventional photoreactor designs often suffer from poor light-harvesting efficiency, which limits overall performance and economic feasibility. In response to these limitations, the strategic evolution of photocatalytic technology has led to the development of PMRs. This innovative approach, which integrates the photocatalytic process with membrane separation technology, has proven particularly effective in surmounting the aforementioned obstacles and represents a significant advancement in reactor engineering (Figure 1). In this setup, the membrane acts as a physical support for immobilizing photocatalyst nanoparticles, directly solving catalyst separation and recovery problems. Simultaneously, the photocatalyst degrades contaminants and organic foulants at the membrane surface, reducing membrane fouling—a common operational problem in conventional filtration [17]. This combined approach not only solves separation issues but also improves degradation efficiency by ensuring prolonged, close contact between pollutants and catalytic sites, representing a promising direction for advanced water purification [16]. The primary thrust of research in this field has been directed toward two principal avenues: the enhancement of photocatalyst efficiency and the innovation of reactor design. While considerable scientific effort has been dedicated to the synthesis and characterization of novel catalyst materials with superior properties, a notable disparity is evident in the literature [18,19,20]. Specifically, rigorous and systematic investigations focusing on comprehensive reactor modeling and process optimization have received comparatively less attention, representing a critical knowledge gap that may hinder the scalable implementation of this technology. Developing photocatalytic reactors that boost catalytic efficiency and minimize separation steps has thus become an urgent technical goal. The integration of inorganic photocatalyst nanoparticles with organic polymeric matrices to fabricate composite photocatalytic membranes represents a particularly effective strategy [21]. This approach capitalizes on the proven versatility and efficiency of membrane separation technology, a cornerstone of water purification that has achieved widespread commercial acceptance [17]. In this synergistic configuration, the degradation of contaminants at the membrane surface is mediated by the immobilized photocatalyst nanoparticles, which concurrently mitigate membrane fouling while ensuring intimate contact with pollutants to enhance degradation efficiency. Such progress in catalyst fabrication and reactor engineering is pivotal for resolving these persistent operational challenges [22].

The integration of membrane filtration with photocatalysis represents a significant advancement in developing energy-efficient and sustainable wastewater treatment methodologies. Photocatalytic membranes (PMs) have been extensively studied due to the sustainability and environmental friendliness of solar energy in an eco-friendly manner [23]. Historically, the synergy between photocatalysis and membrane technology was principally confined to the downstream separation and recovery of photocatalyst nanoparticles from suspension [24]. However, the practical implementation of photocatalytic systems for water treatment necessitates a more sophisticated approach to reactor design, where multiple critical parameters must be simultaneously optimized. Effective photoreactor engineering must address the optimization of catalyst performance, the maximization of light energy utilization, the mitigation of mass transmission limitations, and the efficiency of catalyst retention. The central challenge in reactor design lies in creating a system that concurrently achieves high catalytic activity and rapid pollutant degradation. This imperative has driven the development and optimization of PMRs, which are a key innovation in advanced photocatalytic systems [25]. By immobilizing the photocatalyst directly onto or within the membrane structure, PMRs not only resolve the issue of catalyst recovery but also facilitate more effective control of membrane fouling. Ultimately, rigorous research and development in reactor design are crucial for transitioning these processes from laboratory-scale experiments to industrially viable applications. Advanced PM technologies have garnered considerable attention for their high efficiency and energy-saving potential [26]. This innovative system is predicated on the synergistic integration of photocatalysis with membrane separation, a concept developed in the early 21st century to achieve the dual objectives of contaminant degradation and physical filtration [27]. A key advantage of the PMR configuration lies in its ability to address the persistent challenge of catalyst recovery encountered in conventional slurry-based systems. By immobilizing or confining the photocatalyst, the membrane module ensures its retention within the reactor, which prevents catalyst loss and maintains the continuous stability of the system [23,28]. In addition, this integration enhances the interfacial contact area between the catalyst and pollutants, thereby leading to a significant improvement in degradation efficiency. Compared to traditional water treatment methods, PMRs offer simplified operation, lower energy consumption, high reaction efficacy, and a substantially reduced risk of secondary pollution [29].

A substantial body of review literature has documented the progress of PMRs, providing a solid theoretical foundation for the field. Early reviews systematically described various configurations of photocatalytic-membrane hybrid processes and their potential applications in water treatment [30]. Subsequent research has delved deeper into process optimization, particularly using multivariate analysis to determine optimal operating parameters to maximize photo-oxidation performance [31,32]. A significant portion of subsequent literature has been dedicated to membrane fouling, a critical bottleneck for practical applications. Currently, the application scope of PMRs has expanded significantly beyond pollutant degradation to encompass emergent fields, including visible light implementation. Related reviews systematically summarize the evolution, types, and consequences of membrane fouling [33]; deeply explore the influence of photocatalytic reaction conditions on fouling behavior; and compare and analyze physical cleaning strategies, such as aeration and membrane oscillation. Simultaneously, the application areas of PMRs have expanded from their initial focus on pollutant degradation to a wider range [34], encompassing cutting-edge areas such as visible light-based water splitting for hydrogen production, selective synthesis of organic compounds (such as phenol and vanillin), and air purification of volatile organic compounds (VOCs) and nitrogen oxides (NO_x) [35]. In addition, some reviews have examined the technology from a more macro perspective. For example, one work systematically reviewed the evolution of PMRs over the past two decades, focusing on their progress in reducing membrane material degradation, improving catalyst reusability, enhancing visible light catalytic activity, and improving synthetic reaction selectivity [36,37]. Other studies have focused on the fundamental principles of reactor design, aiming to decouple the intrinsic kinetic properties of catalysts from reactor performance, providing theoretical guidance for establishing a standardized evaluation system [38]. Despite the extensive coverage provided by these reviews, the reviews have predominantly focused on discrete aspects of PMR technology—such as membrane fouling, catalyst development, or specific applications—while existing literature often examines specific aspects—such as membrane fouling, catalyst development, or particular applications—in relative isolation [39]. Specifically, the key question of how the intrinsic properties of the catalyst, the multi-physics coupling within the reactor (such as the light field, flow field, and mass transmission), and the microscopic mechanisms of membrane fouling synergistically influence the overall performance and long-term stability of the reactor remains underdeveloped [40]. Despite the demonstrated advantages of PMRs, including their capacity for high-efficiency separation [41], in situ catalyst recovery [42], and continuous operation [43], their transition from laboratory-scale proof-of-concept to industrial-scale application is impeded by several critical scientific questions [44]. A significant challenge lies in bridging the pronounced gap between laboratory success and viable industrial implementation, an issue often defined as the scale-up challenge. The present review is therefore motivated by the need to bridge this gap. Concurrently, an intrinsic trade-off frequently exists between maximizing catalytic efficiency for broad-spectrum pollutant degradation and achieving high selectivity toward specific target contaminants [35,45]. Furthermore, the suboptimal energy utilization efficiency of many current PMR configurations remains a substantial barrier to their sustainable and cost-effective deployment. Therefore, a focused investigation into these limitations is imperative [46]. This paper is motivated by the necessity to address these unresolved issues, aiming to elucidate key mechanisms and propose novel strategies that will advance the practical application of PMRs in environmental remediation. We will systematically review the research progress of PMRs; discuss their structural design, performance influencing factors, and deep-rooted mechanisms, as well as the challenges in practical applications; and look forward to the future development trend.

2. Principles and Developments of PMRs

The integration of photocatalysis with membrane separation has enabled the development of multifunctional composite systems that overcome the intrinsic limitations of each technology when employed independently. Membrane separation, a well-established technology in modern water treatment, owing to its high efficiency and compact configuration, is nevertheless hindered by fouling [47]. The accumulation of foulants on the membrane surface causes a progressive decline in permeate flux, an increase in transmembrane pressure, and a reduction in service life, thereby elevating operational costs. In addition, conventional membrane processes generate a concentrated retentate stream that necessitates secondary treatment [48,49]. PMRs provide a transformative approach by shifting from passive separation to active, in situ remediation. Their operation relies on immobilized semiconductor photocatalysts that, under light irradiation, generate ROS [50]. These ROS mineralize adsorbed organic macromolecules and microbial biofilms—the principal contributors to fouling—into harmless end products such as carbon dioxide and water. The continuous, light-driven self-cleaning process alleviates pore blocking and cake layer formation while simultaneously eliminating the retentate stream, converting potential waste into purified effluent [51,52,53]. Practical implementation of PMRs has been achieved through three main fabrication strategies: incorporation of photocatalyst nanoparticles into the membrane matrix during phase inversion, deposition or grafting of a photocatalytic layer onto pre-formed membranes, and confinement of a photocatalyst suspension using a membrane barrier [54,55]. Each configuration entails distinct trade-offs among catalyst stability, mass transmission efficiency, and light utilization. By embedding contaminant degradation directly within the separation process, PMRs shift fouling control from reactive measures such as pre-treatment or chemical cleaning toward an integrated, preventive solution [56]. This coupling of separation and degradation processes represents a significant step toward the realization of sustainable and high-performance technologies for environmental remediation.

Recent advancements in PMRs have concentrated on three principal areas: the development of novel catalyst materials, the optimization of membrane structures, and the innovation of reactor configurations (Figure 2). The integration of photocatalysis with membrane technology has emerged as a promising approach to sustainable wastewater treatment [23]. Since their conceptualization around 2000, PMRs have been predominantly investigated for water purification applications, leveraging solar-driven photocatalysis to degrade recalcitrant organic pollutants [57]. Solar-driven photocatalysis has garnered considerable attention, with significant efforts directed toward catalyst innovation and reactor design [58]. Integrating catalytic materials into non-thermal plasma reactors, while challenging, has opened new avenues for performance enhancement [59]. Significant research efforts have been directed toward the synthesis of high-performance photocatalysts, such as graphitic carbon nitride, and their effective integration with advanced membrane materials [40]. The reactor configuration, which fundamentally comprises a light source, the photocatalytic membrane, and the reaction vessel, is a critical determinant of overall system efficiency [60]. Innovations in reactor design, such as the development of split-type PMRs, have been pivotal in enhancing mass transmission, light distribution, and pollutant removal efficacy. A typical PMR comprises a light source, a membrane, and a photocatalyst, underscoring the critical role of reactor configuration in system performance [61]. Future research should prioritize the development of novel, low-cost adsorption materials for the removal of hazardous pollutants [62]. Innovations in photocatalysis-membrane separation reactors have emphasized advancements in membrane materials, reactor design, and integration with complementary technologies [63]. The overarching objective is to transition PMR technology from a laboratory-scale concept to a practical and sustainable solution for environmental remediation [64].

The integrated nature of PMRs confers significant operational advantages over conventional photoreactors, including the capacity for continuous operation, in situ catalyst recovery and reuse, and simultaneous substance separation. The modularity and scalability of PMR designs have consequently spurred considerable research interest. However, the transition of this technology from controlled laboratory settings to industrial-scale applications is impeded by several formidable scientific and engineering challenges [65]. The catalytic efficiency and long-term stability of the system can be significantly compromised by the competitive adsorption of inorganic salts and the fouling effects of natural organic matter, factors often excluded from idealized laboratory assessments. This challenge is directly coupled to the critical issues of scalability and economic feasibility. At present, PMR development remains largely confined to laboratory research and small-scale experimental studies, with limited industrial applications [66]. Despite this, PMRs exhibit several advantages that surpass conventional advanced oxidation and separation technologies, such as simultaneous photocatalytic degradation and separation, along with complete retention of the photocatalyst. Despite significant progress in PMR technology, its application in wastewater treatment remains fraught with challenges and uncertainties. A key issue is the incomplete understanding of reaction kinetics during PMR operation, particularly in complex wastewater systems [48]. Additionally, there is a lack of systematic quantification of the interactions between chemical reaction pathways and catalyst activity. Most existing studies have primarily focused on performance evaluation under controlled laboratory conditions, often overlooking the influence of real-world factors, such as water complexity, fluctuations in pollutant types and concentrations, and the competing adsorption of inorganic salts and organic compounds [67]. These external factors can significantly reduce the catalytic efficiency of PMRs and undermine the long-term stability and fouling resistance of the membrane. Another critical challenge is the scalability and economic feasibility of PMR technology [62]. While some studies have explored low-cost catalytic materials and modular designs, achieving stable and efficient performance in large-scale operations at high flow rates remains a significant technical hurdle. In addition, optimizing the energy efficiency of light sources and addressing the overall energy consumption in large-scale engineering applications are essential for the widespread adoption of PMR systems. Laboratory-scale studies often rely on idealized conditions that may not accurately reflect the challenges encountered during scale-up, such as catalyst deactivation, membrane fouling, and uneven light distribution [68]. The uniform distribution of photons, the management of hydrodynamics to mitigate mass transmission limitations, and the maintenance of membrane integrity and catalyst activity over extended operational periods represent significant technical hurdles in large-scale systems. Accordingly, future research must prioritize a holistic approach to reactor design and process optimization. This necessitates the development of robust kinetic models that account for complex water chemistry and the optimization of reactor configurations to enhance light-harvesting efficiency, potentially through the strategic use of low-energy light sources such as LEDs or natural sunlight [69]. In addition, energy efficiency and economic feasibility must be integral components of reactor design, particularly in selecting and distributing light sources (e.g., natural light or LEDs) [70]. The goal should be to balance uniform light intensity distribution with overall energy control [71]. Furthermore, advancing the scalability of PMRs is predicated on the development of multifunctional, integrated systems that may combine photocatalysis with complementary processes, such as adsorption or biological treatment. The overarching objective is the creation of robust, energy-efficient, and economically viable systems capable of operating effectively at an industrial scale.

The evolution of PMRs is driven by advances in materials and membrane technology and pressing environmental demands. The efficacy of these systems arises from a profound synergy between photocatalysis and membrane separation [72]. By confining the photocatalyst, the membrane establishes a microenvironment with a high catalyst concentration, which optimizes photon utilization and promotes the complete mineralization of pollutants [73]. Reciprocally, the photocatalytic process provides a continuous self-cleaning mechanism that mitigates membrane fouling; however, this potent oxidative environment concurrently raises concerns about the long-term stability of the membrane material [74]. Therefore, a comprehensive understanding of this bidirectional interaction is crucial for the rational design and optimization of advanced PMR systems. Overall, PMRs effectively address many of the limitations associated with traditional photocatalytic technologies. They significantly enhance resource utilization efficiency, simplify operational processes, and reduce operating costs. By integrating photocatalysis with membrane separation, PMRs provide an optimal pathway for efficient photocatalyst recovery and sustainable system operation. Owing to their broad application potential in modern environmental management, PMRs have emerged as a promising and innovative technological direction (Figure 3).

3. Photocatalytic Membrane Reactors

The progression of photocatalytic technology toward industrial-scale commercialization is contingent upon the design and implementation of highly efficient photoreactors. Judicious reactor engineering is paramount, as it can substantially enhance the quantum yield of reactive oxygen species, thereby maximizing the efficient utilization and conversion of photonic energy [75]. Among the various advanced reactor configurations, the photocatalytic membrane reactor (PMR) has emerged as a particularly innovative technology [30]. This system synergistically integrates photocatalysis and membrane separation within a single unit, a design conceived to address the critical challenges of catalyst recovery and the management of photodecomposition products. Within this integrated framework, the photocatalytic component facilitates the mineralization of organic pollutants into benign substances, such as carbon dioxide and water. Concurrently, the membrane module serves a dual purpose: it acts as a physical barrier for the complete retention of the photocatalyst, enabling its recovery and reuse, and as a selective filter for the treated effluent [76]. Attributable to its high efficiency, operational simplicity, and inherently sustainable design, the PMR exhibits considerable promise as a viable and advanced technology for water and wastewater treatment [61]. Therefore, the design and development of efficient PMRs have emerged as a focal area of research, garnering significant attention in the field [61].

PMRs offer distinct operational and efficiency advantages over conventional photocatalytic slurry systems, primarily due to the synergistic integration of photocatalysis and membrane separation [77]. Through the action of membrane components, PMRs can confine the photocatalyst to a specific region during the reaction process while allowing the treated products to selectively permeate the membrane, thus realizing an efficient separation of the catalyst from the products [78]. This design facilitates a highly controlled reaction environment where the photocatalyst is physically confined, while the treated effluent selectively permeates the membrane [79], further enhancing the system’s economy and applicability [30]. Furthermore, the operational stability and process control are significantly improved within a PMR. The membrane module provides a defined reaction zone, which helps to optimize reaction kinetics by regulating the residence time of reactants and intermediates. In PMR systems, photocatalysts can be present in a variety of forms, including loaded on the surface or inside of the membrane matrix, and directly mixed in the reaction medium [33]. This integrated system enables continuous operation with superior catalyst reusability, mitigating the catalyst loss that is often unavoidable in conventional reactor designs. An additional key advantage is the mitigation of membrane fouling. The photocatalytic activity at or near the membrane surface can degrade adsorbed organic foulants, creating a self-cleaning mechanism that extends the operational lifespan and preserves the performance stability of the membrane. This design is particularly suitable for the degradation and separation of environmental pollutants, as it allows for multi-step operation within a single system, thereby simplifying the process flow. The structural design of the PMR also has potential additional advantages. The structural configuration of the reactor extends membrane longevity and enhances performance stability, which is a critical attribute for the long-term treatment of complex wastewater streams. Furthermore, when compared to conventional treatment processes, PMRs exhibit a significantly reduced physical footprint and lower energy consumption, underscoring their potential for widespread industrial application.

3.1. Classifications

The efficacy of a photocatalytic process is fundamentally dictated by the photoreactor’s design and the specific mode of catalyst deployment [80]. Critical design parameters that govern overall performance include the nature of the irradiation source (e.g., natural sunlight versus artificial lamps), its spatial configuration relative to the reaction medium (i.e., submerged or external), and the physical form of the catalyst, which may be deployed as a suspended slurry or immobilized on a solid support [32]. Possessing distinctive structural and functional attributes, PMRs can be classified according to multiple criteria. This inherent versatility allows for tailored optimization of the system across a diverse range of application scenarios.

Depending on the mode of operation, PMRs can be categorized into two types: continuous flow and intermittent flow (Figure 4a,b). The integration of continuous flow operation with multiphase photocatalysis is a particularly promising strategy for the development of sustainable chemical processes. Although conventional filled-bed reactors have been widely used in industrial catalytic applications, they encounter limitations related to energy transfer in photocatalytic systems [81]. Continuous photocatalytic reactors, which operate on the principle that photocatalysts absorb light energy to stimulate a chemical reaction, have a significant advantage over conventional discrete reactors in that they achieve continuous feed and product discharge. This design not only enables stable operation for a long period of time but also significantly enhances the reaction rate, thus providing a novel alternative to conventional synthesis and reaction chemistry [82]. For instance, a continuous fixed-bed photoreactor employing Bi₂WO₆ photocatalysts coated on silica sand was reported to achieve a degradation efficiency of 95.40% for sodium isobutylxanthate within 70 min under optimal conditions [83]. However, despite these advantages, the industrial-scale implementation of continuous PMRs is often hindered by challenges related to scale-up, potential for catalyst deactivation, and overall operating costs. In addition, photochemical reactions conducted in microchannels (inner diameter < 1 mm) demonstrate significantly shorter reaction time due to higher and uniform photon fluxes, while reducing by-product generation induced by over-irradiation, a phenomenon that is more common in conventional intermittent flow reactors. Batch reactors with intermittent flow have traditionally been used for small-scale studies to explore reaction mechanisms and conversion efficiencies. A significant limitation of conventional batch reactors is the non-uniform distribution of light intensity, which typically attenuates from the reactor wall toward the center, resulting in reduced reaction rates and uneven irradiation. Nevertheless, intermittent operation can be strategically advantageous in certain PMR configurations [84]. In contrast, intermittent flow reactors exhibit unique advantages in small-volume reactions due to their simplicity of fabrication, ease of operation, and higher residence times [71]. However, standard intermittent flow reactors, typically on the centimeter scale in diameter, suffer from uneven distribution of light intensity, which attenuates significantly from the vessel wall toward the center of the reaction mixture, resulting in reduced reaction rates and uneven irradiation [84]. In an effort to address such limitations, Ahmad et al. fabricated a partially coated TiO₂ (pc-TiO₂) layer on a porous Al₂O₃ membrane substrate for application in a photocatalytic membrane reactor designed for the treatment of organic dye pollutants; low-cost polyvinyl chloride (PVC) was utilized to generate interstitial voids. Their investigation revealed that intermittent flow membrane filtration markedly enhanced the photocatalytic activity of both uncoated (uc) and pc-TiO₂ composite membranes when compared to the performance of bare Al₂O₃ membranes [85]. Furthermore, different PMR configurations can be distinguished based on different locations of the light source. In particular, the radiation source may be (i) above/inside the container containing the feed solution; (ii) above/inside the cell containing the membrane; and (iii) above/inside an additional container that may be located between the feed tank and the cell containing the membrane (Figure 4c–f) [86].

The most common classification method is based on whether the catalyst is dispersed in a slurry or immobilized on a carrier. PMRs can be designed in two main configurations, as shown in Figure 5: (i) reactors characterized by the suspension of the photocatalyst within the reaction mixture are designated as slurry PMRs and (ii) PMRs with photocatalysts immobilized in/on a substrate material used as a photocatalytic membrane (IPMR). Both configurations have specific advantages and limitations depending on the specific application [69].

3.1.1. Slurry Photocatalytic Membrane Reactor (SPMR)

While photocatalytic membranes with immobilized catalysts offer a promising route for simultaneous pollutant degradation and fouling mitigation, ensuring sufficient and uniform irradiation of the catalyst layer on the membrane surface remains a significant engineering challenge. However, how to ensure that the membrane surface receives sufficient light remains a critical issue to be solved. As a practical alternative, the slurry photocatalytic membrane reactor (SPMR) configuration has been widely investigated. The slurry photocatalytic membrane reactor, a type of photoreactor in which the photocatalyst is dispersed in the feed solution, effectively increases the chance of contact between the pollutant and the photocatalyst. The system utilizes the excellent separation performance of the separation membrane to achieve efficient separation and recovery of photocatalytic particles. This design confers several distinct advantages. The suspended nature of the catalyst provides a very large specific surface area for reaction and mitigates mass transmission limitations, which can significantly accelerate photocatalytic degradation rates [87]. Significantly, the membrane acts as an absolute barrier, ensuring the complete retention and recovery of the photocatalyst particles. Simultaneously, the application of dispersed particles increased the number of suspended photocatalysts, thus accelerating the photocatalytic degradation process and further improving the treatment efficiency.

The SPMR represents a prevalent configuration wherein the photocatalyst is maintained as a suspension within the reaction medium. This design is intended to capitalize on the superior mass transmission characteristics inherent to slurry-based systems, where the high specific surface area of the dispersed nano- or micro-sized catalyst particles optimizes reaction kinetics and pollutant degradation rates. Furthermore, the turbulence induced by mechanical agitation or aeration serves a dual purpose: it mitigates the deposition of foulants on the membrane surface and enhances the mass transmission of dissolved oxygen, which acts as a critical electron scavenger, thereby improving the overall process efficiency. However, excessive bubble formation can scatter light, which may reduce photocatalytic reaction efficiency [25]. Slurry photocatalytic systems are widely recognized for their superior mass transmission performance and high photocatalytic efficiency. Semiconductor photocatalysts, typically in the nanometer or micrometer size range, enhance mass transmission rates and optimize reaction conditions. Slurry-type reactors have been extensively employed to address mass transmission limitations due to their ability to enhance efficiency and reduce residence time [88]. A primary limitation is the phenomenon of light scattering by the suspended particles, which can impede uniform photon distribution throughout the reactor and adversely affect the overall quantum yield [89]. SPMRs have been developed to overcome these limitations by integrating catalyst separation and recycling mechanisms. Concurrently, the membrane module itself is susceptible to two distinct operational issues: potential material degradation resulting from prolonged exposure to UV irradiation and a decline in permeate flux due to the deposition of the catalyst nanoparticles themselves, which can form a resistive cake layer on the membrane surface. In parallel, prolonged operation may expose the membrane to UV radiation, potentially causing damage. This issue can be addressed by incorporating an opaque baffle between the light source and the membrane. Alternatively, placing the membrane outside the reactor can protect it from UV damage, though this arrangement may increase the pressure drop and cause fluctuations in catalyst concentration within the reactor [65]. The design of slurry-based reactors involves optimizing the balance between mass transmission and radiation. The convective movement of catalyst particles can cause significant light scattering, which affects light distribution within the reactor and subsequently impacts reaction efficiency [71]. Numerous studies have demonstrated that PMRs with slurry photocatalysts generally achieve higher efficiencies compared to those with immobilized photocatalysts. Submerging the membrane directly within the photoreactor offers a simplified and compact system, although protective measures such as opaque baffles may be necessary to shield the membrane from direct irradiation [57]. Conversely, positioning the membrane in an external loop effectively protects it from UV degradation but can introduce hydraulic complexities, including increased pressure drop and potential fluctuations in catalyst concentration within the reactor. As an illustration of the former configuration, a suspended catalyst in an SPMR was developed by Gomaa et al., utilizing ZnO as the photocatalyst and an immersed LED-UV lamp as the irradiation source. This system demonstrated approximately 86% dye decolorization efficiency in the treatment of methylene blue (MB) and achieved nearly complete oil-water separation, showcasing its dual functionality for simultaneous pollutant degradation and physical separation [90].

SPMRs are principally classified into two configurations based on the spatial relationship between the photoreaction and membrane separation modules: immersed and split-type systems [36]. In the immersed configuration, the membrane module is submerged directly within the slurry photoreactor, creating a highly integrated and compact unit. This design minimizes the system footprint and hydraulic resistance, thereby reducing capital and operational expenditures. Typically, inorganic or polymer membranes are submerged in a slurry-based photocatalytic reactor for operation [91]. In this system, the membrane module is submerged directly in the photocatalytic reactor, preventing catalyst loss and maintaining a stable catalyst concentration in the reactor. This design provides greater operational stability than other reactor types. In addition, the compact structure of the immersed reactor can shorten the pipeline length and reduce the system resistance loss, which in turn significantly reduces the investment and maintenance costs [92]. The direct immersion ensures complete catalyst retention, which promotes a stable catalyst concentration and high operational stability. Early iterations of this design, such as the system developed by Zhang et al. for sodium dodecylbenzene sulfonate surfactant degradation, utilized a PMR with a silica/titania nanorods/nanotubes composite photocatalytic membrane [93]. While this ensured intimate contact between the catalyst and photons, it lacked a dedicated catalyst recovery mechanism. Subsequent innovations have addressed this limitation [94]. The filtration mechanism relied on a dynamic filter cake layer, making it particularly suitable for handling larger particles of suspended pollutants. However, this system did not include a catalyst recovery method, and the internal placement of the UV light source ensured close interaction with the catalyst, thereby enhancing the excitation and efficiency of the photocatalytic reaction. Building on this approach, Li et al. introduced a novel enhancement by integrating an external magnetic field into the submerged photocatalytic reactor system, enabling efficient recovery of the catalyst. They proposed a photo-Fenton catalytic system, the submerged magnetic separation photocatalytic membrane reactor, which combined photocatalysis and Fenton oxidation processes [95]. The synthesized TiO₂-Fe₃O₄ composite catalyst demonstrated exceptional degradation performance, particularly for the removal of amoxicillin, significantly improving photocatalytic reaction efficiency and catalyst reusability. Extending the application of PMR technology, Zeitoun et al. designed a system combining a polyvinylidene fluoride (PVDF) membrane with TiO₂ and an ultraviolet light source. This PVDF-TiO₂ system exhibited notable hydrophobicity and high filtration precision, making it especially effective for treating dye wastewater and other high-pollution-load effluents [96]. The direct immersion ensures complete catalyst retention, which promotes a stable catalyst concentration and high operational stability. The slurry photocatalytic membrane reactor by Janssens et al. treated secondary wastewater with anticancer drugs, retaining TiO₂. This indicates PMRs are promising for advanced water treatment, but a catalyst for real wastewater is still needed [97]. While this ensured intimate contact between the catalyst and photons, it lacked a dedicated catalyst recovery mechanism. Subsequent innovations have addressed this limitation. Immersed PMR integrates membrane materials and light sources into one system, which can adapt to a variety of treatment scenarios by changing different membrane materials or operating modes. In contrast, split reactors consist of two distinct units for photocatalytic reaction and membrane separation, connected via piping to form an integrated system. The photocatalytic module and the membrane separation module are connected via piping to form a coordinated system [98]. The external structure of this design has the advantages of clear operation, easy assembly, and low maintenance. The split photocatalytic membrane reactor with an external membrane separation module can be operated in continuous mode. In this case, the permeate is discharged as a treated effluent, and the photocatalyst is returned to the reactor for reuse; in the batch mode, both the permeate and the concentrate are returned to the photoreactor to maintain a constant volume of the reaction system [99,100].

From an industrial application perspective, the choice between immersed and split-slurry PMR configurations is often governed by specific operational priorities. Immersed systems are generally favored for applications where high reaction efficiency and a compact footprint are paramount. Slurry-based systems are inherently advantageous as they can minimize the mechanical shear stress on catalyst particles and reduce the potential for pump abrasion and clogging [101]. Despite these advantages, slurry-based PMRs require aeration, which can result in significant energy consumption [102]. Therefore, optimizing the aeration rate, along with the size and distribution of bubbles, is crucial for improving efficiency [103]. Parallel to reactor configuration, the selection of the membrane material is a pivotal design decision. Organic polymeric membranes represent a more economical option, whereas inorganic ceramic membranes are distinguished by their superior chemical, thermal, and mechanical stability [104]. A primary research challenge associated with polymeric membranes is enhancing their long-term stability and resistance to degradation by photogenerated reactive oxygen species. Conversely, while ceramic membranes are inherently more robust, a significant body of research has been dedicated to leveraging photocatalysis for the effective mitigation of biofouling on their surfaces.

3.1.2. Immobilized Photocatalytic Membrane Reactor (IPMR)

Photocatalysts in suspended form are widely used in PMRs, but the complexity of their recovery from the reaction system has become a major challenge in realizing large-scale applications. To cope with this problem, the research focus in recent years has shifted to photocatalytic membrane technology, in which the catalyst recovery challenges are effectively avoided by immobilizing the photocatalyst on the membrane surface or inside [104]. Immobilized PMRs (IPMRs) are designed by fixing the photocatalyst directly on the membrane surface or embedding it in its porous structure, which effectively eliminates the need for downstream catalyst recovery processes. This approach is further motivated by the inherent limitations of conventional photoreactors, which often suffer from low surface-area-to-volume ratios and non-uniform light distribution, thereby constraining overall quantum efficiency. The immobilization of photocatalysts on polymeric membrane carriers, thus utilizing membrane reactors to carry out the reaction, is a promising and efficient solution. The efficacy of the immobilized PMRs concept is well-illustrated by a study from Biao et al. on the removal of microplastic fibers from synthetic laundry wastewater. This system was designed to harness the synergistic effects of photocatalytic degradation by the Ag-TiO₂ layer, enhanced by the plasmonic effect of the silver nanoparticles, and physical rejection of the microfibers by the membrane support. The study demonstrated the significant potential of immobilized PMRs as a scalable and effective technology for mitigating pollution from emerging contaminants [105]. The method combines photocatalytic degradation with membrane filtration to effectively reject and degrade fibrous MPs, thereby reducing their environmental impact.

The integration of the photocatalyst with the membrane support, accomplished via either surface anchoring or matrix entrapment, yields a bifunctional composite. This architecture enables the membrane to serve the dual purpose of a catalytic reaction platform and a selective separation barrier [106]. The preeminent advantage of this design is the complete elimination of the catalyst recovery and recirculation loop required in slurry-based systems. Although the immobilization of the catalyst may introduce mass transmission resistances that can potentially lower the apparent reaction kinetics, this limitation is frequently offset by the substantial gains in operational stability, catalyst reusability, and process simplification. These attributes are instrumental for the development of continuous, long-term, and sustainable photocatalytic processes for environmental remediation.

In PMRs, the immobilization of the catalyst onto a support is a key strategy to overcome the limitations inherent to slurry-based systems. In this approach, the photocatalyst is anchored to the membrane, forming an integrated, bifunctional unit that performs both catalytic degradation and physical separation [25]. Systems with a split design can effectively isolate the photocatalytic and membrane units, thus avoiding potential damage to the membrane material from UV irradiation and photogenerated reactive oxygen species. This design fundamentally obviates the challenges associated with the recovery, agglomeration, and potential loss of suspended nanoparticles [57]. Photocatalysts can be deposited on commercial membranes by surface loading or embedding, and the resulting catalytic membranes prepared are capable of simultaneous pollutant separation and photocatalytic degradation [107]. In this configuration, the function of the membrane can be categorized into two types: only as a carrier for the photocatalyst, or both separation and immobilization of the catalyst. In the latter case, the membrane not only acts as a selective barrier to confine the pollutant to be degraded to the reaction environment, but also assumes the role of a photocatalyst carrier [65]. Integrated PMRs (IPMRs) are considered an advanced solution. Although suspended photocatalysts exhibit high pollutant degradation efficiencies due to the large specific surface area of their nanoparticles, the difficulty of their separation and recycling is their main limitation. In addition, the tendency of nanoparticles to agglomerate may further reduce the reaction efficiency. By immobilizing the photocatalyst on a support material, these problems can be alleviated to a certain extent, as embodied in the design of the IPMR. In addition, membrane contamination triggered by photocatalyst deposition in suspended PMRs (SPMRs) is a pressing issue. Based on the concept of process enhancement, the application of immobilized photocatalysts avoids the subsequent recovery step of the catalyst and can effectively mitigate the fouling caused by photocatalysts and contaminants [65]. In the reactor with immobilized catalysts, since the catalysts are immobilized in the form of membranes, the light scattering effect can be neglected, which simplifies the modeling process of the radiation field. Therefore, the focus of the study is more concentrated on internal and external mass transmission phenomena [108].

The morphology of the photocatalytic membranes and the distribution of nanoparticles largely depend on the specific synthesis method used to incorporate the photocatalysts into the membranes [57]. In recent years, the focus of research has gradually shifted to stationary PMRs, aiming to overcome their lack of efficiency through appropriate catalyst modification [47]. An immobilized photocatalytic membrane reactor can be further divided into the following two categories according to the different photocatalytic membrane preparation methods: (1) photocatalysts are deposited into the membrane structure; (2) photocatalysts are loaded on the surface of the membrane material [88]. The photocatalytic membrane can not only serve as a physical barrier to achieve efficient separation but also degrade the organic pollutants attached to the membrane surface or through the membrane pores through photocatalysis. This dual functionality positions photocatalytic membranes as an innovative water treatment solution that is both efficient and sustainable.

3.1.3. Other Types

Submerged Photocatalytic Membrane

In slurry systems, the flow of photocatalytic particles may cause mechanical damage to the membrane surface, thus shortening the lifetime of the membrane [109]. For slurry-based PMRs, the permeation of photocatalysts may reduce the permeate quality during the early stages of filtration [110]. Although slurry reactors can provide higher mass transmission efficiencies and reaction rates, significantly improving treatment performance, additional separation steps are required to remove suspended catalyst from the treated water [111]. Conversely, the suspended catalyst reactor exhibits a larger mass transmission area and a higher reaction rate for the same catalyst dose. However, this design faces an inherent challenge in that the catalyst particles must be recovered from the treated water [112]. When the catalyst is loaded onto the carrier material in an immobilized form, this usually leads to a significant reduction in the accessibility of the photocatalytic active surface, which weakens the photocatalytic performance [113]. A primary operational constraint is the necessity for a downstream separation unit to recover the catalyst, a requirement that invariably elevates operational expenditures. Internally, the system’s performance is often compromised by two phenomena: particle agglomeration at elevated catalyst loadings, which diminishes the effective specific surface area, and the light-scattering effect induced by suspended particles, which impedes uniform photon penetration and reduces the overall quantum yield. Although photocatalytic membrane systems (PMSs) with suspended catalysts have shown significant advantages in mitigating membrane contamination, the effective recovery and regeneration of the catalysts remain an urgent challenge [25]. Insights from existing theoretical models and slurry system experiments indicate that compact “membrane-type” configurations are generally more effective for achieving high treatment efficiency. By contrast, bulk water treatment demands extended or sequential photocatalytic reactors to maintain adequate reaction efficiency [75]. In response to these limitations, submerged photocatalytic membranes have been proposed as a promising alternative. A submerged photocatalytic membrane reactor is a hybrid system integrating photocatalysis and membrane filtration, which effectively overcomes the difficulty of separating catalysts from wastewater in traditional processes. In this system, the suspended photocatalyst can significantly improve the pollutant removal efficiency due to its large specific surface area and the generation of highly active free radicals. However, the separation of suspended catalysts and the continuous operation of the system face certain challenges. To solve these problems, a submerged photocatalytic membrane reactor was developed. The submerged photocatalytic membrane reactor represents a more compact and structurally integrated configuration when compared to conventional split-loop designs. In the submerged photocatalytic membrane reactor, the core function of the membrane is to act as a barrier, both to prevent the loss of catalyst and to separate the products and by-products generated during the photocatalytic oxidation process. In this system, the core function of the membrane, typically a microfiltration or ultrafiltration module, is to serve as a dynamic barrier. Its role extends beyond the mere retention of suspended catalyst particles to include the separation of products and by-products generated during the photocatalytic oxidation process. Furthermore, the membrane can be engineered to selectively sequester target pollutants, a functionality that allows for the precise regulation of their residence time within the photoreactive zone, thereby enhancing the overall degradation efficiency [114]. Studies have demonstrated that catalyst particles deposited on the membrane surface can further enhance photocatalytic reactions, and optimization of membrane loading and catalyst concentration is critical for improving system performance [115]. In addition, submerged membrane filtration technology, which is widely used in conventional membrane bioreactors, is cost-effective in mitigating particle deposition [116]. Another advantage of combining photocatalysis with submerged membranes is that photocatalytic degradation of contaminants and physical separation through membranes can be achieved simultaneously in a single device. However, the problem of membrane contamination significantly reduces membrane permeability, which in turn shortens membrane lifetime, increases operating costs, and weakens system performance. Especially in a submerged photocatalytic membrane reactor, the deposition of photocatalysts on the membrane surface not only leads to a decrease in membrane permeability but may also affect the photocatalytic efficiency [39]. In recent years, several research groups have made important progress in submerged photocatalytic membrane reactors. Jiang et al. investigated the potential of a membrane photocatalytic reactor (MPR) for degrading soluble extracellular organic matter (SEOM), achieving over 60% removal at a hydraulic retention time of 140 min (Figure 6a). This was largely due to the synergistic effect of adsorption and photocatalysis on the TiO₂ surface, showcasing the foundational benefits of integrating membrane filtration with photocatalysis for pollutant degradation [117]. Building on this foundation, Wang et al. developed a submerged membrane photocatalytic reactor (SMPR) specifically tailored for the treatment of p-nitrophenol (PNP) wastewater. Their system not only achieved a high PNP removal rate of 91.6% under simulated sunlight but also demonstrated a significant advantage over slurry systems by ensuring a 100% retention rate of the catalyst on the MF membrane, thus preventing catalyst loss and secondary pollution, which are common issues in slurry reactors [118] (Figure 6b). Further advancing this concept, Li et al. introduced a submerged ceramic membrane photocatalytic reactor (SCMPR) that utilized suspended TiO₂ for antibiotic removal. The antifouling ceramic membrane with its hollow structure facilitated efficient catalyst separation, offering stability and high removal efficiency for amoxicillin across a broad pH range. This design exemplifies how submerged reactors can handle complex, pH-sensitive pollutants more effectively than slurry systems by providing a stable environment for catalyst activity [119]. Lastly, Kertèsz et al. took the submerged PMR approach to optimize dye wastewater treatment with a hollow fiber microfiltration photocatalytic membrane reactor (Figure 6c). Their system not only achieved complete catalyst separation from the treated water but also operated at a sustainable permeate flux, emphasizing the importance of balancing the photocatalytic and membrane separation capacities for enhanced system performance. This study further underscores the advantage of submerged PMRs in maintaining catalyst integrity and enhancing treatment efficiency compared to slurry reactors, where catalyst recovery is more challenging [120]. In aggregate, these studies illustrate a progressive enhancement in submerged PMR technology, from basic pollutant removal to sophisticated treatment of specific contaminants like dyes and antibiotics, offering clear advantages in catalyst retention, pollutant degradation, and system sustainability over traditional slurry photocatalytic systems.

Oscillating Photocatalytic Membrane

The operation of photocatalysts in PMRs using immobilized photocatalysts also presents several challenges, such as the leaching and settling of the catalysts, as well as the process of separating and recycling the catalysts from the treated water, which are complex, time-consuming, and costly. In addition, the absorption and scattering of light by the catalyst can limit the depth of light penetration [89]. Nevertheless, the deposition of photocatalyst nanoparticles (NPs) on the membrane surface triggers a decrease in flux, while the fouling problem due to light scattering remains a major limiting factor in the performance of such membrane photoreactors [36]. It has been shown that the introduction of oscillatory motion can effectively reduce the deposition of particles on the membrane surface [89].

In submerged photocatalytic reactors without oscillations, the transverse osmotic flow directs catalyst particles to the membrane surface, leading to the formation of a particle/filter cake layer on the membrane surface. This deposit not only increases the pressure drop across the membrane but also reduces the fraction of catalyst in suspension, which decreases the photocatalytic efficiency of the system. In contrast, membrane oscillation can provide significant advantages in terms of membrane cleaning and photocatalytic efficiency. This is due to the development of vortices, especially during shear reversal, which may dominate most of the oscillation cycle. In addition, enhanced shear dynamic filtration (DF) systems have been recognized as an effective means of improving the hydrodynamic conditions of membrane separation systems and reducing membrane contamination [121]. This method creates high shear forces at the membrane surface by generating relative motion between the membrane and the neighboring fluid, such as rotational or oscillatory motion. This mechanism not only reduces membrane contamination and increases system flux but also achieves separation of the permeate stream from the surface shear, thus providing additional flexibility to optimize the reactor residence time to suit the photocatalytic reaction kinetics. The application of oscillatory motion has further demonstrated its significant effect in reducing membrane contamination and promoting catalyst suspension in PMRs (Figure 7) [111]. It has been shown that ZnO exhibits excellent photocatalytic activity by virtue of its excellent UV light absorption and unique crystal structure. These properties make ZnO an efficient photogenerated electron capture site, which significantly enhances charge separation while reducing the need for oxygen as an electron acceptor [112].

3.2. Kinetics in PMRs

The study of reaction kinetics in PMRs is particularly important because it determines the reaction rate, conversion efficiency, and reactor design and operating conditions. As shown in Figure 8, photocatalytic reaction kinetics is the study of the rates, pathways, and influencing factors of chemical reactions of reactants on the surface of photocatalysts, which can help predict and optimize the performance of photocatalytic reactors.

3.2.1. Kinetic Model

A quantitative understanding of reaction kinetics is fundamental to the design, optimization, and scale-up of PMRs. The heterogeneous photocatalytic degradation of organic pollutants in PMRs is most frequently described by the Langmuir–Hinshelwood (L–H) kinetic model, which accounts for reactant adsorption on catalytic sites, surface photochemical reactions, and subsequent desorption of intermediates and products. Under conditions of low initial substrate concentration, the L-H model can often be simplified to an apparent pseudo-first-order kinetic model. The determination of the apparent rate constant, k, is therefore a central objective in experimental and modeling studies, as it provides a quantitative metric for evaluating and comparing system performance. The magnitude of this rate constant is a complex function of multiple interdependent variables. These include intrinsic catalyst properties, such as specific surface area and surface activity; external operational parameters, including incident light intensity (photon flux), pH, and temperature; and the mass transmission characteristics of the system, which are governed by the initial reactant concentration and the hydraulic properties of the membrane module. A thorough understanding of these kinetic relationships is thus indispensable for elucidating reaction mechanisms, identifying rate-limiting steps, and systematically optimizing the operational parameters of the reactor for maximal degradation efficiency. The rate of the photocatalytic reaction can be expressed by the following reaction equation, as shown in Equation (1):

$$r={\displaystyle \frac{kKC}{1 + KC}},$$

(1)

where r is the reaction rate, k is the apparent reaction rate constant, $K$ is the adsorption equilibrium constant, and $C$ is the concentration of the reactants. The model shows that the reaction rate is proportional to the reactant concentration at low concentrations and saturates at high concentrations. The apparent kinetic parameters of photodegradation in immobilized PMR systems are determined by the kinetics of pollutants and intermediates to and from the immobilized photocatalysts on or in the photocatalytic membrane. Most of the degradation reactions in immobilized PMRs follow the proposed first-order kinetics [122]. The rate constant ($k$) is an important factor affecting the reaction rate and conversion (in this case, degradation efficiency). In most studies, the kinetic rate constant takes into mass transmission effects in the reactor. Therefore, it has been reported that the kinetic constant usually varies with size, stirring speed [71]. The kinetics of pollutant degradation in aqueous multiphase catalytic reactions were investigated using the Langmuir-Hinshelwood model and power law [123]. The reaction kinetics provide information about the reaction rate and the mechanism of pollutant removal. The Langmuir-Hinshelwood model is commonly used in multiphase reaction kinetic studies, such as photocatalytic processes using TiO₂ nanoparticles. The general form of the model is shown in Equation (2):

$$R=-{\displaystyle \frac{d{C}_L}{dt}}={\displaystyle \frac{k_{L-H}{K}_{ads}{C}_L}{1 + K_{ads}{C}_L}},$$

(2)

where $C_L$ denotes the concentration of leachate COD at each time, k_L−H is the Langmuir-Hinshelwood reaction constant for the removal of COD, and $K_{ads}$ is the equilibrium constant for the adsorption of organic matter onto TiO₂ nanoparticles.

3.2.2. Fluid Mechanics Model

The integration of kinetic, hydrodynamic, and radiative transfer models enables a priori prediction of photocatalytic reactor performance and provides a foundation for systematic optimization. In order to allow for appropriate development and scale-up strategies for PMR design, it is important to understand the interaction effects of hydrodynamic and colloidal forces on catalyst particle aggregation [121]. Despite its importance, a significant gap persists in the literature concerning the analytical and numerical modeling of these interactions, a deficit that is especially pronounced for advanced configurations such as DF systems. Dynamic filtration is an advanced operational strategy engineered to mitigate membrane fouling by inducing high shear at the membrane surface, typically through rotational or oscillatory motion of the membrane or an adjacent component [124]. Beyond the effective mitigation of concentration polarization and cake layer formation, a salient advantage of DF is the decoupling of the permeate flux from the cross-flow hydrodynamics. This decoupling provides an independent degree of freedom for the precise control of the reactor residence time, a parameter dictated by the intrinsic kinetics of the photocatalytic reaction [121].

3.2.3. Particle Aggregation

The degradation kinetics within slurry PMR systems are profoundly influenced by the interplay between initial substrate concentration and the prevailing hydrodynamic conditions [125]. Particularly in photocatalytic reactors, particle aggregation can negatively affect semiconductor photocatalytic activity by reducing the production of hydroxyl radicals and reactive oxygen species [121]. This phenomenon, known as the inner filter effect, diminishes the quantum efficiency of the process by reducing the rate of electron-hole pair generation. Conversely, operation at lower substrate concentrations, particularly when coupled with higher flow rates, has been demonstrated to yield superior degradation efficiency. This enhanced performance is attributable not only to the mitigation of the inner filter effect, which increases photon availability for the catalyst, but also to the improved mass transmission of reactants to the catalyst surface under conditions of increased turbulence [71].

3.2.4. Mass Transmission

In PMRs, the integration of a physical separation barrier introduces complex mass transmission phenomena that can become the rate-determining step for the overall process. Consequently, the observed degradation rate is frequently governed not by the intrinsic kinetics of the surface photochemical reaction, but by the transport of reactants and products to and from the active catalytic sites. A comprehensive analysis of mass transmission limitations within a PMR must consider several distinct transport resistances. The first is external mass transmission, which involves the convective-diffusive transport of pollutant molecules from the bulk fluid across the hydrodynamic boundary layer to the exterior surface of the photocatalyst or the catalytic membrane. The second is internal mass transmission, or intraparticle diffusion, which pertains to the diffusion of reactants within the porous structure of the photocatalyst or the catalytically active membrane layer. Finally, transmembrane mass transmission—the transport of molecules through the membrane pores—is also a critical factor, influenced by membrane properties, such as pore size and surface chemistry, and by operational conditions that dictate the formation of polarization layer. A rigorous understanding of these coupled transport processes is therefore a prerequisite for the accurate kinetic modeling and rational design of high-performance PMRs.

3.2.5. Photon Utilization

A pivotal performance metric for any photocatalytic system is the photon utilization efficiency, which is defined as the fraction of incident photons that are productively absorbed by the catalyst and result in the desired chemical transformation. This parameter provides an integrated measure of the system’s capacity to convert light energy into chemical potential, inherently accounting for losses due to reflection, transmission, and scattering. In kinetic investigations, the photon utilization efficiency is an indispensable parameter for quantifying and comparing the intrinsic activity of different photocatalytic materials. Its magnitude is fundamentally dictated by the inherent optoelectronic properties of the semiconductor, most notably its band gap energy and spectral absorption characteristics, as well as the degree of spectral overlap with the emission profile of the irradiation source. From a reactor engineering standpoint, the maximization of this efficiency is a primary design objective. This requires the careful optimization of the reactor geometry, the catalyst loading, and the system hydrodynamics to ensure a uniform radiation field and to minimize non-productive light absorption and scattering.

3.2.6. Collision Theory

Within the porous architecture of photocatalytic membranes, where pore diameters typically range from nanometers to several micrometers, the internal fluid dynamics are characterized by Stokes flow. Under this low-Reynolds-number regime, the transport of micropollutants to the catalytically active pore walls is governed predominantly by radial diffusion. ROS are subsequently generated at these surfaces through the interaction of adsorbed pollutants with the immobilized photocatalyst. Within the porous architecture of photocatalytic membranes, where pore diameters typically range from nanometers to several micrometers, the internal fluid dynamics are characterized by Stokes flow. A reduction in pore size to the nanometer or sub-nanometer scale can markedly enhance the apparent reaction kinetics, primarily due to the significant shortening of the diffusional path length required for reactant molecules to access active sites. In such highly confined spaces, the distinction between aqueous-phase and absorbed-phase reactions becomes effectively negligible, as interactions between pollutants, ROS, and the catalytic surface occur almost instantaneously at the solid–liquid interface [126].

Collision theory provides a robust framework for characterizing the rate of homogeneous reactions and serves as an effective tool for assessing the performance and mechanical limitations of PMRs. According to collision theory, reactants must collide to undergo a chemical reaction; however, only a fraction of these collisions lead to successful reactions. For a reaction to occur, the colliding molecules must possess sufficient energy to overcome the activation energy barrier, enabling the breaking of existing bonds and the formation of new ones. An increase in reactant concentration enhances the number of collisions within a given timeframe, thereby increasing the reaction rate. This relationship underscores the importance of collision frequency in determining reaction kinetics. The collision frequency is intrinsically linked to the reaction rate constant, k (mol L⁻¹ s⁻¹), as described by the Arrhenius equation, which quantitatively connects the reaction rate to the temperature and activation energy of the system (Equation (3)).

$$k=Zexp\left({\displaystyle \frac{{-E}_a}{R_{g}T}}\right),$$

(3)

where Z (mol L⁻¹ s⁻¹) corresponds to the product of the concentrations of the reacting species. The activation energy, E_a (J mol⁻¹), represents the minimum energy required for a reaction to occur, while R_g (J K⁻¹ mol⁻¹) denotes the universal gas constant, and T is the absolute temperature. exp$\left({\displaystyle \frac{{-E}_a}{R_{g}T}}\right)$ signifies the fraction of molecular collisions with sufficient energy to overcome the activation energy barrier, thereby facilitating a chemical reaction. Characterizing the reaction kinetics in a PMR based on collision theory requires the identification and quantification of ROS generated during the process. Assuming that a minimal quantity of micropollutants, such as nanogram-per-liter concentrations of steroid hormones, participates in the quenching of singlet oxygen (¹O₂), the concentration of ROS generated can be determined based on three key factors: (i) the absorbed photon flux (φ_abs, V), (ii) the lifetime or self-decay rate of the ROS, and (iii) the photon conversion efficiency, represented by the quantum yield (Φ_Δ), as expressed in Equation (4).

$$[{}^1\mathrm{O}_2]={\displaystyle \frac{{\phi }_{abs, v}{\varnothing }_{∆}}{k_{∆}+k_q\left[MP\right]N_{avo}}}\approx {\displaystyle \frac{{\phi }_{abs, v}{\varnothing }_{∆}}{k_{∆}{N}_{avo}}}$$

(4)

where k_Δ (μs⁻¹) is the rate constant of ¹O₂ decay (inverse of the ¹O₂ lifetime), k_q (L mol⁻¹ s⁻¹) is the bimolecular rate constant of ¹O₂ quenching by micropollutant molecules, and [MP] (mol L⁻¹) is the molar concentration of micropollutants. When the micropollutant concentration is very low ([MP]≪k_Δ/k_q [MP]), the condition k_Δ≫k_q [MP] is satisfied. The quantum yield (Φ_Δ) of ¹O₂ generation is defined as the ratio of the number of ROS formed to the number of photons absorbed. Palladium (II)-containing porphyrins exhibit a Φ_Δ for ¹O₂ generation, attributed to their ability to significantly enhance spin–orbit coupling. This enhancement facilitates intersystem crossing, thereby increasing the efficiency of porphyrins in generating ¹O₂.

The operational flow rate is a critical parameter that profoundly influences the performance of a photocatalytic membrane reactor by governing two key factors: the convective mass transport of pollutants and the hydraulic residence time. An increase in the flow rate enhances the molar flux of micropollutants to the catalytically active surfaces, which can alleviate external mass transmission limitations. Concurrently, however, this elevated flow rate corresponds to a diminished hydraulic residence time, thereby reducing the contact time available for the photocatalytic degradation to occur. The interplay between these opposing effects gives rise to a characteristic kinetic profile, wherein the overall degradation rate is observed to increase with flow rate up to a distinct threshold, beyond which the rate either plateaus or declines. This behavior reflects transitions between reaction regimes: at low flow rates, performance is restricted by mass transmission to catalytic sites, whereas at higher flow rates, this limitation is alleviated, and degradation improves. Beyond the optimal range, however, the system becomes kinetically controlled, with insufficient residence time rendering the intrinsic surface photochemical reaction the rate-determining step.

Previous conceptual frameworks have sought to rationalize the photodegradation efficiency within porous photocatalytic membranes by invoking the qualitative concept of “contact”. This concept correlates the hydraulic residence time with the characteristic “mixing time”—the average time required for a micropollutant to diffuse across the pore radius. While this heuristic provides a rudimentary explanation for performance variations with respect to pore size and flow rate, its utility is fundamentally limited, as it fails to quantitatively account for the specific reaction locus or for critical system parameters such as photon flux, light penetration depth, photosensitizer concentration, and the intrinsic reactivity of the target micropollutants. In contrast, the application of a framework grounded in collision theory represents a significant conceptual advancement by offering a more fundamental and comprehensive approach to describing these nanoscale reactive events. The adaptation of collision theory to describe reactions within highly confined nanoporous environments is a novel approach that is anticipated to provide a more rigorous and quantitative understanding of the kinetic interplay between pollutants and catalytic sites within PMRs.

3.3. Factors Affecting PMR Performances

The operational advantages of PMRs over conventional photoreactor configurations are substantial, arising from the synergistic integration of reaction and separation. This unified approach facilitates the complete retention of the photocatalyst within the reaction environment, affords precise control over molecular residence times, and enables the continuous removal of products [113]. Despite these intrinsic merits, the practical implementation of PMR technology is encumbered by several persistent challenges. These include suboptimal pollutant degradation efficiencies under certain conditions, insufficient long-term membrane durability, a pronounced susceptibility to severe membrane fouling, and the limited availability of robust, recyclable membrane materials [77]. The overall performance of any PMR system is dictated by a complex interplay of physicochemical and engineering parameters. As shown in Figure 9, the key variables influencing the reaction chemistry include the photocatalyst loading, the intensity and wavelength of the irradiation source, the nature and concentration of the target pollutant, and the bulk solution properties, such as temperature, pH, and the presence of inorganic salts. Concurrently, reactor-specific parameters, such as the light intensity distribution, hydraulic residence time, and the mode of catalyst deployment (i.e., suspended or immobilized), are critical determinants of the overall process efficiency [71].

Increased catalyst loading implies a more significant surface area available for adsorption and degradation of target compounds. Nonetheless, overloading of the catalyst will increase operating costs and may lead to a decrease in pollutant removal efficiency by impeding light penetration in the native solution [30]. This phenomenon was exemplified in a study on the degradation of amoxicillin (AMX) within a submerged ceramic membrane photocatalytic reactor (SCMPR), which employed a TiO₂ photocatalyst. They used a TiO₂ photocatalyst and Ceraflo CF-3604-DH hollow plate ceramic membrane in the SCPMR. The results showed that the TiO₂ catalyst (average particle size of 1.4 μm) could be easily separated by the ceramic membrane (pore size less than 500 nm). Although membrane contamination was observed during operation, it was effectively resolved by backwashing. The effect of TiO₂ loading was also investigated, and it was found that excess TiO₂ resulted in decreased degradation efficiency due to light blocking and scattering effects [119]. Hairom et al. successfully synthesized four different types of ZnO nanoparticles using oxalic acid and zinc acetate solutions by precipitation and constructed them into a PMR system to investigate the effect of ZnO nanoparticles on the treatment of Congo red (CR) dye in the MPR. The effect of the ZnO nanoparticles on the optimum photocatalyst loading for the MPR system was found to be 0.3 g/L⁻¹ at an initial dye concentration of 20 mg/L⁻¹ at pH 7, due to the effective surface area of ZnO-PVP-St and adsorption of UV light. It can be concluded that the optimum dosage of ZnO-PVP-St nanoparticles in the MPR for the treatment of industrial dye effluent has a great potential for very low NF membrane contamination [127]. Sakhaie et al. investigated and developed a zirconium-doped titanium dioxide-coated silicon carbide (Zr/TiO₂-SiC) photocatalytic membrane and activated it with an ultraviolet light-emitting diode (UV-LED) to establish the optimum conditions for photocatalytic synthesis and immobilization. The photocatalytic membrane was applied to the degradation of rhodamine B, a typical pollutant in textile wastewater. The design and operating parameters of zirconium molar ratio, photocatalyst loading, and UV-LED irradiance all affected the performance of the photocatalytic membrane. The results showed that zirconium doping increased the photocatalytic activity by more than 20% at an optimal concentration of 10% [128]. Shen et al. designed a continuous-flow fixed-bed photoreactor for the degradation of sodium isobutyl xanthate (SIBX) under visible-light irradiation using a Bi₂WO₆ photocatalyst coated on silica sand. After 30 min of visible light irradiation, the catalyst loading increased from 27.54% to 52.79% as it increased from 30% to 100%. These results indicate that the increase in catalyst loading improved the photodegradation of SIBX. This is due to the utilization of light energy to produce active substances, which are gradually utilized to reach the maximum degradation efficiency after the start of the photoreaction. The increase in catalyst loading leads to an increase in the ratio of [total catalyst surface area]/[SIBX molecules] [83]. Heredia et al. investigated the effect of the thickness of the coated catalysts and proposed a model capable of explaining this effect. Nine photocatalytic nanofiltration ceramic membranes with thicknesses ranging from 0.26 to 21.9 μm were prepared and evaluated. The function of these photocatalytic membranes was experimentally investigated in a one-way dead-end membrane reactor for the decolorization of methylene blue [129]. The obtained surface reaction rate constants indicated that the multilayers provided enhanced reactivity compared to the individual catalytic layers.

The initial and determinative step in a heterogeneous photocatalytic system is the photoexcitation of the catalyst surface, a process fundamentally governed by the quantum efficiency of the incident photons. Consequently, the overall reaction rate is intrinsically dependent on the specific type and concentration of the photocatalyst employed [130]. A significant constraint on the widespread implementation of photocatalytic reactors, particularly PMRs, for wastewater treatment is the requisite availability of large quantities of photocatalyst material and the limited diversity of effective photocatalyst composites [101]. For instance, TiO₂ is the most extensively utilized photocatalyst in slurry PMR systems, a preference attributable to its exceptional photochemical stability in aqueous media, robust catalytic activity, relatively long electron-hole pair lifetime, low cost, and minimal toxicity. A notable limitation of this material, however, is its inactivity under visible light irradiation. Its wide bandgap restricts its absorption profile to the UV portion of the solar spectrum, which constitutes only approximately 5% of the total solar radiation [61]. The efficacy of a photocatalytic process is contingent upon the band gap energy of the semiconductor, which determines its ability to drive chemical reactions. Within this framework, innovative strategies for catalyst modification have been developed to enhance performance. Yin et al. reported a strategy to deposit a Pd-Cu alloy on titanium dioxide bonded to a polyvinylidene difluoride membrane to improve membrane permeability and resistance to contamination and to achieve a high humic acid (HA) rejection efficiency (100%). Detailed characterization revealed a dual functionality of the PdCu-TiO₂ alloy: it provided paired Pd-Cu sites that facilitated enhanced HA adsorption and electron transfer, thereby improving photocatalytic removal, while simultaneously inducing a photothermal effect via carrier damping that directly augmented the membrane’s permeation flux. The remarkable performance of these functionalized membranes validates the significance of the alloying effect for the treatment of aqueous organic pollutants. Notwithstanding these advancements, photocatalyst deactivation remains a critical impediment to the widespread industrial application of photocatalytic degradation technologies. Research on this issue has focused on catalyst lifetime, deactivation mechanisms, and regeneration strategies. In an investigation of the continuous photocatalytic degradation of methyl orange (MO) within a PMR, the lifetime, deactivation causes, and regeneration of Degussa P25 TiO₂ were examined. The study concluded that photocatalyst deactivation had occurred once the concentration of MO in the PMR effluent reached a stable plateau, signifying a cessation of degradation activity [131].

The performance and reaction kinetics of integrated processes, such as CO₂ capture and its subsequent conversion to formic acid, have been evaluated using combined membrane contactor and photocatalytic systems. In such configurations, the rate of CO₂ photoreduction to formic acid was identified as the rate-limiting step, highlighting the necessity for further process optimization [132]. Among the operational variables, the solution pH is a pivotal parameter that profoundly influences photodegradation efficiency. Variations in pH significantly alter the surface charge of the photocatalyst and the speciation of contaminants, thereby governing adsorption behavior and overall degradation performance [83,133]. The pH also affects catalyst aggregation in suspension and the interaction of reactive species with the catalyst surface, making it a decisive factor in process efficiency [31]. Investigations of membrane fouling in PMRs across various pH levels have shown that operation under highly alkaline conditions (e.g., pH = 12) can minimize the membrane contact angle and reduce the deposition of TiO₂ particles [114]. Wang et al. [134] investigated membrane fouling of PMRs at several pH values (3, 6, 9, and 12) and reported that the membranes had the smallest contact angle and the lowest deposition of TiO₂ particles when operated at pH = 12. The effect of pH on fouling is intricate, and the optimal pH must be determined through preliminary studies for specific applications. Generally, alkaline conditions are favored to enhance photocatalytic degradation and optimize the interactions between TiO₂ particles and the membrane. However, the ideal pH is also contingent on the feed composition [33]. When investigating the effect of feed composition and pH on membrane contamination, it was found that the presence of Cl⁻ and SO₄²⁻ ions adversely affected the permeate flux. They also reported that the highest permeate flux was achieved when a near-neutral pH was used [135].

Figure 9. Key parameters influencing the performance of PMR, categorized by photocatalyst properties, operating conditions, and reactor configuration.

Figure 9. Key parameters influencing the performance of PMR, categorized by photocatalyst properties, operating conditions, and reactor configuration.

PMRs have become one of the most effective technologies for the treatment of contaminated water. A significant operational constraint, however, is the unilateral irradiation of the membrane, which utilizes only a single side for photo-activation. As shown by Mathieu et al., a reactor was developed where the membrane can be irradiated on both sides to overcome this problem. Polyacrylonitrile membranes incorporating up to 60% TiO₂ nanoparticles were fabricated by electrospinning and subsequently applied in a 3D-printed staggered-flow photocatalytic membrane reactor for the degradation of methylene blue under various light conditions. Utilizing both membrane surfaces substantially enhanced photocatalytic activity, leading to improved decolorization efficiency of methylene blue in water [136]. Further progress was achieved by Le et al., who designed visible light-responsive TiO₂@polydopamine (TiO₂@PDA) core–shell structures for integration into polyvinylidene difluoride (PVDF) membranes. Better dispersion of TiO₂@PDA in mixed-matrix membranes (MMMs) resulted in higher photodegradation performance compared to surface-coated membranes. At comparable TiO₂ loading (3.5–4.0%), the MMMs degraded 95.1% of the dye, while the surface-coated membrane degraded 89.2%. This performance differential underscores the critical role of intra-pore degradation, which was found to constitute 72% of the total degradation rate in surface-coated membranes and was the predominant mechanism within the MMMs [137].

Incident light serves as the primary energy input for any photocatalytic reaction, as the absorption of photons is responsible for the generation of electron-hole pairs within the photocatalyst, thereby initiating the degradation process. In a photoreactor, the catalyst must be activated by the incident light [104]. In general, the reaction rate increases with augmenting light intensity until a plateau is reached, whereupon the system becomes limited by mass transmission. The relationship between the reaction rate and light intensity is distinctly non-linear and can be delineated into three regimes. At low light intensities, typically in the range of 0–20 mW·cm⁻², a first-order relationship is observed where the reaction rate is directly proportional to the intensity. This is attributable to the fact that the generation of electron-hole pairs is the rate-determining step, with recombination being negligible. As the light intensity increases to a moderate level (~25 mW·cm⁻²), the reaction rate becomes a function of the square root of the intensity. This dependency arises from the competitive kinetics between electron-hole pair formation and their recombination. At high light intensities (>25 mW·cm⁻²), the reaction rate becomes independent of this parameter, as the catalyst surface becomes saturated with photons and the overall process is constrained by other factors, such as the rate of mass transmission of the pollutant to the catalyst surface [30,36]. In practical operation, moderate irradiation intensities typically promote higher pollutant degradation rates while reducing membrane fouling [33], as increased light intensity supports both electron–hole formation and recombination, whereas at low intensities, formation predominates without significant recombination [101]. The effect of light transmission also plays a critical role, since in most photoreactor designs, light must traverse fluids, gases, or transparent walls before reaching the catalyst, with partial absorption occurring along the path [104]. Experimental studies conducted under conditions free from transmission losses have consistently shown that reaction rates follow dependence on light intensity at values exceeding the solar equivalent. Conversely, at sufficiently low intensities (depending on the catalyst), the rate is first order in intensity. Because photon absorption is linear with respect to light intensity, quantum efficiency (molecules converted per absorbed photon) remains constant at low levels but decreases significantly at higher intensities, often to values between 0.7 and 0.5. This decline reflects efficiency loss under strong artificial irradiation or concentrated solar energy. Overall, increasing light intensity enhances volumetric reaction rates until the mass transmission limit becomes the controlling factor [82]. As reported in previous studies, photons with energies equal to or greater than the bandgap energy can be absorbed in the photocatalytic activity, leading to the production of electron-hole pairs. Consequently, the performance of a photocatalytic system is intrinsically linked to the spectral characteristics and intensity of the incident irradiation. For maximal quantum efficiency, a significant overlap between the emission spectrum of the light source and the absorption spectrum of the photocatalyst is required. This principle has driven considerable research into visible-light-responsive photocatalysts, which are of particular interest for developing environmentally sustainable and economically viable PMR systems [138]. Mercury lamps are among the most widely used sources, available in low-pressure and medium-pressure forms. At comparable power levels and without catalyst suspension, LP lamps were shown to induce higher degradation rates of cyclophosphamide than MP lamps, owing to their shorter emission wavelength of approximately 254 nm [139,140]. Upon irradiation, photons with energies greater than or equal to the band gap promote electrons from the valence band to the conduction band, initiating redox reactions through electron–hole pair formation. Since synthesized photocatalysts exhibit varying band gap energies, optimization of the light source is a critical parameter for enhancing performance [101]. Both the type and intensity of irradiation significantly affect photocatalytic efficiency. In multiphase systems, incident photons excite electrons, generating electron–hole pairs and subsequent free radicals [141]. The efficiency of this process depends on the wavelength and intensity of the applied light, while electron–hole generation itself is constrained by the intrinsic band gap of the photocatalyst material. Light introduction into photocatalytic systems typically follows two approaches: external illumination, with lamps positioned on the reactor wall or axis, and internal illumination, where lamps are immersed directly in the reaction medium [71]. Many evidences indicate that the reaction rate constant (k) is related to the illumination intensity (I) and wavelength according to the power law: k = κI^α, where κ is the irradiance constant and α the exponent. At low intensities (I < 200 W·m⁻²), the rate constant increases linearly with intensity (α ≈ 1). In the intermediate range (200–250 W·m⁻²), the dependence weakens (α ≈ 0.5), while at very high intensities, the reaction rate becomes independent of illumination (α = 0). Beyond intensity, the spectral characteristics of the light source are equally critical, as optimal quantum efficiency requires a strong overlap between the emission spectrum of the lamp and the absorption spectrum of the photocatalyst. From a reactor engineering perspective, these principles emphasize the necessity of delivering photons efficiently and uniformly to catalytically active surfaces. This requirement dictates careful consideration of reactor design, particularly light source placement, such as the placement of the light source—either via external illumination or internal immersion within the fluid, to maximize irradiation efficiency while minimizing scattering and parasitic absorption losses.

The chemical composition of the feedwater matrix is a critical determinant of PMR performance, with various co-solutes exerting either inhibitory or synergistic effects on the degradation process and membrane stability. A key consideration is the nature of the oxidant employed; for instance, peroxymonosulfate (PMS) exists in aqueous solutions as an acidic oxidant in the form of HSO₅⁻ and SO₅²⁻ ions. To enhance the efficacy of PMR systems, novel catalytic membranes have been developed. In one such study, 5CoMIL-88B(Fe) catalysts were synthesized in situ on an Al₂O³ carrier, creating a specialized membrane for phenol degradation. A comparative analysis revealed that both the base MRS-MIL-88B(Fe)@Al₂O₃ and the cobalt-doped MRS-5CoMIL-88B(Fe)@Al₂O₃ membranes demonstrated superior phenol removal efficiency under illumination compared to dark conditions, confirming their photo-responsive characteristics. The introduction of PMS into the MRS-5CoMIL-88B(Fe)@Al₂O₃ membrane system further augmented the degradation efficiency. Notably, a direct correlation was observed between increasing PMS concentration and improvements in both degradation rates and permeate flux, indicative of enhanced reactivity. Conversely, the presence of certain inorganic and organic constituents in the feedwater can adversely affect PMR operation [142]. Darowna et al. systematically examined the influence of feedwater composition—specifically the concentration of inorganic anions (SO₄²⁻, HPO₄²⁻, HCO₃⁻), solution pH, and the presence of humic acids (HAs)—on the fouling behavior and stability of commercial polyethersulfone (PES) ultrafiltration membranes in PMRs. Their findings revealed that elevated concentrations of inorganic anions reduced permeate flux and diminished the photocatalytic degradation efficiency of humic acids, largely as a result of ionic hole and hydroxyl radical scavenging. Conversely, in the absence of inorganic salts, adsorption of HAs onto TiO₂ particles played a significant role in enhancing treatment performance within the PMR system [109]. The presence of inorganic salts in the solution also has an effect on the treatment effect of PMR. A significant decline in membrane flux was observed with increasing NaCl concentration, a phenomenon attributed to ionic strength-dependent conformational changes in HA molecules. At low ionic strengths, HA molecules adopt a more linear structure with pronounced intermolecular repulsion, leading to the formation of a looser deposition layer on the membrane surface and consequently higher permeate flux. Conversely, at high ionic strengths, electrostatic shielding effects diminish both intermolecular repulsion and the electrostatic repulsion between the membrane and HA molecules. This induces a more coiled conformation in the HA molecules, resulting in the formation of a dense cake layer that exacerbates membrane fouling and leads to a more pronounced flux decline [98]. Molinari et al. reported a decrease in the flux of humic acids in a saline solution due to the presence of ROS in the oxidizing environment. However, it is crucial to note that configurations employing photocatalysts immobilized directly on the membrane surface can render the polymer susceptible to photodegradation, as the membrane itself is directly irradiated to facilitate contaminant degradation on its surface or within its pores. Beyond their role in fouling, co-solutes can also modulate the efficiency of target pollutant degradation [143]. Petsi et al. investigated the reduction of nitrate in a photocatalytic membrane reactor in the presence of organic acids. A systematic study was carried out in a laboratory pilot PMR using a hybrid TiO₂/UV-A catalytic-ultrafiltration process using formic acid as the most favorable cavity scavenger for nitrate reduction, and the results showed the superior performance of formic acid in nitrate reduction [144]. Peñas-Garzón et al. conducted a comprehensive evaluation of graphitic carbon nitride immobilized on polymeric membranes (CN/F) within a continuous-flow planar reactor for the removal of pharmaceutical mixtures, including venlafaxine, citalopram, carbamazepine, tramadol, ketoprofen, and diclofenac (3 μM each) [145].

The integration of novel photocatalytic materials with innovative reactor configurations is a promising strategy for enhancing PMR performance. When incorporated into a specially designed slant plate photocatalytic reactor, this system demonstrated the rapid and effective degradation of Rhodamine B from simulated wastewater, while also facilitating the efficient, loss-free recovery of the photocatalyst [146]. The development of bifunctional composite membranes, capable of concurrent physical separation and chemical degradation, represents a significant advancement in membrane science. These membranes are capable of simultaneously achieving efficient oil-in-water emulsion separation and photocatalytic degradation of organic dyes in wastewater. To enhance degradation capacity without compromising separation efficiency, strategies combining photocatalytic active nanoparticles with hydrophilic membrane matrices have been widely adopted in water treatment research [147]. Li et al. prepared a bifunctional separation membrane through PVDF/PEMA electrospinning followed by in situ deposition of anatase TiO₂ nanoparticles containing Ti³⁺ sites and oxygen vacancies. The resulting composite membranes exhibited superior hydrophilicity (WCA = 15.65°), strong underwater oleophobicity (UOCA = 156.69°), and excellent photocatalytic activity [148]. Furthermore, the reactor configuration itself is a critical determinant of performance. Yang et al. developed three flat-plate immobilized PMRs based on Bi₂WO₆-g-C₃N₄/PVDF photocatalytic membranes and evaluated their performance in atrazine (ATZ) removal under simulated sunlight. The vertical PMR configuration achieved 78.98% degradation of ATZ within 6 h and maintained stable removal efficiency (~65%) under continuous-flow operation for 18 h [149].

4. Advances of PMRs in Different Applications

PMRs have been instrumental in various areas of environmental remediation, playing a significant role in the green energy transition, carbon neutrality, and the development of a circular economy. Current research focuses primarily on high-efficiency, laboratory-scale applications. Innovations in nanomaterials and membrane architectures have achieved impressive pollutant degradation and conversion efficiencies under highly optimized, albeit energy-intensive, conditions. However, translating these promising laboratory results to industrial-scale applications remains a daunting challenge, requiring overcoming key hurdles such as designing efficient large-scale illumination systems, in-depth integration and optimization of reaction kinetic models, mitigating persistent membrane fouling, and establishing stable, cost-effective operation. Premature scale-up, prioritizing cost reduction at the expense of catalytic efficiency, can lead to degraded system performance and inefficient energy utilization. Therefore, the future direction of PMR technology is to achieve the ideal of high performance at an industrial scale, a goal that requires a shift from simple pollutant removal to more complex functional models. A comprehensive four-quadrant overview of PMR development, as depicted in Figure 10, offers an in-depth comparative analysis of various PMR types based on existing applications. This comprehensive summary, based on existing applications, provides an in-depth comparative analysis of the various types of PMRs within each category, summarizing representative research and applicable scenarios, and guides the selecting of the appropriate PMR type. These advanced applications include leveraging the membrane’s selective separation capabilities to guide reactions to valuable products, integrating PMRs with processes such as adsorption or biological treatment to create synergistic hybrid systems, and facilitating the recovery of valuable resources from wastewater streams. Achieving this vision relies on key engineering and design innovations, such as the adoption of modular reactor configurations for enhanced scalability, the implementation of AI-driven operational control optimization, and the development of coupled photoelectrochemical systems. Ultimately, the successful deployment of PMRs in environmental remediation requires bridging the gap between laboratory feasibility and industrial practicality, supported by rigorous techno-economic evaluations and a sustained focus on maximizing energy efficiency and process robustness.

4.1. Photocatalytic Water Splitting to Produce Hydrogen

As a green, clean, and highly efficient advanced oxidation technology, photocatalysis shows great potential for photocatalytic hydrogen production in water. A variety of innovative strategies, including nanostructure control, doping, co-catalyst integration, and bandgap and defect engineering, have been proposed to enhance photocatalyst performance. Nevertheless, significant challenges persist in photocatalytic hydrogen production, notably charge recombination, limited light absorption, and insufficient catalyst stability, which currently impede large-scale implementation. Therefore, reactor design is crucial for scaling up photocatalytic hydrogen production. The development of photocatalytic reactors for hydrogen evolution has garnered considerable attention, primarily due to their potential for efficient solar energy utilization, which balances material use, light capture, and reaction efficiency, thereby offering a pathway to reduce hydrogen production costs. PMRs have emerged as a particularly effective technology for sustainable hydrogen production. By inhibiting the deleterious back-reaction between H₂ and O₂, PMRs facilitate the generation of pure hydrogen in a single step, obviating the need for downstream purification processes. Foundational work in this area demonstrated a TiO₂-Nafion-Pt based membrane system (Figure 11a) where, under UV irradiation and without an external bias, methanol oxidation on the TiO₂ side drives proton transport across the Nafion membrane for subsequent reduction to H₂ on a Pt surface, achieving a hydrogen production rate of up to 69 µL h⁻¹ cm⁻² [150]. Building upon this principle, subsequent research has focused on enhancing system stability and efficiency. Building on this, Tsydenov et al. proposed an innovative approach by modifying porous polymer membranes with Pt and TiO₂ for hydrogen production through ethanol dehydrogenation. This system not only exhibited excellent physicochemical stability under illumination but also incorporated PTFE filters with polypropylene modifications to improve mechanical robustness, thereby enhancing the durability and practical viability of PMRs for hydrogen production [151,152]. Further advancing the field, a sophisticated dual-fiber optic setup has been introduced, integrating LED light sources with optical fibers coated with a Metal–Organic Framework (MOF) catalyst (Figure 11b), specifically POF-MIL-101(Fe). This advanced configuration, coupled with a re-oxygenated hollow membrane fiber, was engineered not only for efficient H₂ evolution but also for the simultaneous, high-yield production of hydrogen peroxide, showcasing a significant progression toward multi-functional photoreactor systems where performance is optimized through sophisticated light delivery and catalyst integration [153].

While the immense terrestrial solar flux presents a compelling resource for sustainable hydrogen production, a conspicuous disparity persists between laboratory-scale potential and tangible, large-scale implementation [154]. Historically, the research impetus in photocatalytic overall water splitting (POWS) has been overwhelmingly directed toward the synthesis of novel materials to overcome quantum efficiency limitations, a strategy justified when solar-to-hydrogen (STH) efficiencies remained well below the benchmark for commercial viability. However, the recent advent of photocatalysts achieving STH efficiencies approaching the 10% threshold signals a critical inflection point [155]. This progress indicates that the primary impediment to practical deployment is no longer solely the intrinsic activity of the photocatalyst, but rather the absence of sophisticated reactor engineering, mandating a paradigm shift in research priorities. The critical challenge now resides in translating high-efficiency materials into functional systems, a goal for which the holistic design of PMRs is paramount [156]. Future efforts must therefore prioritize the engineering of these integrated systems, where photon management, mass transport, and in situ product separation are co-optimized to bridge the gap between fundamental material discovery and industrial-scale solar hydrogen production.

Progress in solar hydrogen production is marked by the successful translation of nanoscale photocatalytic principles into progressively larger and more robust reactor systems. At the laboratory scale, performance is enhanced through the engineering of composite nanoreactors that exploit localized photothermal effects; silver sulfide quantum dots on carbon nitride nanovesicles, for example, create thermal hotspots that accelerate reaction kinetics. A critical step towards scalable architectures is demonstrated by hydrogen-bonded organic frameworks (HOFs), where engineering micropore-confined exciton transfer has led to exceptional apparent quantum yields (>25%) [157]. This principle has been successfully embodied in a 0.5 m² flexible panel reactor, achieving a notable hydrogen productivity of over one mole per square meter per day [44]. The ultimate validation of this technology’s potential resides in meter-scale demonstrations under real-world conditions, where a panel reactor utilizing immobilized mesoporous carbon nitride has proven its long-term viability by maintaining stable operation over one month under natural sunlight, reaching a maximum solar-to-hydrogen conversion of 0.12% [158]. This developmental trajectory—from engineering nanoscale thermal effects to achieving meter-scale operational stability—delineates a viable pathway for translating fundamental photocatalyst design into practical solar hydrogen technologies. As a clean and efficient advanced oxidation process, photocatalysis holds significant promise for hydrogen production from water. To enhance photocatalyst performance, strategies such as nanostructure engineering, elemental doping, co-catalyst integration, and bandgap or defect engineering have been extensively explored. Despite these advances, key limitations persist, including rapid charge recombination, restricted light absorption, and insufficient catalyst stability. Overcoming these challenges is essential for translating photocatalytic hydrogen production into large-scale sustainable energy systems.

4.2. Removal of Organic Pollutants from Water

Advanced oxidation process (AOP), represented by photocatalysis, has been considered as an effective approach for the removal and degradation of a broad spectrum of non-biodegradable organic pollutants in water bodies [159] (Figure 12a). It has been popularized for its mild reaction conditions, absence of secondary pollution, and low operational cost. However, a significant challenge remains in the separation and recovery of catalysts, especially in large-scale water treatment applications. The photocatalytic membrane reactor, which combines photocatalysis and membrane separation processes, can effectively solve this problem and has been applied to the removal of organic pollutants in water many times. Zhang et al. utilized TiO₂ as a photocatalyst and ultraviolet (UV) light as the irradiation source to treat wastewater contaminated with the azo dye Acid Red B in a slurry photocatalytic membrane reactor (Figure 12b). The reactor design included a dual-layer cylindrical photocatalytic reaction zone and a plate-and-frame membrane separation section. The study demonstrated that the combined photocatalytic and ultrafiltration processes effectively degraded Acid Red B from textile wastewater, highlighting the potential of this system for treating azo dye-contaminated wastewater [160]. To overcome the energy cost limitations associated with UV-dependent systems, subsequent research has shifted focus toward visible-light-driven photocatalysis. Building on this, Nur et al. shifted the focus towards visible light photocatalysis by incorporating ZnO nanoparticles in an MPR (Figure 12c) to treat Congo Red dye, enhancing the treatment capability and addressing membrane fouling, thus offering an improvement over UV-dependent systems by potentially using a broader light spectrum. The results indicated that the optimal dosage of ZnO-PVP-St nanoparticles significantly enhanced the MPR’s capability to treat industrial dye wastewater while minimizing fouling of the nanofiltration (NF) membranes. The ZnO-PVP-St nanoparticle formulation enhanced pollutant removal efficiency while mitigating nanofiltration membrane fouling, thereby improving performance compared with UV-dependent systems [127].

Hu et al. further expanded the scope of PMR applications by evaluating an Al₂O₃ hollow fiber membrane photoreactor (Figure 13a) for the degradation of mixed pollutants, including methylene blue (MB), methyl orange, and phenol. The Al₂O₃ substrate exhibited high thermal and chemical stability, making it suitable for photocatalyst deposition and advanced hollow fiber membrane reactor design [161]. In pursuit of more sustainable materials, Hu et al. also reported a metal-free photocatalyst, phosphorus-doped g-C₃N₄ (PCN), coated on Al₂O₃ hollow fiber membranes. This visible-light-driven PMR achieved efficient and stable removal of MB, methyl orange, phenolic compounds, and their mixtures [162]. Catalyst modification has also been explored to enhance performance under simulated solar irradiation. Ashar et al. developed PMRs by growing undoped and Fe³⁺-doped ZnO nanoparticles on polyester fabrics using a low-temperature hydrothermal method. The ZnO/PMR and Fe³⁺@ZnO/PMR achieved maximum degradation rates of 88.89% and 98.34%, respectively, for the reactive dye RB5 under simulated sunlight (D65, 72 W) irradiation within 180 min (Figure 13b). Subsequent experiments revealed that the photocatalytic efficiency of the Fe³⁺@ZnO/PMR slightly decreased after eight cycles of continuous operation [163]. Concurrently, efforts to mitigate membrane fouling have led to the development of photocatalytic membranes (PMs) with anti-fouling properties. Sahar et al. developed a photocatalytic membrane (PM) by coating a silicon carbide membrane with a Zr/TiO₂ sol. The degradation performance of various concentrations of humic acids was evaluated under UV irradiation (275 nm and 365 nm). The results demonstrated that the PM effectively reduced fouling formation while efficiently degrading humic acids, thus mitigating the loss of membrane permeability [128].

4.3. Heavy Metal Ion Removal

Typical industrial methods employed in conventional heavy metal-containing wastewater treatment, including adsorption, chemical precipitation, ion exchange, ozonation, biological methods, and activated carbon electrochemical methods, often struggle to effectively reduce metal concentrations in water to levels that meet regulatory standards. Photocatalysis has emerged as a promising alternative for heavy metal remediation [164], primarily owing to its high versatility and the ability to convert toxic, high-valence heavy metal ions into their less toxic, low-valence counterparts [165]. The primary mechanisms governing the photocatalytic removal of heavy metal ions include the direct reduction of metal ions by photogenerated electrons from the conduction band and the indirect reduction of highly oxidized metal species by photogenerated intermediates. For direct photocatalytic reduction to be thermodynamically feasible, the conduction band energy level of the semiconductor must be more negative than the reduction potential of the target metal ion couple (Mⁿ⁺/M). Innovations in membrane technologies, such as those based on nanoparticles, carbon nanotubes, or graphene, have demonstrated significant potential for heavy metal ion removal due to properties like high water permeability [166]. Recent innovations in membrane technologies incorporating nanoparticles, carbon nanotubes, and graphene have demonstrated notable potential for heavy metal ion removal due to their high water permeability and functional versatility. However, only nanocomposite membranes have been scaled up and commercialized for heavy metal ion removal [167], while other membranes are still in the basic research and development stage [168]. An advanced strategy for wastewater treatment is the integration of photocatalysis with membrane filtration, forming PMRs. In this configuration, the membrane functions as a selective barrier, while the immobilized photocatalyst provides active sites for contaminant degradation or reduction. This dual action not only enhances pollutant removal but also mitigates membrane fouling, a major operational limitation. For example, Silva et al. reported that a recycled reverse osmosis membrane modified with a graphene oxide–titanium dioxide (GO–TiO₂) nanocomposite effectively polished petroleum refinery effluent. Under UV irradiation, the membrane exhibited self-cleaning behavior, reducing flux decline by degrading surface foulants while simultaneously improving the removal of organic pollutants (Figure 14a). The underlying mechanism—the generation of reactive photogenerated electrons at the membrane surface—can also be applied to heavy metal remediation. These electrons enable the reduction of toxic high-valence metal ions to less harmful lower-valence states. Thus, PMRs offer a promising route to achieve efficient heavy metal removal while maintaining membrane performance through photocatalytic self-cleaning [169]. Building on this foundation, as shown in Figure 14b, Rathna et al. advanced the concept by synthesizing TiO₂-WO₃ nanoparticles through hydrothermal methods and integrating them into polyaniline (PANI) membranes for chromium (VI) removal from aqueous solutions (Figure 14b). They varied the concentration of the photocatalyst within the membrane, which not only enhanced the photocatalytic activity under visible light but also introduced a self-cleaning mechanism due to the combined photoanode and cathode system within the membrane. This approach not only demonstrates a step towards multifunctional membranes that can both degrade and reduce heavy metals but also shows the versatility of PMRs in handling different forms of pollutants in various media, from gas to liquid. These studies collectively illustrate a clear technological progression from foundational, gas-phase applications to more sophisticated, multifunctional liquid-phase systems that employ advanced composite materials, thereby significantly broadening the scope of PMR technology for heavy metal remediation [170].

4.4. CO₂ Reduction

The capture and subsequent utilization of carbon dioxide (CO₂) are now recognized as crucial strategies for advancing sustainable development [171]. Within this context, photocatalytic processes represent a promising technology for the reduction of CO₂ emissions and the concurrent production of energy-containing compounds [172]. Membrane reactor technology, in particular, offers a viable solution for the integrated capture and conversion of CO₂ into valuable products, with PMRs being especially well-suited for this application [173]. While PMRs can maintain high product yields and selectivity by controlling reaction pathways and preventing reverse reactions, their efficiency has historically been constrained by limitations in electron and proton mobility, as well as by mass transmission issues. The enhancement of proton transport and mass transmission is therefore critical to realizing the full potential of membrane reactors for the photocatalytic conversion of CO₂ into fuels. Initial investigations into this application, such as the work by Sellaro et al., demonstrated the fundamental feasibility of using TiO₂-Nafion composite membranes for the continuous conversion of CO₂ to methanol. Building upon this, Brunetti et al. developed more advanced membranes incorporating a C₃N₄-TiO₂ heterojunction photocatalyst, which exhibited enhanced efficiency for CO₂ reduction. However, Brunetti et al. took this further by exploring coupled reactions, embedding a C₃N₄-TiO₂ photocatalyst in a dense Nafion matrix, showcasing high efficiency in continuous CO₂ reduction to methanol, thus expanding on the initial idea of membrane-catalyzed CO₂ conversion [129]. They designed a dual-function Janus membrane, integrating a CO₂ concentration layer (gas separation) and a photocatalytic reaction layer. This membrane demonstrated the ability to directly convert trace CO₂ from air into CH₄. The structure of the gas separation layer and the CO₂ conversion efficiency were optimized by incorporating amine-functionalized graphene oxide, while graphene was introduced into the photocatalytic reaction layer to effectively inhibit electron-hole recombination and enhance the likelihood of excited electron collisions with CO₂ [129]. Building upon these foundational studies, Cheng et al. introduced a novel approach with a photofluidic membrane microreactor. This system leverages photofluidic technology to enhance photon and mass transport, coated with TiO₂ on carbon paper treated with PTFE for hydrophobicity. Their reactor achieved an unprecedented methanol production rate from CO₂ reduction, establishing a new benchmark in yield and efficiency, demonstrating a significant leap in reactor design for CO₂ photocatalytic conversion. The performance of the membrane was evaluated in terms of its ability to reduce CO₂, yielding a maximum methanol production of 111.0 mmol/g-cat/h at a flow rate of 25 mL/min and a light intensity of 8 mW/cm². This result represents the highest reported yield to date, demonstrating the effectiveness and superiority of the proposed photofluidic membrane microreactor for CO₂ photocatalytic reduction (Figure 15a) [174]. Liu et al. approached the challenge from a different angle by focusing on CO₂ separation alongside conversion. They synthesized branched polyethyleneimine (PEI)-functionalized UiO-66 for mixed matrix membranes (MMMs), which not only separated CO₂ from CH₄ but also facilitated CO₂ conversion through reversible amine reactions, achieving high selectivity and permeability, thus broadening the application scope of PMRs to include both separation and conversion processes. The UiO-66-PEI MMMs exhibited optimal performance at a filler concentration of 15 wt%, achieving a CO₂ permeability of 28.23 Barrer and a high CO₂/CH₄ selectivity of 56.49% [175] (Figure 15b). Saleh et al. addressed the challenge of CO₂ conversion by immobilizing TiO₂ photocatalysts on PTFE membranes (Figure 15c) to enhance stability and reusability. The PTFE membrane surface was modified through defluorination and grafting with carboxylic acid, achieving a high grafting degree of 8.5% using 30 wt% acetic acid. TiO₂ was successfully immobilized (11.32 wt%), confirmed by FESEM-EDX and TGA [173]. Although leaching tests showed 59% TiO₂ loss after 4 days, the membrane demonstrated good reusability over 5 cycles. The photocatalytic CO₂ reduction in a rich amine medium produced methanol, albeit at a lower yield (7 μmol/g·h) compared to slurry systems (134.5 μmol/g·h), attributed to reduced surface area and light exposure. This study highlights the potential of immobilized systems for sustainable CO₂ conversion while addressing catalyst recovery challenges. Taken together, this body of research illustrates a clear and progressive evolution of PMR technology for CO₂ conversion, from foundational proof-of-concept studies to the development of highly sophisticated, multifunctional reactor systems.

4.5. VOC Removal

With the great progress in photocatalytic materials, photocatalytic reactor design has a broad prospect and urgency [176]. The use of PMRs for the treatment of volatile organic compounds (VOCs) has become a promising measure due to their high efficiency, sustainability, and flexible operation. By combining selective membrane separation with photocatalytic oxidation, PMRs can achieve the complete mineralization of a wide range of VOCs to CO₂ and H₂O, thereby minimizing the formation of hazardous byproducts and the risk of secondary pollution. A distinct advantage of this technology is its ability to operate at ambient temperature and pressure, which confers a significant energy-saving benefit over conventional, high-temperature abatement methods such as thermal or catalytic combustion. The photocatalytic membrane reactor can be operated at ambient temperature and pressure, offering significant energy-saving advantages over other energy-intensive thermal decomposition or catalytic combustion technologies. In addition, its modular design is easy to expand and can be adapted to different waste gas treatment needs. The treatment of low concentration VOCs is usually a difficult issue in environmental engineering, while the photocatalytic membrane reactor can maintain good degradation performance at low VOC concentrations through the selective separation of the membrane and the high activity of photocatalysis [177]. A particularly promising strategy for the treatment of low-concentration VOCs, a persistent challenge in environmental engineering, is the coupling of photocatalysis with dense, selective polymer membranes. Toshinori et al. laid the groundwork by synthesizing porous titanium dioxide (TiO₂) membranes via the sol–gel method, with pore sizes tailored for the effective degradation of methanol and ethanol. Their setup allowed for unidirectional flow through the membrane, where oxidation occurred both on the surface and within the pores, achieving near-complete oxidation of methanol at high concentrations. The oxidation reactions occurred both on the TiO₂ membrane surface and within its pores, while the reactants flowed unidirectionally through the membrane. The Pt-modified TiO₂ membrane demonstrated near-complete oxidation of methanol, even at concentrations up to 10,000 ppm, with a feed flow rate of 500 × 10⁻⁶ m³/min [68]. However, this approach primarily focused on the membrane’s physical structure and catalyst placement. Further pushing the boundaries, Khalilzadeh et al. employed a jet reactor system with modified TiO₂ nanoparticles, which demonstrated high efficiency in VOC removal (Figure 16a) [178]. This method shifted from static membrane systems to a dynamic one, suggesting improvements in mass transmission and catalytic exposure, thus enhancing the removal efficiency. Lastly, Favre et al. explored an integration of dense polymer membranes with photocatalytic processes by placing the catalyst within the permeation chamber, aiming at low-concentration VOC removal, like n-hexane from air. Their configuration not only improved the membrane’s separation efficiency but also achieved complete mineralization of hexane under UV-A light, illustrating a significant step forward in combining photocatalysis with membrane technology for trace contaminant removal (Figure 16b). Experimental results demonstrated that the coupled device significantly enhanced membrane separation efficiency for hexane concentrations as low as 1 ppm. Moreover, complete mineralization of hexane was achieved under various UV-A light irradiation conditions (W/m²) [179]. Together, these studies illustrate a progression from basic catalyst-membrane interaction, through the enhancement of photocatalytic activity with novel materials, to dynamic reactor designs, and finally to sophisticated integration of photocatalysis with dense membrane systems for low-level contaminant removal, each study adding layers of complexity and efficiency to the application of PMRs in VOC abatement.

4.6. Amplification of Photocatalytic Reactors

The scaling-up of photocatalytic reactors represents a technically and economically viable process [180]. To facilitate the transition of PMRs from laboratory-scale research to industrial applications, the development and utilization of scale-up strategies are essential. Investigating the changes in reaction kinetics during the transition from small-scale experiments to industrial-scale systems is critical. This process necessitates a comprehensive understanding of how key performance-determining factors, such as hydrodynamic regimes, mass transmission phenomena, and the distribution of the radiation field, evolve with increasing reactor size and complexity. The primary objective of reactor scale-up is the design of amplified photoreactors that can process large volumetric flow rates while maintaining high energy efficiency and catalytic performance. A significant challenge in this endeavor is the issue of low solar energy utilization in many photocatalytic systems. Amplified photocatalytic reactors play a crucial role in translating laboratory-scale photocatalytic reactions into industrial applications. These reactors improve the efficiency of light energy utilization and accelerate reaction rates by optimizing key parameters, including light source, reactor design, catalyst support, and fluid dynamics [70]. The primary advantages of amplified photoreactors lie in their ability to maximize light energy utilization, streamline operation, and reduce costs, making them highly applicable in areas such as environmental remediation, energy conversion, and hazardous substance treatment. In recent years, the integration of amplified photocatalytic reactors into PMRs has gained significant attention, particularly in the context of sustainable development goals. The introduction of amplified photoreactors in the design of PMRs offers substantial benefits. By immobilizing catalysts on membrane materials and optimizing light sources, photocatalytic efficiency can be significantly enhanced, particularly for the treatment of high pollutant concentrations or large-scale water treatment applications [181]. Unlike conventional photoreactors, which often face challenges such as uneven light distribution, oversized reactor volumes, and inconsistent catalyst distribution, amplified photoreactors address these limitations through a systematic design approach. These improvements enable the treatment of larger wastewater or gas flows without compromising efficiency. In the pursuit of enhancing photocatalytic efficiency and scalability, several innovative approaches have been explored for PMRs. Starting with the fundamental challenge of low energy utilization in solar-driven photocatalysis, Dittmeyer et al. at Karlsruhe Institute of Technology developed a cost-effective, highly efficient, and easily manufacturable panel-type photochemical reactor (Figure 17a). This reactor addresses the issues of low photocatalytic efficiency (typically around 1% energy utilization) and high costs through its simplicity, mass production potential using polymers, and adaptability to various photocatalysts [182]. Building on this foundation, Yoon’s group at the Korea Institute of Energy Research took a step further by comparing rotary and flat-type photoreactors for treating emerging contaminants in water under UV or solar irradiation. Their studies demonstrated significant improvements in solar-driven photocatalytic efficiency by optimizing reactor geometry and light source configurations, tackling the challenges of photocatalyst recovery and low photon utilization (Figure 17b) [183]. Advancing the design complexity, Spasiano et al. conducted comparative analyses of different photoreactor types, including parabolic trough collectors (PTCs), non-concentrating collectors (NCCs), and compound parabolic collectors (CPCs) [184]. His findings suggest that by rationally designing the light distribution and hydrodynamic parameters, photocatalytic performance can be significantly enhanced, which is crucial for scaling up PMRs for both environmental remediation and improving catalyst recovery and reactor stability [58]. Pushing the boundaries of reactor design, Hyeon’s research team at the Center for Nanoparticle Research, Institute for Basic Science (IBS) in Seoul, Korea, proposed a novel floating photocatalytic platform based on a porous elastomer-hydrogel nanocomposite [185]. This platform, which operates at the air-water interface, offers advantages like efficient light transmission, ease of gas separation, and reduced hydrogen oxidation, making it particularly scalable for photocatalytic applications [186]. To bridge the gap between laboratory-scale materials and industrial applications, a highly photostable organic-inorganic membrane catalyst has been engineered to address key challenges in scaling up solar fuel production. This robust design integrates CdS@SiO₂-Pt composites into durable PVDF membranes, demonstrating exceptional recyclability and achieving 0.68% efficiency in solar-to-hydrogen conversion (Figure 17c). This successful transition from novel materials to larger integrated panel-scale H₂O to H₂ conversion systems provides a viable pathway for the scalable implementation of photocatalytic membrane reactor technology [187].

Fu et al. designed a scalable photocatalytic membrane reactor system for solar-driven water splitting by spatially separating H₂ and O₂ evolution using a MoSe₂-loaded perovskite photocatalyst (FAPbBr₃₄I₄) and NiFe-LDH/BiVO₄, connected via an I₃⁻/I⁻ redox shuttle (Figure 18a). The system achieved 2.47% solar-to-hydrogen efficiency in lab tests and maintained 1.21% efficiency in a 692.5 cm² outdoor demonstration while effectively suppressing back-reactions through its modular design. This approach demonstrates the feasibility of large-scale photocatalytic hydrogen production through reaction decoupling and redox mediation [188]. Lee et al. developed a floatable photocatalytic hydrogel nanocomposite that achieves a high hydrogen evolution rate of 163 mmol h⁻¹ m⁻² using Pt/TiO₂ aerogel (Figure 18b). The system’s air-water interface design enhances light absorption and gas separation while suppressing back-oxidation. When scaled to 1 m², it produces 79.2 mL H₂/day under natural sunlight and maintains performance in seawater and turbid conditions. The platform also enables PET photo-reforming (0.154 L h⁻¹ m⁻²) and demonstrates scalability to 100 m², offering a practical solution for large-scale solar hydrogen production [185]. Together, these studies reflect a progressive journey from addressing basic efficiency and cost challenges in photocatalytic reactors to exploring advanced, scalable designs that enhance both the application spectrum and the economic viability of PMRs in environmental remediation.

5. Conclusions and Outlooks

This review traced the development of photocatalytic membrane reactors (PMRs), highlighting their potential to extend beyond conventional water purification by integrating membrane separation with solar-driven catalytic conversion. This synergistic combination allows PMRs to not only efficiently remove pollutants but also transform waste streams into valuable chemical feedstocks and energy carriers. Based on a comparative evaluation, we systematically categorized PMR configurations according to their functional characteristics and optimal application scenarios. Slurry-type PMRs (SPMRs) are particularly suitable for high-value-added applications at the laboratory scale. The suspended catalyst particles offer extensive surface areas for reactions, which is critical for achieving the high catalytic efficiencies required in emerging fields such as solar-driven hydrogen production and photocatalytic CO₂ conversion into fuels and chemicals. These systems represent the most promising architecture for energy-focused applications. Immobilized PMRs (IPMRs) provide indispensable platforms for fundamental research where system stability and reproducibility are essential. They are primarily used for removing persistent organic pollutants under well-controlled conditions, enabling precise validation of reaction kinetics, performance evaluation of novel photocatalysts, and rigorous assessment of material-membrane compatibility over prolonged operation.

PMR technology progresses toward industrial adoption, and modular high-performance designs—exemplified by panel-type reactors—are envisioned to bridge the gap between laboratory success and scalable deployment. This approach facilitates a systematic increase in processing capacity without compromising catalytic efficiency, making it particularly suitable for demanding industrial environmental and energy applications. Despite significant progress in material design and reactor engineering, a considerable gap remains between laboratory achievements and the demands of industrial-scale implementation. A primary challenge is attaining high performance at scale—an objective that entails more than simply enlarging reactor dimensions. It requires a paradigm shift toward viewing PMRs as multifunctional platforms for resource recovery, sustainable energy production, and waste valorization. Balancing manufacturing cost with catalytic efficiency ensure that scaling up does not undermine energy utilization or overall performance—core aspects of the technology’s value proposition. The next generation of PMRs should be designed not as static reaction-separation units but as adaptive systems capable of continuous adjustment, multifunctionality, and resource integration. Realizing this vision will depend on merging advances in photocatalytic materials with system-level design strategies that enable predictive control of photon, mass, and fouling dynamics. Digital modeling and real-time process monitoring can provide the basis for moving beyond empirical optimization, paving the way for reactors that are efficient, resilient, and scalable.

PMRs should be regarded as nexus technologies that interconnect water treatment, solar hydrogen production, CO₂ conversion, and resource recovery. In this expanded role, they directly support decarbonization and circular economy goals. Embedding sustainability from the design phase—through recyclable catalysts, regenerable membranes, and low-energy manufacturing routes—will be essential to minimize both economic and environmental costs. Integrating PMRs into renewable-energy-powered distributed treatment networks could further amplify their impact, transforming them into decentralized infrastructures capable of delivering both clean water and clean energy.