Applicability of Clay/Organic Clay to Environmental Pollutants: Green Way—An Overview

: Natural clay mineral and its modifier called modified clay have been used in many envi ‐ ronmental applications for a number of years. However, they are not capable enough to achieve a higher conversion rate and so ‐ called ecological sustainability. This can be due to a lack of under ‐ standing of the selectivity of the clay and its modifier or a lack of compatibility between clay and pollutants. Recently, the development and implementation of green principles into practice have become an emerging field that brings together green chemistry and engineering practices to achieve a pollutant ‐ free environment (air, water, and soil). This review summarizes the role of clay/modified clay in pollution control and discusses the role of green chemistry in creating global sustainability. In this context, this review sheds light on the complete classification of the clay family to identify its properties and to critically examine the applicability of clay and modified clay for air, water, and soil pollution control over the past decade. This is the unique point of this review, showing how the properties of clay/modified clay can be useful for removing any type of pollutant without focusing on a single type of pollutant or clay. Furthermore, the importance of green materials in clay research, as well as the future area of application, was discussed. Overall, this review places value on multi ‐ disciplinary researchers to determine the role of the green pathway in the application of clay and modified clay in achieving environmental sustainability.


Background
Pollution is the contamination of the physical and biological components of the earth to an extent that poses a risk of harm to the environment and living beings. Factors such as globalization, industrial modernization, population growth, and technological advances in the use of natural resources beyond natureʹs capacity have resulted in three main types of pollution, namely, soil, water, and air [1,2]. In addition, noise, thermal, and radioactive pollution pose a major problem in the 21st century [1]. On average, 40% of deaths are due to air, water, and soil pollution [3]. It is known that these types of pollution have existed since the beginning of life. However, the significant environmental impacts, such as acid rain, ozone layer depletion, global warming, etc., began to increase after industrialization [4]. Pollution is not just a problem for industrialized countries, developing countries also play an equal role.
Soil pollution caused by contamination of the land via an industrial process and domestic sources can be classified into three categories, i.e., degradable or non-persistent pollutants; slowly degradable or persistent pollutants, and non-degradable pollutants [2,5]. The main pollutants that lead to soil pollution are hydrocarbons, solvents, and heavy metals [5]. Water pollution is caused by the release of waste into water, i.e., municipal waste, food processing waste, livestock pollutants, chemical waste from processing, insecticides, herbicides, organic compounds, and others [6]. Meanwhile, the main air pollutants are SOx (sulfur oxides), NOx (oxides of nitrogen), COx (carbon oxides), ozone, volatile organic compounds (VOCs), radioactive pollutants, and others [1]. The general effects of all pollutants on humans include irritation of the eyes and nose, skin infections, bronchitis, headaches, dizziness, cardiovascular problems, and sometimes cancer and premature death.
It is therefore important to control the amount of contaminants emitted from the source, either before treatment or after treatment. A number of methods, such as adsorption, absorption, coagulation, precipitation, advanced oxidation, filtration, evaporation, ion exchange, solvent extraction, ozonation, etc., have been used for the treatment of water pollutants [7]; meanwhile, adsorption, absorption, condensation, and combustion (incineration) are used to treat air pollutants [8]. While composting, adsorption, chemical conversion, and incineration are the most important soil-contaminant-removal techniques. The above techniques can be used individually or in combination with other abatement techniques. Among the possible pollutant removal techniques, adsorption by a solid adsorbent represents the most efficient and widely accepted technique due to its conversion capacity, availability, lower cost, ease of use, energy efficiency, and possibility of being used at a lower concentration [9].
In order to achieve efficient pollutant removal, careful selection of the adsorbent is of paramount importance. In most cases, activated carbon, silica, activated alumina, zeolites, clays and polymers, and resins are used as adsorbents to remove pollutants [10]. Compared to clay, natural resources are limited, the adsorption capacity is small, regeneration is difficult, and the cost is higher. Thus, in recent years, adsorbents based on clay minerals with unique properties, such as high surface area, easy availability, lack of toxicity, and the possibility of modification, have attracted much attention from researchers [11]. Figure 1 shows the number of publications (research articles) on the application/role of clay/organic clays in pollution control or pollution abatement in the last decade (from 2013 to August 2023). It can be seen that clay-based research for environmental applications is continuously growing exponentially, showing that the applicability of clay research has become a current hot topic. In addition, researchers from all over the world participate in this research area. Even in times of COVID-19, we can observe that the concern in this area is also increasing, which shows the great importance of this topic in recent times.

Recent Trend of Pollution Control
Despite today's development of science and technology, the pursuit of a pollutionfree world is still not enough; although, a large part of the gross domestic product (GDP) is spent on sustainability and the number of deaths from pollution is one in six deaths worldwide. Global Burden of Diseases data show that air pollution accounts for 6.7 million deaths and water pollution accounts for 1.4 million deaths while other pollution, such as lead poisoning as part of soil pollution, caused nearly a million deaths [12]. Thus, air, water, and soil are the three main areas of pollution that need to be controlled to ensure global sustainability. There are many reports stating that introducing a new lifestyle, such as switching to electric and natural gas cooking from legacy fossil fuels and traditional Liquefied Petroleum Gas (LPG)-based cooking, has reduced pollution; but, it is also a fact that more than half of the countries worldwide have had more deaths from outdoor air pollution than from indoor air and water pollution [12]. This situation has recently increased the pressure on countries' GDPs. This situation, and the death toll, will not change until the technology, which mainly focuses on pollution prevention and resource sustainability, comes into practice.

Role of Green Chemistry Approaches for Environmental Sustainability
Green chemistry, also known as sustainable chemistry, is an emerging field of chemistry that puts twelve principles into practice to create a pollution-free environment in air, water, and soil [13]. Basically, this approach consists of urging the world to prevent waste generation by improving the atom economy, using bio-based materials to avoid pollution, changing the synthesis route to minimize solvent consumption and the use of hazardous chemicals in synthesis, and preventing the formation of pollutants and by-products and derivatives [14]. In this way, these principles ensure resource sustainability, which is a very essential requirement for global sustainability. In addition, these principles ensure accident prevention, which is also a higher priority for human health.

Strategies to Implement Green Chemistry in Environmental Applications
After considering the seriousness of the problem, it is time to start thinking about natural adsorption materials that can be used for pollutant-removal applications; the recent trend shows that clays/organoclay-based materials show promise for environmental remediation. Basically, clay is a natural material that is less hazardous in nature and can be used in catalysis and pollution-prevention applications. Thus, the purpose of this article is to examine the role of clay in the field of pollution control without limiting the specific pollutants or clays. There are many clays used for many commercial and pollution control applications that have steadily increased over the last 10 years. However, they fail to catch on and achieve a wide range of removal rates and environmental credentials. Failure to achieve a high conversion rate may be due to a poor understanding of the properties and types of clay and their compatibility with modifiers. This problem can be solved by understanding the classification of clay and putting into practice the principle of green chemistry.
In the latest study, modified clay was most commonly used to remove water pollutants, such as dyes, organic pollutants, and heavy metals. However, this paper not only focuses on the adsorption of pollutants in organic clay water but also pays attention to the adsorption effect of pollutants in the soil and air. This is the unique selling point of this article, which shows the properties and classification of clay as beneficial for the adsorption of pollutants. In addition, this overview focuses on all clays and not just a single type.
The major goals of the paper are (1) to determine the properties of clay and the importance of modifications to improve its properties that are applicable to pollution control applications and (2) to conduct a comprehensive literature review to summarize some examples over the last ten years where these materials have been used to remove soil, water, and air pollutants. This comprehensive overview is a compilation of several publications discussing the use of original clay or modified clay for pollution control applications. In this regard, sufficient effort has been made to include the nature of the material, its sources, the name of the modifier, and its effectiveness based on publications over the past 10 years. This review is summarized using individual pollutant tables, such as air, water, and soil. Finally, (3) the barriers associated with clay materials and modifications are discussed and the role of green chemistry in overcoming these barriers and future recommendations are identified.

Clays and Organoclays
The word clay is used to denote a type of mineral or rock that is classified based on its structure and composition. There are basically 78 mineral classes, of which 27% are clay materials. Most minerals extracted from the rocks of the earthʹs crust are silicates, including clay [15]. The chemical composition of clay consists primarily of silica, alumina, iron, and structural water [16]. In order to improve the properties of clay, such as hydrophobicity, hydrophilicity, surface area, porosity, exchange capacity, and electrostatic attraction, clay surfaces need to be modified; a modified clay product is called organoclay [11,16]. To understand the differences between each clay and their properties, it is necessary to understand the classification of clays.

Classifications of Clays
The basic structure of minerals contains silica tetrahedra, a combination of four oxygen atoms and one silicon atom. This is the most basic structure, where the +4 charges of silicon are capped by the −2 × 4 charges of four oxygen atoms, giving a net charge of −4 [11]. Now these tetrahedrons are assembled into frameworks in simple to complex ways. Figure 2 shows the basic classification of silica clay, which is broken down into six distinct categories, namely, orthosilicate (single unit), disilicate (two units), ring silicate (closed unit), chain silicate, layered silicate (typically referred to as phyllosilicates), and framework silicates (i.e., quartz, feldspars, and zeolite). The clay material that we most commonly refer to as phyllosilicate, a layered structure, comprises silicon tetrahedrons (SiO4) 4− and aluminum octahedrons [Al(OH)6 3− ], which exist in various combinations [16,17]. To understand its combination, it can be divided into five groups based on the octahedron-tetrahedron ratio. The only octahedral structure without a tetrahedral structure is called octahedral clay, which is further subdivided into dioctahedral and trioctahedral clays (see Figure 2). The clay falls into this category and is referred to as gibbsite or brucite [18].
The second layered silicate category is based on the 1:1 ratio of tetrahedrons to octahedrons, represented as TO. These TO clays are also divided into dioctahedral and trioctahedral clays. The kaolinite clay group and the serpentine clay group fall into this category. Kaolinite, dickite, nacrite, and halloysite belong to the kaolin group, which has a lower cation exchange capacity (CEC) and surface area and is widely used in the cosmetics industry.
Meanwhile, the most important and most commonly used phyllosilicate is the 2:1 group, consisting of two tetrahedra and one octahedron (TOT) [18]. In this structure, an octahedral sheet is sandwiched between two tetrahedral sheets. This group of clays is divided into two groups, namely, non-interlayer charge and interlayer charge. Non-interlayer charges are further classified into dioctahedral (pyrophyllite clay) and trioctahedral (talc). The currently most applicable clay category is the TOT interlayer charge group. This TOT group is further divided into expandable clays and non-expandable clays [16]. TOTexpandable layers with an interlayer charge of <0.6 belong to the so-called smectite group, containing dioctahedral clays, such as beidellite, montmorillonite, and nontronite, and trioctahedral clays, such as hectorite, saponite, and sauconite. On the other hand, TOTexpandable clays with an interlayer charge of >0.6 are referred to as vermiculite clay. Now the TOT interlayer charge group clays with non-expandable layers are classified into three groups, with respect to their layer charge. The expandable TOT group with an interlayer charge of <0.5-0.75 is called the dioctahedral illite clay. Meanwhile, the expandable TOT group with an interlayer charge of 1 is called true mica, which is further classified into muscovite and paragonite and biotite and phlogopite on the basis of them being dioctahedral and trioctahedral. Finally, the third group of the expandable TOT group with an interlayer charge of 2 is called brittle mica, which is further classified into margarite and clintonite based on being dioctahedral and trioctahedral. The above-mentioned TOT groups of clays have a wide application due to their higher CEC and surface area properties.
The fourth category is 2:1:1, i.e., TOT-O-TOT [16,18]. Chlorite is the only clay belonging to this structural group. Finally, the fifth class of clays is referred to as inverted ribbons; it contains attapulgite and sepiolite clays. Based on the merits of the clays, they may now be suitable for application. Some clays are commonly used for landfills and the storage of toxic waste.

Types of Organic Modifiers
The organic molecules most commonly used for clay modifiers are surfactants, followed by silane, polymers, and other organic materials [19]. The modifier selection is based on the type of clay and the CEC of the clay. There are basically two possible sites for modifications in clay: first, the outer layer, where the chemical reaction for adsorption takes place, and second, the interlayer adsorption, where ion exchange takes place. Table  1 contains the list of commonly used modifiers for the production of organoclay [19]. The first modification of organic clay was started in 1988 with cetyltrimethylammonium bromide (CTAB) surfactants [20], which are now being developed with many polymer, pyridine, and silane materials. Not limited to cationic and anionic modifications, dimetric surfactants (gemini surfactants) are widely used for their physicochemical properties on clay surfaces. Basically, cationic clays are responsible for the cation exchange capacity; so, they have selectivity for cationic surfactants or cationic molecules where anionic clays have this with anionic surfactants and molecules. In addition to ion exchange, hydrogen bonding, π-π interactions, and van der Waals interactions are the proven efficient approaches to modifying clay [19]. Concerning selectivity, the factors of concern are the exchange capacity (loading), adsorption ratio, time, reusability, and environmental toxicity. In addition, the energy, environment, and economy are the most important factors affecting the industrial use of clay. This is the main reason that the synthesis of organoclay needs to be improved and further developed and that the incorporation of the principle of green chemistry into the synthetic approach will be of crucial importance for the coming period.

Preparation
There are several methods of making organoclay that can be used to modify a given clay to create a larger surface area for each particle in the clay. This first and most common method of producing organoclay is ion-exchange modification (cation/anion exchange) [19]. The ion-exchange method is used to improve the clay's ability to hold positively/negatively charged ions, such as heavy metal ions and phenols that are regularly found in wastewater. To prepare the clay by cation exchange, the clay must be dissolved and mixed with an amount of modifier equal to the CEC of the clay. The second modification method is the solid-state reaction method [21], in which two solid starting materials, the clay and the modifier, react directly with each other. A solid-state reaction is environmentally friendly because it does not require solvents in its manufacture and makes the process more amenable to large-scale industrialization. Since solids do not react with each other at room temperature, it is necessary to heat them to extreme temperatures ranging from around 1000 to 1500 °C. In this process, a volatile liquid, such as alcohol or acetone, is added to create a paste that eventually evaporates back into a solid. Another manufacturing method is the grafting of copolymers [19,21]. This happens when the polymer chains of the surfactants are applied to the surface of the clay. There are two ways to achieve this. First, the pre-polymerized chains can be grafted onto a surface when the chain has a mushroom-like structure, allowing chemicals penetrating through the surface to adhere to the spiral structure. On the other hand, grafting from a surface means that the surface chains are in a linear form, whereby the grafting density can be higher when grafting from a surface than when grafting onto a surface.

Properties of Clays
There is no country in the world without clay sources. However, they exhibit different porosity, electrical conductivity, CEC, and family type depending on the rock structure, elemental contribution, and impurities. Based on the clay family, they have several fundamental properties that are enhanced after modifying clay minerals with organic modifiers. These changes affect the chemical and physical properties of the clays, such as structural, thermal, colloidal, and surface charges.

Plasticity
Clay is famous for its plasticity property, where clay deforms its shape under limited force and retains its shape after the applied force is removed [22]. Essentially, the intensity of this property is based on the clay family. When it has a plate-like structure, it exhibits higher plasticity when water is supplied. The maximum water requirement to obtain plastic clay is called the plastic limit of clay. Once the plastic limit is exceeded, the clay becomes inherently sticky and the water content for that stage is called the liquid limit. Based on the number of properties, the applicability of the clay material can be decided.

CEC
The CEC of clay depends on the availability of exchangeable ions and their position in the clay structure [19]. In addition, the level of structural charge deficit determines the rate of exchange capacity. The CEC depends on the pH of the replaceable atmosphere. For instance, 2:1 clays have a higher cation exchange capacity than other geometric clays. In addition, Na + has a higher affinity for CEC than K + and Ca 2+ ; so, Na-rich clay has a higher medication rate in most cases.

Swelling Capacity
When the clay comes into contact with water, the interlayer starts to expand and the interlayer distance of the clay increases, which is called swelling. The reason for swelling is the hydration energy force associated with particle interactions, which depends on the granularity and surface activity of the clay. For instance, 2:1 clay, namely, montmorillonite, has a higher swelling capacity compared to 1:1 kaolinite and 2:1 mica [23].

Specific Surface Area
The specific surface area of clay depends on the particle size, shape, and purity of the clay. The fact is that with a decrease in particle size, the specific surface area increases. In addition, expanding clays, such as montmorillonite and vermiculite, exhibit greater surface areas than other clays. These surfaces are destroyed by the presence of the contaminants; so, an acid treatment can improve the surface by diluting the contaminants [24].

Surface Charge
Surface charge is the key property for functionalizing or modifying clay surfaces. The fact is that due to the structural tetrahedral and octahedral layers, clay minerals have a negative charge [11]. However, the charge of the clay varies depending on the interlayer ions available. Therefore, the intermediate layer of the clay is crucial to the clay's overall charge and this charge is strongly influenced by the pH of the clay. Therefore, the protonated clay has a positive charge and the basic clays have a negative charge.

Properties of the Modified Clay
The above-mentioned original properties of the clay change with the application of modifications to the clay, resulting in a change in the physical and chemical properties.
Research into organically modified layered clays has been going on for a decade. Some research into the intercalation of cations in clays began in the 1920s after Lawrence Bragg introduced the X-ray diffraction method in 1913, which allowed scientists to observe the atomic and molecular structure of clays and silicates. From then on, many scientists began to observe and experiment with the intercalation of different molecules in the interlayer spaces of different clays.

Basal Spacing (d-Space) of Clays
The properties of modified clay/organic clay depend largely on the structural properties of the composite and the interlayer environment. If an organic modifier is added to the clay and the clay has exchangeable ions, it will start exchanging in the intermediate layer. As a result of this exchange, the static charge on the two platelets changes and they swell, or shift, increasing the distance between the clay platelets [25]. This space is called the basal space. When a long-chain modifier is introduced for modification, the interlayer distance increases more due to the molecular size, thus resulting in a higher d-spacing compared to the original clay. Similarly, when a structure modifier is introduced as a modifier, the d-spacing increases based on their orientation.

Morphology of the Modified Clay
Since the d-spacing proved that the presence of a modifier affects the properties of clay, the surface properties of modified clay were also affected by the modification and can be analyzed using scanning electron microscopy and transmission electron microscopy. From the d-spacing results, it can be demonstrated that the presence of modifiers affects the structure of the clay, which in turn affects the geography of the clay by changing the packing density within the interlayer [19,25]. In general, removing the hardened slabs from the structure disrupts the layered structure of the clay. These changes enlarge the cavities on the clay surfaces. In addition, the presence of unreacted modifiers on the surfaces increases the roughness of the clay surfaces. This roughness and these changes are identified by scanning electron microscope (SEM) and transmission electron microscope (TEM) images.

Hydrophobicity/Hydrophilicity of the Organoclay
Different arrangements of organoclays are observed after modification, where organic modifiers can lie flat as a monolayer, bilayer, or multilayer. The clay colloid is now formed on the basis of the layer. When the organic modifier is firmly attached to the surfaces and blocks the interlayer, the water adsorption capacity decreases and the resulting organoclay behaves like hydrophobic clay. On the other hand, long molecules increase the d-spacing of the clay, leading to an increase in d-spacing and allowing more water to penetrate the organoclay, resulting in a hydrophilic nature [26]. The hydrophilic and hydrophobic nature of clay has been characterized by contact angle and wettability experiments. Organic modifiers with large structures and higher stearic activity are inherently hydrophobic while long-chain surfactants increase the hydrophilicity of clay molecules.

Surface Porosity and Volume
The surface area, the particle size, and the pore structure depend on the arrangement of the organic molecules in the interlayer distance. After modification, organoclay forms a microspore structure or a mesoporous structure. When the clay is modified at low pH or in a highly acidic state, the clay geography turns amorphous by destroying the layered structure; so, controlled modification is extremely important for modifications. When an organic modifier is incorporated into the clay molecules, the organic molecules are packed densely in the clay interlayer, reducing the specific surface area and increasing the pore volume of the modified clay [27]. Additionally, when the benzyl substitutes organoclay colloids due to the stearic hindrance of structural molecules, it occurs, which reduces surface area and increases particle size. These changes in surface properties also indicate the presence of modifiers on the surfaces.

Surface Charge (Zeta Potential)
Clay has an Al-OH and Si-OH structure at its structural end and is therefore negatively charged. However, the edges of the clay are very sensitive to protonation and, therefore, pH can affect the overall charge of the modified clay. As the suspension increases, the clay's net negative charge increases because structural hydroxide ions reduce the positive charge on the particle edge, converting it to a more negative charge [26]. From this, it can be concluded that organoclay has an amphoteric character. This surface charge plays an important role in identifying the presence of modifiers and also in determining the capacity of the adsorbent on the surfaces. This charge sensitivity is important for the selectivity of the modifier and also for the removal of pollutants. In addition to the above properties, thermal stability, colloidal stability, dispersion properties, etc., have been improved after clay modification.

Environmental Applications of Organoclays
Clay is the best candidate for green chemistry materials because it is a natural material that is readily available and offers an attractive and cost-effective pollutant treatment option. From the classification, it can be concluded that the smectite clay group has a larger surface area, higher adsorption capacity, and higher CEC compared to other clays. In the smectite clay group, montmorillonite is the clay best suited for removing inorganic and organic pollutants from the atmosphere [26,27]. Smectite can be used as a barrier in landfills to prevent leaching of pollutants. While natural kaolinite has a lower CEC, it is hardly suitable for environmental applications. From the above classification and identification of surface modifiers, a pair of organic clays have been identified that can be used for environmental applications. Various modifiers with different treatments increase the adsorption efficiency of natural minerals and improve the performance of pollutant removal. In this segment, environmental pollution is divided into solids, liquids, and gases and their elimination by clay or modified clay is emphasized.

Soil Pollution
A variety of organic and inorganic pollutants can enter the soil and water environment. In this part, we focus on toxic substances, such as heavy metals, and some solidphase organic pollutants, such as anthracene, which badly affect drinking water and plants. In most cases, heavy metals can accumulate in the plants and affect the ecosystem. Table 2 focuses on the applicability of clay and organoclay for removing solid contaminants. It can be seen from the table that limited clay was used to remove this type of contaminant, which is due to the weak bond strength between the contaminants and the clay. For the removal of heavy metal waste by clay and modified clay, Bhattacharyya and Sen Gupta [28] studied the phenomenon, where they found that montmorillonite clay and kaolinite clay are enriched with many polyoxycations, such as Zn 2+ , Si 4+ , Ti 4+ , Al 3+ , Fe 3+ , Cr 3+ , Ga 3+ , etc. A similar trend of works over the past 10 years has also been observed when montmorillonite clay is enriched with Mg, Fe2O3 nanoparticles, Fe 3+ , Iron, Ferrihydrite, Goethite, etc., that are used to remove phenol, Hg 2+ , Cr 3+ , Pb 2+ , Zn 2+ , As 5+ , Cd 2+ , Cr 6+ , Co 2+ , Cu 2+ , Ni 2+ , etc. Ma et al. [29] synthesized a Fe3O4-CuO-modified montmorillonite clay catalyst for the degradation of anthracene-contaminated soil. The task of the montmorillonite clay was to prevent the agglomeration and crystallization of Fe3O4-CuO. Using a clay catalyst (1 g/kg) with ClO2 as the oxidative degradation material (1 mol/kg), a 96.2% degradation in anthracene was reported; most importantly, this Fe3O4-CuO-modified montmorillonite clay catalyst was successfully reused for eight cycles.
Yang et al. [30] synthesized montmorillonite clay modified with tetramethylammonium (TMA) and hexadecyltrimethylammonium (HDTMA) for Cr(VI) immobilization capacity and found that TMA-modified clay has a larger surface area and pore volume compared to HDTMA-modified clay. A modified toxicity characteristic leaching (TCLP) method was used to assess Cr(VI) immobilization capacity and it was found that both modified clays may have a higher Cr(VI) stabilization capacity. However, HDTMA has a higher capacity than TMA.
Wang et al. [31] studied the grafting of thiol-functionalized onto organically modified montmorillonite used for Hg stabilization in soil contamination. The stabilization efficiency was over 90% after grafting the thiol group onto the modified clay. This graft increased the stability efficiency to 96.7%, which is 82.4% higher than regular organically modified clay.
Wang et al. [32] used humic acid to modify montmorillonite for Cd-and Hg-contaminated soils. In total, 5 wt% of humic acid-modified clay reduced the Cd and Hg concentrations in TCLP leachates by 94.1% and 93.0%, respectively. In the long-term immobilization, a quantitative accelerated aging method was performed and the reliability of both metals in the modified clay was found to be over 0.95.
Qin et al. [33] used magnesium-based montmorillonite for heavy metals immobilized in contaminated soil and found lower TCLP extractability for Cu, Pb, Zn, and Cu heavy metals in soil. According to their proposed mechanism, heavy metals are first induced mainly through electrostatic attraction, then precipitation, and finally chelation with water-soluble organic carbon.
Wu et al. [34] prepared FeSO4.7H2O-modified Ca-montmorillonite and applied it for pyrene removal and found a removal capacity of 843.9 μg g −1 . This higher removal rate efficiency was due to the formation of γ-Fe2O3 nanoparticles generated on the surface of the clay and at the edges of the clay. It was found that not only the efficiency but also the reusability is at five cycles. According to them, this γ-Fe2O3-modified montmorillonite is a green material for removing organic pollutants from soils and sediments.
Li et al. [35] prepared Fe(III)-modified montmorillonite for the oxidation of As(III) to As(IV) and the degradation of anthracene. They found that Na montmorillonite without Fe(II) has a conversion rate of 60% for As and <30% for anthracene, which would be 90% and almost 100% converted after using Fe-modified montmorillonite with 15 days of incubation.
Li et al. [36] used MnO2 loading on vermiculite-montmorillonite to remove Hg in contaminated soils. They found that adding 15 g/kg of modified vermiculite-montmorillonite reduced the Hg concentration in the soil to 98.2%. In addition, the plant height and biomass of Brassica chinensis L also increased. Similar Hg removal work by Liu et al. [37] involved modifying montmorillonite clay with Fe to control the release of Hg. After deployment, the root of Brassica pekinensis increased in Hg content but the leaf decreased in Hg content. This is because Fe-modified montmorillonite clay can adsorb Hg0 and decrease ionic Hg mobilization through surface adsorption, complexation, and in situ precipitation.
Huang et al. [38] studied montmorillonite clay modified with gemini cationic surfactant (butane-1,4-bis(dodecyl-dimethyl-ammonium bromide)) and tetrachloroferrate (FeCl4 − ) to study the retention performance of W and/or Cr in soil and found that W and Cr were immobilized in soil within 5 min. This is because modified clay along with FeCl4 − turns exchangeable W and Cr to reducible fractions.
Jiang et al. [39] studied the applicability of humus-like substances to modified montmorillonite for the remediation of Pb and Zn in contaminated soils. They found that the mobility, bioavailability, and leachability of Pb and Zn decreased significantly. They found that the TCLP of Pb and Zn decreased by 9.27-13.96% and 10.3-13.18%, respectively, after the introduction of modified clay.
Recently, Zhao et al. [40] investigated montmorillonite clay loaded with goethite to assess the simultaneous adsorption of Cd(II) and As(III) in the water and soil system. They reported removal rates of 50.61 mg/g and 57.58 mg/g for Cd(II) and As(III) after the modified clay introduction. In another recent study, Zhang et al. [41] researched the modification of montmorillonite by exopolysaccharides and requested the removal of Sr and Cs heavy metals. They found that the adsorption capacity of montmorillonite clay increased by 53.8 and 54.5%, reaching adsorptions of 256 mg/g and 90.9 mg/g for Cs and Sr, respectively.
Jiang et al. [42] synthesized amorphous ferrihydrite-modified montmorillonite clay for the removal of As, Sb, and Pb in soil. They found that the stability efficiency by TCLP reached 86.28% for Sb and 94.6% for Pb after 56 days. And, the applicability of modified clay improved soil properties, bacterial richness, and diversity in the soil.
From Table 2, it can be concluded that the montmorillonite and its modification forms dominate for this application due to their higher surface capacity and higher adsorption capacity. This adsorption phenomenon is mainly due to two phenomena in which the structural silanol and aluminol groups of the clay at the edge of the clay attract the heavy metals and trigger adsorption at the sites. On the other hand, functional modifier groups are introduced in a targeted manner to adsorb specific heavy metals. In particular, Hg 2+ can be effectively removed by organoclay when the modifying organic molecule contains the SH-functional group. These solid wastes are not only heavy wastes; more recently, radioactive wastes have also been the focus of attention and they have also been used successfully. In this case, however, it is the clay that prepares the blocks and not the adsorbent material. Figure 3 shows how the movement of heavy metals can be accumulated by clay. When multi-mineral components, such as clay, are introduced into the soil in agriculture, they accumulate heavy metals in their structural units and control the conversion of heavy metal pollutants into minerals and plants [43]. In this type of research, the role of hydrophilic clays with greater surface areas and higher binding sites is preferable to low-swelling clays, such as kaolin.

Water Pollution
According to Sustainable Development Goal 6, clean water and sanitation are priorities for everyone in the 21 st century. Over the past two decades, many researchers have worked on clay-based adsorption materials for removing contaminants from water systems. There are many reports which are regarding the applicability of clay for water and wastewater treatment. Srinivasan Rajani [44] published a report on clay and clay-based composites for removing organic, inorganic, and bio-based contaminants from drinking water. In that report, the author has reviewed natural clay and its application for heavy metal removals, such as Cd 2+ , Cr 3+ , Cu 2+ , Zn 2+ , Se 2+ , Ni 2+ , Fe 3+ , Pb 2+ , U 4+ , Co 2+ , etc. The author also considered its application for inorganic contaminants, fluoride and nitrates, and organic compounds, such as dichloroacetic acid, carbontrachloride, phenol, humidity and O-dichlorobenzene, algae removal, atrazine, sulfentrazone, imazaquin and alachlor, naphthalene and phenolic derivative, salicylic acid, carbamazepine, etc., and pathogens. Gu et al., 2019 [45], reported a review on clay minerals being used as an adsorption material to remove heavy metals from the water system. In this review, they focused specifically on the removal of water and wastewater contaminants. At the same time, Guegan Regis [46] published a report on the use of organoclay in the environment. Shen and Gao [47] published a review highlighting the applicability of gemini-surfactant-modified clay to combating water pollution. All of these investigations concluded that clay is suitable for heavy metal and organoclay contaminants. In addition, high surface area and CEC clays, such as montmorillonite clay, played a dominant role. In the 2020-2023 period, claybased reviews were published by Undabeytia et al. [48], Zhang et al. [49], Sultana et al., [50], Ayalew A. A [51], and Hnamte and Pulikkal [52]; they focused on the implementation of wastewater treatment technologies using clay as a composite with a polymeric material, a flocculating material, or other nanomaterials. So many works on this water pollution related to clay materials have been reported and reviewed. Therefore, in this segment, we have focused on the chronology of research over the last 10 years and placed the emphasis on pollutants and their efficiency. Table 3 focuses on the applicability of clay and organoclay for removing solid pollutants.  [57], and Ali [58] have published work on modifying montmorillonite/bentonite with quaternary ammonium modifier to remove dye and phenol molecules and found that long-chain modifiers, such as hexadecyltrimethylammonium bromide, have a higher removal tendency compared to short-chain quaternary ammonium modifiers. Although modified clay comes from different sources and has different CEC values and surface areas, the removal rate trend is higher in long-chain modifiers due to the hydrophobic behavior of clay.
Similar work was conducted in 2014 by Park et al. [59], where they used clay to remove heavy metals and phenol. In 2015 the applicability of various organic modifiers, such as Arquad 2HT-75 and palmitic acid from Iqbal and Khera [61], was investigated. Specifically, bis-N,N,N-hexadecyldimethyl-p-phenylenediammonium dibromide by Yang et al. [62], bis-imidazolium salts by Makhhoukhi et al. [64], and aminopropyltriethoxysilane by Marcal et al. [66] were combined with a quaternary ammonium modifier by Zhang et al. [60]. Anirudhan and Ramachandran [63] and Huang et al. [65] found that modified organoclay has a higher pollutant removal capacity than unmodified clay.
Yang et al. [76] studied the effect of different alkyl chain head groups on the removal of 2,4,6-trichlorophenol and found that a higher number of head groups has a higher removal capacity due to packing density and ion-dipole interactions. Ghavami et al. [78], and Bahmanpour et al. [80] addressed phenol removal by using a quaternary ammonium modifier. Kahraman [77] addressed the removal of heavy metals by modified chitosanbased clay. Similarly, Zhu et al. [79] studied the effect of quaternary ammonium modifier and chitosan hybrid on removing phenol, Pb 2+ heavy metals and removing conger red and crystal violet dyes and found that hydrophobic integration is the main factor for adsorption. Figure 4 shows the role of the binding sites in the adsorption of pollutants. In 2018, our publication [81] provided a detailed mechanism for phenol and nitrobenzene adsorption. In addition, the role of hydrophilic materials like phenol in adsorption and the role of hydrophobic materials like nitrobenzene in adsorption were studied with their behaviors. Long-chain modifiers have been found to have the affinity to increase d-spacing and provide more sites for adsorption. In Figure 4, it can be seen that phenol is at adsorption Site 1, i.e., the outer layer of clay, while Site 2 is for the intermediate layer and Site 3 for molecules. This was the phenomenon in which Langmuir monolayer adsorption was studied layer by layer. In the same year, the removal of heavy metals, phenols, fluorides, and dyes was studied by Almasri [82], Li et al. [83], Xu et al. [84], Dessalegne et al. [85], and Belbel et al. [86]. . Applicability of (mono-; bi-; tetra-tails and benzyl substituted) quaternary ammonium modified montmorillonite clay as a potential binding site provider for phenol removal in aqueous media [81], copyright 2018, Elsevier.
In 2019, Rahmani et al. [89] used magnetic nanoparticles along with surfactant for the modification of montmorillonite for the removal of methylene blue dye. In this year, Peng et al. [87] and Mahmoodi et al. [88] used the cation-exchange method for the removal of dye. Meanwhile, Iriel et al. [90], Kameda et al. [91], and Zhu et al. [92] had used Fe(NO3)3.9H2O, chitosan, and L-lysine for heavy metal removal. Meanwhile, in the same year, Huang et al. [93] and Seyedi [94] used gemini surfactant and the silane modifier modification of montmorillonite clay for ester removal and phenol removal, respectively. The removal of perfluoro-octanoic acid by hexadecyltrimethyl ammonium (HDTMA) and poly-4-vinylpyridine-co-styrene (PVPcos)-modified Na montmorillonite was reported by Chen et al. [95].
In 2020, Choi and Shin [96] and Luis Malvar et al. [97] modified montmorillonite with hexadecyltrimethylammonium and octadecylamine for phenol removal. Meanwhile, Song et al. [98] studied montmorillonite and biochar composite for heavy metal removal. Liu et al. [99] used Cetylpyridinium chloride as a modifier for heavy metal removal. At the same time, Van et al. [100] used starch and montmorillonite clay for Pb and Cd removal. Meanwhile, Haghigha and Mohammad Khan [101] used chlorosulfonic acid as a modifier to degrade trihalo methane. Dye removals through montmorillonite composite were performed in the same year by El-Kousy et al. [102], Bayram et al. [103], and Pormazar and Dalvand [104].
In 2021, Rahmani and Koohi [105] used cetyltrimethylammonium bromide for dye removal. The xanthan gum with poly(vinylimidazole) as a dye removal modifier was developed by Abu Elella et al. [106]. Another modifier, (3-mercaptopropyl)trimethoxysilane, was used in conjunction with montmorillonite for strong mental clearance and was described by Miao et al. [107]. Ali et al. [108] reported on dioxin removal, which was studied with oregano clay. L-methionine as a phenol removal modifier was studied by Imanipooe et al. [109]. Meanwhile, the removal of tungstate by an alkyne modifier was reported on by Xiao et al. [110]. As the most common pollutant of the 21st century, the micropollutant benzotriazole was identified by Zhang et al. [111]. In addition, Luo et al. [112] conducted a nitrophosphate removal study during this period.
In 2022, phenol removal [113], dye removal [114][115][116], and heavy metal removal [117][118][119][120] with modified organoclay were explored, with a dominance of montmorillonite observed. During this period, the method of introducing magnetic particles to remove dyes was observed. In the current year, 2023, Wei et al. [121], Xie et al. [122], and Hu et al. [123] reported on the removal of phenol by using modified clay. Wang et al. [124], Zeng et al. [125], and Nazarizadeh et al. [126] reported heavy metal removal by modified organoclay. The removal of dyes from polluted water has been reported by Dai et al. [127], Zhao et al. [128], and Ly et al. [129]; chlorine contamination in water being removed by Iranian bentonite was reported by Moradi et al. [130]. From the chronological data and the trend, it can be deduced that, in all cases, organoclay was predominantly used for the removal of aqueous pollutants.

Air Pollution
The adsorption of CO2 on amide-based surfaces has been very famous and, hence, the modification of clay or the grafting of clay by amide modifiers for the adsorption of gas pollutants has been the main focus of the last 10 years of research. Several studies have been published on the use of clay materials as an adsorption material for air pollutants, such as CO2, H2, NH3, VOC, etc. Table 4 summarizes the list of modifiers and their applications that are specific to gas adsorption. Nousir et al. [131] used Boltorn polyol dendrimers to modify montmorillonite to enhance the hydrophilic nature of montmorillonite for CO2 adsorption under dry and wet conditions. They found that the OH of clay has slightly less adsorption than OH-H2O. They also found that the dry-state desorption of CO2 gas is very easy and can be performed at 20-50 °C while wet-state gas desorption requires a higher temperature range, such as 60-70 °C, for desorption. The result obtained confirms the role of physical adsorption and the nature of physico-chemical interactions in wet conditions. Azzouz et al., [132] used three polyglycerol dendrimers with different molecular weights, like 500, 1100, and 1700, from soybean oil and modified Na montmorillonite clay to perform reversible CO2 capture. The CO2 retention capacity (CRC) was measured by temperature program desorption and revealed that the CO2 retention increased with the increasing OH group. They found that higher generation and higher loading of the clay surface with dendrimers markedly reduced the accessible sites for OH groups and decreased adsorption. Thus, montmorillonite with a 1700 modifier has a CRC of 3.88-7.14 compared to the 5.14 CRC with the montmorillonite with a 1100 modifier and the 11.7 CRC with the montmorillonite modified with a 500 molecular weight modifier. In the same year, Azzouz et al., [133] used a low molecular weight dendrimer, like ethylene glycol, and found that at a lower loading availability of -OH, the group is higher and the adsorption of CO2 improves.
Roth et al. [134] modified montmorillonite nano clay with polyethylenimine to provide sites for CO2 capture. The CO2 capture capacity was 7.5 wt% at atmospheric pressure and 17 wt% at 2.07 MPa pressure at 75-85 °C.
Elkhalifah et al. [135] modified bentonite clay with mono-, di-, and tri-ethanolamine to study the CO2 adsorption capacity of bentonite clay. The CO2 adsorption capacity was measured using a magnetic suspension balance. They found that the CO2 adsorption amount was increased to 3.15 mmol/g compared to 0.93 mmol/g for the untreated clay.
In a study conducted by Stevens et al. [136], diamine-modified montmorillonite clay was prepared by a water-based grafting method and applied for isothermal CO2 adsorption. They found that the maximum adsorption capacity of modified montmorillonite is 2.4 mmol/g.
Nousir et al. [137] used 3-aminopropyltriethodysilane to modify Na-bentonite in an ethanol-water mixture and an ethylene-glycol solvent mixture. They found that the ethanol-water mixture has a higher affinity for CO2 than ethylene-glycol solvent mixtures with a retention efficiency factor above 16 μmol/m 2 .
Shah et al. [138] used laponite, sericite cationic clay, and hydrotalcite clay as anionic clays to adsorb CO2 and found that cationic clay has the highest affinity for CO2 adsorption. In addition, higher adsorption at 0.017 g/g of CO2 was achieved by modifying it with dendrimers with cationic end groups.
In 2016, Alhwaige et al. [139] and Atilhan et al. [140] demonstrated the adsorption of CO2 on bio-based chitosan-polybenzokazine-modified clay-based nanocomposite and amine-impregnated porous montmorillonite nanoclay, respectively. The modified chitosan-based composite had a CO2 adsorption of 5.72 mmol/g at ambient conditions. In the study by Atilhan et al. [140], they found that unmodified montmorillonite adsorbed 3.34 mmol/g CO2 at room temperature and a bar pressure of 50 while modified clay under the same conditions adsorbed 3.47 mmol/g of CO2.
Nousir et al. [141] modified bentonite and montmorillonite clay with perhydroxylated glucodendrimenr for CO2 adsorption and CRC data revealed that CO2 adsorption involves physical adsorption with the OH of the dendrimer. In the same year, Shah et al. [27] conducted a study of the adsorption of CO2 and NH3 gas regarding cationic clay and anionic clay. The phenomenon obtained is shown in Figure 5. It can be seen that the NH2-terminated dendrimer has an affinity for adsorbed CO2 and the COO-terminated group has an affinity for NH3 adsorption. Furthermore, desorption experiments showed that the adsorbed CO2 is desorbed from the clay and is retained on the dendrimer since dendrimers have the affinity to keep the gas at room temperature due to their nanopores. Figure 5. Applicability of cationic and anionic dendrimer-modified cationic and anionic clays as a potential binding site provider for CO2 and NH3 gas adsorption [27], copyright 2017, Elsevier.
Pires et al. [142] investigated amino-acid-modified montmorillonite clay as a suitable material for CO2 adsorption and separation from other gases in large-scale processes. They found the adsorption value of CO2 to be 0.8 mmol/g at a temperature of 25 °C and a pressure of 8 bar. In addition, they achieved a selectivity values of 170 in CO2/CH4 separation under a bar pressure of 9.
In 2019, Nousir et al. [143] reported reversible CO2 capture with acid-activated bentonite achieved by the chemical grafting of 3-aminopropyltriethoxysilane or 3-diethanolaminopropyltriethoxysilane. They found that acid treatment increases the number of silanol groups and silylation capacity but compromises CRC capacity. This decay was revived by refining clay with a modifier. In the same year, Gomez-Pozuelo et al. [144] modified montmorillonite, bentonite, saponite, sepiolite, and palygorskite clays via three methods: (1) grafting with aminopropyl and diethylenetriamine organosilanes; (2) impregnation with polyethylenimine; and (3) dual functionalization by the impregnation of previously grafted samples for CO2 adsorption. They found that the CO2 uptake of a grafted and impregnated sample ranged from 61.3 to 67.1 mg/g.
Horri et al. [145] performed acid activation on montmorillonite for CO2 adsorption. They found that three hours of acid activation increased the surface area of the clay from 39 to 202 m 2 /g while the pore volume increased from 0.05 to 0.31 cm 3 /g, showing higher CO2 adsorption capacity.
In 2021, Penchah et al. [147] used nano-montmorillonite impregnated with diethanolamine for CO2 adsorption and found that it had a CO2 adsorption of 219.9 mg/g. At the same time, Khajeh and Ghaemi [146] modified montmorillonite clay with strontium hydroxide for CO2 adsorption and found that it had a CO2 adsorption of 102.21 mg/g at 25 °C and a bar pressure of 9.
Sun et al. [148] synthesized organo-montmorillonite by a simple dry ball-milling method with tetramethylammonium bromide as a modifier. This modified vessel was used to adsorb gaseous toluene. The dynamic adsorption of modified clay was found to result in 55.9 mg/g of toluene adsorption compared to 8.8 mg/g of toluene with unmodified clay. In the next year, Sun et al. [151] synthesized tetramethylammonium bromidemodified clay by ball milling for the application of gaseous acetone adsorption.
Ansari et al. [149] synthesized modified montmorillonite with choline chloride-urea as the eutectic solvent and used it for CO2 adsorption. They found that it had an adsorption of 252 mg/g of CO2 at 30 °C, a bar pressure of nine, and 50% of the solvent-modified clay. Meanwhile, Ghosh et al. [150] studied the roles of different clay minerals, such as montmorillonite, illite, and kaolinite, in the adsorption of gaseous hydrogen at a lower temperature with lower pressure and a higher pressure with a higher temperature. They found that the specific surface area and the micropore volume have a positive effect on hydrogen adsorption and a negative effect on the pore size. Zhu et al. [152] recently modified montmorillonite by a combined thermal and acid activation treatment to adsorb gaseous PbCl2.
From the above data, it can be concluded that organically modified clay and acidtreated clay have tremendous potential for air pollution control. According to the data, the role of the amide-terminated group was dominant for CO2 adsorption due to its physicochemical interactions. Meanwhile, acid treatment has been the most advantageous treatment for improving pore size and surface area for gas adsorption.

Concept of Green Chemistry in Organoclay
From the discussion so far, it is concluded that organoclay is highly efficient in removing contaminants from all types of environments. The recent trend shows that clay/organoclay-based materials show promise for pollution control. Green Chemistry Principle #1 and Principle #11 suggest the benefits of waste prevention and pollution control. If we use clay as an adsorbent, there is a tremendous opportunity to meet the requirements of the two principles above. In addition, clay is a less hazardous material as it is also used for many cosmetic products; so, Principle #3, less hazardous chemical synthesis, and Principle #8, reduced derivatives, are clearly justified. Then, the question arose about the use of modifiers. There are many reports where the modification of clay by biosurfactants and bio-based modifiers, like chitosan, has been observed. With the development of an environmentally friendly concept, the trend towards the applicability of this type of material will increase, which will support Principle #4, design safer chemicals, and Principle #10, design for degradation. With Principle #2, with atom economy in mind, by-product formation does not occur in most clay modifications and research continues to improve modification yields. Similarly, the energy efficiency principle (#6) is also justified by the clay modification as the conversion method and process is very simple and energy efficient compared to other modifiers. Clay is also applied as a catalyst (#9) for the degradation of many organic pollutants through solvent-free (#5) green routes. Not only limited to that, clay is also a good candidate for fire retardants to prevent accidents. In general, the role of the modifier can be determined depending on the application. In addition, recent advances in biodegradable modifiers have been developed to avoid the toxicity of surfactants [153]; the regeneration of the research area of modified surfactant-based clay supports the green path to the sustainable use of clay-based materials for environmental applications.

Current Research Status
The applicability of clay and modified clay is a very large field and has tremendous potential due to its low cost, large surface area, easy availability, and promising material. Tables 2-4 summarize the applicability of clay to combating air, water, and soil pollution using different types of clays, their modifiers, and their adsorption capacities. From summarizing more than 100 major publications of the literature over the past 10 years, it is clear that clay and organoclay are heavily used in pollution control applications, with bentonite or montmorillonite modified by cation exchange techniques for production and application being predominant among other clays and modifiers. Approximately 85% of the papers indicated the use of cation-exchange methods. As can be seen from Tables 1-3, when applying clay for the removal of heavy metals in soil, dyes, phenols, and heavy metals in water, as well as CO2 and CH4 in the air, the removal effect is good. From this review, it is concluded that natural clay and its components are suitable for pollution control. In addition, in most cases, it has been demonstrated that the applicability of clay and organoclay is better than or comparable to the existing commercially available adsorbents. In addition, it can be seen that the applicability of clay and organoclay to treating water pollution is higher than that of air and solid waste. However, recent developments with a trend toward these applications cannot be neglected.
This review also suggests the role of green chemistry in clay application and clay modification. It can be said that this release trend supports the applicability of natural clay to supporting the environment and playing the role of pollution prevention; in addition, the atom economy, less hazardous synthesis, the development of a safer catalyst, solventfree synthesis, energy efficiency, the use of renewable/natural materials and fewer production plays of derivatives, a natural catalyst, design material for decomposing pollutants, the prevention of environmental pollution, and the prevention of accidents due to simple synthesis is preserved.

Future Research Directions
However, there are several gaps that should be considered as potential research areas for the future, such as:  The applicability of clay and modified clay to improve efficiency in soil and air pollution removal. This gap is due to clay's ease of processing as a water absorber, which is difficult in the air and soil environment but still not negligible;  Further research is needed to use natural modifiers instead of surfactant molecules or to use biosurfactant as a modifier to aid in a less hazardous synthesis;  More urgent research is needed to use clay materials to remove microplastics as COVID-19 has increased the use of masks, which are potential water pollutants compared to other organic pollutants;  In addition, there is high potential for the applicability of clay and organoclay materials to remove and treat emerging contaminants;  It would also be interesting to determine the role of microbes in breaking down pollutants or converting them to less harmful forms in the presence of clay or organic clay media.
The search for the above question is very important because clay is a naturally occurring, low-grade, easily treated, and economically viable adsorbent material that has the potential to be used as a large-scale material for environmental applications. At the same time, green chemistry development is also a promising emerging field in which key natural modifiers and modified technologies can be identified to develop environmental entrepreneurs that have less toxic effects on the ecosystem.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.