Advances in sustainable purification of natural clays: Methods, characterizations, challenges, applications and perspectives: A review

3.1. Chemical methods

The purification of clays requires chemical methods to eliminate mineral and organic impurities that are resistant to physical separation. Specific reagents are used in these processes to dissolve different impurities without altering the fundamental properties of the clay minerals. This method provides precise control over the removal of metal oxides, carbonates, and organic compounds from natural clay. This section outlines the primary chemical clay purification techniques and their operational mechanisms, benefits and shortcomings, as well as the changes in clay material attributes after purification. Specific reagents play a crucial role in clay purification by eliminating impurities with minimal alteration to the properties of clay minerals. According to literature, mineral acid reagents such as HCl, HNO₃, and H₂SO₄ remain the main solution agents for dissolving metal oxides, carbonates, and phosphates. Using organic acids, such as oxalic and acetic acids, offers an environmentally friendly approach to clay purification, thereby expanding the range of possibilities based on operational requirements. Combining chemical agents with EDTA provides metal-ion capture, while the addition of NaOH eliminates organic content and H₂O₂ oxidises organic compounds. The success of these processes depends significantly on operating conditions, including reagent concentrations, treatment duration, and temperature. Studies on clays, including kaolin and montmorillonite, demonstrate that suitable chemical approaches enhance purification levels. While this technology provides accurate solutions, it has several disadvantages: it generates expensive products, requires extensive handling of acid residues, and causes environmental damage through its waste liquid emissions. Combining chemical purification methods with physical processing techniques is the most effective way to achieve optimal purification results that satisfy the requirements of industrial and scientific applications [4]. Veiskarami et al. [21] demonstrate in their research that the quality of the purification of montmorillonite affects its hexadecyltrimethylammonium bromide (HDTMA) modification process. The research explores various combinations of physical and chemical methods to investigate how these procedures affect the structure of montmorillonite and preserve its potential for modification. The purification process involved size fractionation prior to completing impurity removal through the chemical and physical treatment of the montmorillonite material. Purifying the material using physical methods alone marginally enhanced the CEC of montmorillonite, but combining physical and chemical methods brought about the greatest increase in CEC, thus improving the HDTMA binding properties. XRD analysis of physically and chemically treated montmorillonite indicated better peak definition, showing that the distribution of interlayer spacing was more organized due to improved HDTMA arrangements inside the spaces. When HDTMA interacted with untreated montmorillonite, it formed distribution arrangements that matched the original clay patterns. The order of the structural arrangement during HDTMA treatment depends on the refinement of the montmorillonite, with surface characteristics becoming clearer through efficient purification. These results are particularly important for montmorillonite applications in sensitive fields, such as drug delivery and polymer-clay nanocomposite production, where precise surface modification is crucial. According to Takahashi et al. [22], chemical treatment with NaOH did not significantly alter the coordination structures of Si and Al within the tetrahedral and octahedral sheets, respectively. However, this chemical process resulted in slight Al-to-Si substitution at the tetrahedral sites. The impact of chemical reagents on clay purification varies depending on the distinct properties of these compounds and the associated minerals.

The clay purification process uses hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃) as essential dissolving agents to remove carbonates and metal oxides. The strength of HCl as an acid makes it efficient at dissolving carbonates, yet it is ineffective against silicate minerals, including quartz. Research conducted by Lu et al. [23] proved that HCl pretreatment successfully eliminated impurities of dolomite, calcite and gypsum from the sample material. Applications of HCl require maintaining pH levels above 4.5 to prevent structural damage to the clay mineral. Typically, HCl at a concentration of approximately 2 M is added drop by drop. In some cases, however, a buffer solution of sodium acetate and acetic acid is preferred to enhance the clay’s properties. H₂SO₄ has the advantage of dissolving metal oxides and phosphates, although it can alter sensitive clay minerals. The oxidizing properties of HNO₃ allow scientists to use it to remove persistent organic compounds and specific metal oxide traces. Oxalic and acetic acids are sustainable, alternative chemical agents for treating mineral contaminants. Oxalic acid acts as a complexing agent for iron and aluminium ions, removing them from other materials. Simultaneously, acetic acid treats carbonates in situations requiring gentle acid treatment [11]. The various ways in which reagents interact with clay impurities enable the precise selection of targeted pollutants without compromising the integrity of the clay materials themselves. The appropriate combination of reagents enables scientists to develop protocols that suit industrial and scientific requirements while minimizing the environmental impact of effluent disposal. Lu et al. [23] explored an innovative process for bleaching natural red clay rich in palygorskite (Pal-R-C), combining HCl pretreatment with microwave-assisted oxalic acid (OA) leaching. This process effectively removed impurities, such as dolomite, calcite and gypsum, before targeting the substance responsible for the color (hematite) through oxalic acid leaching. The treatment partially extracted palygorskite and illite metal ions without altering their mineral structures. The bleached Pal-R-C material exhibited a high L* whiteness measurement of 82.38, as well as a specific surface area of 129.35 m₂/g. This treatment improved the physical and chemical properties, while leaving the nanorod structure of palygorskite and the illite nanosheets intact. When used as an inorganic filler, the bleached clay strengthened a chitosan polymer film while maintaining elongation at break at 36.17%. The tensile strength of the film increased from 27.22 to 47.15 MPa. Research showed that palygorskite-rich red clay, which underwent combined acid treatment, could be used as an economical alternative to conventional methods for bleaching.

Chemical oxidizing agents function as decomposing agents for organic matter in clays, breaking down existing organic compounds in the process. Hydrogen peroxide is a widely used chemical product for this purpose. Laboratory operators recognize H₂O₂ as an efficient oxidizing agent for organic matter that preserves the crystalline structures of clay minerals, provided that the experimental conditions are consistent. Clay samples require a solution of diluted H₂O₂ to be added gradually with stirring, either at room temperature or with moderate heat to speed up the oxidation process [24]. The decomposition of H₂O₂ produces water and active oxygen; the latter serves as the primary oxidizing agent, converting organic matter into carbon dioxide and water. The removal of organic matter is revealed through air bubbles during the process.

Effective clay purification involves strict control of pH and reaction time, since excessive conditions can damage clay structures, particularly in clay smectites. Most chemical treatments include washing and centrifugation to remove soluble compounds and organic matter. Among oxidizing agents, hydrogen peroxide is widely used due to its efficiency, low cost, and minimal environmental impact compared to stronger chemicals like chromic acid or potassium permanganate. A key step in purification is the removal of iron, aluminum, and manganese oxides/hydroxides, which strongly affect clay’s optical, chemical, and industrial properties. The sodium dithionite-sodium citrate-sodium bicarbonate system [24], is commonly applied, where heating and stirring reduce and eliminate iron oxides, followed by rinsing and centrifugation. Other methods include the use of TiCl₃ with EDTA, which shows higher selectivity for iron and aluminum hydroxides, and acidified sodium oxalate, which effectively extracts both. For bentonite purification, Qiao et al. demonstrated that physical fractionation by centrifugation alone cannot fully separate opaline silica from montmorillonite (Mt). However, combining NaOH treatment (90°C, 2 h) with centrifugation improved purity by converting Ca-Mt to Na-Mt, effectively removing opaline silica while preserving the structural integrity, swelling ability, and textural properties of montomorillonite (Mt).

The hybrid purification method shows effectiveness in montmorillonite purification through a process that sustains the base characteristics of the mineral. Shah et al. [25] investigated three chemical purification techniques that used Na(PO₃)₆ (Method-I), NaCl (Method-II) and Na₂CO₃ (Method-III) solutions to evaluate textural alteration of Pakistani bentonite containing more than 70% Ca₂⁺-montmorillonite. All purification approaches decreased particle dimensions but Methods II and III proved most effective for generating fine particles. A Sodium purified bentonite clay (Na-Bentonite) produced by Method-III showed a combination of exceptional microporous features and tiny particles that yielded high specific surface area and larger mesopore volume than other methods. The purified samples from Methods I and II displayed equivalent microporosity features yet used different mesoporosity distribution patterns resulting in higher mesopore volume production from Method I. Bentonite structures undergo significant structural changes because of purification methods and Method-III demonstrates the best performance for manufacturing bentonite materials with both strong adsorption ability and high catalytic power because it produces fine particles that organize their interlayer structure more effectively.

Using strong acids during clay chemical purification procedures may destroy part and alter the entire structural integrity of clays. The acid treatment causes the stepwise vanishing of interlayer cations together with the structural deterioration of the aluminosilicate network which destroys layer organizational structure. The clay textural properties undergo modifications through acid treatment which reduces CEC while creating active sites as well as raising its specific surface area. The modified type of clays known as activated clays have attracted growing interest because of their outstanding features. The chemical properties of activated clays receive substantial enhancement from acid activation processes which enables their usage in key environmental applications. The results presented by Frini-Srasra and Srasra [26] show that Tunisian palygorskite exposed to HCl demonstrated a notable rise in specific surface area because octahedral layers broke down while forming mesoporosity. The acid-activated palygorskite samples absorbed Cd₂⁺ more efficiently than the untreated natural form thus showing improved adsorption ability. Ayati et al. [27] examined how changing HCl dosage together with reaction time and heat influenced acid activation of smectitic clays as well as their pozzolanic behavior. Acid treatment first dehydroxylates the sample before dissolving both octahedral and exchangeable cations, which leads to the creation of an amorphous silica-rich end-product. A 5 M HCl activation with 8 h at 90°C yielded optimal results for smectite activation which produced a high pozzolanic activity of 1117 mg/g through formation of 91% silica amorphous phase.

Acid treatment requires the usage of HCl as a corrosive agent that also leads to the addition of incorporated carbon. The environmental problems related to acid treatment can be minimized with optimal acid-to-solid ratios combined with reagent recycling methods. The authors present for the initial time how acid-activated smectitic clays serve well as pozzolanic materials within cement systems. The researchers recommend enhancing knowledge about clay hydration properties and improving activation processing parameters to increase the potential applications of activated clay minerals for eco-friendly cement production. The investigation by Zhou et al. [28] examined how H₂SO₄ treatment at room temperature affects both the microstructure and surface properties of different kinds of kaolins while demonstrating the structural and chemical effects of this acid activation method. Acid activation processes result in increasing distances between the clay mineral layers because of Al₃⁺ cation dissolution alongside interlayer force breakdown. The activated samples, labeled Kaol-24, Kaol-48, Kaol-72, C-Kaol-24, C-Kaol-48, and C-Kaol-72 based on the treatment duration (24, 48, and 72 h), revealed that the crystalline structure of coal kaolinite (C-Kaol) is more sensitive to acid leaching than that of ordinary kaolinite (Kaol). These results offer crucial insights into the microstructure and surface charge evolution of kaolins under activation, enhancing our understanding of geochemical epigenetic processes and silicate alteration mechanisms. In addition, this research is helping to control surface and interface reactions in acidic conditions, whilst opening the way to wider applications in areas such as ion enrichment and transport within crystalline structures. Yassin et al. [29] examined the potential application of acid-activated natural clays as bleaching agents for the treatment of Niger seed oil (Guizotia abyssinica Cass). Their research concentrated on three clay types collected in the North Shoa region of Ethiopia, notably Zemero, Seladengay and Mehal Meda. These clays were subsequently treated with three different acids (H₂SO₄, HCl and HNO₃) at concentrations of 15%, 20% and 25% to further enhance their adsorption characteristics. Bleaching tests have shown that the clay activated with 25% sulfuric acid (H₂SO₄) gives the greatest efficiency, with a maximum efficiency of 94.5% achieved by Zemero clay under optimum conditions. Acid treatment increased the active surface area of the clays and enhanced their adsorption potential by modifying their chemical and physical characteristics. Xu et al. [30] studied the effect of acid activation on the mechanical properties and shrinkage of cement-based materials by integrating different acid activators, including H₂SO₄, HCl, and acetic acid (CH₃COOH). They have demonstrated that acid activation significantly benefits the compressive strength and decreases the shrinkage of cement-based materials. The refinement of cement particles, the higher concentration of particles in the fresh paste, and the increased density of the microstructure explain this enhancement after hardening. Among the activators tested, acetic acid had the most pronounced effect, with an increase in compressive strength of 43.7%, 29.1% and 24.8% at ages of 3, 28 and 60 days, respectively. In comparison, the gains with HCl were slightly lower, whilst those with sulfuric acid were the smallest. They report that acetic acid’s higher effectiveness results from its partially ionized properties, thereby limiting the formation of salts on the surface of cement particles and promoting more uniform dissolution. In addition, acid activation accelerates cement hydratation and prevents shrinkage through denser microstructure and more complete hydration of cement particles. These results strongly suggest that acetic acid is a most promising activator for improving the performance of cement-based materials, even though the optimum dosage must be carefully adjusted to maximize the benefits and avoid the negative impacts of excessive activation. In the same context, Trabelsi and Tlili [31] studied the effect of acid activation of raw clays on the purification of crude phosphoric acid produced by the Tunisian Chemical Group. Activation was carried out by heating a mixture of 3 N HCl and clay at 90°C for 4 h under continuous stirring. Findings indicated that this activation resulted in partial dissolution of the clays, in particular dissolution of the carbonate and degradation of minerals like calcite, dolomite and feldspar. The clays activated at this temperature exhibited the best textural qualities, with a high specific surface area for the Jebel Hamadi clay (225 m₂/g) and maximum porosity for the Jebel Stah clay (46%). In addition, the activated clays showed better purification efficiency for crude phosphoric acid, with an optimal specific consumption of 30 kg of clay per ton of P₂O₅. These results indicate that activation at 90°C significantly improves the properties of clays, especially those rich in smectite, and yields better purification outcomes compared to raw clays. Whilst chemical methods can effectively purify clays by specifically addressing inorganic and organic impurities, they can have certain limitations in terms of environmental impact and waste management. In this respect, biological processes are showing promise as an alternative, employing micro-organisms or enzymes to clean clays more ecologically and selectively. The following section examines these biological methods of purification, outlining their principles, advantages and applications.

3.2. Physical approaches

Natural clay purification techniques through physical methods serve as fundamental operations for purifying clay materials to produce uniform clay fractions that maintain their mineral composition. The enrichment methods use straightforward mechanical separation systems which exploit the size and wetting characteristics of clay and non-clay materials together with their magnetic properties and gravitational properties. These distinct properties allow the use of various physical techniques, including grinding, sieving, sedimentation, centrifugation, flotation, ultrasonic treatment, magnetic separation, and particle size fractionation, among others (Figure 3). Zhuang et al. [32] proposed a single-step method of purifying and organically modifying sepiolite (Sep) that efficiently removes impurities such as quartz, talc, and calcite. This process involved treating the sepiolite with a dispersant in water, agitating it, and then centrifuging it to obtain the purified mineral. This was then functionalized with organic surfactants to form organo-sepiolite (OSep). These surfactants act as both surface modifiers and flocculants, without altering the mineral’s structure. This method has several advantages, including simple operation, water and energy efficiency, and environmental compatibility, making it ideal for the industrial surface modification of sepiolite. The selection of purification techniques generally depends on the composition of the clay, with physical treatments favored for smectites, palygorskites and sepiolites, whereas kaolin can be easily enriched through simple dispersion in water.

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Figure 3. Illustration of laboratory-scale separation and purification techniques: (a) centrifugation, (b) sedimentation [44], (c) pulsed high-gradient magnetic separation, (d) hydrocycloning [39], (e) size-selective sieving [45].

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Grinding is a fundamental technique in the purification of clay minerals, as it reduces particle size and facilitates the separation of impurities. Using mechanical grinders, ball mills or agate mortars, raw aggregates are broken down into finer particles while contamination is minimized. This process releases clay minerals from impurities such as sand, rock fragments, quartz, calcite, and feldspar. By producing standardized particle textures, grinding enhances the efficiency of subsequent purification methods, such as sedimentation, centrifugation, and sieving. Furthermore, grinding improves the physicochemical properties of clays, particularly their adsorption capacity and surface reactivity. Furthermore, the characteristics of the resulting powder depend heavily on various parameters, such as the type and size of the grinding elements used, the rotation speed, and the time duration of the process. Thus, an appropriate grinding time positively affects the crystal structure of the sample, resulting in reduced crystal size, faster diffusion, improved catalytic efficiency, and a larger external surface area. However, grinding has some limitations compared to other physical techniques. The structural composition of weak minerals, such as smectites, becomes modified through this process because it reduces their adsorption ability while minimizing their micropores, causing particles to cluster and breaking down their crystalline structure. The modification of specific applications becomes negatively affected by these modifications. The generation of highly excessive ultrafine particles through this process makes post-enrichment procedures more difficult and lengthens their execution time. Xu et al. [33] investigated the effect of ball milling time on the purification of palygorskite clay, finding that the duration of grinding significantly impacted both the yield of residue and the development of the clay. Insufficient grinding (for less than 6 h) and low dispersant doses (less than 3%) hindered impurity separation and damaged palygorskite crystals. At optimal grinding times, however, ball collisions effectively removed impurities, reducing residue productivity while enhancing expansion capacity. Similarly, Gong et al. [4] purified Zhejiang bentonite using grinding, dispersion, and centrifugation, increasing the montmorillonite content from 44% to 96.5% with a yield of over 70%. These studies emphasize the importance of effective grinding for dissociating clay aggregates, although excessive grinding can hinder the purification process. The success of separating montmorillonite from other minerals depends on proper grinding. However, prolonged grinding diminishes the dispersion gap between the montmorillonite and the other included minerals, thus blocking the purification process. Controlling the grinding time accurately becomes necessary to reach its maximum efficiency when purifying bentonite. The clay fraction isolation through grinding operates as a preliminary procedure for purification along with sedimentation or magnetic separation methods. Grinding proves essential as a necessary initial stage above all else, especially for compact, aggregated, or strongly bound impurity-containing samples. This method provides broad-ranging efficiency when treating clay minerals, offering versatility and simple, low-cost operation. The effectiveness of grinding increases when it is combined with complementary purification processes for obtaining clay that fulfills industrial specifications.

Physical sieving is a common purification technique for clay minerals, involving the separation of mineral particles through mesh-sized sieving. The method efficiently targets quartz and feldspar impurities together with other non-clay minerals by allowing smaller clay-sized fractions to remain. Wet sieving addresses the issue of breaking up agglomerated fine particles by operating under moist conditions. Sieving is a simple and inexpensive technique that requires basic equipment, making it suitable for regions with limited technological resources. It allows for quick implementation and achieves mechanical separation of clay minerals without the use of chemicals or complex treatment conditions, thereby preserving their natural properties. However, it is only effective for particles larger than 50 µm. Smaller particles may either pass through the mesh or remain agglomerated within it; to minimize this issue, a finer mesh should be used. The method fails to distinguish between clay particles and comparable-sized rock minerals, including fine quartz and feldspars, which hinders the purity level of final products. The separation method of sieving demonstrates lower efficiency than centrifugal or flotation devices when attempting to isolate submicron fractions and achieve high purity outcomes. The use of sieving as a first purification step enables operators to team it up with sedimentation or centrifugation for enhanced overall purification processes. Natural clay was collected from the Meknes site in Morocco, sieved to 63 µm, and subsequently treated with acetic acid [11]. The resulting product showed significant quartz removal, thereby improving the clay’s physicochemical properties. Several studies have used natural clay sourced from various regions of Morocco to develop ceramic microfiltration and ultrafiltration membranes. For example, Ouaddari et al. [34] sieved Moroccan clay using an 80 µm mesh to produce membranes that could effectively remove dyes from industrial wastewater. Similarly, Abderrazek et al. used natural clay from the Midelt region, selecting grain sizes between 63 µm ≤ Φ < 250 µm. Flat supports measuring 40 mm in diameter and 2 mm in thickness were prepared by uniaxial pressing of a dry mixture containing 97 wt% clay powder and 3 wt% activated carbon. The resulting clay membranes exhibited high contaminant rejection efficiency . A previous study by Arkame et al. [24] assessed Moroccan lizardite as a ceramic geomaterial. The scientists sieved the ground sample at 50 µm to remove large particles, dolomite, and other impurities. Analysis of the results showed that the purified material contained mainly lizardite, along with chlorite, and minimal impurities, leading to the production of high-quality ceramic materials. Das Graças Silva-Valenzuela et al. [13] extracted a raw green clay through successive stages of 74 µm sieve screening and granulometric fractionation. The procedure generated suitable fractions, which could be applied in cosmetic and pharmaceutical products. The use of smaller sieves results in better clay purification performance yet introduces performance-related challenges, which demand complementary methods and require more time and spending.

Centrifugation functions as a widespread physical method to purify and concentrate clay minerals through density-based separation of particles without altering their physical or chemical characteristics. A centrifugal force of high intensity accelerates the sedimentation of dense quartz and feldspar particles, while retaining the suspension of clay particles. The sample suspension in deionized water enables centrifugal force to separate buffering fractions that contain traces of very small particles. Centrifugation proves suitable for obtaining thin clay fractions because other procedures, such as sieving or gravity sedimentation, have limited success. The small particle size of clay minerals enables this method to perform accurate purification procedures. Centrifugation is a widely used method for purifying clay, and studies have demonstrated its effectiveness under various laboratory conditions, including speed and duration. Numerous research studies throughout the literature evaluate the successful application of centrifugation in the purification of clay. For example, Chipera and Bish [35] reported that clay fractions smaller than 2 µm can be separated by centrifugation at 8,000 rpm for 40 min. Similarly, Ouaddari et al. [34] purified Moroccan clay at 2000 rpm for 15 min to produce ultrafiltration membranes for dye removal. In another study, Alshameri et al. [36] purified natural bentonite before modifying it for use as an adsorbent to remove ammonium from aqueous solutions. The researchers separated the bentonite particles from the suspension by centrifuging them at 1000 rpm for 10 min. This procedure was repeated three times to achieve total sodium ion saturation. The solid was then washed multiple times with distilled water to remove excess chloride ions. Pereira et al. [37] examined the effect of centrifugation on the purification of bentonite samples from new deposits in the Paraíba state of Brazil to determine their suitability for use in water-based drilling fluids. In this study, the samples were first purified using a centrifuge after being dried and ground. The solution went through sieving action to extract substantial particles. A mixture of deionized water and ammonium hydroxide was created to form the suspensions that would be subjected to centrifugation. The optimal centrifugation conditions occurred when the suspensions were spun at 1900 rpm for 5 min. The authors used characterization techniques to analyze particle size distribution and chemical makeup of purified samples, together with measuring rheological properties. Through centrifugation, the researchers successfully extracted accessory minerals, including quartz, iron, and carbonates, while obtaining a reduction in average size and development of clay content. Additionally, the purified samples, once rendered sodium-rich by the addition of sodium carbonate, showed significant improvements in their rheological and filtration properties, making them suitable for use in drilling fluids for the oil industry. Gong et al. [4] tested the effect of centrifugation speed on the purification process of bentonite. They showed that the percentage of montmorillonite increases relative to quartz as the centrifugation speed increases. They found that a centrifugation speed of 700 rpm was optimal when the centrifugation time was fixed at 2 min. Bahranowski et al. [38] studied the effect of purifying Na-montmorillonite on the textural properties of clay/TiO₂ mineral composites prepared by two distinct methods such as traditional grinding and mixing organo-montmorillonite with inverse micelles containing Ti oxo-hydroxy species. The researchers investigated two approaches for purifying montmorillonite through dialysis and multiple centrifugations. The slow pace of dialysis allows for better maintenance of small particles and creates some breakdown of larger components to enhance the texture of materials prepared through conventional grinding processes. The rate of centrifugation is higher than dialysis, but it results in losing fine particles that do not affect the quality of inverse microemulsion-derived composites. Research evidence demonstrates that dialysis purification increases the adsorption features of ground composites, whereas using centrifugation shortens preparation durations while maintaining their porosity values. The hydrocyclone method employed by Gama et al. [39] successfully purified bentonite clay, which then reduced the amount of silt in the clay fraction, thereby enhancing its industrial worth. An experimental data-derived CFD model demonstrated that the hydrocyclone performance simulation included hopper impact evaluation on process efficiency. Laboratory tests demonstrated that feed pressure enhancements, alongside vortex diameter modifications and underflow opening modifications, resulted in superior performance by increasing fine clay volume in the overflow, which decreased the overall particle dimensions. Feed applications beyond 30 bars did not show increased benefits, but also increased the possibility of cavitation events. Achieving a 6 mm underflow opening combined with 30 bar feed pressure resulted in a 40.16% decrease in average particle size that reached 3.39 µm with a fine particle volume fraction of 39.94% in the overflow product. The established empirical correlation and scaling method adoption for other hydrocyclone models enables maximum recovery of fine clays.

Boylu et al. [40] evaluated the separation of sodium bentonites from the Reşadiye deposit in Turkey using hydrocyclone testing. The Reşadiye deposit, one of the largest in the world at around 300 million tonnes, was processed using multi-stage hydrocycloning with variable vortex and apex diameters and feed pressures. CEC was found to be a key indicator of separation efficiency and industrial value, particularly for applications such as drilling fluids and cosmetics. A high-quality bentonite concentrate that met commercial CEC standards was obtained using a two-stage hydrocyclone process, while lower CEC residues were suitable for soil rehabilitation. This approach enables the large-scale production of purified bentonite without the need for chemical treatments and offers a potential alternative to Wyoming bentonites. Centrifugation complements these separation methods by precisely sorting particles based on size and density. This process effectively isolates fine and deflocculated clays while preserving fragile smectite structures, in contrast to grinding, which can damage clay properties. Centrifugation also outperforms sieving for submicron particles, resulting in a cleaner clay. However, it requires specialized, energy-intensive equipment, which limits its feasibility in settings with limited resources.

Sedimentation is a purification method for clay minerals that utilizes physical parameters to effectively separate clay minerals according to Stokes’ law, whereby small particles rise more slowly than larger ones. Dense, large-sized particles such as quartz descend rapidly to the bottom of containers, whereas fine clay materials remain suspended for longer. This process is typically carried out in columns or containers filled with water or another liquid, and the addition of dispersants can optimize it by preventing particle aggregation. During sedimentation, clay minerals, which are characterized by their very small particle sizes, remain in suspension while coarser minerals settle due to gravity. The separation of clays from other minerals is then carried out by decantation. To improve purification efficiency and reduce particle interactions, the recommended solid content in the water is between 0.5% and 1.0%. This method thus enables the economic recovery of fine clay fractions while preserving their structural properties. We previously enriched a Ghassoul-type clay using sedimentation extraction with a NaCl solution, resulting in enhanced properties of the clay [2]. Sedimentation is notable for its simplicity and low cost, as it requires no sophisticated equipment. It is particularly useful in settings where resources are limited. Furthermore, this method preserves the structural properties of clays, as it does not involve the application of significant mechanical forces that could alter them. Sedimentation maintains better mineral crystallinity than advanced separation methods such as grinding and centrifugation. However, sedimentation has some limitations. The speed of sedimentation is slower than that of centrifugation, making it less suitable for time-sensitive analytical work. Sedimentation is ineffective at distinguishing particles with similar densities, and very fine fractions under 2 µm settle at a reduced pace. Furthermore, the sedimentation method is not well-suited to large sample quantities due to the significant amount of water required. Locations with limited water supplies are disadvantaged by the technique’s high-water requirements.

The flotation technique remains a common method for purifying clay minerals because it utilizes differences in the surface properties of particles to achieve its results. The method utilizes the selective adhesion principle to cause particles to adhere to air bubbles inserted into fluid suspensions. A froth layer containing hydrophobic particles forms on the surface of the liquid as it approaches the floating air bubbles, while hydrophilic clays settle to the bottom of the container. The flotation process achieves better results when collectors, depressants and modifiers are included because these substances modify the surface characteristics of essential minerals. Luz et al. [41] conducted research to improve the beneficiation of kaolin clay from northern Brazil using the R6973 alkylhydroxamate collector in a flotation process. This collector’s absorption process functions without the need for activators to successfully adsorb impurities. The flotation process reduced TiO₂ content from 1.9% to nearly 1%, while maintaining a recovery rate of between 70 and 76%, despite the fact that the majority of clay particles (90%) were smaller than 2 µm, making it difficult to eliminate colored impurities such as TiO₂ and iron oxides. The TiO₂ content decreased to 0.8% when performing selective flocculation, resulting in only a 56% recovery rate. Experimental testing of clay from eastern Georgia demonstrated that flotation produces less efficient results when processing submicron particles. Research continues to overcome the constraints of flotation when processing submicron particles, as current experiments have highlighted these challenges. Flotation is exceptional at separating particles of identical density and is best suited to purifying clay fractions by removing mica or pyrite impurities. This separation method is fast and can be applied to diverse sample types by adjusting the selected reagents. Flotation outperforms sedimentation and centrifugation when processing uniform physical particles. However, the technique has several technical constraints. Operators require specialized equipment and in-depth technical knowledge to handle important operating parameters, including reagent concentration and suspension pH. Using chemicals for the process is costly and generates environmental issues related to waste effluent management. Flotation is ineffective in treating fine-grained or strongly hydrophilic clay particles, as these clays do not adhere adequately to air bubbles for separation. Despite this, flotation is still widely used for clay purification, often in combination with techniques such as sedimentation or centrifugation, to achieve optimal results while handling operational and environmental limitations.

Ultrasonic treatment is an advanced physical method of purifying clay minerals that involves applying high-frequency sound waves to aqueous suspensions. These waves generate acoustic cavitation, which is the formation and implosion of microbubbles. This process produces strong mechanical forces capable of breaking up particle aggregates, separating clay minerals from impurities, and improving the dispersion of fine particles. Barakan et al. [42], studied the ultrasonic purification of bentonite clays rich in montmorillonite. They compared the effectiveness of standing ultrasound (90 W for 2 min) and bath ultrasound (400 W for 30 min), finding that bath ultrasound preserved clay structures while improving purification efficiency and montmorillonite quality. Combining bath ultrasound with a high centrifugal force of 1914 g increased the montmorillonite content from 15% to 84% and achieved an 80% separation yield. Ultrasound separates microflocs from aggregated illite sheets while minimally affecting crystallinity, producing particles with relatively uniform dimensions. Although ball milling is more effective at enhancing UV-blocking properties, ultrasonic treatment is a gentler method of morphological modification. Ultrasonic purification is a relatively fast process that is compatible with other techniques, such as flotation or sedimentation. It is particularly suitable for fine clay particles. However, it has several limitations, including high equipment costs, high energy demands, the potential for structural alterations with prolonged exposure and unsuitability for processing large sample volumes. This restricts its use to laboratory-scale studies.

The magnetic separation technique is the primary method used to purify clay minerals, as it leverages variations in the magnetic susceptibility of the mineral phases. This method involves placing ground, mixed particles through non-homogeneous magnetic field systems. When subjected to a magnetic field, magnetic particles, including those of hematite, magnetite, and various iron oxides, deviate from their path due to magnetic attraction, while non-magnetic particles, such as kaolinite, montmorillonite, and halloysite, remain on course. Controlling the separation points correctly enables the separation of magnetic minerals from their non-magnetic counterparts. This technique is used in the purification of clay, producing several products such as polymer nanocomposites, sorbents, and ceramics. The method performs exceptionally well technologically for extracting iron and titanium oxides from large-sized fractions found in aluminosilicate deposits. The technique offers benefits such as straightforward operation, maintenance of clay properties, minimal environmental impact due to chemical abstention, and adjustable magnetic field strength according to requirements. However, magnetic separation is ineffective with non-magnetic materials such as quartz or carbonates, and the use of high-intensity equipment increases costs. This method also requires complex adjustments to process weakly magnetic impurities. Magnetic separation is an effective and eco-friendly purification technique that works best in conjunction with other methods to optimize clay purification. Sakiewicz et al. [43] explored the use of a chemical treatment alongside magnetic separation processes to enhance the quality of halloysite at the Dunino deposit in Poland. The clay purification process involves grinding the material, followed by the application of HCl and subsequent sedimentation steps, leading to polygradient magnetic separation at a low magnetic field strength. The experimental process yielded an aluminium and silicon mineral fraction consisting mainly of halloysite and kaolinite, with an estimated purity of 98%. The magnetic separation residues consisted of iron oxides, while the intermediate product contained aluminosilicates and iron chlorides resulting from the chemical treatment. These products can be used to manufacture polymer nanocomposites, sorbents, and ceramics. Magnetic separation was highly effective in removing difficult-to-eliminate magnetite (Fe₃O₄) minerals and other heavy magnetic minerals. The researchers found that grinding was fundamental to enhancing the liberation of light aluminosilicates and optimizing magnetic recovery. This integrated physico-chemical method offers promising opportunities for obtaining pure, industrial-grade materials.

Particle size fractionation is a simple and gentle physical method for purifying clay minerals based on differences in particle size. It includes processes such as sieving, sedimentation, and centrifugation, which separate fine clay fractions from coarser impurities, including quartz, feldspar, and metal oxides [44,45]. As particle size distribution is one of the most stable characteristics of soil and is rarely altered by cultivation or other practices, particle analysis is a reliable method. Traditional fractionation methods require particles to be dispersed in an aqueous solution, allowing them to be detached and suspended in the liquid. Particle size fractionation is commonly used to extract fine clay minerals, typically measuring less than 2 µm, via processes such as wet sieving or controlled sedimentation. Kaolins, for example, are mainly composed of one primary clay mineral, accompanied by other clay minerals as impurities. To obtain high-purity clay minerals, these clays are often subjected to large-scale particle-size fractionation processes to enrich their composition. By adjusting parameters such as sedimentation speed or sieve mesh size, homogeneous and pure fractions can be obtained. This technique is simple, cost-efficient, and non-damaging, protecting clay chemicals and crystal structure [13]. The method can be precisely adjusted to work with different minerals and impurities through proper modifications. However, particle size fractionation becomes ineffective when trying to separate impurities that are comparable in size to the clays themselves. To maximize the effectiveness of this technique, it must be combined with both magnetic separation and chemical treatments. This method uses significant amounts of water during operation and incurs ecological costs for waste disposal management, making it impractical for large quantities of material despite its effectiveness. Despite these limitations, particle size fractionation remains a valuable solution and is often used in conjunction with other processes to enhance the purity of clay minerals. According to Thuc et al. [46], particle size fractionation improved the CEC of montmorillonite and imparted important modification characteristics to the enriched clay. Physical methods of clay purification, such as particle size fractionation, magnetic separation, and flotation, effectively remove a variety of impurities by exploiting differences in particle properties. These techniques preserve the crystalline and chemical integrity of clay minerals, making them environmentally friendly. However, they are less effective at removing very fine or chemically bound impurities, necessitating additional treatment for complete purification. While physical methods are an essential step in clay processing, they often cannot achieve the required level of purity for specific applications alone. Consequently, chemical purification methods are typically employed as a subsequent phase, building on the natural separations achieved through physical treatment.

Using plasma to enrich clay remains a promising approach and could provide an environmentally friendly alternative method. Sousa et al. activated palygorskite, a clay mineral, using plasma treatment (H₂/N₂) for 2, 4, and 6 h in order to create new adsorbents for the removal of methylene blue (MB). Both natural and activated palygorskite were characterized using XRD, fourier transform infrared spectroscopy (FTIR), TGA/DTA, SEM-EDS, and N₂ adsorption/desorption techniques. Eliminating coordinated and zeolitic water molecules, among other structural flaws caused by plasma treatment, altered the specific surface area and pore volume of palygorskite. The adsorption capacity of the activated samples increased significantly. This increased from 35.66 mg/g for natural palygorskite to 54.17 mg/g following plasma treatment. Based on the experimental results and kinetic models, a mechanism was proposed to describe the interaction between the adsorbent and methylene blue. The effect of factors such as temperature, concentration, duration, pH, and adsorbent dose on methylene blue adsorption was evaluated. This plasma activation of palygorskite represents a promising approach to developing new, effective adsorbent materials for removing pollutants from water.

3.3. Biological methods

To overcome the limitations of chemical and physical techniques in terms of cost and environmental impact, biological methods are emerging as a promising alternative for removing associated minerals. These methods can play a significant part in removing contaminants such as carbonates, silica, mineral oxides, and hydroxides by utilizing complexation, acid leaching, and reduction mechanisms. Biological techniques are based on the principles of biohydrometallurgy, where microorganisms play a crucial role and can significantly influence the mineralogical composition of clays. This mineralogical modification can be selectively achieved between the minerals and metals present in the material. The appeal of these techniques is specifically linked to their low cost, use of non-toxic products, and low energy footprint, which are beneficial for sustainable development approaches. Such techniques require mastering the interactions between minerals and microorganisms, as well as creating ideal conditions to stimulate the desired biological activity. Compared to other chemical and physical techniques, biological methods take longer and their efficiency varies from one ore to another. Moreover, these biological techniques generally require specific conditions to control the biological activity of microorganisms. Biological purification of minerals involves several microbial processes that remove impurities and enhance material quality. Key mechanisms include bioflotation, bio-flocculation, biolixiviation, biosorption, bioreduction, and enzymatic bioremediation, each contributing to the recovery of purified fractions suitable for technological and environmental applications.

Additionally, the use of eco-friendly solvents, such as supercritical water and ionic liquids, for clay purification is an expanding field of study aimed at optimizing procedure performance while minimizing ecological footprints. Ultimately, the scientific community must commit to developing and scaling these methods for large-scale applications to achieve more sustainable and environmentally friendly purification. Table 2 [47] illustrates a comparison of the principles, advantages, disadvantages, and environmental impact of the three purification processes discussed.

4.1. Mineralogical and structural characterization

The mineralogical characterization of clay is mainly based on techniques such as XRD, FTIR, Raman spectroscopy, and nuclear magnetic resonance (NMR). XRD is used to identify crystal structures and mineral phases, while FTIR provides information on functional groups and chemical bonds. Raman spectroscopy gives an idea of the structural order/disorder of silicates. NMR provides information on the coordination environment of aluminum and silicon, helping to detect structural changes after the purification process. The diverse chemical composition, structural disorder, and wide structural variety of clays make the characterization of clay minerals highly challenging. Indeed, XRD has become one of the most widely used techniques for qualitative and quantitative analysis of these minerals. To simplify the distinction between clay minerals present in a given clay fraction, purification is recommended to concentrate these minerals and obtain intense peaks that are sensitive to slight variations under specific treatments. The principle of XRD differentiation is primarily based on determining the basal spacing of the (hkl) planes. Furthermore, the interaction with the applied treatment differs from one clay mineral to another. For this purpose, the authors proposed four tests to differentiate clay minerals based on the changes in interlayer distance. The normal test involves air-drying for reference. The ethylene glycol (EG) test identifies swelling clays. The 300-500°C heating test distinguishes kaolinite (disappears at ∼500°C) and collapses vermiculites/smectites to 10 Å. It also identifies interlayered minerals. Heating between 100-350°C preserves kaolinite while reversibly dehydrating smectite and vermiculite. Finally, the hydrazine monohydrate test distinguishes kaolinite from chlorite by swelling the kaolinite without affecting the chlorite, based on the peak at 7 Å. To confirm the results of XRD analysis, several researchers have used FTIR technique to analyze clays. This technique is commonly employed to characterize clay by identifying its functional groups and mineralogical composition, enabling the detection of specific vibrations of chemical bonds present in clay minerals. In general, the FTIR analysis was performed by mixing the finely ground material powders with potassium bromide (KBr) and then dried at 60°C (w/w:4/96 ratio of sample/KBr), the FTIR spectra were recorded in the range 400–4000 cm⁻¹. FTIR spectra generally reveal characteristic bands attributed to dominant minerals such as kaolinite, illite, montmorillonite, and quartz. For example, a band located between 3600 and 3700 cm⁻₁ is associated with the vibrations of hydroxyl (–OH) groups present in the layers of clay minerals, particularly kaolinite and illite. Another band, around 1650 cm⁻₁, corresponds to the bending vibration of adsorbed water molecules. Bands between 900 and 1100 cm⁻₁ are attributed to the vibrations of Si–O bonds in silica tetrahedra, characteristic of silicates such as quartz, while a band at 800 cm⁻₁ is often linked to the presence of quartz. Finally, bands around 500-600 cm⁻₁ are associated with the vibrations of Al–O and Si–O bonds, indicating the presence of phyllosilicates such as kaolinite and illite. According to A. El-Kordy et al. [48] analyzed clay collected from the Midelt region using XRD, revealing the presence of quartz (Q), calcite (C), kaolinite (K), smectite (S), and illite (I). The main diffraction peaks were identified at the following 2θ values: Quartz (20.85°, 26.55°, 30.67°, 39.31°, and 42.28°); Calcite (22.90°, 35.90°, and 39.56°); Kaolinite (12.18° and 27.70°); and Smectite (19.71° and 34.54°). The clay’s reddish color was attributed to its iron oxide content (Fe₂O₃, 5.2 wt%). Upon sintering at 1000°C, kaolinite and smectite phases disappeared, while mullite (3Al₂O₃·2SiO₂) formed through the thermal transformation of metakaolin (Al₂O₃·2SiO₂), confirming matrix stabilization at elevated temperatures. A. El-kordy [48] used FTIR analysis to complement the results of XRD and confirm the mineralogical composition of the raw clay and clay supports. The FTIR spectrum of the raw clay showed broad bands between 3386 and 3640 cm⁻₁, which were attributed to vibrations of hydroxyl groups (Si–OH, Al–OH, Fe–OH, and Mg–OH) and adsorbed water molecules. A band at 1440 cm⁻₁ indicated carbonate stretching vibrations due to the presence of calcite (18.79 wt%), while peaks at 460 cm⁻₁ and 1027 cm⁻₁ corresponded to Si–O vibrations. Additional peaks at 518 cm⁻₁ and 867 cm⁻₁ were linked to quartz and kaolinite, respectively. Following calcination at 1000°C, the hydroxyl-related bands disappeared, leaving only illite, quartz, and the newly formed mullite phase. In an important study, Belghazdis et al. [11] confirmed the disappearance of calcite-related peaks, the decrease in the intensity of quartz peaks, and the increase in the intensity of clay mineral peaks. These three events confirm the success of the purification process. In addition, Raman spectroscopy was introduced by Gibbons et al. [49] to improve the identification of clay minerals present. They effectively detected the minerals present as well as associated minerals such as quartz, calcite, anatase, and topaz based on the position of the Raman peaks. The FTIR analysis reported by Bouna et al. [50] highlights the main characteristic bands in the raw clay, corresponding to kaolinite, muscovite, quartz, and physisorbed water. After sintering at 800°C, the clay exhibits an almost complete disappearance of the bands associated with OH groups (from water and structural hydroxyls), confirming dehydroxylation and the formation of metakaolinite. For the geopolymer samples (TANT-GP, TANT-GPS, TANT-GPSH), new broad bands were observed at 3000–3700 cm⁻₁ and 900–1200 cm⁻₁, characteristic of amorphous structures. A shift in the Si–O–Si vibration band from 1038 to 1026 cm⁻₁ confirmed alkaline activation, while the appearance of a shoulder at around 692 cm⁻₁ indicated the formation of Si–O–T bonds (T = Si or Al). XRD analysis further supported these findings. The raw clay displayed the typical diffraction peaks of kaolinite (d001 = 7.15 Å), muscovite (d001 = 9.9 Å), and quartz. After calcination, kaolinite peaks disappeared entirely, and muscovite was transformed into an aluminum–potassium silicate phase, consistent with the formation of metakaolinite. In the geopolymers, a broad amorphous hump in the 2θ range of 20–35° appeared, typical of geopolymer phases, though attenuated by the persistence of quartz and other non-reactive silicates [50].

4.2. Chemical characterization

The chemical characterization of clay can be performed using X-ray fluorescence (XRF) spectroscopy, inductively coupled plasma (ICP) atomic emission spectroscopy, and CHNS analysis. XRF spectroscopy is effective in determining the elemental composition of clay minerals in terms of oxides. Monitoring changes in this composition therefore confirms the success of the purification process. In addition, ICP remains highly sensitive to trace and even ultra-trace elements in clay minerals, making it suitable for detecting any changes in the chemical composition of clay, particularly before and after treatment. Finally, CHNS analysis is used to quantify carbon, hydrogen, nitrogen, and sulfur content, making the technique suitable for determining the amounts of organic matter in clay minerals. In this context, Egole et al. [5] determined the chemical composition of two mixed clays using the XRF technique. Based on the oxide contents (SiO₂, Fe₂O₃, Al₂O₃, K₂O, CaO, P₂O₅, MgO, TiO₂), they were able to detect the clay minerals present as well as impurities.

4.3. Surface charge

Identifying surface charge is another important indicator in detecting clay minerals and associated minerals, as well as confirming the removal of mineral impurities. For example, CEC is used to estimate the total amount of negative charges present in clay. Monitoring changes in this property confirms the success of the purification process. Similarly, depending on the CEC values, the mineralogical composition of the clay can be estimated, since each clay mineral is characterized by CEC values . Belghazdis et al. [51] reported that the CEC of natural clay from Meknes increased from 22.5 meq/100g to 29 meq/100 after purification using a combination of physical and chemical processes. This increase was attributed to the removal of associated minerals characterized by low CEC values. At the same time, zeta potential is another parameter that warrants monitoring during the treatment of clay minerals. It provides information on the surface charge of clay particles as a function of pH, making it suitable for assessing colloidal stability. Monitoring this parameter as a function of pH allows the detection of associated minerals, particularly metal oxides and hydroxides. Treatment efficiency can also be assessed by the pH at the pH_PZC. This test is sensitive to the mineralogical composition of the surface of the clay sheets. In the study by Belghazdis et al. [51], the pH changes from 11 for raw clay to 8 for purified clay, indicating the removal of basic phases such as calcite. In summary, the combination of these three tests is an excellent approach for detecting clay minerals and validating the purification process.

On the other hand, electrochemical techniques are emerging as highly suitable tools for estimating the surface charge of clay minerals. Such approaches improve the reliability, sensitivity, and speed of surface charge research compared to conventional methods. Electrochemical techniques include electrochemical impedance spectroscopy, potentiometry, voltammetry, and flow potential measurements. Their principle is based on monitoring the electrical response at the interface between an electrolytic solution and clay minerals [52]. The changes recorded in current, potential, and impedance provide insight into variations in surface charge. At the same time, these innovative approaches are more environmentally friendly, making them preferable and attractive for characterizing the surface charge of clay minerals.

4.4. Morphology and microstructure

Morphological analysis is another crucial step that can detect the presence of clay minerals and impurities, as well as confirm the removal of associated minerals after purification. The most commonly used techniques for viewing the morphology of clay minerals are SEM, transmission electron microscopy (TEM), and atomic force microscopy (AFM). These techniques allow the structure to be visualized at micrometric and nanometric scales. SEM coupled with Energy Dispersive Spectroscopy (EDS) is a powerful technique widely used to investigate the microstructure, morphology, and elemental composition of clay materials. Several studies have employed SEM/EDS to analyze clays [53], providing valuable insights into their textural properties and chemical composition. SEM micrographs of clay typically reveal an irregular, porous, and layered structure, characteristic of phyllosilicate minerals. The morphology varies depending on the clay type and its mineralogical composition, with kaolinite, illite, smectite, and quartz being the most commonly identified phases. The presence of aggregated and plate-like particles is often observed, indicating the natural compactness and textural arrangement of the clay. Additionally, sintered clay samples exhibit significant changes in morphology, with particle densification and grain growth due to thermal transformations. EDS analysis further complements SEM observations by providing qualitative and semi-quantitative elemental composition data. The primary elements detected in natural clay include silicon (Si), aluminum (Al), oxygen (O), and iron (Fe), confirming the presence of aluminosilicates. Other elements, such as calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na), may also be present, depending on the clay’s geological origin and mineralogical impurities. In some studies, EDS spectra of sintered clay samples indicate the formation of new mineral phases, such as mullite and calcium silico-aluminates, resulting from high-temperature treatments. At the same time, TEM allows for greater resolution than SEM, reaching the nanometric scale. It is useful for distinguishing between the clay phases present, allowing even the values of interlayer distances to be detected. After the purification process, a comparison of the images can confirm the elimination of foreign phases. Finally, AFM provides three-dimensional images of the surface topology at the nanometric scale. It is capable of detecting changes in clay morphology, measuring sheet thickness, and the degree of exfoliation of clay particles. Belghazdis et al. [51] reported the characterization of a clay from Meknes purified by MET during the preparation of clay-polymer hybrids for the construction industry. The results showed a clear structure allowing the determination of clay minerals (kaolinite, illite) and associated minerals, particularly quartz. A similar study was carried out by Ghyati et al. [54] on Meknes clay using SEM. They showed the presence of illite, kaolinite, and quartz in the clay fraction analyzed. In summary, the combination of these three techniques simultaneously constitutes a strategy for visualizing the morphology of clay at different scales. It will also make it possible to establish a link between microstructure, composition, and surface properties. According to El-Kordy et al. [48] SEM/EDS analysis revealed that the clay support had a porous morphology with both mesopores and macropores, smooth surfaces without cracks, and particles that lacked clear crystallinity. EDS analysis showed that the material was mainly composed of silicon (Si), aluminium (Al), and calcium (Ca) (53 wt%), with oxygen (46 wt%) corresponding to their oxides (SiO₂, Al₂O₃, CaO). N₂ adsorption-desorption isotherms of both the raw clay and the clay support were classified as type IV with H3 hysteresis, which is typical of mesoporous materials. This indicates irregular pore size, distribution, and shape, which is consistent with the SEM observations.

4.5. Textural properties

Clay purification processes can also be evaluated by examining textural properties, in particular specific surface area (BET) and porosity distribution. These techniques, based on gas adsorption, isotherm analysis, and porosimetry, also enable the detection of clay minerals and associated minerals. The BET method is widely used to characterize clays, providing essential data on their specific surface area and porosity. This technique is based on the physical adsorption of nitrogen gas at a temperature of liquid nitrogen, enabling the total surface area of a given clay sample to be determined. The BET method also enables the study of pore size distribution, which plays a crucial role in determining the application efficiency of clays. The surface area is influenced by factors such as mineral composition, purification treatments, and thermal modifications, making BET analysis an essential tool for optimizing clay applications in environmental and industrial fields. This parameter can be used to confirm the success of the purification process. The removal of associated minerals with low porosity, such as quartz, feldspar, and calcite, leads to an increase in BET. At the same time, mineralogical identification is possible thanks to knowledge of the specific surface areas of swelling clays (smectite, bentonite, stevensite) and non-swelling clays (illite, kaolinite) [11]. Furthermore, interesting information can be obtained from the analysis of adsorption-desorption isotherms. The type of hysteresis gives an idea of the stacking of clay layers and pore geometry. Techniques based on this principle include the Barrett-Joyner-Halenda (BJH) method and density functional theory (DFT). Finally, the mercury intrusion pyrometer (MIP) is a complementary technique to BJH and MIP at the macropore scale, providing pore distribution at the mesopore and micropore scales. It allows visualization of particle organization at the micrometer scale and confirmation of the presence of compact aggregates. In the study by El-Kordy et al. [48], BET analysis showed that the specific surface area decreased from 34.40 m₂/g for the raw clay to 0.48 m₂/g for the clay support, which was attributed to grain consolidation at high temperatures. BJH analysis revealed pore diameters of 47.77 Å for the raw clay and 35.50 Å for the clay support, confirming mesoporosity. However, the pore volume dropped significantly from 0.04108 cm₃/g for the raw clay to 2.13 × 10⁻₃ cm₃/g for the clay support after sintering. N. Chowdhury et al. [55] investigated the effect of drying methods and initial moisture content on pore structure using natural bentonite from Columbus Co., USA. A standard Proctor test (ASTM D698–12) revealed a maximum dry density of 1200 kg/m₃ and an optimum water content of 27%. Based on these results, two sets of samples were prepared with the same dry density (1121.6 kg/m₃) but with different water contents. First 15% to represent a dry state, and the second 36% to represent a wet state. The clay was first mixed with water to produce large lumps, which were then broken down by sieving through a 3 mm sieve. The sieved soil was then stored in sealed plastic bags for two days to ensure uniform moisture distribution. Specimens were then statically compacted in oedometer rings (70 mm diameter × 19 mm height) in two layers, with careful control of compaction to ensure uniform void ratios. Identical cubes were taken from both the dry and wet samples and subjected to different drying methods before characterization. These included freeze-drying, oven-drying, and air-drying (with and without outgassing), as well as outgassing-only procedures performed at 120°C for 150 min (O-d, O-w) and at 25°C for 24 h (O24-d, O24-w). The microporosity of the bentonite was analyzed using the BET/BJH and MIP methods to evaluate the pore size distribution (PSD) and the specific surface area (SSA). The results showed that drying methods have a strong influence on both pore volume distribution and total pore volume. Of the techniques studied, outgassing at 120°C for 150 min (O-d and O-w) was found to best preserve the original pore structure, with dry and wet specimens exhibiting nearly identical PSDs. In contrast, freeze-drying, oven-drying, and air-drying resulted in significant changes to the pore distribution, particularly following additional outgassing. The highest specific surface area was found in specimens treated at 120°C, with O-d and O-w exhibiting values of 25.99 and 25.76 m₂/g, respectively. This indicates the efficient removal of both free and adsorbed water, as well as the exposure of more adsorption sites. Dry samples consistently exhibited higher cumulative pore volumes than wet samples across all methods, since wet specimens underwent greater shrinkage and microstructural densification during drying. Outgassing-only treatments (O-d, O-w, O24-d and O24-w) produced similar pore size distributions in the meso- and macropore regions, reflecting the uniform effect of this technique.

4.6. Thermal behavior

Analyzing the thermal behavior of clay is an effective strategy both for detecting clay minerals and confirming the success of the purification protocol. Thermogravimetric analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC) are well-known thermal methods capable of producing relevant information about clay minerals. They can capture mass losses, the disappearance of clay phases, and phase transitions as the temperature increases. For example, monitoring mass loss using TGA is an effective approach for detecting the presence of organic matter in clay. After clay purification, this analysis effectively detects the removal of these materials and confirms the success of the enrichment process. The same applies to calcite and dolomite, which show mass losses above 700°C. ATG also confirms the presence of clay minerals, such as kaolinite, which disappears at around 490°C. DSC and ATD complement ATG analysis by detecting endothermic and exothermic phenomena triggered after high-temperature heating. They are very useful in detecting phenomena not monitored by mass analysis, such as the structural reorganization of clay. The ability to combine these three tests gives thermal analysis a unique strength in identifying the minerals present and the final quality of the purified clay [11]. For example, A. Elgamouz et al. [56] used clay collected in the Safi region which is adjacent to the Middle Atlas, two clay samples were collected named SA and CH. The sampling site is located approximately 10 km from the center of Safi. The thermal analyses (TGA/DTA) shown in this document were recorded for the SA clay material only, due to similarities. Three main losses could be noted, the first occurring at the beginning of the process, which was attributed to the loss of adsorbed water on the surface. The second loss between 428°C and 580°C is due to kaolinite decomposition. The third loss, which begins at ̴ 660 and ends at ̴ 750°C, is linked to carbonate decomposition. In DTA, four endothermic peaks and one exothermic peak are shown. These peaks correspond to the losses observed in the TGA analysis. The first, at 93°C, was attributed to the loss of adsorbed water, while the peak at 388°C could be attributed to the loss of structural water in the clay mineral. The third peak at 520°C, with a large amplitude, was attributed to dehydroxylation of kaolinite. The peak at 735°C, also of high amplitude, was attributed to the decomposition of carbonate minerals, which produce CO₂ as they decompose. Finally, the peak at 928°C was attributed to the phase transformation of the raw clay mineral.

4.7. Geotechnical properties

Geothermal properties, particularly Atterberg limits, swelling, and water retention, are essential for assessing the long-term stability of the material. Atterberg limits (liquidity limit (LL), plasticity limit (LP)) are evaluated in order to characterize the water behavior and plasticity of clay minerals. The presence of swelling clay minerals such as smectite and bentonite is indicated by an increase in LI and PL. Such an increase confirms the removal of the associated minerals and therefore the success of the purification process. Conversely, low LL and LP values indicate the presence of non-swelling clay minerals such as illite, kaolinite, etc., or contamination of the clay fraction by associated minerals. Similarly, swelling tests are an excellent strategy for detecting and quantifying the presence of swelling clay minerals. Estimating this parameter after and before purification is a robust strategy for confirming the removal of associated minerals and the concentration of clay minerals. High swelling indicates the dominance of swelling minerals over very low levels of associated minerals. Similarly, water retention tests can provide interesting information about the interaction of clay with water. This interaction provides access to the surface charge, the interlayer environment of the clay, and the specific surface area. When clay shows a high water retention capacity, the presence of swelling clays is most likely, while the presence of non-swelling clay minerals or associated minerals reduces this characteristic. Finally, estimating geotechnical properties is a direct strategy for detecting clay minerals and confirming the success of the purification process applied. In his study, Polidori [57] introduced Atterberg limits to confirm the relationship between these parameters and clay content. He evaluated the liquid limit and plastic limit of six clays and their mixtures. The results reveal a linear relationship between Atterberg limits and clay content.

4.8. Advanced and emerging techniques

Advanced techniques capable of performing detailed characterization of the surface properties, chemical composition, and microstructure of clays have emerged in recent years. They have been developed to provide complementary analysis to conventional and standard analytical techniques and play an essential role in validating the purification process. In this context, X-ray photoelectron spectroscopy (XPS) is one of these emerging techniques capable of estimating the chemical composition of surfaces and the oxidation states of the elements present. Changes detected in the signals of Si, Al, O, Mg, Fe, Na, Ca, etc. atoms confirm the presence of clay minerals and the effect of the purification process on the clay structure. In addition, secondary ion mass spectroscopy (SIMS) remains highly effective for the efficient detection of associated minerals, even in trace amounts. It is particularly suitable for detecting alkali and alkaline earth metals, which are difficult to detect using conventional techniques. The success of the purification process is determined by the reduction in signals from these residual elements. 3D tomography is another technique used to assess the impact of the purification protocol applied to the internal structure of clays. It is designed to evaluate and quantify the microstructure of these remarkable materials without altering their structure. Based on the information obtained, in particular the pore size distribution and total porosity, the method is able to determine the nature of the clay minerals and the quality of the purification process. Temperature-programmed desorption/adsorption (TPD/TPR) techniques are another emerging complementary tool for analyzing the density, nature, and strength of active surface sites. Temperature-programmed desorption (TPD) is based on the search for NH₃, CO₂, and H₂O molecules, which helps to detect acidic, basic, or hydrophilic sites and subsequently identify the clay phases. The detection of associated minerals before and after the purification process focuses on the disappearance of the characteristic signals assigned to these impurity minerals. As for TPD, it is more effective in searching for targeted impurities, particularly metal oxides such as Fe₂O₃ and MnO₂, which are often detected in natural clays. Thus, multi-scale characterization is possible thanks to the combination of these emerging techniques for the detection of clay phases and associated minerals, as well as the validation of the success of a specific treatment.

The integration of different previous characterization techniques provides a comprehensive understanding of the structural, mineralogical, and textural properties of clay. This holistic knowledge is essential for optimizing its performance and ensuring its effective use across a wide range of applications, including environmental remediation, adsorption processes, cosmetic formulations, pharmaceutical products, and ceramic manufacturing.

5.1. Environmental remediation

High-purity clay also has interesting applications in environmental fields, particularly for the removal of heavy metals and highly toxic organic pollutants in wastewater. Their excellent adsorption capacity makes them strong competitors to activated carbon in this area . For instance, Iboustaten et al. [59] studied the adsorption of methylene blue (MB) using Moroccan purified clay/alginate beads. They optimized the adsorption process conditions using the response surface methodology (RSM). The composite beads were made by the drop method, and their effectiveness in removing methylene blue was evaluated based on parameters such as pH, adsorbent mass, and initial MB concentration using the Box-Behnken method. Total removal of methylene blue reached 100% when using a pH of 11.8 along with 4.9 g of adsorbent with an initial MB concentration of 191 ppm. The adsorption mechanism operated through a single adsorption layer, which accepted the data points of pseudo-second-order kinetics and the Langmuir theory. Five consecutive regenerations demonstrated that the adsorbent beads retained their original adsorption efficiency without any decline in performance. Moroccan purified clay/alginate beads prove themselves as an effective, sustainable adsorbent, which shows reliable waste management capabilities regarding methylene blue removal from wastewater. The scientific team of Ouaddari et al. [34] developed new UF ceramic membranes from purified Moroccan natural clays to eliminate textile industry wastewater dyes. Physical and chemical purification was performed on two raw clay materials, known as rough clay A and smooth clay B. The purified clays exhibited enhanced mineral content for smectite, kaolinite, and illite, and reduced mineral content for calcite, dolomite, and quartz, as determined by chemical and mineralogical evaluations. The production of membranes relied on purified clays that required thin-layer application on support base materials before undergoing centrifugal separation and high-temperature treatment (800°C). The filtration tests using membranes performed dyes elimination from liquid at 3 bars pressure leading to 99% rejection with Clay A and 97% rejection with Clay B. Numerous research studies have proven the successful application of ultrafiltration ceramic membranes derived from Moroccan clay purification techniques for wastewater dye removal operations that use economical resources. Salem et al. [60] employed acid treatment to produce nanoporous adsorbents from natural clay for use in the regeneration of used lubricating oil. The authors examined how acid type combined with concentration and contact duration influenced the porous nature of the adsorbent material. The montmorillonite layer spacing decreased by 0.27 nm through acid treatment that used sulfuric acid as the activator agent because of layer protonation. New nano-pores of 10 to 12 nm formed during the process to boost the clay’s absorption abilities. Sulfuric acid treatment yielded the most effective results in increasing the specific surface area (SSA) of the clay, thereby enhancing its capacity for contaminant removal, particularly in the extraction of used oil color. The utilization of nitric acid activation produced a larger number of nano-pores compared to sulfuric acid activation, although it reduced the decolorization efficiency of used oil. Research findings demonstrated that acid concentration levels, together with acid types, directly affect the performance of the regeneration process, and the montmorillonite layer spacing needs precise management to enhance bentonite activation for used oil purification. Taxiarchou and Douni [61] reported activating a bentonite from Milos, Greece, using oxalic acid and studied the effect of acid activation conditions on sunflower oil bleaching. They found that the activated materials obtained had excellent bleaching properties, making them suitable for industrial use as bleaching earth. Scientists modified their reaction conditions to achieve performance results equivalent to those of commercial bleaching earth Tonsil Optimum 210 FF. This study demonstrates that activated oxalic acid enables a bleaching replacement solution that performs comparably to conventional bleaching materials used in vegetable oil processing. Melo-López et al. [62] examined electrochemical purification procedures as they studied how ultrasonic pretreatment affects iron (Fe) and titanium (Ti) impurity removal in four types of kaolinitic clays (ACE, CFL, ASC, and AAM). Oxalic acid caused the dissolution of Fe and Ti oxides at a pH of less than 1.5. The results showed that impurity removal was more significant (82–92% for Fe and 35–79% for Ti) when ultrasonic pretreatment was applied to CFL, ASC, and AAM clays, while ACE clay showed no significant change with ultrasonic treatment. Voltammetry, chronoamperometry, SEM/EDS, ICP, and whiteness index (WI) analyses confirmed that ultrasonic pretreatment enabled better purification, increasing the whiteness index of certain clays (ACE and CFL) above 90 WI. The electrochemical purification method, when used together with ultrasonic pretreatment, effectively extracts Fe and Ti impurities from kaolinitic clays, thereby enhancing their value for ceramic applications in addition to industrial applications. Raw clay and HCl-activated clay were found to be the most efficient clays for wastewater treatment, according to research conducted by Gharbi-Khelifi et al. [63]. HCl-activated clay produced the highest successful results among all tested activated clays. The research demonstrated that acid or base activation of clays reduced the required clay volume for purification from 100 g to 30 g, while simultaneously conserving clay resources for large-scale applications. Scientists conducted tests using HCl activation on natural clay as well as sodium carbonate activation of clay materials. Test results showed that two clays were incompatible with acid activation because their carbonate content was minimal; however, the clay with 50% carbonate met the conditions for this activation method. The examination data indicated that water treatment using this method produced significant enhancements in the water’s physicochemical properties while substantially reducing the content of suspended solids, nitrogen consumption, and phosphorus levels. The researchers at Ouallal et al. [64] analyzed phenol adsorption patterns within aqueous solutions by experimenting with raw red clay (RRC) and treated red clay (TRC) extracted from Drâa-Tafilalet region. The treatment procedure for the clay material involved two stages: first, using hydrogen peroxide to remove organic materials, and afterward, using HCl to eliminate carbonates and expand the surface area. Characterization of both samples through X-ray fluorescence, FTIR, XRD, thermogravimetric and differential thermal analysis (TGA/DTA), and BET analysis showed a significant increase in specific surface area, from 40.96 m₂/g for RRC to 64.26 m₂/g for TRC. These clays are mainly composed of silica, alumina, kaolinite, illite, and quartz. Overall, the treated clay material achieved superior absorption rates as compared to the untreated clay material. Budash et al. [65] investigated how acid modification affected pore structure and adsorption properties of Cherkasy deposit montmorillonite (MMT) and palygorskite (PAL) clays, which originated from Ukraine, for water filtration purposes. The clay samples were refined and then modified with HCl, after which their structural and adsorption characteristics were analyzed. The results showed that acid modification significantly increased the BET surface area and pore volume for both types of clays. Mmt showed an enhancement of the BET surface area from 83.61 m₂/g to approximately 2.3 times the original value, whereas PAL achieved approximately 1.7 times the growth in BET surface area, starting from 140.66 m₂/g. The pore size of PAL experienced an approximately 3.2 times increase through acid activation, which led to new pores forming in interlayer regions. This study demonstrates that acid modification significantly improves the structural and sorption properties of clays, paving the way for their use in water purification systems. Shiferraw et al. [66] conducted an in-depth study on the removal of methylene blue dye using functional beads composed of acid-modified montmorillonite (MMT) and carbon derived from coffee waste (CWC). A composite material was synthesized by mixing PMMT with CWC to boost the dye adsorption performance. Scientists applied phosphoric acid treatment to montmorillonite clay because it enhanced both anionic and cationic exchange abilities and increased the ion density at a cost-effective level. The specific surface area of the modified montmorillonite clay reached 9.93 m₂/g, enabling the binding of 489.9 mg/g of methylene blue dye, resulting in an 80% removal efficiency for the dye. Acid treatment transformed the clay structure by activating cation exchange mechanisms while enlarging its surface area to enhance access for the adsorption process. Coffee waste carbon added as a component optimized the composite adsorption properties, alongside providing environmental benefits for waste disposal. The combination of economically affordable, sustainable material-based water treatment processing demonstrates potential as an effective solution when dealing specifically with removing dyes from water. Xu et al. [33] developed a cascade purification method that yields high-purity nano-palygorskite, suitable for wastewater treatment through dye decolorization. The processing starts with hydrothermal ultrasonic purification and proceeds to acid dissolution activation, then utilizes complexation bleaching for removing carbonate minerals and colored impurities. A Box-Behnken model-based optimization of purification conditions in the first step resulted in nanostructured palygorskite with a purity level of 99.9%. The purified palygorskite displays crystallite distributions ranging between 20–50 nm diameter and 50–200 nm length, which provides an ideal adsorption substrate for methylene blue and Congo red organic dyes. The purified palygorskite formed a porous structure through channel expansion, which caused major improvements in specific surface area, along with pore volume and surface activity results validated through mineralogical analysis. The study demonstrates that cascade purification techniques offer an attractive pathway for producing highly pure palygorskite at an industrial level, as they achieve sustainability alongside strong adsorption properties and economic benefits. This approach opens the way for large-scale applications of purified palygorskite in wastewater treatment, contributing to the sustainable development of the Chinese palygorskite clay industry. Furthermore, high-purity clay is essential in other important fields such as lubricants in drilling fluids and in the paint industry. Pastukhov et al. [67] investigated the chemical degradation of the herbicide metsulfuron-methyl (MSM) in aqueous solutions using acid-activated montmorillonite (Mt-K10) and natural zeolite, with the aim of potential soil remediation of sulfonylurea-based herbicide contamination. While the study did not address the mechanical structure of Mt-K10, nitrogen adsorption tests provided data on the porous characteristics of both aluminosilicates. The results showed that Mt-K10, with its distributed crystallites, outperformed acid-treated zeolite as a catalyst for MSM degradation. Within two days, MSM concentration in liquid media dropped fivefold, achieving over 97% degradation after twenty days. Acid treatment of natural zeolite removed iron, calcium, and sodium cations, replacing them with protons, which increased pore volume and surface area nearly fivefold. This modification also generated new mesopores (2.0–3.5 nm) while eliminating those previously ranging from 10–30 nm. The catalytic activity of these aluminosilicates was attributed to their enhanced hydrogen ion content and expanded surface area, derived either from dispersed Mt-K10 particles or mesopore formation in activated zeolite. Acid-activated minerals thus present a promising option for agricultural applications, particularly in remediating polluted soils and protecting canola and corn seeds prior to planting.

Although the performance of treated clay in water treatment has been confirmed, its effectiveness varies depending on the type of clay, the enrichment process, and the type of pollutant. Challenges arise, particularly in the presence of organic and inorganic contaminants, which render these systems unsuitable. Similarly, colloidal properties make the recovery of clay particles after use difficult, indicating poor regeneration capacity. In order to overcome these limitations, organic-inorganic hybrids have emerged as promising solutions capable of revolutionizing the water treatment industry (Figure 6). Indeed, the combination of these two phases makes it possible to multiply the functionalities of the developed clay. Among the most widely reported hybrids in the literature are those based on biochar, cellulose, chitosan, and metal oxides (TiO₂, ZnO). They are capable of removing various pollutants such as phenolic compounds, heavy metals, dyes, inorganic ions, and others. For example, introducing biochar into the clay matrix makes it possible to produce composites suitable for regeneration and reuse due to the exceptional characteristics of the organic phase, such as its porous structure, high carbon content, and compatibility with the matrix [68,69]. In the case of cellulose- and chitosan-based hybrids, the biopolymer makes functional groups (-OH, -NH₂) abundant, thus facilitating the removal of heavy metals. As for metal oxides, they give the clay matrix remarkable characteristics such as photocatalysis, enabling both the capture and degradation of organic pollutants. Finally, layered double hydroxides (LDHs) can combine with clay to produce dual-function composites due to their anionic structure, making the hybrid effective at removing toxic anions and cations simultaneously.

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Figure 6. Schematic diagram of the development of clay-polymer nanocomposites for water treatment [69].

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5.2. Biomedical and healthcare

All civilizations have shown exceptional interest in the use of clay in biomedical and health applications due to the unique characteristics of this material. Indeed, the value of clays in the fields of medicine, pharmacy, and cosmetics is linked to their physicochemical properties, biocompatibility, and remarkable rheological properties . In the biomedical field, the requirements for high clay purity are particularly stringent in order to produce excipients, absorbents, and active ingredient carriers. Such requirements make clay processing and nano-clay preparation essential in order to meet quality, safety, and reproducibility criteria. In addition, the nanometric size of these fabulous materials aids in the synthesis of organic-inorganic nanocomposites capable of overcoming several challenges encountered when using clay alone. Exploring the remarkable characteristics of clays in biomedicine has made it possible to improve the process of administering bioactive agents, increase therapeutic efficacy, and reduce side effects. Swelling and tubular clays are the most preferred in this field of biomedicine, particularly for controlled drug release. They are also of great interest in cancer treatment, wound healing, tissue regeneration, etc., thanks to their structural versatility, abundance, and cost-effectiveness. In the pharmaceutical field, clays are utilized as excipients and absorption agents in drug formulations, thereby enhancing bioavailability and increasing safety [58]. However, their introduction in these applications requires that the samples meet pharmacopoeia standards, particularly regarding essential physicochemical properties such as CEC, specific surface area, and swelling. Additionally, high chemical and mineral purity is necessary, which involves low biological load and the removal of undesirable minerals, especially quartz, which must be reduced to less than 2% to comply with current regulations. Awad et al. [70] conducted an extensive study to evaluate the potential use of 65 Egyptian kaolin deposits in the pharmaceutical industry. Their investigation employed a combination of techniques, including XRD, XRF, SEM, TEM, and laser diffraction particle size analysis, along with pharmacopoeia standard tests such as pH, buffer capacity, organic matter content, adsorption, and swelling capacity. Microbiological analyses were also performed, including total aerobic microbial counting and combined yeast and mold counting, to ensure the samples were fit for healthcare use. The mineralogical study showed that kaolin was the predominant mineral (75–96%), with impurities including quartz, mica, and hematite present. Of all the samples examined, only three met the necessary requirements for use in manufacturing cosmetic products. Of these, one also met the requirements for pharmaceutical applications as its lead (Pb) concentration remained below 25 ppm. Abdel-Motelib et al. [58] investigated the health benefits of bentonite for human applications through pharmaceutical formulation analysis. Researchers evaluated sodium carbonate-activated bentonite through investigations into its material properties that increase its CEC while making it appropriate for pharmaceutical manufacturing processes. Activated bentonite has three main pharmaceutical applications, including drug adsorbent action, use as controlled-release carrier systems, and oral formulation suspension agent application. Researchers conducted alkaline activation through the combination of bentonite with Na₂CO₃ amounts from 2.5% to 10% followed by complete suspension and 24-h rest, then filtration and 105°C drying. Measurement of d-spacing, along with CEC, identified the best amount of activator dosage. The test results demonstrate that purified or active bentonite from Egypt fulfills all drug product standards described in the pharmacopoeia for pharmaceutical use. The potential application of activated bentonite stands strong for oral suspension formulations as well as drug release systems.

Shah et al. [71] studied the suitability of Pakistani bentonite for pharmaceutical use, both in its raw and purified forms, using two distinct methods: simple sedimentation and classical NaCl treatment. Mineralogical analysis revealed that the raw sample primarily consists of montmorillonite, with minor amounts of illite, kaolinite, and quartz. The removal of quartz impurities from bentonite through purification processes improved its physicochemical properties, most notably swelling capacity, sedimentation volume, and CEC. The pharmacological characteristics of purified bentonite fit well into pharmaceutical usage through its functions as a tablet suspension agent as well as disintegration component and as a drug delivery vehicle for controlled-release systems. Gamoudi and Srasra [72] studied a Tunisian clay to evaluate its potential for use in pharmaceutical and cosmetic applications. After purification using the classical Van Olphen method, the clay showed significant improvements, including a reduction in associated impurities. Chemical and textural results, along with favorable porosimetric properties, demonstrated that this clay is suitable for use in formulations such as creams, powders, and emulsions. Testing methods from the pharmacopoeia demonstrated that the purified clay meets required standards through evaluations of swelling capacity and sedimentation volume, since both properties affect excipient functionality. Laboratory findings demonstrate that the purified clay possesses attributes suitable for product applications, therefore presenting a promising commercial opportunity for using local materials in industrial development. The research conducted by Gamoudi and Srasra [6] investigated Tunisian smectite minerals from the Elfahs region in northeastern Tunisia to evaluate how purification affected their potential usage in pharmaceutical and cosmetic products. The initial clay composition of Ca₂⁺ smectite, with quartz and calcite impurities, underwent sedimentation treatment using deionized water to retrieve particles smaller than 2 μm prior to the classical Van Olphen (1963) purification methodology. This purification process removed a significant portion of the impurities, resulting in a purified smectite noted as Na+-Sm. The purification significantly altered the clay’s properties, as confirmed by structural and functional analyses. XRD results revealed changes in basal spacing (d001) values and a significant reduction in peak intensity, indicating a structural rearrangement. FTIR analysis revealed changes in functional groups and surface properties, including the appearance of characteristic bands associated with stretching vibrations of C=O, C-H, and aromatic methoxyl groups. These modifications reflect improvements in the clay’s characteristics for specific uses. Thermogravimetric stability tests (DSC) showed high thermal resistance of the purified clay, reinforcing its suitability for stable formulations. A study by Viseras and López-Galindo [73] examined the potential integration of four types of clay, including two sepiolites, one palygorskite and one bentonite, into the pharmaceutical sector. Mineralogical analysis of the samples showed that the two sepiolites exhibited remarkable consistency, with a purity exceeding 90%. However, bentonite is characterized by its high smectite content. The palygorskites, meanwhile, have a more diverse mineral composition, including notable occurrences of illite and quartz. Chemical analysis showed that bentonite and palygorskite predominantly contain SiO₂ and Al₂O₃ oxides, whereas sepiolite mainly consists of SiO₂ and MgO oxides. Each clay type plays a vital role in specific industries due to its chemical properties. Furthermore, a comparative study of the mineral and chemical compositions of the examined clays, in relation to three other clays already present in the pharmaceutical industry, showed that the analyzed samples had comparable or more suitable attributes than current commercial products. Finally, microbiological analysis revealed the absence of infectious germs, including E. coli, P. aeruginosa, and S. aureus. Additionally, C. albicans levels were within the required standards. These encouraging findings confirm the potential of these clays for use in the pharmaceutical industry. Furthermore, clays intended for cosmetic applications must undergo rigorous selection and purification, as certain impurities and heavy metals can pose risks to the skin. Silva-Valenzuela et al. [13] studied a clay sample from Vitória da Conquista in north-eastern Brazil using particle size fractionation by dispersion and centrifugation. This method enabled the isolation of three distinct granulometric fractions: pink, red, and light green. The study aimed to characterize the physical and chemical properties of the raw clay and the obtained fractions to evaluate their potential use in cosmetic products. Analysis revealed that the extracted fractions mainly consisted of smectite and kaolinite, with no quartz present. These fractions exhibited beneficial cosmetic properties, as they absorbed oils well during swelling and were microbiologically safe and chemically inert. The pink and red fractions exhibited maximum swelling properties in accordance with pharmacopoeia specifications, as well as displaying the maximum CEC values, which scientists have linked to their elevated smectite concentration. Despite the green fraction having a marginally better CEC and no matching swelling properties, the pink and red fractions demonstrate the best potential for cosmetic uses because they achieve the highest CEC.

The microbiological analysis proved this clay to be free of dangerous microorganisms, making it safe for cosmetic application. The solid-state reaction between dried flowers and colored materials generated new absorption bands that fell among 400 to 700 nm wavelengths thus boosting their usage potential. Through detoxification methods, Tunisian smectite achieved proper cosmetic utilization standards by improving its physical properties and protective aspects against microorganisms. Cortés et al. [74] studied five clay samples originating from Teruel province Spain to determine how purification affected their potential use in pharmaceuticals and cosmetics. The researchers explored thermal muds and sunscreen products as potential ceramic applications of these clays but the ceramic industry mainly utilizes them. Of the tested samples, 28% contained crystalline quartz exceeding 20%, so further treatment became necessary before product application because the minerals negatively affect performance. Markets’ valuation of treatment muds, as well as UV-protecting formulations, increases when carrier clays possess a better mineral content combined with reduced quartz components. The phyllosilicate content modification leads to shape alterations in the clays while proper processing methods enable maximum control of this effect. The research shows positive performance potential for innovative cosmetic and therapeutic products because of promising elements in these clays but additional purification methods remain essential.

A study by Dardir et al. [75] investigated newly discovered bentonite deposits in the Eastern Desert of Egypt, which investigated property modifications and pharmaceutical and cosmetic applications following purification and drug Praziquantel carrying capabilities. The investigators purified the main montmorillonite clay found in bentonite by applying physical and chemical processes through attrition, grinding, acid leaching, and hydrocyclone separation, followed by hydrogen peroxide washing. Tests showed that the purification process removed all detectable mineral impurities from the bentonite making it comparable to the mineralogical standards of pharmacopoeia-grade Wyoming bentonite. The superior properties of purified bentonite create desirable characteristics for manufacturing powder along with emulsion and cream delivery systems. Pharmacopoeia testing proved the product met its high swelling requirements and pH specifications while showing no sign of harmful pathogens so it demonstrates good potential for use in topical applications. After purification, the encapsulation ability of the material increased to 78.4% for purified bentonite, compared to the 62% encapsulation of the untreated raw bentonite. The alkaline activation improved the minimum percentage of drug release capacity to 93.2% along with an increased release speed from 71% for the raw bentonite to 79.2% for the purified bentonite and 87.4% for the activated bentonite during the 420-min intestinal buffer incubation period. The purification and alkaline treatment of bentonite enhanced its functional properties, allowing for its use in pharmaceutical practices and cosmetic production, as well as its status as an effective drug transport agent. Maneetong and Thuadaij [76] studied volcanic clay purification to identify its impact on physicochemical attributes and antioxidant actions suitable for potential cosmetic applications as bio-clay substances. The clay obtained high CEC results along with extensive surface area and 8.51 nm average pore width after purification procedures thus representing a valuable inorganic material. The purification sequence enhanced the efficiency of the clay material in interacting with plant compounds and phytochemicals. When woven into a bio-clay system, three medicinal plants, namely Curcuma longa L., Bougainvillea spp., and Pandanus amaryllifolius Roxb, exhibited exceptional antioxidant performance in purified clay. The purification technique optimized the cosmetic nature of clay through lower L* measurements and higher b* results in its physical attributes. The bio-clay material, with its stable antioxidant properties, exhibits suitable characteristics to serve as a primary ingredient in exfoliating soaps and body scrubs. The ingredient displays three distinctive elements that substantiate its position as a new anti-aging substance for abrasive cosmetics because of its antioxidant activity, phenolic stabilization system, and chemical stability. The research performed by Da Silva Favero et al. [77] analyzed four clay samples collected from sand extraction remains located in the Sao Paulo State Brazilian hinterland. The research established standards for treating contaminants in clay materials, which could be applied to cosmetic and pharmaceutical products through an examination of both physical and chemical properties. Researchers applied 70% ethanol solution during constant dry heat exposure at 120°C for 24 h to reduce biological contaminants in the samples. The decontamination process reduced biological contamination in clays to meet cosmetic requirements by both eliminating biological elements beyond standards and maintaining product stability. The decontamination treatment successfully rendered these clays usable for cosmetic and pharmaceutical purposes, as effective cleaning enables their participation in future research projects.

The pharmaceutical and cosmetic sector uses swelling clays, particularly smectite and bentonite, as their primary clay choice because of their exceptional physicochemical properties. The pharmaceutical industry extensively utilizes montmorillonite, which serves as a fundamental smectite in the creation of dermatological products and gastrointestinal treatment adsorbents. The smectite clay known as Bentonite has gained high value because it develops colloidal gels that enable its use as a stabilizing and thickening ingredient in pharmaceutical and cosmetic products. The preparation exists in multiple dosage forms, which include liquid, semi-solid, and solid formulations, and function well as both oral and topical medications. Kaolinite, a type of non-swelling clay, has its primary application in cosmetics for dermatological purposes, topical anti-inflammatory use, and local anesthesia. This is due to its gentle chemical structure. Gastrointestinal protectants utilize this material because it possesses a large specific surface area together with noteworthy cation exchange capabilities. The antibiotic features of illite clay are utilized in three medical applications where it treats stomach wounds and sustains burn injuries and manages skin health. A study carried out by Khiari et al. [78] analyzed eleven Tunisian natural clay samples to determine their suitability for cosmetic and medical applications. The research documented the analysis of clay pastes’ cooling dynamics after performing physical, chemical, and mineralogical characterization through XRD, XRF, SEM, and TGA techniques. Scientific testing showed illite and kaolinite prevailed in the composition of most samples yet some contained higher levels of smectite and calcite. The XRF examination highlighted the dominant presence of oxides such as SiO₂, Al₂O₃, MgO, Fe₂O₃, and K₂O, confirming that samples with less than 20% quartz content were suitable for skin applications. In contrast, the remaining samples required a purification process before use in the cosmetic sector. With the exception of one sample, the other nine showed promising therapeutic results, due to cooling kinetics that met pharmacopoeial standards.

In summary, in the pharmaceutical and cosmetics industries, the choice of purification method depends on a compromise between its effectiveness and the operational and sustainability criteria that are essential for industrial adoption. Physical methods have a very low environmental impact but require more processing time. Despite their low cost, these techniques have limitations related to the purity of the clay and large-scale adoption. As for chemical methods, the pharmaceutical and cosmetics industries are highly sensitive to the chemicals used in these processes. Chemical methods surpass their physical counterparts in terms of purity and processing time. However, the high cost and environmental impact encourage the search for innovative solutions based on green chemistry. Finally, biological approaches and emerging greener technologies are suitable in terms of sustainability, but face challenges related to processing speed, cost, and large-scale implementation. Thus, no single method satisfies all criteria, leaving the field open for further research, particularly in the direction of green chemistry or the adoption of cascade purification schemes aimed at exploiting the advantages of each process.

5.3. Advanced materials and nanocomposites

The introduction of clays into the synthesis of nanocomposites and advanced materials has undergone a revolution due to their potential to produce materials with controlled characteristics. The low cost and anisotropic structure of clay further encourage this strategy aimed at replacing carbon nanotubes, particularly in large-scale applications. Clay-based nanocomposites are in high demand in the automotive industry, membranes, catalysts, packaging, medicine, pharmaceuticals, and other advanced materials. The shift to combining clay with other materials is dictated by the challenges encountered when using these nanomaterials in highly technological fields that require new functionalities. In addition, their performance in nanocomposites requires a shift to purified clay and nano-clay to overcome one of the challenges of these nanocomposites, which is the exfoliation of clay sheets. This exfoliation results in a homogeneous dispersion in the matrix and positively affects mechanical, rheological, thermal, and other properties. Clay minerals, particularly montmorillonite of the 2:1 type, are the most preferred for the preparation of nanocomposites due to their swelling characteristics. Recently, considerable interest has been shown in other minerals such as kaolinite, illite, sepiolite, and palygorskite, particularly in specific fields. They are very interesting as polymer reinforcements because of their porosity, structure, catalytic, rheological, and sorption characteristics. Improving the quality of clay through purification or preparation of nanoclay has revolutionized the field of nanocomposites by increasing siloxane surface reactivity. This good reactivity leads to the creation of favorable interactions between the matrix and the reinforcement, giving rise to materials with unprecedented properties. The favorable interactions increase the affinity between the two phases, promoting the intercalation of chains in the interlayer environments of the clays and thus the exfoliation of the clay sheets in the matrix. In their study, Devi et al. [79] reported the use of nanoclay for the preparation of poly(lactic acid) (PLA)-clay bionanocomposites as a suitable material in the food packaging industry. The candy spinning technique was very decisive in the exfoliation of nano-clay in the polymer matrix, thereby improving the properties of the final product. The nanocomposites were characterized using DSC, SEM, XRD, TEM, SEM, and dynamic mechanical analysis techniques. The results reveal a significant improvement in mechanical and thermal properties compared to the initial properties of PLA (Figure 7).

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Figure 7. Preparation of bio nanocomposites with improved mechanical and thermal properties using the candy floss spinning method [79].

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5.4. Catalysis and energy technologies

In recent years, researchers have focused their attention on designing solid materials with high-quality catalytic properties. They have taken an interest in synthesizing solid, heterogeneous, environmentally friendly catalysts capable of replacing homogeneous catalysts, which are polluting and take longer to produce. In this context, clay minerals are a class of inorganic materials that are suitable for producing high-quality catalysts due to their structure and composition. They are ideal candidates for further research in this field, particularly for the design of green and sustainable catalysts. The layered structure of clay minerals allows for the separation of sheets at the nanoscale and thus the design of active catalysts, leading to a variety of products such as activated clay-based catalysts, ion exchange catalysts, intercalated catalysts, and inorganic-organic hybrid catalysts. By combining several strategies, it is possible to design multifunctional catalysts in optical, magnetic, electrical, and photonic materials. In this context, upgrading the functionality by switching to purified clay and nano-clay makes the clay more reactive and highly efficient from an energy standpoint. Thus, purifying the clay allows the full potential of this material to be exploited in the catalysis industry. Adekayero et al. [80] reported the use of montmorillonite as an inexpensive, natural, and environmentally friendly catalyst in a fuel production process based on polypropylene waste. The recovery of this waste is of dual interest: on the one hand, it is an approach of great environmental importance and, on the other hand, it meets the standards of a circular and sustainable economy. The results show an improvement in oil production in the case of catalytic pyrolysis (34.45%) compared to the non-catalytic process, which has an efficiency of around 31.25%. At the same time, the kinematic viscosity of the synthesized oils varies between 0.61-1.83 cSt for catalytic pyrolysis and 0.57-1.71 cSt in the non-catalytic case (Figure 8).

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Figure 8. Insertion of a clay-based catalyst for the pyrolysis of polypropylene waste into fuels [80].

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Ali et al. [81] synthesized a bifunctional heterogeneous catalyst from bentonite treated with a NaOH solution and combined with graphene oxide (GO) for simultaneous free fatty acids (FFA) treatment, oil transesterification, and esterification reactions. The catalyst making process consisted of first subjecting bentonite to 500°C calcination for impurity removal before its exposure to a 5% NaOH solution at 60°C with agitation for 12 h. Several assessment procedures included thermogravimetric analysis in combination with X-ray diffraction examination, X-ray photoelectron spectroscopy, Raman spectroscopy, and field emission scanning electron microscopy for evaluating the catalyst’s end structure. The optimized GO-NaOH-bentonite catalyst demonstrated excellent performance, yielding 98.5% biodiesel in 4.5 reaction hours at 62°C when reacting a 6:1 methanol-to-oil mixture with 6% catalyst, in combination with a 1:20 GO/NaOH-bentonite ratio. The research showed that weak acid sites located on GO-NaOH-bentonite permitted the catalyst to achieve sufficient acidity required during FFA esterification. The transesterification of rapeseed oil required basic sites from the GO-NaOH-bentonite catalyst to operate effectively. The following research seeks to investigate how reaction times for biodiesel production from FFA-rich oils can be improved through kinetic studies of GO-NaOH-bentonite as a catalyst.

In the field of renewable energy and energy storage, purified clays are used as components in lithium-ion battery electrodes and supercapacitors. Due to their high surface area and thermal stability, these materials are favored to improve the performance and longevity of these devices. Purified clays enable new development possibilities for composite materials, which advance multiple industrial sectors beyond their classical applications. The incorporation of these exceptional materials into composite formulations gives manufacturers a significant market advantage by improving multiple product characteristics in the final product. Thuc et al. [46] conducted a study to separate and purify montmorillonite (MMT) from bentonite deposits in the LamDong and ThuanHai regions of Vietnam, with the aim of using it as a filler in polymer nanocomposites. The researchers tested several particle size treatments and chemical processes to optimize the extraction of MMT while preserving its physical and chemical properties. The analytical methods included X-ray diffraction, laser granulometry, nitrogen adsorption at 77 K, CEC measurement, and infrared spectroscopy (IR) to evaluate the obtained MMT fractions. The combination of sedimentation at 55 h with centrifugation and sonication produced the purest MMT fraction from the raw material solution. Quartz impurities were removed, though traces of kaolinite were still detected by IR spectroscopy. Additionally, chemically purified bentonites sonicated in an aqueous PEO (polyethylene oxide) solution allowed for the direct preparation of MMT/PEO intercalated composites, free from quartz impurities and suitable for the manufacture of polymer nanocomposites.

5.5. Agriculture

The importance of clays in agriculture is indisputable and is becoming more significant every day. They can enhance the sustainability of agricultural systems, fertility, and input efficiency. These remarkable materials can act as controlled-release fertilizers, pesticides, herbicides, and eliminate toxic pollutants in agriculture [82]. Thanks to their remarkable characteristics, they help meet several sustainability expectations for producing safe food. The physicochemical properties of clays, particularly CEC, make them effective as reservoirs for essential nutrients (K⁺, Ca²⁺, Mg²⁺, NH₄⁺) and for eliminating pollutants. These properties are further enhanced when purified clay and nanoclay are used, offering distinct and beneficial functionalities. Indeed, the emergence of nanoclay has revolutionized the field of agriculture due to its high specific surface area at the nanometric scale and its high accessibility to active sites. This nanomaterial therefore has significant capabilities for releasing, protecting, and encapsulating agents, nutrients, and growth regulators that improve soil quality. With the aim of eliminating oil mill wastewater (OMW), which is a highly dangerous pollutant rich in toxic chemicals, Dehmani et al. [83] reported on the use of clay from the Meknes region rich in illite, kaolinite, and vermiculite. The study focuses on the elimination of OMW by natural clay with a view to using it as a source of irrigation in agriculture. Grover et al. [84] showed that introducing clay minerals into agriculture can help mitigate climate change by reducing greenhouse gas emissions and increasing the carbon storage capacity of soils. They were particularly interested in reducing CO₂ emissions from soil organic matter and plant residues. The results indicate that adding clay reduces CO₂ emissions, making this strategy promising for enhancing carbon sequestration in the soil.

5.6. Construction/Geopolymer

The integration of clays in the construction industry and geopolymers is beginning to take on greater importance due to its ecological and economic benefits. Indeed, the use of Portland cement as a base product makes the construction industry one of the largest sources of greenhouse gases. The challenges facing the construction industry are intensifying due to resource depletion, population growth, and the acceleration of industrial activities. These challenges highlight the need to move towards sustainable and circular construction by incorporating materials that can partially or totally replace cement, such as clays and agri-based composites. Since the earliest civilizations, clay has always been a key component of the construction industry, particularly for producing bricks, tiles, and blocks. This interest is driven by several desirable characteristics offered by this nanomaterial, such as fire resistance, thermal and acoustic insulation, and durability. Illite, kaolinite, and smectite remain among the most preferred clay minerals in the construction industry due to their stability, swelling characteristics, and good high-temperature resistance. However, the presence of impurities such as quartz, calcite, and organic matter negatively affects the performance of clay in this field of construction. Therefore, a purification process is necessary to control the mineralogical composition, reduce firing defects, and improve mechanical properties and chemical reactivity. This purification process is already widely used, particularly in ceramics and cement, despite its moderate cost. Similarly, nanoclay is beginning to gain ground, particularly for high-quality concrete, but more research is needed to overcome cost limitations. In addition, to overcome some of the limitations of clay alone in construction and to reinforce its strengths, efforts have been focused on the development of clay-based hybrids. Clay has been mixed with lime, cement, and other industrial and agricultural waste as a promising strategy for protecting the environment, reducing landfill waste, saving energy, and managing waste effectively. Another promising use for clays is in the preparation of geopolymers to improve sustainability in the construction industry. The high silica and alumina content of these natural materials makes them ideal candidates for the preparation of geopolymers designed to reduce carbon footprints. The quality of geopolymers is directly related to the type of clay used, particle size, porosity, treatment process, and Si/Al ratio. Therefore, optimizing these factors allows for the synthesis of high-strength geopolymers that can effectively replace conventional cementitious materials . In an interesting study, Shilar et al. [85] combined the advantages of a clay-based geopolymer and bamboo to improve sustainable construction. The results reveal that bamboo fibers make the final product resistant to load transfer and cracking. Similarly, compressive strength was improved by approximately 10.71%, while resistance to HCl acid shows good durability for this biocomposite, positioning it as an innovative, environmentally friendly, and robust material.