The Application of Mineral Kaolinite for Environment Decontamination: A Review

Abstract

Kaolinite clay mineral with a layered silicate structure is an abundant resource in China. Due to its advantages of excellent stability, high specific surface area and environmental friendliness, kaolinite is widely used in environment decontamination. By using kaolinite as a carrier, the photocatalytic technology in pure photocatalysts of poor activities, narrow spectral responses, and limited electron transport can be overcome, and the nano-Ag disinfectant’s limitation of the growth and aggregation of nanoparticles is released. Moreover, pure kaolinite used as an adsorbent shows poor surface hydroxyl activity and low cation exchange, leading to the poor adsorption selectivity and easy desorption of heavy metals. Current modification methods including heat treatment, acid modification, metal modification, inorganic salt modification, and organic modification are carried out to obtain better adsorption performance. This review systematically summarizes the application of kaolinite-based nanomaterials in environmental decontamination, such as photocatalytic pollutant degradation and disinfection, nano silver (Ag) disinfection, and heavy metal adsorption. In addition, applications on gas phase pollutant, such as carbon dioxide (CO₂), capture and the removal of volatile organic compounds (VOCs) are also discussed. This study is the first comprehensive summary of the application of kaolinite in the environmental field. The review also illustrates the efficiency and mechanisms of coupling naturally/modified kaolinite with nanomaterials, and the limitation of the current use of kaolinite.

1. Introduction

Kaolinite is an abundant clay mineral resource with a layered silicate structure, which is yielded from the natural alteration of feldspar and other silicate minerals. It is a kind of aluminosilicate formed with Al-(O, OH) octahedrons and Si-O tetrahedrons bonded by common oxygen. Kaolinite possesses the advantages of excellent stability, high specific surface area, and environmental friendliness. Moreover, China is a country with rich amounts of kaolinite, with an estimated reserve of 14.3 billion tons. Therefore, kaolinite is competitive and can be used as potential materials for the decontamination of environmental pollutants, especially regarding water treatment and air pollution control. It is often used as a photocatalyst for organic pollutants, adsorbent for heavy metals, CO₂ capture, and removal of VOCs.

Photocatalytic technology is extensively studied using nano-semiconductors such as TiO₂ and ZnO nanostructures. However, these pure nanostructures suffer from poor activities, narrow spectral responses, and limited electron transport, which restrains the real applications in environmental decontamination. Kaolinite is an optimal candidate to act as nanomaterials’ carrier because it has the advantages of low cost, rich reserves, recyclability, and high adsorption capacity [1,2,3]. In order to boost the generation of active radicals under light from the range of ultraviolet (UV) to visible light and improve the photocatalytic performance on environment decontamination [4], kaolinite has been combined with nano-photocatalysts to synthesize composite materials, such as TiO₂ (titanium dioxide)/kaolinite [5,6,7], ZnO (zinc oxide)/kaolinite [8,9], g-C₃N₄ (graphitic carbon nitrides)/kaolinite [10,11], and g-C₃N₄/TiO₂/kaolinite [9,12]. For example, the deposition TiO₂ nanoparticles on the kaolinite carrier (TiO₂/kaolinite composite) promoted the dispersion of TiO₂ nanoparticles, which benefited the transportation of photoinduced carriers and improved the photocatalytic performance towards the degradation of ciprofloxacin [5]. The g-C₃N₄ growth on kaolinite (g-C₃N₄/kaolinite composite) gave g-C₃N₄ pore structure and rich reactive sites, which showed better photocatalytic performance than pure g-C₃N₄ [3].

Ag nanoparticle is a disinfection agent towards bacteria, fungi, algae and viruses, due to the ability of Ag to interact with DNA and thiol-containing proteins after it transports across the cell membrane. However, the practical application of Ag nanoparticles is limited because disinfection technology based on nanomaterials has limitations such as the growth of nanocrystal, the aggregation of nanoparticles, and the changing of the surface morphology, which hampers the disinfection performance and ultimately restrains its real application. Kaolinite is a suitable candidate for the immobilization of Ag nanoparticles to construct nano-Ag/kaolinite composite with well dispersed nanoparticles and improved disinfection performance. Moreover, the recovery and reuse of Ag/kaolinite is easier (especially in water environment), which overcomes the problem of secondary pollution caused by pure Ag nanoparticles [13].

Kaolinite is also an adsorbent for environmental pollutants. It could work as an adsorbent for heavy metals of Cd(II), Fe(III), Co(II) and Ni(II) [14,15,16]. However, the low cation exchange capacity and specific surface area limit the removal efficiency of pure kaolinite [14]. Therefore, researches have made great efforts to increase the specific surface area, produce a large number of pore structures, and change the surface property of kaolinite. Thermal modification, acid modification, transition metal modification, and organic modification are all efficient methodologies for improving its adsorption of heavy metals [15,17].

This review provides comprehensive information on recent progress in the development and synthesis of kaolinite-based materials. This review systematically summarizes the application of kaolinite-based nanomaterials in environmental decontamination, such as photocatalytic degradation, nano silver (Ag) disinfection, photocatalytic disinfection, and heavy metal adsorption. In addition, the application of kaolinite in gas phase pollutant removal, such as CO₂ reduction and volatile organic compounds (VOCs) degradation, are also discussed. This is the first comprehensive summary of the application of kaolinite in the environmental field. This review also illustrates the efficiency and mechanisms of coupling naturally/modified kaolinite with nanomaterials, and the limitation of the current use of kaolinite.

2. The Properties of Kaolinite

The molecular formula of kaolinite is Si₂AlO₅(OH)₄. It consists of 39.50% Al₂O₃, 46.54% SiO₂, and 13.96% H₂O. Kaolinite is an aqueous aluminosilicate mineral composed of the Al-centered octahedra ([Al₂(OH)₄]²⁺) and the Si-centered tetrahedra (Si₂O₅²⁻) in a two-dimensional arrangement. The crystal structure of kaolinite offers a certain degree of charge non-parallelism because both the Al-centered octahedral layer and the Si-centered tetrahedral layer are negatively charged [18]. Therefore, the oxygen atoms connect the charge-repulsive Al-centered octahedra and Si-centered tetrahedra, and form the layered structure of kaolinite, which promotes the formation of hydrogen bonds to tightly bond kaolinite (Figure 1). Therefore, an abundance of the hydroxyl group is supplied on the surface of kaolinite, which is active group and benefits the further chemical treatment, including anchoring nanoparticles (such as TiO₂, ZnO, g-C₃N₄, Ag) and surface modification (such as thermal-modified kaolinite, acid-modified kaolinite, transition metal-modified kaolinite, and organic-modified kaolinite).

3. Photocatalytic Degradation of Organic Pollutions

Aqueous organic pollutants can cause serious harm to human health and the ecological environment. Traditional wastewater treatment processes of coagulation, sedimentation, filtration, and biological treatment have the disadvantages of non-recyclability, high cost, and low efficiency [19,20]. In contrast, photocatalytic degradation is a promising technology with extensive applications for the direct harvesting and conversion of the solar energy to generate active radicals for the pollutants’ degradation [21,22]. Because the nanostructures in pure, such as ZnO, TiO₂, and g-C₃N₄, suffer from poor activities, narrow spectral responses, and limited electron transport, kaolinite can be used as a carrier for nano-photocatalysts to generate synergistic effects, including promoting dispersion of nano-photocatalysts, increasing the pollutants’ contact with surface active components, and enhancing the restorability, which significantly improves the photocatalytic degradation performance [1,10].

3.1. Photocatalysis Mechanism

The reactive species of photocatalytic degradation of organic pollutants in semiconductor materials mainly includes photogenerated holes (h⁺), superoxide radicals (O₂∙⁻), and hydroxyl radicals (HO·). When light with wavelengths below the absorption threshold of the semiconductor is used, the semiconductor produces photogenerated electrons (e⁻) and h⁺ (Equation (1)). In addition, e⁻ are generated from the conduction band (CB) of the semiconductor and h⁺ are generated from the valence band (VB); e⁻ react with O₂ to generate O₂∙⁻ (Equation (2)), and h⁺ are captured by hydroxyl groups to generate HO· in aqueous solution (Equation (3)). Then, the formed h⁺, O₂∙⁻, and HO·act as oxidants to interact with organic pollutants, which finally degrade into small molecules such as CO₂, H₂O, and inorganic acids [4,9,23]. The schematic illustration of the photocatalytic reaction mechanism is shown in Figure 2 [22].

$$\mathrm{photocatalysts}+\mathrm{h}\mathsf{\nu }\to {\mathrm{e}}^{-}{+\mathrm{h}}^{+}$$

(1)

$${\mathrm{e}}^{-}{+\mathrm{O}}_2\to {\mathrm{O}}_2^{·-}$$

(2)

$${\mathrm{h}}^{+}{+\mathrm{HO}}^{-}\to {\mathrm{HO}}^{-}$$

(3)

The high adsorption capacity and excellent sedimentation performance of kaolinite make it an ideal carrier for photocatalytic semiconductor materials, thus it is widely used in organic pollutants such as pharmaceuticals, personal care products (PPCPs), and dyes [24]. In this section, the applications of different kaolinite-based photocatalysts in the degradation of organic pollutants are summarized.

3.2. TiO₂/Kaolinite Photocatalysis

Despite the fact that semiconductor TiO₂ has been reported as possessing the advantages of simple and mild synthesis conditions and excellent photo-corrosion resistance, its photocatalytic ability is not high because of its limited specific surface area. Coating TiO₂ on the surface of kaolinite is an efficient way to increase the specific surface area. Li et al. [25] synthesized a TiO₂/kaolinite photocatalyst by using a simple sol-gel method. The synthesized TiO₂/kaolinite was 1.42 times higher than the pure TiO₂. Moreover, the degradation rate constant of the prepared TiO₂/kaolinite is 7.90 times higher than pure TiO₂, ascribing to the introduction of micropores and mesopores, which enhanced the adsorption, diffusion, and desorption of organic pollutant ciprofloxacin in the composite, and further increased the photocatalytic degradation performance. In addition, this research found that the calcination temperature for TiO₂/kaolinite preparation is a key factor for affecting the specific surface area. When the calcination temperature increased from 105 to 300 °C, the specific surface area of the TiO₂/kaolinite increased from 74.77 to 106.19 m²/g, resulting in an increased decay efficiency from 82.72% to 92.4%. However, when the calcination temperature was beyond 300 °C, the photocatalytic performance dramatically decreased to 60.26%, accompanied by the specific surface area’s decrease to 74.77 m²/g. The decline of the specific surface area is mainly attributed to the damage and collapse of the micropore structure caused by the exceeded calcination temperature. Another TiO₂/kaolinite photocatalyst prepared by hydrothermal method [26] showed that its specific surface area and average pore volume increased 15.1 fold and 2.1 fold, respectively, compared to pure TiO₂. As a result, the photocatalytic degradation efficiency on dye RhB reached 99.51% in 120 min, which was better than the pure TiO₂ of 88.35%. The specific surface area and photocatalytic degradation performance were also affected by the morphologies of kaolinite [27]. Li et al. [28] prepared TiO₂/kaolinite by adjusting the kaolinite morphologies to nanoflake- and nanorod-like. The TiO₂ nanoparticles/nanoflake-like kaolinite achieve a higher tetracycline hydrochloride degradation efficiency of 98% than TiO₂ nanoparticles/nanorod-like kaolinite (84%), pure TiO₂ (10%), and pure kaolinite (20%). Compared to the small specific surface area of pure TiO₂ nanoparticles (S_BET = 13 m²/g), the specific surface area of TiO₂ nanoparticles/nanorod-like kaolinite and TiO₂ nanoparticles/nanoflake-like kaolinite increased to 109 m²/g and 114 m²/g, respectively, which promoted the dispersion of TiO₂.

Because TiO₂ is a wide band gap material and absorbs only in the UV region, another efficient way to enhance the photocatalytic performance is to explore the light absorption to visible light. Wang et al. [29] fabricated broadened light absorption TiO₂/kaolinite composites by using the titanium tetrachloride (TiCl₄) hydrolysis method, where the kaolinite played a key role in the crystal phase transformation because it promoted the growth of anatase TiO₂ by inhibiting the transformation of the anatase phase to the rutile phase. Consequently, the light absorption was increased with absorption edge shifting from 400 nm to 417 nm, and the photocatalytic performance on dye methyl orange was improved.

Surface modification of TiO₂/kaolinite photocatalysts by changing the surface properties of kaolinite is effective in improving the degradation performance. Xu et al. [30] synthesized TiO₂/kaolinite composites by using HCl acid-treated kaolinite as a carrier. After acid treatment on kaolinite, hydroxyl groups were introduced on the surface of kaolinite, which offered more sites for loading TiO₂ grains, resulting in an enhanced performance of 93.94% on the organic dye methylene blue, compared to the 77.71% at the untreated kaolinite. Oliveira et al. [20] applied H₂SO₄ to alter the surface property of TiO₂ and kaolinite, which positively charged TiO₂ and negatively charged kaolinite. The electrostatic attraction promoted the interaction between TiO₂ and kaolinite. Therefore, the degradation performance of organic dye methylene blue followed the order of H₂SO₄ acid treated TiO₂/kaolinite > TiO₂/kaolinite > TiO₂.

3.3. Other Semiconductor/Kaolinite Photocatalysis

Other photocatalytic semiconductors, such as ZnO, g-C₃N₄, and bismuth oxychloride (BiOCl), were also used for loading on the kaolinite carrier. The morphologies of kaolinite affected the specific surface area and thereby the photocatalytic degradation performance of the ZnO/kaolinite and g-C₃N₄/kaolinite composites. Kutláková et al. used a simple hydrothermal method to synthesize the ZnO/kaolinite composites [31]. Its decay efficiency of dye acid orange 7 reached 95%. Abukhadra et al. [32] revealed that the morphology of kaolinite showed an important influence on the photocatalytic performance. The ZnO/nanotube-like kaolinite showed a 99% levofloxacin (an organic pollutant) degradation rate, which was 76.7% higher than that of ZnO/bulk kaolinite. This is because the higher specific surface area of kaolinite nanotubes benefited the formation of homogeneous ZnO nanoparticles and further enhanced the light absorption capacity. A two-dimensional lamellar g-C₃N₄/kaolinite composite was synthesized and its apparent rate constant on photocatalytic degradation of dye RhB is 2.1 times higher than that of bare g-C₃N₄ because the two-dimensional lamellar structure offered tight interfacial bonding between g-C₃N₄ and kaolinite, and benefited the separation of the photogenerated electron-hole pairs [9]. Furthermore, the treatment of kaolinite by cyanuric acid led to the generation of abundant pore structures and reactive sites. As a result, the apparent rate constant of cyanuric acid modified g-C₃N₄/kaolinite is 1.9 times than that of g-C₃N₄/kaolinite. In addition, Sun et al. [3] found that the loading amount of g-C₃N₄ significantly affected the photocatalytic performance of the g-C₃N₄/kaolinite composites, where the optimum loading amount was determined as 40% (mass ratios of g-C₃N₄ with respect to kaolinite) because it presented the higher adsorption capacity on RhB. Considering the fact that the photocatalytic reaction always occurred on the catalyst surface [33], the higher adsorption of RhB led to the better degradation performance. Moreover, the good dispersity of g-C₃N₄ on the g-C₃N₄/kaolinite composite promoted the separation of photogenerated electrons and holes, providing more reactive radicals for RhB degradation [3].

3.4. Dual Semiconductor-Based Kaolinite Photocatalysis

Despite being above the single semiconductor TiO₂, ZnO and g-C₃N₄ based kaolinite composites showed good performance on organic pollutant decay, but they still suffered from narrow spectral response and limited electron transport, which exhibited limited degradation rates for organic pollutants [34]. Dual semiconductors-based kaolinite composites provided an efficient way to alleviate the aforementioned limitations [4]. Li et al. [35] constructed a g-C₃N₄-TiO₂ heterojunction supported on kaolinite (g-C₃N₄-TiO₂/kaolinite) to accelerate the internal charges’ separation. The g-C₃N₄-TiO₂/kaolinite had an enhanced visible light absorption region with a band gap of 2.72 eV and an enhanced photocatalytic removal efficiency for ciprofloxacin, showing an apparent rate constant 5.35 times than that of pure TiO₂, and 6.35 times than that of g-C₃N₄. The ternary g-C₃N₄-TiO₂/kaolinite composite had a looser porous structure, higher specific surface area, and smaller TiO₂ grain size, which facilitated the adsorption, migration, and degradation of the organic pollutant ciprofloxacin. Huang et al. [36] prepared a ternary cerium dioxide (CeO₂)/g-C₃N₄/kaolinite composite via combining the sol-gel and hydrothermal method. Compared with the decay efficiency of pure CeO₂ (42%) and g-C₃N₄ (51%), the ternary CeO₂/g-C₃N₄/kaolinite composite showed an enhanced degradation efficiency (90%) on the organic pollutant. The superior performance of the ternary composite was attributed to its larger specific surface area, lower band gap, and bigger pore volume. Dong et al. [10] introduced g-C₃N₄ and BiOCl on kaolinite carrier to form a ternary BiOCl/g-C₃N₄/kaolinite composite. The use of BiOCl increased the light absorption intensity in the visible light range, and further enhanced the photocurrent and the interface charge transfer. As a result, the degradation rate on RhB by BiOCl/g-C₃N₄/kaolinite is 10 times that of g-C₃N₄/kaolinite and 5.5 times that of BiOCl.

4. Disinfection with Ag Materials and Photocatalysis

4.1. Disinfection with Ag Materials

Pathogenic microorganisms present a serious threat to human and animal health especially in the aqueous environment [37]. Ag nanoparticle is a disinfection agent towards bacteria, fungi, algae, and viruses because Ag can interact with DNA and thiol-containing proteins after passing through the cell membrane. However, Ag nanoparticles are easy to aggregate owning to van der Waals forces and the high surface energy of nanoparticles, which limits its practical application. Constructing the Ag nanoparticles on the kaolinite surface can solve these problems. E. Ward et al. [38] prepared a Ag/kaolinite composite that showed good antibacterial and antifungal activity, in which the incorporation of Ag nanoparticles into a kaolinite carrier retarded the aggregation of the Ag nanoparticles. Furthermore, to benefit the composites’ separation, Woldegebreal et al. [39] synthesized a magnetic Fe₃O₄-Ag/kaolinite composite by introducing the magnetic Fe₃O₄ into Ag/kaolinite, which can be easily separated from water after application, and achieved efficient disinfection performance on Salmonella paratyphi and Escherichia coli bacteria. Moreover, the increment of the loading amount of Ag nanocomposite contributed to a significant enhancement in the disinfection performance with the loading amount increased from 0.1 g to 0.7 g. Apart from combining with the magnetic materials, the combination of the organic antibacterial agent also improved the disinfection performance. Jou’s study [40] deposited an organic antibacterial agent, i.e., chlorhexidine acetate, onto the Ag/kaolinite surface to prepare a chlorhexidine-Ag/kaolinite composite with enhanced antibacterial activity, which presented excellent antibacterial activity on both Gram-positive bacteria and Gram-negative bacteria. The antibacterial activity of chlorhexidine-Ag/kaolinite composite was better than Ag/kaolinite and chlorhexidine/kaolinite because it produced a larger total inhibition zone and gave a lower minimum inhibition concentration.

4.2. Disinfection with Photocatalysis

Photocatalytic disinfection is a promising technology for environmental disinfection due to the use of solar energy and no secondary pollution [41,42,43]. Numerous studies have shown that photocatalytic disinfection can be used to remove a series of bacteria such as Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Enterococcus faecalis. The mechanism of photocatalytic disinfection is similar to the photocatalytic degradation, where generated radicals such as O₂∙⁻ and HO· via photocatalysis are the main species for the destruction and inactivation of micro-organisms [13]. However, the drawbacks of photocatalytic disinfection with pure photocatalysts are similar to that of photocatalytic pollutant degradation, i.e., poor activities, narrow spectral responses, and limited electron transport. Using kaolinite as the catalyst carrier can alleviate these problems to a certain extent. A TiO₂/kaolinite was prepared via sol-gel method and achieved 5 bacterial log-reduction units on Escherichia coli sp. (ATCC 11775) under 120 min irradiation, which was superior to the pure TiO₂ [44]. The operational factors, such as TiO₂/kaolinite dosage, pH, and aeration rate, were found showing great impact on disinfection activity. Fatimah et al. [45] synthesized TiO₂/kaolinite photocatalytic composites, and the photocatalytic disinfection activity on bacterial Escherichia coli achieved 100%. This antibacterial performance was affected by the loading amount of TiO₂ (of kaolinite mass), and the optimum loading amount of TiO₂ is determined as 10% in mass ratio because it gave a higher band gap energy (3.21 eV) with a broadened light absorption spectrum and a larger specific surface area (40.928 m²/g).

The TiO₂/kaolinite only responds to UV light, which hinders its photocatalytic performance under visible light or solar light. Doping is an efficient method to expand the response spectrum to visible light. For example, B. Aritonang et al. [46] prepared a Bi-doped TiO₂/kaolinite photocatalyst via sol-gel method followed by calcination at 450 °C. With the irradiation of visible light, the prepared Bi-doped TiO₂/kaolinite showed a higher bacterial inactivation performance on Escherichia coli and Staphylococcus aureus than TiO₂/kaolinite composite. This is because the light response was expanded from ultraviolet to visible light, and the band gap significantly decreased from 3.22 eV (TiO₂) to 2.76 eV (Bi-doped TiO₂), which originated from the incorporation of Bi³⁺ to form Bi-O-Ti bonds. Misra et al. [47] prepared Fe-doped ZnO/kaolinite composites via co-precipitation method, which achieved the highest disinfection efficiency (100%) on multidrug resistant Enterobacter sp. under visible light irradiation compared with Fe-doped ZnO and kaolinite. The excellent disinfection efficiency was due to the smaller negative surface charge of Fe-doped ZnO/kaolinite than ZnO and kaolinite, thereby the surface of Fe-doped ZnO/kaolinite composite can interact more closely with the negatively charged Enterobacter sp. cell membrane and improve the bactericidal performance. Moreover, the Fe doped in the composite decreased the band gap of the photocatalyst and accelerated the separation of the photogenerated electron and hole [48], thereby increasing the photocatalytic activity. The photocatalytic disinfection performance is greatly affected by the reaction temperature. In Wang’s study, when the reaction temperature varied in a range of 35–55 °C, the TiO₂/kaolinite showed the highest disinfection efficiency at 55 °C because of the faster cell death at higher temperatures [26].

To improve the utilization of solar energy and regulate the rate and path of photo-generated electron and hole pairs, Li et al. fabricated a novel sandwich structure g-C₃N₄/TiO₂/kaolinite composite using a mild sol-gel method, which is an efficient antimicrobial photocatalyst [35]. Under the visible-light irradiation, the sandwich structure g-C₃N₄/TiO₂/kaolinite displayed better disinfection performance on bacterial S. aureus than bare g-C₃N₄, TiO₂, and kaolinite. The CuO-ZnO/kaolinite composite synthesized by Ugwuja et al. [49] showed improved efficiency on E. coli disinfection compared to CuO/kaolinite and ZnO/kaolinite, because the CuO-ZnO/kaolinite nanocomposite was operated by three disinfecting mechanisms, i.e., electrostatic interaction, metal toxicity of the leaching Zn and Cu, and photocatalysis.

5. Adsorption of Heavy Metals

Heavy metal pollution has attracted much attention due to its serious damages to water, air, and soil. Adsorption is one of the most efficient approaches to remove heavy metals from the environment [17,23,50]. Heavy metal adsorbents mainly include carbon materials derived from biological wastes (activated carbon), clay minerals (kaolinite), and so on [4,51]. Therefore, kaolinite possesses research value for the adsorption of heavy metals due to its advantages of having a wide source, low cost, and no secondary pollution to the environment by using the physical adsorption principle [52]. However, kaolinite showed poor performance as an adsorbent to removal heavy metals such as Fe(III), Ni(II), Co(II), and Cd(II) [14,15,53]. To improve the adsorption capacity of kaolinite, different methods such as thermal modification, acid modification, transition metal modification, and organic modification were adopted [54].

5.1. Natural Kaolinites

Natural kaolinites show a certain ability to adsorb heavy metals, e.g., Ni(II), Zn(II), and Cu(II). The adsorption performance is affected by operating parameters such as reaction time, solution pH, and temperature. Yavuz et al. [55] removed the heavy metal Ni(II) by using natural kaolinite as an adsorbent. The adsorption of Ni(II) on kaolinite fitted the Langmuir adsorption equation well with a Langmuir constant of Ni(II) of 1.669 mg/g. The adsorption of Ni(II) was significantly affected by the adsorption temperature because adsorption is a endothermic process, and the adsorption rate will be accelerated at high temperatures. Shahmohammadi et al. [56] investigated the removal of Zn(II) by raw kaolinite in aqueous phase. The adsorption capacity for Zn(II) was measured to be 4.95 mg/g, indicating kaolinite is an effective adsorbent to remove Zn(II). Chai et al. [57] studied the adsorption of Zn(II) by kaolinite under different operating conditions. The results revealed that the adsorption equilibrium was achieved in 60 min at 25 °C, the maximum adsorption capacity reached 6.35 mg/g, and the adsorption efficiency exceeded 95%. Chai et al. also pointed out that the mechanism of Zn(II) interaction with kaolinite includes surface complexation, cation exchange, electrostatic attraction, and precipitation. Moreover, the adsorption capacity increased with the increment of the reaction time, the solution pH and the operating temperature. Farsi et al. [58] evaluated the removal of Cu(II) by kaolinite in an aqueous solution. The results revealed that solution pH and operating temperature significantly affect the removal efficiency on Cu(II), i.e., when the pH level increased from 2 to 6, the removal efficiency gradually increased. Moreover, the removal efficiency improved when the operating temperature increased from 25 °C to 35 °C.

However, pure kaolinite showed poor surface hydroxyl activity and low cation exchange, leading to the poor adsorption selectivity and easy desorption of heavy metals, which restrains the practical application of kaolinite. Therefore, researchers are devoted to modifying kaolinite to obtain better performance, and the current modification methods for kaolinite mainly include heat treatment, acid modification, metal modification, inorganic salt modification, and organic modification [59].

5.2. Thermal-Modified Kaolinite

Thermal modification refers to the heating of kaolinite at a certain temperature so as to effectively remove H₂O molecules, carbonates, and other organic impurities attached to the pore structure of kaolinite [60]. The disappearance of different forms of water molecules such as adsorbed water, crystalline water, and structural water can effectively increase the specific surface area and the pore volume of kaolinite, and the removal of structural water will transform it into metakaolinite, which exposes it to the generation of a large number of sites for reaction with acid. Meanwhile, its activity is enhanced and easily reacts with acid, which eventually improves the ion exchange performance and adsorption performance of kaolinite for pollutants [61]. Yan et al. [62] converted natural kaolinite into amorphous metakaolinite through a simple thermal activation method, leading to the removal of structural water in kaolinite, and formed two-dimensional sheet morphology with the increased surface area. TiO₂ was then attached to the thermal activation kaolinite to prepare a highly dispersed and stable TiO₂/modified kaolinite composite, which was used as an adsorbent to remove Pb(II). The result showed that the highly dispersed titanium species on the surface of the composite could react with the exposed hydroxyl groups in kaolinite to form the titanium hydroxyl groups. The strong bond connection between the formed titanium hydroxyl groups and Pb(II) caused the high adsorption capacity of Pb(II). It was known that the high hydrophobicity of TiO₂ restrained its real application in water purification. In comparison, the modified kaolinite is hydrophilic, thus the TiO₂ dispersed on the modified kaolinite gave a hydrophobicity–hydrophilicity balance. Therefore, the Pb(II) adsorption capacities via TiO₂/modified kaolinite composite (184 mg/g) were much higher than the modified kaolinite (64 mg/g). Bhattacharyya [15] reported that a thermal modified kaolinite prepared by calcination at 600 °C was efficient for the adsorption of heavy metals (Pb(II), Cd(II), and Cu(II)) because the calcination at the high temperature brought out the collapsed kaolinite, giving rise to the porous structure on kaolinite.

5.3. Acid-Modified Kaolinite

Acid modification refers to the pretreatment of kaolinite by organic or inorganic acids to remove impurities carried by kaolinite itself [63]. Acid modification of kaolinite not only increases the surface acidity and specific surface area of kaolinite, but also results in the production of a large number of pore structures [64]. Bhattacharyya and Gupta [65] treated kaolinite with H₂SO₄ to further improve its properties. The modified kaolinite with H₂SO₄ opened the edges of kaolinite sheet crystals, resulting in the increased pore size and the enlarged specific surface area, and the adsorption rate constant of kaolinite for Cu(II) increased from 9.5 × 10⁻² g/mg·min to 12.0 × 10⁻² g/mg·min. Chai et al. [66] prepared an acid-activated kaolinite by refluxing the kaolinite in the concentrated H₂SO₄ solution. This H₂SO₄ activated kaolinite showed excellent adsorption performance on both Ni(II) and Cu(II). Moreover, the H₂SO₄ activated kaolinite possessed a higher adsorption capacity than raw kaolinite, and the maximum adsorption capacities of H₂SO₄ activated kaolinite towards Ni(II) and Cu(II) achieved 39.62 mg/g and 41.11 mg/g, respectively, while the adsorption capacity of raw kaolinite was slightly lower, showing 37.64 mg/g for Ni(II) and 40.01 mg/g for Cu(II), respectively. The improvement in the H₂SO₄ activated kaolinite should be ascribed to two reasons. First, the acid-activated kaolinite became smaller in size and appeared to be disintegrated, which increased the specific surface area by 84.58%. Second, the acid treatment with H₂SO₄ changed the surface property of kaolinite, such as the number of effective binding sites and surface negative charges. Unuabonah et al. [67] prepared a humic-acid-modified kaolinite and used it for the adsorption of several heavy metals, including Pb(II), Cd(II), Zn(II), Ni(II), and Cu(II). It was found that using humic acid for kaolinite modification partially increased the total charges on the kaolinite surface, which further increased the cation exchange capacity. Therefore, the adsorption capacity of the humic-acid-modified kaolinite on heavy metals was increased.

5.4. Co-Modified Kaolinite with Thermal Treatment and Acid Treatment

Although thermal modification and acid modification alone can enhance the properties of kaolinite, previous studies revealed that the co-modified kaolinite by both thermal modification and acid modification showed higher microporous volume and specific surface area than the sole [68]. Abdallah [69] modified the kaolinite with acid treatment with 2 mol/L HCl followed by a heating treatment at 600 °C. The co-modification increased the adsorption properties of natural kaolinites by enhancing the cation exchange capacities. The crystal structure of kaolinite transformed to a collapsed structure after being co-modified with acid treatment and thermal treatment, and the collapsed kaolinite possessed a large number of porous structures, which assisted in the massive adsorption of heavy metals Zn(II) and Cu(II) from wastewater. Moreover, the co-modified kaolinite showed a lower Zn(II) adsorption than Cu(II) because when the Zn(II) desorbed into the solution, it could be combined with the chloride ions to form a stable chloride complex, which could not be re-adsorbed by the co-modified kaolinite.

5.5. Transition Metal-Modified Kaolinite

Transition metal modification can introduce transition metals between kaolinite layers or on the surface to protonate the surface and broaden the application range of kaolinite; because the transition metal ions are positively charged, they have strong electrostatic attraction and ligand exchange with the negatively charged natural kaolinite minerals [70]. Ç. Üzüm et al. [71] synthesized zero-valent iron (nZVI) with kaolinite (nZVI/Kaolinite), which showed a significant effect on the iso-electric point. The iso-electric point for the natural kaolinite and the nZVI/kaolinite is pH 4.2 and 7.0, respectively. Therefore, the nZVI/kaolinite is more suitable for removing heavy metals at environmental pH levels. Moreover, the adsorption capacity of Cu(II) is obviously related to the modified amount of nZVI in the nZVI/kaolinite composite, where the removal capacity on Cu(II) increased from 32 mg/g to 140 mg/g as the mass ratio of nZVI/kaolinite increased from 0.2/1 to 1/1. However, the uptake capacity of Co(II) increased slightly from 23 mg/g to 25 mg/g. This different adsorption performance was explained by the removal of Co(II), which was determined through adsorption and precipitation, while the removal of Cu(II) was mainly attributed to a reduction reaction with zero valent iron, leading to the insoluble Cu₂O and Cu(0) phases. Modified kaolinite with aluminum sulphate (Al₂(SO₄)₃) [72] was carried out as adsorbents (Al₂(SO₄)₃/kaolinite) to remove heavy metal Pb(II) from the aqueous phase. It was found that the adsorbed amount of Pb(II) with Al₂(SO₄)₃/kaolinite was 4.5-fold higher than that of raw kaolinite. This is because the layers of kaolinite were broken after the modification, leading to the formation of nano/microsized particles with a big surface area. Moreover, ion exchange was another important mechanism for Pb(II) removal. The kaolinite modification with Al₂(SO₄)₃ (Al₂(SO₄)₃/kaolinite) improved the ion exchange capacity because the cation exchange capacity of the Al₂(SO₄)₃/kaolinite is 2.5 times that of the raw kaolinite. Ahmet Sari et al. [73] investigated the adsorption efficiency of MnO₂/kaolinite for the removal of cadmium ions Cd(II) from waste water. The results revealed that a Mn-OH group was formed by the introduction of MnO₂, thus the adsorption of Cd(II) was mainly carried out through chemical ion exchange. Moreover, the higher negative charge density of MnO₂ generated on the modified kaolinite surface increased its adsorption capacity, where the adsorption capacities of MnO₂/kaolinite and kaolinite for Cd(II) were 36.47 and 14.11 mg/g, respectively. Egirani et al. [74] modified the kaolinite with copper oxide (CuO/kaolinite) via the titration method and then used it on Pb(II) removal from water. The modification with CuO gave a good adsorption on Pb(II) because the Pb(II) adsorption by CuO/kaolinite was mainly controlled by the reorganization of active sites. Abou-El-Sherbini et al. [75] explored a ZrO₂/kaolinite adsorbent for the Cd(II) removal. The insertion of ZrO₂ improved the laminar packing of kaolinite and created new micropores, which increased the specific surface area by 250%. As a result, the adsorption performance of ZrO₂/kaolinite was three times higher than that of unmodified kaolinite.

5.6. Organic-Modified Kaolinite

Inorganic salt modification improved the adsorption properties of kaolinite via changing the surface morphology and the stretching vibrations of Al-O and Si-O bonds without destroying the crystal structure of kaolinite [76]. There are still limitations in some of the modified methods mentioned above, especially acid modification, which may lead to structural decomposition of kaolinite and thus affect the adsorption of heavy metals through some groups located at the edges of kaolinite [77]. Therefore, these drawbacks have accelerated the research on the organic modification of kaolinite, which can impart different surface properties to clay minerals depending on the type and nature of the loaded organic functional groups [78]. Organic modifications are often used to modify kaolinite with polymers, surfactants, and other organic substances [79]. These organic modifications enable the introduction of highly reactive groups, such as -SH and -NH₂, attaching to the surface of kaolinite, resulting in high adsorption capacity and unique selectivity for heavy metals [80]. Bhattacharyya and Gupta studied the removal of Cu(II) from the aqueous solution [81]; in order to improve the adsorption performance on Cu(II), tetrabutylammonium (TBA) was used to modify kaolinite, and the intercalation of TBA formed a porous skeleton in kaolinite. The specific surface area of TBA/kaolinite was 3.7 times that of the raw kaolinite. E.I. Unuabonah et al. [82] synthesized a polyvinyl alcohol (PVA) modified kaolinite (PVA/kaolinite) with excellent adsorption capacity on heavy metals Pb²⁺ and Cd²⁺ as the adsorption capacity of the PVA/kaolinite increased to 29.85 mg/g, which was six times higher than that of unmodified kaolinite. Anna Koteja et al. [83] obtained organically modified kaolinite by replacing the intercalation layers with diethanolamine (DEA) and triethanolamine (TEA). It was found that the modified kaolinite with both DEA and TEA showed higher adsorption capacity on the heavy metals of Zn(II), Cd(II), Cu(II), and Pb(II) than the unmodified kaolinite.

6. Removal of Gas Phase Pollutants

Apart from the application of kaolinite in photocatalytic pollutant degradation, disinfection, and adsorption, it still presents some applications in air pollutants, such as carbon dioxide (CO₂) capture, the removal of volatile organic compounds (VOCs), etc. In this part, the applications on the gas phase pollutants of CO₂ capture and the VOCs thermal catalysis are summarized.

6.1. CO₂ Reduction and Adsorption

The combustion of fossil fuels leads to the increase of CO₂ content in the atmosphere and intensifies the greenhouse effect. Therefore, reducing CO₂ to avoid greenhouse effect is the focus of current research. CO₂ photocatalytic reduction refers to the use of light to simulate a photosynthesis system. First, CO₂ is adsorbed on the catalyst, then electron hole pairs are generated and separated. Subsequently, it reacts with CO₂ and H₂O adsorbed on the photocatalyst surface, and is finally converted into green and pollution-free substances such as methane and methanol. The use of kaolinite as a carrier for the photocatalyst can reduce CO₂ to hydrocarbons. Kočí et al. [84] prepared a TiO₂/kaolinite composite via hot solution method. It was found that the grain size of TiO₂ decreased when it deposited on the surface of kaolinite, leading to the fast transport of photogenerated electrons and holes and making the yield of methane and methanol produced by CO₂ photocatalytic higher than that of commercial TiO₂. Reli et al. [85] also found that the CO₂ photocatalytic reduction yield by TiO₂/kaolinite composites was affected by the size of TiO₂ nanocrystals. The calcination temperature and duration time in the TiO₂/kaolinite catalyst preparation significantly influenced the grain size of TiO₂. Moreover, the reduction efficiency of CO₂ was the highest when the TiO₂ grain size was smaller than 18–23 nm. Knížek et al. [86] modified the TiO₂/kaolinite through the addition of HCl, creating a clear edge with a band gap of 2.8 eV in the UV-vis reflectance spectrum, thus offering the HCl modified kaolinite a certain degree of photocatalytic ability on CO₂ reduction. Chen et al. [87] synthesized g-C₃N₄/kaolinite composites via an in-situ synthesis method. The g-C₃N₄/kaolinite presented the specific surface area of 61.9 m²/g, which is three times larger than that of raw kaolinite, originating from the intercalation reactions that occurred between the kaolinite and melamine (g-C₃N₄ precursor). This augment increases the active sites’ number and the charge transfer. Therefore, the photocatalytic activity of the g-C₃N₄/kaolinite was better than bare g-C₃N₄ and kaolinite.

Schaef et al. [88] studied the adsorption of CO₂ onto kaolinite using density functional theory (DFT) methods, which suggested that the interactions between CO₂ and the kaolinite surface resulted in the increase of CO₂ adsorption to 0.34 g/cm³. A previous study proved that Al in the electronic structure of kaolinite could be replaced by other elements to change its adsorption capacity for CO₂ [89]. Hou et al. [90] calculated the variation of electronic structures when the Al element in kaolinite was substituted by Si element, indicating that the Si dopant induced a strong interaction between kaolinite and CO₂, which led to a significant enhancement on the adsorption capacity of CO₂. This improvement benefited from the small radius of Si ion, and the distance between the kaolinite and CO₂ became shorter after replacing.

6.2. Removal of VOCs by Adsorption, Photocatalysis, and Catalytic Combustion

In recent decades, with the fast application and development in industry, VOCs have received widespread attention due to their serious impact on atmospheric environmental pollution and human health [91]. Kaolinite is also often used as a carrier or to prepare component sub-sieves to remove VOCs.

The porous structure of kaolinite provides a big surface area for VOC adsorption. Liu et al. [92] reported that the kaolinite showed excellent methane adsorption performance compared to other clay minerals such as montmorillonite and illite. Heller et al. [93] revealed that the factors affecting the maximum adsorption capacity of kaolinite were the specific surface area and the amount of adsorbed water. Deng et al. [94] found that the adsorption of benzene mainly occurred on the external surface of kaolinite. The procedure of hearting kaolinite at 120 °C increased the adsorption of benzene because the adsorption active sites occupied by H₂O molecules were exposed by heating, and the specific surface area was increased accordingly. Lain et al. [95] found that the kaolinite could selectively remove toluene and phenol in the atmosphere. The results revealed that its adsorption of toluene (−68.2 kJ/mol) and phenol (−87.1 kJ/mol) is stronger than that of CO₂ (−35.0 kJ/mol) and H₂O (−62.4 kJ/mol). Swasy et al. [96] prepared a polyethylenimine-functionalized kaolinite to adsorb butyric acid, butyraldehyde, and dimethyl disulfide vapor. The raw kaolinite was poor in the remediation of these VOCs. Fortunately, the polyethylenimine-functionalized kaolinite showed an excellent ability on VOC capture, and the removal efficiency for butyraldehyde, dimethyl disulfide, and butyric acid reached 100%, 99%, and 90%, respectively.

Dong et al. [10] prepared a “sandwich” structured BiOCl/g-C₃N₄/kaolinite composite by fixing g-C₃N₄ and BiOCl layer by layer on the kaolinite surface, which showed a high formaldehyde photodegradation efficiency of 74.55% under visible light. This “sandwich” heterojunction built the close interface contact among BiOCl, g-C₃N₄, and kaolinite, which effectively promoted the transport and separation of photogenerated electron and hole pairs, and enhanced the photodegradability on formaldehyde. Mora et al. [97] prepared the TiO₂/kaolinite and used it as a photocatalyst in the degradation of VOCs’ toluene. The study showed that the thermal treatment on TiO₂/kaolinite by calcination at 400 °C was an effective approach to improve the toluene degradation performance (up to 90%) because the thermal treatment strongly affected the structural evolution by changing the clay from kaolinite to metakaolinite, and TiO₂ was in an anatase phase at 400 °C. Kibanova et al. [98] prepared a TiO₂/kaolinite composite using the sol-gel method and examined the photocatalytic efficiency for the removal of toluene and d-limonene, which exhibited a comparable photocatalytic performance to the commercial TiO₂ (Degussa P25). A CeO₂-MnO_x/kaolinite [99] was prepared and showed an excellent thermal-catalytic property on benzene. The molar ratio of Mn/Ce significantly affected the activity for benzene combustion. Among different Mn/Ce ratios, the Mn/Ce = 9:1 showed the highest catalytic degradation performance on benzene, and the benzene could be completely oxidized at 260 °C. Moreover, the performance was kept almost constant via a continuous reaction of 800 h. The good thermal-catalytic performance of the CeO₂-MnO_x/kaolinite mainly originated from the well dispersion of CeO₂ nanoparticles on the catalyst surface, and the high oxidation state of MnO_x.

7. Conclusions and Perspective

Kaolinite is a rich mineral in China. Its advantages of being low-cost, non-toxic, environmentally friendly, and stable make it a good candidate for a carrier or adsorbent of environment decontamination. In this review, the utilization of kaolinite as the photocatalysts’ carrier was investigated by constructing the ZnO/kaolinite, g-C₃N₄/kaolinite, g-C₃N₄-TiO₂/kaolinite, and CeO₂-g-C₃N₄/kaolinite composites. Their synthesized methods, photocatalytic performance, and the intrinsic mechanisms were deeply discussed for organic pollutant degradation and disinfection. The efficient approaches for enhancing the photocatalytic performance included increasing the specific surface area, expanding the light absorption from UV light to visible light, changing the surface properties of kaolinite, and so on. In addition, we analyzed the application of kaolinite as a carrier for nano Ag disinfection. The adjustment of the morphology of Ag/kaolinite, combined with the functions of magnetic and organic antibacterial agents, also significantly improved the disinfection performance. Moreover, kaolinite can be used as an adsorbent for heavy metal removal. The removal performance was improved by thermal modification, acid modification, transition metal modification, and organic modification. Finally, a few applications in CO₂ capture and VOC removal were considered.

Despite a few valuable studies based on kaolinite being reported, the application of kaolinite as a carrier for nano-catalyst and nano-disinfectant, as well as an adsorbent, is still in the primary stage and needs to be further improved. In addition, the application of kaolinite in industry is still limited because the modification of kaolinite is difficult to achieve. In order to better solve the problem of environmental decontamination and alleviate the crisis of energy supply, the research on kaolinite-based composites should focus on the following aspects: (1) The photocatalytic mechanism of the synergistic effect between photocatalysts and kaolinite should be deeply investigated. (2) It is better to find convenient methods to grow Ag nanoparticles in the interlayer of the layered structured kaolinite, which can effectively control the size and prevent the agglomeration of Ag nanoparticles. (3) Using the modified kaolinite as an absorbent only cannot meet the needs of actual heavy metal removal. Therefore, the effective combination of kaolinite with other treatment processes should be used to satisfy the demand. (4) Future efforts should focus on synthesizing kaolinite-based materials with energy saving and low-cost methods.