A Critical Review of Clay Mineral-Based Photocatalysts for Wastewater Treatment

Abstract

This review critically examines the latest advancements in clay mineral-based photocatalysts for water purification. Clay minerals, owing to their natural abundance, low cost, and unique physicochemical properties, have emerged as promising candidates for enhancing photocatalytic efficiency. This article delves into various activation methods for clay minerals, including acid, alkali, calcination, and mechanochemical activation, highlighting their roles in enhancing surface area, creating active sites, and improving photocatalytic performance. Moreover, the review explores various modification strategies for photocatalysts, such as doping with metal and non-metal ions, deposition of metals, and the design of heterojunctions, to further boost photocatalytic activity. In particular, the utilization of kaolinite, montmorillonite, attapulgite, and sepiolite as clay mineral supports for photocatalysts is discussed in detail, showcasing their potential in wastewater treatment. The review underscores the significant strides made in the development of clay mineral-based photocatalysts, highlighting their effectiveness in degrading organic contaminants under light exposure. Nevertheless, there are persisting challenges such as the optimization of loading quantities, improvement in compatibility between clay minerals and photocatalysts, and reduction in preparation costs for large-scale applications. In summary, this review offers valuable insights into the current status of clay mineral-based photocatalysts for water purification, thereby stimulating future research in this promising field.

1. Introduction

Water contamination has emerged as a pressing global challenge, imperiling the sustainability of ecosystems and posing significant risks to human health. With rapid industrialization and population growth, the demand for clean water has skyrocketed, while the quality of available water resources has declined drastically. This dilemma has necessitated the development of efficient and cost-effective water-treatment technologies to ensure the safety and accessibility of potable water [1,2]. Taking China as an example, Figure 1 illustrates the water quality status in different regions across various years, evaluated using a grading system where lower grades indicate better water quality [3]. Despite some regions showing improvement in water quality, there are still areas with poor water quality, highlighting the urgent need to address water pollution in these regions. The escalating water crisis emphasizes the necessity for efficient and cost-effective water-treatment technologies to combat the issue effectively.

Among the various water-treatment methods, conventional approaches such as coagulation, flocculation, sedimentation, filtration, and disinfection, while effective to a certain extent, often require high energy inputs and may generate secondary pollutants. Therefore, there is an urgent need for advanced technologies that can address the full spectrum of water pollutants in an environmentally friendly manner.

Photocatalysis, harnessing the power of light, has garnered significant attention as a promising alternative for water treatment [4,5]. This technology leverages the ability of photocatalysts to generate reactive oxygen species (ROS) upon exposure to light, which can subsequently degrade organic pollutants and inactivate harmful microorganisms [6]. Photocatalysis offers several advantages over conventional methods, including its high degradation efficiency, environmental friendliness, and self-regenerating nature. Importantly, photocatalysis can operate under ambient conditions, making it an attractive option for large-scale water-treatment applications [7].

The efficacy of photocatalysis heavily relies on the properties of the photocatalyst employed [8]. Conventional photocatalysts, such as TiO₂, encounter challenges like limited visible light absorption and high recombination rates of photogenerated electron–hole pairs [9]. To address these issues, developing novel photocatalyst materials represents a viable approach to enhance photocatalytic performance [10]. Clay minerals, due to their widespread availability, cost-effectiveness, and distinctive physicochemical characteristics, have emerged as promising candidates for enhancing the efficiency of water purification through photocatalytic processes. Clay minerals offer significant surface areas, robust adsorption capacities, and ion-exchange capabilities, rendering them excellent supports for photocatalysts [11]. By integrating clay minerals with photocatalysts, not only are the dispersion and stability improved but also the effective separation and transfer of photogenerated charge carriers are facilitated, ultimately enhancing photocatalytic activity [8].

This review article critically examines the latest advancements in clay mineral-based photocatalysts for water purification. It delves into the unique properties of clay minerals that make them suitable supports for photocatalysts, including their substantial surface area, strong adsorption capacity, and ion-exchange capabilities [12]. Furthermore, the review explores various activation methods employed to enhance the surface properties and photocatalytic performance of clay minerals, such as acid, alkali, calcination, and mechanochemical activation [13,14,15,16].

This review also focuses on the modification strategies used to further boost the photocatalytic activity of clay mineral-based photocatalysts. These strategies encompass doping with metal and non-metal ions, deposition of metals, and the design of heterojunctions [10]. By incorporating these modifications, researchers have achieved remarkable enhancements in the photocatalytic efficiency of clay mineral-based composites, making them effective for degrading organic contaminants and eliminating harmful microorganisms in water.

The utilization of specific clay minerals as supports for photocatalysts is discussed in detail, showcasing their potential applications in wastewater treatment. This review underscores the significant strides made in this field, highlighting the effectiveness of clay mineral-based photocatalysts in improving water quality.

However, despite these promising developments, challenges remain, including optimizing loading amounts, enhancing compatibility between clay minerals and photocatalysts, and reducing preparation costs for large-scale applications. This review aims to provide valuable insights into the current state of clay mineral-based photocatalysts for water purification, fostering further research in this promising area and ultimately contributing to the development of more efficient and sustainable functional materials.

2. Clay Mineral Activation Methods

The activation of clay minerals involves a series of physical or chemical treatments aimed at modifying their structure and properties [8,17]. These treatments not only eliminate impurities and increase the specific surface area but also introduce new functional groups and active sites, thereby boosting their reactivity and photocatalytic efficiency [8]. The most frequently employed methods for activating clay minerals include acid activation, alkali activation, calcination activation, and mechanical activation (Figure 2). The importance of clay mineral activation lies in its ability to unlock the full potential of these minerals and tailor their properties for specific applications. Through activation, clay minerals can serve as effective supports for photocatalysts, providing a stable platform for the deposition of active species and facilitating electron transfer during photocatalytic reactions [18].

2.1. Acid Activation

Acid activation is a widely employed pretreatment method involving the use of acidic solutions to process clay materials for the purposes of eliminating impurities, enhancing surface area, and creating additional active sites [19,20]. Through this procedure, the acid has the capability to dissolve certain elements or minerals present in the clay, such as carbonates and oxides, thereby cleansing its surface and amplifying its surface area [21]. These alterations facilitate the uniform deposition of photocatalysts onto the clay surface and result in the provision of more reactive sites. Furthermore, the presence of Si-OH groups on the surface serves to provide active centers during photocatalytic reactions, aiding in the segregation and transmission of photogenerated electrons and holes. Additionally, acid treatment plays a role in augmenting the porosity of clay materials, thereby promoting the adsorption of reactants and the diffusion of products [12].

2.2. Alkali Activation

Alkaline activation is a highly effective method of clay pretreatment, particularly suitable for enhancing the interfacial interaction between clay and photocatalysts [22]. Alkaline treatment induces exfoliation and swelling of the silicate layers within the clay, leading to the creation of additional interlayer spaces and active sites at the edges [23]. These alterations promote both the deposition and dispersion of photocatalysts as well as the transfer of photogenerated electrons between the clay and the photocatalysts. Furthermore, this process introduces negatively charged functional groups, such as hydroxyl groups and oxide ions, enhancing the capacity of clay to capture photogenerated electrons and ultimately enhancing photocatalytic efficiency [23,24].

2.3. Calcination Activation

Calcination activation is a thermal treatment process carried out on clay materials at elevated temperatures to eliminate volatile components and organic substances while also promoting a more stable and ordered structure by crystal phase transformation, etc. [25]. Throughout calcination, both moisture and organic impurities within the clay are eliminated, leading to a reorganization and agglomeration of the silicate layers and resulting in a more compact crystal structure. These structural modifications not only bolster the mechanical resilience and chemical robustness of the clay material but also enhance its light-absorption capacity and the mobility of charge carriers. Additionally, calcination can introduce defect sites and oxygen vacancies, which can act as active centers in photocatalytic reactions, thereby further improving the efficiency of photocatalysis [26,27].

2.4. Mechanical Activation

Mechanical activation is a physical method used to process clay minerals by applying mechanical force to crush and refine the clay materials, aiming to enhance their specific surface area and reactivity [28,29]. During the mechanical process, clay particles are fragmented into smaller sizes, exposing a greater number of active sites. The mechanical stress and heat generated during grinding can also introduce active functional groups on the clay surface. These groups have the capability of forming chemical bonds or undergoing physical adsorption with photocatalysts, aiding in electron transfer and chemical bonding processes during photocatalytic reactions. Furthermore, mechanical grinding in the production of clay mineral-loaded photocatalysts promotes intimate contact and homogeneous blending between the clay and photocatalysts. These sites can serve as hubs for capturing photogenerated electrons and holes or act as central points for reactions, thereby enhancing the efficiency of photocatalysis [28,30,31].

In conclusion, recent research has focused on understanding the effects of different activation methods on clay minerals. This knowledge can help develop applications for clay-based photocatalytic materials.

3. Photocatalyst Modification Strategies

The development of highly efficient photocatalysts is paramount for achieving superior performance in wastewater-treatment applications [32]. To this end, various modification strategies have been devised to enhance the inherent properties of photocatalysts, particularly in the context of clay mineral-based composites [32,33]. These strategies primarily focus on tuning the electronic structure, light absorption capabilities, and charge carrier dynamics of the photocatalysts, ultimately leading to improved photocatalytic activity. The key modification strategies employed in this context can be broadly categorized into three main approaches: doping, deposition, and heterojunction design [34]. In the modification strategies, clay is predominantly employed as a support material for the attachment of semiconductor materials, such as TiO₂, due to its considerable specific surface area and stable chemical properties.

3.1. Doping

Doping, as a straightforward and efficient modification technique, can significantly conduct the electronic and bandgap structures of semiconductors [35]. This method allows for the reduction in the bandgap of semiconductors, thereby greatly enhancing their optical, electrical, and luminescent properties [36]. Metal doping, non-metal doping, and co-doping are among the common doping methods in practical applications. Each method offers distinct application advantages and effects, collectively offering robust technical support for enhancing the performance of semiconductors [37].

3.1.1. Metal-Ion Doping

Doping metal ions represents a valuable approach for enhancing photocatalytic performance [37]. By introducing an optimal quantity of metal ions into pivotal catalyst matrices like TiO₂ and g-C₃N₄, their electronic structure, band positions, and light-absorption properties can undergo significant modifications, thereby enhancing their photocatalytic efficiency [38]. Initially, the addition of metal ions can generate new energy levels within the forbidden band of the primary catalysts, thereby narrowing their bandgaps and enhancing light-absorption capabilities. Specific transition metal ions such as Cu, Zn, Ni, and Fe can modify the electronic structure of the primary catalysts by introducing electron donors or acceptors within the bands, enabling activation by low-energy photons and demonstrating increased efficacy under visible light [37]. The diverse impacts of various metal ions on the light-absorption properties of the primary catalysts highlight the importance of selecting suitable metal ions for doping according to specific application prerequisites. Moreover, metal-ion doping can influence the separation and movement of photogenerated charge carriers within the catalysts. Additionally, the introduction of metal ions can alter the surface properties of the catalysts, enhance their active surface sites, and facilitate photoreactions [39]. For example, Mahadadalkar et al. [40] employed a combination of sol–gel and electrospinning techniques to fabricate Fe³⁺-doped TiO₂ fiber photocatalysts for the degradation of dye wastewater (Figure 3). Their research revealed that the incorporation of Fe³⁺ into TiO₂ fibers significantly boosts their absorption in the visible light range of the solar spectrum, thereby enhancing their degradation efficiency. The TiO₂ fibers doped with 5% Fe³⁺ demonstrate an outstanding photocatalytic degradation performance against rhodamine B, achieving a degradation rate of 99% within 120 min, in contrast to the 42% degradation rate observed for pure TiO₂. Moreover, the doped photocatalyst retains a high photocatalytic activity (97%) even after undergoing five cycles of reuse, indicating the structural stability of the doped photocatalyst.

3.1.2. Nonmetallic-Ion Doping

Non-metal-element doping is a crucial strategy for modifying photocatalysts, with the aim of adjusting their electronic structure and optical properties through the introduction of non-metal elements to enhance their photocatalytic performance. Initially, the incorporation of non-metal ions can significantly broaden the light absorption range of photocatalysts. For instance, while traditional TiO₂ is solely responsive to ultraviolet light, doping non-metal ions like nitrogen, carbon, and fluorine can introduce new energy levels into the bandgap of TiO₂, enabling its absorption of visible light and thereby enhancing solar utilization. Moreover, non-metal-ion doping facilitates the efficient separation of photogenerated electrons and holes within the catalyst. Non-metal ions can serve as traps for these charge carriers, reducing the chances of electron–hole recombination, prolonging their lifetimes, and ultimately increasing their participation in photocatalytic processes. Lim et al. [41] proposed a simple one-step underwater plasma method to synthesize nitrogen-doped TiO₂ (N-TiO₂) photocatalysts with enhanced reactivity in the visible light spectrum (Figure 4). The N-TiO₂ material possesses a nanostructured porous rutile polycrystalline architecture incorporating nitrogen atoms as dopants. In the visible light region, N-TiO₂ demonstrates remarkable light-absorption capabilities. As a result, the N-TiO₂ photocatalyst exhibits efficient photocatalytic activity under both natural sunlight and visible light, demonstrating significant degradation efficiency towards methylene blue, rhodamine B, and tetracycline, approximately 4.5 times higher than that of pure TiO₂.

3.2. Deposition

The deposition method is widely employed in the synthesis of multi-component catalysts [36]. By anchoring cocatalysts onto the photocatalyst surface in the form of minute nanoparticles, promoting intimate contact between the different constituents, it enhances rapid electron transfer across interfaces. This effective electron-transfer mechanism leads to the acceleration of catalytic reactions [42].

3.2.1. Metal Deposition

The integration of metals with clay can be employed to fabricate semiconductor-clay mineral photocatalysts [43]. This architectural approach offers two key benefits: prolonging the lifespan of charge carriers and enhancing the capability to absorb visible light. At the interfaces of materials such as Bi, Cu, and TiO₂, as metals typically have a higher work function than the conduction band position of TiO₂, electrons will flow from TiO₂ to the metal nanoparticles, achieving Fermi level alignment [44]. During this procedure, the surplus negative charges gathered on the metal nanoparticles, along with the excess positive charges on the semiconductor, contribute to the creation of a Schottky barrier. The improved efficiency of photocatalysis is credited to the successful capture of conduction-band electrons by the Schottky barrier and the lowering of the recombination rate of photogenerated charge carriers [37].

3.2.2. Noble Metal Deposition

Noble metal deposition is a pivotal technique to enhance photocatalyst performance. Through methods like impregnation, photodeposition, and surface sputtering, noble metals adhere to the surface of photocatalysts, creating minuscule nanoparticles. This process significantly enhances the light absorption of catalysts and optimizes their electronic structure, thereby improving photocatalytic efficiency [45]. Furthermore, noble metal particles induce localized surface plasmon resonance (LSPR) effects, resulting in distinct spectral absorption in the UV–visible range that aligns with the solar spectrum. Noble metals exhibit outstanding chemical stability and corrosion resistance, enabling the catalyst to sustain activity and stability even under harsh environmental conditions. This attribute increases the viability of utilizing noble metal-modified photocatalysts in real-world applications [46]. As a unique catalyst component, noble metal nanoparticles not only demonstrate exceptional visible-light absorption but also establish stable interface structures with the photocatalyst, promoting swift electron transfer in light-driven reactions. Consequently, illuminated noble metal-modified photocatalysts efficiently separate and utilize photogenerated electrons and holes, thus enhancing photocatalytic activity [6,46].

3.3. Common Heterojunction Design

The integration of heterojunctions created by different semiconductor composites in clay mineral-based composite materials is a pivotal modification approach with the capacity to significantly boost the photocatalytic efficiency of catalysts [47]. Heterojunctions, which serve as interfaces between two semiconductors possessing unique band structures, promote band alignment. Moreover, these heterojunctions combine the exceptional characteristics of varied semiconductor materials, resulting in a photocatalyst that demonstrates synergistic effects [48]. Different heterojunction designs to prepare composites and their photocatalytic efficiency in decomposing various pollutants are comprehensively summarized in

Table 1. Different heterojunction designs to prepare composites and their photocatalytic efficiency in decomposing various pollutants.

Photocatalyst	Heterojunction Design	Pollutants	Pollutant Dosage (mg/L)	Degradation Rate	Irradiation Time (min)	Ref.
MoS2/TiO2/clinoptilolite	Type I	Xanthate	20	over 90%	180	[49]
BiVO4/g-C3N4/Diatomite	Type II	Rhodamine B	20	99%	60	[50]
CdS/Zn2In2S5/g-C3N4	Type II	Tetracycline	20	94.7%	60	[51]
α-Fe2O3-CeO2-SiO2	Type II	Tetracycline	25.1	95.9%	94.2	[52]
Bi2WO6/C-TiO2	Z-scheme	Oxytetracycline	15	93.6%	100	[53]
Ag2ZnGeO4/g-C3N4	Z-scheme	Tetracycline	10	94.3%	140	[54]
Ag/α-Fe2O3/g-C3N4	Z-scheme	Rhodamine B	20	97.6%	55	[55]

.

3.3.1. Conventional Heterojunctions

Traditional heterogeneous photocatalysts are classified into three categories: bridging gap (Type I), staggered gap (Type II), and broken gap (Type III) structures (see Figure 4) [56].

In photocatalysts (see Figure 5a), the conduction band (CB) and valence band (VB) of semiconductor A are, respectively, lower and higher than those of semiconductor B, leading to an accumulation of electrons and holes at the CB and VB levels of semiconductor B. However, the effective separation of electron–hole pairs is impeded in Type I heterojunctions due to the confinement of both carriers within the same semiconductor. Furthermore, redox reactions occur on the semiconductor with lower redox potential, significantly diminishing the redox capacity of this photocatalyst [56,57].

Type II photocatalysts (see Figure 5b) exhibit higher CB and VB levels in semiconductor A compared to semiconductor B. This setup facilitates the transfer of photogenerated electrons to semiconductor B under light exposure, while the photogenerated holes move to semiconductor A, enabling the spatial separation of electron–hole pairs. Similar to Type I structures, Type II heterojunctions experience a reduced redox capacity, as redox reactions occur on semiconductor B with a lower reduction potential and semiconductor A with a lower oxidation potential [56,58].

Although Type III photocatalysts (see Figure 5c) share a structure similar to Type II, a significant staggered gap difference leads to non-overlapping bandgaps. Consequently, electron–hole transfer and segregation do not typically occur between the two semiconductors in Type III heterojunctions, rendering them unsuitable for enhancing the separation of electron–hole pairs.

In conclusion, Type II heterojunctions, with their ability to promote the spatial separation of electron–hole pairs, are clearly the most effective traditional structures for enhancing photocatalytic activity [59].

3.3.2. Type “Z” Heterojunction Design

In 2013, Yu et al. [60] introduced the concept of a direct Z-scheme heterojunction, mimicking natural photosynthesis. This heterojunction facilitates the migration of free electrons when the semiconductors contact, causing band-bending and charge redistribution to align Fermi levels, thus generating a unique internal electric field (Figure 6). This field plays a pivotal role in the separation and transport of photo-induced charge carriers. Specifically, it promotes the recombination of electrons in the conduction band of component A with holes in the valence band of component B, while inhibiting the migration of electrons and holes from component B to the corresponding energy levels of component A. Additionally, it prevents the recombination of electrons in the conduction band of component B with holes in the valence band of component A. The direct Z-scheme heterojunction enhances charge carrier transport efficiency by fostering strong interactions between the semiconductor materials and reducing resistance. Contrary to Type II heterojunctions, the direct Z-scheme heterojunction facilitates easier charge transfer within photocatalysts [60,61,62].

4. Clay Mineral-Based Photocatalyst

4.1. Layered-Structure Clay Mineral-Based Photocatalyst

4.1.1. Kaolinite-Based Photocatalyst

Kaolinite features a typical 1:1 layered structure, comprising alternating aluminum–oxygen octahedra and silica–oxygen tetrahedra stacks. Its crystal structure is represented as Al₄[Si₄O₁₀](OH)₈ [63]. Theoretically, kaolinite has a chemical composition of 46.54% SiO₂, 39.50% Al₂O₃, and 13.96% H₂O, with a Si/Al molar ratio close to 2:1 [64]. In its natural state, kaolinite is abundant in SiO₂ and Al₂O₃ and contains trace amounts of oxides such as TiO₂, MgO, MnO₂, Na₂O, and K₂O, etc. [65,66]. The presence and levels of these oxides are greatly influenced by geological conditions and environmental factors during kaolinite formation. By meticulously analyzing the kaolinite composition, it is possible to infer the geological era and background of its deposition. Consequently, the chemical composition of kaolinite rock layers can be used to determine the quantity, nature, and quality of the kaolinite within the deposit [65,67].

Due to its unique lamellar shape and abundance of active surface groups, kaolinite is an important carrier material for active components in catalysis [68]. It provides numerous sites for loading active components, enhancing their dispersion, and facilitates the adsorption and activation of reactants during catalytic processes due to its rich surface active groups, effectively promoting reactions [69]. Consequently, kaolinite has attracted the attention of numerous researchers and scholars, who have sought to gain a deeper understanding of its properties.

In the field of dye wastewater treatment, the incorporation of kaolinite as a support not only enhances the stability and efficiency of the photocatalyst, but also improves the adsorption capacity of the photocatalyst for the dye through its physicochemical properties. This, in turn, enhances the efficiency of the photocatalytic degradation process. Kaolinite-based photocatalysts have demonstrated remarkable efficacy. Awad et al. [70] successfully prepared TiO₂ quantum dot catalysts loaded on kaolinite. The utilization of kaolinite served to enhance the surface properties and microstructural and micromorphological features of the catalyst, thereby facilitating the achievement of synergistic effects. This approach effectively reduced the agglomeration of the catalysts and enhanced the stability of the ultrafine nanoparticle dispersion, which in turn resulted in the highly exposed photocatalytic active sites and optimized the balance between the dye adsorption rate and photodegradation rate. In a previous study, Ighnih et al. [71] developed a BiOCl/kaolinite photocatalytic material with the objective of photodegradation of rhodamine B dye in aqueous solution (Figure 7). The results demonstrated that the total organic carbon (TOC) removal of BiOCl and optimized BiOCl/kaolinite were approximately 41.60% and 95.51%, respectively. The photocatalytic degradation efficiency of BiOCl/kaolinite was found to be approximately four times that of pure BiOCl nanosheets. In the reaction, kaolinite served as a carrier, effectively dispersing the BiOCl nanoparticles and preventing the agglomeration of BiOCl nanosheets, thereby increasing the effective specific surface area of BiOCl and enhancing its photocatalytic activity. Furthermore, kaolinite itself exhibits a certain degree of adsorption capacity, which can pre-adsorb RhB dye molecules and provide additional reaction sites for BiOCl nanoparticles, thereby accelerating the photocatalytic degradation process. Furthermore, the negative charge on the kaolinite surface can attract photogenerated holes on the BiOCl valence band, reducing the rate of compounding of photogenerated electron–hole pairs and enhancing the separation efficiency of photogenerated carriers, thus improving the photocatalytic performance. As a naturally occurring mineral, kaolinite also exhibits favorable chemical and thermal stability, which can effectively safeguard BiOCl nanoparticles from photocorrosion or structural deterioration throughout the photocatalytic process, thereby enhancing the stability of the photocatalyst. The complexation of BiOCl with kaolinite resulted in enhanced light absorption, thereby improving photoresponsiveness in the visible light range. This broadened the potential applications of the photocatalyst.

In the field of medical wastewater treatment, kaolinite serves not only to enhance the porosity and surface area of the material, thereby facilitating contact with target pollutants, but also to significantly improve the photocatalytic degradation efficiency of the photoreaction by combining with the catalyst. Li et al. [72] successfully synthesized a G₃N₄/TiO₂/kaolinite ternary composite, which demonstrated the effective photocatalytic degradation of ciprofloxacin (CIP) and Staphylococcus aureus (S. aureus) in water. The photocatalytic degradation efficiencies were enhanced by 5.35 and 6.35 times, respectively, in comparison to those of pure TiO₂ and g-C₃N₄. The incorporation of kaolinite resulted in a reduction in the size of TiO₂ nanoparticles and an enhanced dispersion within the ternary material, as well as an increase in the exposure and surface area of g-C₃N₄ sheets, which in turn enhanced the visible-light absorption. Furthermore, Li et al. [73] developed SnS₂/ZnIn₂S₄/kaolinite direct Z-type heterostructured photocatalysts with a specific focus on the degradation of tetracycline hydrochloride. In the presence of visible light, the composite exhibited a 20.81-fold and a 2.37-fold enhancement in its degradation efficiency for tetracycline hydrochloride relative to that of SnS₂ and ZnIn₂S₄, respectively. While kaolinite is not directly involved in the photocatalytic degradation reaction, its role as a carrier of the ternary heterostructure provides a larger surface area and stronger adsorption capacity, which contribute to the dispersion of the nanocatalysts and facilitate the migration of photogenerated charges and the adsorption of pollutants. Additionally, the layered structure of kaolinite provides a stable foundation for the construction of a ternary heterostructure, which enhances the recyclability and overall performance of the photocatalyst.

4.1.2. Montmorillonite-Based Photocatalyst

Montmorillonite is a typical 2:1-type dioctahedral layered silicate clay mineral, comprising two layers of silica–oxygen tetrahedral sheets alternating with a layer of alumina–oxygen octahedral sheets (Figure 8) [74]. Its chemical formula is (Na,Ca)_0.33(Al,Mg)₂(Si₄O₁₀)(OH)₂·nH₂O, belonging to the monoclinic crystal system [74,75]. In this system, two layers of silica–oxygen tetrahedral sheets are sandwiched between a layer of alumina–oxygen octahedral sheets [76,77]. The negatively charged surface of montmorillonite lamellae attracts exchangeable cations, like Na⁺ or Ca²⁺, between layers to maintain charge balance. These cations in the interlayer space can be replaced by other positively charged cations through a reaction, inducing swelling behavior. Meanwhile, in aqueous solutions, electrostatic forces cause layers of montmorillonite to stack, creating interlayer gaps typically ranging from 0.8 to 2.0 nm [74]. This arrangement facilitates the interaction of potential environmental contaminants, prolonging substance contact time and thereby accelerating the reaction rate.

In the field of dye wastewater treatment, montmorillonite not only enhances the efficacy of catalysts, but also facilitates the implementation of efficient and environmentally benign dye wastewater treatment. Zhang et al. [78] conducted a comprehensive investigation into the photocatalytic degradation of azo dye acid red 18 (AR18) using AgCl/montmorillonite composites under UV irradiation. Montmorillonite, serving as a carrier, provided a stable base for AgCl particles, ensuring their uniform distribution on both the surface and interlayer of montmorillonite. This arrangement increased the specific surface area of AgCl and enhanced its reactivity. The inherently stable interlayer structure of montmorillonite played a crucial role in preventing the agglomeration of AgCl particles, maintaining continuous exposure of its highly active crystalline surfaces, thereby improving photocatalytic efficiency. With its substantial specific surface area and notable cation exchange capacity, montmorillonite efficiently adsorbed a greater amount of dye molecules, facilitating their interaction with AgCl particles and accelerating the photocatalytic degradation process. Noteworthy, montmorillonite effectively prevented the potential photocorrosion of AgCl, enhancing the stability and reusability of AgCl/montmorillonite composites. Furthermore, Fatimah et al. [18] enhanced the photocatalytic performance of zinc–silica hybridized montmorillonite through a microwave-assisted method for degrading methylene blue. The results demonstrated exceptional photocatalytic activity of this material under both UV and visible light, maintaining efficiency across multiple cycles and confirming its outstanding stability. Montmorillonite acted as a matrix, providing an ideal platform for integrating zinc oxides and silicon, resulting in a porous clay heterostructure that increased specific surface area and porosity, promoting favorable conditions for the photocatalytic reaction. The stable layered structure of montmorillonite further ensured the integrity of the heterostructure, preventing the aggregation of zinc oxides and silica and maintaining optimal catalytic activity. Moreover, the adsorption capacity of montmorillonite facilitated enhanced dye molecule adsorption, facilitating thorough interactions between the photocatalyst and dye molecules and promoting the photocatalytic degradation process. Overall, the combined influence of montmorillonite, zinc oxides, and silicon significantly boosted the photocatalytic performance of the composite material compared to individual components.

In the realm of photocatalytic degradation of medical wastewater, montmorillonite plays a pivotal role in the synthesis and performance enhancement of photocatalysts. It not only effectively adjusts the microstructure of the catalysts but also significantly improves the photocatalytic activity and stability of the materials. Yu et al. [79] utilized montmorillonite as a template to synthesize the novel conjugated microporous polymer composite PAF-Mt, with the aim of enhancing its photocatalytic performance. The findings revealed a substantial enhancement in the photocatalytic activity of PAF-Mt upon the inclusion of montmorillonite, resulting in a noteworthy increase in degradation efficiency for various antibiotics, ranging from 325% to 766%, compared to PAF-45 without a template. Notably, the material maintained stable performance after multiple usage cycles, demonstrating its durability. Montmorillonite not only served as a template to convert the polymerization mode of PAF-Mt from disordered to ordered, thereby enhancing the crystallinity and orderliness of the material, but also offered abundant active sites for PAF-Mt, facilitating pollutant adsorption and photocatalytic degradation. Its substantial specific surface area and high porosity further contributed to this role. Additionally, montmorillonite altered the energy band structure of PAF-Mt, imparting a higher oxidation potential, thus enhancing its photocatalytic degradation capacity. Acting as a carrier, montmorillonite fortified the structural stability of PAF-Mt, effectively averting the loss and aggregation of the catalyst during use. In conclusion, montmorillonite played a multifaceted role in PAF-Mt composites by acting as a template, enhancing the specific surface area and porosity, modulating the energy band structure, improving stability, and promoting adsorption, collectively enhancing the photocatalytic degradation performance of the composites. Moreover, Al-Musawi et al. [80] developed CuFe₂O₄/montmorillonite composites for the sonophotocatalytic degradation of ciprofloxacin (CIP). The study illustrated that montmorillonite served as an optimal platform for loading CuFe₂O₄ nanoparticles, attributable to its layered structure and extensive specific surface area. This functionality not only efficiently inhibited the agglomeration of CuFe₂O₄ nanoparticles but also enlarged the specific surface area of the composites, facilitating adsorption and catalytic reactions. Montmorillonite inherently possessed an adsorption capacity which, when combined with CuFe₂O₄, intensified the adsorption of CIP, thereby expediting the degradation process. Furthermore, montmorillonite played a crucial role in bolstering the stability of CuFe₂O₄ nanoparticles in aqueous environments, diminishing the occurrence of loss and aggregation, thus enhancing the reusability of the catalyst. Consequently, in the CuFe₂O₄/montmorillonite nanocomposites, montmorillonite exerted diverse effects, including enhancing the specific surface area, preventing agglomeration, improving adsorption, and enhancing stability. These advantageous effects significantly optimized the performance of the composites in the ultrasonic photocatalytic degradation of CIP.

4.1.3. Attapulgite-Based Photocatalyst

Attapulgite, also known as palygorskite, is an aqueous magnesium–aluminum silicate mineral characterized by a chain-layer structure with a 2:1 ratio [81,82,83]. The ideal chemical formula is (Mg,Al)₄(Si,Al)₈(O,OH,H₂O)₂₆·nH₂O [83]. In its crystalline form, the structural unit layer is typically manifested as an aggregated bundle comprising eight Si-O tetrahedra arranged in 2:1 layers that extend along the C-axis. Additionally, cations like magnesium occupy the coordinated octahedral sites, composed of oxygen and hydroxide anions, thereby establishing a structure incorporating two layers of silicate tetrahedra intercalated with a layer of magnesium–aluminum oxide octahedra. Attapulgite has a significant specific surface area (100.8~216 m²/g) and pore volume (0.38~0.58 cm³/g), showing it has a strong adsorption property [84].

Characterized by a layered structure, attapulgite features numerous nanopores, a high concentration of structural and coordination water, a notable presence of surface hydroxyl groups, and an exceptional specific surface area, all attributing to its outstanding adsorption capabilities [85]. Simultaneously, the porous nature of attapulgite facilitates accelerated reaction rates as reactants are adsorbed within its internal pores [86,87]. Importantly, the diffusion of reactants from the internal pores to the exterior does not compromise the crystal lattice of attapulgite, positioning it as an optimal loaded catalytic-support material [88,89]. This property guarantees the homogeneous loading of metals, metal oxides, metal salts, and other particles with strong catalytic properties on attapulgite, enabling the full exposure of active sites and the efficient transfer of substances, ultimately leading to significant enhancements in catalytic performance.

In the treatment of dye wastewater, Attapulgite functions as a carrier, enhancing the dispersion and stability of other nano-components. It significantly boosts the photodegradation efficiency of dye wastewater, reducing the cost of photocatalyst preparation by improving adsorption performance and photocatalytic activity. Ma et al. [90] loaded Ag₃PO₄ nanoparticles onto attapulgite and assessed the photodegradation performance of this composite for Orange II in wastewater under visible-light irradiation. The study revealed that the attapulgite-loaded Ag₃PO₄ nanocomposites achieved nearly 99% degradation of Orange II under natural sunlight, demonstrating high photocatalytic activity and stability. The addition of Attapulgite notably improved the dispersibility of Ag₃PO₄, leading to an increase in photocatalytically active sites. The inherent adsorption capacity of attapulgite enabled the pre-adsorption of Orange II molecules in wastewater, enhancing the interaction between pollutants and the photocatalyst, thereby improving photocatalytic efficiency. The enhanced light absorption efficiency and photon utilization further heightened the photocatalytic activity. The hydroxyl groups on the surface of attapulgite could form hydrogen or chemical bonds with Ag₃PO₄, facilitating electron transfer and separation, consequently enhancing photocatalytic efficiency. In a study by Shang et al. [91], the impact of Fe and Ce co-doping into attapulgite as a catalyst for methylene blue degradation in wastewater in the presence of ozone was investigated. Besides offering sites for active component loading and enhancing methylene blue adsorption in wastewater, Attapulgite altered the reaction pathway, accelerating ozonolysis to generate hydroxyl groups, thus expediting methylene blue degradation. The synergistic effect of attapulgite in advancing ozonolysis and augmenting adsorption and catalysis notably boosted methylene blue degradation efficiency in wastewater.

In the realm of medical wastewater treatment, eggplant plays a significant role across various facets. This includes enhancing adsorption and photocatalytic properties, facilitating the separation and transport of photogenerated carriers, fortifying the stability and reusability of the material, and enhancing its adaptability to the environmental conditions. These advantages greatly augment the efficiency of composites in removing molecules like tetracycline from medical wastewater. Tan et al. [92] undertook a thorough study on the design and controllable synthesis methods of composite photocatalysts utilizing natural eggplant and Bismuth Molybdate photocatalysts, aiming for a more efficient elimination of tetracycline (Figure 9). Compared to pure Bi₂MoO₆, the Bi₂MoO₆/attapulgite composite exhibited approximately a 1.7-fold enhancement in the reaction rate constant of the oxidation and mineralization rate. This improvement can be attributed to the abundant hydroxyl groups present on the surface of attapulgite, serving as attachment sites for Bi₂MoO₆ particles and reaction sites for contaminant degradation. This enhances degradation and photon efficiency by expanding the light-receiving area and augmenting the number of active sites. Moreover, the layered structure of eggplant effectively prevents the aggregation of Bi₂MoO₆, boosts the specific surface area and porosity of the material, and offers robust support for the Bi₂MoO₆ particles, ensuring the structural integrity of the composite during reactions and preventing the detachment and loss of particles. Simultaneously, the interface between Bi₂MoO₆ and attapulgite may act as a pathway for electron transfer, further facilitating the efficient separation of photogenerated electrons and holes.

4.1.4. Sepiolite-Based Photocatalyst

Sepiolite is a clay mineral classified as a 2:1 phyllosilicate, with a chemical structure represented as Mg₈(OH₂)₄[Si₆O₁₅]₂(OH)₄·8H₂O. This clay was initially described by German scholar Werner in 1789, and it was officially named “sepiolite” in 1847. Global sepiolite reserves are estimated at around 80 million tons, primarily found in Spain, followed by China, the USA, and Turkey [93]. Its structure consists of magnesium oxide octahedra and silica oxide tetrahedra, providing strong adsorption properties and a molecular sieve capacity. Moreover, the presence of a negative charge at the edge of the fiber enhances the adsorption and ion-exchange capabilities of sepiolite for metal cations [94,95,96]. The properties of sepiolite can be exploited in conjunction with the photocatalyst to enhance the efficiency of photocatalytic degradation.

In the field of dye wastewater treatment, due to the sepiolite acting as an important support, adsorption, electron transfer, and particle size regulator, and synergist, the introduction of sepiolite into composite photocatalysts for dye degradation significantly improved the performance and stability of these materials. Wang et al. [97] pioneered a novel ternary composite photocatalyst, sepiolite/Cu₂O/Cu (SCC), demonstrating its exceptional efficacy in degrading organic dyes (Figure 10). The experimental results unveiled a significantly higher degradation efficiency of Congo red under visible light exposure using the composite material, surpassing that of pure Cu₂O or Cu₂O/Cu nanoparticles. The distinctive layered fiber structure and substantial specific surface area of sepiolite make it an optimal host for Cu₂O/Cu nanoparticles. The uniform dispersion and anchoring of Cu₂O/Cu nanoparticles by sepiolite prevent aggregation, exposing more active sites. Moreover, sepiolite itself exhibits remarkable adsorption capabilities, efficiently capturing numerous organic dye molecules. During the photocatalytic process, the adsorption of Congo red molecules on the SCC nanocomposite surface facilitates subsequent degradation through photocatalysis. By employing sepiolite as a carrier, the stability of Cu₂O/Cu nanoparticles is protected against photocorrosion, thereby enhancing the resilience of the catalyst. Mechanistically, sepiolite aids in the transfer of photogenerated electrons from Cu₂O to Cu, restraining electron–hole pair recombination and enhancing photocatalytic efficiency. Ren et al. [98] detailed the synthesis of a ternary heterostructured photocatalyst, Ag/Ag₃PO₄/sepiolite, using a simple precipitation technique. The ensuing examination highlighted the superior photocatalytic performance of this composite in degrading rhodamine B under visible light, outperforming pure Ag₃PO₄ with a degradation rate constant 3.68 times higher. The utilization of seafoam as a carrier effectively dispersed Ag₃PO₄ particles, preventing aggregation and enhancing photocatalytic activity. The in situ reduction of Ag⁰ particles on seafoam resulted in Ag/APO-S exhibiting a higher degradation rate under visible light compared to pure Ag₃PO₄. The reductive Si-OH groups on the seafoam surface facilitated the reduction of Ag⁺ ions to Ag⁰ nanoparticles, serving as electron acceptors in photocatalysis to enhance electron transfer and separation efficiency. Additionally, seafoam incorporation regulated the particle size of Ag₃PO₄, reducing it to minimize the migration distance of photogenerated carriers and reduce electron–hole pair recombination, ultimately boosting photocatalytic activity.

In the realm of medical wastewater treatment, sepiolite played crucial roles as a carrier, enhancing photocatalytic performance, enabling charge transfer, and increasing hydrophilicity in these studies, elevating the performance and applicability of photocatalysts in aqueous environments by binding with the catalysts. Cheng et al. [99] introduced a CeO₂/sepiolite composite for the efficient degradation of tetracycline under visible light irradiation. The CeO₂/sepiolite combination, at 50% CeO₂ loading, demonstrated the highest tetracycline degradation efficiency (92.7%), with a reaction rate constant 4.88 times greater than that of pure CeO₂. This enhancement is attributed to the increased specific surface area of the photocatalyst and the promotion of Ce³⁺/Ce⁴⁺ redox cycling by the CeO₂/sepiolite amalgamation, leading to the generation of more reactive radicals. The presence of sepiolite facilitated the formation of smaller particles on the surface of the photocatalyst, creating additional active sites and establishing a synergistic adsorption–degradation system. The absorption edge of the CeO₂/sepiolite complex shifted to approximately 458 nm, indicating an improved photoreactivity due to the carrier role of sepiolite for CeO₂. Furthermore, incorporating sepiolite into the catalyst formulation improved stability and recyclability. Yi et al. [100] proposed an innovative approach to enhance antibiotic degradation efficiency under visible light by combining covalent triazine frameworks (CTFs) with sepiolite through hydrogen bonding. The fibrous layered structure and high specific surface area of sepiolite facilitated the uniform dispersion and immobilization of CTF nanoparticles on its surface, preventing agglomeration and enhancing active-site exposure. The Si-OH groups abundant on surface of sepiolite increased the hydrophilicity of the SEP/CTF composites, improving antibiotic adsorption and proximity to the photocatalyst surface for enhanced photocatalytic efficiency. The hydrogen-bonding interactions between sepiolite and the CTF not only stabilized the CTF structure but also facilitated electron transfer from the CTF to antibiotic molecules, inhibited electron–hole pair formation, and boosted photocatalytic activity.

4.2. Porous-Structure Clay Mineral-Based Photocatalyst

4.2.1. Diatomite-Based Photocatalyst

Diatomite, a significant mineral resource, has received increased attention in recent years due to its potential in scientific research. This siliceous sedimentary rock is formed through biogenic processes and primarily consists of amorphous SiO₂ [86]. Diatomite is renowned for its lightweight nature, high porosity, and exceptional adsorption properties. Global crust fracturing and basaltic eruptions have resulted in the formation of numerous faulted landforms on continents and the differential alteration in marine sediments. These geological processes have created faulted and volcanic tectonic basins, providing ideal conditions for diatom proliferation and serving as key sites for the accumulation of diatom remains [101,102]. According to industry standards in mineral resources, diatomite deposits are categorized into three types: marine sedimentary diatomite deposits, lacustrine clastic sedimentary diatomite deposits, and sedimentary diatomite deposits associated with basaltic eruptions [103]. The United States holds the largest known reserves of diatomite, predominantly situated along its east and west coasts and adjacent inland regions. China ranks second as the largest holder of diatomite resources [104].

The continuous advancement of catalytic science and technology has gradually brought its application value as a carrier in the preparation of materials to the fore [105,106,107]. The surface of diatomite is rich in active hydroxyl groups, which play a crucial role in the preparation of materials and catalytic reactions [106]. Hydroxyl groups can not only form a stable coupling with the target pollutants, thus ensuring their tight attachment to the surface of diatomite, but also can be directly involved in the catalytic reaction through the activation of the target pollutants. This participation effectively promotes the photocatalytic degradation reaction [108]. Consequently, numerous research studies have been conducted on diatomite as a carrier for catalytic reactions in recent years, demonstrating its broad application potential and scientific research value.

In the treatment of dye wastewater, the chemical stability and non-toxicity of diatomite make it a promising component of photocatalysts, playing multifaceted roles in dye degradation, including enhancing adsorption, promoting electron–hole pair separation, providing support, optimizing composite ratios, and improving stability. Zhu et al. [109] developed a novel Ag₃VO₄/diatomite composite photocatalyst for the catalytic degradation of rhodamine B (RhB) in a one-step process. The photocatalytic degradation efficiency of Ag₃VO₄/diatomite was found to be 3.57 times higher than that of pure Ag₃VO₄, and the catalytic efficiency was maintained after multiple recycling cycles. Meanwhile, diatomite played a crucial role in enhancing the stability, efficiency, and adsorption capacity of the photocatalysts due to its physicochemical properties. The layered structure of diatomite provided a greater number of active sites for Ag₃VO₄, facilitating interfacial contact between diatomite and Ag₃VO₄ to trap photogenerated carriers and promote surface charge separation, thereby improving the photocatalytic efficiency. This effective separation of photogenerated electrons and holes further enhanced the photocatalytic performance. The incorporation of diatomaceous earth improved the structural stability of the composites, enabling them to maintain high photocatalytic activity after multiple cycles of use. Furthermore, in a study by Van Viet et al. [110], a 3% Fe₂O₃/diatomite composite was prepared to degrade RhB by a Photo-Fenton process. The degradation efficiency of the 3% Fe₂O₃/diatomite composite was approximately 81% in 150 min, twice as high as that of pure diatomite. Furthermore, the composite demonstrated high efficiency after five cycles, indicating good durability and stability. Diatomite provided high porosity and a large surface area, facilitating mass transfer of reactants and products, increased contact opportunities with organic dyes, and enhanced structural stability when combined with Fe₂O₃, significantly improving the photocatalytic degradation efficiency of the photo-Fenton reaction.

Within the domain of medical wastewater treatment, diatomite serves as a versatile carrier by providing a vast specific surface area, a porous microstructure, enhanced adsorption capabilities, prevention of nanomaterial agglomeration, and the establishment of an optimal catalytic reaction environment. Sun et al. [111] conducted a comprehensive investigation on the degradation efficacy of MnFe₂O₄/diatomite composites concerning tetracycline hydrochloride and the underlying mechanism in the photo-Fenton process (Figure 11). Diatomite, with its substantial specific surface area, efficiently adsorbs tetracycline hydrochloride at the onset of the catalytic reaction, thereby augmenting light absorption. The dispersion of MnFe₂O₄ nanoparticles on the diatomite surface not only mitigates inter-particle aggregation but also furnishes ample reaction centers and adsorption sites for the photocatalytic reaction. Furthermore, the porous microstructure present on both the exterior and interior of diatomite offers additional active sites for MnFe₂O₄, facilitating pollutant adsorption and H₂O₂ decomposition. Additionally, diatomaceous earth enhances the concentration of acidic sites on its surface, creating an acidic milieu on the catalyst surface that fosters the synergistic interplay between Fe and Mn, leading to an increased generation of OH radicals.

4.2.2. Zeolite-Based Photocatalyst

Zeolite, first discovered and named by the Swedish scientist Cronstedt in 1756, is a mineral that exhibits a boiling phenomenon when exposed to a flame [112]. Over 40 natural zeolites have been identified globally; however, their industrial utilization is constrained by their properties’ reliance on crystal structure. This limitation stems primarily from the small pore size of natural zeolites, necessitating pre-treatment to expand their pore dimensions. The structural framework of zeolite comprises T-O tetrahedra, with atoms like Si, Al, B, P, and Ti, interconnected by bridging oxygens. As predominantly microporous materials, most zeolite topologies feature intracrystalline pore diameters less than 2 nm. Nonetheless, the diverse yet ordered pore arrangement and stable yet adaptable skeletal structure confer outstanding attributes upon zeolites, encompassing a high specific surface area, excellent hydrothermal stability, elevated adsorption capacity, and remarkable selectivity. These characteristics enable zeolites to undertake various roles, serving as catalysts, adsorbents, and carriers [49,113,114]. Additionally, since the Al in zeolite possesses only three lone electron pairs, supplementary sodium, potassium, calcium, or organic cations are necessary to neutralize the negative charge of the aluminum–oxygen tetrahedra in the zeolite framework [115]. In addition, the formation of Brönsted acid sites occurs when a proton is bonded to oxygen in the Al tetrahedron ([Si-O(H)-Al]), while defective skeletal, skeleton-associated, and non-skeletal aluminum in zeolites contribute to Lewis acid sites. The aforementioned sites offer optimal conditions for catalytic reactions [116,117].

In the field of dye wastewater treatment, zeolite functions as a support, bolstering the structural stability and photocatalytic efficacy of these composites, while simultaneously reducing costs and improving their reusability. Zeolite emerges as a promising candidate for the photocatalytic treatment of dye wastewater using zeolite-based catalysts. Badvi et al. [118,119] developed a series of zeolite-based catalysts for the degradation of dyes. This demonstrated that zeolite, acting as a support, with its high specific surface area and complex pore structure, facilitates the uniform dispersion and immobilization of nickel and TiO₂ nanoparticles. The structural characteristics of zeolites exert a profound influence on the dispersion state of the photocatalysts and the exposure of the active sites. The mesoporous structure of zeolite enhances the light utilization efficiency and increases the internal space through a multilayer structure, which in turn improves the photocatalytic efficiency. Moreover, zeolite exhibits a robust adsorption capacity, which enables the effective enrichment of dye molecules on the catalyst surface, thereby enhancing the photocatalytic degradation efficiency. The contact interface between zeolite and nickel and TiO₂ is conducive to stabilizing the charge transfer-excited state generated by photoexcitation and promoting the separation of photogenerated electrons and holes, which in turn enhances the photocatalytic reaction efficiency. The zeolite structure provides protection for nickel and TiO₂ molecules, enhancing their thermal and chemical stability. This prevents the agglomeration and loss of TiO₂ and extends the service life of the catalyst. Latha et al. [120] investigated the photocatalytic performance of composites comprising rare earth metal oxide CeO₂ and zeolite for the removal of Congo red (CR) and methyl orange (MO) dyes. The significant surface area of zeolite facilitates the homogeneous dispersion of CeO₂ nanocubes, preventing nanoparticle aggregation. Moreover, the extensive specific surface area and adsorption properties of zeolite notably boost the adsorption capacity of the photocatalyst. The porous structure of zeolite aids in photocatalyst separation and retrieval from the reaction system, promoting recyclability. The reduced band gap energy of CeO₂/zeolite nanomaterials results in a redshift in the absorption edge in the UV–visible range, thereby enhancing light-absorption capacity. The efficient transfer of photogenerated electron–hole pairs at the zeolite-CeO₂ interface diminishes the recombination rate of these pairs. Additionally, the acidic environment of zeolite potentially stabilizes the Keggin structure of CeO₂, thus enhancing catalyst reusability.

In the realm of medical wastewater treatment, zeolite is essential in material preparation and the synthesis of photocatalysts. It not only effectively influences the structure and enhances the hydrophilicity of catalysts but also significantly boosts adsorption capacity, promotes charge separation, enhances stability, and contributes to the creation of an efficient synergistic catalytic mechanism. Liu et al. [121] successfully fabricated MoS₂ loaded on modified zeolites photocatalysts (MoS₂/Z). The impact of these MoS₂/Z photocatalysts on visible-light-induced tetracycline degradation was extensively evaluated. The findings revealed that the MoS₂/Z photocatalyst achieved a remarkable 87.23% efficiency in tetracycline degradation, surpassing pure MoS₂ and alkali-modified zeolite. Notably, NaOH-modified zeolite displayed a mesoporous structure among the tested materials, effectively influencing MoS₂ morphology and exposing more active sites. Furthermore, zeolite incorporation enhanced the hydrophilicity of MoS₂/Z, aiding photocatalyst dispersion in aqueous solutions. Zeolite functions not only as a MoS₂ support but also facilitates the efficient separation and migration of photogenerated charges through its distinctive structural and physicochemical properties, thus enhancing overall photocatalytic efficiency. Conversely, Farhadi et al. [122] explored the elimination of ibuprofen from water and wastewater using an ultrasound/UV/hydrogen peroxide/zeolite–titanate photocatalyst system. In addition to shaping the morphology of the catalyst, improving ibuprofen adsorption, and boosting catalyst stability, the introduction of zeolite promoted a more effective separation of photogenerated electrons and holes, enhancing the physicochemical attributes of the photocatalyst and significantly increasing ibuprofen removal efficiency.

4.3. Differences among Clay-Based Materials

Within a variety of clay-based photocatalysts, each displays unique traits in pollutant treatment. To explore this further,

Table 2. Different clay-based composites and their photocatalytic efficiency in decomposing various pollutants.

Photocatalyst	Heterojunction Design	Radiative Type	Pollutants	Pollutant Dosage (mg/L)	Degradation Rate	Irradiation Time (min)	Ref.
TiO2/kaolinite	/	solar light	Rhodamine B	5	91%	120	[70]
SnS2/ZnIn2S4/kaolinite	Z-scheme	solar light	Tetracycline	40	97.8%	60	[73]
AgCl/montmorillonite	/	UV light	Acid red 18	50	90%	4.5	[78]
CuFe2O4/montmorillonite	/	sonophotocatalytic	Ciprofloxacin	50	Nearly 100%	60	[80]
Ag3PO4/attapulgite	/	visible light	Orange II	70	99%	90	[90]
Bi2MoO₆/attapulgite	/	visible light	Tetracycline	30	90%	120	[92]
Sepiolite/Cu2O/Cu	/	visible light	Congo red	10	95.1%	50	[97]
CeO2/sepiolite	/	visible light	Tetracycline	40	92.7%	120	[99]
Ag3VO4/diatomite	/	visible light	Rhodamine B	10	96%	40	[109]
MnFe2O4/diatomite	/	photo-Fenton process	Tetracycline Hydrochloride	80	91.8%	60	[111]
Ni/TiO2/zeolite	/	UV light and H2O2	Methylene blue	10	99%	120	[119]
MoS2/zeolites	/	visible light	Tetracycline	200	87.2%	180	[121]

summarizes the photocatalytic degradation effects of the selected photocatalysts. The investigation identifies three primary factors contributing to the variances in the treatment efficiency of clay-based materials.

Firstly, the diversity in light sources significantly impacts the efficiency of separating photo-induced electron–hole pairs on the photocatalyst surface due to varied energy intensities. Specifically, adequate light energy prompts the migration of photo-induced electrons to the conduction band, leaving holes in the valence band, resulting in the formation of electron–hole pairs. These pairs not only facilitate direct pollutant decomposition but also stimulate the generation of highly active free radicals, like hydroxyl radicals, effectively catalyzing pollutant degradation.

Secondly, the types of semiconductor materials utilized in clay-based photocatalysts play a decisive role. Acting as essential components in light response, semiconductors directly influence light absorption and utilization efficiency. Variations in the conduction and valence band positions of different semiconductors lead to distinct free radical types and quantities during the photocatalytic process, thus affecting the rate of degradation. Notably, co-loading multiple semiconductors on clay surfaces can form heterojunction structures, significantly enhancing the separation of photo-induced electron–hole pairs and boosting photocatalytic efficiency.

Additionally, the inherent characteristics of clay are crucial. The diverse microstructures of clay impact not only the morphology of loaded semiconductor materials but also indirectly modify pollutant-transportation pathways, thereby influencing degradation outcomes. Moreover, differences in clay composition elements may interact with loaded photocatalytic semiconductor materials, resulting in novel heterostructures that expedite the photocatalytic reaction process.

Lastly, the treatment effects of various pollutants during photocatalytic degradation differ. The chemical properties of the target pollutant dictate its degradation pathway and mineralized form under the influence of photo-induced charge carriers on the photocatalyst. For example, when handling complex pollutants such as phenolic compounds in dye and medical wastewater, optimizing photocatalytic conditions (e.g., extended reaction time, enhanced light intensity, utilization of more effective light sources) or employing combined treatment methods (e.g., integrating with Fenton reaction) can achieve favorable catalytic outcomes and meet mineralization objectives.

5. Conclusions and Perspectives

In conclusion, this critical review of clay mineral-based photocatalysts for wastewater purification has uncovered several significant findings. Firstly, clay-based photocatalysts show promising photocatalytic activity for degrading organic pollutants in water thanks to their unique physicochemical properties and large surface area. Secondly, these materials offer a cost-effective and environmentally friendly alternative to conventional water purification methods, contributing to sustainable environmental practices.

The contributions of this study to both academic and practical fields are diverse. Theoretically, our analysis enhances the understanding of the mechanisms behind the photocatalytic activity of clay minerals. On a practical level, this review emphasizes the potential of these materials for large-scale water-treatment applications, particularly in regions with limited access to advanced purification technologies.

The significance of this research lies in its comprehensive evaluation of the use of clay-based photocatalysts in water purification. Not only does it underscore their efficacy, but it also discusses their scalability and economic viability. The long-term implications of this work could lead to the development of more efficient and sustainable water-treatment systems, especially in light of global water-scarcity challenges.

Looking ahead, future research should concentrate on optimizing the photocatalytic performance of clay minerals. This could include exploring novel clay-based composite materials, investigating the impact of dopants and impurities on enhancing photocatalytic activity, and studying the influence of different types of clay minerals on the degradation of specific pollutants. Simultaneously, the exploration of clay-based composites should extend to the degradation of more recalcitrant substances through photocatalysis. Detailed analyses of the pathways for photocatalytic pollutant degradation and subsequent mineralization should be conducted using advanced techniques like LC-MS. Furthermore, deeper exploration is warranted into the mechanisms of radiative transfer and the excitation of free radical groups, necessitating thorough investigation. Researchers should utilize sophisticated detection methods to further study the conversion of radiant solar energy, consolidating universally applicable mechanisms to guide future industrial applications.

Acknowledging the limitations of this study is essential. While this review offers a broad overview of the field, the specific performance characteristics of individual clay minerals may vary depending on the source and processing methods. Moreover, the practical application of these materials may be affected by factors such as water chemistry, temperature, and pH, which were not fully addressed in this review. Future studies should aim to address these knowledge gaps.

Finally, the findings of this review have potential implications for policy makers and practitioners in the water-treatment sector. The cost-effectiveness and sustainability of clay-based photocatalysts make them a viable option for water-purification projects, particularly in developing countries. By integrating these materials into existing water-treatment infrastructure, significant improvements in water quality and public health outcomes could be attained.