Characteristics and Applications of Aluminosilicate Clays: A Review

Abstract

Allophane, halloysite, and imogolite are aluminosilicate clays, whose structures mainly contain silicon and aluminum. However, their morphologies differ markedly: allophane occurs as hollow/porous nanospheres, imogolite as single-walled aluminosilicate nanotubes, and halloysite as multi-walled rolled nanotubes. These structural contrasts control surface hydroxyl chemistry, pH-dependent (variable) charge, porosity, and the accessibility of inner/outer surfaces, which in turn govern the performance of adsorption, catalysis/photocatalysis, enzyme immobilization, biomedical delivery, hydrogel, and membrane technologies. Here, we critically synthesize recent advances with an emphasis on structure–property–function relationships and on the transition from natural/synthesized extracts to engineered/functionalized nanohybrids. At the end of the review, we show that targeted control of morphology, surface charge, and surface functionalization consistently shifts selectivity and stability, enabling application-specific optimization. We also highlight priorities for future research and explain how they contribute to Sustainable Development Goals.

1. Introduction

Allophane, halloysite, and imogolite are natural aluminosilicate clays (ACs), whose characteristic dimensions fall within the nanometer scale (approximately 1–100 nm). These phases are major constituents of many volcanic ash soils (VASs; Andisols), formed through weathering of volcanic glass under typically acidic conditions and humid climates, where silicon (Si) and aluminum (Al) mobilization as well as reprecipitation drive the development of short-range-order minerals [1,2]. Andisols occur widely across volcanic provinces in the Asia-Pacific region, Europe, Africa, and Oceania [3], and the Americas, particularly along the Pacific Ring of Fire [4,5,6,7,8,9,10].

The aluminosilicate composition is built from Si tetrahedra and Al octahedra; however, these materials differ sharply in morphology and surface architecture: allophane forms hollow/porous nanospheres [11], imogolite forms single-walled nanotubes (NTs) [12,13], and halloysite forms rolled multi-walled NTs [14,15]. This morphological diversity translates into distinct distributions of Si-OH and Al-OH functional groups on inner versus outer surfaces, creating pH-dependent variable charge and highly reactive interfaces in aqueous environments. Moreover, allophane and imogolite exhibit a gibbsite-like Al-octahedral outer surface with Si-bearing groups oriented toward the inner surfaces. Halloysite exhibits kaolinite-like 1:1 layering with Si-rich outer and Al-rich inner surfaces [16,17]. This “inside–outside” chemistry is central to selective adsorption, confinement, and functionalization strategies that differentiate these nanostructures from conventional ACs. In fact, over the past few years, the number of publications on allophane, imogolite, and halloysite ACs has increased markedly (Figure 1a). This is because these ACs possess important physical properties, including a high specific surface area (SSA), hydrophilicity, and pH-dependent charge behavior, which enable their use across diverse fields [18,19].

Figure 1. The search was performed in the Scopus database (26 August 2026) using the keywords (a) “allophane” or “imogolite” or “halloysite” limited to articles’ titles, abstracts, and keywords, in the English language, and research articles, and (b) an increase in the number of reviews by year based on the keywords “allophane” or “imogolite” or “halloysite”.

Due to their chemical stability, biocompatibility, and low cost [20], a large body of work has explored their use in the adsorption of oxyanions [21,22] and heavy metals (HMs) [9,23], heterogeneous catalysis, photocatalysis [24], enzyme immobilization [25,26,27], membranes [28], hydrogel reinforcement [29,30,31], and biomedical carrier systems [32,33,34]. However, the literature is often fragmented by application area, reports noncomparable performance metrics, and underconnects outcomes to mechanistic surface chemistry. In addition, the surface properties, structure, and composition of ACs can be modified through physical, chemical, or intercalation treatments to enhance their performance. In the same way, ACs can also be integrated with other phases or matrices to form clay-based composite materials, producing hybrid systems with improved physicochemical properties and a broader range of potential applications [35,36]. However, until now, reviews of these three ACs and their clay-based composite forms have been limited (Figure 1b).

Based on the above, this review aims to address this gap by systematically examining allophane, halloysite, and imogolite as well as their clay-based composites through a structure–property–function framework, with particular emphasis on the differences between natural and synthetic materials and on how surface charge, structural features, and composition influence their properties and applications.

2. Aluminosilicate Clays

2.1. Formation

Allophane and imogolite are formed due to precipitation of soluble Al and Si and by the weathering of any parent material ranging from sandstone to granite [12,37]. The assembly and persistence of allophane or imogolite depend on the organic matter (OM) content and the rainfall regime. These ACs generally occur in soil horizons with low OM content, and Al can exist in inorganic complexes rather than organic acid complexes or humic substances [38]. Since the structure of allophane on a local scale is similar to that of proto-imogolite (the precursor of imogolite) [12], some studies note that imogolite and allophane are nanopolymorphs due to their local-scale structures [39]. The conversion from a spherical shape to a tubular shape occurs at an early stage of precipitation because of the edge tension in the proto-imogolite [39]. Allophane can be obtained from Andisols through a series of reactions that generally involve the removal of OM, iron (Fe) oxides, and nanometric fractions, but it can also be synthesized. For its part, imogolite is a paracrystalline secondary Al mineral [40] that forms a gel film on the surface of volcanic rocks. Some imogolites that originate in soil require a complex purification process because they often coexist with allophane and other ACs [41]. Hence, it is common to use synthetic imogolite, which is usually obtained through the sol-gel process [42], rather than natural imogolite. Like allophane and imogolite, halloysite is formed by weathering, pedogenesis, or hydrothermal alteration of ultramafic rocks, volcanic glass, and pumice. Although halloysite can be derived from the conversion of primary silicates (feldspars, amphiboles, and micas), its formation in Andisols is favored in humid tropical climates with low rainfall. This induces less desilication, unlike allophane, which is predominant in soils with a high rainfall regime [7].

Taken together, these findings indicate that formation reflects coupled control by Al and Si concentration, OM complexation, rainfall-driven desilication, and precursor chemistry rather than by pH alone. Closely related proto-imogolite building blocks can assemble into allophane nanospheres or imogolite NTs, thereby imparting distinct curvatures, porosities, and inner-surface accessibility [7,8,12,38,39,41]. Cross-study comparisons must also distinguish natural extracts, which retain source-dependent coexisting phases, from phase-pure synthetic materials.

2.2. Characteristics

Allophane particles are usually described as porous spheres (Figure 2a) [43,44] with outer diameters of 3.5–5 nm [11] and inner diameters of 2–4 nm (Figure 2b). The thickness of the wall is 0.7–1 nm, and the pore diameter is 0.3–0.4 nm [45,46] (Figure 2b) and has a specific surface area (SSA) of 400–900 m² g⁻¹ [46,47]. The outer layer consists of Al octahedral units, and the inner layer is composed of Si tetrahedral units. Al can be found in both tetrahedral and octahedral coordinates [48].

Figure 2. (a) Scanning electron microscopy (SEM) image of the allophane structure (source: authors’ own work) and (b) allophane structure (reprinted with permission under a Creative Commons Attribution 4.0 International Public License) [49].

Natural allophane can be categorized as Al-rich (Al/Si molar ratio of 2) or Si-rich (Al/Si molar ratio of 1), and its chemical formula is expressed as 1–2 SiO₂·Al₂O₃.5–6H₂O [50]. The Si sites in the structure, usually denoted as Q_n for silicates (where n ranges from 0 to 4), represent the number of directly linked Si tetrahedra. Si-rich allophanes exhibit a framework like that of Al-rich allophones, in addition to other polymerized species of Si (Q₁–Q₄) in addition to isolated Si tetrahedra (Q₀). The Si-OH sites on the inner wall and the Al-OH sites on the outer wall of the particle enable allophane to acquire different charges (negative or positive) depending on the pH conditions and isomorphic substitution of Al³⁺ for Si⁴⁺. Wang et al. [51] reported that the structure of allophane can be altered under acidic conditions, with an increase in pore size at pH > 3 and dissolution of the structure at pH < 3, as observed with imogolite.

On the other hand, imogolite aluminosilicate is microporous and may begin to form immediately in soils derived from vitreous volcanic ash or weathered pumice beds, although it has also been found in many podzols [52]. However, it can also be easily synthesized at relatively low temperatures (100–150 °C) [53] using chemical reagents containing Si and Al [12,41] with uniform diameters [53], where values of SSA are about 200–400 m² g⁻¹ [13]. The formation of synthetic imogolite is easily recognizable, as a gelatinous product forms from the suspension upon the addition of an ammonia solution. The quality of the synthetic product can be easily assessed by the turbidity of the suspension and the formation of a white precipitate [54]. The structure of imogolite is a single gibbsite-like sheet bound by unpolymerized Si tetrahedra [40]. Its structure corresponds to a hollow tube, consisting of a single, modified gibbsite curved sheet with an outer diameter of 2 nm and inner diameters ranging from 0.6 nm for natural imogolite to 2.8 nm for synthetic imogolite. The nanotube length ranges from approximately 100 nm to several μm (Figure 3a) [12,13,52].

Figure 3. (a) SEM images of the imogolite structure (reprinted with permission under a Creative Commons Attribution 4.0 International Public License) [55] and (b) imogolite nanotubes (source: figure created by the authors using Adobe Illustrator).

The tubule wall consists of an outer curved gibbsite sheet (Al octahedral) with SiOH groups attached inside and AlOH groups outside. The unit composition of imogolite corresponds to (OH)₃Al₂O₃SiOH). The AlOH groups have a positive structural charge and are not very reactive; the SiOH groups found in the internal part acquire a negative charge at high pH values [40] (Figure 3b). The size of the imogolite can be varied by changing the temperature [45] or the type of chemical precursor or by replacing Si atoms with other atoms [45], such as germanium (Ge) [12] or Fe [56]. The interior of the imogolite is believed to be accessible to water and aqueous species; this has been studied using classical molecular dynamics simulations in NTs of various radii, which show that water molecules can diffuse along the center of the nanotube [57]. Imogolite NTs form transparent aqueous liquid crystalline solutions with strong birefringence and high X-ray scattering power [53].

Meanwhile, halloysite is a rolled, tube-shaped aluminosilicate with a kaolinite-like structure. A monolayer of water molecules separates the unit layers in halloysite. Consequently, halloysite has a structural formula of Al₂(OH)₄Si₂O₅·nH₂O. The hydrated form of halloysite (n = 2) is called “halloysite-(10 Å)”, where a layer of water molecules is present between multiple layers of halloysite. The dehydrated halloysite structure (n = 0) is referred to as “halloysite-(7 Å)”. It is obtained by the loss of water molecules through gentle heating and/or under vacuum pressure (Figure 4a) [58].

Figure 4. (a) SEM halloysite image (reprinted with permission under a Creative Commons Attribution 4.0 International Public License) [59] and (b) halloysite nanotube structure (source: figure created by the authors using Adobe Illustrator).

The outer surface of halloysite nanotubes (HNTs) is similar to that of SiO₂, whereas the inner surface of the NTs is similar to that of Al₂O₃ [16]. Like allophane and imogolite, halloysite can adopt a positive or negative charge depending on the pH. The external surface, which is like silica, adopts a negative charge at high pH values. In contrast, the internal part, which is similar to alumina, adopts a positive charge at low pH [33]. Halloysite has lengths of 400 to 1000 nm [14,15,60], and in some cases they reach 5000 nm. The inner diameter ranged from 5 to 70 nm, and the outer diameter ranged from 30 to 190 nm. A study by Churchman et al. [60,61] using different types of halloysite tubes revealed that their properties vary according to origin.

2.3. Functional Applications of Allophane, Imogolite, and Halloysite

2.3.1. Adsorption Technologies

Adsorption is the process by which ions or organic molecules are retained on the surface of ACs by physical or chemical forces through interactions with functional groups [62]. Because ACs have a high SSA and functional groups that can adopt charge depending on the pH, they can remove both anionic and cationic contaminants.

The adsorption of several ions on allophane has been studied because of the excellent adsorption capacity for both cations and anions resulting from functional groups located in both the inner and outer parts of the material [50,63]. One of the most frequently used ions for this purpose is phosphate, which is an anion with high retention by allophane due to the presence of Al groups [64,65,66,67]. The importance of studying this anion stems from the need to understand phosphorus retention in volcanic soil ash [68]. It has also been studied in conjunction with anions such as arsenate [65,69,70,71,72], borate, chromate, selenite [69], sulfate [5,67,73], oxalate [64,66,67], fluoride [74], citrate [67], iodate, and iodide [75]. The adsorption of cations has rarely been studied, which included zinc (Zn), cobalt (Co), strontium, barium (Ba) [23], cesium (Cs) [76], copper (Cu), and cadmium (Cd) [9]. Moreover, the organic species worthy of note are inositol hexakisphosphate [68], 2,4-dichlorophenol, pentachlorophenol [77], benzene [78], benzoic acid, phthalic acid, benzaldehyde, ethyl benzoate, diethyl phthalate [78], detergents, and fatty acids [79]. All these studies have shown that the adsorption of anions and cations affects the charge of allophane molecules and the surface properties of particles [80]. In addition, ammonia gas adsorption has only been studied with allophane [81].

Similarly, few studies have investigated the adsorption of neutral molecules [82] and ions [83,84,85,86] on imogolite. For instance, Wilson et al. [82] examined the movement of benzene molecules adsorbed on imogolite and the ability of water or air vapor to remove benzene from imogolite pores. Guerra et al. [83] reported that imogolite is a suitable adsorbent for the rubidium present in drinking water because of its rapid adsorption of anions onto the support. Arancibia-Miranda et al. [85] and Clark et al. [86] investigated the adsorption of HMs such as Cd, Cu, and Co. The adsorption of anionic species such as chloride, perchlorate, boron [87], carboxylate [88], selenate [89], and arsenic [90] has also been studied by imogolite. Kurokawa et al. [87] analyzed the adsorption of boron in relation to the contents of imogolite and ferrihydrite and they reported that the adsorption increased from pH 5 to 8, peaked at pH 8 to 9, and decreased from pH 9 to 1. In recent years, the functionalization of imogolite has led to changes in their inner and outer surface properties, as well as their hydrophilicity, porosity, and thermal stability [56]. Guerra et al. [91] compared the ability of imogolite and imogolite functionalized with 2-mercaptothiazoline (IMOMTZ) to remove methylene blue (MB) from an aqueous solution. It was determined that 40.32 × 10⁻¹ mmol g⁻¹ and 65.13 × 10⁻¹ mmol g⁻¹ were adsorbed by imogolite and IMOMTZ, respectively. Similarly, Arancibia-Miranda et al. [92] evaluated the adsorption of Cu, Cd, and arsenic on magnetic imogolite nanocomposites functionalized with Fe oxide and reported that compared with nonfunctionalized imogolite, Fe oxide (magnetite) favored adsorption of HMs. The adsorption of basic, acidic, nonpolar, and polar amino acids, including arginine, lysine, glutamic acid, and glycine, respectively, has been investigated [93]. Glutamic acid, which has the highest affinity for the imogolite surface, was used to prepare a composite of a synthetic surfactant with a glutamic acid polar head and a hydrophobic C12 alkyl tail [93]. This material was used to evaluate the adsorption of rhodamine B isothiocyanate in aqueous and physiological saline solutions, exhibiting high adsorption efficiency in both media.

Halloysite has also been shown to adsorb organic compounds such as phenol [94] and benzene, as well as HMs. The benzene adsorption rate increased when the solution is heated, as adsorption sites are released (120 °C) due to the removal of water molecules from the nanotube structure [95]. Modifying the structure of halloysite can improve its adsorption properties. Compared with pure halloysite, diethanolamine- and triethanolamine-modified halloysite enhanced the adsorption capacity of metals in the following order: Pb(II) > Cu(II) > Cd(II) > Zn(II) [96]. Similarly, polyethyleneimine (PEI)– HNTs exhibited a 64-fold higher Cr(VI) adsorption capacity than pristine HNTs at pH = 2 after 36 h of shaking [97]. A biopolymer based on chitosan and natural halloysite has also been investigated for Cu(II) adsorption. The incorporation of halloysite into chitosan increased the adsorption capacity of Cu(II) [98]. In most cases, adsorption is primarily due to the hydroxyl groups or other functional groups present in the substances used to modify the structure of the nanomaterial. The magnetic nanohybrid (Fe₃O₄/halloysite) exhibited a Cr(VI) adsorption capacity 100 times greater than that pure halloysite [99].

Dyes are organic compounds used in industries such as paper, rubber, textile, and leather. They are discharged into the environment through wastewater, posing a threat to aquatic life [100].

The use of HNTs modified with Fe oxide nanoparticles has been investigated for the removal of naphthol B [101] or sulfonated HNTs to remove reactive black 5 and reactive red 49 [102]. Halloysite calcination, alkaline treatment, and subsequent acid treatment increase its SSA and porosity. This modification considerably increased the adsorption capacity (618 mg g⁻¹) of MB [103]. The adsorption of 4-chloroaniline was evaluated using pure and acid-activated halloysite, which improved when halloysite treated with sulfuric acid was used [104]. Herbicide adsorption, such as that of prometryn, was better in halloysite treated with 96% sulfuric acid (0.579 mg g⁻¹) than in those treated with 63% nitric acid (0.043 mg g⁻¹) or 30% sodium hydroxide (0.025 mg g⁻¹) [105]. Other compounds, such as pefloxacin (an antibacterial agent), have also been investigated, achieving an adsorption of up to 40 mg g⁻¹ over 100 min at temperatures ranging from 298 to 308 K [106]. In water treatment, polystyrene-halloysite membranes have been studied, yielding promising results for the remediation of wastewater from the paper industry [107].

Overall, adsorption is controlled by solute speciation, pH-dependent Al–OH/Si–OH chemistry, the Al/Si ratio, competing species, and physical access to the relevant interface. Modification can markedly increase uptake: IMOMTZ increased MB adsorption from 40.32 × 10⁻² mmol g⁻¹ to 65.13 × 10⁻² mmol g⁻¹ [91], Fe₃O₄/halloysite produced a 100-fold removal response involving reduction and immobilization of chloroanilines [99], and halloysite-derived mesoporous silica reached 618 mg g⁻¹ for MB [103]. These values describe different transformed materials and test conditions; they do not establish an intrinsic ranking of pristine allophane, imogolite, and halloysite. Precipitation, Al leaching, and incomplete regeneration data further limit comparisons.

2.3.2. Catalysis Processes

The goal of catalysis is to accelerate the rate of a reaction, which is achieved through an alternative mechanism to that by which a reaction normally occurs [108]. Among the factors that influence the reaction is the SSA of the material. The ACs have a high SSA and the advantage of being natural materials, making their cost for industrial use minimal. Aluminosilicate clays do not act as catalysts; rather, they serve as supports or carriers within clay-based composites. In these hybrid systems, the active phase (e.g., transition metals, metal oxides, or noble metal nanoparticles) drives the chemical transformation, while the clay framework provides structural stability, high SSA, and localized pollutant accumulation [109].

One of the reactions used in catalysis is the Fenton-type reaction, in which the oxidation mechanism involves the reversible reaction of hydrogen peroxide (H₂O₂) with Fe²⁺ under acidic conditions [110]. The reversible reactions involve the oxidation of Fe²⁺ to Fe^3+, the generation of the hydroxyl radical (·OH), and the subsequent reduction of Fe³⁺ to Fe²⁺. The highly oxidative radicals produced by the reaction may quickly degrade most organic pollutants to carbon dioxide (CO₂) and water.

Allophane coated with Fe oxides has been used for the degradation of MB. As natural allophane, allophane particles contain Fe; thus, allophane adsorbs MB by chemical and physical interactions, whereas the interaction of H₂O₂ with allophane’s Fe results in the formation of ·OH, which degrades MB [111]. In the oxidation of atrazine using synthetic allophane coated with Fe oxide, greater efficiency was achieved with electro-Fenton reactions than with the heterogeneous Fenton system. Atrazine (1-chloro-3-ethyl amino-5-isopropylamino-2,4,6-triazine) is an organic herbicide frequently detected as a pollutant in groundwater sources and in drinking water sources [112]. In addition, Garrido-Ramírez et al. [113] evaluated the catalytic activity of Fe and Cu oxides supported on allophane for the oxidation of phenol using H₂O₂ under ambient conditions by a Fenton-type reaction. The oxidation of phenol has also been studied by Marco et al. [114] through heterogeneous electro-Fenton reactions using bimetallic Fe–Cu (Fe oxide–Cu oxide) particles supported on allophane, where they achieved greater efficiency by using bimetallic nanoparticles rather than ACs alone because of the synergistic effect between allophane and Fe as well as Cu oxides.

The use of imogolite as a catalyst for the selective isomerization of glucose to fructose has been explored. It is a step in biomass conversion processes for obtaining biofuel and chemical products. Germanium (Ge) was replaced by Si, and the internal surface was functionalized with methyl groups, yielding 30% glucose conversion and a fructose selectivity of 45%. Kuroda et al. [115] reported that imogolite ACs are useful for manufacturing functionalized materials containing gold (Au) nanoparticles. Owing to the thermal stability and high porosity of the imogolite, the material produced presented optical properties like those of Au nanoparticles.

Owing to its large SSA, halloysite is a material used to support diverse types of nanoparticles. For example, halloysite hybridized with Ce and Zr increased the catalytic efficiency of the conversion of CO, C₃H₈, and NO_x [24]. Composite magnetite-halloysite has also been studied for the catalytic degradation of pentachlorophenol, a highly recalcitrant chlorinated organic compound found in aquatic systems and soils [116]. Halloysite–polyacrylamide–cyclodextrin combined with palladium nanoparticles contributes to the Sonogashira C–C coupling reaction [117]. Pd-Co, Pd-Cu, and Pd-Ni supported on HNTs have been used as catalysts in CO oxidation studies. Among them, Pd-Ni/HNT nanocomposite exhibited higher catalytic activity than the other nanomaterials evaluated [118]. Jaine et al. [119] used a simple one-step synthetic method to immobilize Rh, Pd, or Pt nanoparticles on the surfaces of halloysite, alumina, and silica. These composites were used to evaluate the catalytic hydrogenation of olefins such as cyclohexene, 1,5-cyclooctadiene, and 1-octene. Higher catalytic activity was observed for the catalysts supported on halloysite than for those supported on other materials. This difference was due to a more homogeneous distribution of metallic nanoparticles over the halloysite surface. Halloysite has been studied with Au nanoparticles for the oxidation of carcinogenic organic molecules [120,121]. Manganese (Mn) oxides have also been supported on halloysite for the catalytic thermal oxidation of formaldehyde, achieving 90% CO₂ at 149 °C [121]. Sahiner et al. [122] modified halloysite with different types of amines containing modifying agents. The resulting material was used as a catalyst in the methanolysis of sodium borohydride for hydrogen production, yielding positive results because of its biocompatibility.

2.3.3. Photocatalytic Processes

Photocatalysis is a light-induced catalytic process in which a photocatalyst absorbs photons, generates electron–hole pairs, and promotes redox reactions that produce reactive oxygen species (ROSs), which then degrade organic pollutants into less harmful products [123]. In this sense, ACs do not possess an intrinsic bandgap activity and cannot act as photocatalysts on their own. Instead, they function strictly as photocatalyst supports, adsorbents, or electron-mediating matrices [124].

The degradation of trichloroethylene, a solvent, and acetaldehyde by photocatalysis [125] was studied by Nishikiory et al. [126,127] using allophane-TiO₂ compounds; trichloroethylene is transformed into products such as phosgene and dichloroacetyl chloride, which are rapidly adsorbed on allophane and gradually degraded after diffusion into TiO₂. Ono and Katsumata [128] enhanced the photocatalytic activity of an allophane-TiO₂ mixture through acid treatment, leading to improved photocatalytic decomposition of gaseous acetaldehyde compared with that of TiO₂ or allophane treated with acid alone. For the photocatalytic degradation of gaseous acetaldehyde, Hojamberdiev et al. [129] used allophane-wakefieldite (CeVO₄)–cesium (Cs) composite obtained by mechanical mixing and hydrothermal synthesis. They reported that, compared with allophane or CeVO₄, both composites showed higher photocatalytic activity than individual material, with the allophane in the composites increasing the degree of adsorption. Moreover, compared with the compounds obtained by mechanical mixing, the synthesized hydrothermal compound shows greater photocatalytic activity because of a longer, more homogeneous distribution of CeVO₄-Cs and allophane. In another study on the photodegradation of gaseous acetaldehyde, Hojamberdiev et al. [130] used Bi₂WO₆-allophane and bismuth oxyiodide (BiOI)-allophane, prepared by mechanical mixing and hydrothermal synthesis. Using a batch at room temperature, with 50 mg of materials and 200 mg kg⁻¹ of acetaldehyde under irradiation from an LA-251Xe lamp (520–600 nm), they found that BiOI and BiOI-allophane achieved complete acetaldehyde degradation (100%) within 5–7 h, whereas Bi₂WO₆-allophane reached 75.5% after 8 h, while the mixed and synthesized composites achieved 100% and 85.6% degradation, respectively, over the same period.

In recent years, imogolite-doped composites have been used for the photodegradation of organic pollutants [131,132]. For example, the photodegradation of tartrazine, a widely used dye in the food industry and a relevant chemical pollutant in wastewater as well as groundwater [131], has been investigated by methyl-imogolite and Fe-doped methylimogolite composites. In addition, Katsumata et al. [132] reported that upon UV irradiation, imogolite-TiO₂ exhibited superior efficiency in the photodegradation of acetaldehyde. Acid Orange 7 is also a pollutant present in wastewater and groundwater. In this regard, Shafia et al. [38] reported that the compound imogolite–Fe (0.70% and 1.4% by weight) improved the adsorption on the external surface of the material because >90% of the Azo Acid Orange 7 was removed. Moreover, the catalytic properties are affected; apparently, the Fe³⁺ and H₂O₂ species do not come into direct contact, thereby preventing reactions driven by the Fenton process.

Compared with commercial TiO₂, a halloysite–TiO₂ composite showed greater photodegradation of NO_x [133]. In their research on photocatalysts, Gómez et al. [134] demonstrated that Au-decorated HNTs degrade organic dyes. Thus, this represents an ecological method for light-induced oxidation reactions carried out at room temperature. Similarly, the experiments conducted by Li et al. [135] demonstrated that the halloysite–CeO₂–AgBr nanocomposite favored the photocatalytic degradation of methyl orange. The enzyme could serve as a base for the growth of nanoparticles. In this sense, compared with the halloysite–TiO₂ compound, the amylose–halloysite–TiO₂ compound exhibited high dispersion and SSA, and more effective photocatalytic activity for the photodegradation of MB and 4-nitrophenol (4-NP) under UV irradiation [136]. The photodegradation of aniline in the presence of HNTs, activated halloysite (AHal), TiO₂, and commercial titania P25 under UV irradiation has also been studied. The photodecomposition rate constant decreased in the order P25 > TiO₂ > AHal > HNTs [137]. The photodegradation of chloroanilines such as 2-chloro and 2,6-dichloroaniline has been studied under UV radiation using halloysite coated with TiO₂ and halloysite coated with Fe oxide. Compared with those of TiO₂ (anatase) and P25, the activity of these composites was greater [138]. Halloysite–rose bengal compounds were used to evaluate the photodegradation of 4-n-nonylphenol, a pesticide and a water pollutant. Compared with halloysite alone, the hybrid catalyst was more efficient for the oxidation of 4-n-nonylphenol [16]. On the other hand, Cipta et al. [139] evaluated BiOI supported on natural halloysite and reported that, compared with bismuth oxyiodide alone, its use prevents oxyiodide agglomeration, allows modulation of the energy bandgap, and reduces electron–hole recombination in the photocatalyst.

In catalysis and photocatalysis, ACs usually act as a hydroxylated support, at the adsorption interface, or in the confinement domain, whereas Fe, Cu, noble metals, or semiconductors constitute the principal reactive phase. Allophane couples contaminant capture with Fe-mediated or TiO₂-driven conversion, imogolite offers a tunable lumen for selective catalysis, and halloysite disperses metal or semiconductor nanoparticles and limits agglomeration. The performance must therefore be assigned to the complete composite rather than the pristine mineral. Removal should also be separated from chemical transformation and mineralization: >90% Acid Orange 7 removal by Fe-imogolite was attributed mainly to adsorption because Fe³⁺ and H₂O₂ did not contact effectively [42], whereas BiOI/halloysite was linked to reduced agglomeration and electron–hole recombination [139].

2.3.4. Enzyme Immobilization

Owing to their SSA, ACs have been evaluated for enzyme immobilization because they act as enzymatic nanoreactors. This procedure is used to increase the availability of the enzyme on the substrate, as well as its stability and reusability, which in turn allows for greater productivity, an essential requirement for today’s industry.

Organic phosphorus mineralization in decomposed cattle manure has been evaluated with allophane ACs [25]. The results revealed increased specific enzymatic activity of phosphatase and the kinetics of organic phosphorus mineralization [26]. The immobilization of phosphatase on allophane enables the activity of the enzyme by protecting its conformation [26]. The immobilization of phytases obtained from Escherichia coli and Aspergillus niger on allophane covered with Fe oxides and natural montmorillonite has also been studied [27]. The authors determined that synthetic allophane improved thermal stability and increased the proteolysis resistance of phytases. In addition, the residual activity of both phytases increased under acidic conditions.

As a support, halloysite offers greater thermal stability, good storage, and reuse stability. These findings suggest that halloysite is a favorable support material for the immobilization of other enzymes that have yet to be studied. Pandey et al. [33] immobilized α-amylase after functionalizing the surface of halloysite with 3-aminopropyl triethoxysilane (APTES) to improve the affinity for the enzyme, which was achieved by the interaction of the carboxylate groups of the enzyme with the aminopropyl groups generated in APTES [33]. Wang et al. [140] immobilized lipase on halloysite through electrostatic interactions, which allowed it to maintain 80% of its catalytic activity for several days compared with free lipase, which lost its activity during the same period. Additionally, laccase, glucose oxidase, and lipase are immobilized on HNTs through adsorption on both the inner and outer walls, mainly through electrostatic interactions between the net charge of the protein and the internal negative and external positive charges of the HNTs [141]. Similarly, Rouster et al. [142] investigated the encapsulation of β-lactamase in HNTs and its use in resistant hydrophilic bioactive coatings. The enzyme was found to be adsorbed within the lumen of the HNTs because of the interaction of the positive charge that it presents and the negative charge of the β-lactamase, resulting in greater enzymatic activity than that of coatings that do not contain NTs. This evidence links immobilization performance to accessible hydroxylated surfaces, protein support electrostatics, and lumen dimensions. Allophane protected phosphatase conformation and improved phytase stability [25,26,27], whereas halloysite offered both an exterior and a lumen and could be functionalized with APTES to increase enzyme affinity [33]. Quantitative analysis showed that halloysite supported α-amylase and urease retained >80% of their catalytic activity after 60 min of heating and >90% after 15 d of storage [143], and immobilized lipase on halloysite retained approximately 80% of the catalytic activity for 14 d [140]. These benefits must be balanced against possible diffusion limitations or activity loss from overly strong binding. Comparable imogolite-specific evidence is not synthesized in the manuscript and should be considered a gap in the literature.

2.3.5. Biomedical Uses

Since ancient times, ACs have been used in healthcare, specifically in medical therapy [144]. Over the years, this use has become more sophisticated and advanced, and ACs are used in the pharmaceutical and biomedical fields, such as in the transport and release of drugs [145], among other applications.

Medical applications of allophane are recognized for their bactericidal properties, which can be enhanced by forming nanocomposites with Cu and Si nanoparticles. Allophane has also been used as a support for Ag nanoparticles, showing strong bactericidal activity against Escherichia coli and Staphylococcus aureus [32]. Like other ACs, such as halloysite [32], allophane possesses these properties, and its application inhibits [46] the formation of edemas [32,145]. Allophane induces lipid peroxidation and the oxidative degradation of lipids in cell membranes [146]. To explore new methods for drug release, Kawachi et al. [31] investigated the preparation, characterization, and properties of hydrogels based on deoxyribonucleic acid (DNA) using natural allophane. The strong affinity of phosphate for allophane promotes the adsorption of DNA because of interactions with the phosphate groups of DNA.

Halloysite has been shown to possess antiseptic properties and is used to fill polymeric fibers. Such fibers could be used to prepare wound dressings, fabric coverings, tissue engineering fibers, and dental and suture materials. Karolina et al. [147] demonstrated the ability of multilayer polylactic acid/halloysite/gentamicin porous membranes to prevent infections during bone regeneration. Antibacterial sprays with water–halloysite dispersions loaded with antiseptics could be interesting. Halloysite NTs coupled with Ag nanoparticles are bactericidal against Escherichia coli and Staphylococcus aureus, achieving 100% efficiency [148]. Similarly, the essential oil Zataria multiflora encapsulated in halloysite efficiently eliminated Escherichia coli during its controlled release [149]. These NTs are also used in drug delivery systems, especially when a topical effect is needed [150]. According to the research by Barman et al. [151], who demonstrated that HNTs exhibited controlled release and strong antimicrobial activity against Gram-positive and Gram-negative bacteria. Moreover, Rao et al. [152] suggested that halloysite nanocomposite hydrogels are effective at delivering drugs to treat colon cancer. Another hydrogel based on chitosan/oxidized starch/halloysite was evaluated for its metronidazole release capacity [153]. Hydrogels prolong the release of metronidazole because of the presence of halloysite [153]. Hence, they could be ideal materials for the controlled administration of medications. Halloysite has been studied for the design of systems that transport and release drugs (α-lipoic acid and moxifloxacin hydrochloride) and that prevent photodegradation simultaneously; the results revealed that encapsulation promotes controlled release and increases the photostability of drugs [154,155,156]. Their research has demonstrated that halloysite has the potential to be an excellent pharmaceutical, economic, natural, and abundantly available excipient.

Halloysite ACs have the most developed carrier evidence because their large lumen supports loading and sustained release, while their exterior integrates with polymers; the reported systems include antibacterial agents and pH-responsive drug delivery hydrogels [147,156]. However, the particle composition, size, dose of ACs and exposure route, residual metals, sterilization, and long-term fate must be reported before clinical or pharmaceutical claims are made.

2.3.6. Hydrogel Systems

Hydrogels are hydrophilic three-dimensional networks that can absorb large amounts of water. The poor mechanical properties of hydrogels obtained from natural or synthetic sources limit their application in industrial and biomedical fields. One way to enhance these properties is by incorporating additional materials. In this sense, different studies have added imogolite to improve the properties of hydrogels [29,157]. For example, an acrylamide hydrogel was reinforced with imogolite to study its mechanical and optical properties [29]. Compared with simple hydrogels, the resulting hydrogels had better mechanical properties [29]. Similarly, to obtain a hydrogel based on poly (acrylic acid) and improve its mechanical properties, imogolite nanofibers were used, which interlace between the polymeric units and align along the elongation force, allowing an extension of up to 1800% and subsequent recovery [157]. Apart from acrylic acid, 2-hydroxyethyl acrylate, acrylamide, and N-isopropylacrylamide have also been used to generate hydrogels with imogolite via in situ copolymerization, resulting in hydrogels with adjustable physical properties that respond to stimuli such as deformation and temperature [158].

Compared with native nonhybrid hyaluronic acid (HA)-based hydrogels, hybrid imogolite fibrous (IG) nanotube hyaluronic acid (IG/HA) hydrogels have shown superior mechanical properties and lower degradation rates [159]. Furthermore, compared with hydrogels containing imogolite, these hydrogels had better mechanical properties and lower degradation rates. The crosslink density increased when an additional IG network was incorporated into the crosslinked HA network [159]. The imogolite-polyacrylic acid hydrogel synthesized by γ-ray radiation showed excellent mechanical properties because of the imogolite-based structure, characterized by overlapping layers. Imogolite can contribute to both stress relaxation and the maintenance of structures during applied stress [158,160]. Finally, in bone tissue engineering, imogolite has been studied as a nucleation site for calcium phosphate growth. Gelatin-based hydrogels, in the presence of a biocompatible crosslinking agent within the gelatin network, are treated with cryogenic techniques to obtain particles with pores of 100 µm [161]. The interactions among gelatin, imogolite, and the crosslinking agent result in microscale structural conversion of the hydrogel. Minor effects at the nanoscale are shown. Cryogenic treatment enables the formation of pores with the required size range (100 μm) for the permeation of bone cells. Imogolite promotes the formation of carbonated hydroxyapatite and hydroxyapatite [161].

Imogolite provides the clearest structure–property–function relationship in hydrogels: its high-aspect ratio NTs interlace polymer chains, align under strain, transfer load, and contribute to stress relaxation [157], while cryogenic processing of gelatin/imogolite systems produced ~100 µm pores, and the imogolite surface promoted hydroxyapatite nucleation [161]. Allophane contributes strongly to phosphate-mediated binding in DNA hydrogels [31], whereas halloysite mainly serves as a lumen reservoir in sustained-release hydrogels [152,153]. Direct comparison remains limited by differences in polymer chemistry, nanotube loading, crosslinking, and mechanical testing.

2.3.7. Membrane Systems

Membranes are thin films of materials or combinations of materials created for different purposes and used in industry, medicine, and the environment, among other areas.

In the case of allophane, a titanium alkoxide sol-gel has been used to disperse allophane ACs to create a membrane for the photocatalytic degradation of trichloroethylene and acetaldehyde [125]. The distribution of allophane on the surface of the film promoted the adsorption of dichloroacetyl chloride or phosgene, a product of the degradation of trichloroethylene, and CO₂ in the case of acetaldehyde [125].

Hybrid films have been manufactured from cellulose fibers and natural imogolite NTs to address environmental issues. The mechanical and thermomechanical properties of the hybrid films were improved by the addition of up to 1% w/w imogolite. Thin hybrid films of phosphorylated cellulose (P-CNC) nanocrystals with imogolite NTs were also obtained. The presence of phosphate groups results in a strong affinity for imogolite. This allows P-CNC/imogolite films to be quite stable in water over a wide pH range compared to cellulose-imogolite nanocrystal films under the same conditions [162]. Instead of imogolite, Ge-imogolite with cellulose nanocrystals has also been studied to obtain films.

Halloysite has also been used in combination with hydroxypropyl cellulose to form a film that subsequently becomes a geopolymer with flame-retardant properties and high thermal stability. Halloysite NTs are responsible for reducing the activation energy of degradation and reducing the combustion rate [163]. Membranes have been developed with modified and unmodified imogolite NTs on the surface for reverse osmosis applications [158]. They were obtained by interfacial polymerization of polyamide on macroporous polyacrylonitrile supports. The modification was performed with polyvinylpyrrolidone and polydimethylsiloxane to improve the dispersion of the NTs. This process resulted in increased water permeability while maintaining ion rejection [158]. Nanocomposite membranes have also been synthesized by integrating aluminogermanate imogolite (GeINT) NTs, both in their original form and with modified surfaces, into thin polyamide films. When vertically modified GeINT was incorporated, the specific flow increased by 205% when the concentration of GeINT was 5 mg g⁻¹ compared with that of the pure membrane [28]. This was due to the greater water permeability in the incorporated GeINT than in the pure polyamide. When the direction of the GeINTs was parallel to the membrane surface, the specific water flow was only 105% [28]. New direct osmosis membranes of thin-film nanocomposites (polyamide–aluminosilicate NTs) were compared to the pure polyimide membrane. The permeability and selectivity of the nanocomposite membrane with 0.2% w/v of aluminosilicate NTs improved simultaneously.

The membrane performance depends on the particle distribution, polymer compatibility, and nanotube orientation. Allophane–TiO₂ films couple the adsorption of trichloroethylene intermediates with photocatalytic conversion [125]. Natural imogolite reinforces cellulose films, while phosphate groups increase imogolite–cellulose affinity and water stability [162]. Halloysite contributes to a relatively large lumen and a polymer-compatible exterior, but sulfonation, antimicrobial loading, or other modifications often provide the decisive function [102,107,147]. Natural, synthetic, and modified NTs must therefore be distinguished explicitly.

To provide an integrated overview of the materials discussed in this review, Table 1 summarizes the key structure–property–function relationships of natural, synthetic, and modified aluminosilicate clays and composites. This comparison highlights how structural and physicochemical characteristics are associated with their functional applications.

Table 1. Comparative structure–property–function evidence for natural, synthetic, and modified aluminosilicate clays (ACs) discussed in this review.

ACs	Key Structural/Surface Property	Application	Target/Process	Main Quantitative Result	Mechanism	Advantage	Limitation	References
	Natural/Synthetized
Allophane	Hollow porous nanospheres; outer: 3.5–5 nm, inner: 2–4 nm; 400–900 m2 g−1; outer Al-octahedral/inner Si-tetrahedral; Al/Si = 1–2	Structure/surface platform	pH-dependent interfacial reactivity	Pore: 0.3–0.4 nm; dissolution below pH 3	Inner Si–OH and outer Al–OH produce pH-dependent charge	High reported area and differentiated interfaces	Acid alteration, Al leaching, and restricted inner access	[11,43,44,45,46,47,48,50,51]
Imogolite	Single-walled nanotubes; outer: ~2 nm; inner: ~0.6 nm natural to 2.8 nm synthetic; 200–400 m2 g−1; water-accessible lumen	Structure/mass transport	Aqueous species in lumen	Synthesis at 100–150 °C; length: ~100 nm to several µm	Water diffuses along tube center; chemistry tunable by substitution	Uniform synthetic dimensions and tunable lumen	Natural material requires complex purification	[12,13,40,41,45]
Halloysite	Rolled multi-walled nanotubes; outer: 30–190 nm, inner: 5–70 nm; silica-like exterior/alumina-like lumen	Adsorption/delivery/reinforcement	Surface and lumen interactions	Length: 400–1000 nm, up to 5000 nm	Outer surface negative at high pH; lumen positive at low pH	Large lumen and spatially differentiated chemistry	Origin-dependent morphology; acid-induced Al leaching	[14,16,33,164]
Allophane	pH-dependent hydroxyl chemistry; Al/Si ratio and competing solutes affect binding	Adsorption	Oxyanions, cations, and organics	-	Binding changes particle charge; composition and cosolutes regulate retention	Can retain anionic and cationic species	Cation evidence is sparse; metrics are noncomparable	[50,63]
	Natural Supported
Phosphatase/allophane	High-area variable-charge surface	Enzyme immobilization	Organic-P mineralization	Activity and kinetics increased; exact values -	Allophane protects enzyme conformation	Improved activity/stability	Al/Si and inner-surface effects not compared	[25,26]
Enzymes/halloysite	External surface and lumen support enzyme binding	Enzyme immobilization	Alpha-amylase and urease	>80% activity after 60 min heating; >90% after 15 d storage	-	Thermal and storage stability	Heating temperature not stated in manuscript	[143]
	Synthetized/Modified
Imogolite/IMOMTZ	2-Mercaptothiazoline-functionalized imogolite	Adsorption	Methylene blue	40.32 × 10−2 vs. 65.13 × 10−2 mmol g−1	-	Functionalization increased uptake	-	[91]
PEI/halloysite	Polyethylenimine adds binding functionality	Adsorption	Cr(VI)	64-fold relative to pristine halloysite	-	Large relative enhancement	Precipitate formation confounds adsorption	[97]
Fe3O4/halloysite	Magnetic carbonaceous nanohybrid	Adsorption/reduction	Cr(VI)	100-fold relative to pristine halloysite	Reduction and immobilization accompany removal	Large enhancement and magnetic separation	Metric should not be described as adsorption alone	[99]
Halloysite-derived silica	Calcination plus alkaline/acid treatment increases area and porosity	Adsorption	Methylene blue	618 mg g−1	More accessible area and porosity	High dye uptake	Transformed derivative; regeneration not reported	[103]
Treated halloysite	96% H2SO4, 63% HNO3	Adsorption	Prometryn	0.579 mg g−1,	-	H2SO4 gave the highest reported value	Severe acid treatment can leach Al	[105]
Fe/Cu oxide–allophane	Metal oxides dispersed on allophane; activity depends on Si/Al	Catalysis	Phenol oxidation	Bimetallic system more efficient	Fe–Cu synergy and support composition affect activity	Improved relative catalytic activity	Conversion, leaching, and reuse not harmonized	[113,114]
Imogolite/germanium and imogolite/methyl	Internally methylated imogolite-type nanotube; framework substitution	Catalysis	Glucose to fructose	30% conversion; 45% fructose selectivity	Tuned inner-surface polarity/confinement	Demonstrates lumen tunability	verify substitution direction and add inline citation	[13]
MnOx/halloysite	Manganese oxides supported on halloysite	Thermal catalysis	Formaldehyde oxidation	90% CO2 at 149 °C	-	Effective conversion as reported	define conversion/selectivity/yield	[121]
Allophane–TiO2	Adsorptive allophane coupled to photoactive TiO2	Photocatalysis/membrane	Trichloroethylene and acetaldehyde	-	Intermediates adsorb on allophane and diffuse to TiO2	Couples capture with transformation	Hazardous intermediates form before degradation	[125,126,127]
BiOI/halloysite	BiOI dispersed on natural halloysite	Photocatalysis	Semiconductor stabilization	-	Reduced agglomeration and electron–hole recombination; bandgap modulation	Improved dispersion and charge utilization	-	[139]
DNA/allophane	Strong phosphate affinity of allophane	Biomedical hydrogel	DNA adsorption	-	DNA phosphate groups bind allophane	Supports DNA-mineral hydrogel formation	-	[31]
Ag/halloysite/chitosan	Ag-bearing halloysite composite	Antibacterial material	Echerichia coli and Staphylococcus aureus	100% efficiency as reported	-	Complete response reported	Exposure conditions and definition of efficiency absent	[148]
Imogolite/PAA	Nanofibers interlace polymer chains and align during elongation	Hydrogel reinforcement	Deformation/recovery	Extension up to 1800% with recovery	Fiber alignment transfers and relaxes stress	Large reversible extensibility	Test condition comparability not reported	[157]
Ge-imogolite/polyamide	Aluminogermanate nanotube orientation controls channel access	Membrane	Water transport	+205% vertical at 5 mg g−1; +105% parallel	Orientation-dependent water transport	Large permeability response	Clarify percentage basis and loading unit	[28]

3. Sustainability Opportunities and Constraints

The reviewed evidence identifies credible sustainability opportunities but does not establish sustainability from natural origin alone. Allophane combines a high reported SSA and variable-charge hydroxyl chemistry for oxyanion retention and adsorption-assisted catalysis [21,112]; imogolite provides a chemically differentiated, tunable lumen for oriented films, hydrogels, and functionalized catalysts [115]; and halloysite provides a larger lumen and a polymer-compatible exterior for adsorption, catalytic support, membranes, and delivery systems [121]. These structure-specific functions can support contaminant control, resource-efficient catalysis, enzyme stabilization, and controlled release. Consequently, ACs contribute to various Sustainable Development Goals (SDGs): mainly SDG 6 (Clean Water and Sanitation) through the removal of contaminants from water and SDG 13 (Climate Action) by supporting environmentally sustainable technologies (Figure 5). Furthermore, the potential sustainability benefits of ACs are closely linked to their performance throughout their entire life cycle. Factors such as energy requirements during extraction, chemical inputs used for modification, transportation needs, and material recovery options can significantly influence their overall environmental footprint. Therefore, life cycle assessment approaches are essential for identifying trade-offs between different aspects and avoiding the shifting of environmental burdens between the production, use, and disposal stages Finally, the sustainability of ACs depends on how they are extracted/synthesized, processed, used, and discarded after application.

Figure 5. Schematic representation of the structural characteristics, material processing, key applications, and future research directions of aluminosilicate clays from volcanic ash soils, highlighting their role in contributing to the Sustainable Development Goals of the United Nations (source: figure created by the authors using Biorender).

4. Conclusions

Allophane, imogolite, and halloysite are aluminosilicate clays (ACs), the application performance of which follows a structure–property–function relationship. The functionalization of ACs can introduce binding sites, improve the dispersion of metals or semiconductors, suppress agglomeration, and stabilize immobilized enzymes. These benefits are nevertheless formulation- and condition-specific rather than intrinsic to the mineral alone. Future studies should investigate the long-term behavior of these ACs under realistic conditions and develop predictive structure–property models to guide their optimization for different applications. Furthermore, integrating life cycle analysis and techno-economic analyses will be fundamental to evaluating large-scale viability and environmental sustainability. Such efforts will help identify applications that maximize both functional performance and contributions to the Sustainable Development Goals.