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Review

Recent Trends in the Application of Photocatalytic Membranes in Removal of Emerging Organic Contaminants in Wastewater

by
Kipchumba Nelson
1,
Achisa C. Mecha
1,2,*,
Humphrey Mutuma Samuel
1 and
Zeinab A. Suliman
1,3
1
Renewable Energy, Environment, Nanomaterials, and Water Research Group, Department of Chemical and Process Engineering, Moi University, Eldoret P.O. Box 3900, Kenya
2
Department of Environmental Science, University of Arizona, Tucson, AZ 85721, USA
3
Department of Manufacturing, Industrial and Textile Engineering, Moi University, Eldoret P.O. Box 3900, Kenya
*
Author to whom correspondence should be addressed.
Processes 2025, 13(1), 163; https://doi.org/10.3390/pr13010163
Submission received: 24 December 2024 / Revised: 4 January 2025 / Accepted: 7 January 2025 / Published: 9 January 2025
(This article belongs to the Special Issue Electrochemical and Photocatalytic-Based Degradation Technologies for the Treatment of Emerging Contaminants)
Browse Figures
Graphical abstract
"> Figure 1
<p>Schematic representation of titania stocky junction during UV light excitation and generation of radicals.</p> "> Figure 2
<p>Schematic representation of titania surface plasmon resonance during visible light excitation and generation of radicals.</p> "> Figure 3
<p>Schematic of dye sensitization mechanism resulting in generation of radicals.</p> "> Figure 4
<p>Schematic structure of type I heterojunction showing charge separation and generation of radicals.</p> "> Figure 5
<p>Structure of type II heterojunction showing charge separation and generation of radicals.</p> "> Figure 6
<p>Structure of type III heterojunction showing requirement of large driving force to cause charge separation.</p> "> Figure 7
<p>Schematic representation of surface-immobilized photocatalytic membrane showing modified TiO<sub>2</sub> fixed on the membrane surface.</p> "> Figure 8
<p>Schematic representation of internally immobilized membrane showing modified TiO<sub>2</sub> fixed within membrane matrix.</p> ">
Versions Notes

Abstract

:
Increasing water pollution by bio-recalcitrant contaminants necessitates the use of robust treatment methods. Individual treatment methods are not effective against these emerging organic pollutants due to their stability in the environment. This has necessitated the use of advanced integrated systems such as photocatalytic membranes. Synergy in the reactive photocatalytic membranes effectively degrades the emerging organic pollutants. This review presents the state of the art in the synthesis and application of photocatalytic membranes in water and wastewater treatment. The study critically evaluates pertinent aspects required to improve the performance of photocatalytic membranes, such as tailored material synthesis, membrane fouling control, improved photocatalyst light absorption, use of visible light from sunlight, enhanced reaction kinetics through synergy, and regeneration and reuse. Previous studies report on the effectiveness of photocatalytic membranes in the removal of organic contaminants in synthetic and actual wastewater. As such, they show great potential in wastewater decontamination; however, they also face limitations that need to be addressed. The review identifies the challenges and provides a way forward in increasing the photoactivity of titanium oxide, fouling mitigation, scalability, improving cost effectiveness, enhancing membrane stability, and other aspects relevant in scaling up efforts from the lab scale to industrial scale.

Graphical Abstract">
Graphical Abstract

1. Introduction

Growth in the human population, industrialization, and agricultural activities have led to water scarcity and poor sanitation [1]. This has been accelerated by climate change-related impacts, resulting in 2.3 billion people worldwide expected to face water scarcity by 2025 [2]. High consumption has generated vast amounts of wastewater that can be utilized as an additional source to reduce water scarcity [3]. However, wastewaters are loaded with emerging organic contaminants (EOCs) such as pharmaceuticals [4], pesticides [5], agrochemicals [6], hormones [7], dyes [8], and cosmetics [9], making wastewater reuse difficult due to the need to satisfy high quality standards required by relevant authorities. Conventional treatment methods make reusability of wastewater challenging due to the technological inability of currently designed wastewater treatment techniques. The EOC removal efficiency of conventional methods is not satisfactory [10]. EOCs such as phenol, sulfamethoxazole, diphenhydramine, trimethoprim, amoxicillin, bisphenol A, rhodamine, and ibuprofen [11] have high mobility and toxicity at trace concentrations. Additionally, they are bio-accumulative and can cause endocrine disruptions and microbial resistance to drugs [12]. This has highlighted the necessity of advanced, alternative treatment methods such as membrane filtration and advanced oxidation methods.
Membrane filtration utilizes a barrier membrane where contaminant-free or of lower concentrations are allowed to pass as permeate. It has high separation efficiency with high contaminant rejection. Despite this enormous potential, it is faced with the challenges of membrane fouling. Membrane fouling is a scenario where contaminants are deposited on the surface of the membrane (concentration polarization) and in membrane pores [13]. This is accelerated by the hydrophobicity of the membrane due to the mostly hydrocarbon materials used in the development of membranes. This hinders permeate from passing through the membrane. As membrane filtration proceeds, more pressure is required, as fouling increases continuously. This becomes costly, and membrane change is finally needed [14]. Advanced oxidation processes AOPs are a set of powerful techniques that utilize radical reactivity to degrade and mineralize contaminants into carbon dioxide, water, and low-level organics [15]. Examples of AOPs include ozonation, Fenton, and photocatalysis [16]. Among the AOPs, photocatalysis is the most attractive, as it utilizes light and semiconductors such as titanium dioxide (TiO2), zinc oxide, iron oxides, cadmium sulfide, etc. [17]. When enough light containing photon energy falls on a semiconductor and is of a particular wavelength, the electrons (e−) are excited and migrate from the valence band (VBD) to the conduction band (CBD), leaving a positively charged hole (h+). The hole and electron can themselves carry out contaminant degradation or lead to the formation of powerful radicals such as hydroxyl (OH·), superoxide (O2), anionic radicals, and contaminant radicals, leading to degradation and mineralization [18]. Radicals are unselective and highly reactive due to the unpaired electrons they contain. Photocatalysis is more popular, as it occurs in mild operating conditions and requires no specialized equipment. Despite the advantages, photocatalysis’ use of semiconductors has a high potential of causing secondary pollution, which poses challenges of recovery and reuse [19]. Among the semiconductors, titanium dioxide is mostly used, as it is less toxic, less costly, thermally stable, and inert in various reaction conditions [20]. To fully utilize the capability of TiO2, research efforts have been made to make it more photoactive through modifications such as doping, heterojunction construction, and dye sensitization [21]. This is attained through the reduction of bandgaps and requires light of longer wavelength, hence the utilization of visible light, to cause charge separation. This extension toward the use of visible light (>400 nm) enhances the generation of radicals and better degradation of EOCs. The level of photoactivity directly impacts the level of performance [22].
The integration of membrane filtration and photocatalysis can greatly enhance their performance and address their individual shortcomings through flux enhancement, antifouling, and enhanced catalyst recovery [23]. Moreover, prolonged use and reusability will be enhanced [24,25]. They can be applied in two ways: as slurry or immobilized photocatalytic membranes. Slurry systems are unattractive due to photocatalyst aggregation, light scattering, membrane abrasion, and the potential to foul the membrane [26]. In immobilized membranes, the membrane performs normal filtration as well as mechanically supporting visible light-driven photocatalysis in or on membrane surfaces.
This study is among the few studies on the application of solar (visible light) TiO2 photocatalytic membranes in the photodegradation of emerging organic contaminants. The novelty of the study lies in the critical evaluation of pertinent aspects required to improve the performance of photocatalytic membranes, such as tailored material synthesis, membrane fouling control, improved photocatalyst light absorption, use of visible light from sunlight, enhancement of reaction kinetics through synergy, and regeneration and reuse. This study explores the techniques and methods used in developing a visible light-driven immobilized photocatalytic membrane (IPM) through photosensitizing titania, its immobilization, and its recent applications in removing EOCs in water and wastewater. The review proposes ways of addressing the challenges arising from the photoactivity of titania, fouling mitigation, scalability, improving cost effectiveness, enhancing membrane stability, and other aspects relevant in scaleup efforts from lab scale to industrial scale.

2. Modification of Photocatalysts

Typical photocatalyst semiconductors used in photocatalysis include titanium dioxide (TiO2), iron III oxide (Fe2O3) [27], zinc oxide (ZnO) [28], zinc sulfide (ZnS) [29], oxides of copper (Cuo, Cu2O) [30], cadmium sulfide (CdS) [31], tungsten oxide (WO3) [32], and nickel oxide (NiO) [33]. These photocatalysts have different characteristics that make them applicable in photocatalytic membranes for different environmental and energy applications. Examples of semiconductor photocatalysts and their strengths and weaknesses are presented in Table 1.
TiO2 is the most used of the semiconductors, as it is stable in various reaction media, easily activated by UV light, readily available, less costly, and has reduced potential to cause secondary pollution [40]. Despite the numerous advantages, more photosensitization is needed to harvest not only UV light but also the larger visible light portion of the light spectrum and increase its capability to generate more radicals efficiently. To increase the photosensitivity of titania, efforts such as doping [20], creation of heterojunctions [41], and dye sensitization [42] have been implemented. These efforts are vital in reducing reliance on the use of UV electric lamps that consume power. This offers an opportunity for the use of solar green energy that is environmentally friendly.

2.1. Doping

Doping is the introduction of material atoms into the TiO2 lattice either interstitially or substitutionally to generate new lower-valued band gaps and to cause a desired effect by modifying the parent TiO2 catalyst [43]. Among the improvements are bandgap narrowing, extension of light wavelength utilization, reduction of recombination centers, and better charge separation [44,45]. However, metallic doping has been shown to have potential to cause thermal instability [46].

2.1.1. Noble Metal Doping

Noble metals (Nm) such as platinum (Pt), nickel (Ni), gold (Au), palladium (Pd), rhodium (Rh), and silver (Ag) [47] have Fermi levels (FL) lower than that of TiO2. They have been utilized to improve photocatalytic activity by using the visible light region. This phenomenon is created mainly by electrons on the surface of the applied noble metal [48]. These electrons oscillate collectively, creating a plasmon resonance effect [49]. Under UV irradiation, the electrons are transferred from the CBD of TiO2 due to Fermi level equilibrium (creating a stocky barrier) as holes remain in the VBD of TiO2 [50], as shown in Figure 1.
The reverse happens under visible light irradiation, as electrons are transferred to the CBD of TiO2 as holes are created in the dopant atom, as shown in Figure 2. This leads to the creation of an interfacial electric field, encouraging charge separation and suppression of charge recombination.

2.1.2. Cation Doping

Transition metals such as copper (Cu), chromium (Cr), iron (Fe), molybdenum (Mo), and cobalt (Co) [51] have been utilized to improve the photocatalytic activity of TiO2 in their cationic forms as Cu2+, Cr3+, Fe3+, Mo5+, and Co3+. They introduce interbands close to the CBD and VBD of titanium dioxide, reducing the bandgap of titania [52]. When doped, charges from the d orbital of cations transfer to the CBD and VBD of titanium dioxide, causing red shifting and, hence, visible light responsiveness. The cations also maintain an equilibrium of charges by trapping electrons and holes [53]. Using higher valence dopants such as Mo5+ causes additional electrons in the CBD of TiO2 and, hence, a negative drift of the Fermi energy level in normal hydrogen electrode. Similarly, using lower valence dopants such as Cu2+ causes a positive drift of the Fermi energy level in normal hydrogen electrode.

2.1.3. Anion Doping

Anions such as carbon, phosphorus, sulfur, and nitrogen have been introduced into titanium dioxide lattice interstitially and substitutionally to improve the photocatalytic activity of TiO2. The anions create localized states and smaller bandgaps by red and blue shifting of the absorption edge in TiO2 [54]. This requires less photon energy to be activated and, hence, a smaller wavelength of light to be responsive in visible light. Also, anionic doping leads to the formation of charge traps within TiO2 bands. This increases the life of separated holes and electrons, increasing photocatalytic activity [55]. Carbon and nitrogen doping have been shown to cause a red shift of 50 nm [56]. Anions suppress the formation of recombination centers, unlike cationic dopants. Fluorine does not cause the shifting of the bandgap, but it improves the acidic properties of TiO2 surfaces, reduces Ti4+ to Ti3+, and hence promotes the separation of charges [57].
Anion doping instills superior thermal stability compared to metallic doping. Moreover, the process used in doping requires less specialized equipment and is cost-effective [58]. Most anionic dopants are found in widely available precursors.

2.2. Dye Sensitization

A variety of dyes, such as 4-(diphenylamino) phenylcyanoacrylic acid and 5-[4-(diphenylamino)phenyl]thiophene-2-cyanoacrylic acid, have been used in improving the photocatalytic characteristics of TiO2 [59]. Dye molecular structures attached to TiO2 are responsible for improving its photocatalytic activities. Once enough light in a given wavelength falls on the dye, molecule-attached electrons are excited and move from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the dye molecule [60]. The electrons then migrate to the CBD of TiO2, where they easily attach to surface molecules such as dissolved oxygen to form superoxide radicals that degrade pollutants, as shown in Figure 3. Dyes are made up of long organic chains. This makes them easily disintegrate at elevated temperatures during the synthesis of dye-sensitized TiO2 and during radical degradation, which can generate heat. Scenarios of dye accumulation on TiO2 interpores have also been observed and may negatively affect the number of radicals [21]. The degradation of pollutant dyes acts as a dye sensitizer whereby the dye sensitizes titania photocatalyst as it degrades the dye [61].

2.3. Construction of Heterojunctions

Heterojunctions are constructed when other semiconductor materials such as bismuth sulfide (Bi2S3), cadmium sulfide (CdS), zinc oxide (ZnO), iron iii oxides (Fe2O3), and tin iv oxide (SnO2) are placed and attached to TiO2. The boundary formed between TiO2 and the other semiconductor is called a heterojunction [62]. TiO2 and the different semiconductor used in heterojunction construction can be activated with enough light in the wavelength of each semiconductor, generating holes and electrons. The difference in the VBD and CBD location of TiO2 and other semiconductors results in an increase in the time of separation of e− and h+, reducing recombination chances, the stabilization of TiO2, and hence an improvement in photocatalytic activity. Better charge separation is shown through high values of charge densities [63]. Heterojunctions can be classified as type I, II, III, and z-scheme.

2.3.1. Type I Heterojunction

Type I heterojunctions are formed on TiO2 with any other p-type semiconductor. The VBD energy level of TiO2 is usually higher than that of the corresponding semiconductor (CS), whereas the CBD energy level of TiO2 is usually lower, giving a straddling band alignment [64]. The electrons and holes from TiO2 are transferred to the other semiconductor, resulting in better charge separation (Figure 4). By virtue of different semiconductors having lower potential, radicals are generated in the other semiconductor, and therefore, all the photocatalytic reactions take place on the other semiconductor. A type 1 heterojunction provides additional active sites for adsorption of organic pollutants and their intermediates; this leads to improved kinetics and high catalytic activity. Examples of type 1 heterojunctions are ZnFe2O4/TiO2 [65], TiO2/Fe3O4 [66], and Cu2O/D-TiO2 [67].

2.3.2. Type II Heterojunction

In a type II heterojunction, the CBD and VBD of TiO2 are higher than the CBD and VBD of the other corresponding semiconductor used. The excited electrons, activated by light, move from the CBD of TiO2 to the CBD of the other corresponding semiconductor. The hole would then be transferred to TiO2 from the other semiconductor, which would give a staggered gap band alignment [64], as shown in Figure 5.
This formation favors better charge separation since electrons will be on the corresponding semiconductor and holes in titania. This yields better effectiveness in generating radicals. Examples are Ag10Si4O13/TiO2 [68], CdS/TiO2 [69] CuSbSe2/TiO2 [70], and g-C3N4/TiO2 [71].

2.3.3. Type III Heterojunction

In a type III heterojunction, the CBD and VBD of TiO2 are significantly lower than the CBD and VBD of the other corresponding semiconductor used, giving a broken gap band alignment [64], as shown in Figure 6. For electrons to move from the CBD of TiO2 to the CBD of the different semiconductor, more driving force will be required. Similarly, more driving force is needed for a hole to migrate from the other semiconductor to TiO2. This arrangement displays weak photocatalytic activity due to the suppression of efforts to separate charges [72].

2.3.4. Z Scheme Heterojunction

A Z scheme heterojunction can be constructed by using TiO2 and another semiconductor, whereby either TiO2 or the other semiconductor will be the electron donor or receiver. The photoexcited electrons or holes cross over from TiO2 or the different semiconductor through a mediator material, creating an appropriate channel [73]. Most mediator materials are usually noble metals such as gold or silver. The electrons or holes then accumulate on TiO2 or the other semiconductor, depending on the individual potential for holes and electrons. This results in the effective separation of charges. The electrons and holes will then participate in the creation of radicals for degradation. Examples are P–TiO2/g-C3N4 [74], ZnFe2O4/P25 [75], and g-C3N4/TiO2 [76].

3. Synthesis and Fabrication of Immobilized Photocatalytic Membrane

3.1. Phase Inversion

A stable polymer solution consisting of polymers and appropriate solvent is first prepared in phase inversion. Modified titania nanoparticles are then added to the polymer solution and mixed thoroughly and evenly to form a casting solution. The phases will then be transformed from the stable casting solution with nanoparticles to solid phase film with TiO2 nanoparticles by various techniques, such as immersing in the non-solvent phase, precipitating by cooling, and precipitating by evaporation [77].
Immersing in non-solvent: The casting solution is first cast evenly on a flat surface plate and is immersed in a non-solvent [78]. Precipitation will then occur by solvent movement from the casting solution to the non-solvent phase [79]. This results in a thin membrane film containing nanoparticles. The movement of solvent from the casting solution to non-solvent starts at the surface. It slows down towards the film membrane’s inner region, forming an internal pore structure containing nanoparticles.
Precipitation by cooling: The casting solution is spread evenly on a flat surface and allowed to cool down. This destabilizes the solubility characteristic of the casting solution. The solvent phase will separate and form a thin film membrane containing photocatalytic nanoparticles [80].
Evaporation: The casting solution is spread uniformly on a flat surface. Because the solvent has a lower boiling point, it then evaporates from the spread casting solution [81], leaving a thin film membrane with photocatalytic nanoparticles.
A phase inversion technique is applied in the fabrication of IPM immobilized in the internal structures of membranes such as PVDF, PS, PE, CA, PP, and cellulose acetate/PolyPyrrole [82]. By varying the combinations of polymer, nanoparticles, and the technique used to invert phases, different characteristics can be imparted on the membrane type [83]. Examples of visible light-driven photocatalytic membranes fabricated through this design technique are GO/TiO2-CA [84], TiO2/ZnO-PVC [85], Fe-N-TiO2-CA [86], and Ce-Y-ZrO2/TiO2/SiC [87].

3.2. Dip Coating

Dip coating is a technique used to physically immobilize photocatalyst nanoparticles on surfaces of polymeric and ceramic membranes [88]. The method is highly favorable for membranes sensitive to harsh conditions like pH, temperature, and pressure, which are applied to produce nanoparticles [89]. Modified titania nanoparticles are first evenly suspended in solvents such as water, water-polyvinyl alcohol mixture, or ethanol to form coating solutions. The prepared or commercially available membranes are then dipped in the coating solutions. By virtue of the porosity of the membrane, the coating solution is sucked by capillary forces into membrane pore structures, hence depositing visible light-driven titania nanoparticles into the membrane and on its surface.
Using low boiling point solvents that easily evaporate gives a more uniformly packed coating [90]. The factors that determine the quality of coatings are dipping and withdrawal time, number of repeated coatings, solvents used, and characteristics of the supporting membrane. Increasing the number of coatings increases the nanoparticles’ loading, hence the photocatalytic ability. Despite the rise in photoactivity, this increases the photocatalytic membrane resistance to the permeate flow.
Membranes coated by dip coating have weak adherence of nanoparticles, which might lead to the loss of immobilized nanoparticles in operations that are violent, time-consuming, or demand high pressure [91]. Additional procedures must be performed to reinforce the coatings. For polymeric membranes, a crosslinking solution is prepared by the use of glutaraldehyde [92], adipic acid, and hydrazide. The crosslinkers introduce more robust binding functional groups such as carboxylate, amine, and hydroxyl groups [93] that firmly attach to titania ion (Ti4+). For ceramic membranes, heating and annealing techniques are used to firmly attach the nanoparticles to membranes [94]. The addition of titania nanoparticles has the potential to reduce permeate flux but eventually increases relative flux as compared to pristine membranes due to better antifouling capability [95].

3.3. Electrocoating and Electrospraying

Electrocoating and electrospraying are electrical techniques used for making photocatalytic membranes immobilized in a membrane structure, on the membrane as a coating, as well as titania-based self-photocatalytic membranes. Visible light-driven titania nanoparticles are first evenly mixed with the appropriate polymer or solvent and put inside a syringe [96]. Facing the syringe directly is a collecting surface at a proper distance. The syringe and the collecting surface are then connected to a high electric potential difference that is high enough to overcome the surface tension of the polymer or solvent used. The syringe then ejects the polymer or the solvent loaded with nanoparticles into thin fibers or atomized droplets that are deposited on the bare collecting surface in the form of fibers or thin sheets of membrane material [97]. When a polymeric or ceramic membrane is placed on the collecting surface, it results in evenly spread coatings in nanofibers [98,99]. Thin fibers or atomized droplets ejected are produced depending on the molecular weight of the polymer or solvent and syringe opening size used.
The intensity of the potential difference, the distance between the syringe and collecting surface, the ejection rate, and the type of polymer or solvent determine the characteristics of the membrane or coating formed [100].

3.4. Spin Coating

Spin coating is a physical technique used to immobilize visible light-driven TiO2 nanoparticles in membrane structure and on the surface of an already available membrane. Appropriate polymer solutions are first made. The nanoparticles are then dispersed uniformly in the polymer solution and poured at the center of a rotating disk at a set speed. Centrifugal forces from the center of the rotating disk toward the outside evenly and radially distribute the polymer solution containing nanoparticles [101]. This gives a thin-layer sheet containing nanoparticles [102]. As spinning proceeds, the solvent part of the polymer solution evaporates, leaving a thin membrane with immobilized nanoparticles in the membrane structure.
For coating of membranes, appropriate polymers are used, and an already-prepared membrane is placed on top of the rotating disk before the polymer loaded with nanoparticles is poured. The characteristic of membranes or coating produced is determined by factors such as spin velocity, solvent boiling point, and viscosity of the prepared polymer-loaded solutions [103].

3.5. Chemical Vapor Deposition

Chemical vapor deposition is a chemical technique for immobilizing visible light-driven titania and modified titania on polymeric and ceramic membranes. The titania-carrying precursor is first changed from liquid to gaseous phase by applying energy forms such as heat, light, or plasma discharge in an enclosed, unreactive environment [104]. The precursor then reacts, disintegrates, and deposits titania dopants on the membrane surface, giving a thin film coating [105]. The coating produces layers of high surface area suitable for photocatalytic degradation.

3.6. Vacuum Filtration

Vacuum filtration is a physical technique for surface-immobilizing modified titania nanoparticles on polymeric and ceramic membranes. Nanoparticles are first suspended uniformly in a fluid. The fluid is then directed to the membrane surface, where suction pressure is applied on the other side of the membrane. The nanoparticles are then deposited on the membrane surface and bonded by hydrogen bonding. Hydrogen bonding does not offer strong bonding between the membrane and nanoparticles and may lead to leaching of nanoparticles and, hence, loss of performance. To promote firmer attachment, additional techniques such as sintering or dopamine treatment are applied [106]. By varying the concentrations of suspended nanoparticles and suction pressure, coatings that yield different photocatalytic membranes can be achieved.

3.7. Electrochemical Anodization

Electrochemical anodization is a technique of producing self-photocatalytic membranes in the form of nanotubes by using direct current electricity, an anode, and a cathode and inserting in the electrolytic solution. A thin titanium flat sheet on the anode allows the growth of self-photocatalytic membrane nanotubes [107]. Another metallic sheet, such as platinum, is connected as a cathode, where reduction and oxidation occur and lead to hydrogen and oxygen gas generation. The electrolytic solution is composed of deionized water, bromine salts, chloride salts, sulfate salts, ethylene glycol, and other chemicals such as graphene oxide in different compositions and pH depending on the characteristics of nanotubes required [108,109].
The power supplied may be constant potential difference, current, or variable. However, in most of the literature, electrochemical anodization is usually applied with a constant potential difference [110,111].

4. Types of Immobilized Photocatalytic Membranes

Immobilized photocatalytic membranes incorporate titania or titania-doped nanoparticles and membrane as the main parts. The membrane offers physical support to nanoparticles in addition to performing its normal function of acting as a barrier for selectively separating pollutants and partially degraded pollutants. In IPMs, the modified TiO2 nanoparticles are fixed and therefore address the issue of challenges in nanoparticle recovery, nanoparticle pollution, and regeneration of photocatalytic membrane [102]. Because of immobilization, the membrane surface must be irradiated with light generated from UV lamps or direct sunlight or through sunlight concentrating mechanisms for photocatalytic degradation of pollutants in the wastewater. Despite lower degradation as compared to the slurry photocatalytic membrane (SPM), the issues of nanoparticle fouling and light scattering experienced in SPM are mitigated. IPMs can be classified as (1) surface immobilized photocatalytic membrane and (2) internally immobilized photocatalytic membrane.

4.1. Surface Immobilized Photocatalytic Membranes

In surface-immobilized photocatalytic membranes, modified titania nanoparticles are placed and rigidly attached to the surfaces of membranes [112], as shown in Figure 7. The attachment of nanoparticles on the membrane surface is attained through physical techniques or chemical means, producing a coating. The coated surface is then irradiated with solar light or lamp, resulting in photocatalytic degradation on the surface. Among the membranes utilized for surface-type immobilization are polymeric and ceramic membranes such as PVDF, PSF, PES, PAN [113], alumina, and silica [114].
Techniques used to immobilize nanoparticles on the surface include dip coating, electrospinning, electrospraying, vacuum filtration, and chemical vapor deposition. Pristine membranes present negative traits such as hydrophobicity, easy fouling, and low mechanical ability [115]. With immobilization coatings, these negative traits are reduced by controlling fabrication parameters and conditions.

4.2. Internally Immobilized Photocatalytic Membranes

In internally immobilized photocatalytic membranes, visible light-driven titania nanoparticles are placed permanently within the membrane structure, as shown in Figure 8. The membrane is then irradiated with solar light or lamp, resulting in photocatalytic degradation of organic contaminants. Due to nanoparticles becoming fixed in membrane structures, direct contact between pollutants and nanoparticles is hindered, and lower photocatalytic degradation is observed [116]. Photocatalytic activity and mechanical properties can be determined by controlling the amount and size of nanoparticles to be immobilized. However, an increasing amount of nanoparticles beyond a specific limit causes an increase in membrane resistance and agglomeration of visible light-driven TiO2 nanoparticles, leading to a decline in photocatalytic degradation [117].
The techniques used to immobilize nanoparticles in the membrane structures include wet spinning, phase inversion, electrospinning and electrospraying (Table 2).

4.3. Self-Photocatalytic Membranes

Self-photocatalytic membranes are made of pure titania-based photocatalyst as the membrane material. This type requires no support through immobilization techniques, massively reducing leaching experienced with the other immobilized photocatalytic membranes. Self-photocatalytic membranes are fabricated in forms such as nanofibers [127], nanotubes [103], and nanowires [128,129]. They are made through electrospinning, hydrothermal growth, and electrochemical anodization. The membrane itself is highly photocatalytically active due to the direct transfer of electrons and, hence, holes [130]. Self-photocatalytic membranes also have a higher anatase phase crystallization and high hydrophilicity [131]. Examples and the performance of self-photocatalytic membranes are listed in Table 3.

5. Application of Immobilized Photocatalytic Membranes

Photodegradation of organic pollutants is attained mainly by hydroxyl radicals, partly superoxide, and partly by holes and electrons. Radicals, mainly hydroxyl, are unselective and attack organic pollutants as well as their intermediate photodegradation products. Therefore, zero or first order should be applied, assuming that the pollutants exist at very low concentrations [136].
The kinetics of degradation do not fit a linear function. Degradation increases as irradiation increases until zero rate order is reached, and hence, best fit Langmuir–Hinshelwood kinetics (LG–HS) [137]. The LG–HS kinetic equation encompasses all effects of the surface area of nanoparticles, dynamic rates of reaction, and reaction times, which change as radicals consume pollutants and, therefore, provide for more photocatalysts to take part in degradation reactions.
For applicability, all reactions are assumed to take place in aqueous phases between radicals and organics. Then, the rate of reaction (r) will be proportional to the surface area (Ωx) of nanoparticles in contact with the organic pollutant [138], as shown in Equation (1):
r = d c d t = k r Ω x = k r K C 1 + K C
where kr is the reaction rate constant, C is the concentration of organic pollutants, and K is the Langmuir constant.
Photo reactions depend on the surface area of the photocatalyst. Assuming that the intermediates of degradation reactions are degraded by other forms of radicals other than hydroxyl radical, then kr will be in the magnitude of 10−3 m/s since the reactions are taking place in water, and hence, it will be proportional to the concentration of the pollutant.
Applying boundary conditions such that at the beginning of photoreactions, t = 0 and C=CO, and that at any given time of the reaction, the concentration of pollutant remaining is C, then, as shown in Equation (2),
ln ⁡ C C o − K C − C o = − k r K t
The value of the Langmuir adsorption constant K is given by a plot of 1/r versus 1/C in the equation above. The value of K is minimal because it is countered by desorption when a pollutant is exposed to nanoparticle surfaces. Pollutants in real wastewater usually exist at low concentrations; therefore, as shown Equation (3),
r = d c d t = k r K C
Integrating the above equation gives first-order reaction kinetics, as shown in Equations (4) and (5):
C = C O e − k r K t
and
ln ⁡ C C o = − k r K t = k ′ t
where k′ is represented in per-minute units.
Various contaminants have different levels of degradability, pathways, and intermediates that still consume radicals and have the potential to cause fouling, hence different performance. Intermediates can be degraded by allowing more degradation time. This can be achieved by reducing feed pressure on the photocatalytic membrane, resulting in increased residence time for degradation.
Research on most visible light-driven photocatalytic membranes has been performed at a lab scale. Efforts to scale up involve detailed evaluation and modeling before implementation. It has been shown that degradation scaleup depends on the diameter of the reactor vessel [139]. Other considerations, such as performance and durability, material and operation costs, patent rights, and engineering aspects have been identified as areas of concern to realize environmental and economic viability [140]. Additionally, scaled up systems have been shown to have reduced area to total pollutant volume, an indication of economic viability [141]. Additionally, some studies have demonstrated real wastewater treatment using photocatalytic membranes [142,143]. This should encourage the scaling up of photocatalytic membranes.
A few studies have attempted to scale up photocatalytic membrane applications to the pilot scale, but with pristine TiO2, and most depend on electric UV lamps. This is despite the high degradation ability displayed in lab-scale studies using visible light-driven TiO2. This is considered a step toward the development of pilot-scale applications that use visible light in titanium dioxide membranes. Examples of attempts at the pilot-scale application of photocatalytic membranes are listed in Table 4. All these studies have been performed in a continuous mode of operation.

6. Factors Determining Performance of Photocatalytic Membranes

6.1. Nature and Quantity of Visible Light-Driven TiO2 Based Photocatalysts

The TiO2-based nanoparticles are essential to visible light-driven photocatalytic membranes in effecting degradation. The ability of the nanoparticles depends on the characteristics of the bandgap energy caused by doping and other modifications, the phases involved, and the porosity, surface area, and distribution of nanoparticles [148]. Doping lowers bandgap energy and extends its ability to utilize visible light in photocatalytic reactions [149]. The doping level and type of dopant determine its effectiveness and how far in wavelength it can utilize visible light. Titania exists mainly in two polymorphs, namely anatase and rutile. The anatase phase is more photocatalytically active than the rutile phase [150]. Hence, the combination and composition of nanoparticles determine their overall ability to cause photocatalytic reactions. The surface area and porosity are critical characteristics of nanoparticles. The smaller the nanoparticles, the larger the surface area and porosity. Surface area and porosity determine the interactions between nanoparticles and pollutants [151]. This controls the rates of radical interactions with pollutants and subsequent degradation.
Increasing titania-based nanoparticles in immobilized-type photocatalytic membranes increases the surface area for pollutant interactions. A larger surface area produces more sites for hole and electron generation, resulting in more hydroxyl and superoxide radicals [152]. An optimal number of nanoparticles per unit volume of water exists. Adding more than the limit of photocatalyst nanoparticles causes (1) scattering, reflections of light, and hindered penetration, thus limiting performance; (2) agglomeration of nanoparticles, which reduces the surface area and hence its effectiveness in creating sites for charge separation [153,154].

6.2. Concentration and Characteristics of Pollutants

The concentration and inherent characteristics of organic pollutants affect the performance of photocatalytic membranes and are therefore important in designing degradation by photocatalytic membranes in real wastewater. When the number of pollutants is small compared to the radicals produced, fewer pollutants will interact with nanoparticles, resulting in underutilization. High amounts of contaminants result in blockage of light reaching the photocatalyst and reducing degradation [115]. Therefore, optimization of the balance between nanoparticle active sites and pollutants should be considered to maximize interactions [155]. Different organic pollutants have different structures and chemical characteristics, such as iso-electric points; hence, their resistance to degradation varies, as well as the related intermediate products formed [156]. Intermediate products also consume radicals, as they also undergo degradation. This will increase the amount of time required for degradation.

6.3. The Light Source and Its Intensity

Light sources such as lamps and sunlight facilitate the separation of holes and electrons. The type and composition of the light source depend on the type of light wavelength in the spectrum it emits. The UV-A wavelength ranges from 315 nm to 400 nm, which creates equivalent bandgap separation from 3.10 EV to 3.94 EV. The UV-B wavelength ranges from 280 nm to 315 nm, and its equivalent bandgap separation is from 3.96 EV to 4.43 EV. The UV-C wavelength ranges from 100 nm to 280 nm, and its equivalent bandgap separation ranges from 4.43 EV to 12 EV [157]. The sunlight UV region of the light spectrum is approximately 5% [158] and gives power ranging from 20 W/m2 to 30 W/m2 depending on the weather conditions [159]. At low intensities of less than 20 W/m2, the separation and recombination of electrons and holes are significantly reduced. The photocatalytic reaction rate at low intensities tends to be proportional to the intensity. At the intermediate intensity of approximately 25 W/m2, electron and hole separation is faced with an increase in recombination. This makes the photocatalytic reactions proportional to the square root of the intensity of light. Electron and hole recombination at high intensities is more remarkable than separation [160]. This is due to the limitation caused by the mass transfer of scavenging radicals. This hinders an increase in photocatalytic reactions [153].

6.4. Effects of Solution pH

The pH is a measure of the amount of hydrogen ions or hydroxyl ions. A point in pH exists where the surface of visible light-driven titania is neutral or isoelectrically uncharged in water. This point is close to pH 7 [161]. Changes in pH below or above the isoelectric point cause the photocatalyst surface to be charged [162]. Reducing the pH below the isoelectric point causes the photocatalyst surfaces to be positively charged. An increase in pH above the isoelectric point causes titania surfaces to be negatively charged, as shown in Equations (6) and (7).
pH < pHiso: TiOH + H+ → TiOH2+
pH > pHiso: TiOH + OH− →TiO− + H2
At higher pH, nanoparticles have negatively charged surfaces. This causes repulsion between hydroxide ions and immobilized nanoparticles, hindering their interactions and reducing the number of hydroxyl ions that scavenge for electrons [163]. This reduces the amount of radicals and, hence, photocatalytic reactions. Additionally, different pollutants have different points of zero charge. This determines the type of interaction between the pollutant molecules and the surface of the charged photocatalyst at different pH values. At points close to zero charge, it has been shown that photocatalyst solubility is significantly reduced and can result in higher photocatalytic activity [164].

6.5. Inorganic Ions

Inorganic ions such as chloride, nitrate, and sulfate ions influence degradation ability [165]. These ions cause inhibition and scavenging competition for separated charges and active sites by interacting with nanoparticles, forming less powerful radicals and few hydroxyl radicals [166]. This lowers the degradation ability of photocatalysts. Ions such as magnesium, calcium, and zinc slightly affect the formation of radicals [167]. The high presence of ions indicates the high presence of salts, which cause a decline in flux by scaling [168].

6.6. Aeration

Aeration of wastewater in the operation of photocatalytic membranes has been employed as a fouling reduction measure through the turbulent displacement of deposited particles and gelling [169]. This also increases dissolved oxygen [170]. Oxygen is an electron acceptor and scavenges for electrons separated from titania-based photocatalyst [171], reducing charge recombination. Oxygen scavenging produces superoxide radicals, which may also form hydroxyl radicals. Both radicals carry out degradation. In addition to forming radicals, aeration improves pollutant contact with titania-based nanoparticles. This promotes photocatalytic reactions. In the application of aeration, an optimum point exists. Beyond this limit, bubbles hinder light penetration, directly reducing photocatalytic reactions [172] and reducing interactions between pollutants and titania-based nanoparticles.

7. Challenges and Prospects

This comprehensive review has highlighted the joint use of visible light-driven photocatalysis and membrane filtration as an efficient means of enhancing both technologies through joint use in surface or internal immobilization of visible light-driven titania-based photocatalysts. Despite this excellent performance potential in the removal of organic contaminants, challenges that hinder its full implementation exist. The study identifies the challenges and proposes a way forward:
(i)
Membrane fouling is still a matter of concern. Pollutants accumulate on the membrane surface, reducing its efficiency and requiring frequent cleaning. However, the presence of photocatalytic nanoparticles makes the membranes reactive and reduces the fouling propensity. Studies should focus on tailoring photocatalytic membranes to specific pollutants or groups of pollutants to eliminate fouling and minimize cleaning or replacement. To enhance membrane cleaning and reduce membrane deterioration, there is a need to develop cleaning agents that are effective against foulants but less harmful to the photocatalytic membrane structures.
(ii)
Low light utilization in systems using immobilized photocatalytic membranes is a major challenge. Therefore, ensuring uniform light distribution across the membrane surface needs to be addressed to improve overall efficiency. Access to more light enhances the number of generated radicals, which are crucial in the degradation of organic contaminants. To address this, modification of photocatalysis is necessary, for instance, to reduce the bandgap and increase the absorption and utilization of visible light. Additionally, the design of the photocatalytic membrane cells needs to be improved to increase the amount of light reaching the surface of the photocatalytic membrane to irradiate the immobilized photocatalysts and generate degradation radicals.
(iii)
Deterioration of the photocatalytic membrane over time or under prolonged exposure to light can diminish its effectiveness. This can also be caused by immobilized titania leaching, radical degradation of the membrane polymeric chains, and photodegradation of the membrane polymeric chains by UV light. This weakens the polymeric membrane mechanically, dramatically reducing its lifespan and reusability. During turbulent operation, titania-based nanoparticles detach, resulting in a lower number of nanoparticles that carry out degradation and reducing the ability to degrade organic contaminants. Radicals are highly reactive and indiscriminate and will, therefore, attack the polymeric chains of the base membrane. This lowers the mechanical strength of the membrane in prolonged use. Additionally, irradiation of the membrane surface has a negative effect, with UV rays photodegrading the membrane and weakening it further. More studies are required to develop new membrane materials that promote firmer immobilization of visible light-driven titania and resist membrane radical degradation and UV light photodegradation. This will prolong the lifespan of the membranes and enhance their reusability.
(iv)
Scalability of operation is another issue. Most studies reviewed have been performed at the laboratory scale, and very few at the pilot scale. There is a need to fast-track scale-up studies to develop large-scale applications at industrial scale and serve large territories. This can be attained though modeling and simulation to predict the economic cost and residence time for degrading different pollutants.
(v)
The cost-effectiveness of photocatalytic membrane processes is another pertinent aspect. For instance, high production costs for advanced materials and fabrication techniques can limit the economic viability of photocatalytic membranes. However, with recent advances, the costs of commercial membranes have significantly decreased. Furthermore, the use of visible light-driven photocatalysts has enabled the use of natural sunlight, which is abundant especially in the tropics, thereby reducing reliance on UV lamps that require electricity.

8. Conclusions

This review discusses the recent advances in the application of visible light-driven photocatalytic membranes in the degradation of organic pollutants in water. A critical evaluation of the membrane separation technique, its operation, materials used, the potential of titania photocatalyst, and the mechanism of degradation was undertaken. It was noted that doping, the creation of heterojunctions, dye sensitization, and novel fabrication of photocatalytic membranes significantly impact their performance. The typical performance of photocatalytic membranes was evaluated, comparing different membrane and photocatalyst types as well as pollutants. Despite the good performance of the photocatalytic membranes reported, the study identified key challenges and proposed a way forward. These include increasing the photoactivity of titania, fouling mitigation, scalability, improving cost-effectiveness, enhancing membrane stability, and other aspects relevant in scaleup efforts from lab scale to industrial scale. Of notable concern is the fact that most studies in the literature have been conducted with synthetic wastewater. There is a need for more studies conducted with a real wastewater matrix to understand this technology’s actual degradation performance in multicomponent pollutants and how interactions among pollutants affects performance. This will be attractive for scaling up efforts to pilot and eventually full-scale systems.it will also be a step closer to implementation and incorporation in present municipal treatment methods for wastewater.

Author Contributions

Conceptualization, K.N. and A.C.M.; formal analysis, K.N.; investigation, K.N. and A.C.M.; writing—original draft preparation, K.N.; writing—review and editing, K.N., A.C.M., H.M.S. and Z.A.S.; supervision, K.N. and A.C.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

No new data were created or analyzed in this study.

Acknowledgments

We would like to express our gratitude to the Department of Chemical and Process Engineering, Moi University.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic representation of titania stocky junction during UV light excitation and generation of radicals.
Figure 1. Schematic representation of titania stocky junction during UV light excitation and generation of radicals.
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Figure 2. Schematic representation of titania surface plasmon resonance during visible light excitation and generation of radicals.
Figure 2. Schematic representation of titania surface plasmon resonance during visible light excitation and generation of radicals.
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Figure 3. Schematic of dye sensitization mechanism resulting in generation of radicals.
Figure 3. Schematic of dye sensitization mechanism resulting in generation of radicals.
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Figure 4. Schematic structure of type I heterojunction showing charge separation and generation of radicals.
Figure 4. Schematic structure of type I heterojunction showing charge separation and generation of radicals.
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Figure 5. Structure of type II heterojunction showing charge separation and generation of radicals.
Figure 5. Structure of type II heterojunction showing charge separation and generation of radicals.
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Figure 6. Structure of type III heterojunction showing requirement of large driving force to cause charge separation.
Figure 6. Structure of type III heterojunction showing requirement of large driving force to cause charge separation.
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Figure 7. Schematic representation of surface-immobilized photocatalytic membrane showing modified TiO2 fixed on the membrane surface.
Figure 7. Schematic representation of surface-immobilized photocatalytic membrane showing modified TiO2 fixed on the membrane surface.
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Figure 8. Schematic representation of internally immobilized membrane showing modified TiO2 fixed within membrane matrix.
Figure 8. Schematic representation of internally immobilized membrane showing modified TiO2 fixed within membrane matrix.
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Table 1. Examples of semiconductor photocatalysts and their characteristics.
Table 1. Examples of semiconductor photocatalysts and their characteristics.
PhotocatalystTypeStrengthsWeaknessesReference
TiO2Metal oxide
  • Photo responsive
  • Stable
  • Inert
  • Non toxic
  • Low cost
  • Low charge separation
  • Wide bandgap
[34]
ZnOMetal oxide
  • High catalytic activity
  • Non toxic
  • Low cost
  • Wide bandgap
  • High charge recombination
  • Photocorrosion
[35]
MoSMetal sulfide
  • Narrow bandgap
  • Low charge recombination
  • Flexible
  • Mechanical strength
  • Poor quantum yield
  • Low surface area
[36]
CdSMetal sulfide
  • Utilizes visible light
  • Low activity
  • Photocorrosion
  • Photodissolution
[37]
gC3N4Carbon based
  • Easy synthesis
  • Excellent stability
  • High charge recombination
  • Low light utilization
[38]
Carbon Nano tubesCarbon based
  • Delayed charge recombination
  • Utilizes visible light
  • Varied bandgap
  • Toxic
  • Low reusability
[39]
Table 2. Application and performance of immobilized photocatalytic membranes.
Table 2. Application and performance of immobilized photocatalytic membranes.
TypeImmobilization TechniquePollutantPower SourceDegradation/Mineralization/FluxRef.
TiO2/GO/CElectrospinningmethylene blueVisible light98.5% degradation[118]
TiO2-coated YSZ/silicaDip coatinghumic acid, methylene blue, tetracycline each 20 mg/LVisible light88.2% humic acid degradation, 92.4% methyl blue degradation, and 99.5% tetracycline degradation[119]
Au-TiO2/PVDFPhase inversionTetracycline 200 mLXenon lamp 300 W75% degradation[120]
PVP/La3+: TiO2ElectrospinningCiprofloxacin 10 mg/LVisible light99.5% degradation[121]
Au-TiO2-CelluloseVacuum filtrationrhodamine B (1–9 mol/L)Sunlight95% degradation[122]
Boron doped-TiO2
SiO2/CoFe2O4/PES		Phase inversiondirect red 16, biologically treated palm oil mill effluentVisible light98% removal[123]
TiO2/Al2OSpin coatingAO dye300 W UVA lamp85% degradation[124]
GO/g-C3N4/TiO2Vacuum filtrationoilSunlight95% flux for 10 runs[125]
NTiO2-PVDFDip coatingSulphamethoxazole and NaClSunlight76.5% degradation and 9.8 mL/7 cmD/min flux[126]
Table 3. Application and performance of self-photocatalytic membranes.
Table 3. Application and performance of self-photocatalytic membranes.
TypeTechniquePollutantPower SourceDegradation, MineralizationRef.
g-C3N4/TNA/TiO2Electrical anodizationRhB (3 mg L−1)500 W xenon arc lamp60% removal[132]
Fe2O3/g-C3N4@N-TiO2Electrical anodizationBisphenol A (4.5 mg/L)Simulated sunlight lamp100% removal, 40 min[133]
TiO2/PtElectrical anodizationParaquat (37.4 µg/L)6 UV fluorescent lamps (8 W)86% degradation[134]
PbS-Ti/TiO2Electrical anodizationIfosfamide (20 mg L−1)Simulated sunlight (550 W m−2)44% TOC removal[135]
Table 4. Pilot-scale applications of photocatalytic membranes.
Table 4. Pilot-scale applications of photocatalytic membranes.
ModeCapacityPollutantsCatalystPerformancePower SourceRef.
Continuous1.2 m3/dayThiabendazole and acetamipridTiO241.5% thiabendazole and 25% acetamiprid removal in 3 hlamp[144]
Continuous18.32–27.63 L/m2 hTextile wastewaterTiO2/halloysite>98% COD removallamp[145]
Continuous1.2 m3/daydiclofenacTiO252% total organic carbon removallamp[146]
Continuous300 gallonsestrogensTiO2 P-25>70% estrogenic removallamp[147]
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Nelson, K.; Mecha, A.C.; Samuel, H.M.; Suliman, Z.A. Recent Trends in the Application of Photocatalytic Membranes in Removal of Emerging Organic Contaminants in Wastewater. Processes 2025, 13, 163. https://doi.org/10.3390/pr13010163

AMA Style

Nelson K, Mecha AC, Samuel HM, Suliman ZA. Recent Trends in the Application of Photocatalytic Membranes in Removal of Emerging Organic Contaminants in Wastewater. Processes. 2025; 13(1):163. https://doi.org/10.3390/pr13010163

Chicago/Turabian Style

Nelson, Kipchumba, Achisa C. Mecha, Humphrey Mutuma Samuel, and Zeinab A. Suliman. 2025. "Recent Trends in the Application of Photocatalytic Membranes in Removal of Emerging Organic Contaminants in Wastewater" Processes 13, no. 1: 163. https://doi.org/10.3390/pr13010163

APA Style

Nelson, K., Mecha, A. C., Samuel, H. M., & Suliman, Z. A. (2025). Recent Trends in the Application of Photocatalytic Membranes in Removal of Emerging Organic Contaminants in Wastewater. Processes, 13(1), 163. https://doi.org/10.3390/pr13010163

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