# Heterogeneous Photocatalysis for Advanced Water Treatment: Materials, Mechanisms, Reactor Configurations, and Emerging Applications

Maria Paiu, Doina Lutic, Lidia Favier, Maria Gavrilescu

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Maria Paiu, Doina Lutic, Lidia Favier, Maria Gavrilescu. Heterogeneous Photocatalysis for Advanced Water Treatment: Materials, Mechanisms, Reactor Configurations, and Emerging Applications. Applied Sciences, 2025, 15 (10), pp.5681. 10.3390/app15105681. hal-05094705

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Review

# Heterogeneous Photocatalysis for Advanced Water Treatment: Materials, Mechanisms, Reactor Configurations, and Emerging Applications

Maria Paiu<fnref n="1" />, Doina Lutic<fnref n="2" />, Lidia Favier<fnref n="3" />,*, and Maria Gavrilescu<fnref n="1" />, 4, 5, *

**Abstract:** Heterogeneous photocatalysis has emerged as a versatile and sustainable technology for the degradation of emerging contaminants in water. This review highlights recent advancements in photocatalysts design, including band gap engineering, heterojunction formation, and plasmonic enhancement to enable visible-light activation. Various reactor configurations, such as slurry, immobilized, annular, flat plate, and membrane-based systems, are examined in terms of their efficiency, scalability, and operational challenges. Hybrid systems combining photocatalysis with membrane filtration, adsorption, Fenton processes, and biological treatments demonstrate improved removal efficiency and broader applicability. Energy performance metrics such as quantum yield and electrical energy per order are discussed as essential tools for evaluating system feasibility. Special attention is given to solar-driven reactors and smart responsive materials, which enhance adaptability and sustainability. Additionally, artificial intelligence and machine learning approaches are explored as accelerators for catalyst discovery and process optimization. Altogether, these advances position photocatalysis as a key component in future water treatment strategies, particularly in decentralized and low-resource contexts. The integration of material innovation, system design, and data-driven optimization underlines the potential of photocatalysis to contribute to global efforts in environmental protection and sustainable development.

**Keywords:** emerging contaminants; environmental remediation; heterogeneous photocatalysis; hybrid systems; operational parameters; reactor design; visible-light photocatalysts

## 1. Introduction

Access to safe and clean water is increasingly threatened by diverse environmental pressures and intense human activity. Industrial discharges, agricultural runoff, and continuously increased amounts of municipal wastewater introduce a broad spectrum of chemical pollutants into natural waters, many of which are resistant to degradation and not effectively removed by conventional treatment systems [1,2]. Among these, emerging contaminants (ECs) represent a growing category of concern due to their persistence, bioactivity, and potential to cause long-term ecological and human health effects [3,4].

<figure><img src="image_2.png" /></figure>

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Academic Editor: Juan García Rodríguez

Received: 15 April 2025

Revised: 7 May 2025

Accepted: 14 May 2025

Published: 19 May 2025

**Citation:** Paiu, M.; Lutic, D.; Favier, L.; Gavrilescu, M. Heterogeneous Photocatalysis for Advanced Water Treatment: Materials, Mechanisms, Reactor Configurations, and Emerging Applications. *Appl. Sci.* **2025**, *15*, 5681. https://doi.org/10.3390/app15105681

**Copyright:** © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

<footnote n="1">¹ Department of Environmental Engineering and Management, "Cristofor Simionescu" Faculty of Chemical Engineering and Environmental Protection, "Gheorghe Asachi" Technical University of Iasi, 700050 Iasi, Romania; maria.paiu@student.tuiasi.ro</footnote>

<footnote n="2">² Faculty of Chemistry, "Al. I. Cuza" University of Iasi, 700506 Iasi, Romania; doilub@uaic.ro</footnote>

<footnote n="3">³ Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR-UMR6226, F-35000 Rennes, France</footnote>

<footnote n="4">⁴ Academy of Romanian Scientists, 3 Ilfov Street, 050044 Bucharest, Romania</footnote>

<footnote n="5">⁵ Academy of Technical Sciences of Romania, 26 Dacia Blvd., 010413 Bucharest, Romania</footnote>

<footnote>* Correspondence: lidia.favier@ensc-rennes.fr (L.F.); maria.gavrilescu@academic.tuiasi.ro (M.G.)</footnote>
Detected at concentrations ranging from nanograms to micrograms per liter, these substances often escape regulatory monitoring frameworks and are continuously introduced into aquatic environments. Their resistance to conventional biological and physicochemical treatments has been widely documented, along with the risk of generating transformation byproducts that may be equally or more toxic than the parent compounds [5,6]. Studies have confirmed their presence in surface waters, groundwater and even drinking water supplies, raising concerns over chronic exposure, endocrine disruption and the propagation of antimicrobial resistance [7,8].

These risks are further amplified by the limited efficiency of standard treatment plants, which were not designed to remove complex organic micropollutants. Even where partial degradation occurs, byproducts may retain or exceed the toxicity of the parent compound, contributing to bioaccumulation and ecosystem stress [9,10]. Additionally, the detection of pharmaceuticals, artificial sweeteners, and industrial additives in effluents and downstream drinking water underscores the inadequacy of current systems to prevent environmental dissemination [10,11].

Recent global assessments estimate that more than 2 billion people rely on drinking water sources contaminated by fecal or chemical pollutants, while nearly 80% of wastewater is discharged untreated into the environment [11]. Monitoring campaigns have identified pharmaceuticals, polyfluoroalkyl substances (PFAS), and personal care products in rivers and lakes across multiple continents [12]. For instance, a European survey reported frequent detection of antibiotics and anti-inflammatory drugs at concentrations exceeding ecotoxicological safety thresholds [13].

In response to these challenges, attention has increasingly turned toward advanced treatment processes capable of degrading complex contaminant mixtures at trace levels. Heterogeneous photocatalysis has emerged as a promising approach due to its capacity for complete mineralization of organic pollutants, operation under mild conditions, and compatibility with solar energy. It offers significant advantages in environmental compatibility, treatment efficiency, and adaptability to diverse water matrices, making it a compelling candidate for sustainable water purification.

## 1.1. *Main Classes of Emerging Contaminants and Their Environmental Significance*

Emerging contaminants encompass a wide variety of chemical substances with diverse sources and properties; the main classes include the following (Figure 1) [2–4,7–9].

*   Pharmaceuticals and Personal Care Products (PPCPs):

This category includes prescription and over-the-counter drugs, antibiotics, analgesics, hormones, and ingredients found in cosmetics, fragrances, sunscreens, and soaps. These compounds enter water systems through excretion, improper disposal and runoff from agricultural activities involving veterinary drugs [10,11].

*   Endocrine-Disrupting Compounds (EDCs):

EDCs are a subset of ECs that interfere with hormonal systems in animals and humans. They include natural and synthetic hormones, certain pesticides, plasticizers (like bisphenol A), and industrial chemicals. These compounds can lead to reproductive and developmental abnormalities even at very low exposure levels [12,13].

*   Per- and Polyfluoroalkyl Substances (PFASs):

PFASs are a group of highly persistent, synthetic, fluorinated compounds used in firefighting foams, non-stick cookware, water-repellent clothing, and food packaging. Their strong carbon-fluorine bonds make them resistant to degradation, leading to their nickname as "forever chemicals" [14,15].
* Industrial and Household Chemicals:

This category encompasses solvents, flame retardants (e.g., polybrominated diphenyl ethers or PBDEs), plasticizers (e.g., phthalates), surfactants (e.g., nonylphenol), and other compounds used in manufacturing or domestic cleaning agents [16,17].

* Pesticides and Herbicides:

While many are regulated, newer formulations or breakdown products may still be unregulated and classified as emerging. These substances can leach into surface and groundwater from agricultural fields and urban landscapes [18–20].

* Nanomaterials:

Engineered nanoparticles such as silver, titanium dioxide, and carbon nanotubes are increasingly used in consumer products. Their small size and unique properties raise questions about their environmental fate, transport, and potential toxicity [7,21,22].

* Artificial Sweeteners and Food Additives:

Compounds like sucralose and acesulfame-K are widely used and excreted unchanged, making them markers of anthropogenic pollution in water bodies. Though not acutely toxic, their environmental persistence is a concern [23,24].

* Microplastics and Nanoplastics:

Fragmented plastic particles less than 5 mm (microplastics) or smaller than 100 nm (nanoplastics) have been identified in freshwater, marine, and drinking water sources. The complexity, diversity, and persistence of emerging contaminants present significant challenges for water treatment systems worldwide [25,26].

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Types of ECs (schematic representation of major classes of emerging contaminants (ECs), including pharmaceuticals, endocrine-disrupting chemicals (EDCs), personal care products (PCPs), industrial chemicals, and nanomaterials). Each category is associated with potential environmental or health impacts, highlighting the complexity of EC sources and their relevance in water treatment research (reused from Li et al., 2024 [7], under the Creative Commons CC-BY-NC-ND license and permits non-commercial use of the work as published, without adaptation or alteration provided the work is fully attributed).</figcaption></figure>
These trace-level contaminants pose major challenges to conventional water treatment technologies, which often fail to ensure complete removal and may generate harmful byproducts [2,7].

## 1.2. Advanced Treatment Strategies for Emerging Contaminant Mitigation

Many ECs elude traditional treatment barriers and may accumulate in ecosystems or re-enter through human consumption cycles. A multi-pronged approach involving source control, advanced treatment technologies (e.g., ozonation, adsorption on activated carbon, membrane filtration, advanced oxidation), and regulatory frameworks is essential to mitigate the long-term risks posed by these substances.

The removal of emerging pollutants presents significant challenges, requiring advanced treatment methods beyond conventional approaches. The most prominent technologies developed and investigated for ECs removal fall into several categories as outlined below (Figure 2).

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Overview of technologies used for the removal of emerging contaminants, including their targeted pollutant classes, specific treatment methods (e.g., adsorption, advanced oxidation processes, membrane filtration), and a summary of their respective advantages and limitations. The figure highlights the need for integrated approaches to address the complexity of ECs in water treatment systems.</figcaption></figure>

### (a) Advanced Oxidation Processes (AOPs)

AOPs involve the generation of highly reactive species, such as hydroxyl radicals (•OH), capable of degrading a wide range of organic contaminants [27–29].
* Ozonation ($O_3$), UV/$H_2O_2$, Fenton, and photo-Fenton processes are widely applied.

* Effective in degrading persistent molecules such as artificial sweeteners, antibiotics, and dyes.

* Limitations include energy requirements and possible toxic byproduct formation.

### (b) Adsorption Techniques

Adsorption is one of the most effective and widely used techniques for EC removal, due to its simplicity and cost-effectiveness [1,9,30]. Activated carbon (powdered or granular) is extensively applied for the uptake of pharmaceuticals, endocrine disruptors, and food additives.

* Biochar, carbon nanotubes, and graphene-based materials offer enhanced adsorption capacity, particularly for nanomaterials and microplastics.

* Challenges include adsorbent regeneration and disposal.

### (c) Membrane Filtration Technologies

Membrane processes physically remove emerging contaminants based on size exclusion or charge repulsion [31–33].

* Nanofiltration (NF) and reverse osmosis (RO) are effective for removing low-molecular-weight ECs, including artificial sweeteners and pharmaceuticals.

* Ultrafiltration (UF) and microfiltration (MF) can remove microplastics and some nanomaterials when combined with coagulation or flocculation.

* Membrane fouling, large scale application limitation, and high operational costs remain concerns.

### (d) Biological Treatments

While traditional biological treatments (e.g., activated sludge) show limited efficacy for many ECs, recent innovations have improved biodegradation capacity [4,34–36].

* Membrane bioreactors (MBRs) and moving bed biofilm reactors (MBBRs) show enhanced performance due to longer sludge retention times.

* Engineered microbial consortia and microbial cell factories are explored for specific contaminant degradation.

* Biological processes are less effective for highly recalcitrant substances like sucralose or nanoplastics.

### (e) Electrochemical and Photocatalytic Methods

Electrochemical oxidation and photocatalysis are gaining attention for the degradation of persistent and non-biodegradable ECs [37–40].

* Electrochemical oxidation using boron-doped diamond (BDD) or Ti/Pt anodes enables direct electron transfer to degrade pharmaceuticals and food-related ECs.

* Heterogeneous photocatalysis using $TiO_2$, $ZnO$, and doped semiconductors under UV or solar light are effective in degrading a wide range of ECs, including pesticides and sweeteners.

* These methods are costly and energy-intensive, but could be integrated with solar energy for sustainability.

### (f) Coagulation–Flocculation and Sedimentation

This conventional method is often used as a pre-treatment or in combination with other technologies [41–43].

* The method alone is ineffective for most ECs, but it can assist in the removal of microplastics, nanomaterials, and particulate-associated pollutants.
* Use of advanced coagulants (e.g., magnetic flocculants or biocoagulants) enhances efficiency.

### (g) Hybrid and Integrated Systems

Combining physical, chemical, and biological methods often enhances removal efficiency and addresses the limitations of single techniques [1,41,44].

* Examples include AOP-MBR hybrids, adsorption-photocatalysis systems, and membrane-biochar filters.

* Integration allows for targeted removal of both particulate and dissolved ECs.

* These combinations can add system complexity and cost implications.

Advanced oxidation processes (AOPs) have emerged as effective strategies for degrading a broad spectrum of organic and inorganic pollutants through the generation of highly reactive oxidative species. Among them, heterogeneous photocatalysis has garnered considerable attention due to its potential to achieve complete mineralization of contaminants under mild conditions, using light as the driving force and a solid photocatalyst to initiate redox reactions [45–47].

Although numerous advanced treatment techniques have been developed for removing emerging contaminants, a comparative analysis reveals significant trade-offs in performance, cost, and environmental impact. For instance, ozonation, and UV/H₂O₂-based AOPs are widely accepted for their strong oxidation potential; however, several studies have highlighted concerns about toxic byproduct formation and energy-intensive operation, particularly at large scales [7]. Conversely, adsorption-based methods using activated carbon or biochar are praised for their simplicity and low cost, but still suffer from non-specificity and challenges in adsorbent regeneration, raising questions about long-term sustainability [7].

Photocatalysis, particularly using semiconductors like TiO₂ or ZnO, presents a promising alternative due to its potential for complete mineralization of organic pollutants. However, its scalability remains contested. While some authors emphasize its versatility and environmental compatibility [11], others caution that rapid charge recombination, low visible-light response, and reduced activity under real water matrices severely limit its practical applicability [11]. Notably, studies diverge on the actual impact of heterojunction formation. While TiO₂/g-C₃N₄ and BiVO₄/graphene composites show promising synergy, some reports fail to replicate enhanced activity due to interface instability or inconsistent synthesis quality [11].

Furthermore, a persistent controversy surrounds the use of noble metal dopants (e.g., Ag, Pt, Au). Although these elements boost efficiency via plasmonic enhancement, critics argue that cost, toxicity, and environmental persistence undermine their real-world viability [14]. A recent trend toward earth-abundant, metal-free photocatalysts attempts to address these issues, though their performance and stability under dynamic conditions remain under investigation [3].

In terms of system-level integration, the literature increasingly supports hybrid configurations (e.g., photocatalysis + membranes or Fenton processes), yet consensus is lacking regarding optimal reactor design. Some researchers advocate for modular, immobilized reactors for field use [1], while others find that slurry reactors, despite recovery challenges, offer superior degradation rates [1]. These discrepancies underscore the need for standardized testing protocols and more pilot-scale demonstrations to reconcile laboratory results with real-world expectations.

Therefore, heterogeneous photocatalysis is particularly attractive for water treatment applications because it does not require the addition of chemical reagents and can utilize renewable solar energy. Over the past two decades, extensive research has been devoted to
developing more efficient photocatalytic materials and their reuse over numerous cleaning cycles, optimizing reactor designs, and evaluating photocatalytic performance under real-world conditions. Nonetheless, challenges remain in scaling up laboratory successes, improving the selectivity and durability of photocatalysts, and ensuring their environmental safety.

This review focuses specifically on the use of heterogeneous photocatalysis for the removal of emerging organic contaminants in water and wastewater, including pharmaceuticals, endocrine disruptors, personal care products, and industrial chemicals. The scope includes laboratory-scale research, pilot-scale studies, and practical considerations for integration into existing treatment systems. Particular attention is given to systems operating under solar or simulated visible light and to environmental conditions typical of surface water, municipal effluents, and decentralized applications.

The manuscript is structured as a narrative literature review. Relevant peer-reviewed articles were selected through a targeted search of scientific databases (e.g., Scopus, Web of Science, and ScienceDirect) using combinations of keywords such as “heterogeneous photocatalysis”, “emerging contaminants”, “visible-light photocatalysts”, “hybrid water treatment”, and “photocatalytic reactors.” The selection emphasized studies published between 2020 and 2025 to ensure coverage of recent advancements, while also including earlier foundational works where necessary. Priority was given to research articles, reviews, and case studies that presented experimental data, pilot-scale results, or novel conceptual frameworks relevant to the removal of emerging pollutants in water and wastewater. The included sources were evaluated for their scientific quality, relevance, and alignment with the manuscript’s thematic structure and objectives.

The paper aims to provide a comprehensive overview of recent advances in heterogeneous photocatalysis for water purification. It synthesizes and critically analyzes findings from recent literature on heterogeneous photocatalysis for water treatment, focusing on materials, mechanisms, operational parameters, and scale-up strategies. The work covers the underlying principles of photocatalytic processes, critically examines various classes of photocatalytic materials, highlights key applications in the removal of organic micropollutants and pathogens, and discusses the technological progress and limitations associated with current photocatalytic systems. Emerging trends and future directions are also considered, with an emphasis on strategies to enhance photocatalytic efficiency and facilitate practical implementation in decentralized and large-scale water treatment systems.

## 2. Core Concepts of Heterogeneous Photocatalysis

Harnessing solar energy, a clean, renewable, and abundant resource to drive advanced oxidation and reduction reactions is widely recognized as an environmentally friendly strategy for sustainable chemical processes. Transitioning away from fossil fuel dependence is a critical step toward achieving long-term environmental and energy stability. Photocatalysis, which plays a key role in solar energy utilization, can be applied in three main areas [38,48–52]: (i) transforming solar radiation into usable chemical energy that can be conveniently stored, such as generating hydrogen fuel through water splitting, where a photocatalyst activated by light enables electron-hole separation as photons excite electrons from the valence band to the conduction band; (ii) facilitating chemical reactions by enabling the reactants to surpass the activation energy threshold, thanks to the energy provided by the excited photocatalyst; and (iii) breaking down harmful chemical contaminants through photocatalytic activation that allows the process to overcome its energy barrier.

Heterogeneous photocatalysis is a light-driven process that occurs at the interface between a solid semiconductor catalyst and water containing target contaminants. It relies
on the specific semiconductors' capacity to absorb photons and generate reactive species that initiate redox reactions. These reactions can lead to the degradation of a wide variety of pollutants, including organic micropollutants, pathogens, and inorganic ions.

## 2.1. Basic Mechanism of Photocatalysis

Photocatalytic processes have gained increasing attention for their ability to degrade a broad range of persistent emerging contaminants through light-induced oxidative reactions. These methods typically involve semiconductors such as TiO₂ or ZnO, which, upon light irradiation, generate reactive oxygen species (ROS) capable of breaking down complex pollutants into less harmful substances [53,54]. The fundamental mechanism underlying this process is illustrated in Figure 3, where charge carrier dynamics and contaminant interactions are depicted [55,56].

When a semiconductor photocatalyst is irradiated with photons possessing energy equal to or higher than its band gap, electrons (e⁻) in the valence band (VB) are excited to the conduction band (CB), leaving behind holes (h⁺) in the VB (Equation (1)) [57,58].

$$ \text{Semiconductor} + h\nu \rightarrow \text{eCB}^{-} + \text{hVB}^{+} \quad (1) $$

The photogenerated electrons and holes can migrate to the surface of the catalyst, where they participate in redox reactions (Figure 3) [59]. Electrons can reduce dissolved oxygen to form superoxide radicals (•O₂⁻), while holes can oxidize water or hydroxide ions to produce hydroxyl radicals (•OH) (Equations (2) and (3)).

$$ \text{O}_2 + \text{e}^- \rightarrow \text{•O}_2^{-} \quad (2) $$

$$ \text{H}_2\text{O} + \text{h}^+ \rightarrow \text{•OH} + \text{H}^+ \quad (3) $$

These reactive oxygen species (ROS), especially •OH and •O₂⁻, are highly oxidative and play a central role in the degradation of organic contaminants by breaking chemical bonds and transforming them into less harmful substances or mineralizing them to CO₂ and H₂O (Equations (4) and (5)).

$$ \text{ROS} + \text{contaminants} \rightarrow \text{degraded products (indirect oxidation)} \quad (4) $$

$$ \text{Contaminants} + \text{eCB}^{-} + \text{hVB}^{+} \rightarrow \text{reduced/oxidized products (direct oxidation)} \quad (5) $$

The general reactions involved in photocatalytic water purification can be summarized as in Equation (6) [29].

$$ \text{Pollutant} + \text{•OH/•O}_2^{-} \rightarrow \text{Intermediate products} \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{Minerals} \quad (6) $$

The degradation pathway typically involves the formation of transient intermediates, which are further oxidized until mineralization is achieved. Selectivity is often low, leading to non-specific degradation of organic molecules.

## 2.2. Factors Affecting Photocatalysis Efficiency

The overall performance of photocatalytic systems is influenced by several interconnected factors that determine their efficiency and applicability in environmental remediation [60–64]. These key factors are summarized in Figure 4.

Understanding and optimizing these factors are essential to improving the degradation efficiency and sustainability of the photocatalytic treatments.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** The fundamental mechanism of photocatalytic degradation of contaminants in water under light irradiation (reused from Parida et al., 2023 [59], under the CC-BY-Non-Commercial (NC)-No-Derivatives (ND) license where users can download and distribute the work without any alterations of any kind, after giving appropriate credit to original source).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Key parameters influencing the efficiency of photocatalytic degradation processes, including light source and intensity, charge carrier recombination, catalyst surface properties, solution chemistry, and catalyst dosage. These factors collectively determine the effectiveness and reliability of photocatalytic water treatment systems.</figcaption></figure>

**Figure 4.** (i) Light source and intensity

Photocatalysis is inherently light-driven, and the energy of the incoming photons must match or exceed the band gap energy of the semiconductor material [65]:
* UV light (especially UV-A) is commonly used with materials like TiO₂ due to its strong photocatalytic activity. UV-A radiation, which is most commonly used in photocatalysis, typically spans the wavelength range of 315–400 nm, while UV-B ranges from 280 to 315 nm and UV-C from 100 to 280 nm. These distinctions are important when evaluating the activation thresholds of different photocatalysts, especially those responsive to near-visible light.

* Visible-light-responsive photocatalysts (e.g., doped TiO₂, g-C₃N₄, ZnFe₂O₄) (wavelength range of 400–700 nm) are gaining popularity for solar-driven applications, enhancing environmental compatibility and energy efficiency.

* Both light intensity and irradiation duration directly affect the rate of electron-hole pair generation. Higher light intensity increases, in principle, the number of incident photons on the photocatalyst surface per unit time, thereby enhancing the excitation of electrons from the valence band to the conduction band and increasing the generation rate of electron-hole pairs. Similarly, prolonged irradiation duration extends the cumulative exposure, enabling more excitation events over time. However, excessively high intensity or duration may also lead to recombination or thermal effects that reduce the overall efficiency.

The light source is a critical component in photocatalytic systems, directly influencing the degradation efficiency. Key aspects such as illumination intensity, emission spectrum, and source design and positioning affect the activation of the photocatalyst [66,67]. In setups using artificial light, uniform photon distribution and energy-efficient operation must be ensured. The light must penetrate the reaction medium effectively, despite potential scattering, and its spectral output should match the photocatalyst's band gap to initiate the electron-hole pair generation [68,69].

### (ii) Recombination of charge carriers

A major limitation in photocatalysis is the rapid recombination of photogenerated electrons (e⁻) and holes (h⁺), which reduces the availability of reactive oxygen species (ROS) such as hydroxyl radicals (•OH). Once excited by light, the electrons may quickly recombine with holes if they are not effectively separated or transferred. This recombination suppresses photocatalytic activity and lowers overall efficiency [70,71]. Strategies to suppress recombination and facilitate charge separation and prolong carrier lifetimes include the following [72,73]:

* Heterojunction constructions (e.g., Z-scheme or type-II)

* Noble metal deposition (doping) (e.g., Ag, Au) as electron sinks

* Co-catalyst integration (e.g., reduced graphene oxide, carbon quantum dots)

### (iii) Surface properties of the photocatalyst

The surface characteristics of a photocatalyst are essential for its reactivity and pollutant degradation performance. The surface characteristics of a photocatalyst directly impact its interaction with contaminants [74–76].

* High specific surface area increases the number of active sites.

* Porosity and surface functional groups (e.g., hydroxyl or amine groups) promote adsorption and improve photocatalytic kinetics.

* Morphology (nanorods, nanosheets, hollow spheres) and particle size also influences light absorption and diffusion behaviors.

### (iv) Solution parameters

The efficiency of the photocatalytic processes is not determined by the catalyst alone; the composition of the surrounding aqueous environment also plays a crucial role. Several
solution-related factors can alter the surface interactions, photon absorption, and the generation of reactive species, ultimately impacting the overall degradation performance.

The chemical composition of the aqueous medium can significantly influence photocatalytic activity [77–79]:

* The pH affects the surface charge of the photocatalyst and the ionization state of pollutants.

* The ionic strength and the presence of natural organic matter (NOM) or inorganic ions (e.g., Cl⁻, HCO₃⁻) can compete for active sites or scavenge the ROS.

* Turbidity and colored substances may interfere with light penetration.

Catalyst loading as an efficiency-determining parameter. The quantity of photocatalyst used in a reaction system is a key parameter that directly influences the photocatalytic efficiency. Achieving an optimal catalyst loading is essential to ensure sufficient active sites for pollutant degradation, while maintaining effective light utilization and mass transfer. Both underloading and overloading can adversely affect performance, making careful optimization necessary based on the specific system configuration and contaminant characteristics.

An optimal amount of photocatalyst is necessary to balance the efficient photon absorption with effective mass transfer [80,81].

* Low solid loading limits the degradation due to insufficient active sites.

* High solid loading can cause light scattering or shielding effects, decreasing the photon penetration and reducing efficiency.

* The optimum concentration depends on the reactor design, contaminant type, detailed composition of the aqueous mixture, and catalyst nature.

One major limitation of current photocatalytic systems is the fast recombination of photogenerated electron–hole pairs, which significantly reduces quantum efficiency. While many strategies, such as doping, heterojunction formation, and plasmonic enhancement are proposed to overcome this, their effectiveness is often inconsistent across different experimental setups. Some studies report up to a fivefold increase in degradation rate using heterostructures, such as Z-scheme TiO₂/g-C₃N₄ and BiVO₄/graphene composites, due to the improved charge carrier migration, as presented by Li et al. [71] and Lettieri et al. [72]. However, other investigations have failed to reproduce these enhancements consistently, attributing the limitations to interfacial charge trapping, instability of heterojunctions, or synthesis variability under real water matrices [74,75]. These discrepancies underscore the need for more standardized testing conditions and reproducibility studies to guide the rational design of robust photocatalytic systems.

### 3. Photocatalytic Materials for Water Treatment

The choice of the photocatalyst plays an essential role in determining the efficiency, selectivity, and practical applicability of heterogeneous photocatalysis in water treatment. An ideal photocatalyst should exhibit strong light absorption, high quantum efficiency, low recombination rate of charge carriers, chemical stability in aqueous environments, non-toxicity, and cost-effectiveness. Over the years, a wide range of photocatalytic materials have been explored, with ongoing efforts to improve their performance under solar or visible light and enhance their reusability.

#### 3.1. UV-Responsive (Conventional) Semiconductors

Titanium dioxide (TiO₂) is the most widely studied and commercially used photocatalyst due to its strong oxidizing power, chemical stability, low cost, and non-toxicity. It exists mainly in anatase and rutile crystalline forms, with anatase typically exhibiting higher pho-
tocatalytic activity. However, TiO₂ has a wide band gap (~3.2 eV), limiting its activation to the UV region, which accounts for only a small fraction of the solar spectrum [53,79,82,83].

Zinc oxide (ZnO) is another UV-active semiconductor with photocatalytic properties comparable to TiO₂. It features a similar band gap (~3.3 eV) but often suffers from photocorrosion and reduced long-term stability in aqueous systems [84–86].

Their characteristics are shown in Table 1. This comparison highlights that while both TiO₂ and ZnO are effective photocatalysts, ZnO often demonstrates higher electron mobility and faster photocatalytic disinfection rates [40,84,85,87]. However, ZnO susceptibility to photocorrosion can affect its long-term stability. TiO₂, on the other hand, offers greater chemical stability but may experience limitations due to lower electron mobility and a narrower light absorption range. The choice between these materials should consider the specific requirements of the application, including desired efficiency, durability, and operating conditions.

<table><caption>**Table 1.** Comparison of TiO₂ and ZnO photocatalysts.</caption><thead><tr><th>Property/Performance Metric</th><th>TiO<sub>2</sub></th><th>ZnO</th></tr></thead><tbody><tr><td>Band gap energy</td><td>~3.2 eV (anatase phase)</td><td>~3.3 eV</td></tr><tr><td>Electron mobility</td><td>0.1–0.4 cm<sup>2</sup>/V·s</td><td>200–300 cm<sup>2</sup>/V·s</td></tr><tr><td>Surface area</td><td>Generally higher</td><td>Generally lower</td></tr><tr><td>Photocatalytic activity</td><td>Effective; limited by recombination</td><td>Comparable or superior; prone to photocorrosion</td></tr><tr><td>Photocatalytic disinfection</td><td>Effective; may have lag time</td><td>Faster with no lag time</td></tr><tr><td>Stability and durability</td><td>High chemical stability</td><td>Prone to photocorrosion</td></tr><tr><td>Light absorption range</td><td>Primarily UV</td><td>UV and potentially visible with modifications</td></tr><tr><td>Applications</td><td>Environmental remediation, self-cleaning, water purification</td><td>Antibacterial coatings, sensors, water purification</td></tr><tr><td>Composite systems</td><td>Effective in heterostructures</td><td>Improves performance when combined with other semiconductors</td></tr></tbody></table>

In addition to TiO₂ and ZnO, other conventional photocatalysts have been widely studied in earlier photocatalytic research and continue to serve as reference materials. WO₃ (tungsten trioxide), with its visible-light responsiveness and oxidative capability, has been employed in the degradation of organic pollutants and photocatalytic water splitting. Fe₂O₃ (hematite) is another abundant and stable semiconductor with a narrow bandgap; however, its performance is often constrained by short charge carrier diffusion lengths and rapid recombination. CdS (cadmium sulfide) has drawn attention due to its high visible-light activity and tunable bandgap, though its practical application is limited by photostability and toxicity concerns. Additionally, SrTiO₃ has been explored for photocatalytic hydrogen production and degradation applications, particularly in UV-active systems. The exploration of these materials has laid the foundation for the development of modified and doped photocatalysts with improved performance and environmental compatibility [31,42,44,53].

## 3.2. Visible-Light-Responsive Photocatalysts: Composition and Properties

To overcome the limitations of UV activation, considerable efforts have been made to develop photocatalysts able to operate under visible light (~400–700 nm). Examples include the following (Table 2) [65,68,75,88–90]:

* Graphitic carbon nitride (g-C₃N₄): As a metal-free semiconductor with a moderate band gap (~2.7 eV), g-C₃N₄ can harness visible light and has shown promising results
for degrading organic pollutants. Its layered structure allows for easy modification and heterojunction formation.

* Bismuth-based compounds: Materials like BiVO₄, Bi₂O₃, and Bi₂WO₆ have received attention due to their visible-light responsiveness and photocatalytic activity for oxidation reactions.

* Silver halides (AgX, where X = Cl, Br, I): These materials exhibit strong visible light absorption and can be used alone or as part of composite systems, though stability and toxicity are concerns.

<table><caption>**Table 2.** Comparison of visible-light-responsive photocatalysts.</caption><thead><tr><td>Photocatalyst</td><td>Band Gap Energy (eV)</td><td>Key Features</td><td>Applications</td></tr></thead><tbody><tr><td>WO<sub>3</sub></td><td>2.5–2.8</td><td>Effective under visible light. High stability. Suitable for environmental remediation.</td><td>Photocatalytic degradation of organic pollutants.</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub></td><td>~2.7</td><td>Metal-free. Strong visible light absorption. Tunable electronic structure.</td><td>Photocatalytic degradation of organic pollutants.</td></tr><tr><td>BiVO<sub>4</sub></td><td>2.4–2.5</td><td>Strong visible light absorption. High stability.</td><td>Photocatalytic degradation of organic pollutants.</td></tr><tr><td>CdS</td><td>~2.4</td><td>Narrow band gap. Effective visible light absorption. Prone to photocorrosion.</td><td>Photocatalytic degradation of organic pollutants.</td></tr><tr><td>ZnO/g-C<sub>3</sub>N<sub>4</sub> Composite</td><td>Varies based on composition</td><td>Enhanced visible light absorption. Improved charge separation.</td><td>Photocatalytic degradation of organic pollutants.</td></tr></tbody></table>

Table 2 provides a concise overview of various visible light responsive photocatalysts, emphasizing their band gap energies, key features, and typical applications. These materials have been studied for their effectiveness in environmental remediation, particularly in the degradation of organic pollutants under visible-light irradiation.

For example, titanium dioxide (TiO₂), a wide bandgap semiconductor, is frequently studied in its rutile and anatase crystalline forms, which exhibit bandgap energies of approximately 3.0 eV and 3.2 eV, respectively. Its sensitivity to ultraviolet (UV) light has spurred not only extensive research in photocatalysis, but also in areas such as superhydrophilic surface behavior, environmental decontamination, and solar fuel generation. When TiO₂ is photoexcited across its bandgap, it generates electron-hole pairs that are subsequently involved in redox reactions, with surface-adsorbed species acting as electron or hole scavengers, as depicted in Figure 5 [91].

### 3.3. Modified and Doped Catalysts

Band gap engineering using metal and non-metal doping has proven effective in enhancing visible-light absorption and reducing recombination. Bandgap engineering refers to strategies used to modify the energy band structure of photocatalytic semiconductors to enhance their light absorption, especially in the visible range. Traditional materials like TiO₂ and ZnO have wide bandgaps that mostly restrict activity to UV light [14,31]. To overcome this, several approaches have been employed. Doping with metal or non-metal ions (e.g., N, S, Fe, Cu) introduces intermediate energy states within the bandgap, facilitating visible-light activation [49,51,60]. Heterojunction formation between semiconductors with different band structures improves the charge separation and extends the absorption range [65,69]. Sensitization with dyes or narrow-bandgap semiconductors, as well as cou-
pling with plasmonic nanoparticles, are also effective in tuning optical properties [70,71,73]. These bandgap engineering strategies are central to the development of high-performance photocatalysts that are active under visible light. The main bandgap engineering strategies used to enhance visible-light-driven photocatalysis include non-metal doping, metal doping, and the integration of noble metal nanoparticles.

* Non-metal doping: Elements such as nitrogen, sulfur, or carbon are commonly used; they can narrow the band gap and improve the visible-light activity.

* Metal doping: Iron, copper, and silver serve to modify the electronic structure or act as electron traps [75,92]. They can introduce new energy levels in the solid and enhance the charge transfer.

* Noble metal nanoparticles (e.g., Ag, Au, Pt): These nanoparticles can enhance photocatalysis via surface plasmon resonance (SPR), which increases light absorption and promotes charge separation when integrated with semiconductors, boosting photocatalytic efficiency [93,94].

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Schematic of semiconductor excitation by band gap illumination leading to the creation of “electrons” in the conduction band and “holes” in the valance band (reused from Ibhadon and Fitzpatrick, 2013 [91], under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/)).</figcaption></figure>

Table 3 provides an overview of various modified and doped photocatalysts, emphasizing their enhancements and specific applications in water treatment and environmental remediation.

Defect engineering, including the intentional introduction of oxygen vacancies or surface defects, is another effective strategy to improve the photocatalytic performance. These defects can introduce localized states within the bandgap, enhance visible-light absorption, and serve as active sites for redox reactions. In materials like TiO₂, ZnO, and BiVO₄, oxygen vacancies have been shown to improve charge separation and reduce recombination rates by acting as shallow traps for the photoinduced carriers. Moreover, surface defects can also increase the adsorption of target pollutants, contributing to higher degradation efficiency [91,92,95].

These structural modifications significantly influence the surface properties and electronic behavior of photocatalysts. For example, the presence of oxygen vacancies introduces localized energy levels within the bandgap, which can act as electron traps that prolong charge carrier lifetimes and reduce recombination. These vacancies also increase surface
reactivity by creating unsaturated coordination sites that facilitate the adsorption and activation of target pollutants, thus improving degradation efficiency. Moreover, such defects can promote the chemisorption of molecular oxygen and water, aiding the generation of reactive oxygen species (ROS) like hydroxyl radicals (•OH) and superoxide anions (O₂•⁻), which are key to the photocatalytic process.

<table><caption>**Table 3.** Comparison of modified and doped photocatalysts.</caption><thead><tr><td>Photocatalyst</td><td>Modification/Doping</td><td>Key Features</td><td>Applications</td></tr></thead><tbody><tr><td>N-TiO<sub>2</sub></td><td>Nitrogen doping</td><td>Enhanced visible light absorption. Improved photocatalytic activity.</td><td>Photocatalytic degradation of organic pollutants.</td></tr><tr><td>Ag@N-TiO<sub>2</sub></td><td>Silver and nitrogen doping</td><td>Enhanced visible light absorption. Antibacterial properties.</td><td>Photocatalytic inactivation of pathogenic bacteria in wastewater treatment.</td></tr><tr><td>ZrO<sub>2</sub> doped with noble metals</td><td>Noble metal doping</td><td>Tuned bandgap energy. Enhanced photocatalytic activity.</td><td>Photocatalytic degradation of organic pollutants.</td></tr><tr><td>WO<sub>3</sub>/Ti-WO<sub>x</sub>/TiH<sub>y</sub>Oz</td><td>Homo/heterojunction</td><td>Extended light absorption above 400 nm. Improved charge separation.</td><td>Photocatalytic degradation of azo dye water pollutants under visible light.</td></tr><tr><td>ZnO/NiFe<sub>2</sub>O<sub>4</sub> nanocomposite</td><td>Composite formation</td><td>High degradation efficiency under UV light. Effective against methylene blue dye.</td><td>Photocatalytic degradation of organic dyes in wastewater.</td></tr><tr><td>MgO/graphene nanoplatelets</td><td>Composite formation</td><td>Significant photocatalytic activity. Enhanced antibacterial performance.</td><td>Purification of industrial wastewater. Potential applications in nanomedicine.</td></tr></tbody></table>

The structural and chemical impacts of these modifications are often confirmed by advanced characterization techniques. X-ray photoelectron spectroscopy (XPS) is widely employed to detect oxygen vacancies, reduced oxidation states (e.g., Ti³⁺, Bi³⁺), and changes in surface composition through shifts in binding energies or the appearance of new peaks associated with defect states. For instance, the XPS spectra of reduced TiO₂ or doped BiVO₄ have shown an increase in surface oxygen vacancies and Ti³⁺ sites, in direct correlation with the improved photodegradation of contaminants such as antibiotics and azo dyes under visible-light irradiation [92,95–97].

Similarly, Raman spectroscopy provides insight into lattice distortion, structural disorder, and phonon confinement effects caused by doping or defect formation. The appearance or broadening of characteristic Raman bands (e.g., Eg or B1g modes in TiO₂) can indicate increased disorder or defect density. The detection of disordered lattice vibrations and Ti³⁺-induced shifts further supports the presence of active surface defects that enhance the charge separation. In combination, these tools validate the successful implementation of defect engineering strategies and help explaining how surface-level changes translate into enhanced photocatalytic performance [98].

In well-established photocatalysts like TiO₂ and ZnO, the defect engineering, especially through the creation of oxygen vacancies and cationic defects, has proven to be particularly effective in enhancing the photocatalytic performance. In TiO₂, the oxygen vacancies act as shallow electron traps, improving charge separation and extending activity into the visible spectrum. For instance, reduced TiO₂ with Ti³⁺ surface states had demonstrated increased
degradation efficiency for pollutants such as methyl orange and tetracycline under visible light [91,95]. Similarly, ZnO is prone to intrinsic defects such as zinc interstitials and oxygen vacancies, which can modulate the band structure and facilitate light absorption beyond the UV range. Studies have shown that ZnO with controlled defect concentration exhibits improved photodegradation rates of dyes and antibiotics, attributed to enhanced ROS generation and pollutant adsorption [86,99,100]. These defect-based modifications are typically confirmed using XPS and photoluminescence (PL) spectroscopy, which provide insight into trap states and recombination dynamics. As such, defect engineering presents a cost-effective and scalable route to improve the efficiency and applicability of TiO₂- and ZnO-based photocatalysts.

Figure 6 illustrates the role of modified and doped catalysts in enhancing photocatalytic performance. It emphasizes mechanisms such as band gap narrowing, improved charge separation and surface plasmon resonance. The left side of Figure 6 shows a semiconductor material modified by non-metal dopants like nitrogen, sulfur, and carbon, which integrate into the crystal lattice. This doping narrows the band gap, enabling the material to absorb visible light more effectively. Metal dopants such as iron, copper, and silver are also shown, and these can either introduce new energy levels within the band gap or act as electron traps, reducing the electron-hole recombination. On the right, noble metal nanoparticles (Ag, Au, Pt) are depicted on the semiconductor surface. They enhance the photocatalytic activity via surface plasmon resonance (SPR), where collective oscillations of conduction electrons under light exposure increase the local electromagnetic fields, boosting light absorption and promoting an efficient charge carrier separation. Arrows indicate the movement of electrons and holes, illustrating the reduced recombination and enhanced photocatalytic efficiency under visible-light irradiation.

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Schematic representation of modified and doped photocatalysts: Band gap engineering using metal (Fe, Cu, Ag) and non-metal (N, S, C) doping for enhanced visible-light absorption and reduced recombination, and photocatalytic enhancement with noble metal nanoparticles (Ag, Au, Pt) using surface plasmon resonance (SPR) and improved charge separation.</figcaption></figure>

### 3.4. Composite and Heterojunction Photocatalysts

Forming heterojunctions by combining two or more semiconductors with different band structures can significantly improve the photocatalytic performance. These systems facilitate charge carrier separation and broaden the light absorption range and benefit from
synergistic effects between the components. Examples include TiO₂/g-C₃N₄, ZnO/WO₃, and BiVO₄/graphene composites [95,101].

Carbon-based supports such as graphene oxide (GO), reduced graphene oxide (rGO), and carbon nanotubes (CNTs) are also employed to enhance electron transport and surface area [81].

Table 4 provides an overview of various composite and heterojunction photocatalysts, emphasizing their compositions, enhancements, and specific applications in water treatment and environmental remediation.

<table><caption>**Table 4.** Comparison of composite and heterojunction photocatalysts.</caption><thead><tr><td>Photocatalyst</td><td>Composition</td><td>Key Features</td><td>Applications</td></tr></thead><tbody><tr><td>Fe₂O₃/TiO₂ heterojunction</td><td>Iron oxide (Fe₂O₃) and titanium dioxide (TiO₂)</td><td>Optimized weight ratio enhances photocatalytic activity. Effective in antibiotic removal.</td><td>Photocatalytic degradation of antibiotics in wastewater.</td></tr><tr><td>g-C₃N₄/TNP composite</td><td>Graphitic carbon nitride (g-C₃N₄) and TNP-based perovskite materials</td><td>Formation of p-n junction enhances charge separation. Increased photocatalytic activity and stability.</td><td>Photocatalytic degradation of organic pollutants in wastewater.</td></tr><tr><td>BiVO₄@ZIF-8 composite</td><td>Bismuth vanadate (BiVO₄) and zeolitic imidazolate framework-8 (ZIF-8)</td><td>High efficiency in photocatalytic wastewater treatment. Enhanced light absorption and charge transfer.</td><td>Photocatalytic degradation of pollutants in wastewater.</td></tr><tr><td>WO₃/Ti-WOₓ/TiH₃Oz heterojunction</td><td>Tungsten trioxide (WO₃) and titanium-based compounds</td><td>Extended light absorption above 400 nm. Improved charge separation.</td><td>Photocatalytic degradation of azo dye water pollutants under visible light.</td></tr><tr><td>MoS₂/Ag₂Mo₂O₇ composite</td><td>Molybdenum disulfide (MoS₂) and silver molybdate (Ag₂Mo₂O₇)</td><td>S-scheme heterojunction improves electron-hole separation. Enhanced photocatalytic efficiency.</td><td>Photocatalytic degradation of organic pollutants in wastewater.</td></tr><tr><td>CQDs/TiO₂@Ti-TPA-MOF triple heterojunction</td><td>Carbon quantum dots (CQDs), titanium dioxide (TiO₂), and titanium-based MOF (Ti-TPA-MOF)</td><td>Excellent visible-light harvesting. Enhanced photocatalytic performance.</td><td>Photocatalytic degradation of organic pollutants.</td></tr></tbody></table>

Further insights into the application of semiconductor heterojunctions for the degradation of pharmaceutical pollutants are provided in Table 5, which compiles representative studies highlighting various material combinations, operational conditions, and observed efficiencies, demonstrating the versatility and effectiveness of these systems in advanced water treatment [102].
<table><caption>**Table 5.** Photocatalytic degradation activity toward antibiotics and pharmaceutical pollutants (CIP, hydrochloride, tetracycline, and TOC in pharmaceutical wastewater) using different heterojunction photocatalysts (reused from Jabbar et al., 2022 [102], under Elsevier License 6000951320125, 2 April 2025).</caption><thead><tr><th>Photocatalyst</th><th>Target Pollutant</th><th>Light Source</th><th>Irradiation Time</th><th>Method of Synthesis</th><th>Removal Efficiency, %</th><th>Ref.</th></tr></thead><tbody><tr><td>BiVO<sub>4</sub>-Bi<sub>2</sub>WO<sub>6</sub></td><td>CIP</td><td>Visible light (150 W Xenon lamp)</td><td>60 min</td><td>Organic additive-free, microwave-assisted method</td><td>76.8</td><td>[68]</td></tr><tr><td>Bi<sub>2</sub>S<sub>3</sub>/BiOBr</td><td>CIP (10 mg/L)</td><td>Visible-light irradiation (λ > 420 nm)</td><td>180 min</td><td>Anion exchange strategy</td><td>42 of COD</td><td>[73]</td></tr><tr><td>SnS<sub>2</sub>/BiMoO<sub>6</sub></td><td>CIP (10 mg/L)</td><td>Visible light (5 W LED white lights)</td><td>120 min</td><td>Hydrothermal</td><td>90</td><td>[96]</td></tr><tr><td>CdS-TiO<sub>2</sub></td><td>Hydrochloride (TCH), 50 mg/L</td><td>Visible light (500 W Xenon lamp)</td><td>8 h</td><td>–</td><td>87.06</td><td>[65]</td></tr><tr><td>Type II AgI/CuBi<sub>2</sub>O<sub>4</sub> Z-scheme AgBr/CuBi<sub>2</sub>O<sub>4</sub> 3D CdIn<sub>2</sub>S<sub>4</sub>/2D ZnO nanosheet heterojunctions</td><td>Tetracycline (TC), 10 mg/L TCH (10 mg/L)</td><td>Visible light (300 W Xenon lamp) Visible light (250 W Xenon lamp)</td><td>60 min 40 min</td><td>Hydrothermal Impregnation-hydrothermal</td><td>80 and 90 94.04</td><td>[87,90]</td></tr><tr><td>Z-scheme AgI/BiVO<sub>4</sub></td><td>Tetracycline (TC), 20 mg/L</td><td>Visible light (300 W Xenon lamp)</td><td>60 min</td><td>Deposition-precipitation</td><td>94.91</td><td>[97]</td></tr><tr><td>TiO<sub>2</sub>/Fe<sub>3</sub>O<sub>4</sub></td><td>Tetracycline (TC), 50 mg/L</td><td>10 W UVC lamp</td><td>60 min</td><td>Wet chemical ion exchange</td><td>98</td><td>[97]</td></tr><tr><td>In<sub>2</sub>S<sub>3</sub>/InVO<sub>4</sub></td><td>Tetracycline (TC), 10 mg/L</td><td>Visible light (300 W Xenon lamp)</td><td>60 min</td><td>Hydrothermal</td><td>71.4</td><td>[98]</td></tr><tr><td>MnO<sub>2</sub>/CNK-OH-M</td><td>TOC in pharmaceutical wastewater</td><td>Visible light (300 W Xenon lamp)</td><td>120 min</td><td>Calcination-impregnation</td><td>74.9</td><td>[103]</td></tr></tbody></table>

### 3.5. Nanostructured Photocatalysts

Nanoscale materials offer larger surface areas, shorter diffusion paths for charge carriers, and more active sites for reactions. Nanostructures such as nanoparticles, nanorods, nanotubes, and core-shell structures are engineered to optimize the photocatalytic behavior by increasing the surface area, shortening the diffusion paths for the charge carrier, and increasing the number of active sites [68,104,105]. Nanoparticles provide high surface-to-volume ratios. Nanorods and nanotubes improve directional charge transport. Core-shell structures enable precise control over interface properties and reactivity.

Table 6 provides an overview of various nanostructured photocatalysts, emphasizing their nanostructures, enhancements, and specific applications in water treatment and environmental remediation. This categorization supports a clearer understanding of material development trends and provides a structured reference for researchers exploring the heterogeneous photocatalysis in water treatment.
<table><caption>**Table 6.** Comparison of nanostructured photocatalysts.</caption><thead><tr><td>Photocatalyst</td><td>Nanostructure</td><td>Key Features</td><td>Applications</td></tr></thead><tbody><tr><td>ZnS-based nanostructures</td><td>0D, 1D, 3D morphologies</td><td>Versatile nanostructures enhance photocatalytic activity. Effective under simulated and sunlight irradiation.</td><td>Photocatalytic degradation of organic pollutants in wastewater.</td></tr><tr><td>WO<sub>3</sub>-based nanostructures</td><td>Various nanostructures</td><td>Enhanced photocatalytic activity through nanostructuring. Improved application in water treatment.</td><td>Photocatalytic degradation of contaminants in water.</td></tr><tr><td>ZnO nanostructures</td><td>Controlled microscale arrangement and nanoscale structure</td><td>Optimized light absorbance capacity. Enhanced photocatalytic activity for organic pollutant removal.</td><td>Photocatalytic degradation of organic pollutants in water.</td></tr><tr><td>MgO/graphene nanoplatelet nanocomposites</td><td>Nanocomposite with graphene nanoplatelets</td><td>Significant photocatalytic activity. Enhanced antibacterial performance.</td><td>Purification of industrial wastewater. Potential applications in nanomedicine.</td></tr><tr><td>ZnO/NiFe<sub>2</sub>O<sub>4</sub> nanocomposite</td><td>Composite nanoparticles</td><td>High degradation efficiency under UV light. Effective against methylene blue dye.</td><td>Photocatalytic degradation of organic dyes in wastewater.</td></tr><tr><td>WO<sub>3</sub>/Ti-WO<sub>x</sub>/TiH<sub>y</sub>O<sub>z</sub> heterojunction</td><td>Homo/heterojunction nanostructure</td><td>Extended light absorption above 400 nm. Improved charge separation.</td><td>Photocatalytic degradation of azo dye water pollutants under visible light.</td></tr><tr><td>TiO<sub>2</sub> nanotubes</td><td>1D tubular arrays</td><td>High surface area, directional electron transport, improved light harvesting.</td><td>Degradation of pharmaceuticals and endocrine-disruptors under UV/visible light.</td></tr><tr><td>TiO<sub>2</sub>/graphene oxide composite</td><td>2D layered nanocomposite</td><td>Enhanced charge separation, extended visible-light response.</td><td>Removal of antibiotics and organic dyes from wastewater.</td></tr><tr><td>N-doped TiO<sub>2</sub> nanoparticles</td><td>0D nanoparticles (doped)</td><td>Narrowed band gap, visible-light activation.</td><td>Photocatalytic degradation of emerging pollutants under solar light.</td></tr><tr><td>TiO<sub>2</sub>/BiVO<sub>4</sub> heterojunction</td><td>Z-scheme nanocomposite</td><td>Effective charge separation, expanded light absorption range.</td><td>Treatment of pharmaceutical residues and PFAS-contaminated water.</td></tr></tbody></table>

Figure 7 illustrates the key types of nanostructured photocatalysts designed to enhance the photocatalytic performance for environmental applications. These include nanoparticles, nanorods, nanotubes, and core-shell structures, each offering unique structural and functional advantages [42,50,104,106].

Nanoparticles are depicted as small, spherical entities with high surface-to-volume ratios that provide abundant active sites for photocatalytic reactions. Nanorods are elongated structures that facilitate directional charge transport and improved light absorption due to their geometry. Nanotubes, represented as hollow cylindrical forms, offer confined spaces
that enhance the reaction efficiency and reduce the charge recombination by promoting rapid carrier migration [107,108].

<figure><img src="image_9.png" /><figcaption>**Figure 7.** Representative nanostructured photocatalysts, illustrating various morphologies (e.g., nanoparticles, nanorods, nanotubes) and their associated functional enhancements that contribute to improved photocatalytic activity, such as increased surface area, charge transport efficiency, and light absorption.</figcaption></figure>

The core-shell structures, composed of a central core enveloped by a distinct shell layer, illustrate how combining different materials can optimize the charge separation, band alignment, and structural stability. Collectively, these engineered nanostructures aim to increase the surface area, improve the charge carrier dynamics, and enhance light-harvesting capabilities, thereby significantly improving the photocatalytic efficiency in water treatment and other environmental applications.

However, while many photocatalysts demonstrate promising results under controlled conditions, their long-term stability in real water matrices remains a concern. For example, ZnO is more susceptible to photocorrosion and leaching than TiO₂, particularly in acidic environments, which limits its field applicability despite its high electron mobility [24,25]. Moreover, the activity of BiVO₄ and similar bismuth-based materials has been shown to deteriorate due to surface instability and poor charge separation in complex water matrices [32,33].

## 4. Applications of Heterogeneous Photocatalysis in Water Purification

Heterogeneous photocatalysis has shown remarkable potential in addressing various water contaminants that are often resistant to conventional treatment processes. Its non-selective oxidative capacity enables the degradation or removal of a wide range of pollutants, including trace organic compounds, microbial pathogens, and natural organic matter [109,110]. Figure 8 provides a symbolic overview of selected photocatalytic applications, while detailed descriptions of the underlying processes and operational features are presented in the text.
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Application of catalysts in water purification: (a) photocatalytic degradation pathway of organic micropollutants in water treatment; (b) mechanism of photocatalytic pathogen inactivation in water treatment; (c) photocatalytic transformation pathways of inorganic contaminants in water treatment; (d) photocatalytic mitigation of natural organic matter and disinfection by-product precursors; (e) pilot-scale applications of photocatalytic systems in real water matrices.</figcaption></figure>

### 4.1. Degradation of Organic Micropollutants

Organic micropollutants such as pharmaceuticals, personal care products, pesticides, endocrine-disrupting compounds (EDCs), and industrial chemicals are increasingly detected in surface and drinking water sources. These substances, even at low concentrations, pose risks to human health and ecosystems due to their persistence and bioaccumulative properties [2,4,9,17,110].

Photocatalysis has proven to be effective in degrading these compounds by breaking their complex molecular structures into simpler intermediates, ultimately leading to mineralization into CO₂ and H₂O. Figure 8a illustrates the photocatalytic degradation process of organic micropollutants, highlighting the breakdown of complex contaminants such as pharmaceuticals, hormones, and pesticides into simpler, less harmful intermediates, ultimately leading to complete mineralization into carbon dioxide and water [55,111]. It visually supports the role of photocatalysts like TiO₂ and g-C₃N₄ in enabling effective treatment under UV and visible light conditions. Studies using TiO₂ and g-C₃N₄ have demonstrated successful degradation of antibiotics (e.g., sulfamethoxazole, ciprofloxacin),
hormones (e.g., 17α-ethinylestradiol), and herbicides (e.g., atrazine) under both UV and visible light [92,112].

## 4.2. Pathogen Inactivation and Disinfection

Disinfection is a critical aspect of water treatment to prevent the transmission of waterborne diseases. Traditional disinfection methods, such as chlorination, may generate harmful byproducts like trihalomethanes. Photocatalysis offers a chemical-free alternative for microbial inactivation, by generating reactive oxygen species that damage cell membranes, proteins, and nucleic acids [113,114]. Figure 8b suggests the mechanism by which photocatalysis inactivates waterborne pathogens through the generation of reactive oxygen species (ROS). These species attack critical cellular components, such as membranes, proteins, and nucleic acids, leading to the destruction of bacteria, viruses, and protozoa. Photocatalysis is emphasized as a chemical-free and effective alternative to conventional disinfection methods, including its potential in addressing antibiotic-resistant microorganisms [115,116].

Photocatalytic systems have been reported to effectively inactivate a variety of pathogens, including bacteria (e.g., *E. coli*, *S. aureus*), viruses (e.g., adenovirus, MS2 bacteriophage), and protozoa (e.g., *Giardia lamblia*) [116,117]. Recent advances also show promise in targeting antibiotic-resistant bacteria and genes, contributing to efforts in antimicrobial resistance mitigation [118,119].

## 4.3. Removal of Inorganic Pollutants

Although less common, heterogeneous photocatalysis can also facilitate the transformation of certain persistent inorganic contaminants. For example, photocatalytic reduction can convert toxic Cr(VI) to less toxic Cr(III), or nitrate to nitrogen gas. Figure 8c illustrates the photocatalytic reduction pathways for selected inorganic contaminants, such as the conversion of toxic hexavalent chromium [Cr(VI)] to the less harmful trivalent form [Cr(III)], and the reduction of nitrate to nitrogen gas [120,121]. It highlights the role of tailored photocatalysts and sacrificial agents in enhancing the selectivity and efficiency of these redox reactions within advanced water treatment processes. Tailored catalysts and sacrificial agents enhance the selectivity and efficiency of these reactions [99,122].

## 4.4. Natural Organic Matter and Disinfection Byproducts

Natural organic matter (NOM), ubiquitous in surface and ground waters, can act as a precursor to harmful disinfection byproducts during chlorination. Photocatalytic treatment can reduce the aromaticity and molecular weight of NOM, thereby mitigating the formation of byproducts such as trihalomethanes and haloacetic acids [100,111]. Figure 8d shows how photocatalysis alters natural organic matter (NOM) by reducing its aromatic content and molecular weight. This transformation limits the formation of harmful disinfection byproducts, such as trihalomethanes and haloacetic acids, during chlorination [100,123], supporting the role of the photocatalytic processes in improving water safety and reducing chemical risks from treatment systems.

## 4.5. Pilot-Scale and Real Water Matrix Applications

While most photocatalytic studies are conducted under laboratory conditions using synthetic water, there is growing interest in evaluating photocatalytic systems in real or natural water matrices. These environments contain competing ions, dissolved organic matter, and turbidity, which can affect the photocatalytic performance [124,125].

Several pilot-scale photocatalytic reactors, including solar-driven systems and those powered by artificial light sources such as UV lamps and visible-light LEDs, have demonstrated promising results in degrading emerging contaminants under realistic operating
conditions. Figure 8e illustrates the implementation of photocatalytic systems at pilot scale under real-world conditions, including the treatment of surface water, secondary effluents, and drinking water. It emphasizes the influence of complex water matrices, such as the presence of ions, organic matter, and turbidity on system performance, while showcasing the potential of solar-driven reactors for practical, decentralized water purification [126,127]. However, further development is needed to address operational challenges and ensure regulatory compliance.

# 5. Reactor Systems and Process Parameters

The performance of heterogeneous photocatalytic water treatment systems is not solely dependent on the choice of photocatalyst, but also on the design of the photocatalytic reactor and operational parameters. Effective reactor configurations must maximize light utilization, optimize catalyst–pollutant contact, and facilitate scalability while ensuring energy efficiency and practical applicability.

## 5.1. *Photocatalytic Reactor Designs*

Photocatalytic reactors can be broadly categorized based on how the catalyst is deployed, either as a suspended powder (slurry systems) or as immobilized on a support (fixed systems) (Figure 9) [128–132].

* Slurry reactors: These systems involve the dispersion of catalyst nanoparticles throughout the aqueous phase, offering high surface area and mass transfer. However, post-treatment separation of the catalyst is necessary, which can increase operational complexity and cost.

* Immobilized systems: In these reactors, the catalyst is fixed on substrates such as glass, ceramic, polymer membranes, or stainless steel meshes. While they reduce the need for separation, mass transfer may be limited, and surface deactivation can occur over time.

* Annular reactors: These consist of a cylindrical configuration with a light source at the center and photocatalyst-coated surfaces arranged around it. They are commonly used in lab-scale and pilot studies for their uniform light distribution.

* Flat plate and falling film reactors: Suitable for solar applications, these designs maximize light exposure and are effective for shallow water layers, making them ideal for outdoor use.

* Fluidized bed and membrane photocatalytic reactors: Emerging designs such as these integrate advanced features like enhanced catalyst suspension, reduced fouling, or combined separation and reaction zones.

A comprehensive comparison of various photocatalytic reactor configurations commonly employed in environmental applications, particularly in water and wastewater treatment, is presented in Table 7. Each reactor type exhibits distinct design principles, operational advantages, and associated challenges that influence its suitability for specific applications and scales. These systems address limitations related to fouling and scalability, although they often involve complex engineering and higher initial costs. Overall, Table 7 underscores the trade-offs among catalyst efficiency, operational complexity, and scalability, guiding the selection of reactor configurations based on specific treatment goals and implementation contexts.
Slurry Reactor for Photocatalysis

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>Annular Reactor</figcaption></figure>

<figure><img src="image_13.png" /></figure>

Immobilized System
for Photocatalysis

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Flat Plate and
Falling Film Reactor</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Figure 9.** Representative designs of reactors employed in photocatalytic processes, including batch, fixed-bed, and slurry configurations. These reactor types influence light distribution, catalyst contact, and scalability, thereby affecting the overall performance of photocatalytic water treatment systems.</figcaption></figure>

Slurry reactors, characterized by the dispersion of catalyst nanoparticles within the aqueous phase, offer high photocatalytic efficiency due to enhanced surface area and mass transfer. Fouad et al. (2023) investigated the photocatalytic degradation of methylene blue dye using an anatase-titania slurry in a falling film reactor [133]. The results demonstrated enhanced degradation efficiency, highlighting the effectiveness of slurry systems in dye removal applications. In a study conducted by Rani and Karthikeyan (2018), a slurry photocatalytic reactor equipped with a 16 W UV lamp emitting at 254 nm was utilized to degrade phenanthrene (PHE), a polycyclic aromatic hydrocarbon known for its persistence in the environment [134]. The researchers explored various operational parameters, including initial PHE concentration (ranging from 1000 to 1500 µg/L), catalyst dosage (between 0.1 and 0.9 g/L), and pH levels (from 3.0 to 9.0), to determine their effects on degradation efficiency.
<table><caption>**Table 7.** Key features and operational considerations of photocatalytic reactors.</caption><thead><tr><td>Reactor Type</td><td>Catalyst Configuration</td><td>Design Features</td><td>Advantages</td><td>Limitations</td><td>Typical Applications</td><td>Scale</td></tr></thead><tbody><tr><td>Slurry reactors</td><td>Catalyst nanoparticles dispersed in liquid phase</td><td>Agitated or recirculating reactor vessel</td><td>- High surface area<br/>- Excellent mass transfer<br/>- High photocatalytic efficiency</td><td>- Catalyst recovery required<br/>- Increased post-treatment complexity<br/>- Risk of nanoparticle aggregation</td><td>- Degradation of pollutants in wastewater<br/>- Lab-scale studies for catalyst testing</td><td>Lab to pilot</td></tr><tr><td>Immobilized systems</td><td>Catalyst fixed on supports (glass, ceramic, polymer membranes, stainless steel meshes)</td><td>Static or flow-through systems</td><td>- No catalyst recovery needed<br/>- Suitable for continuous operation<br/>- Low risk of secondary contamination</td><td>- Lower mass transfer<br/>- Possible surface deactivation over time<br/>- Catalyst replacement can be costly</td><td>- Water disinfection<br/>- Air purification<br/>- Surface reactors in continuous systems</td><td>Lab to full scale</td></tr><tr><td>Annular reactors</td><td>Catalyst coated on inner or outer cylinder surface</td><td>Cylindrical geometry with central light source</td><td>- Uniform irradiation<br/>- Scalable for pilot systems<br/>- Good light utilization</td><td>- Limited surface area<br/>- Difficult to clean and maintain<br/>- Requires precise engineering</td><td>- Advanced oxidation processes<br/>- Photodegradation of organics<br/>- Pilot-scale photocatalysis</td><td>Lab to pilot</td></tr><tr><td>Flat plate and falling film reactors</td><td>Catalyst on flat surface or inclined plate</td><td>Open or closed systems exposed to natural or artificial light</td><td>- Excellent light exposure<br/>- Ideal for shallow water treatment<br/>- Effective use of solar energy</td><td>- Limited water depth<br/>- Potential for drying or channeling<br/>- Lower throughput for large volumes</td><td>- Solar photocatalysis<br/>- Surface water treatment<br/>- Field applications in sunny climates</td><td>Lab to field</td></tr><tr><td>Fluidized bed reactors</td><td>Catalyst particles suspended in liquid by upward flow</td><td>Dynamic bed with mixing and irradiation</td><td>- Good mixing<br/>- Reduced fouling<br/>- Scalable design</td><td>- Complex hydrodynamics<br/>- Particle attrition<br/>- Requires flow rate optimization</td><td>- Industrial-scale treatment<br/>- Decentralized treatment systems</td><td>Pilot to industrial</td></tr><tr><td>Membrane photocatalytic reactors</td><td>Catalyst embedded in or coated on membrane</td><td>Combines photocatalysis with membrane filtration</td><td>- Simultaneous reaction and separation<br/>- Reduced fouling potential<br/>- Enhanced retention of products</td><td>- Membrane fouling still possible<br/>- High fabrication cost<br/>- Long-term stability concerns</td><td>- Treatment of micropollutants<br/>- Water reuse and recycling<br/>- Emerging water purification technologies</td><td>Lab to pilot</td></tr></tbody></table>
Under optimized conditions, specifically a PHE concentration of 1000 µg/L, TiO₂ dosage of 0.5 g/L, and pH set at 3.0, the study achieved an 83.5% degradation of PHE and a 60.2% removal of total organic carbon (TOC) within a 3-hour reaction period. The degradation process adhered to pseudo-first-order kinetics, underscoring the efficacy of slurry reactors in treating water contaminated with persistent organic pollutants. However, the requirement for downstream catalyst separation poses practical and economic challenges, particularly at larger scales.

In contrast, immobilized systems utilize fixed catalysts on solid supports, significantly reducing the need for separation and minimizing secondary pollution. Researchers have explored the use of 3D printing technologies for immobilizing photocatalytic materials, aiming to enhance the stability and reusability of the catalysts in water treatment applications. This method offers a promising avenue for developing efficient immobilized systems [135]. A widely cited study by Malato et al. (2002) investigated the use of immobilized titanium dioxide (TiO₂) on glass beads in a compound parabolic collector reactor for the solar photocatalytic degradation of contaminants in wastewater [49]. The catalyst was fixed on the surface of glass supports, eliminating the need for post-treatment separation. The system was tested for the degradation of herbicides and pesticides, including isoproturon and diuron, under real sunlight conditions. Despite their advantages, these systems may suffer from lower mass transfer rates and potential surface deactivation over time.

Annular reactors, with their cylindrical geometry and centralized light source, provide uniform light distribution and are often adopted in laboratory and pilot-scale studies. Their well-defined structure facilitates reproducibility but limits scalability and cleaning ease. An annular photocatalytic reactor was employed by Kumar and Bansal (2013) for the degradation of organic dyes, with titanium dioxide immobilized on the reactor's surface [136]. The study reported effective degradation, underscoring the reactor's applicability in treating dye-laden effluents. A study conducted by Abracia et al. (2025) explored the degradation of benzo[a]pyrene (BaP), a carcinogenic polycyclic aromatic hydrocarbon, in aqueous solutions using an annular photocatalytic reactor with immobilized zinc oxide (ZnO) as photocatalyst [137]. The reactor featured a 500 mm length with a central UV light source, and ZnO was immobilized on the reactor's inner surface. Using computational fluid dynamics (CFD) simulations, the research analyzed the effects of various operational parameters, including residence time, initial BaP concentration, surface irradiance, and catalyst zone length, on the degradation efficiency. The study demonstrated that optimizing these parameters significantly enhanced the reactor's performance, achieving substantial BaP degradation. These works underscore the potential of annular reactors with immobilized photocatalysts for effectively treating persistent organic pollutants in water.

Flat plate and falling film reactors are particularly suited for solar-driven applications due to their ability to maximize light exposure in shallow liquid layers. These designs are effective in outdoor settings, although they may be constrained by low water volumes and operational variability. A falling film photocatalytic reactor was modeled by Colina-Marquez et al. (2016) for environmental applications, emphasizing its simple construction and high solar radiation utilization [138]. The study highlighted the reactor's potential in harnessing solar energy for pollutant degradation. A study by Aziz et al. (2019) investigated the degradation pathways of the pharmaceuticals diclofenac (DCF) and ibuprofen (IBP) in aqueous solutions using a photocatalytic falling film reactor [139]. In this setup, TiO₂ was coated on Pilkington Activ™ glass (Pilkington Group Limited (part of NSG Group), St Helens, UK) serving both as the photocatalyst and falling liquid film generator. The reactor was illuminated with UV-A light to initiate the photocatalytic process. The degradation kinetics of both pharmaceuticals followed a pseudo-first-order model. Various intermediate byproducts were identified during the photocatalytic degradation of IBP,
including 4-isobutylbenzoic acid and 4-isobutylacetophenone. The study provided insights into the degradation pathways and highlighted the effectiveness of the falling film reactor design in treating pharmaceutical contaminants in water.

Emerging technologies such as fluidized bed and membrane photocatalytic reactors integrate advanced features like enhanced catalyst suspension, improved mixing, and combined separation and reaction zones. Gusmao et al. (2024) explored a gas-liquid-solid mini-fluidized bed reactor for wastewater treatment, demonstrating its potential as a high-efficiency photocatalytic system [140]. The reactor exhibited improved degradation rates of pollutants, indicating its suitability for industrial applications. Another study performed by Xu et al. (2023) investigated the photocatalytic degradation of acetaldehyde, a common indoor air pollutant, using a fluidized bed reactor [141]. In this setup, nano titanium dioxide (TiO₂) was employed as photocatalyst. The reactor design facilitated the effective contact between the gaseous pollutant and the photocatalyst particles, enhancing the degradation efficiency. The study also conducted kinetic analyses to understand the degradation process, proposing an optimal kinetic model based on reactor performance. This research underscores the potential of fluidized bed reactors in addressing volatile organic compounds (VOCs) in air purification applications. This example highlights the practical application of fluidized bed photocatalytic reactors in air purification, demonstrating their effectiveness in degrading harmful indoor pollutants like acetaldehyde.

Membrane photocatalytic reactors integrate photocatalysis with membrane filtration, allowing simultaneous reaction and separation processes. They offer reduced fouling potential and enhanced retention of reaction products but may face challenges related to membrane fouling and high fabrication costs. A novel pilot-scale continuous photocatalytic membrane reactor was tested by Plakas et al. (2016), for the degradation of recalcitrant micropollutants [142]. The reactor successfully removed various contaminants, showcasing its effectiveness in advanced water treatment processes.

A study conducted by Constantin et al. (2024) investigated the performance of a solar-driven slurry photocatalytic membrane reactor for the advanced treatment of municipal wastewater [143]. The reactor utilized iron-doped titanium dioxide (Fe-TiO₂) as photocatalyst and a polysulfone-based membrane for the separation process. Under both simulated and natural solar light irradiation, the system achieved chemical oxygen demand (COD) removal efficiencies ranging from 66% to 95%, over a 7-hour irradiation period. The results demonstrated the enhanced photocatalytic activity afforded by iron doping and established solar-powered slurry as an effective, low-energy, and environmentally friendly alternative for municipal wastewater treatment. Integrating photocatalysis with membrane separation processes and utilizing solar energy can enhance the degradation of organic pollutants in wastewater.

## 5.2. *Light Sources*

The choice of light source significantly impacts system efficiency, energy consumption, and operating costs [65].

* UV lamps: Conventional low- and medium-pressure mercury lamps are effective, but energy-intensive and limited to UV-active photocatalysts.

* Visible light and solar illumination: Increasing research efforts focus on visible-light-responsive photocatalysts compatible with LEDs or sunlight, offering more sustainable and cost-effective operation.

* Light emitting diodes (LEDs): These are increasingly preferred due to their energy efficiency, long lifespan, and tunable wavelength output.

Figure 10 presents a comparison of three primary light sources used in photocatalytic water treatment: UV lamps, visible light (including solar illumination), and light-emitting
diodes (LEDs). It highlights key distinctions in their operational characteristics, including energy efficiency, wavelength compatibility, and suitability for different photocatalysts. The layout supports a clear understanding of how light source selection influences the photocatalytic performance and sustainability.

<figure><img src="image_17.png" /><figcaption>**Figure 10.** Comparative overview of common light sources used in photocatalytic water treatment, including UV lamps (100–400 nm), visible light (400–700 nm), and LEDs. The diagram summarizes their typical wavelength ranges, energy efficiency, and compatibility with different photocatalyst types.</figcaption></figure>

A study conducted by Nadeem et al. (2021) examined the degradation of methylene blue (MB) dye using a CFA-ZnFe₂O₄ photocatalyst under three distinct lighting conditions: white LEDs, natural sunlight, and ultraviolet (UV) light at 254 nm [29]. Among these, UV light (144 W, 254 nm) exhibited the highest degradation efficiency, which is attributed to its shorter wavelength and higher photon energy. In a separate investigation, Jallouli et al. (2018) explored how varying the number of LED lamps influences the photocatalytic degradation of ibuprofen (IBU) using TiO₂ at a concentration of 1.0 g/L [144]. Their results demonstrated a positive correlation between the number of LEDs and the degradation rate of IBU, suggesting that enhanced light intensity leads to increased production of hydroxyl radicals, thereby boosting photocatalytic activity. Additional research findings on the influence of different light sources on the photocatalytic removal of organic pollutants are compiled in Table 8.

<table><caption>**Table 8.** The photocatalytic destruction performance of different organic compounds is affected by different light sources (UV radiation, sunlight, and visible light) (reused from Jabbar et al., 2022 [102], under Elsevier License 6000951320125, 2 April 2025).</caption><thead><tr><td>Photocatalyst</td><td>Target Pollutant</td><td>Light Source</td><td>Photocatalytic Performance</td><td>References</td></tr></thead><tbody><tr><td>9 AC-ZnO</td><td>4-acetylphenol</td><td>UV radiation, sunlight</td><td>Under UV light, 100% of 4-acetylphenol was degraded in 150 min.<br>Under sunlight, 100% of 4-acetylphenol was degraded in 120 min.</td><td>[54]</td></tr></tbody></table>
<table><caption>**Table 8. Cont.**</caption><thead><tr><th>Photocatalyst</th><th>Target Pollutant</th><th>Light Source</th><th>Photocatalytic Performance</th><th>References</th></tr></thead><tbody><tr><td>TiO<sub>2</sub></td><td>Coke water</td><td>UV radiation</td><td>At a light intensity of 400 mWcm<sup>-2</sup>, the removal efficiency was 15% at 60 min.<br>At a light intensity of 1300 mWcm<sup>-2</sup>, the removal efficiency was 30% at 60 min.</td><td>[38]</td></tr><tr><td>TiO<sub>2</sub>-TiO<sub>2</sub> nanorod arrays</td><td>COD in coking wastewater</td><td>300 W UV light (250-380 nm)</td><td>At a light intensity of 400 mWcm<sup>-2</sup>, 85% of COD was removed in 60 min.<br>At a light intensity of 1300 mWcm<sup>-2</sup>, 92% of COD was removed in 60 min.</td><td>[83]</td></tr><tr><td>MWCNT/ZnO</td><td>Acetaldehyde</td><td>Q-switched Nd-YAG laser</td><td>At laser irradiation energy of 60 mJ, 36% of acetaldehyde was degraded in 10 min.<br>At laser irradiation energy of 120 mJ, 60% of acetaldehyde was degraded in 10 min.</td><td>[86]</td></tr><tr><td>Nitrogen-doped TiO<sub>2</sub> (N-TiO<sub>2</sub>)</td><td>Benzene</td><td>Visible light irradiation</td><td>Photoreaction coefficient (k<sub>pm</sub>) is 3.992 × 10<sup>-6</sup> mol·kg<sup>-1</sup> s<sup>-1</sup> at an illumination intensity of 36 × 10<sup>-4</sup> Ix and 11.55 × 10<sup>-6</sup> mol·kg<sup>-1</sup>·s<sup>-1</sup> at an illumination intensity of 75 × 10<sup>-4</sup> Ix.</td><td>[68]</td></tr><tr><td>TiO<sub>2</sub>/organic fibers</td><td>Acid orange 7 (AO7)</td><td>UV radiation, solar light</td><td>The degradation rate of solar light is 1.5 times greater than that of artificial UV light.</td><td>[69]</td></tr><tr><td>TiO<sub>2</sub> powder</td><td>Rhodamine B</td><td>UV radiation</td><td>At a light intensity of 23 W/m<sup>2</sup>, the removal efficiency was 42.1% at 90 min.<br>At a light intensity of 114 W/m<sup>2</sup>, the removal efficiency was 87.8% at 90 min.</td><td>[106]</td></tr></tbody></table>

### 5.3. Operational Parameters

Several operational factors must be carefully optimized to maximize the photocatalytic performance (Figure 11) [51,60,79,99,102,145–150].

#### (i) Catalyst loading strategies in reactor configurations

The concentration of the photocatalyst significantly influences the degradation efficiency. Insufficient catalyst amount results in fewer active sites, limiting the generation of reactive oxygen species (ROS). Conversely, excessive catalyst loading can lead to particle agglomeration and increased solution turbidity, causing light scattering that reduces the photon penetration and hinders the photocatalytic activity.

For example, Boudechiche et al. (2024) demonstrated that higher catalyst doses provide more active sites, accelerating degradation [151]. However, beyond an optimal point, the efficiency declines due to light scattering and reduced photon absorption [151]. Zhang et al. (2023) reported that increasing the concentration of g-C<sub>3</sub>N<sub>4</sub> from 0.1 to 1.0 g/L improved the degradation rate of rhodamine B, but further increases to 2.0 g/L resulted in reduced performance due to light scattering and reduced photoactivation efficiency [152]. A study performed by Tolosana-Moranchel et al. (2021) evaluated the influence of light scattering by different TiO<sub>2</sub> catalysts on photonic energy absorption [153]. It was found that higher concentrations of TiO<sub>2</sub> particles led to increased light scattering, thereby reducing
the number of photons reaching active sites and diminishing the photocatalytic efficiency. For instance, Zhang et al. reported that increasing the g-C₃N₄ dosage from 0.1 to 1.0 g/L enhanced the degradation of rhodamine B, but a further increase to 2.0 g/L reduced efficiency, due to excessive light scattering [152].

<figure><img src="image_18.png" /><figcaption>**Figure 11.** Critical operational parameters influencing the efficiency of photocatalytic degradation processes, including catalyst loading, pH, dissolved oxygen, initial contaminant concentration, and ionic composition. These factors interact to affect light absorption, ROS generation, and pollutant-catalyst interactions and must be optimized to enhance overall system performance.</figcaption></figure>

Additionally, it is important to recognize that the trade-offs associated with catalyst loading extend beyond photocatalytic efficiency. In slurry systems, high catalyst dosages may lead to increased operational complexity, due to the need for downstream separation and recovery of fine particles. These challenges are particularly relevant in scaling up to pilot and industrial levels, where process economy and sustainability are essential. Studies have highlighted that excessive catalyst use, while initially improving performance, may compromise process simplicity and increase sludge handling costs [151,153]. Consequently, future design strategies should integrate optimization of catalyst dosage with reactor configuration and reuse potential to enhance both efficiency and practicality.

Thus, determining an optimal catalyst concentration is essential for balancing active site availability and light absorption.

### (ii) pH of the medium

The pH level of the solution affects the surface charge of the photocatalyst and the ionization state of contaminants, influencing the adsorption behavior and ROS generation. Titanium dioxide (TiO₂) typically exhibits higher photocatalytic activity in acidic to neutral pH ranges. At these pH levels, the TiO₂ surface is positively charged, promoting the electrostatic attraction with negatively charged pollutants, thereby enhancing degradation efficiency. However, the optimal pH can vary depending on the specific photocatalyst and target contaminant. For example, Kumar and Pandey (2017) found that the adsorption of dyes on TiO₂ is minimal at the isoelectric point, but increases under acidic conditions, due to favorable electrostatic interactions [154]. Adjusting the pH to suit the specific photocatalyst and pollutant is essential for maximizing the photocatalytic efficiency. Typically, TiO₂-based systems perform better under slightly acidic to neutral conditions, enhancing
electrostatic attraction between catalyst and pollutants. The research of Hasham Firooz et al. [155] demonstrated that the photocatalytic degradation of tetracycline using TiO₂ was significantly influenced by pH. The photocatalytic degradation of tetracycline by TiO₂ was shown to be most efficient at pH 5–6, while performance dropped significantly at alkaline pH due to decreased electrostatic interactions between the pollutant and the catalyst surface [155]. At higher pH levels, the tetracycline changes to its neutral or anionic forms, reduces its interaction with the positively charged TiO₂ surface, thereby decreasing the degradation efficiency.

Moreover, the photocatalytic behavior of other widely used semiconductors such as g-C₃N₄ and ZnO is also significantly influenced by pH. For g-C₃N₄, acidic conditions typically promote enhanced degradation due to increased electrostatic attraction between its negatively charged surface and positively charged contaminants. However, in alkaline environments, the surface deprotonation can reduce this interaction and lower the process efficiency [92]. For example, the degradation of rhodamine B by g-C₃N₄ was reported to be most effective at pH 4–6, where increased adsorption and suppressed recombination of photogenerated charge carriers were observed [92]. In contrast, at higher pH values, the repulsion between deprotonated catalyst surfaces and anionic pollutants, as well as the decreased oxidative potential of photogenerated holes, led to reduced photocatalytic efficiency.

ZnO, on the other hand, exhibits optimal photocatalytic activity under slightly acidic to neutral pH, as it tends to dissolve in strongly acidic or basic media, compromising the structural stability and active surface area [86,99]. Studies have shown that the degradation of methylene blue and acid orange 7 using ZnO is most efficient in the pH range of 6–8, where both sufficient hydroxyl radical generation and stable surface interactions are maintained [86,99]. Beyond this range, particularly above pH 9, the photocatalyst becomes prone to photocorrosion, leading to Zn²⁺ leaching and a decline in reusability. Additionally, surface charge repulsion at high pH values can hinder the adsorption of anionic pollutants such as azo dyes, further reducing performance. These findings underscore the need to tailor the pH of the reaction system according to the specific photocatalyst and target pollutants to maximize performance and ensure material stability.

### (iii) Dissolved oxygen

Oxygen acts as a crucial electron acceptor in photocatalytic reactions, facilitating the formation of ROS necessary for pollutant degradation. The availability of dissolved oxygen directly influences the reaction rate. Research indicates that increasing the dissolved oxygen content can significantly improve the degradation rate constant. For instance, Bakhtiar Azim et al. (2024) assessed the photocatalytic activity of graphene oxide and hydrogen peroxide (H₂O₂) by photodegrading methylene blue (MB) in an aqueous solution [101]. Increasing the dissolved oxygen content from 2.8 to 3.9 mg/L, resulted in an increase in the degradation rate constant from 0.035 to 0.062 min⁻¹ in graphene oxide-based photocatalysis of methylene blue [101]. Ensuring adequate oxygen levels in the reaction medium is, therefore, vital for effective photocatalysis.

### (iv) Initial contaminant concentration

The starting concentration of pollutants affects also the photocatalytic performance. High contaminant levels can saturate the catalyst's surface, leading to reduced active sites and diminished degradation rates. Additionally, elevated pollutant concentrations may cause light attenuation, further hindering the process. Studies have shown that higher initial concentrations of contaminants result in lower degradation efficiencies due to these factors [156].
A study of Yazdanbakhsh et al. [93] investigated the photocatalytic degradation of diclofenac (DCF) using Mn-WO₃ under LED light irradiation. The results demonstrated that as the initial concentration of DCF increased from 1 to 10 mg/L, the degradation rate decreased. This decline was attributed to the saturation of active sites on the photocatalyst and reduced generation of hydroxyl radicals at higher contaminant concentrations. Research on the photocatalytic oxidation of butyraldehyde (BUTY) performed by Abdelkader et al. [157] revealed that increasing the initial pollutant concentration led to a decrease in the removal efficiency. At a constant flow rate, higher BUTY concentrations resulted in lower degradation rates due to the limited availability of active sites and reduced contact time with the reactive species. An experimental investigation into the degradation of clofibric acid (CA) showed that at initial concentrations of 5 and 10 mg/L, approximately 100% elimination was achieved after 15 min [158]. However, when the initial concentration was increased to 300 mg/L, only 87% removal was observed, indicating that higher pollutant loads can hinder photocatalytic performance. Optimizing the initial pollutant load is necessary to maintain effective photocatalytic activity.

(v) Ionic strength and competing ions

In natural water matrices, various inorganic ions such as chloride ($Cl^-$), sulfate ($SO_4^{2-}$), and phosphate ($PO_4^{3-}$) can influence the photocatalytic reactions. These ions may compete with pollutants for adsorption sites on the photocatalyst or scavenge ROS, thereby reducing degradation efficiency. However, some studies have reported that certain anions can promote the photocatalytic activity under specific conditions [159]. For example, research demonstrated that chloride and sulfate ions enhanced the generation of hydrated electrons, leading to improved degradation of perfluorocarboxylates in a vacuum ultraviolet/hydrogen system. Chloride ions can have dual effects on photocatalytic degradation. At lower concentrations, $Cl^-$ may enhance degradation by forming reactive chlorine species. Conversely, at higher concentrations, $Cl^-$ can inhibit degradation by scavenging hydroxyl radicals (•OH) and competing for active sites on the photocatalyst. For instance, a study demonstrated that chloride ions at a 0.5 mM concentration increased the degradation of methylene blue, while higher concentrations led to inhibition [105,160]. Sulfate ions generally exhibit an inhibitory effect on the photocatalytic processes by adsorbing onto the photocatalyst surface and blocking active sites. Research indicates that sulfate anions had the most significant inhibitory effect on the degradation of methylene blue, likely due to their strong adsorption affinity [160]. Phosphate ions can also inhibit the photocatalytic activity by adsorbing onto the photocatalyst surface, thereby reducing the availability of active sites for pollutant degradation. A study observed that phosphate ions significantly decreased the generation of photogenerated holes and hydroxyl radicals, leading to reduced photocatalytic efficiency [161].

To enhance the efficiency of photocatalytic processes, it is essential to optimize various operational parameters. For example, several studies reported optimal catalyst dosages ranging from 0.5 to 2.0 g/L for $TiO_2$-based systems, with irradiation times typically between 60 and 180 min under visible or UV light sources [97,98]. Reaction temperatures are often maintained between 20 and 35 °C to simulate ambient conditions [104].

Tailoring these factors to the specific photocatalyst and pollutants involved can lead to more effective and sustainable environmental remediation strategies [51,149,162]. A comprehensive understanding of these interactions is essential for optimizing photocatalytic systems for environmental applications.
### 5.4. *Energy and Efficiency Metrics*

The photocatalytic systems must be evaluated not only by contaminant removal efficiency, but also by their energy and cost performance. Common performance indicators include the following:

* Quantum yield (QY): The ratio of degraded pollutant molecules to incident photons, reflecting light utilization efficiency.

QY measures the efficiency of photon utilization in driving photocatalytic reactions, defined as the ratio of reacted molecules to absorbed photons. A higher QY indicates more effective use of light energy. Ghosh et al. (2023) synthesized a reduced graphene oxide-cadmium telluride (RGO-CdTe) composite using a solvothermal method [163]. This composite achieved an apparent quantum yield (AQY) of 22.29% in the photocatalytic degradation of tetracycline antibiotics under visible light, a 2.63-fold increase compared to pure CdTe. The enhancement was attributed to improved charge separation and increased surface area provided by the RGO.

* Electrical energy per order (EEO): The energy required to reduce the pollutant concentration by one order of magnitude in a unit volume, often used for comparing treatment technologies.

EEO quantifies the energy required to reduce a pollutant's concentration by one order of magnitude in a unit volume, expressed in kilowatt-hours per cubic meter per order (kWh·m⁻³·order-1). Lower EEO values signify more energy-efficient systems [164]. The researchers calculated the EEO values to assess energy efficiency, finding that optimal conditions led to significant pollutant degradation with reduced energy consumption. This approach demonstrated the practicality of using EEO as a metric for comparing energy efficiencies across different advanced oxidation processes. These examples underscore the importance of incorporating both QY and EEO metrics when evaluating and optimizing photocatalytic systems for environmental remediation.

## 6. Recent Developments and Innovative Approaches

Advancements in materials science, reactor engineering, and process integration have significantly improved the performance and applicability of heterogeneous photocatalysis in water treatment. Recent developments aim to overcome the limitations of traditional photocatalysts and expand the operational window of the technology toward real-world, energy-efficient, and sustainable applications.

Figure 12 summarizes key advancements in heterogeneous photocatalysis, including the development of visible-light-responsive materials, hybrid systems integrating multiple treatment methods, solar-driven reactor designs, smart responsive catalysts, and the application of AI and machine learning in catalyst discovery and optimization. Each section is illustrated with relevant icons and concise descriptions to support both educational and scientific communication.

### 6.1. *Advances in Visible-Light-Responsive Photocatalysts*

One of the major research directions is the development of photocatalysts capable of utilizing visible light, which constitutes a much larger portion of the solar spectrum than UV. Strategies include the following:

* Band gap engineering through doping (e.g., nitrogen, sulfur, carbon) to extend light absorption.

* Heterojunction formation to promote charge separation and extend absorption range (e.g., TiO₂/g-C₃N₄, BiVO₄/CeO₂).
* Plasmonic enhancement using noble metal nanoparticles (e.g., Ag, Au) to exploit localized surface plasmon resonance (LSPR) effects.

<figure><img src="image_19.png" /><figcaption>**Figure 12.** Summary of recent innovations in heterogeneous photocatalysis for sustainable water treatment. The figure highlights key strategies such as band gap engineering, heterojunction formation, and plasmonic enhancement aimed at improving visible-light responsiveness, charge carrier separation, and overall photocatalytic efficiency under solar irradiation.</figcaption></figure>

Such materials enable the use of solar energy, reducing operational costs and supporting decentralized treatment systems.

Aloni et al. [95] investigated the impact of nitrogen-doped graphene quantum dots (N-GQDs) on the photocatalytic performance of TiO₂. The incorporation of N-GQDs significantly increased light absorption, narrowed the band gap, and improved charge transfer within the TiO₂ matrix. This enhancement led to a notable improvement in the degradation efficiency of organic pollutants under visible-light irradiation, demonstrating the effectiveness of nitrogen doping in extending the light absorption range of TiO₂-based photocatalysts. Some researchers developed novel heterojunctions combining sulfur-doped titanium dioxide (S-TiO₂) nanoparticles with graphitic carbon nitride (g-C₃N₄) sheets. The S-TiO₂ (5%)/g-C₃N₄ catalyst exhibited the highest activity for the photocatalytic degradation of methylene blue (MB) dye under visible-light irradiation [165]. The enhanced performance was attributed to improved separation and transfer of photogenerated charge carriers. The photocatalyst demonstrated stability and reusability over multiple cycles, indicating its potential for wastewater remediation applications.

Figure 13a illustrates the main strategies for developing visible-light-responsive photocatalysts aimed at enhancing solar energy utilization in water treatment and the material-level innovations that underpin recent progress in visible-light-responsive photocatalysis. It offers a clear representation of the structural and functional modifications involved, aiding the reader in understanding the relationships between design strategies and enhanced photocatalytic behavior under solar illumination.
<figure><img src="image_20.png" /><figcaption>**Figure 13.** Conceptual advances and strategic innovations in photocatalytic water treatment: (a) strategies for enhancing visible-light-responsive photocatalysts in water treatment; (b) hybrid and synergistic systems enhancing photocatalytic water treatment; (c) harnessing solar energy in photocatalytic reactor designs for water purification; (d) smart and stimuli-responsive photocatalysts for adaptive water treatment; (e) AI-driven strategies for photocatalyst design and optimization.</figcaption></figure>

## 6.2. Hybrid and Synergistic Systems

To improve treatment performance and tackle a broader range of contaminants, photocatalysis is increasingly combined with other processes:

* Photocatalysis-membrane filtration: Integrated systems use membranes for catalyst recovery and additional separation, improving water quality and reusability.

* Photocatalysis-adsorption: Combining photocatalysts with porous adsorbents (e.g., activated carbon, zeolites, MOFs) enhances pollutant uptake and facilitates degradation.

* Photocatalysis-Fenton or photo-Fenton processes: These synergistic systems increase radical generation, especially under acidic conditions, accelerating degradation rates.

* Photocatalysis-biological treatment: Pre- or post-biological treatments are used to complement photocatalytic oxidation, especially for biodegradable intermediates.
A study performed by Boonya-Atichart et al. [166] investigated the removal of per-fluorooctanoic acid (PFOA) from groundwater using a combination of nanofiltration and photocatalysis. The nanofiltration process achieved a 99.62% removal efficiency, and the subsequent photocatalytic treatment degraded 59.64% of the rejected PFOA. This hybrid ap-proach significantly reduced the environmental release of PFOA compared to nanofiltration alone. Yang et al. [94] studied the synergistic integration of adsorption and photocatalysis for enhanced degradation of organic pollutants. This research focuses on the development of a composite material derived from ZIF-67, resulting in cobalt-embedded nitrogen-doped nanoporous carbon (Co-N/C). The material exhibited a well-distributed structure with a suitable specific surface area and abundant active sites. These characteristics contribute to its dual functionality: (i) Adsorption: The nanoporous structure facilitates the initial uptake of organic pollutants, such as rhodamine B (RhB), from aqueous solutions; (ii) Photocatal-ysis: Upon light irradiation, the material effectively degrades the adsorbed pollutants, leading to their mineralization.

Figure 13b illustrates the integration of photocatalysis with complementary treatment processes to improve the efficiency and broaden contaminant removal. It highlights four key hybrid approaches: membrane filtration for catalyst recovery and enhanced separation, adsorption using porous materials for increased pollutant uptake, Fenton and photo-Fenton processes for accelerated radical generation, and biological treatments that support the degradation of intermediate byproducts. These synergistic combinations enhance the overall water treatment performance and adaptability to complex water matrices.

### 6.3. *Solar-Driven Photocatalytic Systems*

The shift toward solar-driven photocatalysis reflects a broader push for sustainability. Solar reactors, such as flat plate, parabolic trough, and compound parabolic concentrators, are being tested for various water matrices. These systems are particularly attractive for remote or low-resource settings, where centralized water treatment infrastructure is lacking.

Challenges remain in maintaining consistent performance under fluctuating solar irradiance and weather conditions. Integrating solar energy into photocatalytic systems has garnered significant attention for sustainable water treatment, especially in remote or resource-limited areas. Recent advancements have focused on various reactor configurations to optimize solar energy utilization.

Constantin et al. [143] investigated the performance of a solar-driven slurry photo-catalytic membrane reactor (PMR) for the advanced removal of organic pollutants from municipal wastewater. Utilizing iron-doped titania as the photocatalyst and a polysulfone-based membrane for separation, the system achieved chemical oxygen demand (COD) removal efficiencies ranging from 66 to 85% under simulated solar light and from 52 to 81% under natural sunlight over a 7-hour irradiation period. The results demonstrated the enhanced photocatalytic activity afforded by iron doping and established solar-powered PMRs as an effective, low-energy, and environmentally friendly alternative for municipal wastewater treatment. Touaref et al. [167] designed and implemented a parabolic trough solar concentrator (PTSC) aimed at enhancing the efficiency of solar distillation processes for water purification. The study addressed critical challenges in PTSCs, including optical efficiency and thermal performance, and proposed solutions to improve their applicability in water treatment. The findings highlighted the potential of PTSCs in harnessing solar energy for efficient and sustainable water purification, particularly in regions with high solar irradiance. Geng et al. [168] developed a numerical study focused on the particle flow and solar radiation transfer in a compound parabolic concentrator (CPC) photocatalytic reactor designed for hydrogen production. The research developed a comprehensive simulation model to analyze the natural convection flow field, temperature distribution, and radiative
power absorption within the reactor. The results provided valuable insights into the design and optimization of CPC reactors, emphasizing their effectiveness in utilizing solar energy for photocatalytic applications.

Figure 13c illustrates the key reactor configurations utilized in solar-driven photocatalytic water treatment, including flat plate, parabolic trough, and compound parabolic concentrators. These systems are designed to harness solar energy efficiently and are especially suited for decentralized applications in remote or resource-limited regions. By integrating sunlight as a renewable energy source, these reactor designs support low-carbon, energy-efficient solutions for addressing water contamination challenges.

## 6.4. Smart Photocatalysts and Responsive Materials

Emerging research also focuses on stimuli-responsive photocatalysts, which can adapt their activity in response to environmental conditions such as light intensity, temperature, or pollutant concentration. Examples include the following:

* Self-cleaning surfaces to prevent fouling.

* Magnetically recoverable catalysts that allow rapid separation after use.

* Photoelectrochemical (PEC) systems, which couple light and bias voltage to enhance electron-hole separation and pollutant removal efficiency.

Mittal (2023) explored the development of self-cleaning smart photocatalytic coatings designed to prevent fouling in water treatment applications [169]. These coatings utilize photocatalytic materials that, upon exposure to light, degrade organic contaminants on surfaces, thereby maintaining cleanliness and operational efficiency. The research highlights the potential of these coatings to reduce maintenance requirements and enhance the longevity of water treatment systems. Zheng et al. [170] developed magnetically recyclyable nanophotocatalysts (MRNPCs) for the efficient removal of recalcitrant organic pollutants from water. These MRNPCs can be easily separated from treated water using external magnetic fields, facilitating catalyst recovery and reuse. The review covers various synthesis methods, applications, and challenges associated with MRNPCs, emphasizing their role in enhancing the practicality of photocatalytic water treatment processes. A study performed by Kaur et al. [171] investigated the application of photoelectro catalytic (PEC) systems for the treatment of municipal wastewater containing emergent chemical and biological pollutants resistant to conventional treatments. The research focuses on the removal of these pollutants using PEC approaches, highlighting the potential of coupling light with an applied bias voltage to enhance electron-hole separation and improve pollutant degradation efficiency.

Figure 13d showcases innovative smart photocatalytic technologies designed to adapt dynamically to environmental conditions during water treatment. It highlights three key approaches: self-cleaning surfaces that minimize fouling and maintain efficiency, magnetically recoverable catalysts that enable easy separation and reuse, and photoelectrochemical (PEC) systems that enhance pollutant degradation by coupling light with an applied bias voltage. These responsive materials represent a significant step toward more efficient, sustainable, and operationally flexible photocatalytic systems.

## 6.5. AI and Machine Learning in Photocatalyst Design

Artificial intelligence (AI) and machine learning (ML) are being used to accelerate the discovery and optimization of new photocatalysts. These tools enable the following:

* Prediction of band structures and material properties.

* Identification of optimal synthesis conditions.

* Optimization of operational parameters for specific contaminants.
Data-driven approaches can significantly reduce the experimental costs and time, while revealing novel catalyst combinations with high performance.

Researchers developed a data-driven approach integrating ML with experimental validation to design high-performance nitrogen-rich covalent triazine frameworks (CTFs) for photocatalytic applications. Utilizing graph neural networks, the study achieved high accuracy in predicting the photocatalytic properties, leading to the identification and synthesis of a novel CTF structure with an ultrahigh hydrogen evolution rate [172]. A machine learning-based module was developed by Zhai and Chen [173] to expedite the design of Bi₂WO₆/MIL-53(Al) nanocomposites with enhanced photocatalytic activity. By constructing a dataset and employing support vector regression, the study identified key synthesis parameters influencing degradation rates of rhodamine B dye, facilitating the discovery of nanocomposites with improved performance.

Figure 13e provides a conceptual overview of how artificial intelligence (AI) and machine learning (ML) contribute to the development of advanced photocatalysts. It presents a streamlined and symmetrical layout, by linking AI to three core tasks, including predicting material properties, identifying synthesis parameters and optimizing operational conditions, and emphasizing the integration of data-driven tools in accelerating photocatalytic research and design.

## 7. Challenges and Limitations of Heterogeneous Catalysis

Despite the significant progress and promise of heterogeneous photocatalysis in water treatment, several scientific, technical, and practical challenges continue to hinder its large-scale and commercial implementation. While heterogeneous photocatalysis offers many advantages, its large-scale deployment remains limited by a range of technical, environmental and economic challenges, which are reviewed in detail in this section. These limitations include sensitivity to water matrix composition, difficulties in catalyst recovery, and limited efficiency under natural sunlight conditions.

Understanding these limitations is essential to guide future research and promote the transition from laboratory-scale studies to real-world applications.

* Limited photocatalytic efficiency under natural light

Many widely studied photocatalysts, such as TiO₂ and ZnO, require ultraviolet (UV) light for activation due to their wide band gaps. However, UV represents less than 5% of the solar spectrum, which limits the efficiency of solar-driven photocatalytic systems. Although visible-light-responsive materials have been developed, their activity is often lower, less stable, or poorly characterized under realistic conditions.

* Fast recombination of charge carriers

One of the major intrinsic limitations of semiconductor photocatalysis is the rapid recombination of photogenerated electron–hole pairs. This reduces the number of reactive oxygen species (ROS) available for pollutant degradation. Although heterojunction formation, doping, and co-catalyst strategies have improved charge separation, recombination remains a key bottleneck, especially in scaled-up systems.

* Catalyst stability and deactivation

Photocatalysts may suffer from loss of activity over time due to fouling, surface poisoning by reaction intermediates or structural changes under prolonged irradiation. In real water matrices, the presence of natural organic matter (NOM), inorganic ions, or biofilms can contribute to catalyst deactivation and require periodic regeneration, which complicates operation and increases costs.
* Recovery and reuse of catalysts

Slurry reactors, which typically offer higher activity due to greater surface area, require post-treatment separation of the catalyst particles. Recovery of nanoparticles is particularly challenging, leading to secondary contamination risks and resource losses. Immobilized systems offer easier reuse but often at the cost of reduced photocatalytic efficiency and mass transfer limitations.

* Variability in real water matrices

Most studies are performed using synthetic water with single contaminants, which do not reflect the complexity of real water matrices. In actual applications, competing substances (e.g., anions, cations, dissolved organics) can interfere with catalyst performance, ROS availability, and light penetration. The presence of turbidity and suspended solids also limits photon transfer and contaminant-catalyst interactions.

* Energy consumption and cost

Photocatalytic systems relying on artificial UV light are energy-intensive, limiting their sustainability and cost-effectiveness. Even with solar-driven systems, achieving adequate performance under fluctuating light conditions remains difficult. Moreover, the high cost of advanced photocatalysts, especially those using noble metals or rare elements raises concerns for large-scale use.

* Environmental and health risks

The potential release of engineered nanomaterials into the environment during operation or disposal raises questions about long-term safety. The ecotoxicological effects of these materials, particularly under light exposure, are not yet fully understood. Regulatory guidelines and standardized testing protocols are lacking for the safe use of photocatalytic materials in water treatment.

Table 9 summarizes the main scientific and practical challenges currently limiting the large-scale application of heterogeneous photocatalysis in water treatment. For each challenge, key implications are outlined along with targeted research needs aimed at improving catalyst performance, system scalability, operational sustainability, and environmental safety. This structured overview supports the identification of priority areas for future investigation and development.

<table><caption>**Table 9.** Challenges and research needs in photocatalysis.</caption><thead><tr><td>Challenge Area</td><td>Key Implications</td><td>Research Needs</td></tr></thead><tbody><tr><td>Photocatalytic efficiency under natural light</td><td>Limited solar utilization hampers real-world deployment and reduces energy efficiency gains.</td><td>Develop stable and efficient visible-light photocatalysts validated in outdoor conditions.</td></tr><tr><td>Charge carrier recombination</td><td>ROS generation is significantly reduced, lowering pollutant degradation rates.</td><td>Design advanced materials and structures to suppress recombination across scales.</td></tr><tr><td>Catalyst stability and deactivation</td><td>Operational complexity increases due to frequent maintenance and inconsistent performance.</td><td>Enhance catalyst durability and resistance to fouling and degradation.</td></tr><tr><td>Catalyst recovery and reuse</td><td>Trade-off between efficiency and ease of catalyst handling; challenges in sustainable reuse.</td><td>Innovate recyclable and immobilized photocatalyst formats with minimal performance loss.</td></tr></tbody></table>
<table><caption>**Table 9. Cont.**</caption><thead><tr><td>Challenge Area</td><td>Key Implications</td><td>Research Needs</td></tr></thead><tbody><tr><td>Real water matrix variability</td><td>Experimental results may not be representative; scalability becomes uncertain.</td><td>Standardize testing in complex water matrices and assess real-world performance.</td></tr><tr><td>Energy and cost concerns</td><td>High operational costs and dependence on noble materials limit affordability and adoption.</td><td>Optimize solar use, reduce reliance on rare materials, and assess life-cycle costs.</td></tr><tr><td>Environmental and health risks</td><td>Uncertainties in ecotoxicity and lack of regulation hinder responsible implementation.</td><td>Conduct long-term safety studies and establish regulatory frameworks for nanomaterials.</td></tr></tbody></table>

## 8. Future Perspectives

The application of heterogeneous photocatalysis in advanced water treatment holds considerable potential for addressing pressing global challenges related to water quality, sustainability, and public health. To fully realize its potential, future research and development efforts must focus on improving efficiency, scalability, and environmental safety, while aligning with the principles of green chemistry and sustainable engineering.

### (a) Development of efficient visible-light photocatalysts

A key priority remains the design and synthesis of photocatalysts capable of efficiently harvesting visible light, which dominates the solar spectrum. Future work should emphasize the following:

* Low-cost, non-toxic materials with stable photocatalytic performance under natural light.

* Tailored band structures for optimized redox potentials and selective pollutant degradation.

* Multifunctional composites that combine photocatalysis with adsorption or antimicrobial activity.

The exploration of bio-inspired, earth-abundant, and metal-free materials is also expected to grow, helping reduce environmental and economic barriers.

Figure 14 provides a conceptual overview of the fundamental directions in the design and development of visible-light-responsive photocatalysts, which are essential for advancing solar-driven water treatment technologies. As visible light comprises the majority of the solar spectrum, the ability to efficiently harvest this energy source is key to reducing reliance on artificial UV irradiation and enabling cost-effective, decentralized treatment systems.

Figure 14 highlights three major research priorities. First, it underscores the need for low-cost, non-toxic materials that maintain stable photocatalytic performance under natural sunlight, addressing both environmental and economic sustainability. Second, the figure emphasizes the engineering of tailored band structures to fine-tune redox potentials, thereby enhancing the selectivity in pollutant degradation and improving overall efficiency. Third, it features multifunctional composite materials that not only drive photocatalysis, but also incorporate complementary functionalities such as adsorption or antimicrobial activity, broadening the applicability of these systems.

Together, these strategies form the foundation for the next generation of high-performance photocatalysts that align with global sustainability goals and the transition toward circular and carbon-neutral water treatment solutions.
<figure><img src="image_21.png" /><figcaption>**Figure 14.** Key strategic priorities in the development of visible-light-responsive photocatalysts for sustainable water treatment, including material design, enhanced light absorption, charge separation, surface reactivity, and long-term stability under real environmental conditions.</figcaption></figure>

### (b) Scalable and modular reactor designs

The next generation of photocatalytic reactors must be designed with scalability and field deployment in mind. Important directions include the following:

* Modular solar reactors suited for decentralized or rural water treatment.

* Immobilized systems with optimized surface area and durability.

* Integration with existing infrastructure, such as in hybrid membrane-photocatalysis systems or in tertiary treatment steps in water reuse schemes.

Process intensification and automation could further support continuous operation and real-time performance monitoring.

Figure 15 illustrates key design strategies aimed at enabling the practical deployment of photocatalytic reactors across various treatment settings. It highlights modular solar units tailored for decentralized or rural use, immobilized photocatalyst systems engineered for durability and surface efficiency, and configurations that integrate with existing water treatment infrastructure, such as hybrid membrane-photocatalysis systems. Additionally, the visual underscores the role of process intensification and automation in supporting continuous, real-time operation, facilitating broader adoption of photocatalytic technologies in scalable water purification solutions.

### (c) Coupling with renewable energy and circular water systems

Heterogeneous photocatalysis aligns well with global transitions toward carbon-neutral and circular water management strategies. Efforts to integrate photocatalysis with renewable energy systems, especially solar energy, are likely to expand. Additionally, coupling photocatalytic processes with water reuse frameworks, such as greywater recycling and potable reuse can enhance resource recovery and resilience. Greywater refers to relatively low-contaminated domestic wastewater generated from activities such as bathing, handwashing, and laundry, while excluding blackwater that contains fecal matter. It typically represents 50–80% of household wastewater volume and is considered suitable for reuse after appropriate treatment [127]. Photocatalytic processes, particularly those based on visible-light-active semiconductors, have been explored for greywater treatment, due to their efficiency in degrading organic pollutants and microbial contaminants [86,99].
Treated greywater is commonly reused for non-potable purposes such as toilet flushing, landscape irrigation, or industrial wash water, while direct mixing with potable water is avoided, unless subject to advanced multi-barrier treatment and regulatory approval [127].

<figure><img src="image_22.png" /><figcaption>**Figure 15.** Design strategies supporting the development of scalable, modular, and integrative photocatalytic reactors for advanced water treatment applications. The figure outlines key aspects such as reactor configuration, catalyst immobilization, light delivery systems, and process integration to enhance treatment efficiency and adaptability.</figcaption></figure>

Figure 16 provides a comprehensive visual representation of how heterogeneous photocatalysis can be effectively integrated into broader sustainability frameworks through coupling with renewable energy sources and circular water management strategies. On the energy side, solar panels are depicted as the primary renewable power source driving the photocatalytic process, reflecting the technology's alignment with global efforts toward carbon neutrality and low-energy water treatment.

<figure><img src="image_23.png" /><figcaption>**Figure 16.** Integration of photocatalysis with renewable energy and circular water systems for sustainable treatment and resource recovery. Treated greywater is reused in non-potable applications, such as irrigation, toilet flushing, or industrial processes, in accordance with safety and water quality regulations.</figcaption></figure>

The diagram also illustrates how photocatalytic systems can be embedded into water reuse loops, such as greywater recycling and potable reuse schemes. Although not typically
mixed directly with potable water, advanced treatment technologies, such as photocatalysis, can produce effluents of sufficiently high quality to allow safe discharge or reuse, in contexts where water quality is strictly regulated [86,99,127]. These pathways demonstrate how treated water can be redirected into various applications, including household, industrial, or agricultural applications, thereby reducing freshwater demand and enhancing resilience in water-scarce environments.

Overall, Figure 16 emphasizes the synergy between clean energy input and closed-loop water management. It highlights the potential of photocatalysis not just as a stand-alone treatment technology, but as a core component in integrated environmental solutions aimed at optimizing energy use, minimizing waste, and advancing sustainable development goals.

While photocatalytic processes are being explored not only for pollutants degradation, but also for resource recovery, such as nutrient (e.g., nitrogen and phosphorus) and metal reclamation, their feasibility at scale remains debated. Although promising results have been achieved in laboratory studies, including enhanced recovery from synthetic and real wastewater matrices, significant challenges in selectivity, separation efficiency, and cost-effectiveness persist when integrating such processes into existing infrastructure. In particular, the low concentration of target elements, co-existing matrix complexity, and post-recovery purification needs can hinder practical implementation and economic viability. Table 9 summarizes these obstacles, including issues related to catalyst reuse, variability in real matrices, and energy consumption [126,128,130,132]. Future work must address these limitations through system-level optimization, pilot-scale trials, and the development of selective, regenerable materials to support both pollutant degradation and resource recovery in sustainable treatment frameworks.

(d) Standardization and real-world validation

A major gap in the field remains the lack of standardized protocols for evaluating the photocatalytic performance. Variability in experimental conditions—such as light intensity, pollutant type, pH, and reactor configuration—frequently results in non-comparable outcomes across studies, limiting the reproducibility and hindering the benchmarking of photocatalysts [134]. To overcome these discrepancies, there is increasing consensus on the importance of long-term, pilot-scale testing under realistic environmental conditions, as well as the standardization of test matrices and performance metrics [131,132]. Furthermore, techno-economic assessments and life cycle analyses (LCAs) are essential to evaluate the cost-effectiveness, environmental footprint, and energy demand of full-scale systems [133]. Benchmarking catalyst stability, reusability, and regeneration using internationally accepted standards is crucial for transitioning photocatalytic technologies from lab-scale success to real-world implementation [135].

For these reasons, to ensure transition from research to practice, it is essential to standardize testing protocols and validate performance in real water matrices. Future research should prioritize the following:

* Long-term pilot-scale demonstrations under diverse environmental conditions.

* Techno-economic analysis and life cycle assessment (LCA) to evaluate cost, environmental impact, and feasibility.

* Benchmarking of catalyst stability, reusability, and regeneration using internationally accepted standards.

Greater collaboration among academia, industry, and regulatory bodies will be critical to establishing safe and effective guidelines for implementation.

Figure 17 offers a conceptual overview of the foundational elements required to transition photocatalytic technologies from experimental settings to practical use. It emphasizes the interconnected roles of performance validation, system-level evaluation, and collabora-
tive engagement in ensuring reliable, scalable, and accountable deployment in real water treatment scenarios.

<figure><img src="image_24.png" /><figcaption>**Figure 17.** Strategic components required to advance photocatalytic water treatment technologies toward real-world implementation. The figure outlines key factors such as system integration, economic feasibility, regulatory support, scalability, and operational robustness critical for transitioning from laboratory-scale research to practical deployment.</figcaption></figure>

### (e) Data-driven discovery and process optimization

The application of artificial intelligence (AI), machine learning (ML), and computational modeling is poised to transform the design and deployment of photocatalytic systems. These tools can be used to achieve the following:

* Accelerate the material discovery through predictive modeling.

* Optimize the process parameters based on complex datasets.

* Enable intelligent control systems for autonomous operation.

As computational power and data availability grow, these technologies will play a central role in driving innovation and scaling up heterogeneous photocatalysis.

Table 10 presents a comprehensive overview of emerging data-driven tools and their transformative potential in the design, optimization, and implementation of heterogeneous photocatalytic systems. As photocatalysis evolves toward greater complexity and practical application, traditional experimental approaches face limitations in scalability, efficiency, and adaptability.

To address these challenges, the integration of computational methods, ranging from predictive modeling to machine learning and artificial intelligence, is becoming increasingly critical. Table 10 categorizes these tools into seven key types and outlines their specific applications in photocatalysis. These include forecasting the performance of novel materials, identifying optimal synthesis and reaction parameters, and enabling real-time autonomous process adjustments. Beyond their technical roles, each tool offers unique benefits, such as accelerating material discovery, reducing experimental workload, enhancing design precision, and supporting cost-effective scale-up using virtual testing environments.

The use of computational chemistry and simulations adds a molecular-level understanding of reaction mechanisms, while big data analytics helps uncover patterns from large and complex datasets. Meanwhile, digital twins and autonomous experimental platforms represent cutting-edge approaches for replicating and optimizing real systems without the need for physical trials.
<table><caption>**Table 10.** Data-driven tools in photocatalytic process optimization.</caption><thead><tr><td>Data-Driven Tool</td><td>Applications in Photocatalysis</td><td>Benefits</td></tr></thead><tbody><tr><td>Predictive modeling</td><td>Forecasting photocatalyst performance based on electronic structure and composition.</td><td>Speeds up material screening and reduces trial-and-error experimentation.</td></tr><tr><td>Machine learning (ML)</td><td>Identifying optimal synthesis routes and reaction conditions from large datasets.</td><td>Improves process efficiency and reproducibility through targeted parameter tuning.</td></tr><tr><td>Artificial intelligence (AI)</td><td>Integrating multiple models for decision-making in catalyst selection and process design.</td><td>Enhances system adaptability and design precision with cross-domain intelligence.</td></tr><tr><td>Computational chemistry and simulations</td><td>Simulating reaction mechanisms, surface interactions, and energy transfer dynamics.</td><td>Provides molecular-level insights that inform material modifications.</td></tr><tr><td>Big data analytics</td><td>Analyzing complex experimental or real-time data to reveal trends and hidden variables.</td><td>Enables pattern recognition for predictive maintenance and long-term monitoring.</td></tr><tr><td>Digital twins and virtual reactors</td><td>Replicating reactor behavior under varying conditions to test scenarios before deployment.</td><td>Supports safer, cost-effective scale-up with reduced experimental workload.</td></tr><tr><td>Autonomous experimental platforms</td><td>Running self-optimizing experiments that adjust parameters in real time to improve efficiency.</td><td>Accelerates innovation cycles and fosters real-time process optimization.</td></tr></tbody></table>

In essence, Table 10 captures how these technologies collectively support the shift from empirical, trial-and-error development to a more predictive, intelligent, and data-informed framework. This evolution is critical for overcoming existing barriers in photocatalytic water treatment and ensuring the efficient translation of laboratory findings into robust, scalable environmental solutions.

## 9. Conclusions

Visible-light-responsive photocatalysts hold significant promise for solar-driven water treatment due to their ability to harness a larger fraction of the solar spectrum. Materials such as doped TiO₂, g-C₃N₄, BiVO₄, and heterojunction composites have demonstrated enhanced photocatalytic activity under visible irradiation. However, their real-world application requires further attention to challenges such as long-term material stability under environmental conditions, potential release of toxic metal ions (e.g., from Ag, Cu or Bi-based systems), high synthesis or modification costs, and reduced efficiency in complex natural water matrices. Therefore, ongoing research should focus not only on improving photocatalytic performance but also on enhancing material durability, environmental safety, and cost-effectiveness for large-scale implementation.

In parallel, considerable progress has been made in optimizing photocatalytic reactors. Systems such as slurry reactors, immobilized configurations, annular designs, and flat plate and falling film reactors each offer distinct advantages, depending on the application scale, water composition and operational constraints. Emerging configurations like fluidized bed reactors and membrane photocatalytic reactors integrate multiple functionalities, enhanced mixing, reduced fouling, and simultaneous separation, pushing the boundaries of performance and applicability.

To address the limitations of standalone photocatalysis, hybrid and synergistic systems have become increasingly prominent. The coupling of photocatalysis with membrane
filtration enables catalyst recovery and enhances effluent quality. Adsorption and photo-catalysis combinations improve pollutant uptake and pre-concentration, facilitating rapid degradation. The integration of photocatalysis with biological treatment processes has shown potential to enhance overall treatment efficiency, particularly by transforming certain recalcitrant pollutants into more biodegradable intermediates. However, this effect is contaminant specific, and in some cases, photocatalysis may generate transformation products that remain toxic or resistant to biological degradation. Therefore, understanding the nature of intermediates formed and tailoring treatment sequences accordingly is essential to ensure synergistic performance.

Energy efficiency is a critical factor for real-world deployment of photocatalytic systems, particularly in large-scale or decentralized applications. Metrics such as quantum yield (QY), which measures the number of pollutant molecules degraded per photon absorbed, and electrical energy per order (EEO), which quantifies the energy required to achieve a 90% pollutant reduction, are increasingly used to assess and compare system performance. These indicators provide practical insights into the energy cost of treatment, allowing researchers and engineers to optimize operational parameters and select materials that balance efficiency with economic feasibility.

Solar-driven photocatalytic systems, including flat plate, parabolic trough, and compound parabolic concentrators, demonstrate the strong potential for decentralized treatment in low-resource and remote settings. These configurations support low-carbon, off-grid applications and align with global efforts to promote energy-efficient water purification technologies. However, their performance remains sensitive to fluctuations in solar irradiance and weather conditions, necessitating robust design and adaptive operational strategies.

Smart materials, such as magnetically recoverable photocatalysts and stimuli-responsive systems, offer operational advantages by enabling catalyst separation and potential reuse. However, practical implementation is still challenged by factors such as magnetic particle aggregation, reduced recovery efficiency over time, and loss of photocatalytic activity after multiple cycles. Addressing these limitations requires advances in material design to enhance stability, dispersibility, and long-term performance under realistic water treatment conditions.

The integration of artificial intelligence (AI) and machine learning (ML) into photocatalyst development and process optimization holds strong transformative potential. These tools can accelerate material screening, predict performance parameters, and support real-time process control. However, their effectiveness is currently limited by the scarcity of high-quality, domain-specific datasets, which can constrain model training and reduce prediction reliability. Advancing this integration will require standardized data collection protocols and open-access repositories to support reproducible and data-driven photocatalysis research.

Key contributions of this work include:

A structured analysis of photocatalytic materials, with a focus on visible-light-active semiconductors, their functional modifications (e.g., doping, heterojunctions), and challenges related to recombination and stability.

* A critical synthesis of operational conditions (e.g., pH, catalyst dose, light intensity), showing how they influence the photocatalytic performance and identifying bottle-necks in optimization and reproducibility.

* A comparative evaluation of reactor configurations (slurry, immobilized, annular, membrane), including their advantages and limitations for field-scale deployment.
* Discussion of integrated and hybrid systems, such as adsorption-photocatalysis and membrane-photocatalysis, emphasizing how such combinations extend treatment capability and enhance process synergy.

* Inclusion of real-world applications and pilot-scale demonstrations, emphasizing the role of reactor scalability, solar integration, and process automation in bringing laboratory innovations closer to practical implementation.

In terms of practical applications, photocatalysis offers significant advantages: operation without added chemicals, use of solar energy, and potential for integration into circular water systems. However, its widespread adoption depends on overcoming barriers such as energy costs, catalyst durability, scalability, and safety concerns regarding nanomaterials. Looking ahead, future research should prioritize the following:

* Development of stable, low-cost, visible-light-active photocatalysts using earth-abundant materials.

* Modular reactor designs suitable for decentralized and rural water treatment applications.

* Standardization of performance testing protocols and evaluation under real water matrices.

* Life cycle assessments and techno-economic evaluations to assess sustainability and commercial feasibility.

* The integration of machine learning, digital twins, and autonomous experimentation to accelerate material discovery and optimize system performance.

Overall, the heterogeneous photocatalysis is gaining momentum as a promising technology for advanced water treatment, supported by progress in material design, reactor development, and integration with renewable energy. However, its real-world implementation remains limited and further efforts are needed to overcome economic and technical barriers before large-scale commercial adoption becomes viable. Continued interdisciplinary research, standardization of performance metrics, and validation under realistic operating conditions will be essential to overcome remaining barriers. As part of circular and decentralized water management systems, photocatalytic technologies offer promising solutions to address the pressing challenges of water quality, sustainability, and environmental protection in the 21st century.

**Author Contributions:** M.P.: Investigation, Methodology, Visualization, Writing—Original Draft; D.L.: Conceptualization, Formal analysis, Investigation, Methodology, Validation, Writing—Original Draft, Writing—Review and Editing; L.F.: Conceptualization, Formal analysis, Investigation, Methodology, Validation, Writing—Original Draft, Writing—Review and Editing; M.G.: Conceptualization, Formal analysis, Investigation, Methodology, Visualization, Validation, Supervision, Writing—Review and Editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors acknowledge the use of OpenAI's ChatGPT-4o (under subscription) for its assistance in improving English phrasing in some parts of the manuscript and in enhancing the visual quality of several figures.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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