The most prevalent synthesis method for exfoliating vermiculite nanosheets is the two-step ion-exchange method [8]. This method is more environmentally friendly than the method proposed by Hummer, which employs acids to separate GO from graphite. Initially, the vermiculite particles are subjected to thermal expansion, whereby the evaporation pressure of water in the interlayer space facilitates the formation of thermally expanded vermiculite (Fig. 1c) [17]. The vermiculite interlayer typically contains naturally occurring Mg²⁺ and may also include Ca²⁺ or Fe²⁺ [18]. The hydration capacities of these cations are relatively weak. Consequently, the number of intercalated water molecules within the interlayer space is relatively limited and the interactions between the layers are stable, rendering delamination a challenging process. To facilitate the exchange of these cations with cations that possess a stronger hydration capacity, the interlayer ions are replaced with Na⁺ by heating under reflux in an aqueous solution of NaCl. Subsequently, the material is vacuum-filtered and washed with water and ethanol to remove any residual NaCl. The resulting Na-exchanged vermiculite is placed in an aqueous LiCl solution and heated under reflux to replace the interlayer ions with Li⁺. As described in detail in a subsequent section on the application of this process, Li-exchanged vermiculite (LiV) exhibits superhydrophilic properties. Consequently, it can be readily dispersed in water through relatively simple techniques such as ultrasonication. Furthermore, the LiV aqueous solution can be readily fabricated into thin-film membranes via vacuum filtration (Fig. 1d).

Pure vermiculite is widely regarded as a non-toxic and chemically stable mineral with a variety of industrial, agricultural and environmental applications [26, 27]. However, concerns have been raised regarding potential contamination with asbestos, particularly amphibole asbestos found in certain vermiculite deposits. While pure vermiculite itself does not pose significant inhalation hazards, it has been documented that vermiculite mined from mines contaminated with asbestos has been associated with the development of severe respiratory and pulmonary diseases [28, 29]. To mitigate the risks associated with vermiculite mining and processing, modern industries implement strict safety measures, including advanced ore purification processes and asbestos screening techniques. Regulatory agencies such as the Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA) have established guidelines to ensure that commercially available vermiculite products are free from hazardous asbestos contamination [30]. While vermiculite itself is not inherently toxic, sourcing it from certified suppliers who adhere to safety regulations and conduct rigorous testing is essential to prevent health hazards. Continued research and monitoring of vermiculite deposits remain crucial for ensuring its safe use. The two-dimensional nanosheet form of vermiculite is improbable to contain contaminants, as it is processed in solution, separated into nanosheets, washed and filtered. Therefore, vermiculite nanosheets can be considered pure and are unlikely to be biologically hazardous, as they are composed of hydrated magnesium aluminum silicate, which is considered biologically inert.

The study of 2D nanomaterials has been a prominent field of research in numerous disciplines since their emergence. The discovery that 2D nanomaterials such as GO form angstrom-scale interlayer spaces with lamellar structures when stacked layer-by-layer demonstrates their potential as membranes for ion and molecular separation.

Similarly, vermiculite can be dispersed in an aqueous solution and fabricated into a membrane with a lamellar structure through filtration. As previously stated, vermiculite membranes have been documented to possess an interlayer distance of 12–14 Å, which is analogous to the interlayer distance of GO membranes in a wet state. Moreover, vermiculite possesses a distinctive attribute: the interlayer distance and membrane characteristics can be altered by introducing cations into the interlayer. Consequently, research has been conducted to employ this membrane as a multifunctional membrane for the simultaneous separation of ions and molecules.

Nair et al. were the first to demonstrate that the properties of vermiculite membranes can be modified by the insertion of cations between the layers. Utilizing the ion-exchange properties of vermiculite, as discussed in Section 2.2.2 [8], the researchers incorporated K, Li, Ca, La, and Sn into the interlayer space, adjusting the distance from 12.6 to 15.2 Å. This resulted in a notable change in the wetting properties of water, which shifted from superhypophilic to superhydrophobic, spanning a range from 15.1° (Li) to 101° (Sn) (Figs. 2a and b). Superhydrophilic surfaces can maintain a hydration layer that can repel foulants such as biological molecules and oils, thereby exhibiting self-cleaning properties. Studies have reported that LiV membranes possess superhydrophilic properties, which enable the formation of a surface hydration layer during oil–water separation. This layer prevents oil droplets from wetting the membrane surface, thereby maintaining the ability to permeate only water continuously. In the case of oil–water emulsions (oil droplets in water emulsions), the tendency of oil droplets to adhere to the membrane surface and permanently foul the membrane surface is particularly pronounced. The use of LiV membranes has been demonstrated to prevent this phenomenon and maintain sustainable separation properties (Figs. 3a and b).

Owing to its superhydrophilic nature, LiV was employed in a two-step ion-exchange process to exfoliate vermiculite into single sheets. The hydrophilic nature of Li facilitates its wide-spread use as a membrane material. Tian et al. demonstrated that LiV membranes exhibit consistent performance even when subjected to harsh conditions for molecular and ion separation [19]. In organic solvent nanofiltration experiments conducted to assess the molecular-separation properties, LiV with an interlayer distance of 14.4 Å exhibited typical viscous flow characteristics, with permeance dependent on the viscosity of organic solvents in the permeation experiments involving various organic solvents (Fig. 3c). Moreover, in the separation experiments with diverse dye molecules, LiV demonstrated superior molecular sieving, with a significant rejection rate for molecules larger than 452.37 g/mol (orange blue) (Fig. 3d). Furthermore, the measurement of the ion-transport behavior revealed that Mg²⁺ exhibited a higher permeance, despite the fact that its hydration diameter is larger than that of Na⁺. Furthermore, the permeance of chloride salts (MgCl₂ and NaCl) was observed to exceed that of sulfate salts (MgSO₄ and Na₂SO₄), thereby demonstrating an anion valence-dependent outcome (Fig. 3e). The current–voltage curves for potassium chloride solutions at varying concentrations demonstrated that ion transport at elevated concentrations exhibited characteristics similar to those observed in the bulk solution. However, at lower concentrations, the curves exhibited a plateau-like behavior (Fig. 3f). This result indicates that the ion-transport behavior through the vermiculite membrane is predominantly governed by the surface charge rather than size exclusion, owing to the high negative charge of the membrane.

Selective ion permeation using vermiculite membranes has been previously reported. Zhang et al. synthesized membranes not only from vermiculite but also from montmorillonite, another natural clay 2D nanomaterial, and mica [21]. The clay membranes exhibited typical surface-charge governed ionic transport behavior and reported the unique ion transport properties of each membrane through asymmetric structural modification in the form of isosceles trapezoidal (Fig. 4a). The conductance of K⁺, Li⁺ and Na⁺ in the asymmetric montmorillonite membrane with forward bias was much higher than that with reverse bias (Fig. 4b and c). This indicates that the geometry-based asymmetric factor causes unidirectional conductivity, the so-called ionic rectification phenomenon (ICR). In the case of the mica membrane, Li⁺ and Na⁺ pass through the channels of the mica membrane, whereas for the naturally intercalated K⁺ showed a reciprocal motion that resulted in a periodically fluctuating current (Fig. 4d). A 6000-fold difference in the permeation rates of Li⁺ and K⁺ was observed (Fig. 4e). These results indicate that the mica surface has a natural affinity and recognition effect for K⁺ ions. In ion-permeation measurements with an asymmetric vermiculite membrane that is naturally intercalated with Mg²⁺, the permeation rate of Li⁺ was measured to be 856 times higher than that of Mg²⁺ (Fig. 4f), indicating that, similar to mica membranes, vermiculite membranes also have a recognition effect on naturally intercalated cations. Therefore, natural clay 2D materials can be applied as membranes for selective ion permeation and separation from brine or in electrodialysis systems to extract and recover valuable resources, such as Li, by showing high permeation rates for other cations, in addition to the recognition effect on naturally intercalated cations. Pang et al. chemically modified vermiculite with sulfonated polyvinyl alcohol to introduce sulfonated groups into the interlayer and enhance the selective transport of Li ions [20]. This vermiculite membrane selectively permeates Li ions with a more compact hydration structure via a self-confinement ion-recognition mechanism. The selectivity values for Li⁺/Mg²⁺, Li⁺/Na⁺, and Li⁺/K⁺ were 23.8, 14.9, and 19.1, respectively, demonstrating the selectivity of this membrane for Li ions. Therefore, they reported that it could be utilized for the selective extraction and separation of Li.

Tian et al. reported a multifunctional Cobalt-vermiculite membrane (Co@VMT) intercalated with Cobalt for water purification (Fig. 5) [11]. Cobalt can effectively activate peroxymonosulfate (PMS) to produce reactive oxygen species (ROS), which can degrade pollutants such as dyes, pharmaceuticals, and phenols. The ranitidine removal experiment of Co@VMT with PMS filtration showed a 100% removal rate and water permeability that was two orders of magnitude higher than that of LiV. Therefore, Co@VMT is a multifunctional water-treatment membrane that can simultaneously perform advanced oxidation processes and membrane filtration via the catalytic role of Cobalt. The results of this study can be regarded as a representative example of the utilization of functionalized interlayer ions by exploiting the easy replacement of interlayer ions in vermiculite. Similarly, TiO₂@VMT, fabricated by inserting TiO₂ between vermiculite sheets, showed a removal rate of over 98% and permeability of 261 L m⁻² h⁻¹, which was 14 times higher than that of LiV, for dyes above 374 g/mol⁻¹ molecular weight in a filtration study of organic dyes [22]. In addition to the filtration function of this membrane, it exhibits antifouling properties owing to the photocatalytic degradation ability of TiO₂ for organic pollutants.

Pervaporation (for liquid mixtures) and vapor permeation (for vapor mixtures), which are methods for separating liquid or vapor mixtures through membranes, have recently gained attention. The most widely used conventional distillation technology has the disadvantages of high energy consumption and difficulty in separating azeotropes. The separation of azeotropic mixtures by distillation requires the addition of additives (third components such as entrainers and extractants), which causes environmental problems. Therefore, the development of novel energy-efficient and environment-friendly separation technologies is required. Pervaporation and vapor permeation are membrane-based processes for the selective separation of mixtures of liquids or vapors by partial vaporization through a nonporous or porous membrane that acts as a selective barrier between the feed and permeate [31]. A membrane with selectivity for one component of a mixture is a key factor. Studies have reported that GO membranes can be used for the dehydration process for the advanced purification of chemicals by selectively permeating only the water component in a water-containing mixture owing to its hydrophilic nature [32–34]. Moreover, vermiculites, such as GO, are hydrophilic and can be used as a membrane for pervaporation or vapor permeation. Zhang et al. reported that a membrane composed of GO and vermiculite exhibited superior performance in n-butanol dehydration [23]. In this mixed membrane, the applied LiV formed multiple in-plane pores via acid etching to shorten the mass transport distance. The hydrophilicity, permeability, and separation factor were controlled by the GO content. Notably, the GO-vermiculite membrane exhibited stability.

As mentioned in Section 3.1, vermiculite provides 2D nanochannels that enables selective ion transport. This suggests that it functions as a proton (H⁺) conduction channel. Shao et al. reported interesting results on the proton-transport properties of LiV membranes [10]. They determined that the proton-transport mechanism of the LiV membrane exhibited a surface-charge-governed transport characteristic, similar to the transport of other ions, and the isotope and temperature effects revealed that the proton-transport mechanism is based on hopping through water molecules in the interlayer space. Vermiculite has a good intrinsic thermal stability and can be rehydrated by annealing at high temperatures. After annealing at 500°C, the proton conductivity was measured to recover its pre-annealing value (1.2 × 10⁻³ cm² V⁻¹ s⁻¹), demonstrating the unique property of vermiculite membranes of maintaining stable proton conduction at high temperatures. To improve the thermal stability of cellulose used as a separator in Li-ion batteries, Gu et al. synthesized a cellulose–vermiculite composite membrane [35]. Vermiculite not only enhanced the hydrophilicity of the cellulose membrane, but also maintained its structure after annealing at 200°C, which is higher than the decomposition temperature of 135°C for pure cellulose. Moreover, the proton conductivity of the blended film measured at 100°C (0.043 S cm⁻¹) was almost twice that of pure cellulose (0.024 S cm⁻¹), demonstrating stable proton transfer properties at high temperatures. Owing to its superior thermal stability, vermiculite is expected to open a new field of ion-transport membranes with stable performance at high temperatures.

Greenhouse gases, such as CO₂, which have recently been blamed for rising global temperatures, cause greenhouse effects and climate change. Therefore, technologies to capture and recycle greenhouse gases have attracted considerable attention. Li et al. reported a CO₂ adsorbent prepared by the interlayer activation of amines with polyethyleneimine (PEI) on acid-activated expanded vermiculite-derived silica (AEV) with a hierarchical layered porous structure for CO₂ capture [36]. This adsorbent exhibits high CO₂ uptake performance (2.21 mmol/g @ 75°C) owing to the unique hierarchical porous structure and abundant Lewis acid sites, which enhance the amine loading efficiency. Furthermore, the AEV/PEI CO₂ adsorbent was reported to have high cyclic stability, showing only 1% decay after 10 cycles. Similarly, Zhao et al. reported a vermiculite PEI CO₂ adsorbent [25]. Vermiculite was acid etched to create less convoluted and shorter nanochannels for the efficient capture of CO₂ gas. The porous vermiculite was then modified with PEI. Owing to its abundant amino groups, this adsorbent can reversibly react with CO₂ in a humidified state. They reported 568 gas permeation units (GPU) for the permeance of CO₂ and a selectivity of 27.6 for CO₂/CH₄, exceeding the performance of membranes synthesized from other 2D nanomaterials. Thus, the results of the two previous studies suggest that vermiculite is a superior building block material that can increase the loading of amine groups owing to its unique layered and porous structure.

Another application that can exploit 2D nanochannels to allow selective ion transport in vermiculite films is energy harvesting. Deka et al. reported a pressure-responsive energy-harvesting device that exploited the water-holding capacity of vermiculite [24]. By doping one side of the vermiculite membrane with poly(diallyldimethylammonium chloride), a cationic polymer that can hold water, and placing it in contact with an Al foil under pressure, the device generates an interfacial electrochemical potential driven by the intrinsic difference in the redox potential. H⁺ migrates to the negatively charged vermiculite surface and OH⁻ migrates to the Al surface, where it reacts and releases electrons to generate electricity (Fig. 6a). Interestingly, this vermiculite-based power generation device was shown to retain its ability to generate electricity even after exposure to a direct flame (Fig. 6b), liquid nitrogen (−195 °C), high-temperature heat pulses (450 °C), water vapor, and mechanical stress (100 N) (Figs. 6c–e). Cao et al. reported the use of lamellar porous vermiculite membranes to enhance osmotic energy conversion from salinity gradients [37]. These membranes feature artificial in-plane nanopores that reduce the ion-diffusion resistance and increase the ion flux, resulting in a significant increase in the energy conversion efficiency. The study reported a 16-fold increase in power density to 10.9 W/m², outperforming several existing 2D lamellar membranes.

Vermiculite is reported to have a significantly low thermal conductivity (<0.1 W m⁻¹ K⁻¹) [38] and superior thermal insulation properties [39] resulting from the phonon scattering at the interlayer interface. Owing to these properties, studies have reported their excellent performance as a coating material or composite for fire retardants. Sethurajaperumal et al. reported a fire-retardant epoxy paint filled with vermiculite nanosheets (VN) (Fig. 7a) [40]. The fire-retardant properties of VN-epoxy nanocomposite paint were reported by brush coating a 100 μm thick layer of VN-epoxy paint on a wooden surface. The flame height was reduced in the VN-epoxy-paint-coated wood, resulting in a 62% reduction in heat released during combustion and a 42% reduction in thermal degradation. Researchers have also reported a 68.3% reduction in the combustion rate, thereby minimizing toxic gas emissions. Thus, it was experimentally demonstrated to be a superior material for a new flame retardant that minimizes the flame propagation speed and improves char formation (Figs. 7b–d). Cetin et al. reported the thermal stability and fire-retardancy properties of a composite synthesized by incorporating expanded vermiculite into a triblock thermoplastic elastomer copolymer, styrene-b-(ethylene-co-butylene)-b-styrene (SEBS) [41]. Thermal dimensional stability measurements showed that the thermal shrinkage of the composite containing 15 wt% vermiculite was reduced by 87% at 100°C compared to that of pure SEBS. Vertical flame spread analysis showed that the incorporation of vermiculite enhanced the flame retardancy. The increase in flame retardancy was attributed to the formation of a multilayer carbonaceous morphology as a result of the reaction between SiO₂ and Al₂O₃, and Fe₂O₃ was effective for radical trapping. Reports also indicate that the foamy morphology caused by the residue of the composite formed by combustion acts as a barrier layer to delay the spread of the flame by minimizing air contact, thereby reducing heat transfer. The low thermal conductivity, attributed to the layered structure of vermiculite, makes it an excellent material for fire retardancy.

As outlined in Section 2, the vermiculite characteristics are determined by its structural composition. This comprises an octahedral Mg layer situated between two tetrahedral silica layers, comprising two layers of SiO₂ and Al₂O₃ sandwiched by a layer of MgO and Fe₂O₃. Ji et al. presented a universal wet-chemical exfoliation method based on alkali etching that intelligently captured ultrathin and biocompatible functional core layers (MgO and Fe₂O₃) sandwiched between two identical tetrahedral layers (SiO₂ and Al₂O₃) from vermiculite (Fig. 8) [42]. These nanosheets, which comprise the U.S. Food and Drug Administration-approved MgO and Fe₂O₃, are cost-effective [42–45] and scalable and possess a tunable and appropriate electron band structure with a reduction in bandgap from 2.0 to 1.4 eV and increase in conductive band from −0.4 to −0.6 eV, making them suitable for broader biomedical applications. Their ability to combine photothermal, photodynamic, and chemodynamic therapies, as well as multiple imaging techniques (photoacoustic, photothermal, and fluorescence), make them highly efficient for cancer theranostics. Moreover, these nanosheets exhibited enhanced electronic properties such as a decreased bandgap and improved photothermal conversion, further facilitating the generation of ROS for therapeutic purposes. This allows these nanosheets to modulate the tumor microenvironment by catalyzing the conversion of hydrogen peroxide into oxygen and depleting glutathione, thus relieving hypoxia and reducing the antioxidant capabilities of tumors. Their study provides a clear example of how nanotechnology can evolve traditional Chinese medicine into innovative medical nanomaterials and emphasizes the potential of nanosheets for future applications in energy, photocatalysis, and biomedical engineering.