Recent prospects, challenges and advancements of photocatalysis as a wastewater treatment method
0 Abstract
The climate crisis, unrestrained use of water resources, and rising population demands have amplified the urgency for sustainable and eco-friendly wastewater treatment solutions. Conventional methods often fail to degrade emerging and persistent pollutants produced by industries, driving the need for alternative technologies. In this context, photocatalysis has emerged as a promising and effective method, offering superior degradation of contaminants and cost-effective application, especially with its advancements in recent years. As an advanced oxidation process (AOP), photocatalysis is particularly well-suited to address the limitations of traditional treatments. This review paper explores the types of photocatalysts and their operational mechanisms, examines the key parameters affecting degradation efficiency, such as operating conditions and photoreactor design, and analyzes recent developments in wastewater effluents containing emerging contaminants over the past five years. Lastly, this review proposes the integration of photocatalysis with other treatment technologies and presents potential future directions for research.
Keywords
INTRODUCTION
The global climate crisis, coupled with rapid industrialization, urbanization, and unsustainable water consumption, has placed immense pressure on existing water systems. As freshwater resources become increasingly scarce, sustainable wastewater treatment methods are urgently needed[1,2]. In recent years, emerging contaminants (ECs), including pesticides, pharmaceuticals, microplastics, and industrial chemicals, pose a growing concern due to the lack of regulations for monitoring or reporting their presence in water supplies. Municipal wastewater contributes significantly to EC pollution, with conventional wastewater treatment plants (WWTPs) being unable to remove these pollutants effectively[3].
To address these challenges, novel and combined treatment methods have recently been developed and tested at laboratory and pilot scales for enhanced removal of ECs and persistent organic pollutants (POPs) in various wastewater effluents. Some of the proposed methods include physicochemical processes, absorption, membrane filtration, ozonation, electrochemical processes, biological treatment, advanced oxidation processes (AOPs), and others[4,5]. Among these, AOPs have emerged as highly effective wastewater treatment methods due to their ability to generate highly reactive hydroxyl radicals (•OH), which degrade a wide range of organic pollutants, including pharmaceuticals, pesticides, and industrial chemicals. AOPs encompass techniques such as ozonation, reagent activation processes, Fenton oxidation, electrodeionization, electrochemical processes, and photocatalysis. Their main advantages include high removal efficiency, wide-spectrum pollutant degradation, minimal sludge generation, and pathogen inactivation[6].
However, each AOP has intrinsic limitations. For example, ozonation, while capable of generating strong oxidants and enabling fast degradation, involves high operational costs due to the energy required for ozone generation and the need for specialized equipment. Fenton processes require strictly acidic conditions, which limits their applicability in neutral or alkaline wastewater. Additionally, they produce iron-containing sludge, which needs further treatment before disposal, thereby increasing the operational complexity. Electrochemical oxidation, which relies on electrical current to drive redox reactions, leads to challenges such as high energy consumption, scalability issues due to frequent maintenance, and electrode replacement costs[6].
In contrast, photocatalysis stands out for its ability to offset most of these drawbacks due to its efficiency, development potential, and adaptability. It operates under mild conditions and can utilize natural sunlight, making it a more energy-efficient and cost-effective approach in the long term compared to other AOPs, such as ozonation or electrochemical oxidation. Unlike the Fenton process, photocatalysis avoids secondary sludge production and enables the reusability of the catalysts. Additionally, the synthesis and use of low-cost, non-toxic, visible light active and stable semiconductor photocatalysts reinforce its viability, especially when photocatalysis is activated by solar energy[7]. In terms of by-product formation, photocatalysis typically results in fewer harmful intermediates due to the mineralization pathways, whereas ozonation can generate more toxic intermediates if not properly controlled. Figure 1 provides a conceptual overview of the photocatalytic process and the scope of this review.
Figure 1. Schematic diagram highlighting the contents of this review.
This review discusses the progress made over the past five years in the development of novel photocatalysts, the design of photoreactors, the removal efficiency of ECs, and treatment conditions for various wastewater effluents. Additionally, it provides an overview of combined technologies aimed at minimizing drawbacks and accelerating large-scale applications. Finally, it highlights future research prospects, hoping to inspire and generate ideas for overcoming the challenges faced during the implementation of photocatalysis in real-life wastewater treatment.
BASIC PRINCIPLES OF PHOTOCATALYSIS - MECHANISM
The photocatalysis mechanism is based on a light source for energy and a photocatalytic material as a “mediator” to accelerate a chemical reaction, while remaining unconsumed. When a photocatalyst, typically a semiconductor, absorbs photons with energy equal to or greater than its band gap (Eg), an electron (e-) from the valence band (VB) gets excited and moves to the conduction band (CB). This transfer results in a positively charged hole (h+) in the VB. A narrower band gap increases electron-hole pair generation[8].
For effective photocatalysis, electron-hole recombination must be minimized. Strategies such as doping, introducing surface defects, or coupling with other catalysts enhance charge separation and electron transfer to reactive sites[9,10]. The excited electrons in the CB act as reductants, while holes in the VB facilitate oxidation. These species generate reactive oxygen species (ROS) such as •OH and superoxide anions (O2•-), which degrade organic pollutants[8].
In wastewater treatment, pollutants adsorb on the photocatalyst surface, where ROS drive reactions that break them down into less harmful by-products[9,11]. Moreover, a noteworthy variant of the photocatalytic process is the photo-Fenton process, which enhances ROS production via Fe2+ and hydrogen peroxide
An optimal photocatalyst should be efficient, non-costly, environmentally friendly, stable, and sunlight-activated.
Key attributes include eco-friendly synthesis, a narrow band gap for visible light activation, low electron-hole recombination, structural stability, resistance to leaching, high surface area, and strong reactivity. Additionally, it should effectively degrade pollutants into non-toxic by-products, be easily separated from treated water and retain activity over multiple cycles while minimizing reliance on UV light[10,11,13].
Several parameters influence photocatalytic efficiency. Contact time and catalyst dose impact degradation efficiency, as longer exposure enhances pollutant removal until saturation, while excessive catalyst loading can reduce efficiency due to light scattering and particle aggregation. Light intensity and wavelength also play a crucial role. While UV light is most commonly used, recent efforts have focused on developing visible-light-active catalysts to reduce energy costs[13-16]. The pH of the solution affects both the photocatalyst surface charge and the degradation pathways. When the pH is below the point of zero charge (PZC), the surface becomes positively charged, attracting anionic pollutants, while at pH above PZC, the surface is negatively charged, attracting cationic pollutants[16]. pH also influences the generation of ROS, such as •OH and O2•-, with lower pH favoring their production. Additionally, the stability and performance of the photocatalyst are pH-dependent; extremely high pH levels can negatively affect both the catalyst and its degradation efficiency[13]. Temperature plays a significant role in photocatalytic degradation. Although most reactions are conducted at room temperature, studies show that temperatures below 0 °C slow the reaction rate, while excessively high temperatures can degrade the photocatalyst and shorten the lifetime of reactive species. In contrast, moderate temperatures generally accelerate reaction kinetics by enhancing charge carrier generation and improving adsorption[13,17]. Oxidizing agents can improve photocatalytic degradation by promoting ROS formation and enhancing electron-hole pair dynamics. However, their concentration should be carefully controlled, as excessive amounts could break down the catalyst[13,18]. In real wastewater systems, inorganic ions may interfere with photocatalytic reactions by altering pH, scavenging reactive species, or occupying active sites on the catalyst surface. Some ions can even induce photocatalyst aggregation, thereby reducing its efficiency[17,19]. The concentration of pollutants also influences photocatalytic degradation rates. At lower concentrations, degradation is typically faster due to the abundance of available active sites and reactive species. In contrast, higher pollutant concentrations can hinder degradation, mainly due to the difficulty of light penetration, leading to a reduction in accessible active sites[13].
TYPE OF PHOTOCATALYTIC MATERIALS
Photocatalytic materials are primarily used to accelerate chemical reactions when exposed to light. A wide variety of these materials exist, and new ones are continuously being developed to enhance efficiency and minimize drawbacks. They can be categorized based on their composition, structure, or combination with other compounds. Broadly, photocatalytic materials can be classified into semiconductors, materials with point defects, and heterojunctions.
Semiconductors
Semiconductors are the earliest and most widely used photocatalytic materials due to their stability and high photocatalytic ability. Commonly used examples include ZnO, TiO2, Cu2O, MgO, and SnO2. However, conventional semiconductors typically require UV light or activation because of their large band gaps, while UV light accounts for only 5% of the solar spectrum. In comparison to visible light, UV radiation is not only more harmful to human health but also significantly more costly to generate. To address these challenges, research has increasingly focused on developing semiconductors that can be activated by visible light. However, visible-light-responsive semiconductors, which generally have narrower band gaps, often suffer from high rates of electron-hole pair recombination, reducing their efficiency in pollutant degradation. To overcome this limitation, various strategies have been developed, leading to many categories of photocatalysts for wastewater treatment[20]. For instance, Bahadoran et al. proposed using e-waste as a source of raw materials for synthesizing photocatalysts, thereby enhancing both reusability and sustainability[21]. Similarly, Chen et al. suggested recovering metals from e-waste by exploiting the catalytic properties of these photocatalysts[22].
Another challenge in semiconductor photocatalysis is photocorrosion, where prolonged light exposure degrades the material itself. This not only shortens the lifespan and reduces the stability of the semiconductor but can also lead to secondary water contamination from the degraded material. To mitigate this issue, oxide semiconductors are often preferred for their exceptional electrical and physical stability under illumination[23].
Materials with point defects
The simplest way to enhance the degradation ability of a semiconductor is by introducing point defects in its structure. These defects refer to imperfections occurring at a single location or/and on a minor scale within the crystal lattice, and they can significantly influence the material’s photocatalytic properties. However, the greater the amount of point defects, the greater the instability of the photocatalyst. Defects can arise from an atom’s complete vacancy or variation from its conventional lattice position, exchange of the atoms with others, or doping with foreign elements[24]. These structural modifications introduce additional energy levels, allowing electron excitation and transfer to the CB. Hence, the band gap energy limitation can be overcome, enabling the defective photocatalyst to absorb lower-energy photons and operate with improved efficiency[23]. Additionally, p-type (positive) and n-type (negative) doping introduces controlled impurities that create an excess of holes (or vacancies) and electrons, respectively.
Several studies have demonstrated the effectiveness of point defects and doping in enhancing photocatalytic performance. Wang et al. achieved 100% degradation of perfluorooctanoic acid (PFOA) within 6 h under UV illumination using oxygen vacancy BiOF nanosheets[25]. In another research, Wu et al. utilized oxygen vacancies on the surface of In2O3 (MnOx/In2O3), achieving 99.8% degradation of PFOA within 3 h under solar light[26]. Similarly, Núñez-Salas et al. doped ZnO with boron (BeZnO), enabling 89% degradation of cyanide under visible light in just 2 h[27]. Aboutaleb and El-Salam also reported 96% degradation of Congo red (CR) dye within 3 h under visible light by doping CeO2 nanoparticles (NPs) with Fe[28].
Heterojunctions
The formation of heterojunctions/heterostructures is attributed to the need to enhance charge separation and minimize electron-hole recombination. Specifically, a heterojunction consists of two distinct semiconductor materials with different band structures. During its formation, the CB and VB align in a specific way at the interface. This alignment allows electron transfer from one material to the other, depending on the relative energy levels of the bands. Since the materials involved vary, their band alignment also differs, leading to[29] different photocatalytic pathways that reduce electron/hole recombination[30,31].
Type-I heterojunction
Type-I heterojunction, also known as a straddling type of heterojunction, is characterized by a symmetric arrangement of the CB and VB of the two semiconductors. In this configuration, both the CB and VB of one material (M1) lie at lower energy levels than those of the other material (M2). Consequently, both electrons and holes tend to remain within the same semiconductor, typically the one with the narrower bandgap. As a result, charge carriers are confined within a single material, which limits the efficiency of charge separation and increases the rate of electron–hole recombination[32]. Khurram et al. compared the photocatalytic degradation efficiency of CR using bareα-Fe2O3/ZnO andα-Fe2O3/ZnSe nanocomposites[33]. The findings indicated that the degradation efficiency of α-Fe2O3/ZnO (Type-I heterojunction) was lower
Type-II heterojunction
In a type-II heterojunction, also known as staggered-gap type heterojunction, the CB1 and VB1 of M1 are positioned at lower energy levels than those of the M2. This band alignment drives electrons toward the lower energy CB1 and holes toward the higher energy VB2. This results in the spatial separation of electrons and holes across the two materials, decreasing recombination and enhancing photocatalytic activity[34]. Paul et al. synthesized a CsPbBrCl2/g-C3N4 heterojunction for the efficient degradation of textile dye Eosin B under visible light irradiation[35]. Their results showcased an impressive degradation efficiency of 94%, compared to 73% and 86% for the individual components. Moreover, the heterojunction exhibited excellent stability and reusability after testing in 3 consecutive cycles. Another example of type-II heterojunction synthesis and use was implemented by Cui et al., who degraded Rhodamine B (RhB) using In2S3/Bi2MoO6 under 90 min of visible light illumination[36]. The degradation efficiency reached 88.2%, which remained above 81.4% even after 4 cycles. However, a notable disadvantage is the reduced charge carrier mobility, which can slow down reaction rates[23].
Type-III heterojunction
A type-III heterojunction, also known as broken-gap heterojunction, is characterized by the CB1 and VB1 of M1 being located entirely below the VB2 of M2. This results in a broken band alignment with no direct overlap between the conduction and VBs of the two materials. During light illumination, electrons are excited to the CB1 of M1, while holes are generated in the VB2 of (M2). Due to the broken band alignment, the charge carriers are separated across the materials, and electrons tend to migrate to CB2 (higher-energy CB), while holes move to VB1 (lower-energy VB). Therefore, recombination is minimized, and photocatalytic efficiency is increased by enabling charge carriers to participate in different photocatalytic reactions[31]. Nevertheless, the broken band alignment of the two combined materials leads to poor electron transfer between them, as there is no continuous energy pathway for electron flow. This may slow carrier dynamics or hinder the mitigation of electrons and holes to their respective reactive sites[37]. Lastly, type III heterojunctions are generally considered unstable under prolonged irradiation or harsh photocatalytic conditions, limiting their practical applications[38]. In an effort to overcome these limitations, Li et al. proposed incorporating plasmonic metal NPs between the semiconductor components[39]. This strategy facilitated the selective transfer of photothermally generated electrons and holes, effectively converting the type III catalyst into a type B heterojunction photo-thermo-catalyst and thus achieving a 73-fold enhancement in the photothermocatalytic degradation rate of toluene.
Z-scheme heterojunction
A Z-scheme heterojunction exhibits a band alignment similar to that of a type-II heterojunction, while the key difference lies in their charge transfer mechanisms [Figure 2][34].
Figure 2. Differences between Z-scheme and Type-II heterojunction mechanisms.
Jiang et al. synthesized the BiFeO3/ZnFe2O4 for the degradation of methylene blue (MB) and tetracycline (TC) under visible light irradiation[40]. BFO/ZFO-10% achieved the greatest degradation performance on MB and TC. Luo et al. used AgI NP/Zn3V2O8 for TC degradation and observed that within 140 min under visible light irradiation, the degradation efficiency reached 91%[41]. Chen et al. studied the photocatalytic degradation of ephedrine (EHP) by AgBr/P-g-C3N4[42]. The results indicated an efficiency of 99.9% in 1 h[43].
p-n Heterojunction
A p-n heterojunction forms by coupling a p-type with an n-type semiconductor, creating an internal electric field that accelerates charge carrier transfer to the photocatalyst’s surface while reducing recombination. Like type-II, redox reactions occur on the surface but with lower oxidation/reduction potentials than Z-scheme systems[34]. Beshkar et al. used a CuI/FePO4 p-n heterojunction for amoxicillin (AMX) degradation under simulated sunlight, achieving ~90% efficiency versus ~41% (CuI) and ~69% (FePO4) individually[44]. Limitations include narrow light absorption and potential band alignment issues that hinder performance[31].
S-scheme heterojunction
S-scheme (step-scheme) heterojunctions combine two n-type semiconductors with a type-II-like band structure but distinct charge transfer. Semiconductor A (SA), with a lower work function, interacts with semiconductor B (SB), causing electron flow from SB to SA and forming an internal field. This leads to band bending, downward in SB and upward in SA. Upon light irradiation, electrons in SB recombine with holes in SA, while the reductive electrons in SA and oxidative holes in SB remain active for surface redox reactions[31,45]. Fan et al. developed In2S3/Bi2O2CO3 S-scheme for RhB degradation, reaching 91% efficiency in 60 min under simulated light[46]. While efficient, they face limited light absorption and possible charge transfer inefficiencies due to suboptimal band alignment[45].
Schottky heterojunction
A Schottky heterojunction [Figure 3] forms between a semiconductor and a metal due to their Fermi level difference, creating a Schottky barrier that reduces electron-hole recombination and enhances photocatalytic activity[47]. Material compatibility is crucial for stability[48]. Graimed et al. synthesized a SiO2/
Figure 3. Schematic of the Schottky heterojunction mechanism.
While photocatalytic materials present a promising potential for degrading ECs in wastewater, their real-world applicability varies. Semiconductors are effective for degrading pharmaceuticals under UV light, but they face problems such as narrow light absorption ranges and recovery issues. Materials with point defects offer improved visible light activity and charge separation, yet they often suffer from issues related to instability and scalability. In this context, heterojunctions further boost performance by reducing electron-hole recombination, making them suitable for treating complex pollutants. However, they are prone to deactivation and are often expensive to synthesize. It is evident that certain materials perform better for specific contaminants (e.g., g-C3N4-based composites are frequently used for antibiotic degradation)[42]. Nevertheless, their practical application requires careful optimization tailored to each type of contaminant and the specific characteristics of the water matrix.
TYPES OF PHOTOCATALYTIC CONFIGURATIONS
Photocatalysts can be employed in different configurations to maximize efficiency, depending on the materials used, the matrix, and the type of targeted pollutants. The most common configurations are immobilized systems and suspended (slurry) systems[50]. When photocatalysis is combined with other techniques or materials, it leads to hybrid systems, such as photoelectrocatalytic (PEC) systems and photocatalytic membrane systems.
Single photocatalysis systems
Immobilized systems
Immobilized photocatalysis (fixed-bed or solid substrate) is one of the earliest and most widely used configurations. In this system, the photocatalyst is fixed onto the surface of a support material, allowing wastewater to flow above or through it. Common support materials include glass, stainless steel, fibers, polymers, silica, ceramic, cellulose, zeolites, metals, photoreactor walls, concrete, aerogels, low-cost substrates, and organometallic frameworks[51,52]. Upon light exposure, the immobilized catalyst activates and degrades pollutants, offering uniform irradiation, easy catalyst recovery, high reusability (lowers operational costs), and suitability for continuous, large-scale wastewater treatment[53,54] [Figure 4].
Figure 4. Schematic of a fixed bed photocatalyst reactor.
Chairungsri et al. immobilized TiO2 on glass and iron beads to study the photodegradation of textile wastewater in a batch reactor[55]. These immobilized systems of TiO2 showed an efficiency of 64% in four hours of irradiation and high reusability in the second cycle. Additionally, Yusuf et al. employed an immobilized TiO2/β-Bi2O3 on borosilicate glass beads for the photocatalytic degradation of diclofenac amide (DCFA) in a recirculating reactor, achieving 90% degradation efficiency after two years of use[56].
However, there are many challenges present to immobilized systems. First, a considerable part of the surface area of the catalyst is occupied by the support material, limiting its ability to absorb photons and participate in degradation reactions. Second, efficiency may be limited by the reduced contact between pollutants and the photocatalyst surface, especially in low-flow conditions. Limited light penetration can hinder photocatalyst activation in deeper layers, lowering efficiency. Finally, surface fouling by organic/inorganic deposits over time further reduces activity[50].
Suspended (slurry) systems
In suspended systems, powdered photocatalysts disperse in wastewater, maximizing surface area and active site availability for efficient degradation. Another advantage is the flexibility of the system, which can be scaled up or down by adjusting the amount of photocatalyst and light intensity. Moreover, suspended photocatalysts have been proven to be quite useful for degrading a variety of organic pollutants, including those present in complex wastewater matrices[57].
The disadvantages of the suspended system are related to post-treatment catalyst recovery. Once the degradation process is complete, separating the photocatalyst from the treated water is difficult and often requires filtration or centrifugation, which increases operational costs and leads to catalyst loss over time. Due to their limited reusability, slurry systems are mostly used in laboratory experiments, pilot studies, or novel research setups rather than in large-scale industrial applications. Furthermore, their efficiency tends to decrease over time due to catalyst deactivation and/or fouling [Figure 5].
Figure 5. Schematic of a slurry photocatalyst reactor.
Al-Tameemi et al. investigated the use of suspended SnO2 as a photocatalyst in a slurry bubble photoreactor for the degradation of COD in petroleum refinery wastewater[58]. The results indicated the photocatalyst’s reusability and the high efficiency of the suspended system (73.16% COD removal in 99 min under optimized conditions) - even suggesting potential for industrial-scale utilization. Shokry et al. used commercial TiO2 to observe the photocatalytic degradation of methyl violet dye[59]. They concluded that proper adjustment of operational parameters enables effective treatment of low-concentration industrial dye wastewater, with peak degradation at neutral pH (7) before declining at higher values. Ng et al. tried to eliminate the need for mechanical stirring, which requires massive amounts of energy for industrial scale-up, by synthesizing a SiO2-cored ZnO photocatalyst[60]. Combined with O2-bubbling to maintain particle suspension, their system Combined 67.6% degradation of MB within 120 min of UV light irradiation, using a catalyst concentration of 0.2 g/L.
Combined (Hybrid) photocatalysis systems
Photocatalytic membrane systems
Photocatalytic membrane systems combine photocatalysis with filtration membranes, enabling simultaneous pollutant degradation and separation. As wastewater flows through the membrane under light irradiation, contaminants degrade while the membrane acts as both a physical barrier and catalytic surface. This system can operate continuously, reducing fouling and clogging due to in-situ degradation. However, long-term reuse remains a challenge as persistent fouling reduces effectiveness, and high material costs limit widespread application[61-63].
Espíndola et al. utilized a photocatalytic membrane system comparing the efficiency of suspended and immobilized TiO2-P25 for oxytetracycline (OTC) degradation in ultrapure water (UPW) and secondary effluent water from an urban wastewater treatment plant (UWW). While the slurry system exhibited higher degradation efficiency, the immobilized system demonstrated better reusability[64]. Nguyen et al. studied the degradation of diclofenac (DCF) in wastewater under visible light irradiation using a submerged photocatalytic membrane reactor (SPMR)[65]. The DCF degradation efficiency was greater with the suspended N-doped TiO2 (32%-42%), whereas the immobilized N-doped TiO2 (31%) membrane resisted fouling more effectively.
PEC systems
PEC systems combine photocatalysis with electrochemical processes by applying an external electrical field to the photocatalyst, which increases its charge separation and overall performance. Upon light irradiation, the active sites of the catalyst generate electron-hole pairs, while the applied electric field assists in the transfer of electrons, thus boosting pollutant degradation. This approach reduces electron-hole recombination rate, improves efficiency, and can be applied to both gaseous and liquid matrices. However, PEC systems come at a high cost due to their complex design and reliance on external energy supply. For that reason, their scalability is limited, making them economically unfeasible for treating large volumes of wastewater. Nevertheless, these systems are being explored for water splitting for hydrogen production[66-68].
Suhadolnik et al. compared the efficiency of a photocatalytic (10%) and a PEC (100%) system in degrading the textile dye Reactive Red 106 in distilled and tap water[69]. Their results showed that photoelectrocatalysis significantly improved efficiency by hindering the recombination of electron-hole pairs. They also found that the diffusion rate is the parameter that limits the design and implementation of a PEC system for industrial wastewater treatment applications. Meng et al. tried developing a three-dimensional photoelectrocatalytic reactor (3D-PER) in which over 95% of the RhB dye was degraded[70]. It was shown that photocatalysis is responsible for degradation reactions, while electrocatalysis contributes to the mineralization of degradation products and by-products. In an effort to minimize the energy needed for the PEC system, Boschetti et al. developed a solar PEC reactor for the degradation of pharmaceutical pollutants[71]. The system achieved an 84% removal efficiency, underscoring its potential for industrial-scale wastewater treatment.
PHOTOCATALYTIC REACTORS
Since conventional wastewater treatment methods seem inadequate to manage the nature of ECs, photocatalysis has piqued the interest of both scientific and engineering communities seeking sustainable waste management solutions. Photocatalysis is considered a cost-efficient, eco-friendly, and widely applicable method[72]. Although a variety of photocatalytic reactors have been developed for experimental purposes, their adaptation for real-world industrial use - particularly for treating large volumes of wastewater at a relatively low cost - remains a major challenge[73]. Effective reactor design is critical for maximum efficiency, which includes ensuring proper mixing of the photocatalyst and wastewater, achieving uniform light distribution, and optimizing photocatalyst utilization[74,75].
Light source type and position
Photocatalytic reactors can utilize either natural (sunlight) or artificial light (lamps). Solar light, being cost-free, is often preferred for industrial applications, although its use is limited to daylight hours and the intensity is variable. Solar reactors can be categorized as either non-concentrated or concentrated, with the latter offering greater efficiency at the expense of increased cost[76,77].
Artificial UV light sources provide more consistent and controllable irradiation and thus offer greater efficiency. However, they are expensive. Common UV light lamps include low-, medium-, and high-pressure mercury lamps, which have relatively short lifespans, high energy consumption, and pose environmental risks after disposal. Due to their longevity and durability, UV-LEDs are increasingly favored. Based on the position of the light source, photocatalytic reactors can be divided into three types: those with internal light sources, those using external irradiation, and those employing light transmitters to direct the radiation[76].
Reactor processing methods
Batch photocatalytic reactors
In batch reactors, photocatalyst and wastewater are mixed without continuous flow, with stirring to ensure uniform contact. After the reaction, residual wastewater, by-products, and catalyst particles are removed[78]. These reactors are simple, affordable, and easy to operate but unsuitable for large-scale applications due to the lack of continuous flow[79,80]. Balarak and Kord Mostafapour used a batch reactor to optimize AMX degradation with NiO nanostructures under UV light[81]. This setup enabled precise control of experimental parameters and reproducibility, achieving 96% degradation efficiency. They also found that increasing UV exposure time enhanced the degradation rate. Krishnan et al. used a batch reactor to study (TiO2)-based photocatalytic degradation of various dyes under different conditions (catalyst dosage, light source, aeration)[82]. Aeration improved degradation efficiency by ~15% under sunlight and ~20% under UV light, with nearly 100% degradation under UV with aeration. The reactor’s simple design further contributed to its cost-effectiveness and user-friendliness.
Continuous photocatalytic reactors
Continuous photocatalytic reactors are designed for large-scale applications and are divided into Continuous Stirred Tank Reactors (CSTRs) and Plug Flow Reactors (PFRs). In CSTRs, the initial reaction conditions are typically transient and unstable but become stable after a short operational period[83]. Given their ability to provide continuous treatment and scalability, Yang et al. studied the degradation of MB using a floating photocatalyst (expanded polystyrene EPS-TiO2) in a CSTR[84]. They compared the degradation kinetics with those of a batch system and found that the CSTR had a higher degradation rate constant (k = 0.0126-0.0172/min) compared to the batch system (k = 0.0113/min), particularly when the wastewater flow rate was low to moderate. Tang et al. achieved the degradation of reactive black 5 dye using TiO2/UV in an annular photoreactor[85]. Ma et al. developed a PRF-based reactor utilizing an anaerobic process for hospital wastewater (HWW) treatment[86]. This system demonstrated operational stability and resilience under challenging conditions over a two-year period, achieving reductions of 85%-90% and 95%-97% in BOD and COD, respectively.
Mechanics
Bed photocatalytic reactors
Recent studies have focused on enhancing the stability of immobilized photocatalysts via physical or chemical bonding, leading to the development of bed reactors. These systems offer lower mass transfer efficiency than suspended ones, making performance highly dependent on catalyst bed surface area[87]. To address this, various bed reactor designs have emerged, with Fixed, Packed, and Fluidized bed reactors being the most common[56,88,89]. Yusuf et al. used a fixed-bed photocatalytic reactor [Figure 6] to DCFA by TiO2/β-Bi2O3 (TB) under UV-LED irradiation[56]. TB was immobilized onto borosilicate beads and the results showcased the potential for refining and optimizing large-scale fixed-bed photoreactors for wastewater treatment applications. The research reported that the degradation rate constant, k, was 0.0071 at the beginning of the treatment and increased to 0.0085 after 2 years, indicating sustained catalytic activity over time. Wang et al. also used a fixed-bed system to investigate the photocatalytic degradation of RhB using BiFeO3/3DOM-TiO2-x[90]. In addition to achieving excellent degradation efficiency (89.2%) with the synthesized catalyst, the study provided valuable insights into the kinetics of RhB degradation in a fixed-bed reactor system.
Figure 6. Simplified schematic representation of a flat-plate Fixed Bed Photoreactor.
Jaiswal et al. explored the use of a packed-bed system for the photocatalytic degradation of p-cresol under UV light[89]. The degradation efficiency was 98.43% at an initial concentration of 700 mg/L after 120 h [Figure 7].
Figure 7. Simplified schematic representation of an annular Packed-Bed Photoreactor.
Thin-film photocatalytic reactors
Thin-film reactors, suitable for lab-scale applications (50-500 mL), efficiently degrade concentrated solutions due to the minimal light penetration depth required. These systems employ an inclined flat substrate coated with a photocatalyst, enabling gravity-driven or reverse wastewater flow. Precise reaction control is achieved via self-priming pumps that mix air and fluid, enhancing distribution. The thin-film design ensures optimal light exposure and catalytic contact[76,91]. Kahveci et al. synthesized unary ZnO, CuO, and CdO, as well as a ternary ZnO-CuO-CdO composite, as sunlight-driven photocatalysts for use in thin-film reactors to degrade MB[92]. The ternary ZnO-CuO-CdO composite showed superior performance, achieving 70% degradation in 20 min.
Membrane photocatalytic reactors
Photocatalytic membrane reactors are divided into three types: (1) slurry-based systems with separate filtration; (2) integrated membranes for catalyst retention; and (3) reactors with immobilized photocatalysts, which are increasingly adopted, with some already commercially available[93,94]. Despite their advantages, challenges include light scattering, structural complexity, high costs, and membrane fouling[93,94]. Recent advances, such as photocatalytic ultrafiltration membrane reactors, show promising potential[95]. Golshenas et al. developed a TiO2/γ-Al2O3-modified ceramic membrane for synthetic oily wastewater treatment under UV irradiation, achieving a high degradation efficiency of 90%, with industrial-scale potential[96]. Similarly, Deepracha et al. fabricated a ceramic-based membrane by depositing TiO2 on the macroporous (3 µm) side of an asymmetric Al2O3 membrane combined with smectite, achieving the degradation of MB and phenol under UV light, demonstrating the potential for use in industrial continuous-flow reactors[97]. Gomaa et al. designed a SPMR featuring a superhydrophilic ZnO NP-coated layer for simultaneous oil removal and pollutant degradation, under LED-UV light illumination[98]. The oil removal efficiency was approximately 99%, while MB degradation was 86%.
Cascade photocatalytic reactors
Cascade reactors are not a distinct class of photocatalytic reactors but rather a configuration that combines multiple types in a sequential design where effluent undergoes progressive treatment as it flows from one stage to the next[91]. Kubiak and Cegłowski developed a cascade continuous-flow photoreactor for the photocatalytic degradation of sulfamethoxazole (SMX) in HWW[99]. By integrating a carbon sponge-based TiO2 photocatalyst with a cascade system, they achieved 80% degradation efficiency, reduced energy consumption, and demonstrated potential for industrial-scale wastewater treatment in real-world applications.
Closed-loop step photocatalytic reactor
The closed-loop step photocatalytic reactor recirculates wastewater for continuous photodegradation, minimizing catalyst loss and energy use. The system uses specific light sources at various wavelengths or intensities to drive the photoreactions at each stage[91]. Zeghioud et al. utilized a pilot-scale closed-loop step photocatalytic reactor to investigate the degradation of flumequine (FLU), oxolinic acid (OA), and nalidixic acid (NA)[100]. Using TiO2 impregnated on cellulosic paper and activated by UV light, they found that the photocatalytic rates were highest in mono-compound systems, followed by the ternary and then binary mixtures (attributed to competitive adsorption of antibiotics on the catalyst’s active sites). Buehler et al. developed a solar-powered, closed-loop laundry facility (LaundReCycle) for laundry wastewater reuse[101]. Operated over five weeks, the system recovered 69% of washing machine effluent and achieved 92% COD removal per cycle. Modeled data showed that 80% of the energy was internally reused, with 30% overall energy self-sufficiency. However, further optimization is needed to meet hygiene standards.
Micro-photocatalytic reactors
Micro-photocatalytic reactors employ photocatalysis at the microscale to accelerate chemical reactions under light exposure. These reactors operate at dimensions ranging from micrometers to millimeters, enhancing the surface-to-volume ratio and improving both light absorption and mass transfer efficiency[102]. Their main advantages include shorter reaction times and the integration of microfluidic systems, where fluids flow through narrow passageways, enabling efficient mixing and uniform exposure to both the photocatalyst and the light source[103].
Two common types of micro-photocatalytic reactors are microchannel reactors and microcapillary tube reactors, both of which can be integrated into compact systems[104,105]. Microcapillary reactors use extremely small tubes for wastewater transport, offering a higher surface-to-volume ratio and a compact design, though scalability is limited. Moreira and Puma used helical microcapillary film (MCF) photoreactors for the degradation of pharmaceuticals in water by UV/H2O2[106]. The helical design enhanced mixing, improving degradation efficiency from 4.4% to 37.9%. Hakki and Allahyari compared the degradation of methyl orange (MO) by a microcapillary photoreactor, a conventional batch photoreactor, and a tubular flow photoreactor[107]. All systems were coated with CuPc-Bi2O3-ZnO and operated under visible LED light. The microcapillary reactor achieved the highest degradation efficiency (90.83%), followed by the tubular flow photoreactor (65.33%) and the batch photoreactor (58.6%). Their study also found that increasing the tube diameter while decreasing its length further improved degradation performance.
Microchannel reactors, with immobilized photocatalytic NPs in multiple channels, offer enhanced mass transfer, design flexibility, and suitability for large-scale, multi-step reactions[108]. Yusuf et al. proposed a computational fluid dynamics (CFD) model for the photocatalytic degradation of 4-nitrophenol using immobilized N-TiO2/rGO in a continuous-flow microchannel reactor with nine parallel rectangular microchannels[109]. The degradation rate of 4-nitrophenol was observed to be proportional to the Ceq, allowing for straightforward estimation of rate constants. Jia et al. developed a spiral microchannel photocatalytic reactor using suspended TiO2 to degrade OTC, achieving a 99.33% degradation efficiency under optimal conditions (UV-4, flow rate 1.966 mL/min, initial OTC concentration of 20 mg/L, TiO2 dosage of 1 g/L)[110].
Other types of photocatalytic reactors
Various other types of photocatalytic reactors exist, including hybrid designs that combine multiple configurations, such as flat-plate, spinning disk, rotating, Taylor vortex, optical fiber, biofilm, and coupled electrochemical reactors. Among them, spinning disk and rotating photocatalytic reactors are notable, as they are mainly used for large-scale wastewater treatment[111].
Spinning disk reactors offer high efficiency; however, scaling them up to handle massive amounts of wastewater is quite challenging. As a result, they are generally more suitable for medium-scale systems[112]. Fallahizadeh et al. developed a thin-film spinning disk photocatalytic reactor coated with a visible light-activated Fe3O4@SiO2-NH2@CuO/ZnO composite for the degradation of AMX[113]. Under optimal conditions (60 min irradiation, 350 rpm, 0.9 L/min flow, pH 5, 20 mg/L AMX), the system achieved a high degradation efficiency of 98.7%, and over 69.95% COD and 61.2% TOC were also removed. Ghasemi et al. reported the successful degradation of MB by two novel 3D-printed components combined with UV-illuminated glass discs coated with TiO2[114]. Under optimal conditions (pH of 6.05, disc rotation speed of 22.35 rpm, initial dye concentration of 3.15 × 10-5 M, residence time of 7.98 h, and the number of NP layers set at 3.99), the system achieved a 96.63% removal rate.
In contrast, Rotating Photocatalytic Reactors utilize a different rotating element (drum or cylinder), which is coated with photocatalytic material. Due to their three-dimensional design, these reactors offer a larger surface area[115], making them more suitable for large-scale applications. However, their efficiency can be limited by fouling and maintenance challenges[73].
Energy and environmental costs of photocatalytic reactor systems
Photocatalytic wastewater treatment faces significant scale-up challenges, primarily due to the high costs and energy demands associated with light sources, photocatalysts, and photoreactors[32]. These systems often require specialized materials with high UV transmittance and complex designs to ensure uniform light distribution and effective interaction between light, catalyst, and pollutants[77,84,88]. Large-scale reactors also demand extensive surface areas or advanced configurations (e.g., thin-film or immobilized systems), which significantly increase both capital and maintenance expenses[60,78]. While costs vary depending on the specific application and treatment objectives, a common limitation remains: the need for a sustainable, low-cost, efficient, and reliable light source to drive the system[73,87].
Light sources play a critical role in both treatment performance and operational cost. Artificial UV light offers stable and controllable intensity but requires high energy input, whereas natural sunlight is free and renewable but fluctuates with weather conditions, location, and time of day, reducing reliability in outdoor systems[34,57,71]. To overcome this, reactor designs can incorporate solar concentrators, light-storage technologies, or photocatalysts that remain active under diffuse or low-intensity light. Furthermore, optimizing operational parameters and designing photocatalytic systems that simultaneously generate energy, such as H2 production or photoelectrochemical elements (electricity production), can indirectly lower both sustainability and energy costs[37,45,58,59,71]. Ultimately, the ideal photoreactor for large-scale wastewater treatment must balance energy efficiency, cost-effectiveness, and consistent operational performance[91].
REMOVAL OF COMMON AND EMERGING POLLUTANTS
While WWTPs are highly effective in reducing some common contaminants, the majority of common, newly emerging, and POPs[5] often make their way through, bioaccumulate, and disrupt the environment, also posing a health hazard for human health. These contaminants originate from various sources, including urban (municipal, hospital) sewage, industrial wastewater (e.g., textile, tannery, pharmaceutical, petrochemical), and agricultural runoff. The nature of these organic pollutants spans over a range of categories.
Inorganic pollutants
The categories of inorganic pollutants identified as wastewater contaminants include acids, typically released through industrial effluents (e.g., H2SO4, HNO3), bases, primarily generated by the food processing industry and municipal sewage (e.g., NH3), and chemical waste/heavy metals (HMs), which end up in wastewater via various industrial processes and agricultural activities[116]. Among these, HMs are considered the most toxic inorganic compounds, posing significant threats to living organisms, particularly at high concentrations, as they tend to bioaccumulate through the food chain and can lead to mortality[117]. Photocatalysis has emerged as an efficient method for removing HM ions from wastewater. This process works by reducing/oxidizing metal ions to lower/higher valence states or to zero-valance forms, thereby rendering them insoluble in water. During photocatalysis, ROS generated by the photocatalyst upon photon absorption facilitate these redox transformations. This approach offers the following benefits: minimal production of hazardous by-products and reduced processing time[118,119] [Table 1].
Photocatalytic removal of inorganic and radioactive [uranium (VI)] pollutants
| Contaminant | Photocatalyst | Light source | Reusability | Max. efficiency | Ref. |
| Cr (VI) | ZFO and MFO | UV-vis light irradiation, 300-W Xe lamp | 3 h cycles | 99% (MFO-6-ZFO, 3 h) | [120] |
| 30% MnFe2O4/g-C3N4/H2O2 | 350-W Xenon lamp | 5 cycles | 100%, 1 min | [121] | |
| WS2/PANI nanocomposite | Xenon arc lamp | Up to 4 cycles | 99%, 120 min | [122] | |
| Cu (II) | TiO2-Ni-NiO Schottky-heterojunction | Visible light (300-W xenon lamp) | 5 cycles (< 48%) | 86.7 %, 1 h | [123] |
| Hg (II) | Mesoporous 3% CuO/ZnO | Visible light (300-W xenon lamp) | 5 cycles (no significant decline) | 100% in 60 min | [124] |
| Cd (II) | Bentonite/TiO2 and Bentonite/ZnO | UV-A lamp | - | ~70% (B/TiO2), ~60% (B/ZnO) | [125] |
| Pb (II) | TiO2 NPs | UV light irradiation | - | 82.53% in 12 h | [126] |
| As (III), Cr (VI) | HCS@Fe2O3 | 100W xenon lamp | 5 cycles | 100% [Cr (II)], 70% [As (III)], 120 min | [127] |
| Cu (II), Ag (I), Pb (II) | ZnO rod-shape particles | UV light irradiation | - | > 85% in 1 h | [128] |
| Cd (II), Pb (II) | CS/Ag thin films | Natural sunlight | - | 89% [Cd (II)], 88% [Pb (II)] | [129] |
| U (VI) | TiO2/PDA@Fe3O4 | 175 W UV high-pressure Hg lamp | 6 cycles | 100%-83.5%, in 60 min | [130] |
| ZnFe2O4/g-C3N4 S-scheme | Visible LED light irradiation | 5 cycles | 94.62%-90% | [131] | |
| ZnFe2O4 rods | Visible light (300-W xenon lamp) | - | > 95% in 40 min | [132] | |
| MoS2/P-g-C3N4 nanocomposites | Visible light (300-W xenon lamp) | - | 99.75% in 60 min | [133] | |
| Mesoporous g-C3N4 | Visible light (300-W xenon lamp) | 5 cycles | 100% in 20 min | [134] | |
| Defective-enriched K3PW12O40@WS2 heterojunction | Simulated sunlight (300 W Xe lamp) | 4 cycles | 95.6% (60 min) | [135] | |
| Z-Scheme heterojunction Cu2(OH)3F/Bi2WO6 | Simulated sunlight (300 W Xe lamp) | 5 cycles | 90.9% | [136] | |
| CdS_H/CdS_C@PB with homo/heterojunction | Visible light (300 W Hg lamp) | 8 cycles | 98.5% (60 min) | [137] | |
| CdS@NiCr-LDH Z-scheme heterojunction | 25 W visible LED light | 3 cycles | 2,246.14 mg/g of U (VI) | [138] | |
| Low-level con. U (VI) | UPC-H4a | Simulated sunlight (300 W Xe lamp) | 5 cycles | 98.18% simulated solution, 66,14% real wastewater (120 min) | [139] |
Ma et al. employed a Z-scheme photocatalyst system gC3N4-rGO-TiO2 for NH3-N removal from coking wastewater, achieving over 90% efficiency across six continuous cycles[6].
Chen et al. synthesized a conjugated microporous polymer (CMP), designated TT-TPP, to selectively remove uranium (VI) from wastewater under visible light activation. Their results demonstrated removal efficiencies of over 89.9% in a 3 M NaNO3 solution and 97.6% in simulated radioactive wastewater[140].
Organic pollutants
Photocatalysis has emerged as an eco-friendly and highly efficient method for degrading diverse organic pollutants in wastewater, including dyes, petrochemicals, phenolics, fluorinated compounds, microplastics, pesticides, and pharmaceuticals. Dyes from textile and food industries reduce oxygen levels, block light, and bioaccumulate, with compounds like MB and RhB effectively degraded by photocatalysis[141]. Petrochemical pollutants such as benzene, xylene, and PAHs, released from refining and plastic production, are toxic and carcinogenic but can be photocatalytically removed[142,143]. Phenolics (e.g., chlorophenols, bisphenols) from oil, plastics, and pharma industries pose serious ecological and health threats; photocatalysis efficiently degrades them[144]. Micro- and nanoplastics from consumer goods persist in water bodies and organisms; conventional methods fail, but photocatalysis offers a promising route[145]. Pesticides and related agrochemicals contaminate ecosystems and foster resistance; photocatalysis has demonstrated effective removal capabilities in such cases[146]. Pharmaceuticals, including antibiotics and hormones, remain after tertiary treatment, disrupting endocrine systems and contributing to resistance. Their complete degradation via photocatalysis is essential[147-149]. Several examples of photocatalytic degradation of various organic pollutants are presented in Table 2.
Photocatalytic degradation of organic pollutants
| Type | Contaminant | Photocatalyst | Light source | Reusability | Max. efficiency | Ref. | |
| Microplastics | Microplastics | ZnO NPs | UV light lamp | - | - | [145] | |
| High-density polyethylene (HDPE) | N-TiO2 semiconductor | Visible light (400-800 nm) | - | 6.40% in 18 h | [150] | ||
| Polypropylene microplastics | ZnO NPs | Solar light | - | Degradation within 24 h | [151] | ||
| PS and PET microplastics | Fe2O3 (FeO) microparticles | NIR light (980 nm) | - | 97%-98% in 80 min | [152] | ||
| Low-density polyethylene (LDPE), polypropylene (PP), and polyamide (PA) | CuO/BiVO4 nanocomposites | Visible light (8 h/day for 14 days) | - | 100% with more H2O2 and longer contact time | [153] | ||
| Polylactic acid (PLA) | C3N4/Bi12O17Cl2 (CN/BOC) on a polyacrylonitrile (PAN) membrane (CN/BOC/PAN) | Visible light irradiation | 5 cycles | Maximum product conc. At 12 h | [154] | ||
| PFAS | PFOA | FeOCl/(Bi-MOF-BiOCl)100 with oxygen defects | UV light (240 min) | - | > 99.9% PFOA in 30 min and 64.3% defluorination efficiency | [155] | |
| MIL-125-NH2 | UV light | 3 cycles | > 98.9% in 24 h | [156] | |||
| Perfluorononanoic acid (PFNA) | UV light (365 nm) | - | 100% in 22 h | [157] | |||
| Phenolic compounds | Phenolic compounds | gAu/RGO nanocomposite | Visible light irradiation | 6-7 cycles | 95.78% in 130 min | [158] | |
| Mesoporous TiO2 NPs | UVA lamps | 5 times | 100% in 140 min | [159] | |||
| Phenol | LaVO4/MCM-48 nanocomposite | Visible light | 6 cycles | 99.9% in 3 h | [160] | ||
| CeO2 NPs synthesized at dif. temp. | UV lamp | 3 cycles | 62% in 5 h | [144] | |||
| TiO2/SR and ZnO/SR | Visible light | YES | 87% (TiO2/SR), 70% (ZnO/SR) | [161] | |||
| Petrochemical wastewater | Petrochemical wastewater | Co-TiO2/zeolite and K2S2O8 | UV light (254 nm) | YES | 93.40% | [142] | |
| Petroleum wastewater | g-C3N4/Ce-ZnO | Simulated and direct sunlight radiation | - | 88% simulated irradiation, 52% direct sunlight | [162] | ||
| Crude oil wastewater | MIL-101(Fe) | UV light irradiation | > 5 times | 94.73% in 30 min | [163] | ||
| Polycyclic aromatic hydrocarbons (PAHs) | NiO NPs | UV and VIS light for 300 min | - | 70% (UV light), 64% (sunlight) | [164] | ||
| N-TiO2/ASC-PVP | Visible light | 4 cycles | 98.6% in 120min | [165] | |||
| 2,6-dichlorophenol (2,6-DCP) | Fe/Zn@biochar composite | Parabolic solar collector system | - | > 90% in 4 h | [143] | ||
| Dyes | MB | FeWO4@N-TiO2 | Visible light (λ ≥ 420 nm) | 3 times | 77% in 2 h | [141] | |
| Acid violet 7 dye (AV7) | ZnS-SnO2 (60ZS) heterostructure/nanocomposite | Visible light with ~3% UV radiation | 5 cycles | 94.71% in 60 min | [166] | ||
| Tartrazine Y5 | Carbon-containing Cu-based material (Cu@C) | CPC, artificial solar light and sunlight | - | 100% for solar CPC in 10 h | [167] | ||
| MB | SiO2@WO3@Fe3O4 | Visible light irradiation | 4 cycles | > 90% in 120 min | [168] | ||
| CR | BiOCl–ZnFe2O4/CNTs | Natural sunlight | 4 cycles | 95.27% in 2 h | [169] | ||
| Herbicide | Pendimethalin | BiOCl–ZnFe2O4/CNTs | Natural sunlight | 4 cycles | 88.6% in 2 h | [169] | |
| Fungicide | Imazalil | Open-cell ceramic foams covered with TiO2 | Natural sunlight and UVA (lab) | 5 cycles | 95% in 30 min | [146] | |
| Pesticide | 2,4-dichlorophenoxyacetic acid (2,4D) | MI TiO2/2,4D | UV light irradiation | 5 cycles | 74% in 240 min | [170] | |
| Insecticides | Thiamethoxam, imidacloprid, acetamiprid and thiacloprid | TiO2 with Na2S2O8 | Natural sunlight | YES | 100% (thiamethoxam), ~99.7% (imidacloprid), ~97% (acetamiprid), ~99.97% (thiacloprid) | [171] | |
| Pharmaceuticals | Antibiotics | TC | PC/GO-CTS (protonated carbon/graphene oxide-Cu3SnS4) | UV light | - | 95.6% in 45 min | [172] |
| Spiramycin (SPY) | ZnO | UV light | - | 95%-99% in 80 min | [173] | ||
| Ofloxacin (OFL) and SMX | Bi24O31Br10 (BOB) | Visible light irradiation | 5 cycles | OFL > 95.5% SMX 57%, in 2 h | [174] | ||
| Psychiatric drugs | Amisulpride | g-C3N4 | UVA light radiation | - | 86% in water (60 min) | [175] | |
| Sertraline hydrochloride (SERT) | ZnO-NPs | UV light irradiation | - | 98.7% in 30 min | [176] | ||
| Bupropion (BUP) | TiO2 | UV light irradiation | - | 100% in 90 min | [177] | ||
| CBZ | C3N4/Bi12O17C12 (CN/BOC) on a PAN membrane (CN/BOC/PAN) | Visible light irradiation | 5 cycles | 100% in 180 min (for 19 of the 20 pharmaceuticals) | [154] | ||
| Cytostatics | Etoposide (ETO) | TiO2 Degussa P25 | UV light irradiation | - | 40% of ETO at pH 5 and 9 in 2 h | [178] | |
| Kabi cytarabine | Bismuth iron oxide (BFO) | UV light irradiation | 5 cycles | 97.9% in 120 min | [179] | ||
| 5-fluorouracil | Ni3(VO4)2/ZnCr2O4 Z-scheme | Visible light irradiation | 6 cycles | 98.9% in 200 min | [180] | ||
| Hormones | Estriol (E3) hormone | TiO2/Nd polyurethane nanofibers | UV and visible light irradiation | 2 cycles | ~90.2% at pH = 7, 120 min under UV | [181] | |
| Estrone hormone | Silica-supported g-C3N4/WO3 | UV and visible light irradiation for 3 h | 7 cycles | 100% under UV, 96% under visible | [182] | ||
| Lipid regulators | Gemfibrozil | Ag@mpgC3N4 | - | > 5 cycles | > 90% in 60 min | [183] | |
| Anti-histamines | Chlorpheniramine | XFe1.0 (Fe doped carbon material) | Solar light irradiation | 2 cycles | 82%-91% for XFe1.0 | [184] | |
| Anti-inflammatories | Ketorolac tromethamine | Zn0.5Cd0.5S/MoS2 S-scheme | Natural solar light | 5 cycles | 92.54% in 150 min | [185] | |
| Ibuprofen (IBP) | Bi24O31Br10 (BOB) | Visible light irradiation | 5 cycles | IBP > 95.5 % in 2 h | [174] | ||
| Paracetamol (aka acetaminophen, APAP) | NASICON@C | Visible light irradiation | 5 cycles | 99.7% in 30 min | [186] | ||
| Antivirals | Lopinavir (LOPI), ritonavir (RITO) | Ni-doped ZnO/SiO2 nanocomposites | UV light irradiation | 4 times | 100% of RITO in 60 min (by all) and 100% of LOPI in 30 min (by 6-Ni-ZnO/SiO2) | [187] | |
| Multiphase photocatalysts (WU and WW) | Visible light irradiation | 3 cycles | 95% of RITO (15 min) 95% of LOPI (60 min) | [188] | |||
| Favipiravir (FAV) | Bi24O31Br10 (BOB) | Visible light irradiation | 5 cycles | FAV > 95.5 % in 2 h | [174] | ||
| Beta-blockers | Metoprolol (Met), propranolol (Pro) | Biochar-doped TiO2 | Simulated visible light for 60 min | 3 times | 70% (Pro) 60% (Met) | [189] | |
| Atenolol (AT), metoprolol (ME) | ZnO NPs/Zeolite and TiO2 NPs/Zeolite | UV light for 60 min | 5-6 times | 83.2% of AT (TiO2 NPs/Zeolite) and 83.1% of ME (ZnO NPs/Zeolite) | [190] | ||
However, concerns remain regarding the safety of the treated effluent[147,176]. More specifically, incomplete mineralization of target compounds can lead to the formation of intermediate by-products, some of which may be more toxic than the original pollutants[148,153,178]. These degradation products, such as short-chain organic acids, aldehydes, or nitrosamines, require thorough identification and assessment[177]. Additionally, in slurry systems, the potential release of catalyst NPs poses a risk of nanotoxicity to aquatic life[50]. To address these problems, treated effluents should undergo chemical analysis and ecotoxicity tests[175,191]. Additionally, incorporating post-treatment steps, such as activated carbon filtration or biological polishing, or integrating complementary treatment methods (as discussed in Section “COMBINED TECHNOLOGIES”) can further improve safety. It is essential that the final effluent complies with regulatory standards for toxicity and contaminant levels. Thus, a comprehensive safety evaluation is critical before discharging photocatalytically treated water into the environment[191].
Biological pollutants
Disinfection represents a crucial final step in wastewater treatment before its release into the environment and re-entry into the water cycle. Conventional WWTPs typically use chlorination, and less commonly ozonation or UV irradiation, to inactivate various pathogens[191]. However, these methods are not always cost-effective and often fail to completely eliminate the most resistant microorganisms (bacteria, fungi, parasites, viruses, etc.), particularly those present in HWW[192,193]. Escherichia coli, Enterococcus, Pseudomonas, Aspergillus fumigatus, Candida, norovirus, sapovirus, and enteric adenovirus are a few of the common pathogenic microorganisms in wastewater that can cause diarrhea, respiratory system infections, and gastrointestinal diseases in humans[194].
A highly dangerous subcategory of biological pollutants includes antibiotic-resistant bacteria (ARB), which are found in relatively high concentrations in HWW. This comes as a result of the extensive use of antibiotics, which drives bacterial evolution toward expressing antibiotic resistance genes (ARGs) and producing antibiotic-resistant proteins (ARPs). These proteins employ various resistance mechanisms, including reduced permeability, increased efflux, target site mutation or protection, and direct antibiotic modification through hydrolysis or chemical group transfer[195]. Furthermore, it has been proven that certain advanced treatment methods are able to inactivate ARBs even in complex wastewater matrixes[196,197] [Table 3].
Photocatalytic inactivation of bacteria, fungi, parasites, viruses, phage and ARBs/ARGs
| Type | Contaminant | Photocatalyst | Light source | Reusability | Max. efficiency | Ref. |
| Bacteria | E. coli and S. aureus | α/β-Bi2O3 composite | LED lamp (430-800 nm) | 3 cycles | 99% E. coli and 98% S. aureus, in 240 min | [198] |
| Escherichia coli, Staphylococcus aureus, Enterococcus faecalis | Pd-loaded BiFeO3 micro composites | Visible light | 6 cycles | 100% in 180 min | [199] | |
| Pseudomonas aeruginosa | TiO2 | UVA irradiation for 60 min | - | Inactivated by 4 log units | [200] | |
| Escherichia coli and Staphylococcus aureus | WCF@PCN-222/Ag2O-Ag | Visible light for 90 min | Outstanding reusability | 99.99% (inactivated by 6 log units) | [201] | |
| Pathogens in UWW effluent | Gd-BiVO4 | UVA-LED lamp (370 nm) | - | Almost completely (180 min) | [202] | |
| Fungi | Fusarium equiseti | TiO2 | UV light for 4 h/day | - | 50% in 14 days | [201] |
| Aspergillus niger | Pd-loaded BiFeO3 micro composites | Visible light irradiation | 6 cycles | 100% in 180 min | [199] | |
| Parasites | Cryptosporidium parvum | Fe2+/H2O2 | Solar radiation, 4 and 6 h | - | 96.32% (4 h) and 93.61% (6 h) | [202] |
| Giardia lamblia | H2O2 | UV-LED (255/280/400 nm) | - | 92% in one sample | [203] | |
| Virus | Influenza virus | TiO2 NP and nanotubular surfaces | UV light irradiation λ max = 365 nm, 2 h | - | - | [204] |
| Human adenoviruses (HAdVs) | g-C3N4/WO3/biochar | 6-h visible light irradiation | - | 5-log complete inactivation | [205] | |
| Phage | Viral surrogates (phage MS2) | g-C3N4@COF | 60 min visible light irradiation | 4 cycles | 6 logs of MS2 removal | [206] |
| Bacteriophage MS2 | rGO-supported g-C3N4 (7.5% GCN) | λ > 400 nm and λ > 305 nm | - | 3.20 log (λ > 400 nm) and 3.27 log (λ > 305 nm), in 2 h | [207] | |
| PhiX174 bacteriophage | Cu-doped TiO2 nanorods | Visible light (150 W Xe lamp) | 5 cycles | Inactivation rate 0.42 log inactivation/min | [208] | |
| Bacteriophage f2 | Cu-TiO2 nanofibers | Visible light (400 nm) | 4 cycles | 100% in 2 h | [209] | |
| Lactococcus phages | Photocatalytic paint (with carbon-doped anatase TiO2) wall photoreactor coating | Visible light (360-720 nm) | - | 100% in 1.5-5 h (for 8 phages) | [210] | |
| ARBs/ARGs | Shigella flexneri HL | 60%-BiVO4/EAQ Z-heterojunction | Simulated sunlight (500-W xenon lamp) | YES | 100% inactivation in 150 min | [211] |
| Methicillin-resistant Staphylococcus aureus (MRSA) | Cu-BiOCl with nanosheets assembled into spheres | 500 W Xenon lamp (λ ≥ 290 nm) | - | 3.5 log units of removal | [212] | |
| E. coli DH5α with blaTEM-1 and tetA in plasmid pWH1266 and its plasmid DNA carrying eARG | Urea-based g-C3N4 | Simulated solar irradiation | - | 6.2 log units reduction (100%, in water) and 4.3 log units reduction of ARB (real wastewater) | [213] | |
| tetA and ampC ARGs in ARBs | Ag/TiO2/graphene oxide (GO) | Xe light irradiation with a 420 nm cut-off | - | 100% removal efficiency of both ARGs under optimal conditions | [214] | |
| Shigella flexneri HL | MWCNT/M-BiVO4/T-BiVO4 | Simulated solar irradiation | 4 cycles | 100% inactivation by C/BVO-1 in 180 min | [215] |
COMBINED TECHNOLOGIES
To improve photocatalysis and pollutant removal in wastewater treatment, researchers are exploring integrated approaches combining multiple technologies. Studies have merged these with photocatalysis to boost efficiency, minimize limitations, and enable energy recovery[216,217].
The combination of photocatalysis with AOPs, such as ozonation and Fenton/Fenton-like reactions, has been extensively studied for its potential to enhance wastewater treatment efficiency. Photocatalytic ozonation offers significant advantages over individual methods, as it improves the mass transfer limitations of fixed catalysts and generates a higher concentration of reactive •OH. Studies such as those by Xiao et al. demonstrated the high degradation capability of visible light-activated graphitic carbon nitride (g-C3N4) in a light/O3/photocatalyst system. More specifically, using g-C3N4 for photocatalytic ozonation (vis/O3/g-C3N4) causes the mineralization of oxalic acid (model pollutant) at a rate 95.8 times higher than the sum of photocatalytic oxidation (vis/O2/g-C3N4) and ozonation alone (100% degradation in 2 and 20 min, respectively)[218]. Rodríguez et al. investigated the degradation of N, N-diethyl-m-toluamide (DEET) using TiO2-supported mullite ceramic foams and glass Raschig rings, showing that photocatalytic ozonation enhances carboxylate formation and mineralization rates with minimal interference from the wastewater matrix, resulting in a 25% increase in efficiency[219]. The combination of photocatalysis with photo-Fenton reactions enables catalyst regeneration, higher efficiency, and operation across a broader pH range. Jiad and Abbar applied a photo-Fenton catalytic system with TiO2, achieving significant COD reduction in petroleum refinery wastewater (68.122% and 91.26% removal efficiency for photocatalysis and the combined process, respectively, showcasing an increase of ~23% in removal rate)[220]. Xu et al. developed a photocatalytic fuel cell (PFC) system integrating electro-Fenton (WO3/W photoanode) and Fe@Fe2O3/carbon cathode, demonstrating enhanced organic pollutant degradation alongside simultaneous energy generation. This system achieved 91.6% degradation, significantly outperforming the individual methods (77.2% for EF and 68.3% for PC). The system’s performance was highly pH-dependent: optimal degradation (91.6%) occurred at pH 3, while efficiency declined at both lower (82.7% at pH 1.5-3) and higher pH values (71.8% at pH 4.5)[221].
The integration of microalgae systems with photocatalysis enhances pollutant degradation by producing oxygen, promoting ROS formation, and preventing catalyst fouling. Additionally, they absorb nutrients, degrade pathogens, and improve wastewater treatment efficiency[222]. Li et al. demonstrated a sequential system where photocatalysis degraded antibiotics, while microalgae further reduced toxicity. However, the effectiveness varied depending on the microalgae species used. In their study, C. pyrenoidosa achieved a 98% higher degradation efficiency of photocatalysis by-products compared to S. obliquus[223]. Similarly, Lu et al. reported improved enrofloxacin degradation efficiency (rising from 57% to 64%) in a two-step system combining photocatalysis with microalgae treatment[224]. de Souza et al. also applied a sequential photocatalysis-microalgae system to treat Cr(VI)-contaminated wastewater, achieving a 7%-20% higher degradation efficiency compared to either method used individually[225]. Another promising hybrid method is sonophotocatalysis, where ultrasound waves enhance photocatalysis by generating ROS, increasing mass transfer, and improving catalyst regeneration[226]. Schieppati et al. showed that sonophotocatalysis degraded herbicide Isoproturon faster than photocatalysis alone, achieving complete degradation within 60 min, compared to 240 min with photocatalysis alone[227]. Karim and Shriwastav designed a multi-frequency reactor for antibiotic degradation and found that simultaneous application of ultrasound and visible light achieved 44% degradation efficiency in 90 min, compared to 41.4% with sonocatalysis[228]. However, the high cost of sonophotocatalysis remains a challenge. To address this, Khalegh and Qaderi developed a cost-effective design for textile wastewater treatment, calculating that under optimal conditions, complete removal of 1 mg of MB in 13.695 min would cost only 0.001¢[229]. Constructed wetlands (CWs) coupled with photocatalysis offer another sustainable treatment method[230]. Nguyen et al. demonstrated that combining CWs with TiO2-based photocatalysis increased organic matter degradation to levels compliant with THM standards if chlorination was applied to the treated water[231]. Chen et al. compared three CW-photocatalysis configurations for removing ARGs, finding that CWs effectively removed intracellular ARGs, while photocatalysis targeted extracellular ARGs. When combined, the system achieved a 99.8% removal efficiency, compared to only 50%-60% with each method alone[230].
Dindaş et al. investigated the treatment of pharmaceutical wastewater using a combination of electrocoagulation (EC), electro-Fenton (EF), and photocatalytic oxidation processes. Their results highlighted the main role of photocatalysis, especially when paired with EF, reaching a 64% removal rate compared to 26.84% for EF alone and 39.9% for the combination of photocatalysis and EC[232]. Similarly, Aldana et al. integrated membrane filtration, activated sludge, and solar photocatalysis, and compared various membranes for the treatment of table olive processing wastewater, achieving 60%-90% pollutant removal[233]. Kusworo et al. fabricated a UV-light-driven photocatalytic nanohybrid membrane (PVDF/GO-NiFe@SiO2), coupled with bentonite adsorption and an ozonation process, for effective and sustainable textile wastewater treatment, increasing removal efficiency from 55.11% to 97.22%[234]. Majumder et al. employed a multi-stage system comprising a bed-type bio-reactor sedimentation tank (MBSST), an aerated horizontal flow constructed wetland (AHFCW), and photocatalysis to treat real HWW spiked with carbamazepine (CBZ). MBSST and AHFCW in combination removed 85% of COD, 93% of TSS, 99% of NH3, but only 30% of CBZ. However, applying photocatalysis to the effluent from the AHFCW raised CBZ removal to 85%. Furthermore, the intermediate products (IPs) generated through photocatalysis were found to be less toxic than those formed during the earlier biological treatment stages[235].
Overall, combined technologies integrating photocatalysis with membrane filtration, electrochemical oxidation, Fenton reactions, and biological treatments are gaining momentum in industrial wastewater remediation, as they offer synergistic advantages, such as enhanced degradation efficiency and broader pollutant removal spectra[230,231]. However, they often involve high capital, material and maintenance costs due to complex configurations, high energy demands, and issues such as membrane fouling and catalyst deactivation[217,233,234]. Despite these limitations, such integrated approaches can be cost-effective for treating high-strength wastewater (e.g., those from dye, pharmaceutical, or petrochemical industries), especially when solar-driven photocatalysis reduces energy consumption. Optimization of operational parameters (e.g., pH, photocatalysis duration, sequence and combination of methods, etc.) can further improve treatment efficiency and lower costs[224,227,233,235]. Additionally, coupling photocatalysis with PFCs offers indirect cost savings by generating electricity during the treatment process[221]. Although scalability remains a challenge, modular and application-specific system designs are gradually facilitating industrial adoption and offering relatively stable operation, as previously discussed[226,229]. Several studies, such as that by Khalegh and Qaderi, have also examined the cost-effectiveness of removing MB using a photocatalytic/ultrasonic hybrid system[229]. In summary, combined technologies offer high oxidation power, rapid reaction rates, broad-spectrum pollutant removal, and improved energy efficiency, making them promising candidates for future wastewater treatment systems[220,222].
CONCLUSION AND FUTURE PROSPECTS
Photocatalysis presents a promising solution for wastewater treatment, offering effective degradation of pollutants with minimal chemical use and potential for solar energy integration. However, key challenges persist, including limited efficiency, catalyst instability, poor recyclability, and difficulties in scaling up from laboratory experiments to industrial applications. Future research should prioritize the development of durable and cost-effective photocatalysts, the optimization of reactor designs, and the reduction of harmful by-products. Moreover, integrating photocatalysis with other advanced treatment methods - such as AOPs, membrane filtration, absorption, biological treatment, and electrochemical methods - offers significant potential to enhance treatment efficiency, expand the range of removable contaminants, and improve scalability. Addressing these challenges will be crucial for unlocking the full potential of photocatalysis for sustainable, large-scale wastewater treatment.
DECLARATIONS
Authors’ contributions
Writing - original draft, investigation: Chalatsi-Diamanti, P.
Writing - original draft and editing: Isari, E. A.; Grilla, E.
Made substantial contributions to the conception, design and review of the study: Kokkinos, P.
Supervision: Kalavrouziotis, I. K.
Availability of data and materials
Not applicable.
Financial support and sponsorship
None.
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
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