Mineral-Supported Photocatalysts: A Review of Materials, Mechanisms and Environmental Applications

Abstract

Although they are of significant importance for environmental applications, the industrialization of photocatalytic techniques still faces many difficulties, and the most urgent concern is cost control. Natural minerals possess abundant chemical inertia and cost-efficiency, which is suitable for hybridizing with various effective photocatalysts. The use of natural minerals in photocatalytic systems can not only significantly decrease the pure photocatalyst dosage but can also produce a favorable synergistic effect between photocatalyst and mineral substrate. This review article discusses the current progress regarding the use of various mineral classes in photocatalytic applications. Owing to their unique structures, large surface area, and negatively charged surface, silicate minerals could enhance the adsorption capacity, reduce particle aggregation, and promote photogenerated electron-hole pair separation for hybrid photocatalysts. Moreover, controlling the morphology and structure properties of these materials could have a great influence on their light-harvesting ability and photocatalytic activity. Composed of silica and alumina or magnesia, some silicate minerals possess unique orderly organized porous or layered structures, which are proper templates to modify the photocatalyst framework. The non-silicate minerals (referred to carbonate and carbon-based minerals, sulfate, and sulfide minerals and other special minerals) can function not only as catalyst supports but also as photocatalysts after special modification due to their unique chemical formula and impurities. The dye-sensitized minerals, as another natural mineral application in photocatalysis, are proved to be superior photocatalysts for hydrogen evolution and wastewater treatment. This work aims to provide a complete research overview of the mineral-supported photocatalysts and summarizes the common synergistic effects between different mineral substrates and photocatalysts as well as to inspire more possibilities for natural mineral application in photocatalysis.

1. Introduction

As environmental pollution issues have taken place more and more frequently in recent years, sustainable development has become the leading target of energy and environmental policy–making worldwide, determining the public concern for scientific research. Since the TiO₂-based water-splitting phenomenon was discovered by Fujishima and Honda in 1972, photocatalysis has stepped onto the solar energy conversion stage [1]. Many researchers have verified the extensive potential of TiO₂ in the photodegrading of different pollutants, such as bisphenol A [2], tetracycline [3], gaseous formaldehyde [4], S. aureus [5], etc. As a result, photocatalysis is widely regarded as the most promising route to solve both the energy and pollution crises. Several common photocatalysts have been found to possess excellent photocatalytic performance for the degradation of toxic dye and organic pollutants, including ZnO [6,7], In₂O₃ [8], g-C₃N₄ [9], BiOBr [10], etc.

However, large-scale industrial applications of pure photocatalysts often suffer several key issues: (1) small specific surface area and resultant poor adsorptive ability; (2) easy photo-generated electron-hole recombination and resultant low quantum efficiency; (3) difficult solid-liquid separation and photocatalyst recycling; and (4) high cost. For example, CdS has superior visible-light absorption and photocatalytic activity [11], while it suffers severe photo-corrosion issues and easily causes secondary pollution problems. Although TiO₂ has stable and superb photocatalytic performance, it is only UV-light responsive and hard to recycle [12]. Additionally, self-aggregation and high photogenerated electron-hole recombination rates are the other common issues that the purest photocatalysts encounter. Researchers have been trying many different modification strategies to address these problems. So far, several modification strategies have been developed to control the photocatalyst morphology and suppress photo-induced charge carrier recombination, including heterojunction design, noble-metal deposition, support composites, etc. [13,14,15]. Among these modification strategies, the support composite is the most promising method to overcome the photocatalyst recycling difficulty and thus realize the industrialization of photocatalysts. Common photocatalyst support materials are carbon nanotubes, mesoporous carbon, graphene graphite, zeolite, etc. While these carbon materials generally exhibit more regular morphology and better electrical properties, they are not the ideal support candidates for photocatalyst industrial development considering their cost and recycling issues. Natural minerals are generally defined as abundant natural resources on earth that are obtained directly from nature. Compared with other support materials, natural minerals are well known as suitable photocatalyst support candidates for the following reasons: (1) They are environmentally friendly, low cost, chemical resistant, mechanically stable, easily available, and do not cause secondary pollution to the environment; (2) natural minerals have the characteristics of a large specific surface area, providing more active centers and adsorbing pollutants to concentrate catalysis; (3) natural minerals are also excellent photocatalysts, such as ilmenite and pyrite, which play a good role in the degradation of water pollutants. More importantly, the combination of photocatalysts and natural minerals has become one of the most valuable modification strategies due to the vast availability and cost-effectiveness of natural minerals. Until now, many researchers have devoted enormous contributions to this field, as shown in Table 1.

Few reviews have been written about natural mineral supports. Liu et al. chose clay minerals and layered double hydroxides to study the synthesis methods of photocatalytic applications of various clay or LDH-based photocatalysts [16]. Szczepanik et al. found that clay/TiO₂ nanocomposites could achieve the photodegradation of several organic pollutants (organic dyes, herbicides, and aromatic aniline) through pseudo-first-order kinetics based on the Langmuir–Hinshelwood model [17]. Various synthesis methodologies of TiO₂/clay nanocomposites and the effects of clay support on the physical properties and photocatalytic activity of TiO₂/clay nanocomposites have been recently discussed [18]. In addition to synthetic strategy evolution, titanium/clay mineral composite applications in the environmental catalysis and photocatalysis fields were illustrated thoroughly by solid literature support [19]. Papoulis et al. elaborated on the application of halloysite-based nanocomposites in photodegrading inorganic air pollutants, volatile organic compounds, antibiotics, pesticides, azo dyes, and other organic pollutants [20]. More recently, Li et al. discussed the role of the three typical phyllosilicate minerals (kaolinite, montmorillonite, and rectorite) in improving the photocatalytic performance of several photocatalysts, such as TiO₂, g-C₃N₄, ZnO, and MoS₂ [21]. As one of the typical layered clay minerals, kaolinite was selected to analyze the preparation of common kaolinite-based photocatalysts and their environmental applications, such as gas and liquid phase pollutants and bacterial disinfection [22]. Photocatalytic mechanisms and kinetics were introduced in detail, as were kaolinite’s structure and properties. Zou et al. focused on the aluminosilicate clay minerals in photocatalytic applications and divided them into type 1:1 and 2:1 aluminosilicate clay according to their specific layered structures [23]. Several typical 1:1 and 2:1 aluminosilicate clays, such as kaolinite and montmorillonite, were used to elucidate their natural structure and surface activating and interlayered modification methods. However, they only focused on several specific clay minerals, failing to comprehensively overview and generalize valuable combination mechanisms between natural minerals and photocatalysts [24]. More recently, a review work came out to present the latest progress of natural minerals in the photocatalysis field based on their classification (silicate, oxide, sulfide, borate, and other minerals) [25]. However, the influence and reactive mechanisms of natural minerals in the photocatalyst–mineral composite system have not been completely covered in these review articles.

This paper represents a comprehensive review of natural mineral-supported photocatalysts. The purpose and contribution of this review are, first, to offer a comprehensive overview of mineral-supported photocatalysts development. We submitted the search query to the literature databases Web of Science (WOS) and Google Scholar in 2022. This work mainly focused on the research conducted in the last 15 years and initially selected 858 research papers. According to different research subjects of this literature, we categorize natural minerals into four groups: silicate minerals (799 papers, 93%), carbonate or carbon-based minerals (12 papers, 1%), sulfate or sulfide minerals (23 papers, 3%), and several special minerals (24 papers, 3%). As seen in Figure 1, natural silicate minerals are the main part that can be further classified into four groups according to their original mineral morphology, such as 1D (218 papers, 27%), 2D (309 papers, 39%), 3D (81 papers, 10%), and porous minerals (191 papers, 24%). Secondly, recent literature studies on the mineral application of photocatalysis are summarized; a brief introduction of photocatalyst modification methods and their merits are included in this work. Based on this, we summarize the recent research on each mineral application in photocatalysis. Thirdly, the key mechanisms for natural minerals to enhance photocatalytic properties are analyzed. We analyzed the modification mechanism based on its characteristics (i.e., morphology, chemical composition, light absorption ability, etc.). Fourth, the perspective of future work and research directions are mentioned.

Table 1. Photocatalysts prepared using different minerals as support materials.

Photocatalyst	Synthesis Method	Organic Pollutant	Light Source	Quantum Efficiency (%)	Refs.
Chrysotile/SnO2	A direct precipitation process coupled with calcination treatment	MB	250 W 450 nm mercury lamp	99	[26]
Chrysotile @ZnO	Precipitation	MB	250 W 365 nm mercury lamp	99.50	[27]
TiO2/micro-meso porous silica nanofibers	Sol-adhesion	RhB	25 W 254 nm ultraviolet light	95	[28]
Exfoliated kaolinite nanolayers	Intercalation and delamination	RhB	150 W 254 nm mercury lamp	over 95	[29]
g-C3N4/TiO2/kaolinite	A mild sol-gel method associated with chemical stripping and self-assembly	CIP and S. aureus	500 W over 400 nm xenon lamp and 8 W fluorescent lamp	92	[5]
N-doped TiO2/kaolinite	A modified two-step sol-gel method	CR	8 W lamp	99	[30]
Ti-Fe kaolinite composite	A simple precipitation method	Cr(VI)	300 W over 420 nm dysprosium lamp	87 at PH = 3.0	[31]
Ag/g-C3N4/kaolinite	A two-step assembly strategy by employing in situ impregnation-calcination and photo-deposition process	IBP	500 W over 400 nm xenon lamp	99.90	[32]
Kaolinite/TiO2	Sol-gel method	MB	8 W 254 nm high-intensity UV-C radiation lamp	97	[33]
Mica/TiO2/Fe2O3	A sol-gel assisted hydrothermal method	G-MeCHO	400 W xenon lamp	80	[34]
MoS2/mica	Hydrothermal	TH	300 W xenon lamp	90.60	[35]
Ti-Fe-based alkaline muscovite	Impregnation	TBBPA	150 W xenon lamp	90	[36]
TiO2/illite	Hydrothermal	MO	30 W UV light source	73.4	[37]
KNbO3/vermiculite	In situ hydrothermal method	MB	300 W xenon lamp	81	[38]
AgI-Bi2MoP6/vermiculite	Sol-gel and precipitation methods	MG	65 W lamp	98.89	[39]
BiOBr/magnetic bentonite	In situ coprecipitation followed by microwave-assisted hydrothermal method	TC and CIP	500 W over 420 nm xenon lamp	85 and 95 for TC and CIP	[10]
Ag/AgCl/montmorillonite	Dispersion method	MB	100 W over 400 nm tungsten lamp	98	[40]
Calcite/TiO2	Sol-gel method	TC	24 W 254 nm UV-lamp	90	[41]
ZnO-halloysite	Ultrasonic treatment	CIP	160 W mercury lamp	91	[42]
Fe(III)/montmorillonite	Simple adsorption	Cr(VI)	300 W xenon lamp	100	[43]
BiOBr/Ti3C2/exfoliated montmorillonite	In situ co-precipitation coupling with microwave hydrothermal	CIP	500 W over 400 nm xenon lamp	96	[44]
γ-Fe2O3/montmorillonite	Hydrothermal	RhB	40 W xenon lamp	99	[45]
CdS/MoS2/montmorillonite	Direct coagulation casting	TC	50 W LED lamp	90.03	[46]
Coal-bearing strata aolinite/MnFe2O4	Hydrothermal	CTC•HCl	300 W over 420 nm xenon lamp	85.1	[47]
Cu2O/ZnO/kaolinite	Co-precipitation	MB	150 W halogen lamp	93	[48]
ZnO-bentonite	Facile synthesis	MB and eriochrome black-T	Solar radiation	Over 95	[49]
ZnO/γ-Fe2O3/bentonite	Facile co-precipitation	CIP	Solar simulator	95	[50]

Table 1. Photocatalysts prepared using different minerals as support materials.

Photocatalyst	Synthesis Method	Organic Pollutant	Light Source	Quantum Efficiency (%)	Refs.
Chrysotile/SnO2	A direct precipitation process coupled with calcination treatment	MB	250 W 450 nm mercury lamp	99	[26]
Chrysotile @ZnO	Precipitation	MB	250 W 365 nm mercury lamp	99.50	[27]
TiO2/micro-meso porous silica nanofibers	Sol-adhesion	RhB	25 W 254 nm ultraviolet light	95	[28]
Exfoliated kaolinite nanolayers	Intercalation and delamination	RhB	150 W 254 nm mercury lamp	over 95	[29]
g-C3N4/TiO2/kaolinite	A mild sol-gel method associated with chemical stripping and self-assembly	CIP and S. aureus	500 W over 400 nm xenon lamp and 8 W fluorescent lamp	92	[5]
N-doped TiO2/kaolinite	A modified two-step sol-gel method	CR	8 W lamp	99	[30]
Ti-Fe kaolinite composite	A simple precipitation method	Cr(VI)	300 W over 420 nm dysprosium lamp	87 at PH = 3.0	[31]
Ag/g-C3N4/kaolinite	A two-step assembly strategy by employing in situ impregnation-calcination and photo-deposition process	IBP	500 W over 400 nm xenon lamp	99.90	[32]
Kaolinite/TiO2	Sol-gel method	MB	8 W 254 nm high-intensity UV-C radiation lamp	97	[33]
Mica/TiO2/Fe2O3	A sol-gel assisted hydrothermal method	G-MeCHO	400 W xenon lamp	80	[34]
MoS2/mica	Hydrothermal	TH	300 W xenon lamp	90.60	[35]
Ti-Fe-based alkaline muscovite	Impregnation	TBBPA	150 W xenon lamp	90	[36]
TiO2/illite	Hydrothermal	MO	30 W UV light source	73.4	[37]
KNbO3/vermiculite	In situ hydrothermal method	MB	300 W xenon lamp	81	[38]
AgI-Bi2MoP6/vermiculite	Sol-gel and precipitation methods	MG	65 W lamp	98.89	[39]
BiOBr/magnetic bentonite	In situ coprecipitation followed by microwave-assisted hydrothermal method	TC and CIP	500 W over 420 nm xenon lamp	85 and 95 for TC and CIP	[10]
Ag/AgCl/montmorillonite	Dispersion method	MB	100 W over 400 nm tungsten lamp	98	[40]
Calcite/TiO2	Sol-gel method	TC	24 W 254 nm UV-lamp	90	[41]
ZnO-halloysite	Ultrasonic treatment	CIP	160 W mercury lamp	91	[42]
Fe(III)/montmorillonite	Simple adsorption	Cr(VI)	300 W xenon lamp	100	[43]
BiOBr/Ti3C2/exfoliated montmorillonite	In situ co-precipitation coupling with microwave hydrothermal	CIP	500 W over 400 nm xenon lamp	96	[44]
γ-Fe2O3/montmorillonite	Hydrothermal	RhB	40 W xenon lamp	99	[45]
CdS/MoS2/montmorillonite	Direct coagulation casting	TC	50 W LED lamp	90.03	[46]
Coal-bearing strata aolinite/MnFe2O4	Hydrothermal	CTC•HCl	300 W over 420 nm xenon lamp	85.1	[47]
Cu2O/ZnO/kaolinite	Co-precipitation	MB	150 W halogen lamp	93	[48]
ZnO-bentonite	Facile synthesis	MB and eriochrome black-T	Solar radiation	Over 95	[49]
ZnO/γ-Fe2O3/bentonite	Facile co-precipitation	CIP	Solar simulator	95	[50]

2. Photocatalyst Modification Methods

Essentially, photocatalysis is a particular catalysis type involving light absorption by several specific active species [51]. For example, a novel inorganic–organic TiO₂-g-C₃N₄ composite could be excited under visible light (λ ≥ 400 nm) irradiation, activating the electron (e⁻) transfer from the valence band (VB) to the conduction band (CB) [52]. In this case, the photo-induced holes of TiO₂ could be attracted to the facet-coupled interface between TiO₂ and g-C₃N₄ due to the internal static electric fields of TiO₂, and then transferred to VB of g-C₃N₄, oxidizing RhB directly. Meanwhile, the photo-induced electrons of g-C₃N₄ transfer to TiO₂ due to the higher CB potential, reacting with the adsorbed O₂ into superoxide radicals (•O₂⁻¹), as shown in Figure 2. However, the photocatalytic reaction can be easily prohibited due to fast photoexcited electron-hole recombination. In this part, TiO₂, as the most typical photocatalyst, is selected to present the common photocatalyst modification methods and their mechanism, including impurity doping, noble-metal deposition, alkali or alkaline earth modification, heterojunction design, and support combination.

2.1. Ion Doping

Doping is one of the most frequently applied methods to effectively extend the light absorption range of semiconductors via directly altering the bandgap structure [53]. Generally, TiO₂ photocatalysts could be doped with metal and non-metal dopants to enhance their photocatalytic activity [54,55,56]. For example, cobalt-doped TiO₂ samples synthesized via one-step hydrothermal method varied the cobalt dopant concentration from 0.1% to 3.0% to expand its optical response to the visible light region [57]. The photocatalytic phenol degradation reaction reached the highest degradation rate (81.72%) when the cobalt-doped amount was fixed at 0.3%. First principle calculation results indicate the density of the impurity state strengthens with increased dopant concentration, which is beneficial for the visible light absorption range extension and the enhancement of its photocatalytic efficiency. Additionally, the doped ions can also act as shallow traps to promote photoinduced electron-hole separation. However, when the doping concentration exceeds by too much, these doped ions are contrarily exchanged into photo-excited electron-hole recombination centers. Another work found that nitrogen ions could substitute the oxygen lattice of the TiO₂ matrix, therefore narrowing the band gap and shifting the absorption edge to the visible light region [58]. The BET surface areas of N–TiO₂ and N–TiO₂/C nanocomposite were calculated to be 101.4 and 131.6 m²/g, higher than that of TiO₂ (87.7 m²/g).

In order to process acidic gas pollution such as CO₂ and NO_x, special ions, namely alkali or alkaline earth metal ions, are incorporated into photocatalysts for better acidic gas adsorption capacity. For example, alkali (Na, K, Rb, and Cs) or alkaline earth metals (Mg, Ca, Sr, and Ba) were selected to modify anatase TiO₂ photocatalyst and investigate their effect on low-temperature NO_x trapping efficiency [59]. It was proved that the NO_x storage capacity is positively related to the amount of the modified metal species, and Cs-TiO₂ and Ba-TiO₂ have similar capacities (179 and 187 μmol∙g⁻¹, respectively), which is higher than that of conventional NO_x-trapping catalysts. According to in situ diffuse reflectance infrared Fourier transfer (DRIFT) spectra results, adsorbed NO_x species were trapped on the Ti sites of alkaline earth–doped TiO₂ and the alkali sites of alkali-doped TiO₂, while NO_x trapped on Ti sites was found to be more efficient than on other sites for its excellent photocatalytic oxidation ability.

2.2. Noble-Metal Deposition

Coupling with one or more noble metal nanoparticles, such as Pt, Au, Ag, and Pd, is another well-developed surface modification approach. First of all, the noble metal nanoparticles function as electron traps and realize the rapid electron migration from photocatalyst to metal nanoparticles through the Schottky barrier formed at the interface between the semiconductor and the noble metals. The surface plasma resonance properties of noble metals can promote photocatalyst excitation under visible light and thus facilitate interfacial electron transfer [60]. Furthermore, the Fermi levels of the noble metal nanoparticles are generally lower than the CB of TiO₂; therefore, the band gap structure could be modified. For instance, four noble metals (Pt, Pd, Ag, and Au) were loaded onto the TiO₂ surface, and it was found that photoactivity of the noble metal–deposited TiO₂ increased with the amount of noble metals [61]. Among these metals, 0.2 wt% Ag–deposited TiO₂ exhibited the highest phenol degradation rate (0.51 μmol∙dm⁻³∙min⁻¹) under 60 min visible light irradiation, which is five times that of pure TiO₂. It is pointed out that Ag nanoparticles are deposited on the {101} crystal facet of anatase TiO₂ according to TEM analysis, which results in decreasing photoinduced electron-hole recombination and visible light absorption extension via acting as electron traps and the surface plasma resonance effect.

2.3. Heterojunction Design

Heterojunction construction is one of the most commonly applied and effective surface modification approaches, coming from the combination of two or more semiconductors with different VB and CB positions and the resultant narrower band gap structure. The referred heterojunction is the type-II heterojunction and could be classified into four types such as the conventional type-II heterojunction, P–N junction, surface heterojunction and direct Z-scheme heterojunction, depending on the charge carrier separation mechanism [62]. Generally, the photocatalytic activity of TiO₂ could be enhanced by designing a heterojunction structure with several oxides and non-oxide materials [63,64,65]. For example, g-C₃N₄/TiO₂ heterojunction composite was synthesized via the in situ hydrothermal method, and the obtained band gap energy was proved to be 2.8 eV compared with 3.0 eV of TiO₂ [66]. The g-C₃N₄/TiO₂ sample with a mass ratio of 1:4 exhibited the maximum hydrogen evolution rate (1041 μmol/(g∙h)) under visible light irradiation, which could be attributed to the heterojunction formation and its spatial charge separation effect. The XPS valence spectra results confirm the interaction between g-C₃N₄ and TiO₂ as type-II heterojunction, and the mechanism for photocatalytic performance improvement is explained such that the electron migration from VB of g-C₃N₄ to CB of TiO₂ via C-O-Ti bonding can offer electrons more photocatalytic reaction time. TiO₂ is known as a typical n-type semiconductor and can be used to construct p–n heterojunctions by combining it with a p-type semiconductor. As a result, BiOI was selected to couple with TiO₂, and it was found that the 20% BiOI/TiO₂ heterostructure achieved the highest phenol conversion efficiency (13.5%) and the best selectivity (92.1%) [67]. The enhanced photocatalytic performance is ascribed to a narrower band gap and efficient photoinduced electron-hole separation due to the p–n heterojunction structure as well as more exposed reactive facets.

2.4. Support Material Combination

Over the past decade, loading the well-developed photocatalysts onto special supporting materials with unique chemical properties or morphology has become more popular. Compared with photocatalyst modification methods such as ion doping, support material combinations have different characteristics in promoting photocatalysts. Support material generally only influences the photocatalytic reaction rate instead of the reaction process. On the one hand, when deposited onto the surface of minerals, the aggregation of photocatalysts could be greatly meliorated, which considerably decreases the chance of photo-generated electron-hole recombination [68,69]. For example, the addition of reduced graphene oxide into the titanium oxide matrix could hinder the agglomeration of photocatalyst nanoparticles and efficiently promote photoinduced electron-hole immigration [70]. On the other hand, the combination of minerals and photocatalysts could obtain better adsorptive ability, which is favorable for the concentration of pollutants near the photocatalysts and shortens the transfer route of photo-induced charge carriers. With high specific surface area and porosity, porous diatomite could cooperate with silver phosphate to harvest visible light and adsorb dye molecules [71]. In many cases, minerals have some active species that function differently in photocatalytic reaction systems. Some active species in minerals could be combined with photocatalysts and modify the crystalline and electronic structure of photocatalysts, similar to the mechanism of ion doping and heterojunction construction. While other active species could act as the separation center of photo-induced electron and hole pairs and vary the transfer route of the charge carriers. The most promising directions in recent decades are more postulates of mineral support influences and further elucidations of the known influences.

Generally, support materials are mainly divided into natural minerals and artificial minerals. Increasing research on mineral-supported photocatalysts has found that the natural minerals as photocatalyst supports could effectively achieve fast photocatalyst collection and recycling at a relatively low-cost level [21]. The natural minerals as supporting materials are introduced into the photocatalysis system to mainly improve the durability of photocatalysts and decrease the application cost, which could enable the industrialization of photocatalysts. For artificial minerals, there are few reports about them as photocatalyst support. Some studies have pointed out that although it is possible to synthesize specific minerals with better morphology and the same chemical composition in the lab, the cost and time of large-scale production is much higher than using natural minerals due to the strict method of laboratory mineral synthesis. Here, we focus on natural minerals. We take a more detailed look at the four natural mineral groups (silicate minerals, carbonate or carbon-based minerals, sulfate or sulfide minerals, and several special minerals) mentioned in the Introduction section.

3. Natural Silicate Mineral-Supported Photocatalyst

As the main part of natural minerals, silicate minerals have numerous members and abundant reserves. Many efforts have been devoted to exploring suitable clay supports to prepare photocatalyst-supported composites, mainly because of their specific structure, high surface area, and multiscale porosity, including the resultant adsorption properties and ability to be bound to reactive groups. It is promising to disperse photocatalyst particles onto the clay minerals under mild reaction conditions to deal with the universal aggregation problems.

According to the literature, natural clay minerals can be divided into four general types (porous, 1D, 2D, and 3D minerals).

3.1. 1D Mineral-Supported Photocatalyst

3.1.1. Attapulgite-Supported Photocatalyst

As a typical clay soil, attapulgite (ATP) is magnesium aluminium phyllosilicate with special lath or rod-like morphology that is abundant in Jiangsu Province, China. It is characterized as a porous crystalline structure containing tetrahedral layers arranged together along longitudinal sideline chains [72]. Due to its large specific surface area, substantial intrinsic nano-channels, and porous structure, attapulgite has rich hydroxyl groups on its surface and could incorporate many active species and transient metal ions to construct functionalized nanocomposites with superior adsorption ability [73].

Considering its high specific surface area and thermal and chemical stability, attapulgite is regarded as suitable support to compensate for catalyst selectivity and stability deficiency under different photocatalytic conditions. For instance, Bi₂MoO₆ deposited on attapulgite support by a simple solvothermal method showed an improved photocatalytic oxidation rate of tetracycline (0.0109 min⁻¹) under visible light irradiation, which was about 1.70 times higher than that of pure Bi₂MoO₆ (0.00626 min⁻¹) [74]. The high specific surface area of attapulgite (216.4 m²/g) was considered to speed up the adsorption and photocatalytic process, resulting in a better photocatalytic performance of the Bi₂MoO₆/attapulgite composite.

Although its superior visible light-responsive ability, g-C₃N₄ suffers serious agglomeration problems, which affect its photocatalytic performance. Many efforts have been dedicated to controlling its morphology and structure to improve its photocatalytic activity. Zhu et al. prepared a novel magnetic Fe₃O₄ quantum dots@g-C₃N₄/attapulgite (ATP) applying a eutectic method followed by water bath deposition [75]. The specific surface area was enhanced from 45.1 m²/g to 60.9 m²/g after g-C₃N₄ loading due to the formation of rich interior space between attapulgite and the g-C₃N₄ matrix, as shown in Figure 3a. Furthermore, the decay lifetime of Fe₃O₄ quantum dots@ g-C₃N₄/attapulgite ternary composite reached 7.1 ns, which was longer than that of g-C₃N₄/ATP (4.3 ns) and g-C₃N₄ (3.1 ns), confirming that attapulgite and Fe₃O₄ quantum dots in the intercalation structure could effectively promote the separation of photogenerated electron-hole pairs. The later Fe₃O₄@g-C₃N₄/attapulgite photocatalyst could degrade 90.6% of 20 mg/L 2-Mercaptobenzothiazole under 90 min visible light irradiation. Moreover, magnetic Fe₃O₄ particles in the composite could provide efficient magnetic separation of the photocatalyst after the photocatalytic experiments. Zuo et al. prepared attapulgite/Cu₂O/Cu/g-C₃N₄ with unique Z-scheme heterostructure through in situ reduction of ultra-fine CuO nanoparticles on the surface of rod-like attapulgite under NH₃ gas during melamine polycondensation [76]. The obtained photocatalyst exhibited a high BET surface area (79.5 m²/g) and band gap energy of 1.9 eV, significantly improving the separation and immigration of photogenerated electron-hole pairs. This attapulgite/Cu₂O/Cu/g-C₃N₄ composited showed a higher degradation rate of chloramphenicol antibiotic (61%) under visible light irradiation for 120 min than that determined for attapulgite/g-C₃N₄ composite.

Recently, some photocatalysts have been successfully assembled onto the attapulgite surface for CO₂ reduction [77,78]. For instance, CeO₂/attapulgite composite showed CO and CH₄ yields of about 3 and 3.6 times higher than that of pure CeO₂, respectively, and the photocurrent intensity of CeO₂/attapulgite is nearly 2.2 times higher than that of the bare CeO₂ photocatalyst [77]. Adding In₂O₃ to the later composite to obtain In₂O₃/CeO₂/attapulgite ternary hybrid system exhibited even better CO and CH₄ yields, which are 6.11-fold and 6.78-fold higher than that of pure CeO₂ [78]. It is supposed that the introduction of attapulgite into binary or ternary hybrid systems can restrain the accumulation of CeO₂ and In₂O₃ particles and provide a large surface area with abundant active sites. Similarly, CeO₂/g-C₃N₄-hybrid photocatalyst deposited on attapulgite skeleton showed excellent electron immigration capability with the high adsorptive capacity of attapulgite matrix, and it was synergistically beneficial for the photocatalytic desulfurization [79]. However, an excessive attapulgite amount in the composite would minimize the light transmission and thus influence its photocatalytic activity.

Noble metals have many vital applications in the catalysis field, but it is unrealistic to put them into mass production due to the high cost. As a result, attapulgite can be used for photocatalyst support to reduce the usage amount of noble metals. Ma et al. could successfully distribute Ag₃PO₄ nanoparticles on attapulgite surface without apparent spherical particle aggregation [80]. With the improved adsorptive ability and stability, the silver content of Ag₃PO₄/attapulgite decreased from 77.3% to 18.87%. This work solidifies the possibility of reducing the cost of noble metal catalysis in practical production without reducing their photocatalytic performance.

Furthermore, the surface modification of attapulgite with hydrochloric acid could exhibit significantly improved water splitting ability in the presence of photosensitizers such as Eosin Y dye (EY) [81]. The treatment of attapulgite with relatively low concentration HCl solution (1 mol∙L) and the use of EY-sensitizer yielded a maximal hydrogen generation rate of 210 μmol∙h⁻¹ with evident 28.6% quantum efficiency at 490 nm under visible light illumination [82]. It is proposed that electrons excited from sensitizer molecules adsorbed on the attapulgite surface could immigrate to attapulgite efficiently and therefore enhance the photocatalytic performance remarkably, as the process presented in Figure 3b.

3.1.2. Palygorskite-Supported Photocatalyst

Similar to attapulgite, palygorskite is also a fibrous hydrated magnesium aluminum phyllosilicate with rich reactive hydroxyl species on the surface. It possesses a chain-lamellar structure composed of two silica tetrahedral sheets linked by a magnesium or aluminum octahedral sheet, where the mismatch between the tetrahedral and octahedral sheets induced non-inter-connected zeolite-like channels. Apart from fibrous morphology, high specific surface area and moderate cation exchange capacity, palygorskite has a negatively charged surface that could exert electrostatic attraction to adsorb cations [83].

Based on the aforementioned advantages, natural palygorskite clay has been used as a unique fibrous substrate for various photocatalysts. For instance, the nanometric size of TiO₂ particles immobilized on the palygorskite surface could prevent sintering-induced growth from exceeding the nucleus critical size required for the anatase–rutile phase transition [84]. E. Stathatos et al. prepared a porous TiO₂/palygorskite composite by applying nonionic surfactant molecules as a pore directing agent that could be removed by thermal treatment to generate the porosities in the composite [85]. The latter approach resulted in a homogeneous dispersion of TiO₂ on the surface of the palygorskite nanofibers with an average crystallite size of 12–16 nm. Kuang et al. fabricated an Ag-deposited TiO₂/palygorskite composite via a solvothermal-chemical reduction route [86]. The optimized ternary Ag-TiO₂/palygorskite composite could degrade 95.27% of methylene blue under visible light illumination after 210 min compared with 54.11% of TiO₂/palygorskite and 9.85% of commercial TiO₂ after the same time. It is suggested that the improved photocatalytic activity of the composite under visible light was attributed to the positive synergistic effect between palygorskite support and TiO₂, including large specific surface area, evenly distribution, and more adsorption sites as well as the surface plasmon resonance effect of Ag ions responsible for efficient photogenerated electron-hole pair separation. Additionally, Zhang et al. revealed that the inorganic nanonetwork of palygorskite could be modified from rigid nanorods to multiporous structures and then combined with graphitic carbon nitride, resulting in superior photocatalytic activity for Cr(VI) reduction and methylene blue degradation under visible light [87]. The g-C₃N₄/palygorskite composite exhibited removal efficiency of 99.5% of Cr (VI) and 99.2% of MB after 100 min compared with about 14% and 10%, respectively, removed by pure g-C₃N₄. This remarkable improvement in photoactivity could be ascribed to the adsorption ability of the palygorskite matrix and the porous nanonetwork, including the macroporous structure of the as-synthesized composite that is beneficial for preventing the charge carrier recombination.

As a novel p-type bismuth-based semiconductor, bismuth oxyiodide has a suitable band gap and high stability and is available for application in the hydrogen generation and environmental purification fields. Luo et al. have recently assembled 3D flower-like palygorskite/bismuth oxyiodide composites via intercalating natural palygorskite clay into the microspheres [88]. The as-prepared composite was composed of two bismuth oxyiodide phases, Bi₅O₇I and Bi₄O₅I₂, loaded on a palygorskite surface, and it exhibited higher photoactivity with 88% greater degradation efficiency of tetracycline hydrochloride than that of pure Bi₄O₅I₂ (64%) and Bi₅O₇I (59%). This improvement in photocatalytic performance was mainly ascribed to the enlarged specific surface area of palygorskite, the evident absorption shift from UV to visible light, and the raised photogenerated charge carrier density due to type II heterojunction formation between Bi₄O₅I₂ and Bi₅O₇I. Owing to the remarkable effect in upconverting photoluminescent properties of rare-earth ions, He et al. have more recently investigated the doping effect of Yb³⁺ and Tm³⁺ in LaF₃ matrix and the hosting effect of acid-modified palygorskite [89]. On the one hand, they found that Yb³⁺ ions acted as the sensitizer for near-infrared light as energy source transfer to acceptor ions, while the Tm³⁺ ions acted as an activator in the co-doping system of Yb³⁺, upconverting near-infrared light into UV and visible light and realizing full spectrum utilization. On the other hand, the acid treatment of the palygorskite skeleton resulted in a large specific surface area, abundant adsorption sites, and shortened band gap, facilitating rare-earth fluoride particle immobilization and constructing an indirect Z-scheme heterostructure. The formed Z-scheme heterostructure mediated by defective fluorine vacancy favors the separation of electron-hole pairs, preserves high reduction–oxidation potentials, and provides rich active sites beneficial for photocatalytic N₂ fixation. In the long run, the recycling problem of catalysts has become a common concern in the practical application of various catalysts. Yuan et al. revealed that Fe-octacarboxylic acid phthalocyanine supported on palygorskite/Fe₃O₄ nano-composites by the ultrasonic method presented high photocatalytic performance and could be magnetically recycled in an external magnetic field [90].

Furthermore, natural palygorskite could also be used to fabricate Maya blue-like organic–inorganic hybrids as photocatalyst. For instance, Zhang et al. prepared a novel photocatalyst hybrid in which palygorskite clay acted as a matrix, Eosin Y dye as photosensitizer, and triethanolamine as a sacrificial agent [91]. The hydrogen evolution rate of the optimal Eosin Y–sensitized palygorskite is approximately 3247.2 μmol/h under visible light irradiation, which is over 12 times better than that of the counterpart without palygorskite. It was postulated that a negatively charged palygorskite surface could provide a driving force to control the photosensitized electron-transfer reaction via electrostatic attraction.

3.1.3. Halloysite-Supported Photocatalyst

As one kind of hydrothermal alteration or surface weathering of phyllosilicate minerals and a rare derivative of kaolin, halloysite is a two-layered clay mineral with a nanotubular structure formed by rolling layers composed of silica tetrahedral sheets as the outer surface and alumina octahedral sheets as the inner surface [92]. With the empirical chemical formula Al₂Si₂O₅(OH)₄ and a well-defined inherent hollow tubular framework, halloysite nanotubes have a positive octahedral Al–OH array on the internal surface and negative Si–O–Si groups on the external surface, facilitating the functionalization of their surface and composite fabrication, and compared with carbon nanotubes, natural halloysite is an environmentally benign, economically available, and highly stable with a large specific surface area with sufficient hydroxyl groups, making it a promising raw material in many application fields.

Halloysite nanotubes were commonly applied as photocatalyst support owing to their unique tubular network and superior adsorptive ability. For instance, Wang et al. prepared homogeneously dispersed TiO₂ particles on halloysite nanotubes (HNTs) via a one-step solvothermal method for photocatalytic application [93]. The as-synthesized TiO₂/HNTs composite showed pH sensibility during the methanol degradation process, and the CO₂ evolution rate reached about 1744.31 ± 7.63 μmol∙h⁻¹∙g⁻¹, which is much higher than that determined for pure TiO₂ (1053.12 ± 7.23 μmol∙h⁻¹∙g⁻¹) under the same reaction conditions. This superior photocatalytic performance was explained by the excellent adsorptive capacity of halloysite, enhancing the crystallization degree of TiO₂ and inhibiting TiO₂ agglomeration. As presented in Figure 4b,c, the TiO₂ particles were distributed onto the tubular structure of HNTs. Based on the TiO₂/HNTs photocatalytic system, some modification methods have been applied to further improve photocatalytic activity. For example, Yu et al. investigated the modification of TiO₂/halloysite with several rare-earth metal ions as well as conductive polymers via the ion imprinting technique, and as a result, the Y-TiO₂/halloysite-Poly-o-phenylenediamine (POPD) composite exhibited high photodegradation efficiency (78.8%) of tetracycline [94]. The modification of natural halloysite by acid treatment and then loaded TiO₂ and Fe₂O₃ could also lead to high photocatalytic degradation of aniline and chloro-anilines due to the large surface area and homogeneous particle distribution on the composites [95]. Additionally, the loading of the TiO₂ photocatalyst on the surface of two different tubular halloysite clays, which originated from Limnos Island and Utah, via hydrothermal treatment resulted in slight differences between the two halloysite-supported TiO₂ photocatalysts due to the difference in the surface characteristics of halloysite clays [96].

In recent years, heterojunction construction has received more attention in the photocatalysis field, and therefore more work is concentrated on fabricating novel hierarchical ternary hybrid photocatalysts, including g-C₃N₄/TiO₂, g-C₃N₄/ZnO, CeO₂/AgBr, etc. For instance, ternary Ni(OH)₂@g-C₃N₄/halloysite composites with considerably improved photocatalytic activity could be synthesized from porous g-C₃N₄ that could accommodate Ni(OH)₂ nanoparticles, while abundant hydroxyl groups and Si–O groups present on the halloysite surface strongly interacted with nickel ions, resulting in high particle dispersion [97]. The latter study showed that 1wt% Ni(OH)₂@g-C₃N₄/halloysite exhibited a higher photocatalytic hydrogen evolution rate (18.42 μmol·h⁻¹) in comparison with that of Ni(OH)₂@g-C₃N₄ (9.12 μmol·h⁻¹), which was explained by the negatively charged surface of halloysite that efficiently traps photogenerated holes, decreases the electron-hole recombination rate, and prolongs the electron lifetime. Similarly, g-C₃N₄/TiO₂/halloysite heterojunction, prepared via the sol-gel and calcination method, possessed a decreased band gap of 2.5 eV attributed to the interfacial synergistic interaction between halloysites and semiconductors and more efficient separation of photogenerated charge carriers [98]. As shown in Figure 4d, the TiO₂ nanoparticles were firmly attached to the halloysite surface and then evenly inserted into the interlayer space of g-C₃N₄. At the interface of the ternary components, a heterojunction structure is formed by band gap overlapping. Furthermore, Pt–Cu bimetallic nanoparticles have been loaded on the halloysite surface by trapping negative species [PtCl₆]²⁻ inside the halloysite and complexing Cu(II) electrostatically with Pt(IV) to realize photocatalytic performance improvement [99]. The existence of the active species [PtCl₆]²⁻ could help Cu ions to enter the channels of natural halloysite nanotubes via the formation of the heterometallic double complex between Cu(II) cations and Pt(IV) anions. Additionally, the special tubular structure of natural halloysite can protect Pt–Cu bimetallic nanoparticles from size alteration after heat treatment.

By virtue of its unique structure and characteristic surface chemistry, halloysite could be used to modify semiconductor arrangement and structure. For example, MnO₂ nanocomposites could be prepared and placed onto the surface of the hierarchical flower-like architecture of natural halloysite [100]. Xu et al. creatively designed a core-shell fluorescence Ag/HNTs composite by selectively modifying the external tubular walls of halloysite with citrate–europium (Cit-Eu) complex and firmly attaching Ag nanoparticles into the interior surface of natural halloysite nanotubes to achieve the efficient degradation and ultrasensitive detection of tetracycline (TC), as depicted in Figure 4a. It is asserted that these thick halloysite tubular walls can provide natural defense and promote metal-enhanced fluorescence effects, subsequently accelerating TC detection efficiency, while the attached Ag nanoparticles can induce the faster separation of photoexcited electrons and holes and thereby enhancing its photocatalytic activity [101]. It is worthy to note that a novel hybrid photocatalyst, fabricated by incorporating Rose Bengal dye into the halloysite matrix, showed improved photodecomposition ability with zero-order kinetics in the 4-n-nonylphenol photooxidation process [102]. The latter hybrid composite could efficiently adsorb hydrophobic compounds and promote singlet oxygen production.

3.1.4. Sepiolite-Supported Photocatalyst

As a typical hydrated magnesium silicate in the phyllosilicate mineral group, sepiolite consists of alternating blocks and intracrystalline channels that crystallize in the direction of fibers belonging to a chain-type clay. Each structural block is constructed by an octahedral magnesium sheet intercalating between two tetrahedral silica layers. Differing from other 2:1 type silicate clays, discontinuity of the octahedral silica sheets leads to the formation of structural channels and tunnels. Owing to the peculiar pore structure with interior zeolite-like channels, natural fibrous sepiolite has a high specific surface area and strong adsorption ability [103]. Therefore, sepiolite has been widely applied in the catalysis and environmental purification fields.

Considering its abundance and low cost, Karamanis et al. deposited TiO₂ onto sepiolite to construct a multi-functional photo-responsive catalyst for building applications [104]. In this TiO₂/sepiolite nanocomposite, TiO₂ provides certain photocatalytic activity for pollutant degradation, while natural sepiolite, as adsorbent, retains hydrophilicity even in the presence of hydrophobic TiO₂. TiO₂/sepiolite nanocomposite possesses type II adsorption isotherm and achieves the maximum water vapor adsorption capacity of 30% at 93.6% relative humidity. Papoulis et al. recently modified natural sepiolite by substituting first the exchangeable cations with Zn and Cu ions and then incorporating anatase TiO₂ particles into the composite [105]. They found that Zn-exchanged sepiolite-based nanocomposites could exhibit high photocatalytic performance for NO_x gas oxidation. The difference in photoactivity was explained by the synergistic effect of porous sepiolite support and the ZnO formation in the anatase TiO₂ matrix. In order to eliminate the volatile organic compounds in the air, Hu et al. more recently prepared BiOCl/TiO₂/sepiolite (T-B-S-500D) heterogeneous ternary photocatalyst via a facile hydrolysis–precipitation method followed by the calcination and crystallization processes [106]. The photocatalytic activity of the as-prepared composites was evaluated by the decomposition rate of gaseous formaldehyde according to the Langmuir–Hinshelwood model. The BiOCl/TiO₂/sepiolite composite calcined at 500 °C showed a high reaction rate constant of 0.00728 min⁻¹ under visible light, which is approximately 6.62 and 7.43 times higher than that of TiO₂ and BiOCl, respectively, under the same conditions. They suggested that sepiolite in the composite functions as a superior adsorbent to capture and enrich the pollutant concentration near the photocatalyst surface, and the synergistic effect between sepiolite and formed BiOCl–TiO₂ heterojunction enhances the light-responsive ability as well as the separation efficiency of electron-hole pairs as the reaction route, as shown in Figure 5a. Moreover, a hollow PVP/sepiolite/BiOBr hierarchical microsphere, synthesized from acidized sepiolite (AS) and polyvinyl pyrrolidone (PVP) as the treatments, shown in Figure 5b, could possess slightly improved photodegradation efficiency of diclofenac sodium (80%) under visible light irradiation [107]. The increased activity of PVP/sepiolite/BiOBr composite was explained by the unique hollow network, as presented in Figure 5c,d, which improved visible light absorption and efficient charge carrier separation. Acid treatment of sepiolite could also enhance the dispersion of Cu and Cu₂O photocatalysts in the ternary sepiolite/Cu₂O/Cu composite, leading to a redshift of the band gap and enhancing the visible light utilization as well as Cu ions functioning as electron mediator [108]. This composite showed superior photodegradation efficiency of Congo Red dye with approximately 95.1%, which is much higher than that determined for Cu₂O/Cu (71.2%). Anchoring ZnO/Fe₃O₄ dual nanoparticles onto surfactant-modified sepiolite surfaces could not only improve the photocatalytic removal efficiency of ZnO/Fe₃O₄-sepiolite composite for the degradation of pharmaceuticals and organic dyes but also solve its recycling problems [109,110]. Other than enlarged surface area and improved adsorption ability, the ZnO/Fe₃O₄/sepiolite composite exhibits a superparamagnetic property and thus can be easily recovered.

Furthermore, photosensitized sepiolite could be applied for the photodegradation of several organic dyes. Owing to its negatively charged surface, sepiolite is characterized as having an obviously higher adsorption capacity of cationic dyes, such as Rhodamine B, than of anionic dyes, such as trypan blue [111]. The spent sepiolite after Rhodamine B could be reused to degrade trypan blue under visible light illumination, and its photocatalytic performance of trypan blue degradation increases to approximately twice as much as the original sepiolite. These works solidify a compromising route to realize multiple applications of natural sepiolite in a low-cost and environmentally benign method.

3.1.5. Imogolite-Supported Photocatalyst

As a naturally occurring weathered volcanic glassy ash soil, imogolite is a kind of hydrous aluminosilicate, single-walled nanotube clay with a basic unit arrangement of an external curved octahedral Al(OH)₃ sheet on which separated internal SiO₃(OH) tetrahedron units are linked upright to the octahedral vacancy via a covalent chemical bond between three mutual oxygen atoms [112]. With its empirical chemical composition Al₂SiO₃(OH)₄ and unique tubular structure, imogolite is regarded as a promising absorbent, catalyst, and catalyst support in various fields and can be easily synthesized by the low-temperature sol-gel method.

Due to the specific tubular structure and chemical properties, Bottero et al. could successfully introduce SiCH₃ groups to replace the SiOH groups at the inner surface of imogolite and create a hydrophilic outer Al(OH)Al surface with hydrophobic groups inside the nanotubes [113]. The modified imogolite reached a high specific surface area of about 800 m²/g, showing an almost 2.5 times higher adsorption capacity of methane than that exhibited by unmodified imogolite. The organic/inorganic hybrid, composed of Fe-doped methyl-imogolite nanotubes with isomorphic substitution of Fe³⁺ ions for octahedral Al³⁺ ions, possessed lower band gap energy of 2.4 eV compared to 4.9 eV of bare methyl-imogolite nanotubes [114]. In the presence of H₂O₂, bare methyl-imogolite could mineralize 65% tartrazine under UV light illumination for 2 h, which was explained by the reactive -AlOOH group that is favorable to HO∙ radical generation. On the other hand, Fe-doped methyl-imogolite could mineralize 80% tartrazine under the same conditions because the isomorphic substitution of Al³⁺ by Fe³⁺ was beneficial for HO∙ radical production. Additionally, the silicon in imogolite could also be substituted by germanium, improving the catalytic performance and selectivity of glucose isomerization to fructose [115].

Furthermore, Kuroda et al. could prepare a novel functionalized Au/imogolite photocatalyst with morphologically controlled Au nanoparticles on the imogolite surface [116]. The particle size of Au was decreased from 20–50 nm to about 2–4 nm after loading on the imogolite surface. The synthesized imogolite via the hydrothermal method could also be used as support for Cu(II)-grafted TiO₂ photocatalyst that could catalyze acetaldehyde photodegradation [117]. The as-prepared imogolite possesses a high specific surface area of 245 m²/g, and the resultant imogolite-Cu(II)-grafted TiO₂ composite exhibits better photocatalytic activity for acetaldehyde degradation under visible light.

3.1.6. Wollastonite-Supported Photocatalyst

Wollastonite is a naturally forming calcium inosilicate mineral and sometimes contains a minor amount of Fe, Mg, and Mn substitution for calcium. It consists of an infinite chain unit arrangement of a pair of corner-sharing tetrahedrals alternating with a single tetrahedron with one edge parallel to the chain direction and Ca atom layers in octahedral coordination alternate with Si atom layers [118]. Consequently, it has attracted noticeable attention in the ceramic and cement industries due to its abundance, thermal, chemical, and mechanical stability as well as its outstanding biocompatibility. Early in 2009, wollastonite was used to prepare mesoporous TiO₂/SiO₂ nanocomposites with high specific surface areas (around 308 m²/g), large pore volumes, and narrow distributions of pore size range in the absence of any surfactants [119]. Furthermore, a unique core-shell wollastonite@TiO₂ composite synthesized by a mechanochemical method could efficiently improve the dispersion of TiO₂ and reduce the TiO₂ dosage [120]. These wollastonite@TiO₂ particles are composed of the combination of van der Waals force, electrostatic attraction, and chemical bond. The as-prepared composite has superior characteristics in the white painting aspect, such as 96.9% of whiteness, 17.97 g/m² of hiding power, and 22.72 g/100 g of oil absorption.

Moreover, wollastonite is commonly regarded as promising support for immobilizing various semiconductors. For instance, Yao et al. prepared wollastonite/g-C₃N₄ by the facial calcination and acid leaching process with excellent durability and reusability [118]. The wollastonite/g-C₃N₄ photocatalyst exhibited high photodegradation efficiency of Rhodamine B, with a reaction rate constant that was 6.5 times better than that of pure g-C₃N₄, which could be ascribed to not only high surface area and porosity but also to the synergistic effect between g-C₃N₄ and wollastonite, and resultantly promoted the separation of photogenerated electron-hole pairs.

3.1.7. Basalt-Supported Photocatalyst

According to the definition of the International Union of Geological Sciences (IUGS) classification scheme, basalt is a dark-colored, fine-grained aphanitic igneous rock with major compositions of plagioclase and pyroxene minerals. Generally, it forms as an extrusive rock originating from the rapid cooling of magnesium-rich and iron-rich lava flows. As one of the most common rock types in the world, basalt contains rich MgO, CaO, and low amounts of SiO₂ and alkali oxides, which could be used as CO₂ adsorbents [121]. Early in 2012, natural basalt tuff could be modified by HNO₃ to construct Pd(II)-Cu(II)/basalt composite through the impregnating method [122]. This study showed the water contents of catalyst samples were negatively correlated to their reaction activity of low-temperature CO oxidation in a specific range of 1.6–7.2 wt%. Additionally, Qiu et al. used HF solution to modify basalt fibers and combine them with TiO₂ through microwave thermal treatment at 200 °C for 30 min [123], and after 5 h of ultraviolet light irradiation, the basalt fiber/TiO₂ could degrade 94% of Rhodamine B and exhibit excellent recyclability with a photocatalytic degradation rate still maintained at 86% in the fifth cycling test. The basalt fiber@perovskite PbTiO₃ (BF@ PbTiO₃) composite with three-dimensional core-shell nanostructure could be prepared via a sol-gel dip-coating technique yielding 290 μmol∙gcat⁻¹∙L⁻¹ CH₄ from adsorbed CO₂ after 6 h of reaction, as the basalt fiber surface could preferentially adsorb CO₂ molecules [124]. The photocatalytic CO₂ hydrogenation process was confirmed as below:

Basalt fiber: More CO₂ adsorption
PbTiO₃: Easy electron transfer and water adsorption
Reduction: CO₂ + e⁻ → ∙CO₂⁻
Oxidation: H₂O + h⁺ → ∙OH + H⁺ → H⁺ + O₂
CH₄ production: H⁺ + e⁻ → ∙H
CO₂⁻ + ∙H → ∙CH → ∙CH₂ → ∙CH₃ → CH₄

(1)

Equations of CH₄ production from photoreduction of CO₂ with H₂O vapor over the BF@PbTiO₃ core-shell composite.

The 3% Ag/Ce⁴⁺/La³⁺-TiO₂@basalt fibers composite, prepared by sol-gel and coating routes, represented the best activity in the degradation of ammonia nitrogen in wastewater [125]. Moreover, the aggregation problem of TiO₂ particles was hindered, and the hydrophobicity of TiO₂ powder was significantly improved after being coated on basalt fibers. Compared with 93.9% and 70.7% of 3% Ag/Ce⁴⁺/La³⁺-TiO₂ photocatalyst, the ammonia and nitrogen degradation rate of 3% Ag/Ce⁴⁺/La³⁺-TiO₂/basalt fiber achieved 95.3%, and nearly 74% of total nitrogen was removed after a 6 h degradation process. At the same time, Kwak et al. prepared 3D core/shell microstructure by coating Zn_1−xMg_xO on basalt fibers with specific morphology through a modified sol-gel process [121]. The basalt fiber@Zn_0.75Mg_0.25O presented superior photocatalytic activity and selectively for the photoreduction of adsorbed CO₂ to produce 5.0 μmol∙gcat⁻¹∙L⁻¹ of CO gas after an 8 h reaction. This special core@ shell structure could effectively prolong the charge lifetime by restraining electron-hole recombination and thereby improve the photocatalytic efficiency for CO₂ reduction. Basalt fibers could be used as mineral core support to extend the adsorption of reactive species and the incident light properties. The synergistic effect between basalt fibers and Mg-incorporated ZnO shells would contribute to higher CO₂ absorption capacity and reduction efficiency for CO production.

3.1.8. Tourmaline-Supported Photocatalyst

Being classified as a semi-precious stone, tourmaline is a crystalline boron silicate mineral and has a unique trigonal crystal space group of R3m. Its special crystal structure is able to produce a spontaneous and permanent electric field in which electrons can be driven to flow continually from the negative to the positive pole. This electrostatic field on the tourmaline surface arises from its silicon–oxygen octahedral distortion. This polarization equips tourmaline with pyroelectric and piezoelectric properties with an electronic field strength of 106–107 V/m [126]. Aside from the natural electrical polarity, thermoelectricity, and piezoelectricity, tourmaline also possesses an infrared radiation function and negative ion release ability; thus, it is considered as a prospective material in the field of environmental protection.

The incorporation of tourmaline into a mixture of TiO₂ phases could effectively reduce the band gap energy of the as-prepared TiO₂/tourmaline composite to 2.9 eV and increase its specific surface area, exhibiting photocatalytic decomposition efficiency of gaseous formaldehyde approximately 6 times better than that of pure TiO₂ [127]. Fu et al. prepared CDs/tourmaline@ZnO composite by introducing a tourmaline core into a hierarchical ZnO nano-flower matrix decorated with carbon dots (CDs) as an active light sensitizer [6]. The photocatalytic efficiency of CDs/tourmaline@ZnO composite increased by 54% under visible light, where carbon dots broadened the visible light response ability and tourmaline prohibited photo-generated e⁻/h⁺ pair recombination to enhance the photocatalytic oxidization potential via spontaneous polarization and surface electric fields. The modification of the tourmaline surface with nitrogen-doped TiO₂ showed good activity for ethylene photo-oxidation [128]. The optimal percentage of tourmaline in the tourmaline/N–TiO₂ composite was calculated to be 4%, and the spontaneous electric field of tourmaline was applied to mediate the photogenerated electrons from N–TiO₂ to react with ethylene. Baeissa et al. studied the hybridization effect between tourmaline and mesoporous graphene/TiO₂ heterojunction on its photoactivity for CO₂ reduction under visible light [129]. The as-prepared composite had a high surface area of 340 m²/g m and a relatively narrow band gap of 2.28 eV due to the existence of tourmaline. The surface polarization of tourmaline induced spatially varied energy levels and changed band bending of TiO₂, therefore lowering the calculated band gap and restraining electron-hole pair recombination. Li et al. have more recently loaded tourmaline powder into g-C₃N₄/BiVO₄ heterojunction to enhance its automobile exhaust gas purification performance [130]. They found that the presence of tourmaline in the composite could increase the physical adsorption capacity via releasing negative ions, thereby increasing CO and NO decomposition efficiency by factors of 1.74 and 2.52 times, respectively. On the other hand, Bian et al. applied modified tourmaline as a novel photocatalyst to degrade methyl blue dye through a∙OH⁻ mediated system [131].

3.1.9. Chrysotile-Supported Photocatalyst

As the most common asbestos form on the earth, chrysotile is a 1:1 layered hydrated magnesium silicate and belongs to the phyllosilicate kaolinite–serpentine mineral group with the idealized chemical formula Mg₃Si₂O₅(OH)₄ [27]. Generally, the chrysotile basic unit has trigonal symmetry and consists of a SiO₄ tetrahedral sheet layered with a magnesium octahedral sheet. The cation substitution in the chrysotile unit can modify its morphology and chemical or physical properties. Tang et al. fabricated TiO₂/micro-meso porous silica nanofibers (TiO₂/MMPSNF) based on the natural chrysotile via an innovative sol-adhesion synthesis strategy [28]. The Ti-O-Si chemical bond in this TiO₂/MMPSNF system could efficiently hinder the TiO₂ phase transition from anatase to rutile and control crystallite size growth. This TiO₂/MMPSNF composite exhibited higher specific surface area (197.1 m²/g) and pore volume (0.46 cm³/g) than those of the bare TiO₂ (39.4 m²/g and 0.07 cm³/g), which proves that TiO₂ nanoparticles are uniformly distributed on the chrysotile-based silica surface. Additionally, the TiO₂/MMPSNF composite obtained at 800 °C showed a higher degradation efficiency for Rhodamine B (RhB) with about 95% after 90 min compared with commercial TiO₂ (77.29%) and naked TiO₂ (17.6%) under the same conditions. Similar to other generally accepted TiO₂/SiO₂ photocatalyst systems, the hydroxyl groups on the chrysotile support surface could efficiently capture the photo-induced electrons and inhibit electron-hole recombination, which eventually enhanced its photocatalytic activity. Furthermore, this kind of monolayer dispersion structure could also provide a synergistic effect of nano-scale pore structure and adsorption ability [132]. Zhang et al. could successfully immobilize 0D Ag nanoparticles onto the modified 1D chrysotile nanowire surface to achieve an effective framework arrangement and better properties [133]. It was found that the BET surface area of Ag/SiO₂ composite reached 210 m²/g and thus exhibited excellent photocatalytic performance for 4-nitrophenol reduction.

3.1.10. Talc-Supported Photocatalyst

As the softest mineral, talc is generally presented by the chemical formula of Mg₃(OH)₂Si₄O₁₀ and is naturally hydrophobic. As a result, natural talc hardly gets wet in water without surfactant. The surface property of talc can be altered after being coated by TiO₂ particles, and thereby its wettability is evidently improved [134]. It is also considered that talc has an outstanding activity for adsorbing small organic molecules, promoting the photocatalytic reaction of the as-prepared TiO₂/talc composite. Ai et al. intercalated TiO₂ nanoparticles into talc interlayers to increase the specific surface area and reduce average pore diameters [135]. As a photocatalyst carrier, talc has a strong adsorption ability and intensifies the photoabsorption capability. Thus, the talc/TiO₂ nanocomposite could degrade more than 99.5% of 2,4-dichlorophenol after 1 h and maintain this photocatalytic efficiency even after recycling 20 times. Actually, owing to its low hardness, talc is mainly used as a thickening agent and lubricant ingredient in ceramics, paint, and cosmetics products. Consequently, its application in the photocatalysis field is finite.

3.2. 2D Mineral-Supported Photocatalyst

3.2.1. Smectite-Supported Photocatalyst

Smectite is defined as a clay mineral group consisting of a 2:1 layer silicate structure that could expand and contract depending on water content. The basic structural unit is composed of an alumina octahedral sheet sandwiched between two inward-pointing tetrahedral sheets, and the layers extend in the a and b directions but are weakly bonded by a negative charge which is easy to replace with various cations and allows water and other molecules to enter between the layers, causing expansion in the c direction. Due to ion substitution, the members of the smectite group are divided into dioctahedral minerals such as montmorillonite, beidellite, and nontronite and trioctahedral minerals such as hectorite (Li-rich), saponite (Mg-rich), and sauconite (Zn-rich). These subgroup minerals possess orderly stacked lamellar structures and excellent adsorption ability and therefore are regarded as one of the most promising photocatalyst support materials. Here, TiO₂ is taken as a photocatalyst example to introduce some common synthesis methods in

Table 2. Synthesis methods using natural smectite minerals as TiO₂ support.

Photocatalyst	TiO2 Precursor	Method	Synthesis Temperature	Specific Surface Area (m2/g)	Application	Refs.
F-doped TiO2/exfoliated bentonite	Tetrabutyl titanate	A liquid exfoliation and facile sol–gel method	400 °C	49	Gaseous toluene	[136]
Ag/TiO2/bentonite	Titanium isopropoxide	A facile thermal decomposition method	500 °C	80	S. aureus, E. coli and human embryonic kidney cell line	[137]
Au–TiO2/bentonite	Tetra-n-butyl titanate	Deposition–precipitation and calcination method	400 °C	178	Sulforhodamine B photodegradation	[138]
F, V and Mn co-doped TiO2-pillared bentonite	Butyl titanate	A sol-gel method	500 °C	86	NO photocatalytic denitration	[139]
g-C3N4/TiO2/bentonite	Titanium butoxide	A wet impregnation process	60 °C	70	Reactive brilliant red dye	[140]
TiO2 pillared montmorillonite	Tetrabutyl titanate	An intra-gallery template method	500 °C	392	MB	[141]
Montmorillonite Modified TiO2	Tetra-isopropyl orthotitanate	A single sol-gel method	500 °C	83	CO2 reduction	[142]
Ni/montmorillonite supported TiO2	Tetraisopropyl orthotitanate	Sol-gel method	500 °C	58	CO2 reforming of methane	[143]
Montmorillonite dispersed single wall carbon nanotubes (SWCNTs)/TiO2	Tetraisopropyl orthotitanate	A simple sol-gel assisted wet-impregnation method	180 °C	--	H2 production	[144]
Ag@AgCl-TiO2/organic rectorite/quaternized chitosan	Titanium tetraisopropoxide	Sol–gel combing calcination technique, precipitation and photoreduction combined with emulsification/chemical crosslinking method	80 °C	89	MO	[145]
Ag-AgCl-TiO2/rectorite	Titanium tetraisopropoxide	A facile deposition–photoreduction method	Room temperature	--	Acid orange and 4-NP	[146]
Bi-crystalline TiO2/rectorite	Titanium tetraisopropoxide	A facile sol–gel method	80 °C	100	Acid red G and 4-NP	[147]
TiO2-pillared saponite	Titanium tetraisopropoxide	Intercalation, neutralization and calcination process	400 °C	154	BPB	[12]
TiO2-montmorillonite	Tetrabutyl titanate	Hydrothermal	160 °C	77	MB	[148]
TiO2/pillared clays	Titanium isopropoxid	Wet impregnation through stirring followed by calcination	500	29 ± 5	TCS, 2,6-DCP and BPA	[149]
TiO2/sepiolite	Titanyl sulfate	Structural assembly and thermal treatment	400	135	HCHO removal and H2 evolution	[150]
Bi2MoO6/TiO2/sepiolitte	Titanyl sulfate	A facile hydrolysis precipitation and calcination crystallization route	500	100	HCHO	[151]
TiO2-BiOBr-bentonite	Tetrabutyl titanate	Solvothermal	160	125	Phenol	[152]
Bentonite-TiO2	Titanium isopropoxide	Sol-gel	500	124	TC	[153]
Fe3O4 @TiO2/silica aerogel	Tetraethyl orthotitanate	Sol-gel and ultrasound assisted reflux technique	400	347	TC	[154]

.

Bentonite-Supported Photocatalyst

As a mineralogy term to define naturally occurring sedimentary mud-rocks that are composed mainly of swelling smectite clay, bentonite is an aluminium phyllosilicate mineral containing over 70% montmorillonite, a small amount of beidellite, and impurities from other mineral phases such as kaolinite, quartz, and calcite. Its basic structural unit generally consists of one octahedrally coordinated alumina sheet placed between two tetrahedrally coordinated silica sheets. It has net negative charges due to the isomorphous substitution of Al³⁺ for Si⁴⁺ in the tetrahedral site and Mg²⁺ for Al³⁺ in the octahedral site and can be balanced by the presence of exchangeable cations in the lattice structure [136,155]. Owing to its abundant availability, large specific surface area, high cation exchange capacity, and resultant strong adsorption/desorption capacity of ionic or polar compounds, bentonite is widely regarded as the eco-friendly adsorbent and catalyst substrate in wastewater treatment, such as for organic dyes and heavy metals. For instance, Nguyen et al. could disperse Ag-modified TiO₂ onto bentonite surfaces to enhance the phenol removal efficiency from 23.25% to 98.94% due to the inhibiting effect of anatase-to-rutile phase transformation [156]. According to the dominant exchangeable cations, bentonite can also be named as Na, Ca, and K bentonite. Among them, Na bentonite has a remarkable swelling property that can absorb water as much as several times its dry mass to be expanded. The incorporation of Ag/TiO₂ nanoparticles into Na bentonite layers could increase its specific surface area to 80 m²∙g⁻¹ via the cation exchange process, as shown in Figure 6a [137]. The synthetic Ag/TiO₂/bentonite composite exhibited considerable antibacterial effect and biochemical applications for its small particle size, enabling easy penetration through bacterial walls. The composite showed a redshift in absorption wavelength, which might be related to the charge transfer transition of interlayer tetrahedral and octahedral ions in bentonite clay. Due to the bentonite structure modification effect, Ag/TiO₂/bentonite nanocomposite is proved to be an effective nontoxic antibacterial material for human health. Moreover, the exfoliation process could increase the specific surface area of bentonite, thereby improving its adsorption ability. For instance, Ma et al. synthesized exfoliated bentonite/g-C₃N₄/Ag₃PO₄ composite with increased specific surface area and adsorption properties [157]. The adsorption capacity of g-C₃N₄/Ag₃PO₄ photocatalyst for RhB dye increased by more than two times after anchoring the photocatalyst onto the exfoliated bentonite clay surface. Because the negatively charged surface of exfoliated bentonite has more electrostatic attraction to dye molecules, and its enhanced photocatalytic activity is ascribed to the fact that exfoliated bentonite has a restrained effect on Ag₃PO₄ decomposition and synergistic effect between bentonite structure and semiconductors.

Additionally, bentonite can also be utilized as special loading material to fabricate a bifunctional catalyst for pharmaceutical photodegradation and methanol electrooxidation. Thiruppathi et al. found that 6% bentonite/CuWO₄ nanoparticles could degrade 98.8% of a diclofenac sodium solution under visible light irradiation with superior reusability up to 6 cycles [158]. It was revealed that 6% bentonite/CuWO₄ shows an increased active surface area of 21.7 nm and thereby reduced photogenerated electron-hole pairs recombination rate as well as increased electrical conductivity. The metal ions originating from bentonite could make the absorption edge of CuWO₄ shift to higher wavelengths, while the electrostatic attraction between CuWO₄ and negatively charged bentonite could control the colonization of excited charge carriers of CuWO₄. Furthermore, a 2D bentonite nanosheet was employed to couple with Z-scheme WO₃/g-C₃N₄ heterojunction for selective photocatalytic reduction of CO₂ under visible light illumination [159]. The WO₃/bentonite/g-C₃N₄ composite displayed the highest photocatalytic activity for CH₄ production, which is 6.01 times higher than that produced using WO₃/g-C₃N₄ composite. Moreover, bentonite clay enhanced the selectivity of photocatalysts during the CO₂ reduction process to produce 76.07% methane compared with 36.66% for the WO₃/g-C₃N₄ composite. As the electron mediator, bentonite could promote photoinduced charge carrier separation and is favorable for trapping electrons, resulting in efficient CO and CH₄ evolution. To further facilitate the photocatalyst separation and recycling process under an external magnetic field, Liu et al. prepared magnetic Fe₃O₄/bentonite and grew flower-like BiOBr onto its surface via sodium dodecyl sulfate modification (SDS) through the procedure drawn in Figure 6b,c [10]. According to the obvious redshift and increased intensity of light absorbance and decreased band gap energy, it is deemed that bentonite acts as a negatively-charged photocatalyst substrate that could capture electrons, inhibit recombination of photoexcited e⁻/h⁺ pairs, and enhance light absorption ability, as the mechanism is shown in Figure 6d. In addition, the remarkably improved adsorption capacity of tetracycline and ciprofloxacin is achieved by bentonite introduction via physical adsorption force and enlarged surface area. Moreover, Riaz et al. applied polymer poly-naphthylamine (PNA) to modify bentonite clay, obtaining bentonite/PNA nanohybrids with core-shell morphology and better electroactivity [160]. It was revealed that PNA existed in clay shells and the as-prepared bentonite/PNA nanohybrid exhibited higher degradation efficiency of orange G dye, which is two times more than that determined for pure bentonite.

On the other hand, organophilic bentonite is fabricated by intercalating 1,10-phenanthrolinium cation into its interlayer space and proved to be an efficient photocatalyst for the photodegradation of indigo carmine [161].

Montmorillonite-Supported Photocatalyst

Montmorillonite (MMT) is widely considered as a representative 2D sheet-like clay mineral of the phyllosilicate–smectite group and consists of three-layer structural units (TOTs) in which one octahedral alumina sheet is sandwiched between two tetrahedral silica sheets and the silicate layers stack by relatively weak forces. The montmorillonite surface is negatively charged due to the isomorphic substitution of Mg ions on the Al octahedral sites and balanced by cations via strong electrostatic force, resulting in the formation of the interlayer space [40,141]. Owing to its large specific surface area, swelling nature, high cationic exchange capacity, and adsorption capability, montmorillonite is commonly recognized as a promising material as an adsorbent and catalyst host, and it can accept and donate electrons due to the occurrence of Lewis acids and bases. As the most common semiconductor, TiO₂ has excellent photocatalytic efficiency but severe recycling problems. Huang et al. fabricated TiO₂/carbon-montmorillonite composites with superior separability via a bio-mimetic coating method followed by a carbonization process [162]. Although its photocatalytic efficiency remains unchanged after immobilizing onto montmorillonite, the as-obtained photocatalyst could be easily settled by simple gravity sedimentation without any apparent photoactivity reduction. Another early study investigated the montmorillonite modification effect on TiO₂ for photocatalytic reduction of CO₂ with water vapor and achieved high CH₄ and CO yield rates of 441.5 and 103 μmol∙gcatal⁻¹∙h⁻¹, respectively [142]. The study showed that montmorillonite loading could reduce the particle size from 18.73 to 13.87 nm and increase the surface area from 46.98 to 82.62 m²∙g⁻¹. When intercalated with TiO₂ particles, montmorillonite functions as a barrier to inhibit agglomeration and controls the crystal phase formation as well as enhances hydrophilicity of the as-prepared photocatalyst. Later, Tahir et al. reported that Na-montmorillonite/TiO₂ composite could exhibit considerably improved CO conversion efficiency (750 μmol∙h⁻¹∙g⁻¹) and selectivity [143]. During the photocatalytic dry reforming of methane process, montmorillonite acts as a mediator to transport electrons and promotes TiO₂ dispersion on its surface with better stability and large surface area, while nickel ions serve as an electron sink to efficiently prevent charge carrier recombination. Umer et al. designed a simple sol-gel-assisted wet-impregnation strategy to prepare montmorillonite-dispersed single-walled carbon nanotubes (Mt-SWCNTs)/TiO₂ ternary heterojunction composites [144]. They reported that the maximum H₂ yield rate over Mt-SWCNTs/TiO₂ reaches 9780 ppm∙h⁻¹∙g⁻¹ under visible light illumination, which is 2.05 and 1.33 times higher than that of Mt/TiO₂ and SWCNTs/TiO₂ composites, respectively. The SWCNTs in the composite work as a photosensitizer, which induces a slight reduction in band gap energy level from 3.12 to 2.78 eV, and its synergism effect with montmorillonite increases the absorption in the visible light region, promotes the photogenerated electron-hole pairs separation, and improves photocatalyst stability. Peng et al. assembled layered MoS₂ nanosheets uniformly onto montmorillonite surfaces via hydrogen bonds, and the as-obtained composite could degrade methyl orange completely after 2 h due to its larger surface area, more reactive sites, and synergistic adsorption capacity, as depicted in Figure 7 [163]. Obalová et al. deposited ZnS nanoparticles onto montmorillonite after cetyltrimethylammonium bromide stabilization and achieved photodegradation efficiency of 79% for N₂O under 24 h illumination [164]. Additionally, Cu₂O nanoparticles, which can easily aggregate, could be homogeneously distributed in montmorillonite clay support, yielding preferable adsorption-flocculation capacity of Microcystis aeruginosa [165]. It is stated that 26.31% Cu₂O-montmorillonite can remove 90.4% of Microcystis aeruginosa under visible light irradiation after 3 h and effectively reduce its mass and photoactivity loss to ensure environmental safety. Similarly, Zhang et al. incorporated AgCl nanoparticles into montmorillonite interlayers in the presence of HCl vapor to produce AgCl/montmorillonite with high photocatalytic azo-dye degradation efficiency [166]. Montmorillonite could tailor AgCl morphology, mediate the electron transfer via the contained electrochemically active iron species, and provide a larger surface area for pollutant adsorption. Therefore, the more active {100} crystal planes in the as-synthesized cube-like AgCl/montmorillonite composite might further increase the photocatalytic activity and stability. Owing to the net negative charges on its surface, montmorillonite is used to hybridize with g-C₃N₄/NiCoP cocatalyst to achieve a high photocatalytic hydrogen evolution rate (12.50 mmol∙g⁻¹∙h⁻¹) with Eosin Y–sensitization under visible light irradiation [167]. The maximum quantum yield of 40.3% is thought to be related to the better distribution of g-C₃N₄ sheets and the electrostatic repulsion originating from negatively-charged surfaces. Furthermore, other studies reported that g-C₃N₄/montmorillonite composite is also a prospective candidate as a photoelectrochemical sensor for a small concentration of Cu²⁺ [9]. The specific surface area increased from 8.45 m²/g of g-C₃N₄ to 61.63 m²/g of g-C₃N₄/montmorillonite composite, and the photogenerated electron-hole pairs recombination rate was obviously lower after being combined with montmorillonite according to BET and PL analysis.

Nontronite Catalysis

Deriving from the weathering process of biotite and basalts, nontronite belongs to a small group of smectite clays and contains over 30% Fe₂O₃ and a little Al₂O₃, which obtains brownish or dull greenish appearance due to high iron content. It is generally composed of two tetrahedral silica layers and one octahedral sheet in which the octahedrons are only occupied by Fe³⁺, unlike montmorillonite. The net negative charges on the nontronite surface mainly originate from ion substitution in the tetrahedral layers [168]. Liu et al. reduced the iron species in nontronite clay to Fe(II) and achieved efficient Cr(VI) reduction with the assistance of organic ligands [169]. Additionally, the reduced nontronite was investigated to be effective for E. coli degradation as the soluble metal ions released from clays can be antibacterial under non-neutral pH conditions. The dissolved Al³⁺ compromises the outer membrane of bacteria and further facilitates the soluble Fe²⁺ diffusion into the cell. To decrease the repulsion between the negatively charged bacteria and the catalyst surface, positively charged chitosan was intercalated into the reduced nontronite to acquire more persistent and stable antibacterial activity [170]. Unlike pure reduced nontronite, chitosan-reduced nontronite still exhibits antibacterial capabilities at pH 7 and 8 conditions.

Rectorite-Supported Photocatalyst

Consisting of regular 1:1 alternate stacking of non-expansible dioctahedral mica-like layers and expansible dioctahedral smectite-like layers, rectorite is a natural interstratified layered silicate mineral and exhibits high thermal and chemical stability, large specific surface area, as well as high cation exchange capacity. These remarkable properties could enable rectorite to adsorb and stabilize organic substances both on its external surface and within its interlaminar space via interaction or substitution, making it a promising catalyst carrier [171].

The ultrasonic process was early applied to expand rectorite’s interlayer space and then embedded Bi species onto its lamellar layers via ion exchange, which could enhance the specific surface area of Bi-modified rectorite composites and achieve closer contact between the photocatalyst and pollutant [172]. Later, Guo et al. substituted the exchangeable Ca²⁺ or Mg²⁺ ions of rectorite with Ag⁺ and therefore loaded Ag₃PO₄ onto lamellar rectorite layers via the ultrasound-assisted precipitation method [173]. Ag₃PO₄/rectorite calcinated at 400 °C showed photocatalytic degradation efficiency of methyl orange with 6.6 times higher than that determined for pure Ag₃PO₄, which was explained by the higher specific surface (13.6 m²/g) compared with that of bare Ag₃PO₄ (3.3 m²/g) and better visible light adsorption capacity. Additionally, the presence of rectorite could increase the stability of photocatalysts by inhibiting Ag₃PO₄ from photochemical decomposition. Rectorite could capture electrons to generate O²⁻ radicals with the adsorbed oxygen molecules and thereby prohibit electron-hole recombination and Ag₃PO₄ photo-corrosion. As a photosensitizer with adjustive wide spectrum responsive ability, Ag₂O was incorporated into rectorite clay to obtain Ag₂O/rectorite nanocomposites with high efficiency under broader spectrum light, especially near-infrared light irradiation [174]. The Nyquist curve comparison of different Ag₂O/rectorite samples indicated that the existence of rectorite clay could provide more active sites to detach photoinduced electron-hole pairs and effectively improve photocatalyst stability. The RhB dye degradation experimental results showed that the 70% Ag₂O/rectorite nanocomposite exhibits a clear redshift, a strong visible and near-infrared light absorption in the spectrum range of 220–2200 nm, and the highest photocatalytic activity if compared to pure Ag₂O and rectorite. Additionally, the ∙O²⁻ radical was proved to be the primary active species during the photocatalytic degradation process of RhB dye under visible and near-infrared light irradiation. Similarly, in the ternary Ag/AgI/BiOI-rectorite nanocomposite, rectorite, as support, plays an outstanding role in adsorbing the organic pollutants and facilitating the photogenerated charge carrier separation [175]. Bu et al. incorporated In₂O₃ particles into rectorite clay matrices to acquire visible-light photoactivity of In₂O₃/rectorite composite and to achieve a remarkable surface area improvement from 13.5 m²/g of In₂O₃ to 198.51 m²/g of In₂O₃/rectorite composite [8]. The composite showed a noticeable redshift of the absorption edge from 460 nm to 355 nm and improved photocatalytic efficiency, which was explained by the doping effect of some ions that exist in rectorite and the synergetic effect between the ordered rectorite structure and In₂O₃ particles. According to previous reports, the rectorite could influence the TiO₂ growth process and stabilize the anatase phase. For instance, during the synthesis of porous lamellar sulfur-doped TiO₂/rectorite composites, rectorite could stabilize a higher amount of anatase TiO₂. Moreover, rectorite as a disperser and carrier could increase the specific surface area from 57.2 to 187.5 m²/g [176]. The substitution of lattice Ti⁴⁺ by S⁶⁺ creates an impurity energy level that could also reduce the band gap from 3.2 to 2.8 eV. Thus, the intercalation effects of rectorite could improve metronidazole removal efficiency from 71.8% to 93.8% under visible light irradiation. Moreover, iron-modified rectorite prepared via ultrasonic-assisted ion exchange and in situ hydrolysis could show excellent bifunctional adsorbent and photocatalyst properties [177]. It was assumed that iron modification could increase layer-to-layer spacing and specific surface area, which increases the maximum adsorption capacity of Rhodamine B by 11 times. The electron spin resonance spectroscopy (ESR) results showed that it substantiates ∙OH radical produced by iron-modified rectorite functions as primary reactive oxygen species in the presence of H₂O₂ under visible light illumination during the photo-Fenton process. Later, similar iron-modified rectorite could be successfully fabricated via the degradation of Rhodamine B and exhibit excellent simultaneous removal efficiency of tetracycline hydrochloride and Cr(VI) in a Fenton-like process [178]. It was proved that the existence of tetracycline hydrochloride enhanced the Cr(VI) reduction rate by 47.4-fold, which was ascribed to its favorable effect on Cr(VI) reduction and its suppressing effect on Cr(III) reoxidation, while involving RhB dye could increase the interlayer spacing of the rectorite and induce more iron ions into its interlayers, thus facilitating pollutant molecular diffusion, inhibiting iron leaching, and strengthening its electrical conductivity.

Saponite-Supported Photocatalyst

Saponite has a general chemical formula of Ca_0.25(Mg, Fe)₃((Si, Al)₄O₁₀) (OH)₂·n(H₂O) and refers to a trioctahedral group of smectite minerals. Its charge originates from Al substitution for Si in the tetrahedral sheet, which greatly influences its acidity. Generally, saponite is more thermally stable than montmorillonite. The pillarization process is the common technique to prepare semi-synthesized materials by inserting or immobilizing metal oxide into the interlayer space of clay minerals. The swelling ability and exchangeable cations of smectite minerals enable this modification method to obtain unique photocatalytic properties of specific metal oxides. Nikolopoulou et al. could incorporate TiO₂ onto saponite clay via solvothermal reaction, and the obtained TiO₂/saponite composite exhibited the highest photocatalytic NO_x gas degradation activity when the titania/saponite mass ratio reached 0.2 [179]. Moreover, these TiO₂/saponite photocatalysts showed mesoporous structure with improved pore volume (from 0.19 to 0.83 mL/g) and specific surface area (from 125 to 346 m²/g) compared to pure saponite. Additionally, its enhanced photocatalytic efficiency could be ascribed to a better distribution of titania onto saponite surface as well. Later, Fatimah et al. could load TiO₂ onto the pillared saponite followed by applying tris(2,2′-bipyridyl) di-chlororuthenium (II) complex as a photosensitizer for bromophenol blue photodegradation [12]. This pillarization process increased the basal spacing of saponite from 14.95 Å to 16.27 Å and the specific surface area from 110 m²/g to 154 m²/g. Meanwhile, the photosensitization produces an obvious absorption spectra shift to the visible light range. Saponite as titania support could achieve slight catalytic activity improvement and better catalyst reusability.

3.2.2. Kaolinite-Supported Photocatalyst

Kaolinite (Al₂Si₂O₅(OH)₄) is a 1:1-type planar hydrate phyllosilicate clay mineral, in which the basic block consists of octahedral aluminium hydroxide sheets and tetrahedral silica sheets bridged through a co-sharing oxygen atom. The adjoining lamellae are linked to one another via hydrogen and covalent bonds [180], and the strong interaction forces make kaolinite chemically stable and a suitable support for immobilizing the other components. Generally, rocks rich in kaolinite are known as kaolin or China clay and have low swelling and cation-exchange capacity, which are widely used in various industries such as paper filling, ceramic production, painting, and wastewater treatment.

Due to its low cost, high efficiency, and non-toxicity, there are numerous works related to loading TiO₂ onto various substrates for various applications such as wastewater treatment and heavy metal removal. For instance, Kočí et al. compared the pure TiO₂ and kaolinite/TiO₂ composite in terms of CO₂ photocatalytic reduction efficiency [181]. They found that kaolinite could decrease anatase crystallite size, change surface acid–base properties and inhibit electron-hole pair recombination. Later, Sia et al. prepared N-doped TiO₂/kaolinite composite for photodegradation of Congo red in a semi-continuous reactor system [30]. This work showed that the as-prepared N-doped TiO₂/kaolinite composite exhibited higher photocatalytic performance, which was explained by the fact that kaolinite as a TiO₂ support provides excellent adsorption ability, enhanced photocatalytic oxidation ability, and easy recovery. Additionally, a new 0D/2D hierarchical C-doped TiO₂/kaolinite was assembled to promote the generation of Ti(III) species through the synthesis process, shown in Figure 8a, and thus obtain excellent photoactivity under visible light [182]. The latter composite showed high removal efficiency of ciprofloxacin following the pseudo-first-order kinetic model. The simulated reaction rate constant of the as-prepared C-doped TiO₂/kaolinite composite is 0.00597 min⁻¹, which was approximately 3.24 and 24.88 times higher than that of C-doped TiO₂ and pure TiO₂, respectively. The morphology changes before and after modifying kaolinite are shown in Figure 8b,c. The accumulated net charge originating from negative hydroxyl groups on the kaolinite surface could produce a built-in electric field at the interface between the kaolinite and TiO₂ favorable for the fast separation of photogenerated carriers, and lamellar kaolinite as substrate could prompt bare TiO₂ dispersion and enable efficient photocatalyst recovery by virtue of its large crystallite size and high specific gravity. However, the efficiency of these TiO₂/kaolinite photocatalysts under visible light was still relatively low even after modification. Therefore, Li et al. designed a highly efficient self-assembly g-C₃N₄/TiO₂/kaolinite composite to advance visible light-harvesting ability [5]. As illustrated in Figure 8d, the small crystalline TiO₂ particles and more exposed g-C₃N₄ sheets are well disposed on the negatively charged kaolinite surface for more effective electron-hole separation. Furthermore, natural kaolinite clay has a unique sandwich structure that can limit crystallite growth and alter the framework. Zhang et al. showed that g-C₃N₄ loaded on a porous kaolinite-derived template could significantly enhance the photocatalytic hydrogen evolution rate under visible light, approaching 1917 μmol∙g⁻¹∙h⁻¹ compared with 810 μmol∙g⁻¹∙h⁻¹ of unmodified g-C₃N₄ [183]. The natural kaolinite functioned as a morphology template, providing a large specific surface area and more defect active sites to mediate hole reactivity towards interfacial oxidation. In order to explore the interfacial electronic property of heterostructures, a density functional theory (DFT) calculation was utilized to depict the possible electron transfer direction of the Kaol/Degussa P25 composite [184]. This DFT computation revealed an interlayer electron transfer from kaolinite to TiO₂, increasing the electron density of the TiO₂ space charge regions and decreasing the electron density of the kaolinite surface. Thus, Kaol/P25 composite could be a traditional type II heterostructure, and its interfacial Al–O–Ti bonds were efficient charge transfer channels.

On the other hand, natural kaolinite is generally regarded as an excellent adsorbent material, which could also show good photocatalytic activity after doping or modification. For instance, Fida et al. developed a new bifunctional Ti–Fe kaolinite composite via precipitating a certain amount of Ti and Fe species [31]. They reported that the specific surface area of the as-prepared Ti–Fe kaolinite composite was 5.5 times as much as that of the pristine kaolinite, and a low pH value can promote its Cr(VI) removal process. Later, Shawky et al. reported that exfoliated kaolinite nanolayers obtained by the intercalation technique could exhibit outstanding degradation efficiency of Rhodamine B dye that increased slightly with decreasing solution pH [29]. These results were explained by the electrostatic interactions between the siloxane surface of kaolinite and the carboxylic group of RhB molecules that would increase at lower pH, therefore promoting RhB discoloration. Additionally, the exfoliation process exposed more metal oxide impurities on kaolinite nanolayers and thus prompted its photocatalytic properties. The ongoing research will open an auspicious door for extensive applications of natural kaolinite clay in the environmental purification and remediation fields.

3.2.3. Dickite-Supported Photocatalyst

Dickite is a phyllosilicate clay mineral belonging to the kaolin–serpentine group, with kaolinite-type composition but a different polymorph crystal structure. Dickite consists of double 1:1 layered SiO₄ tetrahedral and AlO₆ octahedral sheets within a monoclinic symmetry, distinguished from natrite by different stacking sequences of two 1:1 silicate layers. Expanded dickite layers were obtained via rapid heating of the mixture of the dickite–urea intercalation complex and KClO₃ and the deposited TiO₂ particles on the surface by thermal hydrolysis of the expanded dickite and TiOSO₄ suspension [185]. It was revealed that the addition of KClO₃ could trigger an explosive reaction to produce large amounts of gases in a short time. The results from XRD, EDS, and FTIR showed expanded dickite came from the combination of the edge expansion of layers caused by ‘tiny bombs’ exploded on the cross-section of layered particles and a synergistic effect on expanding the inside of the layers because of the internal urea decomposition.

It was confirmed by XRD patterns that the dickite carrier could exert an obvious inhibition effect on the crystallization and crystal growth of TiO₂, and the inhibition effect differences between raw dickite and expanded dickite were related to the different specific surface areas. Additionally, expanded dickite, with nearly 10 times larger specific surface area, could apparently provide many more active adsorption sites. The enhanced photocatalytic ability for the removal of methyl orange of TiO₂-impregnated expanded dickite was also attributed to the higher specific surface area and the size effect of the dickite carrier.

3.2.4. Mica- and Muscovite-Supported Photocatalysts

As a naturally occurring lamellar silicate mineral group, mica has distinctive perfect cleavage planes parallel to the (001) plane and a characteristic unit consisting of two tetrahedral silica sheets held together by one octahedral alumina sheet via electrostatic forces [186]. In general, mica mineral is hydrophilic due to the hydrogen-bonding character of the surface hydroxyl groups. After substituting most OH⁻ ions present at the hexagonal cavities of mica surface by F⁻ ions, the modified fluorine mica is formed, which is less hydrophilic than other clays and shares similar expandable properties with montmorillonite. Previous studies showed that the photocatalytic efficiency was highly dependent on the ability to concentrate the target pollutants near the vicinity of photocatalysts. The common toxic organic pollutants are highly hydrophobic and thus restrain the performance of many efficient photocatalysts. On this basis, it was desired to design TiO₂ pillared fluorine mica composite via conventional thermal treatment [187]. Several composites consisting of TiO₂ and different mineral supports were tested to clarify the support effect on the photocatalytic degradation activity of hydrophobic persistent organochlorines and found the structures of TiO₂ species on different composites were almost the same. According to the characterization results, it was reasonable to assume that the interaction between the hydrophobic reactant and the rather hydrophobic interlayer surface of fluorine mica was more favored in aqueous solutions. Furthermore, not only the hydrophobicity of the modified support but also the high surface area and relatively small pore size of layered silicate minerals were all significant factors in improving photocatalytic efficiency. Additionally, mica could serve as a highly reflective core of a novel ternary mica-titania@reduced graphene oxide (MT@rGO) composite to improve its light-harvesting capability [188]. As a result, the obtained [email protected] composite exhibited the best photocatalytic degradation performance of gaseous acetaldehyde, which is 2.5 times higher than that of the bare TiO₂ nanoparticles.

In addition, muscovite is a representative hydrated potassium-rich silicate mineral in the mica family and features highly perfect basal cleavage allowing it to split into thin transparent laminae. Zhou et al. synthesized N-doped nano-TiO₂/muscovite composites using urea as nitrogen dopant via the hydrolysis–precipitation method [189]. It was proved by XPS results that TiO₂ particles were coated uniformly on the muscovite surface by forming the Si–O–Ti and O–Ti–N bonds and interfacial Coulomb’s force. The specific lamellar structure of muscovite endowed the composite with excellent dispersion in the aqueous solution of methyl orange, resulting in higher light absorption ability and improving the recycling efficiency of TiO₂ remarkably. Recently, Y. Li et al. applied a liquid precipitation method followed by thermal treatment to prepare TiO₂/muscovite composites and analyzed the effects of muscovite matrix on the crystal phase [190]. Their XRD results revealed that the muscovite matrix exerted a retarding effect on the growth and phase transformation of TiO₂. Moreover, the solid UV-vis diffuse reflectance spectrophotometry (UV-DRS) showed that muscovite matrix loading could reduce the band gap of TiO₂ and enhance light absorption ability. The synergistic adsorption effect between the muscovite matrix and TiO₂ could also promote h+ to oxidize adsorbed H₂O molecules into ·OH radicals, thus improving the photocatalytic efficiency of TiO₂/muscovite composites. A. Senthilnathan et al. could also design thin films of rice-like akaganeite deposited on the muscovite surface by the facile urea-assisted controlled self-assembly technique and investigate their photocatalytic activity under solar irradiation [191]. This study suggested that the adsorbed water molecules on muscovite surfaces had a major influence on the formation of hydroxyl radicals through their reaction with photo-excited holes or superoxide radicals on akaganeite/muscovite composite surfaces. Peng et al. integrated muscovite sheet/SnO₂ structure with g-C₃N₄-based photocatalytic cement and achieved improved photocatalytic performance [192]. They alleged that SnO₂ as an electron acceptor layer could promote interfacial charge transfer and prevent electron-hole recombination, while the muscovite sheets could assist in eliminating the mutual influence between the photocatalytic reaction and cement blocks as well as enhancing its stability. Recently, Bao et al. reported the synthesis of nano zerovalent iron-loaded muscovite composites via the liquid-phase reduction method and optimized its catalytic activity for P-chlorophenol decomposition under different reaction conditions [193]. They found that muscovite could significantly reduce the agglomeration degree of iron particles, increasing their dispersibility and catalytic activity of the composite. Aenthilnathan et al. prepared ZnO/muscovite composite using the surficial attractive forces of mica, and the presence of muscovite might promote the photodegradation efficiency by its adsorptive ability, enriching the pollutant concentration near the photocatalyst composite [194]. Moreover, muscovite modification could change the ZnO morphology from an oval shape to a flower-like structure and expose more reactive sites.

3.2.5. Illite-Supported Photocatalyst

As one of the most abundant natural clays contained in sedimentary rocks, illite belongs to the mica–phyllosilicates group and has a unique 2:1 sandwich framework of silica tetrahedron (T)–alumina octahedron (O)–silica tetrahedron (T) layers with rich hydrophilic aluminum hydroxyls. The hydrated potassium cations are generally localized in the space between the T-O-T layer sequence, which is responsible for its non-swelling property [195]. As a consequence, the cation-exchange capacity of illite is around 20–30 meq/100 g and lower than that of smectite.

Due to its specific structure and outstanding properties, illite has been widely used as support for various applications. It is reported that natural illite clay is usually applied as catalyst support for autothermal dry reforming of methane (DRM)-like nickel-containing Ce- or La-promoted catalysts, which is different from some common methods to improve DRM efficiency of Ni-containing catalysts [196,197,198]. Although iron impurity in natural illite tends to form FeNi₃ alloy, the formation of the britholite phase from the migration of lanthanum into the bulk could inhibit this alloy formation, preserving more active Ni particles available for methane activation. Additionally, a novel self-assembly Co₃O₄@illite composite has been successfully prepared by encapsulating the aggregation of Co₃O₄ nanospheres onto natural illite micro-sheets with abundant oxygen vacancies [199]. The catalytic performance of these composites was evaluated by decomposition of Atrazine(2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) via activation of peroxymonosulfate (PMS). The reaction rate constant determined according to the pseudo-first-order kinetic model of the Co₃O₄@illite composite was about 1.7 times better than that of bare Co₃O₄. The natural illite could not only promote Co₃O₄ nanosphere distribution but also regulate the grain size and crystallinity of Co₃O₄ nanospheres as well as the oxygen vacancies which originated from illite clay as the electron reservoir accelerates electron immigration from PMS to the catalyst surface. Additionally, Dong et al. could synthesize TiO₂/illite composites to explore the influence of pH value on this reaction system [37]. They found that TiO₂ particles were connected onto the illite surface via Si–O–Ti and Al–O–Ti bonding; thus, illite could effectively hinder the phase transition from anatase to rutile at high temperatures. Consequently, the catalytic activity of TiO₂/illite was slightly better than commercial TiO₂ at the optimal pH value of 1.5. On the other hand, the presence of illite in TiO₂/illite composites could evidently reduce the production cost without compromising any physical and chemical properties when applied in the pigment industry [200].

Due to its chemical composition and electronic structure, illite could be firmly dispersed on g-C₃N₄ surface via coordination bonds with bridging Si or Al ions of illite with N ions in g-C₃N₄ [201]. By virtue of its large specific surface area, abundant Lewis acid sites, and high electronegativity induced by ion substitution, the existence of illite could introduce defects at the interface between g-C₃N₄ and illite and make the g-C₃N₄ sheets peel and roll up, increasing oxygen adsorption to generate more reactive oxygen species. The resultant self-built electric field in the composite could efficiently promote the separation of photogenerated charge carriers. Therefore, the removal efficiency of NO by g-C₃N₄/illite composite was 1.8 times higher than that determined for the pure g-C₃N₄.

3.2.6. Vermiculite-Supported Photocatalyst

Forming from the weathering or hydrothermal alteration of biotite or phlogopite, vermiculite is classified as a 2:1 type layered hydrous phyllosilicate mineral constituting two silica tetrahedral sheets which are bonded together with an alumina or magnesia octahedral layer. Therefore, it has abundant deposition, a large surface area, and good thermal and chemical stability, which makes it eligible to be an excellent adsorbent and catalyst carrier. Zhang et al. utilized natural paper-like vermiculite as a host photocatalyst to load CdS quantum dots and evaluated the photocatalytic hydrogen evolution activity of the as-prepared composite using Na₂S and Na₂SO₃ as sacrificial reagents under visible light irradiation [202]. The CdS/vermiculite composite with the optimal ratio of 5% exhibited the highest hydrogen generation rate of 92 μmol/h and the best apparent quantum efficiency of 17.7% at 420 nm. The improved photocatalytic activity and stability accounted for the synergistic effect between vermiculite and CdS induced by the well-matched overlapping band structure and thus accelerating charge carrier separation. Later, Wang et al. could successfully construct a three-dimensional KNbO₃/vermiculite composite via an in situ hydrothermal procedure [38]. They found that KNbO₃ nanoneedles were grown on the outer and inner surface of layered vermiculite and abundant active sites in the 3D framework are favorable for photocatalytic activity. Additionally, Zhu et al. could fabricate an efficient ternary AgI-Bi₂MoO₆/vermiculite adsorption-photocatalyst with superior photoactivity under visible light [39]. This composite showed high photodegradation efficiency of malachite green of about 98.89%, which was much better than that of Bi₂MoO₆ (54.89%) and Bi₂MoO₆/vermiculite (80.74%). These results were explained by the homogeneous distribution of AgI/Bi₂MoO₆ on the vermiculite surface, which increased adsorption ability and effective electron-hole separation. Moreover, vermiculite and bentonite have been used as photocatalysts to degrade sulfamethoxazole (SMX) and trimethoprim (TMP) under UV and solar illumination, and the photodegradation efficiency of SMX and TMP was enhanced by 11.4 and 46.4 times, respectively, after adding vermiculite [203]. These results were explained by the higher percentage of doping ions, such as Fe, Mn, and Ti, in the vermiculite than in the bentonite.

3.3. 3D Mineral-Supported Photocatalysts

3.3.1. Allophane-Supported Photocatalyst

As a natural clay mineral, allophane is a kind of amorphous or poorly-crystallized hydrous aluminosilicate belonging to the kaolin–serpentine group and varies remarkably in morphology from fine-rounded particles to irregular agglomeration. Generally, this mineral shares a 3.5–5 nm sized hollow spherical structure on which 0.3–0.5 nm sized defects are distributed [204]. It is reported that the hollow sphere walls consist of inner silica and outer alumina layers with a hydroxylated or hydrated surface, which is suggested to endow allophane with a significant ability to adsorb ionic or polar pollutants from the gaseous and aqueous phases due to its amphoteric ion-exchange activity and large specific surface area. Allophane’s structure is convenient for even dispersion in host photocatalyst materials and efficient electron transport on the surface [205].

In 2011, highly-adsorptive allophane/TiO₂ composite synthesized by the sol-gel method achieved even distribution of small clay particles on TiO₂, inhibited the emission of intermediate products efficiently, and then degraded gradually during the degradation of trichloroethylene [206]. Compared with the mechanical mixing sample of allophane/TiO₂, the final product’s CO₂ concentration produced from the allophane/TiO₂ nanocomposite was three times higher due to the homogeneous dispersion of allophane into TiO₂. As an amorphous silica source, allophane was selectively dissolved by acid treatment and mixed with TiO₂ at various mass ratios [207]. It was found that the photocatalytic activity evaluated by gaseous acetaldehyde degradation was improved with over 5 mass% mixing ratio of allophane despite the specific surface area decrease. When the mixing ratio of allophane reached 20 mass%, its photocatalytic activity calculated by the CO₂ generation rate increased to 1.8 times higher than non-treated TiO₂. Additionally, the selective leaching of alumina from allophane could leave microporous or mesoporous silica structures, which evidently increase the adsorption capacity and accelerate the photocatalytic acetaldehyde decomposition of the as-prepared TiO₂/silica composite. Recently, allophane was applied as support for heterojunction Bi₂WO₆/CeVO₄ photocatalyst via hydrothermal synthesis combined with mechanical mixing [208]. As illustrated in Figure 9b,c, SEM results suggest that Bi₂WO₆/CeVO₄/allophane composites consist of flake-spherule allophane particles and CeVO₄ nanocrystals dispersed on the hierarchical flower-like Bi₂WO₆ microspheres. According to its photocatalytic performance, the Bi₂WO₆/CeVO₄/allophane composite with the 1:1 mass ratio of Bi₂WO₆: CeVO₄ showed the highest photocatalytic activity, and the CeVO₄ worked as a photosensitizer to absorb more photons from visible light in this reactant system. The enhancement in the photocatalytic activity of the Bi₂WO₆/CeVO₄/allophane composite was presumed to result from a certain extended light absorption range, and the construction of heterojunction, as well as the mineral support, could separate photogenerated electron-hole pairs more efficiently.

3.3.2. Pyrophyllite-Supported Photocatalyst

As a relatively rare clay, pyrophyllite is a layered hydroxy aluminosilicate with a dioctahedral 2:1 structure, as shown in Figure 9a. Its crystal structure is described as being composed of Al-O(OH) octahedral sheets sandwiched between two tetrahedral SiO₄ sheets, with each octahedral Al bonded with a tetrahedral Si by an apical O and with an adjacent Al by two OH groups [209]. Generally, pyrophyllite exhibits excellent physicochemical characteristics, such as low thermal expansion and high reheating stability. Some studies utilize modified pyrophyllite clay as an efficient catalyst for the catalytic wet peroxide oxidation technique, which is an important technique in advanced oxidation processes. Taking phenol as target pollutant, pyrophyllite is impregnated by 2.5 wt% copper content to achieve total phenol degradation and 80% total organic carbon reduction in the presence of H₂O₂ during 4 h at optimized conditions [210]. The as-obtained Cu-supported pyrophyllite was proved to have better stability and detoxification properties with an obvious low copper leaching concentration. Additionally, pyrophyllite/TiO₂ heterostructures could also degrade over 89% of a phenol solution after 2 h with obviously improved stability [51]. The latter study showed that the methanol washing process could reduce the defect sites and raise the specific surface area from 16.58 m²∙g⁻¹ to 56.39 m²∙g⁻¹, which effectively decreases the photoexcited e⁻/h⁺ recombination rate. However, there are just limited works on pyrophyllite clay in the photocatalysis field due to its rarity.

3.3.3. Perlite-Supported Photocatalyst

With main components as alumina and silica, perlite is an amorphous volcanic vitreous rock lava containing crystal water and has porosities of over 95% as well as a density of 64–144 kg/m³. When heated at the temperature range of 700–1200 °C, perlite has a volume expansion of up to 20-fold of the original one, which induces largely reduced density and porous structure, allowing it to stay afloat on the solution surface. As a result, this modified perlite, also named expanded perlite, is considered an excellent adsorbent and support material for immobilization of photocatalysts owing to its chemical inertia and adequate light transparency to provide the passage of photons [211]. M. Długosz et al. coated TiO₂ particles onto perlite granule surfaces to obtain a floating photocatalyst composite [212]. This TiO₂/perlite composite possessed a better solar light adsorptive ability and excellent photocatalytic degradation efficiency for phenol. Xue et al. prepared B-N co-doped TiO₂/expanded perlite composite with a mesoporous structure and high specific surface area (99.23 m²/g), exhibiting more efficient photocatalytic activity that was five times higher than TiO₂/expanded perlite [213]. Song et al. loaded expanded perlite with amorphous Al₂O₃ as a porous floating carrier for g-C₃N₄, and they found that this floating photocatalyst composite could not only inactivate algal cells but also decompose the released organic matter as well as achieve 74.4% of the photocatalytic Microcystis aeruginosa inactivation efficiency [214]. It was elucidated that Al₂O₃ could promote the adsorption process and improve the photogenerated charge carrier separation efficiency of g-C₃N₄. The existence of perlite was proved to effectively suppress g-C₃N₄ agglomeration in solution so that there are more dissolved oxygen molecules for g-C₃N₄ to produce radicals. Khan et al. could successfully construct a floating cabbage-like Bi-Bi₂S₃-Bi₂WO₆@expanded perlite photocatalyst composite which could decompose methyl orange and Rhodamine B completely in 25 min [215]. The expanded perlite furnishes the Bi-Bi₂S₃-Bi₂WO₆@expanded perlite photocatalyst with a mesoporous framework to promote the efficient adsorption of aqueous contaminant solutions. Considering the remarkable properties of CuFeO₂ semiconductor, Darkhosh et al. decorated CuFeO₂ onto expanded perlite to optimize its electronic structure and narrow the band gap energy to 1.81 eV [216]. They found that the combination of CuFeO₂ and perlite could increase the surface area of perlite from 2.302 m²∙g⁻¹ to 142.32 m²∙g⁻¹ and the pore volume from 0.0044 cm³∙g⁻¹ to 0.214 cm³∙g⁻¹. In this binary system, expanded perlite works as an electron sink that could attract the photogenerated electrons from CuFeO₂ to reduce the electron-hole recombination rate. The floating property of expanded perlite provides CuFeO₂/expanded perlite composites with better stability and organic dye mineralization efficiency. The oil spill problem is detrimental to aquatic wildlife and the ecological environment. As a result, Qiu et al. designed a honeycomb-like BiOI/expanded perlite photocatalyst nanocomposite to treat diesel wastewater using its outstanding adsorptive and photocatalytic performance [217]. This excellent BiOI/expanded perlite composite could achieve a diesel removal efficiency of 85% after 2 h under sunlight irradiation, and it could be reused at least five times. Moreover, the presence of expanded perlite could decrease the toxicity of BiOI and facilitate the recollection of the photocatalyst composite after application.

As low-cost and sustainable support, the silanol hydroxyl groups on perlite surfaces are favorable for bonding with other photocatalytic materials. Perlite could easily suppress the aggregation problem of photocatalysts and advance the floating property of the designed photocatalyst.

3.3.4. Quartz-Supported Photocatalyst

As the second most abundant mineral in Earth’s continental crust, quartz belongs to weathering rock, with the chemical formula of SiO₂ [218]. Silicon and oxygen atoms in quartz are linked in a continuous framework of SiO₄ tetrahedra, with each oxygen shared between two tetrahedra. Unlike other clay minerals, most photocatalysts are deposited onto quartz substrate to avoid separation step by centrifugation. Generally, quartz support could not modify the properties and crystalline structure of photocatalysts themselves. Additionally, UV-transparent quartz is beneficial for assisting the light absorption process of photocatalysts. A. Okemoto et al. could load a 50 nm picene thin film onto quartz to acquire very high hydrogen production activity using sacrificial agents under light irradiation [219]. The TiO₂ support quartz fiber filter could efficiently degrade atmospheric fine particulate matter (PM2.5) under UV irradiation and provide a significant increase in CO₂ production [220]. The as-prepared fiber exhibits an obvious capillarity effect beneficial for photocatalytic reaction. Moreover, Zhang et al. synthesized Gd₂Ti₂O₇ supported on quartz spheres and realized remarkable enhancement of photocatalytic ofloxacin degradation efficiency, which could be attributed to the smaller crystallite size and larger specific surface area [221]. Furthermore, although quartz is an insulator, Li et al. found H₂ produced from CH₃OH-H₂O aqueous solution at the quartz surface under light [222]. They suggested that the surface states of quartz located between the band gap might act as electron acceptors and hence successfully produce H₂ from the CH₃OH-H₂O solution. Additionally, quartz is also regarded as an SiO₂ source to prepare target photocatalyst composites [223]. However, quartz as photocatalyst support still has limited research interest and a long way to go.

3.3.5. Bauxite-Supported Photocatalyst

As the main aluminium and gallium source worldwide, bauxite belongs to a lateritic sedimentary rock with a relatively high aluminium content. It is considered as a naturally reddish heterogeneous material mostly consisting of the gibbsite boehmite and diaspore minerals mixed with goethite and hematite, kaolinite, and small amounts of anatase TiO₂ and ilmenite. In 2010, a novel low-cost photocatalyst was synthesized for hydrogen production from water-based bauxite residue material [224]. According to this work, this catalyst reached a 20.85 μmol/h/g hydrogen generation amount under visible light and exhibited excellent physical and chemical stability. Later, Ismail et al. used Malaysian bauxite to explore the calcinating temperature effect on its composition and resultant photocatalytic activity [225]. They found that heat-treated bauxite catalysts had low band gap energy (2.04 eV), and the sample processed under 800 °C could exhibit the best photocatalytic performance, degrading 75% of Reactive Black 5 dye after 6 h of UV and visible light exposure, due to its smaller particle size. It was concluded that bauxite, after heat treatment, possessed a heterostructure photocatalyst allowing fast electron transport and an electron-hole recombination rate decrease.

3.4. Porous Mineral-Supported Photocatalysts

3.4.1. Diatomite-Supported Photocatalyst

Diatomite, also named kieselguhr, is a naturally occurring soft and porous siliceous sedimentary rock and mainly consists of amorphous silica and small quantities of alumina and iron oxide [226]. Diatomaceous earth is originated from the skeleton fossils of dead microscopic single-celled algae known as diatoms. Generally, as depicted in Figure 10a, natural diatomite has a unique, highly ordered porous structure and many superior characteristics, such as high porosity, low density, high adsorption capacity, etc. Accordingly, it has been widely used as a filtration aid, adsorbent, and catalyst support. Up to now, natural diatomite is still widely used as a support material for various photocatalysts for different applications, which can be seen in

Table 3. Applications of various photocatalysts using natural diatomite as support material.

Photocatalyst	Method	Application	Light Source	Photocatalytic Efficiency (%)	Refs.
Nb2O5@diatomite	Hydrothermal	Photoreduction of Cr(VI)	500 W mercury lamp	94.50	[226]
TiO2-diatomite	Thermal hydrolysis and annealing method	RhB	300 W UV lamp	--	[227]
TiO2-diatomite	Sol-gel and calcination or phosphoric acid pretreatment	MO	250 W mercury lamp and 300 W white light lamp	90 and 60	[228]
Diatomite/Fe2O3/TiO2	Co-precipitation and impregnation method	MB	300 W xenon lamp	99	[231]
g-C3N4/diatomite	Calcination	MB	350 W xenon lamp	90	[232]
Diatomite @Ni/NiO	Acid leaching and in situ reduction method	MG and Cr(VI) photoreduction	400 W metal halide lamp	almost 100	[233]
CdS @diatomite	Biological synthesis using Bacillus licheniformis	MB and Cr(VI) photoreduction	360–450 nm UVA lamp	over 90	[234]
BiVO4 @diatomite/microcrystalline cellulose/PVB composite fibers	Hydrothermal and electrospinning method	MB and HCHO	250 W high pressure mercury lamp	66.80 and 56.80 for MB and HCHO	[235]
BiOCl/diatomite	Hydrothermal	TH and gaseous HCHO	300 W over 420 nm xenon lamp	91.02 and 63.01 for TC and HCHO	[236]
BiOCl/diatomite	Hydrolysis	CIP	300 W 320–780 nm xenon lamp	94	[237]
Magnetic g-C3N4/diatomite/Fe3O4	An electrostatic self-assembly method	RhB	500 W over 420 nm xenon lamp	98	[238]
Ag3PO4/Fe3O4/diatomite	Hydrothermal	E. coli and RhB	500 W xenon lamp	100 and 98 for E. coli and RhB	[239]
TiO2/ZnO/diatomite	Two-step precipitation	MB	30 W xenon lamp	80.34	[240]
Bi2MoO6 modified TiO2/diatomite	Hydrothermal	HCHO	300 W over 400 nm xenon lamp	96.32	[241]
ZnO/ZnFe2O4/diatomite	Hydrothermal-precipitation	OTC	300 W xenon lamp	95	[242]
Fe2O3/TiO2/diatomite	Chemical precipitation-calcination	CIP	300 W over 420 nm xenon lamp	88	[243]
Fe-g-C3N4/diatomite	Thermal polymerization	TC	500 W xenon lamp	98.3	[244]
Ag3VO4/g-C3N4/diatomite	Annealing-precipitation	Cr(VI)	500 W over 420 nm xenon lamp	70	[245]
Fe3O4@TiO2/diatomite	Sol-gel and ultrasound assisted reflux technique	TC	500 W high-pressure mercury-vapor lamp	97	[154]
TiO2/recycled diatomite	Acid leaching and alkaline treatment	HCHO	300 W xenon lamp	95.59	[246]

. For instance, S. K. Padmanabhan et al. could deposit TiO₂ nanoparticles on diatomite surfaces, which showed good photocatalytic degradation of Rhodamine B, following pseudo-first-order kinetics [227]. Due to its higher surface area and mesoporous structure, diatomite worked as a host matrix for TiO₂ particles to realize uniform distribution and smaller TiO₂ crystalline size. Xia et al. also found that the anatase-to-rutile phase transition temperature was retarded to nearly 900 °C after loading onto the diatomite surface [228]. The pre-treatment of diatomite in phosphoric acid could modify its surface features and further increase the Si–O–Ti bonding strength between TiO₂ and diatomite, suppressing TiO₂ grain growth via remaining P species. As a result, it is favorable for the thermal-stable meso-porous structure formation at the granular spaces level, which could effectively promote the charge and mass transfer efficiency. Later, Zhang et al. applied TiO₂/diatomite composite to degrade volatile organic compounds and optimized its relative humidity level and reaction atmosphere for different ketone and alcohol degradation processes [229]. The optimal relative humidity is reported to be positively related to the carbon chain length of volatile organic compounds with the presence of oxygen. Additionally, Wu et al. utilized a TiO₂/diatomite catalyst to solve the serious secondary pollution problem of non-thermal plasma technology in reducing the polycyclic aromatic hydrocarbons emission process [230]. They stated that the TiO₂/diatomite composite works as the catalyst packing of dielectric barrier discharge reactor and improves naphthalene conversion up to 40% and CO_x selectivity up to 92%, and the aerosol and secondary volatile organic compound formation are substantially reduced by nearly 90% and 100%, respectively. They suggested that the synergetic effect between the TiO₂/diatomite catalyst and plasma could intensify the electron energy in the dielectric barrier discharge reactor and activate the deep oxidation of polycyclic aromatic hydrocarbons without external heating. Moreover, the Al₂O₃ component in diatomite support could exert a considerable effect on decomposing O₃ into O for naphthalene conversion and CO_x formation. Then, Barbosa et al. introduced Fe₂O₃ into heterogeneous diatomite/TiO₂ composite to achieve easy recovery without any significant activity loss under an external magnetic field [231]. It is known that the self-aggregation of g-C₃N₄ severely inhibits its photocatalytic performance. As a result, porous micro-disk-like diatomite could effectively suppress its self-aggregation process by acting as a low-cost photocatalyst carrier via the treatment presented in Figure 10b [232]. The g-C₃N₄/diatomite composite could display excellent selective adsorption ability, and photocatalytic activity for methylene B removal in mixed dyes, where the multi-interface produced between diatomite and g-C₃N₄ could dramatically accelerate electron migration. In 2020, Liu et al. decorated diatomite with Ni/NiO nanoparticles for the photoreduction of malachite green dye and Cr(VI) under visible light irradiation [233]. The synthesized composite was characterized as having a higher surface area of 400 m²/g and lower band gap energy of 1.71 eV, as well as the improvement of malachite green dye and Cr(VI) photodegradation percentages by 56.5% and 53%, respectively, compared with Ni/NiO nanoparticles. These could be ascribed to the fact that diatomite provides high exposed surface area for the incident photons, and Ni integration reduces the band gap energy. S. Tajmiri et al. could incorporate cadmium sulfide onto diatomite substrate to solve its recycling problem and obtain better adsorptive ability [234]. They found that the methylene B and Cr(VI) reduction process could be better simulated by the pseudo-first-order kinetic model, which means the adsorption and photodegradation process are controlled by diffusion and chemical reaction, respectively. Furthermore, Hua et al. combined BiVO₄@diatomite composite with microcrystalline cellulose and PVB ethanol solution to prepare the visible-light responsive photocatalytic paper by electrospinning [235]. The optimal BiVO₄@diatomite/MCC/PVBCFs (3:3:4) fibers are reported to be able to degrade 66.8% and 56.8% of methylene B and HCHO after 4 h visible light illumination as well as have excellent physical properties, such as tightness of 0.482 g/cm³ and tensile index of 11.28 N∙m∙g⁻¹. More recently, several diatomite-based photocatalysts have been developed that showed high photocatalytic efficiencies for the degradation and reduction of different pollutants, shown in Table 3.

In summary, natural diatomite clay exhibits great potential in the photocatalysis field due to its outstanding properties and abundant availability and might play a different role in future industrial applications.

3.4.2. Zeolite-Supported Photocatalyst

As a microporous hydrated aluminosilicate mineral commonly used as a commercial adsorbent and catalyst, natural zeolite is believed to be a mixture of volcanic rocks and ash layers reacting with alkaline groundwater [247,248]. Generally, it consists of silicon and aluminum, which are arranged in tetrahedral coordination with oxygen atoms. However, some Si⁴⁺ in the SiO₂ network could be replaced by Al³⁺, which leads to a negatively charged surface. As a result, various cations can be accommodated into the zeolite framework and loosely held, resulting in easy ion exchange. Early in 2012, M. Takeuchi et al. utilized Y-zeolite, faujasite zeolite with a structural silica-to-alumina ratio over 3, to construct a TiO₂/zeolite hybrid photocatalyst to achieve efficient gaseous toluene and benzene removal [249]. They found that these aromatic compounds were concentrated within the hydrophobic zeolite cavities due to the weak interaction with silanol groups on the zeolite surface, which enabled their better distribution onto TiO₂ surface. Zhang et al. modified zeolite by acid leaching and hybridized it with TiO₂ via hydrolysis deposition [186]. It was found that the anatase titania nanoparticles with a crystallite size of about 16 nm could be stabilized by the acid leaching of the zeolite surface. Moreover, acid leaching of zeolite surface could increase surface area, generate more surface hydroxyl groups, and provide a better dispersion effect. Later, Kovalevskiy et al. prepared a TiO₂/zeolite composite from a TiOSO₄ precursor for photocatalytic oxidization of volatile organic compounds [250]. They reported that zeolite, as TiO₂ support, could evidently enhance the adsorption capacity of the composite and effectively decrease the concentration of intermediates by suppressing their desorption on the photocatalyst surface during the heterogeneous photocatalytic oxidation process. Combining FeS with Fe₂O₃ to form a heterojunction semiconductor onto the surface of clinoptilolite nanoparticles could also show good photocatalytic activity [251]. The optimum ratio of FeS/Fe₂S₃ synthesized from 0.1 mol/L Fe(II) and 0.15 mol/L Fe(III) solution exhibited the best performance for ciprofloxacin photodegradation, where the electric field of the zeolite framework played an important role in photogenerated electron-hole separation, and hence, obviously improved photocatalytic activity. Additionally, Rajabi et al. took magnetic F₃O₄ as a core and coated it with a mordenite zeolite shell and CuO shell [252]. It was shown that the presence of mordenite zeolite could broaden the visible light absorption ability and enable stability in photocatalytic activity. Moreover, this magnetic core could realize easy separation and recycling of the as-prepared photocatalyst under an external magnetic field. In addition to natural zeolite, Nassar et al. prepared a zeolite nanostructure hydrothermally and modified its morphology and crystallite size by using different aluminum sources [248]. It was exhibited that the produced zeolite using aluminum isopropoxide possessed the optical energy band of 3.29 eV and BET surface area of 20.18 m²/g. This composite could degrade 85% of methylene B after 180 min under UV light irradiation, which followed the pseudo-first-order model. Owing to ion-exchange capacity and surface and structure properties, zeolite is gaining more and more attention in various industrial applications.

3.4.3. Pumice-Supported Photocatalyst

Originating from violent eruptions from volcanos under high temperature and pressure, pumice appears as porous silica rock and consists of highly vesicular rough-textured volcanic glass. Due to the existence of abundant hydroxyl groups on the surface and unique porous structure, pumice has been mainly applied as a support for various photocatalysts. For instance, Yusuff et al. prepared ZnO/pumice composite that was used as a photocatalyst to treat dye-containing textile industry wastewater [253]. It was ascertained that the degradation reaction kinetics of the as-prepared ZnO/pumice composite agree well with the pseudo-first-order approximation of the Langmuir–Hinshelwood model. Additionally, the ternary Fe₃O₄/ZnO@pumice composite showed good photocatalytic activity for methylene blue degradation under different conditions [254]. The intrinsically magnetic property of natural pumice with Fe₃O₄ nanoparticles facilitates the convenient recovery of the composite, and the metallic particles are well integrated into the porous pumice laminates. As a result, the specific surface area of Fe₃O₄/ZnO@ pumice composite decreases from 41.7 to 32.3 m²/g due to the partial loading of Fe₃O₄ and ZnO particles onto pumice pores. Through these works, natural pumice presents numerous promising possibilities in industrial applications.

4. Natural Carbonate or Carbon-Based Mineral-Supported Photocatalysts

4.1. Calcite-Supported Photocatalyst

As the most stable polymorph of calcium carbonate, calcite is extremely common in sedimentary, metamorphic, and igneous rocks throughout the world and is thus considered a rock-forming carbonate mineral. In 2019, Belhouchet et al. made full use of the natural Algerian shale abundantly existing in northeastern Algeria and North Africa to search for highly efficient photocatalytic material [41]. The study results revealed that the natural mineral mainly consisted of calcite coated with TiO₂ by the sol-gel method, and the composite CAL30 sample, had the largest pore volume, higher specific surface area, and, therefore, the best photocatalytic activity. It was analyzed that this calcite/TiO₂ system followed a pseudo-first-order kinetic model from the photocatalytic experiment results. The introduction of the mineral into CAL30 sample could suppress the electron-hole recombination and reach a close tetracycline degradation performance (the rate constant k = 2 × 10⁻² min⁻¹) of pure TiO₂, saving 70% of the TiO₂ amount.

Additionally, limestone and marble generally make up the Earth’s crust, a significant part with primary constituent as calcite, and serve as one of the largest carbon storages on this planet. Generally speaking, limestone as sedimentary carbonate rock is often formed by the skeletal fragments of marine organisms and the chemical precipitation of calcium carbonate. Because of the water solubility, weak acid properties, and wide distribution, limestone is usually used as a building material, white pigment, and product filler [255]. However, regardless of whether it is a building material or wall painting material, limestone always encounters a cleaning problem. As a result, self-cleaning technology design is put forward to solve this issue economically and efficiently. In 2017, N. Fadhilah could successfully synthesize super-hydrophilic self-cleaning limestone coating paint with TiO₂ nanoparticles modified by dispersant polyethylene glycol 6000 (PEG6000) [256]. This new paint with the composition of TiO₂:PEG600 as 1:6 could achieve the highest self-cleaning ability (83.1%) with water contact angles of 7°. Up to 2019, Mohsen et al. prepared CaCO₃/TiO₂ nanostructures with high surface area, mesoporous structure, and lower bang gap energy of 2.05 eV by the precipitation method [257]. Thus, the CaCO₃/TiO₂ nanocomposite showed photocatalytic desulfurization of 95% dibenzothiophene under 550 Watt linear halogen lamp illumination and in the presence of H₂O₂ and acetic acid.

Often referred to as magnesium-rich limestone, dolomite is known as an anhydrous carbonate mineral composed of calcium magnesium carbonate and crystallizes in the trigonal-rhombohedral system. With the ideal formula CaMg(CO₃)₂, dolomite has double carbonate-containing alternating planes of Ca²⁺ and Mg²⁺ ions. In 2019, Belarbi et al. calcined dolomite under 800 °C and then modified it by Ca(NO₃)₂ to obtain the dolomite-based catalyst CaD800, which could degrade 95% pentachlorophenol under UV light irradiation after only 1 h [258].

4.2. Other Carbon Material-Supported Photocatalyst

Apart from graphene, other carbon materials with specific morphology are also excellent candidates for photocatalyst support, such as carbon quantum dots, carbon nanotubes, carbon nanofibers, etc. [259]. Similar to graphene, these carbon nanomaterials also possess superior mechanical and electrical properties. In one benzene mineralization study, carbon nanotubes were combined with TiO₂ by the wet impregnation method. Carbon nanotubes as support could provide a stable electron reservoir and efficient electron transfer route. Moreover, the existence of carbon nanotubes effectively suppresses the agglomeration of TiO₂ particles, and thus more reactive sites could be exposed. Compared with the bare TiO₂, the enhanced photocatalytic performance of TiO₂/CNT composite was proved to come from tight interfacial contact between carbon nanotubes and TiO₂. Due to all these factors, the conversion efficiency from benzene to CO₂ over TiO₂/CNT is much higher than P25, a kind of commercial TiO₂ [260].

5. Natural Sulfate or Sulfide Mineral—Supported Photocatalyst

5.1. Gypsum-Supported Photocatalyst

As the most common sulfate mineral, gypsum is a soft calcium sulfate dihydrate compound that is often used as a fertilizer and as the main constituent in many forms of plaster, blackboard, and drywall. Generally, gypsum deposited from lake, sea, and hot springs crystallizes as a translucent crystal of selenite and forms as an evaporite mineral and hydration product of anhydrite. The previous research mainly focuses on assembling efficient photocatalytic additives with commercial gypsum to develop self-cleaning building materials and exploring their photodegradation efficiency on various pollutants. A novel BiOCl_xBr_1−x@gypsum heterojunction composite was constructed as a self-cleaning surface for building material [261]. The photocatalytic activity results showed that the as-obtained hybrid material could mineralize RhB in 4 min and the recalcitrant Naphthalene contamination in 20 min under visible light irradiation. Additionally, it could maintain superior photocatalytic activity and efficiency for at least 7 months. Janus et al. incorporated N, C co-modified TiO₂ into commercial gypsum to obtain a self-cleaning property and exhibit obvious dye removal capacity [262]. They found that N, C-TiO₂ photocatalyst functioned as the void space fillers in gypsum matrix could reduce gypsum particle size and over 10% mass content of N, C-TiO₂ additive into gypsum causes a considerable decrease of the compressive strength value, pore volume, and specific surface area. The obtained N, C-TiO₂/gypsum sample treated at 300 °C exhibited 85% and 70% degradation of azo dye Remazol red 198 after 15 h of UV and visible light irradiation, respectively. V. Binas et al. fabricated photocatalytic cement-based coating mixed with Mn-doped TiO₂ and compared the influence of different supporting materials on the photocatalytic activity against NO air pollutants [263]. The substrate materials displayed a slight difference in photocatalytic degradation efficiency, and the mixture of Mn-doped TiO₂ composite coating influenced its surface area and porosity.

5.2. Pyrite-Supported Photocatalyst

As a naturally n-type semiconductor material, pyrite is a brass-yellow sulfide mineral with metallic luster and chemical formula of FeS₂. Considering its broad existence in igneous, metamorphic, and sedimentary rocks worldwide, pyrite is widely regarded as a promising candidate for wastewater treatment. Diao et al. employed pyrite as a photocatalyst for the degradation of malachite green (MG) and Cr(VI) reduction [264]. Pyrite photocatalysts showed removal efficiency of MG and Cr(VI) reached 96.2% and 53.1%, respectively, from their mixture solution after 120 min UV light irradiation, which is better than that determined for individual separated solutions. They suggested that the presence of malachite green could accelerate Cr(VI) reduction and vice versa, which is attributed to the synergistic effect in the MG and Cr(VI) coexistence system. They also found that the dissolved oxygen could remarkably promote MG degradation efficiency and Cr(VI) reduction that is mainly controlled by direct electrons of pyrite. Furthermore, natural pyrite could be applied to inactivate Escherichia coli in the presence of citrate and EDTA as chelating agents [265]. These chelating agents would increase the iron solubility and reduce the oxidation of the pyrite surface, which promotes the photo-Fenton process and the E. coli inactivation rate. Additionally, natural pyrite could be transformed structurally into a Z-scheme FeS₂/Fe₂O₃ composite via the heat treatment method, and the synergistic effects between FeS₂ and Fe₂O₃ have a remarkable positive influence on its photocatalytic activity [266]. It was proved that the as-obtained composite exhibits increased photocatalytic degradation efficiency from 36% to 65% with the presence of 30 mg/L Cr(VI), and the addition of Cr(VI) ions can also be almost reduced. That is related to the fact that Cr(VI) could capture the electrons and thus improve photo-induced electron-hole pair separation efficiency. These findings make natural pyrite suitable for designing a low-cost and efficient photocatalyst for wastewater treatment and environmental applications.

6. Natural Special Mineral-Based Photocatalysts

6.1. Ilmenite-Based Photocatalyst

As a wide-bandgap anti-ferromagnetic semiconductor, ilmenite is known as a titanium–iron oxide mineral with the idealized chemical formula FeTiO₃, containing around 40–60% of TiO₂. Generally, pure ilmenite has a rhombohedral crystal structure with the hexagonal packing of the oxygen atoms with cations occupying two-thirds of the octahedral positions, where Fe and Ti occupy alternating layers, as illustrated in Figure 11A,B [267]. Due to its chemical composition and unique properties, ilmenite is always considered the most suitable natural precursor of photocatalyst TiO₂. A. Simpraditpan et al. explored the effect of calcination temperature on the properties of titanate nanofibers originated from natural ilmenite minerals and concluded that the temperature increase could result in the phase transformation from thermodynamically metastable anatase phase to the most stable rutile phase with increasing the crystallite size [268]. They found that the nanofibers calcined at high temperatures of 600–1000 °C were mixtures of three crystalline phases, e.g., anatase, rutile, and Fe₂O₃. Moreover, increasing the calcination temperature induces the increase of crystallite sizes, therefore leading to the absorption edge of the as-synthesized sample shifts to the visible light region. Among these samples, the nanofibers calcined at 400 °C for 2 h exhibited the best photocatalytic activity, even higher than the commercial TiO₂. Shao et al. synthesized iron-doped TiO₂ nanorods from raw ilmenite ore via a direct acid leaching method. They found that the band gap energy decreased from 2.94 eV to 2.49 eV with higher iron concentration, as compared with 3.02 eV of bare rutile TiO₂ [269]. The optimal amount of iron dopant was reported to be 1.69%, leading to the best photogenerated charge carrier separation efficiency. Another study showed that iron species from natural ilmenite minerals could be converted into Fe₃O₄ in addition to titania phases, providing magnetic recyclability of the photocatalyst under an external magnetic field [270]. This composite was characterized to be composed of face-centered cubic Fe₃O₄ and body-centered tetragonal anatase TiO₂ with superparamagnetic behavior and strong visible-light absorption ability. It could be separated and recycled quickly and easily due to the magnetization effect of Fe₃O₄ core. Moreover, superoxide radicals were proved to be the main active species of the Rhodamine B degradation process. Xia et al. introduced persulfate into the natural ilmenite reaction system to achieve efficient E. coli inactivation under visible light irradiation [271]. They suggested that persulfate could repress the photogenerated electron-hole recombination, and the Fe(II) species from ilmenite can be beneficial for persulfate activation and thus promote reactive radical production like singlet oxygen (¹O₂), which was investigated to be the primary bactericidal agent, according to the reaction routes drawn in Figure 11C. Lee et al. investigated the effect of the iron contents in ilmenite on its photocatalytic activity. The acid concentration was able to control the iron content during the acid leaching process [272]. The optimum iron content was found to be 1.62 at% after pre-treatment with 25 v/v% HCl, and hence, it was able to achieve complete removal of the Reactive Black 5 dye within 30 min of visible light illumination owing to obvious surface area increase and low photogenerated electron-hole recombination rate. In addition, natural ilmenite also was applied as a photocatalyst for selective reduction of nitrate using oxalic acid as a reducing agent and hole scavenger, enabling the complete removal of N₂ from the nitrate solution [273]. At the present stage, ilmenite is regarded as the most suitable and promising candidate to realize the cost-effective production of TiO₂ and its commercialized application.

6.2. Monazite-Supported Photocatalyst

A monazite is a group of monoclinic phosphate and arsenate minerals that contains one or several kinds of rare-earth metals. Monazite (Ce), as a thorium-bearing mineral, was selected as TiO₂ substrate for its minor radioactivity and its interior framework of a composite on which TiO₂ grew by forming Ti–O–Ce microstructure [274]. TiO₂ embedded in monazite became a kind of radiation-sensitive photocatalyst, and the degradation principle was similar to photocatalysis in that the band gap of TiO₂ is activated by γ-ray irradiation. It was revealed that the radiations of thorium nuclides in TiO₂/monazite composite endowed energy to trigger TiO₂ excitation, existent cerium ions in monazite served as energy absorber with charge separator, and the TiO₂/CeO₂ structure could provide a unique electron transfer process which is beneficial to separate the charge-carriers more efficiently and thus improve catalytic activity in dark conditions. Furthermore, radioluminescence (RL) is the phenomenon by which luminescence is produced in a material by the bombardment of α-particles, γ-rays, or β-particles. It was proposed that radio-sensitive Ce could produce interior UV because of RL and TiO₂ were excited by RL. This self-activated photocatalytic behavior was similar to the heterojunction reaction mechanism and inner electric field established within TiO₂ semiconductor and CeO₂ due to Fermi level equilibrium and could transfer electron-hole pairs more effectively.

7. Summary and Outlook

To face the challenges of worldwide pollution in 21st century, scientific research and progress in photocatalysis are of great concern all over the world. The widely applied surface modification methods of bare photocatalysts are impurity doping, noble-metal deposition, heterojunction design, and mineral support combination. Among these approaches, the mineral support combination approach is widely considered to be the promising treatment to realize large-scale industrial production owing to the low cost, chemical stability, and specific structure of natural minerals. However, due to regional limitations and wide chemical composition variations, the related research lacks systematism and the potential to pioneer further research. This review aims to sort the current research, point out the commonness of these studies, and propose all possible mineral support modification mechanisms. The proposed mineral support modification mechanisms are listed below according to different mineral groups:

As a major part of natural minerals, silicate minerals are divided into 1D, 2D, 3D, and porous minerals. Owing to their advantages in structure and adsorption capacity, silicate minerals could not only effectively suppress the self-aggregation of photocatalyst particles but also enhance the specific surface area and expose more reactive sites. Moreover, the synergistic effect between clay support and photocatalyst could efficiently promote the photo-induced charge carrier separation and hence improve the photocatalytic activity, which is ascribed to the fact that the introduction of clay mineral can enrich the pollutant concentration near the photocatalyst composite and accelerate the photogenerated electron transfer.

The presence of carbonate minerals could retard the photogenerated electron-hole pair recombination and thus reach higher photocatalytic performance. The other carbon materials can provide a stable electron reservoir and an effective electron transfer route. Therefore, they can efficiently mitigate the photogenerated electron-hole recombination with their morphology and electric conductivity.

Photocatalysts act as natural gypsum additives to endow building materials with photocatalytic activity. Under this circumstance, natural gypsum only functions as a photocatalyst substrate, making it easy to recycle, and natural pyrite belongs to the n-type semiconductor itself and could be applied as a low-cost photocatalyst after some proper treatment.

Natural ilmenite, as an anti-ferromagnetic semiconductor, is regarded as TiO₂ precursor and produces iron-doped titania or iron-oxide/titania composites with excellent photocatalytic activity by controlling treatment parameters. Due to its minor radioactivity, natural monazite as photocatalyst support enhances the photosensitivity and photocatalytic activity of composite photocatalysts.

Additionally, some active ion species sometimes exist in the natural mineral structure and therefore can act as doping species when the mineral is combined with one specific photocatalyst. Under this circumstance, the band gap structure of the photocatalyst composite can also be modified, and the photocatalyst composite could obtain a wider light absorption range and higher photocatalytic efficiency. This work intends to provide a comprehensive description of present mineral-supported photocatalytic applications and modification mechanisms based on the existing research and hopes to lay a solid foundation for future research. The combination of natural minerals and photocatalysts is believed to be the most promising method to realize the industrial production and wide applications of photocatalysis.

Last but not least, although we focus on natural minerals in this work, other mineral supports (artificial) are also worth noting. In particular, how to construct lower-cost, higher-efficiency “artificial” minerals is also a direction that can be further expanded in the future.