Clay-Based Materials for Heavy Metals Adsorption: Mechanisms, Advancements, and Future Prospects in Environmental Remediation

Abstract

Given the severe threats posed by heavy metal pollution to ecological environments and human health, developing effective remediation technologies is of paramount importance. This review delves into the mechanisms, recent advancements, and future prospects of clay mineral-based materials in the adsorption of heavy metals. Clay minerals such as kaolinite, montmorillonite, and bentonite have demonstrated immense potential for the removal of heavy metals from water and soil due to their natural abundance, low cost, and high efficiency. This article summarizes the latest advancements in the adsorption of heavy metals like chromium, copper, lead, cadmium, arsenic and hydrargyrum by clay minerals, while examining how chemical and physical modifications can enhance the adsorption capacity, selectivity, and stability of these minerals. Furthermore, this review discusses how factors such as pH, temperature, and ionic strength affect adsorption efficiency and outlines challenges and future research directions for optimizing clay-based adsorbents in environmental applications.

1. Introduction

Heavy metal contamination is an escalating environmental crisis, posing severe and long-lasting threats to ecosystems and human health [1]. Metals such as chromium (Cr), copper (Cu), lead (Pb), and cadmium (Cd) are persistent pollutants, especially arsenic (As), which, as an extremely toxic pollutant, poses significant environmental and health hazards. These pollutants are capable of bioaccumulating in living organisms and pose a long-term threat to ecosystems and human health. Industrial discharges, mining, agricultural runoff, and improper waste disposal introduce these metals into the environment where they can persist in soils and water bodies for decades, leading to chronic exposure that causes carcinogenesis, neurotoxicity, and organ damage [2]. The ubiquity of this contamination underscores the urgent need for effective, efficient, and sustainable remediation technologies to mitigate environmental impact and protect public health [3,4].

Various methods have been developed to remediate heavy metal-contaminated soil and water, which can be categorized into physical, chemical, and biological approaches, and a combination of these methods may also yield better results [5]. However, chemical precipitation or ion exchange usually requires complex infrastructure and generates secondary waste. Adsorption is an efficient, cost-effective, and easy-to-operate method for removing trace contaminants, as well as for recovering metals and other adsorbed pollutants, with good implementability and a high degree of scalability [6,7,8,9]. This scalability is essential for tackling widespread contamination in both aquatic and terrestrial ecosystems. Clay minerals in particular have proven to be outstanding adsorbents because of their distinctive structural and chemical properties, which are not only naturally abundant and low-cost but also have a large surface area and high cation exchange capacity (CEC), which are highly effective in capturing and immobilizing heavy metals across environments with diverse pH levels and ionic strengths [10,11,12]. The impressive adsorption capacity of clay minerals is largely due to their layered structures, which are composed of silica and alumina sheets and offer numerous active sites that bind metal ions, effectively adsorbing a wide range of contaminants, including Pb, Cd, and As. The high reactivity of these active sites further boosts the overall adsorption capacity, making clay minerals highly versatile for diverse remediation applications. By effectively immobilizing toxic metals, clay minerals reduce their bioavailability and mitigate long-term environmental and health risks. This blend of affordability, availability, and efficiency makes clay minerals adsorption an ideal approach for sustainable heavy-metal remediation [4,13].

Recent research focuses on enhancing the adsorption capacity of natural clay minerals through chemical and physical modifications to improve the selectivity, efficiency, and stability of clay minerals under diverse environmental conditions [14,15,16]. Techniques such as acid activation, organic modification, and nanomaterial incorporation have been explored to increase adsorption capacity and tailor clay properties for specific applications [3,17]. This review provides a comprehensive overview of current research on clay-based adsorbents for heavy metal removal, examining the fundamental properties of clay minerals, the mechanisms governing adsorption behavior, and the factors influencing their efficiency. In addition, it highlights the benefits of modified clays and identifies key areas for future research and development, highlighting the versatility and potential of clay-based adsorbents as a sustainable solution for mitigating heavy metal contamination.

2. Structure and Properties of Clay Minerals

Clay minerals, the primary components of rocks and soils, typically constitute particles smaller than 2 μm [18,19]. Ralph E. Grim et al. classified clay minerals into two major categories based on their crystal structures: crystalline and amorphous. The amorphous type includes allophane, while the crystalline minerals are further divided based on their layered structures. These include 1:1 layered (kaolinite), 1:1 tubular (halloysite), 2:1 layered (montmorillonite, illite, vermiculite), regularly mixed-layered (chlorite type), and 2:1 chain-type (attapulgite, sepiolite) minerals [20,21,22]. Layered silicate minerals are composed of basic structural layers with one or two tetrahedral sheets surrounding an octahedral sheet. The tetrahedral sheet is made up of the silica tetrahedral unit (SiO₄), in which each silicon atom is surrounded by four hydroxyl groups arranged in a tetrahedral configuration. The octahedral structure is formed by the coordination of metal cations with oxygen atoms or hydroxyl groups (M(O, OH)₆, where M represents a cation) [23]. According to the number and charge of metal ions in the octahedral sheet, octahedral sheets can be divided into two types: dioctahedral trivalent cations (Al³⁺) occupy two-thirds of the site of the octahedral sheet, and trioctahedral divalent cations (Mg²⁺) occupy all octahedral sites. Dioctahedral cations are generally more chemically stable and are suitable for environments requiring high stability, such as soil conditioners and heat-resistant materials. Trioctahedral cations have higher interlayer exchange capacities and flexibility and are suitable for environmental remediation such as adsorption, catalysis, and modification.

Due to their small particle sizes, large specific surface areas, and complex porous structures, clay minerals can immobilize heavy metals through surface reactions and adsorption, with adsorption playing a dominant role [24,25]. Additionally, the high surface area of the pores indicates the binding capacity of clay minerals, making them highly effective for this purpose. As a result, many researchers have applied clay minerals in the field of heavy metal removal [26,27,28].

3. Related Mechanism of Clay-Based Adsorbents

Clay minerals, which are primarily composed of layered sheets of silica and alumina, belong to the phyllosilicate class or the phyllosilicate group. Isomorphic substitutions create a permanent negative charge in the structure, which is balanced by cations in interlayer spaces or on external surfaces. This layer structure and negative charge make clay minerals highly effective at retaining metal cations (Figure 1).

Clay-based adsorbents not only embody the natural surface and structural attributes of clay but also incorporate a wealth of functional groups derived from composite reagents. This dual nature endows them with intricate mechanisms for the adsorption of a variety of heavy metals. The related mechanisms such as hydrogen bonding (Figure 2a), ion exchange processes (Figure 2b), electrostatic interactions (Figure 2c), and surface complexation (Figure 2d), which collectively contribute to their multifaceted adsorptive capabilities are introduced in detail below.

3.1. Hydrogen Bonding

The aluminosilicate framework of clays, such as kaolinite and montmorillonite, presents a rich array of Al-OH surface and edge sites in aqueous media. Furthermore, the integration of additives into clay-based adsorbents introduces a plethora of functional groups, including -OH, -COOH, -NH₂, and -HF. These moieties readily engage in hydrogen bonding with the O-, H-, N-, and F-groups of dyes, facilitating contaminant removal [30] (Figure 3).

3.2. Ion Exchange

Ion exchange is the principal mechanism for contaminant sequestration. Isomorphous substitution endows most clays, such as montmorillonite, with a perpetual negative surface charge counterbalanced by exchangeable cations [32]. These cations are readily exchangeable with cationic dyes or heavy metal ions, a property that positions clays as prominent adsorbents. Furthermore, ion exchange also occurs between contaminants and protons associated with functional groups like -COOH or -OH, which are introduced by incorporated organic or inorganic amendments.

3.3. Electrostatic Attraction

Electrostatic attraction, a prevalent force in the adsorption of cationic pollutants onto clay-based adsorbents, is predominantly governed by permanent charge. These adsorbents exhibit pH-responsive charge characteristics, influenced by solution pH and particle dimensions. Typically, clay-based materials maintain a negative surface charge across the entire pH spectrum, which is attributed to the inherent negativity of clay [33]. As the pH escalates, the electronegativity of the adsorbents augments, a consequence of the deprotonation of functional groups [34]. This heightened electronegativity not only escalates contaminant attraction but also impedes the desorption of adsorbed dyes and heavy metal ions.

3.4. Surface Complexation

Surface complexation, a pivotal adsorption mechanism, involves the interaction of heavy metal ions with functional groups on adsorbent surfaces. It includes outer-sphere and inner-sphere complex formation on basal and edge surfaces. Metal ions can adsorb onto external surfaces and interlayer spaces or participate in mineral nucleation and redox reactions. This versatility in surface types and reactivity allows clay minerals to effectively immobilize heavy metals (Figure 4). Wang et al. enhanced the adsorptive properties of MoS₂@2DMMT hydrogel by incorporating -COOH, -OH, and -NH₂ groups, thereby significantly boosting the Cu²⁺ uptake. The selectivity of these groups for metal ions is variable; notably, -COOH groups exhibit a higher affinity for Pb²⁺ ions over -OH and -NH₂ counterparts, as established by prior studies [35].

4. Influencing Factors of Adsorption Reactions

The adsorptive capacity of clay mineral-based adsorbents for heavy metal ions in water is primarily determined by the properties of the clay minerals, such as the CEC, specific surface area (SSA), and structural characteristics [36]. CEC represents the total negative charge present in the clay, which can attract and retain heavy metal cations [37]. SSA refers to the total surface area of clay minerals available for use as active sites. Minerals are sufficient to adsorb reasonable concentrations of heavy metals from contaminated media. Some clay minerals, such as kaolinite and illite, have only external layers, while others, such as montmorillonite and attapulgite, have both internal and external layers. Variations in the compositions of clay minerals across different regions may affect the remediation efficiency and the required dosage [38]. A single type of clay-based adsorbent exhibits varying affinities for different heavy metals, mainly due to the differential affinities between the surface functional groups of the adsorbent and the heavy metals. Modification methods significantly influence their performances. Furthermore, solution conditions and reaction parameters, including the type of heavy metal, pH, temperature, and co-existing ions, affect the adsorption efficiency [39,40,41]. Among them, the adsorption of alkali and alkaline earth metal ions, such as cesium (Cs⁺) and barium (Ba²⁺), in their cationic forms, is less influenced by pH and is primarily dependent on ionic strength. In contrast, the adsorption of more hydrolysable transition metals, such as nickel (Ni²⁺) and cobalt (Co²⁺), as well as rare earth elements like europium (Eu³⁺), is significantly affected by both pH and ionic strength [42].

4.1. Dosage of Adsorption

Effective metal adsorption can be achieved with small amounts of clay (dosages as low as 2.5%). Research shows that 4–8% clay by weight can remove up to 70% of heavy metal contaminants. Higher clay dosages provide more active sites, thus improving the adsorption process. For instance, one study achieved 74% mercury removal using just 0.4 g of clay in 40 mL of contaminated water [43]. The composition of clay minerals varies by region, affecting the necessary dosage for optimal remediation. Therefore, field applications often require pilot trials to determine the best dosage and conditions for each specific environment.

4.2. pH of the System

pH plays a crucial role in the adsorption of heavy metals by clay minerals, affecting the leaching capacity of heavy metal cations. The pH value of the solution not only affects the protonation degree of the adsorbent’s surface functional groups but also influences the chemical properties and speciation of heavy metal ions in water, thereby significantly impacting the adsorption performance of the clay mineral-based adsorbent [35]. Chen et al. found that when the pH increased from 2 to 4, the adsorption rate sharply rose from 35% to 80%. Further increasing the pH from 4 to 8 resulted in even better adsorption rates, which then remained stable [44]. As the pH increases, the surface of clay minerals becomes more negatively charged, which enhances the adsorption of positively charged metal ions such as Cd²⁺, Pb²⁺, and Zn²⁺. The optimal pH range for this adsorption process typically lies between 6 and 8. However, at very high pH levels, metal ions may precipitate, forming insoluble hydroxides that fall out of the solution. Naseem et al. investigated the adsorption of Pb²⁺ by bentonite and found that at a pH of 3.4, each gram of clay could adsorb 20 mg of Pb²⁺. The adsorption rate increased significantly from 30% to 94.5% as the pH of the solution rose from 1.4 to 3.4 but dropped to 40% when the pH reached 5.0 [45]. An increase in soil pH generates more negatively charged adsorption sites on soil colloids and organic matter surfaces, thereby reducing the availability of metals [46]. After the addition of clay minerals to contaminated soil, the pH can be maintained between 6 and 8, which is the ideal pH range for agricultural soils [47]. Adjusting the pH to the optimal range not only enhances adsorption efficiency but also decreases the bioavailability of heavy metals in contaminated environments, thereby reducing their potential harm to plants and animals.

4.3. Temperature

Reaction temperature affects the adsorption kinetics of heavy metals on clay surfaces by altering the adsorption enthalpy, which in turn affects the adsorption performance. Most studies indicate that room temperature (22–29 °C) is ideal for metal adsorption. Although a slight increase in temperature can speed up adsorption, temperatures above 30 °C often lead to desorption, where metals detach from the clay’s surface [48]. Usman et al. reported good results at temperatures as high as 36 °C [49]. Some studies have found that adsorption rates decrease when the temperature exceeds 30 °C [50].

Chemical reactions involved in adsorption generally accelerate with increasing temperature, but overly high temperatures can destabilize the adsorbed metals. In practical applications, ambient or slightly cooler temperatures are preferred to ensure stable and efficient metal adsorption without the risk of desorption. Chai et al. investigated the adsorption capacity and removal efficiency of Cu(II) and Ni(II) ions using raw and acid-activated kaolinite. Optimal conditions were determined for each parameter, including a contact time of 60 min, a pH of 7.0, adsorbent dosage of 0.1 g/50 mL, an initial metal ion concentration of 100 mg/L, and a temperature of 25 °C (Figure 5) [51].

4.4. Contact Time

The adsorption rate of clay minerals gradually increases with contact time and then stabilizes once equilibrium is reached. In laboratory settings, significant metal removal often occurs within 3 to 6 h [52]. However, in some cases, complete heavy metal adsorption may take up to 6 h, and field applications may require longer periods, ranging from days to months, to achieve similar levels of adsorption, depending on factors such as the composition of the clay minerals, the type of heavy metal, and the pH of the system. Usman et al. observed prolonged adsorption behavior, with no heavy metal uptake for the first 21 days, followed by 75% adsorption after 111 days [53]. In addition, researchers treated a multi-heavy-metal-contaminated recreational site over the course of one month. The initial 30 min of the adsorption process were very rapid and spontaneous, but the adsorption rate may begin to slow down due to an imbalance between the number of adsorbates on the clay and the remaining adsorbates in the medium [47].

5. Advancements of Heavy Metals Adsorption by Clay-Based Adsorbents

There has been increasing research on using natural and modified clays as potential adsorbents for the removal of various toxic heavy metal ions from aqueous solutions (Table 1 and Table 2). After modification, the CEC and the specific SSA of clay minerals were significantly enhanced, which in turn increased their adsorption capacities.

5.1. Adsorption of Chromium (Cr)

Chromium contamination, particularly in its hexavalent form (e.g., Cr(VI)), is a major environmental concern due to its toxicity and carcinogenic potential [57]. The most common sources of chromium pollution are electroplating, leather tanning, dye, photographic, and cement industries. Respiration in Cr(VI) for a short time causes irritation of the nasal passages as well as cough, and chest pain, whereas swallowing results in abdominal pain, diarrhea, heart failure, and damage to the gut, liver, and kidneys, as well as dermatitis or skin ulcers [58]. The U.S. Environmental Protection Agency (EPA) has established a maximum contaminant level (MCL) for Cr of 0.1 mg/L in drinking water.

In addition to adsorbing Cr(VI), montmorillonite also adsorbs Cr(III), which is a less toxic form, further enhancing its overall effectiveness for chromium remediation. This dual action—adsorption and reduction—marks montmorillonite as a valuable tool in mitigating chromium pollution [59]. Li et al. also identified attapulgite (ATP) as a cost-effective adsorbent for divalent heavy metals in wastewater. Their synthesized CdS/ATP composites demonstrated multifunctional photocatalytic capabilities, effectively removing heavy metal Cr(VI) and tetracycline hydrochloride, as well as eliminating harmful pathogens in water under visible light [60] (Figure 6). Cellulose hydrogel-coated nanometer zero-valent iron-intercalated montmorillonite (CH-MMT-nFe⁰) composite enhances Cr(VI) removal efficiency to 98% through improved dispersity and stability and the formation of a “microreactor” for adsorption and degradation. Its mesoporous hydrogel structure and multiple interaction mechanisms make it a potential material for Cr(VI)-polluted wastewater treatment [61]. Similarly, copper nanoclusters (CuNCs)-halloysite nanotubes (HNT) composite (CuNCs@HNT), a multifunctional material, effectively removes Cr(VI) with high adsorption capacity and reusability, simplifying heavy metal remediation in wastewater [62]. The CeO₂/Mt composite synthesized via a one-pot thermal treatment of cerium dioxide thermally inserted into the structural framework of montmorillonite (Mt) clay can effectively remove Cr(VI) oxyanions from aqueous solutions, achieving 100% removal efficiency within 90 min at optimal conditions. Characterized by techniques such as FTIR, SEM, and XRD, the composite exhibits a high Langmuir monolayer adsorption capacity of 52.08 mg/g, making it a promising adsorbent for Cr(VI)-contaminated water treatment [59]. Additionally, the polyaniline-based hydroxyapatite–montmorillonite (PANI@Hap-Mt) composite reveals high potential for Cr(VI) adsorption, and the adsorption process followed pseudo-second-order kinetics and Langmuir isotherm with excellent regeneration capacity, making it a promising adsorbent for Cr(VI) detoxification in aqueous solutions [63].

5.2. Adsorption of Copper (Cu)

The diverse sources of Cu²⁺ in the environment are natural/industrial deposits, wood preservatives, plumbing, the chemical industry, the pesticides industry, and mining, posing health risks [64]. The excessive ingestion of copper brings about serious toxicological concerns, such as gastrointestinal irritation, damage to the kidney and liver, convulsions, cramps, vomiting, increased blood pressure and respiratory rates, or even death [65]. The U.S. EPA regulates its level in drinking water at 1.3 mg/L. Effective Cu²⁺ removal from wastewater has been achieved by using adsorbents like bentonite, bentonite/GO composite, and montmorillonite-based nanocomposite hydrogels, which offer promising solutions for mitigating copper ion pollution in aquatic environments [66]. Bentonite and graphene oxide (GO) were combined to create a bentonite/GO composite with enhanced adsorption properties for removing copper and nickel ions from wastewater. The bentonite/GO composite showed a higher adsorption capacity as compared to pure bentonite, with maximum adsorption capacities of 248.9 mg/g for Cu and 558.4 mg/g for Ni, indicating its potential for efficient heavy-metal ion removal from aqueous solutions [67]. Wang et al. investigated the use of activated bentonite–alginate composite beads (ABn-AG) for the removal of copper and lead ions, achieving high removal rates even under highly acidic conditions. The material also demonstrates excellent reusability, making it a highly efficient adsorbent for water treatment [68]. Additionally, the authors synthesized a nanocomposite hydrogel using exfoliated montmorillonite layers as a noncovalent cross-linker, which demonstrated exceptional adsorption capacity for heavy metal ions. The hydrogel, optimized with 4% acrylic acid, balanced adsorption efficiency and stability while maintaining self-stabilization with minimal swelling during the sorption process [68]. The high CEC of the clay allows Cu²⁺ to replace naturally occurring cations (such as Na⁺ or Ca²⁺) on the clay’s surface. Additionally, the formation of inner-sphere complexes, where Cu²⁺ binds directly to oxygen atoms in the silicate layers, further enhances the overall adsorption performance [69,70].

5.3. Adsorption of Lead (Pb)

Lead (Pb²⁺) is found to be a very hazardous heavy-metal ion for human beings, with its sources being natural/industrial deposits, plumbing, solder, and brass alloy faucets. The accumulation of Pb²⁺ in the body disturbs processes such as hemoglobin synthesis and renal function and causes neurological and behavioral disturbances in children [70]. The permissible capacity of Pb²⁺ in drinking water set by the World Health Organization (WHO) is 10 μg/L. Meanwhile, the U.S. EPA established a maximum contaminant level (MCL) for Pb²⁺ of 0.015 mg/L in drinking water. The use of clay minerals like montmorillonite and bentonite for lead removal has been extensively researched, with studies demonstrating high adsorption capacities [71]. Wahba et al. ground zeolite raw material into a powder and applied it in the same proportions to contaminated sandy and clay soils. They found that zeolite effectively remediated soils polluted with Pb²⁺, Zn²⁺, and Ni²⁺ [72]. Shang et al. studied the impact of oxalic acid on kaolinite’s adsorption of Pb/Cd and the role of calcium silicate (CS) and magnesium silicate (MS) in mitigating heavy metal risks. The results showed that increasing oxalic acid concentrations elevated Pb/Cd release, but the addition of CS/MS significantly reduced the free Pb/Cd levels through redistribution in the aging system [73]. Su et al. explored the protective effect of clay minerals on bacterial cells under Pb²⁺ stress. They compared the adsorption capacity of two types of montmorillonite (Mt) and found no significant difference in the adsorbed Pb²⁺ (~55 mg/L), highlighting Mt’s role in shielding microorganisms from Pb²⁺ stress [74] (Figure 7).

5.4. Adsorption of Cadmium (Cd)

The various sources for Cd²⁺ are from welding, fertilizers, nuclear emission plants, steel and plastics industries, cooling tower blowdown, electroplating, metal plating, nickel–cadmium batteries, galvanized pipe corrosion, natural deposits, batteries, and paints. Cadmium exposure also causes prostate and renal cancer bronchiolitis, emphysema, fibrosis, and skeletal and kidney damage [75]. Occupational and environmental cadmium exposure is linked to renal dysfunction and skeletal disorders, including osteomalacia, osteoporosis, and increased fracture risk. Studies in Japan and Europe have demonstrated a dose-response relationship between low to moderate cadmium levels and decreased bone mineral density, with potential direct effects on bone metabolism and increased fracture prevalence [76]. To mitigate these health risks, strategies such as maintaining neutral soil pH to reduce plant uptake of cadmium and ensuring adequate iron status to lower intestinal absorption are crucial, especially considering the fact that the current European cadmium intake is nearing the tolerable limit [76]. The U.S. EPA has established a MCL for cadmium of 0.005 mg/L in drinking water.

Clay minerals with high surface areas and abundant exchangeable cations, such as bentonite and kaolinite, are particularly efficient at adsorbing Cd²⁺ [77]. Studies also indicate that modified clays, incorporating substances like organic ligands or metal oxides, show improved Cd²⁺ removal efficiencies due to the introduction of additional active sites [56]. A novel MgO-modified biochar composite was developed for Cd²⁺ removal, exhibiting an adsorption capacity significantly higher than that of pristine biochar. The enhanced performance was attributed to multiple mechanisms, including ion exchange and chemical bonding, as confirmed by DFT calculations [78]. Li et al. present a one-step hydrothermal synthesis of MoS₂/bentonite composites for heavy metal ion removal, which utilized the large active surface area of MoS₂ and the adsorptive properties of bentonite to reach a maximum Cd²⁺ adsorption capacity of 89.45 mg/g, highlighting the material’s potential for environmental remediation [79]. Wang et al. developed a novel, nontoxic, and magnetic attapulgite-based adsorbent (ATPCFS-CSEs) for efficient Cd²⁺ removal from water. The material, synthesized via solvothermal and sol-gel processes, demonstrated a high adsorption capacity of 127.79 mg/g and selectivity in the presence of competitive ions and maintained over 88% removal efficiency after five cycles, making it a promising adsorbent for Cd-contaminated water purification [80].

5.5. Adsorption of Arsenic (As)

Arsenic exists in two main oxidation states—arsenite (As(III)) and arsenate (As(V))—with As(III) being more toxic and mobile. Water contamination by arsenic salt is one of the big environmental pollutions, from sources such as natural deposits, smelters, glass, electronics waste, orchards, etc. It causes lifetime diseases such as cancer, neurological disorders, nausea, hyperkeratosis, muscular weakness, and many others [81,82,83]. The U.S. EPA has established a MCL for arsenic of 0.010 mg/L in drinking water.

Clay minerals, particularly montmorillonite, have been demonstrated to effectively adsorb both forms of arsenic [84]. Ghorbanzadeh et al. investigated the adsorption and removal of arsenate and arsenite by montmorillonite from river water. They found that montmorillonite is an excellent adsorbent for heavy metals, achieving 99.5% for arsenate and 68.2% for arsenite [85]. Moreover, modification methods such as thermal modification, acid modification, and organic modification are the most widely applied approaches for enhancing the performance of clay minerals. Among them, modification through the addition of iron oxides or organic modifiers has been shown to significantly enhance adsorption capacities [86]. Kazantzis presented an Australian smectite-supported nano zero-valent iron (nZVI) composite for efficient arsenate (As(V)) sorption, characterized by advanced techniques and revealing an even distribution of iron nanobeads on clay particles. The composite exhibited a high As(V) sorption capacity of 23.12 mg/g with negligible iron release across a wide pH range, offering an effective and eco-friendly solution for arsenic removal from water [83]. Guo introduced a zirconia-loaded halloysite nanotubes (ZrO₂/HNTs) nanohybrid for the efficient removal of arsenite (As(III)) from water, featuring a strong adsorption capacity of 36.08 mg/g attributed to its hydroxyl groups and large surface area [87]. Qi et al. investigated different adsorption configurations, analyzed the electronic properties, and found that the E ad values of the four HM atoms on the surface of illite (001) are As > Cr > Cd > Hg. The values for the most stable adsorption configurations are −1.8554, −0.7982, −0.3358, and −0.2678 eV, respectively (Figure 8) [87].

5.6. Adsorption of Hydrargyrum (Hg)

Hydrargyrum (Hg) pollution is a global environmental problem that originates mainly from industrial emissions (e.g., coal combustion, mining, chlor-alkali industry) as well as natural processes (e.g., volcanism), and can be highly persistent in the environment through the atmospheric, soil, and water cycles [88]. Hg(II) is one of the oxidized forms of mercury and often exists as inorganic salts or complexes. It is one of the most important forms of hydrargyrum pollution, mainly originating from industrial activities (e.g., metallurgy, electroplating, the chemical industry). Hg(II) is highly water-soluble and readily disperses in water bodies and enters the soil through deposition or is biologically absorbed. In addition, Hg(II) can be methylated by microorganisms in anaerobic environments to the more toxic methyl hydrargyrum (MeHg), which can be biomagnified in the food chain, posing a serious threat to ecosystems and human health, including neurological damage and developmental disorders. Therefore, Hg(II) is a priority research target for pollution prevention [89,90]. The U.S. EPA has established a maximum allowable concentration of hydrargyrum in drinking water of 0.002 mg/L [91].

Phothitontimongkol et al. synthesized AEPE-montmorillonite and AEPE-hectorite by grafting clay minerals with 2-(3-(2-aminoethylthio)propylthio)ethylamine (AEPE). The modified clay minerals exhibited significantly enhanced chelating properties for Hg(II) ion adsorption as compared to their unmodified counterparts. Under conditions of pH 4 and an initial Hg(II) concentration of 140 mg/L, the adsorption capacities were 46.1 mg/g for AEPE-montmorillonite and 54.7 mg/g for AEPE-hectorite, demonstrating their potentials as effective adsorbents for mercury removal [92]. Moreover, Pei et al. prepared ISH-Mt and GSH-Mt by thiol-functionalizing the clay mineral montmorillonite (Mt) through intercalation with 2-mercaptoethylammonium (ISH) and surface grafting with 3-mercaptopropyltrimethoxysilane (GSH). Surface grafting notably altered the surface morphology and porous structure, resulting in rougher surfaces, larger specific surface areas, and enhanced porosity. The thiol functionalization significantly improved the adsorption capacity for Hg²⁺, with ISH-Mt and GSH-Mt achieving adsorption amounts of 141.55 and 136.92 mg/g, respectively [93]. Wang et al. modified montmorillonite (Mont) and medical stone (Med) using thiol-based material (-SH) and chitosan to produce modified clay mineral adsorbents. The modified clay minerals exhibited greater efficiency in reducing the Hg levels in rice as compared to the unmodified minerals. Among them, the SH-modified Med (Med-SH) demonstrated the highest reduction efficiencies for both total hydrargyrum (THg) and MeHg in rice, achieving reductions of 78% and 81%, respectively. This resulted in THg concentrations in rice that were below the Chinese rice sanitary standard of 20 ng/g (Figure 9) [94].

Table 2

Modified clay minerals for the removal of heavy metal contaminants.

Modified Clays Mineral(s)	Heavy Metal(s) Removed	Removal Efficiency or Adsorption Capacity	References
CdS/ATP composites	Cr(VI)	/	[60]
CH-MMT-nFe0 composite	Cr(VI)	98%	[61]
CuNCs@HNT composite	Cr(VI)	/	[62]
CeO2/Mt composite	Cr(VI)	100%; 52.08 mg/g	[59]
PANI@Hap-Mt composite	Cr(VI)	/	[63]
Bentonite/GO composite	Cu2+ Ni2+	248.9 mg/g; 558.4 mg/g	[67]
ABn-AG composite beads	Cu2+ Pb2+	/	[68]
Eolite powder	Pb2+ Ni2+ Zn2+	/	[72]
Oxalic acid; calcium silicate (CS) and magnesium silicate (MS)	Pb/Cd	/	[73]
Montmorillonite (Mt)	Pb2+	55 mg/L	[74]
MoS2/Bentonite composites	Cd2+	89.45 mg/g	[79]
ATPCFS-CSEs	Cd2+	88%; 127.79 mg/g	[80]
Montmorillonite	As(V), As(III)	99.5%, 68.2%	[85]
nano zero-valent iron (nZVI)	As(V)	23.12 mg/g	[83]
ZrO2/HNTs nanohybrid	As(III)	36.08 mg/g	[87]
AEPE-montmorillonite and AEPE-hectorite	Hg(II)	46.1 mg/g, 54.7 mg/g	[93]
ISH-Mt and GSH-Mt	Hg2+	141.55 mg/g, 136.92 mg/g	[94]
Med-SH	THg, MeHg	78%, 81%	[95]

6. Conclusions and Perspectives

Clay-based adsorbents offer a sustainable and cost-effective approach to removing heavy metals from contaminated environments due to their distinctive structural properties, high cation exchange capacities, and ability to interact with a wide range of metal ions. However, enhancing the adsorption capacities of natural clays through chemical and physical modifications remains a critical area of research. Modified clays, including organoclays, nano zero-valent iron (nZVI) clays, and thermally treated clays, have shown promise in improving adsorption efficiency, selectivity, and stability [95]. Future research should focus on developing more effective modification methods, exploring emerging materials such as nanocomposites and functionalized clays, expanding field trials, and developing standardized application schemes. At the same time, attention should be given to the influence of competing ions and variable environmental conditions. Finally, the potential environmental risks of modified clays should be evaluated to ensure their long-term sustainability and safety.

Author Contributions

Conceptualization, P.W. and X.S.; methodology, J.L.; software, L.Z.; validation, P.W., X.S. and S.Q.; formal analysis, X.S.; investigation, Y.M.; resources, J.L.; data curation, S.Q.; writing—original draft preparation, P.W.; writing—review and editing, S.Q.; visualization, L.Z.; supervision, Y.M.; project administration, J.L.; funding acquisition, X.S. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

Pengsheng Wang, Xinkai Shen, Long Zhang, Yanping Ma and Jinbao Liang were employed by the Shanxi Metallurgical Geotechnical Engineering Investigation Co. The remaining author declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Clay mineral sheet, layer, and particle structures. (Reproduced from Liu et al., 2022 with permission [29]).

Figure 2. Mechanisms of heavy metals and dyes removed by clay-based adsorbents: (a) Contaminants with O-, H-, N-, and F-groups form hydrogen bonds with -OH on the surface (like kaolinite) or edge (like montmorillonite) of the clay-based adsorbents. (b) Cationic contaminants replace the exchangeable cations in clay adsorbents via ion exchange. (c) Electropositive contaminants adsorbed onto the surface of electronegative clay-based adsorbents via electrostatic attraction. (d) Surface complexation occurs among heavy metal ions and surface functional groups on the adsorbent surface.

Figure 3. Graphical representation of the process of heavy metal adsorption by clay minerals. Source: Adapted from (Uddin, 2017) [31].

Figure 4. Interaction mechanisms of metal ions with clay mineral: (a) outer-sphere complex formation on basal surface, (b) outer-sphere complex formation on interlayer space, (c) inner-sphere complex formation on interlayer space, (d) complexation at a redox active site (Reproduced from Liu et al., 2022 with permission [29]).

Figure 5. Factors of the adsorption reactions: (a) N2 adsorption–desorption isotherms of raw and acid-activated kaolinite. (b) Zeta potential–pH behavior of raw and acid-activated kaolinite (1 wt%). (c) Effect of contact time on Cu(II) and Ni(II) adsorption at pH 7, 25 °C, 0.1 g/50 mL dosage, and 100 mg/L ion concentration. (d) Effect of pH on adsorption at 60 min under the same conditions. (e) Effect of adsorbent dosage on Cu(II) and Ni(II) adsorption. (f) Effect of initial ion concentration on adsorption under the same conditions. (Reproduced from Chai et al., 2020 with permission [51]).

Figure 6. Schematic diagram of CdS/ATP composites’ preparation and pollutant treatment: (a) Schematic representation of the preparation process for CdS/ATP-T composite. (b) Diagrammatic illustration of the photocatalytic mechanism of CdS/ATP-T during the treatment process: Upon light irradiation, e−1 in the valence band of CdS are excited to the conduction band, which has higher energy levels, leaving behind an equal number of holes (h+) in the valence band; Photogenerated e−1 can reduce O2 to produce superoxide anions (·O2−), while holes (h⁺) can oxidize water (H2O) and hydroxide ions (−OH) to generate hydroxyl radicals (·OH). (Reproduced from Li et al., 2023 with permission [60]).

Figure 7. Clay-mediated protection of Enterobacter sp. against Pb (II) stress. (Reproduced from Qi et al., 2022, with permission [74]).

Figure 8. The adsorption of As, Cd, Cr, and Hg heavy metal atoms onto the illite (001)’s surface. (Reproduced from Qi et al., 2022 with permission [87]).

Figure 9. Modified clay minerals for the remediation of mercury-contaminated rice soils [94].

Table 1

Clay minerals for the removal of heavy metal contaminants.

Clay Mineral(s)	Remediated Media	Heavy Metal(s) Removed	References
Attapulgite, montmorillonite	Soil	Cu2+	[47]
Bentonite	Wastewater	Co2+, Cu2+, Ni2+, Pb2+, Zn2+	[54]
Kaolinite, illite	Aqueous solution	Cd, Cr, Cu, Ni, Pb, Zn	[55]
Mixed layer clay	Wastewater	Ni	[52]
Montmorillonite	Aqueous solution	Zn, Cd, Cu, Pb, Hg	[56]

References

1. Xiang, M.; Li, Y.; Yang, J.; Lei, K.; Li, Y.; Li, F.; Zheng, D.; Fang, X.; Cao, Y. Heavy metal contamination risk assessment and correlation analysis of heavy metal contents in soil and crops. Environ. Pollut.; 2021; 278, 116911. [DOI: https://dx.doi.org/10.1016/j.envpol.2021.116911] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33740600]

2. Li, R.; Wang, J.; Zhou, Y.; Zhang, W.; Feng, D.; Su, X. Heavy metal contamination in Shanghai agricultural soil. Heliyon; 2023; 9, e22824. [DOI: https://dx.doi.org/10.1016/j.heliyon.2023.e22824] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/38125553]

3. Kinoti, I.K.; Karanja, E.M.; Nthiga, E.W.; M’thiruaine, C.M.; Marangu, J.M. Review of Clay-Based Nanocomposites as Adsorbents for the Removal of Heavy Metals. J. Chem.; 2022; 2022, 7504626. [DOI: https://dx.doi.org/10.1155/2022/7504626]

4. Zaimee, M.Z.A.; Sarjadi, M.S.; Rahman, M.L. Heavy Metals Removal from Water by Efficient Adsorbents. Water; 2021; 13, 2659. [DOI: https://dx.doi.org/10.3390/w13192659]

5. Ma, Y.; Li, X.; Mao, H.; Wang, B.; Wang, P. Remediation of hydrocarbon–heavy metal co-contaminated soil by electrokinetics combined with biostimulation. Chem. Eng. J.; 2018; 353, pp. 410-418. [DOI: https://dx.doi.org/10.1016/j.cej.2018.07.131]

6. Sharma, Y.C.; Srivastava, V.; Singh, V.K.; Kaul, S.N.; Weng, C.H. Nano-adsorbents for the removal of metallic pollutants from water and wastewater. Environ. Technol.; 2009; 30, pp. 583-609. [DOI: https://dx.doi.org/10.1080/09593330902838080]

7. Wang, E.; Huang, T.; Wu, Q.; Huang, L.; Kong, D.; Wang, H. Influence of Hydrothermal Modification on Adsorptive Performance of Clay Minerals for Malachite Green. Molecules; 2024; 29, 1974. [DOI: https://dx.doi.org/10.3390/molecules29091974]

8. Ling, C.; Liu, Z.; Yu, X.; Zhao, Y.; Siringan, F.P.; Le, K.P.; Sathiamurthy, E.; You, C.-F.; Chen, K. Clay minerals control silicon isotope variations of fine-grained river sediments: Implication for the trade-off between physical erosion and chemical weathering. Chem. Geol.; 2024; 662, 122249. [DOI: https://dx.doi.org/10.1016/j.chemgeo.2024.122249]

9. Nair, M.; Best, S.M.; Cameron, R.E. Crosslinking Collagen Constructs: Achieving Cellular Selectivity Through Modifications of Physical and Chemical Properties. Appl. Sci.; 2020; 10, 6911. [DOI: https://dx.doi.org/10.3390/app10196911]

10. Nir, S.Z.; Salem, A.; Salem, S. Application of clay-based waste collected in landfill of vegetable oil refinery for immobilization of heavy metal ions from wastewater of zinc industry through fabrication of zeolite LTA and hydroxysodalite. Sustain. Chem. Pharm.; 2024; 39, 101618. [DOI: https://dx.doi.org/10.1016/j.scp.2024.101618]

11. Kamińska, G.; Marszałek, A.; Puszczało, E.; Fathy Abdel Salam, N. Clay-based nanocomposite membranes for removal of heavy metals from rainwater. Sep. Purif. Technol.; 2024; 346, 127334. [DOI: https://dx.doi.org/10.1016/j.seppur.2024.127334]

12. Zhang, Z.; Chen, Y.; Wang, D.; Yu, D.; Wu, C. Lignin-based adsorbents for heavy metals. Ind. Crops Prod.; 2022; 193, 116119. [DOI: https://dx.doi.org/10.1016/j.indcrop.2022.116119]

13. Nir, S.Z.; Salem, A.; Salem, S. Rapid production of hydroxysodalite from a clay-based waste through microwave-assisted technique for treatment of wastewater contaminated with zinc cation. Appl. Clay Sci.; 2024; 258, 107499. [DOI: https://dx.doi.org/10.1016/j.clay.2024.107499]

14. Meng, Z.; Li, J.; Zou, Y.; Li, N.; Fu, X.; Zhang, R.; Hu, S.; Liu, Q. Advanced montmorillonite modification by using corrosive microorganisms as an alternative filler to reinforce natural rubber. Appl. Clay Sci.; 2022; 225, 106534. [DOI: https://dx.doi.org/10.1016/j.clay.2022.106534]

15. Ashour, E.A.; Tony, M.A. Eco-friendly removal of hexavalent chromium from aqueous solution using natural clay mineral: Activation and modification effects. SN Appl. Sci.; 2020; 2, 2042. [DOI: https://dx.doi.org/10.1007/s42452-020-03873-x]

16. Shapkin, N.P.; Leont’ev, L.B.; Khal’chenko, I.G.; Panasenko, A.E.; Maiorov, V.Y.; Razov, V.I.; Kaidalova, T.A.; Papynov, E.K. Chemical modification of natural clays. Russ. J. Inorg. Chem.; 2017; 62, pp. 1209-1214. [DOI: https://dx.doi.org/10.1134/S0036023617090121]

17. Kausar, A.; Iqbal, M.; Javed, A.; Aftab, K.; Nazli, Z.-i.-H.; Bhatti, H.N.; Nouren, S. Dyes adsorption using clay and modified clay: A review. J. Mol. Liq.; 2018; 256, pp. 395-407. [DOI: https://dx.doi.org/10.1016/j.molliq.2018.02.034]

18. Xu, G.; Deconinck, J.F.; Feng, Q.; Baudin, F.; Pellenard, P.; Shen, J.; Bruneau, L. Clay mineralogical characteristics at the Permian–Triassic Shangsi section and their paleoenvironmental and/or paleoclimatic significance. Palaeogeogr. Palaeoclimatol. Palaeoecol.; 2017; 74, pp. 152-163. [DOI: https://dx.doi.org/10.1016/j.palaeo.2016.07.036]

19. Walaa, R.M.; Nesreen, M.S.; Sayed, S.M.; Saad, E.A. Surface modification of ball clay minerals with gamma irradiation polymerization for removal of cerium and gadolinium ions from aqueous phase. Hydrometallurgy; 2022; 208, 105816. [DOI: https://dx.doi.org/10.1016/j.hydromet.2022.105816]

20. Oliveira, L.; Osajima, J.; Peña-Garcia, R.R.; Silva-Filho, E.C.; Fonseca, M.G. Effect of Oxycations in Clay Mineral on Adsorption—Vanadyl Exchange Bentonites and Their Ability for Amiloride Removal. Minerals; 2021; 11, 1327. [DOI: https://dx.doi.org/10.3390/min11121327]

21. Mokhtar, A.; Ahmed, A.B.; Asli, B.; Boukoussa, B.; Hachemaoui, M.; Sassi, M.; Abboud, M. Recent Advances in Antibacterial Metallic Species Supported on Montmorillonite Clay Mineral: A Review. Minerals; 2023; 13, 1268. [DOI: https://dx.doi.org/10.3390/min13101268]

22. Ghosh, S.; Mukhopadhyay, J.; Chakraborty, A. Clay Mineral and Geochemical Proxies for Intense Climate Change in the Permian Gondwana Rock Record from Eastern India. Research; 2019; 2019, 8974075. [DOI: https://dx.doi.org/10.34133/2019/8974075] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/31922141]

23. Hansen, K. Ralph E. Grim: Applied Clay Mineralogy. McGraw-Hill Book Comp. New York, Toronto, London 1962. 422 s., 120 fig. 16x23,5 cm. 97 sh. Geogr. Tidsskr. Dan. J. Geogr.; 1962; 61.

24. Shichi, T.; Takagi, K. Clay minerals as photochemical reaction fields. J. Photochem. Photobiol. C Photochem. Rev.; 2000; 1, pp. 113-130. [DOI: https://dx.doi.org/10.1016/S1389-5567(00)00008-3]

25. Wilson, I. Applied Clay Mineralogy. Occurrences, processing and application of kaolins, bentonite, palygorskitesepiolite, and common clays. Clays Clay Miner; 2007; 55, pp. 644-645. [DOI: https://dx.doi.org/10.1007/BF03406033]

26. Siyao, H.; Mingbiao, X.; Peng, X.; Yu, Z.; Xinying, W. Composite Hydration Process of Clay Minerals Simulating Mineral Clay Components and Influence Mechanism of Cations. Energies; 2022; 15, 7550. [DOI: https://dx.doi.org/10.3390/en15207550]

27. Marsh, A.; Heath, A.; Patureau, P.; Evernden, M.; Walker, P. Alkali activation behaviour of un-calcined montmorillonite and illite clay minerals. Appl. Clay Sci.; 2018; 166, pp. 250-261. [DOI: https://dx.doi.org/10.1016/j.clay.2018.09.011]

28. Deng, L.; Yuan, P.; Liu, D.; Annabi-Bergaya, F.; Zhou, J.; Chen, F.; Liu, Z. Effects of microstructure of clay minerals, montmorillonite, kaolinite and halloysite, on their benzene adsorption behaviors. Appl. Clay Sci.; 2017; 143, pp. 184-191. [DOI: https://dx.doi.org/10.1016/j.clay.2017.03.035]

29. Liu, X.; Tournassat, C.; Grangeon, S.; Kalinichev, A.G.; Takahashi, Y.; Marques Fernandes, M. Molecular-level understanding of metal ion retention in clay-rich materials. Nat. Rev. Earth Environ.; 2022; 3, pp. 461-476. [DOI: https://dx.doi.org/10.1038/s43017-022-00301-z]

30. Karakışla, M. The adsorption of Cu(II) ion from aqueous solution upon acrylic acid grafted poly(ethylene terephthalate) fibers. J. Appl. Polym. Sci.; 2002; 87, pp. 1216-1220. [DOI: https://dx.doi.org/10.1002/app.11578]

31. Uddin, M.K. A review on the adsorption of heavy metals by clay minerals, with special focus on the past decade. Chem. Eng. J.; 2016; 308, pp. 438-462. [DOI: https://dx.doi.org/10.1016/j.cej.2016.09.029]

32. Rao, W.; Lv, G.; Wang, D.; Liao, L. Enhanced Degradation of Rh 6G by Zero Valent Iron Loaded on Two Typical Clay Minerals With Different Structures Under Microwave Irradiation. Front. Chem.; 2018; 6, 463. [DOI: https://dx.doi.org/10.3389/fchem.2018.00463] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/30356813]

33. Chen, T.; Yuan, Y.; Zhao, Y.; Rao, F.; Song, S. Effect of Layer Charges on Exfoliation of Montmorillonite in Aqueous Solutions. Colloids Surf. A Physicochem. Eng. Asp.; 2018; 548, pp. 92-97. [DOI: https://dx.doi.org/10.1016/j.colsurfa.2018.03.066]

34. Ge, P.; Zhang, J.; Liu, Y.; Wang, S.; Liu, W.; Yu, N.; Wu, Y.; Zhang, J.; Yang, B. Smart Anisotropic Wetting Surfaces with Reversed pH-Responsive Wetting Directions. Adv. Funct. Mater.; 2018; 28, 1802001. [DOI: https://dx.doi.org/10.1002/adfm.201802001]

35. Paoli, L.; Vannini, A.; Monaci, F.; Loppi, S. Competition between heavy metal ions for binding sites in lichens: Implications for biomonitoring studies. Chemosphere; 2018; 199, pp. 655-660. [DOI: https://dx.doi.org/10.1016/j.chemosphere.2018.02.066]

36. Liu, X.; Lu, X.; Sprik, M.; Cheng, J.; Meijer, E.J.; Wang, R. Acidity of edge surface sites of montmorillonite and kaolinite. Geochim. Cosmochim. Acta; 2013; 117, pp. 180-190. [DOI: https://dx.doi.org/10.1016/j.gca.2013.04.008]

37. Ali, A.; Guo, D.; Zhang, Y.; Sun, X.; Jiang, S.; Guo, Z.; Huang, H.; Liang, W.; Li, R.; Zhang, Z. Using bamboo biochar with compost for the stabilization and phytotoxicity reduction of heavy metals in mine-contaminated soils of China. Sci. Rep.; 2017; 7, 2690. [DOI: https://dx.doi.org/10.1038/s41598-017-03045-9]

38. Churchman, G.J.; Gates, W.P.; Theng, B.K.G.; Yuan, G. Chapter 11.1 Clays and Clay Minerals for Pollution Control. Developments in Clay Science; Bergaya, F.; Theng, B.K.G.; Lagaly, G. Elsevier: Amsterdam, The Netherlands, 2006; Volume 1, pp. 625-675.

39. Baimenov, A.; Montagnaro, F.; Inglezakis, V.J.; Balsamo, M. Experimental and Modeling Studies of Sr2+ and Cs+ Sorption on Cryogels and Comparison to Commercial Adsorbents. Ind. Eng. Chem. Res.; 2022; 61, pp. 8204-8219. [DOI: https://dx.doi.org/10.1021/acs.iecr.2c00531]

40. Fuks, L.; Herdzik-Koniecko, I.; Maskalchuk, L.; Leontieva, T. Clay-salt slimes of the JSC “Belaruskali” as potential engineering barriers in the radioactive waste repositories: Sorption of Cs(I), Sr(II), Eu(III) and Am(III). Int. J. Environ. Sci. Technol.; 2017; 5, pp. 2047-2058. [DOI: https://dx.doi.org/10.1007/s13762-017-1597-3]

41. Marmier, N.; Delisée, A.; Fromage, F. Surface Complexation Modeling of Yb(III), Ni(II), and Cs(I) Sorption on Magnetite. J. Colloid Interface Sci.; 1999; 211, pp. 54-60. [DOI: https://dx.doi.org/10.1006/jcis.1998.5968]

42. Yao, Y.; Feng, Y.; Li, H.; Liu, M.; Cui, Y.; Xu, C.; Li, Y.; Wang, J. Investigation of the adsorption performance and mechanism of multi-source mineral composite calcination materials on heavy metal ions. Desalination; 2024; 586, 117847. [DOI: https://dx.doi.org/10.1016/j.desal.2024.117847]

43. Jin, X.; Zha, S.; Li, S.; Chen, Z. Simultaneous removal of mixed contaminants by organoclays—Amoxicillin and Cu(II) from aqueous solution. Appl. Clay Sci.; 2014; 102, pp. 196-201. [DOI: https://dx.doi.org/10.1016/j.clay.2014.09.040]

44. Chen, Y.-G.; Ye, W.-M.; Yang, X.-M.; Deng, F.-Y.; He, Y. Effect of contact time, pH, and ionic strength on Cd(II) adsorption from aqueous solution onto bentonite from Gaomiaozi, China. Environ. Earth Sci.; 2011; 64, pp. 329-336. [DOI: https://dx.doi.org/10.1007/s12665-010-0850-6]

45. Naseem, R.; Tahir, S.S. Removal of Pb(II) from aqueous/acidic solutions by using bentonite as an adsorbent. Water Res.; 2001; 35, pp. 3982-3986. [DOI: https://dx.doi.org/10.1016/S0043-1354(01)00130-0]

46. Akpomie, K.G.; Dawodu, F.A. Acid-modified montmorillonite for sorption of heavy metals from automobile effluent. Beni-Suef Univ. J. Basic Appl. Sci.; 2016; 5, pp. 1-12. [DOI: https://dx.doi.org/10.1016/j.bjbas.2016.01.003]

47. Zhang, G.; Lin, Y.; Wang, M. Remediation of copper polluted red soils with clay materials. J. Environ. Sci.; 2011; 23, pp. 461-467. [DOI: https://dx.doi.org/10.1016/S1001-0742(10)60431-7]

48. Es-sahbany, H.; Berradi, M.; Nkhili, S.; Hsissou, R.; Allaoui, M.; Loutfi, M.; Bassir, D.; Belfaquir, M.; El Youbi, M.S. Removal of heavy metals (nickel) contained in wastewater-models by the adsorption technique on natural clay. Mater. Today Proc.; 2019; 13, pp. 866-875. [DOI: https://dx.doi.org/10.1016/j.matpr.2019.04.050]

49. Alshameri, A.; He, H.; Xin, C.; Zhu, J.; Xinghu, W.; Zhu, R.; Wang, H. Understanding the role of natural clay minerals as effective adsorbents and alternative source of rare earth elements: Adsorption operative parameters. Hydrometallurgy; 2019; 185, pp. 149-161. [DOI: https://dx.doi.org/10.1016/j.hydromet.2019.02.016]

50. Manohar, D.M.; Anoop Krishnan, K.; Anirudhan, T.S. Removal of mercury(II) from aqueous solutions and chlor-alkali industry wastewater using 2-mercaptobenzimidazole-clay. Water Res.; 2002; 36, pp. 1609-1619. [DOI: https://dx.doi.org/10.1016/S0043-1354(01)00362-1]

51. Chai, J.-B.; Au, P.-I.; Mubarak, N.M.; Khalid, M.; Ng, W.P.-Q.; Jagadish, P.; Walvekar, R.; Abdullah, E.C. Adsorption of heavy metal from industrial wastewater onto low-cost Malaysian kaolin clay-based adsorbent. Environ. Sci. Pollut. Res.; 2020; 27, pp. 13949-13962. [DOI: https://dx.doi.org/10.1007/s11356-020-07755-y]

52. Zheng, Q.; Zhou, Y.; Liu, X.; Liu, M.; Liao, L.; Lv, G. Environmental hazards and comprehensive utilization of solid waste coal gangue. Prog. Nat. Sci. Mater. Int.; 2024; 34, pp. 223-239. [DOI: https://dx.doi.org/10.1016/j.pnsc.2024.02.012]

53. Usman, A.R.A.; Kuzyakov, Y.; Stahr, K. Effect of clay minerals on extractability of heavy metals and sewage sludge mineralization in soil. Chem. Ecol.; 2004; 20, pp. 123-135. [DOI: https://dx.doi.org/10.1080/02757540410001665971]

54. Vhahangwele, M.; Mugera, G.W. The potential of ball-milled South African bentonite clay for attenuation of heavy metals from acidic wastewaters: Simultaneous sorption of Co²⁺, Cu²⁺, Ni²⁺, Pb²⁺, and Zn²⁺ ions. J. Environ. Chem. Eng.; 2015; 3, pp. 2416-2425. [DOI: https://dx.doi.org/10.1016/j.jece.2015.08.016]

55. Chantawong, V.; Harvey, N.W.; Bashkin, V.N. Comparison of Heavy Metal Adsorptions by Thai Kaolin and Ballclay. Water Air Soil Pollut.; 2003; 148, pp. 111-125. [DOI: https://dx.doi.org/10.1023/A:1025401927023]

56. Sdiri, A.; Khairy, M.; Bouaziz, S.; El-Safty, S. A natural clayey adsorbent for selective removal of lead from aqueous solutions. Appl. Clay Sci.; 2016; 126, pp. 89-97. [DOI: https://dx.doi.org/10.1016/j.clay.2016.03.003]

57. Zhang, D.; Xu, Y.; Li, X.; Wang, L.; He, X.; Ma, Y.; Zou, D. The Immobilization Effect of Natural Mineral Materials on Cr(VI) Remediation in Water and Soil. Int. J. Environ. Res. Public Health; 2020; 17, 2832. [DOI: https://dx.doi.org/10.3390/ijerph17082832]

58. Mohan, D.; Pittman, C.U. Activated carbons and low cost adsorbents for remediation of tri- and hexavalent chromium from water. J. Hazard. Mater.; 2006; 137, pp. 762-811. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2006.06.060]

59. Anthony, E.T.; Alfred, M.O.; Saliu, T.D.; Oladoja, N.A. One-pot thermal synthesis of Ceria/Montmorillonite composite for the removal of hexavalent chromium from aqueous system. Surf. Interfaces; 2020; 22, 100914. [DOI: https://dx.doi.org/10.1016/j.surfin.2020.100914]

60. Li, H.; Zhang, J.; Zhang, Y.; Huang, H.; Ou, H.; Zhang, Y. In-situ adsorption-conversion recovery of heavy metal cadmium by natural clay mineral for multi-functional photocatalysis. Sep. Purif. Technol.; 2023; 319, 124058. [DOI: https://dx.doi.org/10.1016/j.seppur.2023.124058]

61. Yin, W.; Yishu, G.; Naipeng, L.; Hu, J.; Xue, W.; Ning, L.; Xiaodong, Z. Cellulose hydrogel coated nanometer zero-valent iron intercalated montmorillonite (CH-MMT-nFe0) for enhanced reductive removal of Cr(VI): Characterization, performance, and mechanisms. J. Mol. Liq.; 2021; 347, 118355. [DOI: https://dx.doi.org/10.1016/j.molliq.2021.118355]

62. Deb, A.K.; Biswas, B.; Naidu, R.; Rahman, M.M. Mechanistic insights of hexavalent chromium remediation by halloysite-supported copper nanoclusters. J. Hazard. Mater.; 2021; 421, 126812. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2021.126812] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/34396956]

63. Imgharn, A.; Laabd, M.; Naciri, Y.; Hsini, A.; Mahir, F.-Z.; Zouggari, H.; Albourine, A. Insights into the performance and mechanism of PANI@Hydroxapatite-Montmorillonite for hexavalent chromium Cr (VI) detoxification. Surf. Interfaces; 2022; 36, 102568. [DOI: https://dx.doi.org/10.1016/j.surfin.2022.102568]

64. Xiong, Z.-T.; Wang, H. Copper toxicity and bioaccumulation in Chinese cabbage (Brassica pekinensis Rupr.). Environ. Toxicol.; 2005; 20, pp. 188-194. [DOI: https://dx.doi.org/10.1002/tox.20094] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/15793819]

65. Paulino, A.T.; Minasse, F.A.S.; Guilherme, M.R.; Reis, A.V.; Muniz, E.C.; Nozaki, J. Novel adsorbent based on silkworm chrysalides for removal of heavy metals from wastewaters. J. Colloid Interface Sci.; 2006; 301, pp. 479-487. [DOI: https://dx.doi.org/10.1016/j.jcis.2006.05.032]

66. Awual, M.R.; Ismael, M.; Yaita, T.; El-Safty, S.A.; Shiwaku, H.; Okamoto, Y.; Suzuki, S. Trace copper(II) ions detection and removal from water using novel ligand modified composite adsorbent. Chem. Eng. J.; 2013; 222, pp. 67-76. [DOI: https://dx.doi.org/10.1016/j.cej.2013.02.042]

67. Chang, Y.S.; Au, P.I.; Mubarak, N.M.; Khalid, M.; Jagadish, P.; Walvekar, R.; Abdullah, E.C. Adsorption of Cu(II) and Ni(II) ions from wastewater onto bentonite and bentonite/GO composite. Environ. Sci. Pollut. Res.; 2020; 27, pp. 33270-33296. [DOI: https://dx.doi.org/10.1007/s11356-020-09423-7]

68. Pawar, R.R.; Lalhmunsiama, n.; Ingole, P.G.; Lee, S.-M. Use of activated bentonite-alginate composite beads for efficient removal of toxic Cu2+ and Pb2+ ions from aquatic environment. Int. J. Biol. Macromol.; 2020; 164, pp. 3145-3154. [DOI: https://dx.doi.org/10.1016/j.ijbiomac.2020.08.130]

69. Vezentsev, A.I.; Gorbunova, N.M.; Sokolovskiy, P.V.; Mar’inskikh, S.G.; Chub, A.V.; Nguyen Hoai, C.; Greish, A.A. On the adsorption mechanism of copper ions on bentonite clay. Russ. Chem. Bull.; 2022; 71, pp. 651-655. [DOI: https://dx.doi.org/10.1007/s11172-022-3461-y]

70. Qu, X.; Alvarez, P.J.J.; Li, Q. Applications of nanotechnology in water and wastewater treatment. Water Res.; 2013; 47, pp. 3931-3946. [DOI: https://dx.doi.org/10.1016/j.watres.2012.09.058]

71. Elanchezhiyan, S.S.; Karthikeyan, P.; Rathinam, K.; Hasmath Farzana, M.; Park, C.M. Magnetic kaolinite immobilized chitosan beads for the removal of Pb(II) and Cd(II) ions from an aqueous environment. Carbohydr. Polym.; 2021; 261, 117892. [DOI: https://dx.doi.org/10.1016/j.carbpol.2021.117892]

72. Wahba, M.M.; Rahim, I.S.; Zaghloul, M.A. Application of Clay Minerals to Decrease Hazard of Heavy Metals in Some Egyptian Soils. Am. J. Environ. Resour. Econ.; 2017; 1, pp. 24-31.

73. Shang, Z.; Wang, T.; Ye, Q.; Wu, P.; Wu, J.; Sun, L.; Zhu, N. An environmentally friendly strategy for reducing the environmental risks of heavy metals adsorbed by kaolinite. J. Environ. Manag.; 2024; 355, 120506. [DOI: https://dx.doi.org/10.1016/j.jenvman.2024.120506] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/38447514]

74. Su, M.; Han, F.; Wang, M.; Ma, J.; Wang, X.; Wang, Z.; Hu, S.; Li, Z. Clay-assisted protection of Enterobacter sp. from Pb (II) stress. Ecotoxicol. Environ. Saf.; 2021; 208, 111704. [DOI: https://dx.doi.org/10.1016/j.ecoenv.2020.111704] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33396035]

75. Ihsanullah, N.; Al-Khaldi, F.A.; Abusharkh, B.; Khaled, M.; Atieh, M.A.; Nasser, M.S.; Laoui, T.; Saleh, T.A.; Agarwal, S.; Tyagi, I. et al. Adsorptive removal of cadmium(II) ions from liquid phase using acid modified carbon-based adsorbents. J. Mol. Liq.; 2015; 204, pp. 255-263. [DOI: https://dx.doi.org/10.1016/j.molliq.2015.01.033]

76. Kazantzis, G. Cadmium, osteoporosis and calcium metabolism. Biometals; 2005; 17, pp. 493-498. [DOI: https://dx.doi.org/10.1023/B:BIOM.0000045727.76054.f3]

77. Djukić, D.; Krstić, A.; Jakovljević, K.; Butulija, S.; Andjelković, L.; Pavlović, V.; Mirković, M. Brushite-Metakaolin Composite Geopolymer Material as an Effective Adsorbent for Lead Removal from Aqueous Solutions. Sustainability; 2022; 14, 4003. [DOI: https://dx.doi.org/10.3390/su14074003]

78. Yang, K.; Cheng, Z.; Luo, W.; Guo, B.; Zhang, B.; Zhang, Y. Adsorption performance and mechanisms of MgO-modified palygorskite/biochar composite for aqueous Cd (II): Experiments and theoretical calculation. Appl. Surf. Sci.; 2023; 638, 157965. [DOI: https://dx.doi.org/10.1016/j.apsusc.2023.157965]

79. Wenlong, L.; Xiaoqing, K.; Tianyu, Z.; Shiyao, L.; Rong, C.; Min, Z.; Haobo, H. MoS₂/Bentonite prepared by a facile method for efficient removal of Cd²⁺ from aqueous solution. J. Alloys Compd.; 2022; 907, 164508. [DOI: https://dx.doi.org/10.1016/j.jallcom.2022.164508]

80. Wang, Y.; Zhou, R.; Wang, C.; Zhou, G.; Hua, C.; Cao, Y.; Song, Z. Novel environmental-friendly nano-composite magnetic attapulgite functionalized by chitosan and EDTA for cadmium (II) removal. J. Alloys Compd.; 2020; 817, 153286. [DOI: https://dx.doi.org/10.1016/j.jallcom.2019.153286]

81. Chen, R.; Zhi, C.; Yang, H.; Bando, Y.; Zhang, Z.; Sugiur, N.; Golberg, D. Arsenic (V) adsorption on Fe₃O₄ nanoparticle-coated boron nitride nanotubes. J. Colloid Interface Sci.; 2011; 359, pp. 261-268. [DOI: https://dx.doi.org/10.1016/j.jcis.2011.02.071]

82. Mandal, B. Arsenic round the world: A review. Talanta; 2002; 58, pp. 201-235. [DOI: https://dx.doi.org/10.1016/S0039-9140(02)00268-0] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/18968746]

83. Mukhopadhyay, R.; Manjaiah, K.M.; Datta, S.C.; Yadav, R.K.; Sarkar, B. Inorganically modified clay minerals: Preparation, characterization, and arsenic adsorption in contaminated water and soil. Appl. Clay Sci.; 2017; 147, pp. 1-10. [DOI: https://dx.doi.org/10.1016/j.clay.2017.07.017]

84. Ghorbanzadeh, N.; Jung, W.; Halajnia, A.; Lakzian, A.; Kabra, A.N.; Jeon, B.-H. Removal of arsenate and arsenite from aqueous solution by adsorption on clay minerals. Geosystem Eng.; 2015; 18, pp. 302-311. [DOI: https://dx.doi.org/10.1080/12269328.2015.1062436]

85. Mukhopadhyay, R.; Sarkar, B.; Barman, A.; Datta, S.C.; Manjaiah, K.M. Arsenic Adsorption on Modified Clay Minerals in Contaminated Soil and Water: Impact of pH and Competitive Anions. Clean Soil Air Water; 2021; 49, 2000259. [DOI: https://dx.doi.org/10.1002/clen.202000259]

86. Guo, C.; Chen, W.; Yan, S.; Chen, Y.; Zhao, X.; Zhang, P.; Ma, W.; Yang, A. Novel zirconia-halloysite nanotube material for arsenite adsorption from water. J. Environ. Chem. Eng.; 2022; 11, 109181. [DOI: https://dx.doi.org/10.1016/j.jece.2022.109181]

87. Qi, C.; Xu, X.; Chen, Q.; Liu, H.; Min, X.; Fourie, A.; Chai, L. Ab initio calculation of the adsorption of As, Cd, Cr, and Hg heavy metal atoms onto the illite(001) surface: Implications for soil pollution and reclamation. Environ. Pollut.; 2022; 312, 120072. [DOI: https://dx.doi.org/10.1016/j.envpol.2022.120072]

88. Chen, M.; Neupane, B.; Zhan, X.; Liu, T.; Lin, Z.; Gao, C.; Zaccone, C.; Bao, K. Three thousand years of Hg pollution recorded in mangrove wetland sediments from South China. Environ. Res.; 2024; 420, 127574. [DOI: https://dx.doi.org/10.1016/j.envres.2024.118866]

89. Ma, Y.; Wang, J.; Lv, P.; Zhou, J.; Zhou, J.; Mei, J.; Yang, S. Alternating adsorption of gaseous Hg⁰ in smelting flue gas and aqueous Hg²⁺ in waste acid using FeSx/Fe₂TiO₅: Full utilization of active S resource and centralized control of Hg pollution. Chem. Eng. J.; 2024; 494, 152959. [DOI: https://dx.doi.org/10.1016/j.cej.2024.152959]

90. Wang, L.; Xu, J.; Wang, Y.; Cheng, P.; Liu, G. Hg distribution and risk assessment in soil–Bozhou peony system. Environ. Earth Sci.; 2024; 83, 154. [DOI: https://dx.doi.org/10.1007/s12665-024-11492-7]

91. Li, C.; Enrico, M.; Bishop, K.; Roberts, S.J.; Hodgson, D.A.; Lamentowicz, M.; Mauquoy, D.; Mestrot, A.; Grosjean, M. Perspectives on using peat records to reconstruct past atmospheric Hg levels. J. Hazard. Mater.; 2024; 482, 136581. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2024.136581]

92. Phothitontimongkol, T.; Siebers, N.; Sukpirom, N.; Unob, F. Preparation and characterization of novel organo-clay minerals for Hg(II) ions adsorption from aqueous solution. Appl. Clay Sci.; 2008; 43, pp. 343-349. [DOI: https://dx.doi.org/10.1016/j.clay.2008.09.016]

93. Pei, P.; Sun, T.; Xu, Y.; Sun, Y. Using thiol-functionalized montmorillonites for chemically immobilizing Hg in contaminated water and soil: A comparative study of intercalation and grafting functionalization. Appl. Clay Sci.; 2021; 216, 106381. [DOI: https://dx.doi.org/10.1016/j.clay.2021.106381]

94. Wang, Y.; He, T.; Yin, D.; Han, Y.; Zhou, X.; Zhang, G.; Tian, X. Modified clay mineral: A method for the remediation of the mercury-polluted paddy soil. Ecotoxicol. Environ. Saf.; 2020; 204, 111121. [DOI: https://dx.doi.org/10.1016/j.ecoenv.2020.111121] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32798754]

95. Zhang, T.; Wang, W.; Zhao, Y.; Bai, H.; Wen, T.; Kang, S.; Song, G.; Song, S.; Komarneni, S. Removal of heavy metals and dyes by clay-based adsorbents: From natural clays to 1D and 2D nano-composites. Chem. Eng. J.; 2020; 420, 127574. [DOI: https://dx.doi.org/10.1016/j.cej.2020.127574]