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.

A variety of organic and inorganic pollutants can enter the soil and water environment. In this part, it is focused on toxic substances, such as heavy metals, and some solid-phase 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 1 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 Zn²⁺, Si⁴⁺, Ti⁴⁺, Al³⁺, Fe³⁺, Cr³⁺, Ga³⁺, etc. A similar trend of works over the past 10 years has also been observed when montmorillonite clay is enriched with Mg, Fe₂O₃ nanoparticles, Fe³⁺, Iron, Ferrihydrite, Goethite, etc., that are used to remove phenol, Hg²⁺, Cr³⁺, Pb²⁺, Zn²⁺, As⁵⁺, Cd²⁺, Cr⁶⁺, Co²⁺, Cu²⁺, Ni²⁺, 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 1, 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, Hg²⁺ 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.

According to Sustainable Development Goal 6, clean water and sanitation are priorities for everyone in the 21st 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 Cd²⁺, Cr³⁺, Cu²⁺, Zn²⁺, Se²⁺, Ni²⁺, Fe³⁺, Pb²⁺, U⁴⁺, Co²⁺, 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, clay-based 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.

Dos-Santos et al. [53], Nguyen et al. [54], Park et al. [55], Zheng et al. [56], Egbon et al. [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.

In 2016, a new trend of grafting clay with polymeric material was observed when Zhang et al. [72] and Huang et al. [73] applied organoclay for dye removal. Apart from quaternary ammine [69,70,71,75], HNO₃ acid treatment [67], humic acid, N-2-hydroxy-propyl trimethyl ammonium chloride chitosan [74], and zwetter-ionic surfactant, such as 1,10-didodecyl-4,40-trimethylene bispyridinium bromide and 1,10-dihexadecyl-4,40-trimethylene bispyridinium bromide [68], were applied for dye removal, phenol removal, and heavy metal removal.

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 chitosan-based clay. Similarly, Zhu et al. [79] studied the effect of quaternary ammonium modifier and chitosan hybrid on removing phenol, Pb²⁺ heavy metals and removing conger red and crystal violet dyes and found that hydrophobic integration is the main factor for adsorption.

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.

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.

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/m².

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. 

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 m²/g while the pore volume increased from 0.05 to 0.31 cm³/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 bromide-modified 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 acid-treated 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.