Nowadays, a wide variety of clay modification approaches have been used to improve CO₂ sorption capacity. In addition, various modification combinations to extend the application range of these materials have been approached. The modification typically is carried out using either raw clay material or activated clay material. The textural and chemical properties of clay materials can be changed essentially by clay activation. The most commonly used clay activation methods are mechanochemical activation, intercalation, thermochemical activation, and chemical activation. All these approaches have high potential to promote the generation of new clay pore structures and formation of new or improved active sites on the surface of clay materials ^[1].

The most common method for clay activation is the activation by alkaline or acid treatment (chemical activation). For example, Wang et al. (2013) proposed to improve kaolinite and montmorillonite textural properties through acid or alkaline treatment, and it was found that the acid treatment affected the clay structure to a higher extent than the alkaline treatment ^[1]. In general, the acid activation is a chemical treatment to alter clay textural (surface area, pore volume) and chemical properties (catalytic, adsorptive etc.). In addition, the acid activation also includes clay leaching with inorganic acids, causing disaggregation of clay particles, elimination of mineral impurities and dissolution of external layers ^[1]. Additionally, Horri et al. (2019) have studied the impact of acid treatment time (3, 8, and 24 h) using bentonite and 3 M HCl, and it was found that the content of Al decreased significantly with increasing treatment time, while the percentage of Fe and Mg oxide significantly decreased only after 24 h. Moreover, the specific surface area of 3 h treated bentonite was 227 m² g⁻¹, which was significantly higher than that for raw bentonite (21 m² g⁻¹). It is believed that the acid treatment leads to an increase in the specific surface area and pore volume because of the pore opening that appears due to the octahedral sheet degradation. However, longer acid treatment times reduce the specific surface area, as in the study performed by Horri et al. (2019). After 8 h, the specific surface area reduced to 223 m² g⁻¹ and 177 m² g⁻¹ after 24 h, respectively ^[2]. Even if there is no clear proportionality between sorbed CO₂ amount and the acid treatment time, it can be observed that the acid treatment significantly increases the sorption capacity of the material when compared to a raw bentonite ^[2]. At the same time, it is believed that the acid concentration plays an important role in the activation process. For example, Wang et al. (2013) reported that when using either 3 M, 6 M, or 9 M H₂SO₄ for the activation of bentonite, the use of 6 M H₂SO₄ gave the most optimal results, while more concentrated acid solutions led to pore collapse, which ended up with a smaller surface area and pore volume ^[1].

The efficiency of acid treatment can be optimized using such an approach as the microwave-assisted radiation, because it is possible to increase the specific surface area of the clay minerals using shorter treatment time and more diluted acid solution ^[3][1]. For instance, Cecilia et al. (2018) confirmed that the microwave-assisted acid treatment for sepiolite and palygorskite has resulted in an increased surface area and pore volume of clay minerals ^[4]. Similar results were also obtained by Franco et al. (2020) when studying microwave-assisted acid treatment of kerolitic clay, where after the acid treatment the surface area varied from 250 to 435 cm² g⁻¹ and the CO₂ adsorption increased to a greater or lesser extent, depending on a particular clay material that was used ^[5]. In addition, Cecilia et al. (2018) reported that the acid treatment reduced the CO₂ adsorption capacity for sepiolite but increased it for palygorskite. These findings coincide with other authors’ conclusions that the treatment with acid is causing the loss of cations in the interlayer distance and cations in the octahedral sheet ^[3][4]. It is worth noting that even though the acid treatment is increasing clay textural properties (e.g., pore size, pore volume); however, this process is not always beneficial to improve the CO₂ adsorption capacity. For instance, Irani et al. (2015) have studied sepiolite treatment with 2 M HCl and described the obtained material using energy dispersive and infrared spectra, which have shown that during the acid treatment Mg²⁺ ions were completely extracted from the sepiolite crystalline lattice, whereas Si-OH had formed in the structure of treated sepiolite. The surface area for raw sepiolite used for their experiments was 103.7 m² g⁻¹, but after the acid treatment it significantly increased and reached 272.4 m² g^{−1 [6]}. In yet another study, Cecilia et al. (2018) have reported that the microwave-assisted acid treatment had increased the surface area from 182 m² g⁻¹ to 326 m² g⁻¹, while CO₂ adsorption reduced from 65 mg g⁻¹ to 41 mg g⁻¹ at 25 °C and 1 bar. The possible reason for this is related to the partial collapse of the octahedral layer, which may cause an increase in the size of the nanocavities and thus resemble something similar to molecular sieve that retains CO₂ molecules due to a weaker physical interaction between CO₂ molecules and the wider nanochannels of the acid treated clays. Other possible reason can also be related to loss of cations in the nanocavities that interact with CO₂ molecules ^[4].

A notable approach for clay modification is the creation of pillared interlayered clays (PILCs). It is a class of two-dimensional microporous and synthetic materials, and due to the high surface area and permanent porosity of PILCs, these are very attractive solids for CO₂ capture. In this regard, smectite group is the most preferred for modification. Minerals from the smectite group are layered clays and the charge is balanced between their layers by interlayer cations, such as Na⁺ or K⁺. Exchanging these cations for other inorganic species allows to construct PILCs. The exchange is often performed for large oligomeric polycations, such as Zr, Al, Fe, Cr, and various other oligomers ^[7][8].

Besides the activation, there are several other appropriate methods for clay modification, among which the functionalization by grafting and/or the impregnation (immobilization) with amine containing polymers are showing considerably high efficiency. For instance, the functionalization of clay minerals with amine groups improves the CO₂ adsorption capacity due to the synergetic effect of the molecular sieve of the adsorbent and the chemical interaction between the amine species and CO₂ molecules ^[9][10]. The functionalization is performed by using the grafting and impregnation of amine-rich polymers. The grafting is a chemical process, where silanol groups located on the surface of the adsorbent react with amino alkoxysilane molecules to form a material with available amine groups on its surface ^[11][12]. The impregnation of amine-rich polymers is stabilized by hydrogen bonds with the silanol groups of the adsorbent ^[13][14][15].

Clay minerals can be substantially modified by replacing the natural inorganic interlayer cations with selected organic cations ^[16]. An example of organic compounds used for intercalation are amines and amino acids ^[17][18]. Since amines can react selectively with CO₂, various attempts have been made to incorporate amines into porous materials. These materials are chemically treated with amine containing compounds, thus obtaining new sorbents. When a layer of such a sorbent passes through a mixture of gases containing CO₂, the immobilized amine groups react with CO₂ to form carbamates, resulting in the capture of CO₂. Amine-containing sorbents are receiving increasing attention due to such advantages as low energy consumption, high CO₂ capacity, high resistance against contaminants, and high stability. Furthermore, recent research indicates that amine efficiency increases with increasing amine loading onto the sorbent that consequently provides much stronger CO₂ binding to a high density of amine loading. It is also important to underline that the adsorption efficiency is affected by the reaction conditions, either dry or humid conditions ^[19]. One of the proposed mechanisms is that under humid conditions water or hydroxide ions can act as a base and the amine efficiencies can approach the unity, while another proposed mechanism explains that under humid conditions ammonium bicarbonate species are formed ^[3][19]. In turn, under dry conditions a second amine acts as a base to produce an ammonium carbamate, giving a theoretical maximum efficiency of 0.5. Thus, obtaining materials with high density of amines it is possible to improve the efficiency of CO₂ capture ^[3][2].

The functionalization of porous materials with amine species (primary or secondary) improves the chemical interactions between the given adsorbent and the CO₂ molecules via zwitterion intermediate to form ammonium carbamate species ^[20][3]. The carbamate is formed by a nucleophilic reaction between the lone electron pair on the nitrogen of the amine and CO₂, during the anhydrous adsorption process. This process results in the formation of a zwitterion (equal number of positively and negatively charged functional groups), which is deprotonated in the presence of a base ^[2][19][21]. The tertiary amines interact with the CO₂ and produce bicarbonate ^[3]. Three main clay mineral functionalization methods involve:

• Impregnation of amines onto porous carriers;
• Formation of covalent bonds between amine containing functional groups and porous surface (grafting);
• In-situ polymerization.

• Impregnation of amines onto porous carriers;
• Formation of covalent bonds between amine containing functional groups and porous surface (grafting);
• In-situ polymerization.

Wet impregnation can be used to immobilize amines into pores if the solid support material is porous. Additionally, the amine compounds can be covalently bound to solid support material via hydroxyl functionality. Another approach is a direct polymerization of amine containing polymers on the surface of solid support ^[22]. In this regard, solid support materials with a high specific surface area and a porous structure are commonly used.

Impregnation of porous adsorbent with amine containing polymer is a widely used method to disperse a high amount of amine species on the surface of the adsorbent. Yuan et al. (2018) suggest that wet impregnation facilitates much higher loading of accessible amine molecules than is achievable by grafting or co-condensation, and this is resulting in an enhanced CO₂ adsorption capacity ^[23]. The most often used wet impregnation reagents are polyethyleneimine (PEI), tetraethylenepentamine (TEPA), triethylenetetramine (TETA), diethanolamine (DEA), diisopropanolamine, triethanolamine, and diethylenetriamine (DETA) ^[2][24]. In addition, Chen and Lu (2014) have studied kaolinite wet impregnation using monoethanolamine (MEA), ethylenediamine (EDA), and a mixture of both (4MEA+1EDA) and found that the latter shows the highest CO₂ sorption capacity ^[25]. Furthermore, Atilhan et al. (2016) have modified montmorillonite nanoclays with various amino groups and revealed that the impregnation of montmorillonite nanoclays with a single primary amine (octadecylamine) is more effective than the impregnation with tertiary amine (dimethyl dialkyl) or modification with doubling primary amines (octadecylamine) ^[26]. It is believed that the modification with primary amines is enhancing the hydrophilic character of nanoclays and thus is also affecting the amount of sorption sites. In similar studies, Ouyang et al. (2018) studied the impregnation possibilities of the acid treated sepiolite using five different amines, from which ethylenediamine was the only containing terminal –NH₂ groups, while TETA, TEPA, and PEI contained both terminal (-NH₂) and middle (-NH-) radicals and the length of their chain increased gradually until up to polymers with random networks ^[27]. The highest CO₂ adsorption capacity (up to 3.7 mmol g⁻¹ at 75 °C, 60 mL min⁻¹ CO₂, and 40 mL min⁻¹ N₂ mixture) was obtained using the 50% TEPA-loaded modified sepiolite matrix, when compared to other materials using the same concentration of amines. The authors revealed that the sorption capacity for materials containing other amines were 2.48 mmol g⁻¹ for PEI, 1.99 for TETA, and 1.68 mmol g⁻¹ for ethylenediamine, respectively ^[27].

Several authors have explored the impregnation with amine containing polymers at different loadings ^{[28][23][24][25][2829][2924][30]}. From their research results, it can be concluded that the CO₂ adsorption capacity increases with an increase in amine loading, and this is due to more active amine sites in the surface layer of the sorbent; however, it decreases if there is a blockage in the mesopores. For instance, Wang et al. (2014) have studied various PEI loadings on the acid-treated montmorillonite and found that the most optimal PEI loading amount was ≤50 wt%, which reached the highest CO₂ sorption capacity (112 mg g⁻¹) at 75 °C under dry conditions, while further enhancement up to 142 mg g⁻¹ was observed with the addition of moisture ^[28]. The authors have found that the CO₂ sorption capacity decreased when PEI loadings were further increased, which was possibly due to fast decrease of the accessible amine sites, possibly due to the agglomeration of the particle. Similar results were also obtained by Niu et al. (2016), who used halloysite nanotubes that were pre-treated to produce mesoporous silica nanotubes, which were further impregnated with PEI, where PEI loadings varied from 30 to 60 wt%. It was found that 50 wt% loading of the PEI was the most suitable and the sorbed CO₂ amount reached 2.75 mmol g⁻¹ at 85 °C in 2 h ^[29]. Furthermore, Ouyang et al. (2018) reported that MgO-SiO₂ nanofibers from sepiolite loaded with 50 wt% of PEI reached higher CO₂ sorption capacity (2.48 mmol g⁻¹ at 75 °C) than using 30 wt% to 60 wt% PEI loadings ^[30]. Analogous results were also obtained by Zhang et al. (2020) while studying CO₂ adsorption on an exfoliated vermiculite-TEPA composite, where TEPA content in the composite (0.2 to 50 wt%) strongly affected the textural properties of the composite and sequentially also the sorbed amount of CO₂, indicating that the optimal TEPA loading was 2 wt% ^[24]. Meanwhile, Wang et al. (2013) explored the wet impregnation of bentonite using varied loadings of TEPA ^[1]. The highest adsorption capacity was obtained for the material modified with 6M H₂SO₄ and impregnated with 50 wt% of TEPA at 75 °C, while loading higher than 50 wt% had negative impact on the adsorption performance. Irani et al. (2015) have studied the effect of TEPA loading on the acid treated sepiolite and found that the increase of TEPA loading from 30 to 60 wt% increased CO₂ capacity from 1 to 3.8 mmol g⁻¹. At the same time, further increase of TEPA loading from 60 to 70 wt% decreased CO₂ capacity significantly ^[6]. Liu et al. (2018) have explored CO₂ sorption onto purified, acid-treated sepiolite impregnated with various amounts of DETA and noticed that sepiolite loaded with 0 to 0.2 DETA has lower CO₂ capacity in comparison to acid-treated sepiolite, which was possibly due to the reduction of physisorption because of the blockage of pores with amine species ^[31]. Further increase of DETA loadings (up to 1) led to reduction of adsorbed CO₂ and this finding coincides with previously mentioned results obtained by other authors using PEI and TEPA loadings. Yuan et al. (2018) have studied CO₂ adsorption (50 °C in a pure CO₂ atmosphere with a flow rate of 100 mL min⁻¹) using TETA loaded acid treated sepiolite, where TETA loading varied from 10% to 60% ^[23]. It was found that loading with TETA significantly increased sorption capacity of sepiolite. Moreover, TETA loading increased in sepiolite from 10% to 30% and the resulted increase of sorbed amount of CO₂ was from 1.28 to 1.93 mmol g⁻¹, accordingly. Further increase of TETA loading in sepiolite (30% to 60%), reduced sorbed amount of CO₂ from 1.93 mmol g⁻¹ to 0.74 mmol g⁻¹.

Another functionalization method, grafting, involves a chemical reaction between the available silanol groups of the surface of the adsorbent and amine alkoxysilane compound to obtain hybrid adsorbents with high thermal stability, high water tolerance, and high selectivity towards CO₂ ^[2].

The amine-based materials obtained by the interaction, in which the amine species are incorporated in the surface of the adsorbent, are characterized by high thermochemical stability. Amino-containing organosilanes containing moieties such as amino-propyl (AP), ethylene-diamine (ED), diethylene-triamine (DT), 3-aminopropyl-trimethoxysilane (APTES), N1-(3-trimethoxysilylpropyl) diethylenetriamine (TMSPDEA), tetraethylenepentamine (TEPA), and N-2-aminoethyl-3-aminopropyltrimethoxysilane (AEAPTS) are often used as the grafted compounds.

Grafting involves interaction between amine species and CO₂ molecules via zwitterion mechanism that results in the formation of carbamates ^[2]. A number of authors have researched the sorption of CO₂ on grafted clay minerals. A variety of clays and clay-containing materials have been used, such as bentonite, saponite, palygorskite, montmorillonite, sepiolite, and kaolinite. Stevens et al. (2013) have studied the introduction of amine groups on the surface of montmorillonite or hexadecyltrimethylammonium bromide-intercalated montmorillonite (MMT CTAB N2) using a water-aided exfoliation method ^[32]. Within this approach, the desired amount of material was inserted in toluene and the reaction mixture was then sonicated in an ultrasound bath for 4.5 h at room temperature. The authors explained that the sonication was used to delaminate larger particles and thus to significantly increase the specific surface area and consequently to reach much higher amine grafting rate and less pore blockage. Afterwards, AEAPTS was added to the reaction mixture and sonicated for another 24 h at 60 °C. Samples were then filtered and dried at 80 °C overnight. The authors revealed that the highest CO₂ adsorption capacity (2.4 mmol g⁻¹ at 100 °C) was reached using initially intercalated and then grafted montmorillonite (MMT CTAB N2), and they explained that chemosorption is the predominating process there. Furthermore, such sorbent showed rather good stability in pure CO₂, while its CO₂ sorption ability was significantly reduced in the presence of SO₂ ^[32]. Simultaneously, Gomez-Pozuelo et al. (2019) reported that the adsorption capacity of CO₂ for bentonite, montmorillonite, saponite, palygorskite, and sepiolite functionalized by grafting with the (3-aminopropyl)-trimethoxysilane and N1-(3-trimethoxysilylpropyl) diethylenetriamine ranged from 32 mg g⁻¹ to 61 mg g⁻¹ at 45 °C under 1 bar ^[33]. In addition, Horri et al. (2019) have studied CO₂ adsorption abilities on APTES and diethylenetriamine-trimethoxysilane (DT) grafted bentonite treated with 3M HCl for 3, 8, and 24 h before the functionalization ^[2]. The results have shown that grafting with APTES increased the sorption capacity of the material, while the use of DT reduced the overall adsorption capacity. Different sorption capacities of the materials after their functionalization with either APTES or DT can be explained by the longer DT chain length, which may result in the pore blockage and thus a significant part of the amino groups may not be accessible to CO₂ molecules. The authors explained that the material may have been saturated with DT. The solution to overcome this issue could be grafting with DT using another support material with a larger specific surface area and pore volume, avoiding the saturation of the material ^[2].

Vilarrasa-Garcia et al. (2017) have studied the incorporation of amine species on bentonite and porous clay heterostructures (PCH) using grafting by APTES or impregnating by PEI or TEPA ^[34]. The authors revealed that PCH is the most appropriate material for the functionalization. In addition, CO₂ sorption on grafted and impregnated materials occurred via chemical and physical interaction, due to co-existence of these dual sites ^[34]. Furthermore, functionalization with APTES indicated the contribution of both sites, while functionalization with PEI and TEPA favoured the chemical contribution, especially as the polymer content in the material increased. As the polymer content in the material increased further, CO₂ adsorption capacity decreased; this is due to stacking of the amine-rich polymer. Similar results were also reported by Cecilia et al. (2018) by showing that the grafting with APTES or impregnation with PEI is decreasing CO₂ adsorption, while it increases the chemisorbed amount of CO₂ ^[4].

Several authors have offered to use the aminosilane-modified clay nanotubes to entrap CO₂ ^[35][19][35]. Following this approach, three different aminosilanes—(3-aminopropyl)triethoxysilane, N-[3-(trimethoxysilyl)propyl]-ethylenediamine, and N1-(3-trimethoxysilylpropyl)diethylene-triamine have been used ^[19]. The highest CO₂ adsorption capacity was reached in halloysite nanotubes modified with N1-(3-trimethoxysilylpropyl)-diethylenetriamine; while CO₂ adsorption capacity of halloysite nanotubes without amine loading was extremely low (almost non-existent) even at the same experimental conditions ^[19]. In addition, Jana et al. (2015) have studied the isotopic selectivity of CO₂ adsorption on amine grafted halloysite nanotubes, studying major abundant isotopes of CO₂ ^[21]. This experiment was carried out using an optical cavity-enhanced integrated cavity output spectroscopy. Results suggest that amine-grafted halloysite nanotubes can be regenerated at relatively low temperatures, and thus recycled repeatedly to capture atmospheric CO₂ ^[21]. Cecilia et al. (2018) reported that the acid treatment improves the functionalization by grafting due to a slight increase of the pore size resulting from partial or total pore blockage of the nanochannels of the fibrous clay materials ^[4].

It is suggested to incorporate amine species into the clay minerals by a double functionalization method. This approach involves grafting and impregnation procedures consequently on the same material ^[4][33]. The limited surface groups or adsorption sites may restrict the number of grafted amine groups, whereas the combination of chemical grafting and physical impregnation can incorporate unlimited amine groups onto the carrier. Gomez-Pozuelo et al. (2019) have noticed that as a result of organic functionalization of clays, the texture properties of grafted and impregnated clays decrease ^[33]. The authors presented the decrease of the surface area from 137 m² g⁻¹ for unmodified palygorskite to 55 m² g⁻¹ for grafted clay samples and 42 m² g⁻¹ for impregnated clay materials. Similarly, the pore volume decreased from 0.32 cm³ g⁻¹ to 0.16 cm³ g⁻¹ and 0.11 cm³ g⁻¹ for grafted and impregnated materials, accordingly. The authors have noticed that amino-containing organosilanes that contain more amino groups also have higher CO₂ sorption capacity. Furthermore, PEI impregnation and double functionalization yielded higher organic loadings but lower amino efficiencies than grafted samples. At the same time, the double functionalization showed lower CO₂ adsorption than individual grafting or impregnation due to pore blocking by high organic loadings. Simultaneously, the sorption efficiency was higher in humid environment and grafted materials were more stable than PEI-impregnated materials after three adsorption/desorption cycles ^[33].

Cecilia et al. (2018) suggested an increase of available amine sites or higher proportion of primary amines obtained after grafting with APTES ^[4]. In their study, the CO₂ uptake of sepiolite after double functionalization reached 1.41 mmol g⁻¹ at 760 mmHg and 25 °C, while the raw palygorskite reached 1.04 mmol g⁻¹ under the same conditions. The double functionalization (APTES-PEI) led to the highest adsorption capacity due to the higher amount of available amine sites that favour the chemical interaction with CO₂ molecules ^[4]. Roth et al. (2013) have studied double functionalization of montmorillonite nanoclays using at first grafting with 3-aminopropyltrimethoxysilane (APTMS) and afterwards, the wet impregnation of PEI, to achieve 50% loading of PEI-treated clay. The authors reported that the double functionalization of montmorillonite nanoclays allowed to improve the sorption capacity of CO₂ up to 7.5% at 85 °C at the atmospheric pressure ^[22].

The affinity of untreated clays for CO₂ is enhanced by the intercalation of organic matter with compounds with basic properties, such as amine compounds, polyol, and amino dendrimers with hydroxyl and amino groups, respectively ^[36]. By intercalating clays with polyol dendrimers, it is possible to obtain organoclays, which recently have been used also for CO₂ capture ^[3][37]. Some authors suggest the intercalation of polyols-species in the interlayer spacing, thus it would be possible for CO₂ molecules to interact with weak base -OH sites. Some reports indicate that montmorillonite intercalated with polyols dendrimers shows slightly lower affinity to CO₂, while the regeneration is easier in comparison to raw support material. Thus, this indicates that weak basicity can promote the reverse capture of CO₂. It is suggested that an ideal adsorbent would be able to release the adsorbed gas upon slight heating or under vacuum ^[37]. Furthermore, Azzouz et al. (2013) have explored the role of -OH groups when preparing polyol-montmorillonite using different polyalchohols and found that the incorporation of polyalcohol molecules significantly enhanced the affinity of montmorillonite towards CO₂ and the OH groups of the incorporated polyalcohol were the main adsorption sites ^[37].

There were several attempts to insert a cationic amine-rich dendrimer in clay material, for example, inserting polyamidoamine (PAMAM) to improve CO₂ sorption capacity ^[3]. The organo-clays present are three adsorption sites; two of them are attributed to the clay layers (internal binding unit and external binding unit) and the third one is the adsorption site attributed to the availability of the dendrimer sites, which grows directly with the amount of intercalated-dendrimer. This site is the most determining since the CO₂ adsorption capacity increases directly with the amount of the intercalated dendrimer as the consequence of the strong affinity between the dendrimer and CO₂ molecules ^[3]. Stevens et al. (2013) have prepared hexadecyltrimethylammonium bromide (CTAB) intercalated montmorillonite and used it as a source material for further amine surface modification, as well as studied CO₂ adsorption process using it as adsorbent ^[32]. In this experiment, the highest CO₂ sorption capacity (0.19 mmol g⁻¹) was reached at 25 °C and the capacity was decreasing with increasing temperature, thus indicating that the predominant process is physisorption.

Pires et al. (2018) offered the amino acid-intercalated montmorillonite as an alternative sorbent material for CO₂ sorption. The main advantage of such a material is environmentally friendly synthesis that can be performed in water environment without using noxious solvents. It also has relatively lower raw material costs and therefore no expensive amine alkoxysilanes, instead using renewable non-toxic reactants, as well as easy preparation ^[38]. The authors concluded that amino acid intercalated montmorillonite promotes the sorption of CO₂ in comparison to raw montmorillonite. Furthermore, the number of amino groups per molecule that can interact with CO₂ had a major impact on the ability to capture the gas ^[38]. Simultaneously, Elkhalifah et al. (2015) offered amine-bentonite hybrid sorbent for CO₂ adsorption. The preparation of proposed sorbent involves two steps—preparation of magnesium form of bentonite and sorbent synthetization by intercalating monoethanolammonium cations in the interlayer space of the Mg-bentonite ^[36].

The sorption of CO₂, for instance, on amine-containing porous sorbents includes both chemosorption on amino groups and physical adsorption within pores. Therefore, the adsorption temperature has a significant effect on which of the sorption modes will be predominant should be accounted for when modifying clay minerals. Wang et al. (2014) have studied the impact of temperature and found that with increasing temperature, the sorption capacity of montmorillonite/PEI sorbent increases up to 75 °C but decreases from 75 to 85 °C. It is suggested that the CO₂ diffusion is the predominant at the temperature range from 30 to 75 °C, but above 75 °C the thermodynamics become dominant, and the equilibrium shifts to the desorption, resulting in the decrease of CO₂ sorption capacity ^[28][1]. Similar results were obtained by Chen et al. (2013) when studying CO₂ sorption on PEI-impregnated bentonite at the temperature interval 25 to 100 °C and found that 75 °C is the optimum temperature for CO₂ capture. The authors explained that the temperature increase promotes a flexibility of amine groups and thus increases the affinity of these sites to CO₂, at the same time, too high temperature may also cause the desorption of CO₂ ^[39]. Cecilia et al. (2018) confirm that CO₂ adsorption capacity improves with increasing temperature thanks to a rearrangement of the amine-rich polymer favouring the diffusion of the CO₂ molecules in the adsorbent, therefore enhancing the CO₂/N efficiency ^[4]. Ouyang et al. (2018) also reported that PEI-loaded MgO-SiO₂ nanofibers from sepiolite reached the highest adsorption capacity of CO₂ at 75 °C ^[27][30]. Similar conclusions were reached by Irani et al. (2015) in studying the CO₂ sorption capacity of TEPA-impregnated sepiolite in the temperature range of 25 to 70 °C. As the temperature increased from 25 to 60 °C, the sorption capacity increased, but it decreased with increase of temperature from 60 to 70 °C ^[6]. Yuan et al. (2018) found that the CO₂ adsorption capacity on TETA-impregnated sepiolite decreased with increasing temperature in the interval from 30 to 70 °C ^[23].

In-situ polymerization requires placing a monomer between clay mineral layers and the expanding and dispersing the mineral layers into the matrix by polymerization. This approach can significantly increase the surface area and consequently also improve the CO₂ adsorption capacity. At the same time, the exact properties of clay-polymer composite are largely affected by polymer-polymer and polymer-clay interaction, as well as clay aspect ratio, dispersion, and the alignment, all of which are highly variable characteristics among clay minerals ^[40][41][42].

The Baltic region, or Baltic Sea region, refers to countries in the general area surrounding the Baltic Sea. Countries with shorelines along the Baltic Sea include Denmark, Estonia, Latvia, Finland, Germany, Lithuania, Poland, Russia, and Sweden. In there, it needs tois review, authors focus on the Baltic States to explore the potential of sustainable and cost-effective local natural resource usage of the region, conjointly with the GHG emission reduction in these post-soviet countries. The Baltic States is a geopolitical term used to group three countries on the eastern coast of the Baltic Sea: Estonia, Latvia, and Lithuania. All three countries are members of EU and engage in the implementation of the European Green Deal.

The Baltic States are located on the Eastern European Craton, which is the core of the Baltica proto-plate. Three countries are located on the Baltic Shield of the Eastern European Craton. The Baltic Shield is the exposed Precambrian northwest segment of the East European Craton. The Baltic Shield contains the oldest rocks of the European continent with a lithospheric thickness of 250 to 300 km ^[43][44]. Generally, in the direction from the northeast to southwest throughout the Baltic States, the stratigraphical layers of the craton are monoclinally deepening from the north of terrestrial Estonia to Lithuania, where the craton reaches a depth of more than 2 km.

Under the Quaternary sediment bedrock surface in the Baltic States, deposits of Cambrian, Ordovician, Silurian, and Devonian strata (in Estonia); mainly Middle and Upper Devonian (in Latvia); in the south-eastern part of Latvia and Lithuanian territory also Carboniferous, Permian, Triassic, Jurassic, and Cretaceous (only in Lithuania) are represented ^[45].

Due to the geological history of the region, clay in the Baltic States is among the most common types of deposits. During the Quaternary deglaciation of the region, glaciolimnic sediments on the surface were deposited in the form of varves into waterbodies in front of glacier margins ^[46]. In addition, due to the wide distribution of clay deposits on the surface of Quaternary environment, large territories have undergone paludification and transformed into mires ^[46]. Simultaneously, in smaller quantities also Devonian, Jurassic, Triassic, and even Cambrian clays are present in the region.

Use of clays in the Baltic region has been known for thousands of years, for example, at about 6000 years before present inhabitants of Estonia learned to make earthenware from locally available clay deposits. As civilisation has evolved, so has the pattern of clay use. Locally available clays were used in the production of worldwide recognizable red bricks, which were used as building material for strongholds and churches and provide modern Estonia’s historical buildings and architectural monuments with distinct geological grandeur ^[46]. A major amount of these clays is of Cambrian origin. Lower Cambrian (Lontova formation) clays in north Estonia are represented at shallow depths and contain rare earth element deposits. Clays of Lontova formation can also be found in the outcrops of the Baltic Klint ^[47].

Clay deposits in Latvia generally are related to Devonian and Quaternary systems, although rare deposits of Triassic and Jurassic clays are also present here. The most recognizable clay in Latvia is Liepa (Lode) clay from the Devonian period, Saltiski clay from the Triassic period, and Apriki clay from Quaternary period. These clays are dominated by illite minerals with additional kaolinite minerals in varied proportions ^[48]. Devonian clay is distributed in the north and northeast Latvia and can be extracted from numerous mineral deposits, however, the only mineral deposit currently active for extraction is Liepa (Lode) clay deposit (Cesis municipally) ^[49]. Devonian clays contain a high percentage of illite minerals, with additional kaolinite and chlorite minerals. Triassic clay is rare and can be found only in few mineral deposits in the southeast Latvia. Triassic clays contain high percentage of montmorillonite minerals. Triassic clays have high adsorption capacity. A large capacity of Triassic clay is in Lithuania; however, they are industrially exploited. Jurassic clay is rare and can be found only in close proximity to Triassic clay deposits in the southeast Latvia. One of the major distinctive features of Jurassic clays is the presence of natural organic matter within these deposits ^[50]. At the same time, in Lithuania parts of Jurassic clay deposits are exposed to the surface (Papile region), while in Estonia these clays are not present. The origin of these black clays that are rich in organic matter is still under dispute; perhaps they were deposited in shallow lagoons where large rivers transported organic sediments during Mesozoic era ^[50]. Quaternary clays can be found in mineral deposits throughout Latvia and in less amount in Lithuania and Estonia. These are typical glaciolimnic clays.

It is important to emphasize that the exact amount of clay stocks in the Baltic region is not known. This is due to diverse levels of research work on clay deposits, which therefore cannot be mutually comparable. Moreover, the majority of clay deposits do not have their stocks recalculated to clay reserves. For instance, while the quantity of clays from Lontova formations (Lower Cambrian) are almost precisely known, the quantities of Quaternary glacial clays are often only generalized calculations that were performed during the USSR period. At the same time, the amount of clay stocks is not the key factor for the evaluation of clays for CO₂ capture. In turn, adsorption/absorption parameters of clays play the key role here.