The Prospects of Clay Minerals from the Baltic States for Industrial-Scale Carbon Capture: A Review

Abstract

Carbon capture is among the most sustainable strategies to limit carbon dioxide emissions, which account for a large share of human impact on climate change and ecosystem destruction. This growing threat calls for novel solutions to reduce emissions on an industrial level. Carbon capture by amorphous solids is among the most reasonable options as it requires less energy when compared to other techniques and has comparatively lower development and maintenance costs. In this respect, the method of carbon dioxide adsorption by solids can be used in the long-term and on an industrial scale. Furthermore, certain sorbents are reusable, which makes their use for carbon capture economically justified and acquisition of natural resources full and sustainable. Clay minerals, which are a universally available and versatile material, are amidst such sorbents. These materials are capable of interlayer and surface adsorption of carbon dioxide. In addition, their modification allows to improve carbon dioxide adsorption capabilities even more. The aim of the review is to discuss the prospective of the most widely available clay minerals in the Baltic States for large-scale carbon dioxide emission reduction and to suggest suitable approaches for clay modification to improve carbon dioxide adsorption capacity.

1. Introduction

Carbon dioxide (CO₂) is one of the major greenhouse gasses (GHGs), which, if released into the atmosphere, is reflecting infrared radiation back to the Earth’s surface and traps the radiant heat. Therefore, excessive CO₂ emissions are of global concern as they are amongst the main contributors to climate change. Furthermore, global trends of CO₂ emissions show annual growth, which is then naturally followed by an annual increase in the average temperature. Recent data indicate that in October 2021, the increase of CO₂ concentration was 2.46 ± 0.26 ppm y⁻¹, but according to the National Oceanic and Atmospheric Administration (NOAA) 2020 annual climate report, the temperature (land and ocean combined) has increased at an average rate of 0.08 °C per decade since 1980 [1,2]. The consequences thereof are gradual warming and drying of the climate that, among other threats, is causing major and devastating wildfires worldwide, which in fact release massive amounts of CO₂ to the atmosphere themselves and make carbon emissions into even a larger issue [3]. In reality, an ever-increasing CO₂ concentration in the atmosphere has a number of other secondary effects on the environment, such as changes in the hydrogeological cycle, increased occurrence of various extreme climate events, sea-level rise, species migration, harvest loses, increased occurrence of infectious diseases, and others [4,5,6,7,8].

The evidence suggests that the atmospheric CO₂ concentration has increased from the pre-industrialization level (1750) of 280 ppm to 413.3 ppm in 2021 [9,10]. Furthermore, the rate of CO₂ growth currently is rapidly speeding up and, according to the future predictions, CO₂ concentration will continue to increase and can reach 670 ppm by year 2100 if no action is taken as soon as possible [9]. Essentially, far-reaching measures are required to deal with this growing climate change emergency. It is crucial that forward-looking measures to reduce the impact of climate change caused by increasing CO₂ concentration include alternative, green, and sustainable energy sources. In addition, incentives to increase energy efficiency are required as indicated in recent political declarations of the European Union (EU), United Nations (UN), and other international organisations [11,12,13].

Given that the replacement of energy sources is a slow, gradual, and yet expensive process, measures to absorb industrially produced CO₂ emissions are amidst current primary tasks. At the same time, actions for energy source transition must also be implemented immediately, in parallel with emission reduction. The most sustainable strategies to limit these CO₂ emissions is carbon capture and storage (CCS) and, where it is possible, the subsequent CO₂ utilization [14].

The climate change mitigation plan includes carbon capture to address the emissions mostly coming from burning fossil fuel in the energy production and transport sectors, which is then followed by carbon storage, for example, in the geological environment or industrially designed media. In addition, where possible, the utilization of stored CO₂ as an ingredient or component for practical applications is also supported and advisable. In fact, the development of new approaches on how to use CO₂ as a source material for other substances, therefore transforming this waste product into a valuable resource for other products, is also among modern topical issues that requires immediate solution [15,16,17,18].

Depleted oil and gas reservoirs, or hydrocarbon reservoirs in general, are viewed as one of the top options for CCS technologies; this is because these structures are sealed in by thick layers of hard impervious rocks, which can prevent possible CO₂ leakage. In fact, long-term structural integrity of these sealing structures is a precondition and imperative for avoiding leakage from pools of hydrocarbons [19,20,21]. Therefore, alongside research of these reservoirs for potential CO₂ storage, much attention has also been paid to their sealing geological structures.

The impervious sedimentary rocks, which are isolating hydrocarbon reservoirs, often contain high percentages of various clay minerals, such as kaolinite, smectite, illite, and others, thus their potential in relation to CCS has also been evaluated worldwide. At the same time, it should be noted that finding suitable storage reservoirs or media is only half of the solution for the climate crisis. The other half of the solution includes sustainable applications and technology for carbon capture directly from the emission sources that could be used on an industrial scale. Therefore, this review article addresses possibilities for carbon capture by clay minerals and their prospective for use in applications and technologies for CO₂ emission reduction, including approaches for clay modification to improve the efficiency of use.

Clay minerals are hydrous aluminium (Al³⁺) phyllosilicates. Besides Al³⁺, they often contain variable amounts of Fe²⁺, Fe³⁺, alkali metals (Na⁺, K⁺), alkaline earths (Mg²⁺, Ca²⁺), and other cations. Clay minerals are common weathering products and one of the end results of the low-temperature hydrothermal alteration in rocks. Therefore, they are common in soil and in fine-grained sedimentary and metamorphic rocks [22]. In regard to hydrocarbon reservoir sealing structures, the most common constituents at depths up to 2–3 km and temperature up to 100 °C are smectite clay minerals, among which the most common smectite mineral is montmorillonite [23]. Beyond this depth range and temperature limit, smectites go through the diagenesis and transform into illite clay minerals [24]. Illite comparatively has lower cation-exchange capacity (CEC) than smectite and, in addition, it is also non-expanding clay, whereas smectite is expanding clay and thus can be considered as much more valuable for a variety of uses, including CO₂ adsorption. Moreover, given that most hydrocarbon reservoirs with any reasonable potential to store CO₂, due to pressure and temperature limitations, are within the depth of 2–3 km, smectite interaction with CO₂ has been of special scientific attention for decades [24,25,26,27,28]. In addition, smectite and other clay minerals have been viewed in regard to CO₂ capture not only as the part of sealing structures around hydrocarbon reservoirs, but also individually as materials with reasonable CO₂ adsorption capacity, which can be used in applications and technologies to trap CO₂ from the atmosphere on an industrial level [20,24,28]. Unfortunately, there are major drawbacks of CO₂ sequestration attempts, which reside in the low overall concentration in the main CO₂ emission sources. To overcome these drawbacks, scientists worldwide seek new and alternative technologies that among all also encompass a variety of clay mineral modification approaches [29]. In this context, the review summarizes the prospective of clay minerals for carbon capture on an industrial scale and suggests strategies for clay modification to improve CO₂ adsorption capacity.

Although this review focuses on clay mineral assemblages that are available in the Baltic States to explore the potential of sustainable and cost-effective local natural resource usage, the authors believe that the information provided is comprehensive and representative of clay applications on a global scale to reach the carbon capture aims.

2. Global and Regional Carbon Dioxide Emissions

Global CO₂ emissions are the primary driver of climate change. It is universally recognized that to avoid irreversible damage to the climate, the civilization immediately needs to reduce GHG emissions. If no action is taken as soon as possible, climate change will fundamentally reshape the society and geopolitical relations; in perspective, it will destroy economies and health care systems. This is a global and comprehensive issue, which everyone must be involved in tackling. Unfortunately, due to a lack of public and political support for specific pro-environmental policies, it has been an endless point of contention in international discussions with strong resistance against immediate action [30,31,32].

The majority of anthropogenic CO₂ emissions are produced in urban areas, which therefore is a challenge for a sustainable development. It was expected that during the COVID-19 pandemic in 2020 and 2021, with continuation in 2022, when due to worldwide lockdowns the industry was significantly less active, CO₂ emissions would decrease [33]. However, the decrease did not stand out from any natural CO₂ variations observed before and the average growth rate kept increasing (

Table 1. Mean annual CO₂ growth rates (ppm y⁻¹) for Mauna Loa for the last decade from 2011 through 2021 [10].

Year	2011	2012	2013	2014	2015	2016	2017	2018	2019	2020	2021
Growth (ppm)	1.92	2.65	1.99	2.22	2.90	3.03	1.92	2.86	2.48	2.31	2.46 1

¹ Data for October 2021.

) [34]. In addition, global wildfires that seem to increase annually by the extent of area they occupy were producing perhaps similar if not higher CO₂ emissions than any possible decrease resulting from this pandemic.

Most of the anthropogenic CO₂ emissions on the planet are produced in the Northern Hemisphere, with the largest emitters being Eastern Asia, Western Europe, and the Northeast of North America, where GHG is a major by-product of industrial activities. In general, five global economic sectors with the highest produced CO₂ emissions are:

• Transportation (predominantly non-commercial motor vehicles);
• Electricity production (fossil fuel burning);
• Industry (manufacturing, processing, etc.);
• Residential (heating, cooking, etc.);
• Agriculture (farm equipment, agricultural machinery, etc.).

Although these sectors have varied in correlative sequence depending on the given state economy, emissions within these sectors fundamentally reside in fossil fuel burning. Even though the agriculture is commonly seen as a non-fuel contributor of CO₂ emissions, the agricultural machinery, such as harvesters, tractors, and other technologies are clearly a significant contributor of fossil fuel CO₂ emissions, especially in top agricultural producing countries. In view of this, the largest producers of fossil fuel CO₂ emissions worldwide in 2019 were China, United States (US), India, and Russia [35]. Even though statistics for 2020 at the time of this review had not yet been compiled, predictable trends were similar to those before.

Trends of the annual atmospheric CO₂ emissions typically are measured at the Mauna Loa observatory. Mauna Loa is an island of Hawaii, isolated in the middle of the Pacific Ocean at over 3 km above sea level. The CO₂ measurements are performed at the upper north face of Mauna Loa volcano, where there is no impact from any industrial objects or forests that may cause increase or decrease in CO₂ concentration within their vicinity. The CO₂ sensors are positioned so they can sample an incoming breeze directly from the ocean, unaffected by any factors on the island [36]. Recent measures indicated that the monthly average Mauna Loa atmospheric CO₂ in September 2021 was 413.30 ppm, which was nearly 2 ppm increase from the year before, when in September 2020 it was 411.52 ppm [10]. Even the worldwide suspension of industry in 2020 due to COVID-19 did not yield any significant reduction in CO₂ emissions and trends keep showing annual increase, which therefore requires an immediate action to slow down this trend. In 2020, global CO₂ emissions declined by 5.8%, which is the largest decline ever recorded; however, global energy-related CO₂ emissions remained high and contributed to CO₂ emissions reaching their highest ever average concentration in the atmosphere at 412.5 ppm at the end of the year, followed by yet another increase in 2021 [37]. The only option to slow down climate change is to halt the annual increase of CO₂ emissions, which requires the implementation of carbon neutrality policies, prudent use of fossil resources, and transition to environmentally friendly energy sources. The future of the Earth requires creating and sustaining an effective strategic dialogue among governments, industries, and societies worldwide.

Even though the carbon emissions vary greatly from country to country, reducing these emissions must be a global effort. As for the CO₂ emission level in the Baltic States, it is determined by the range of energy sources of the region. The range, however, differs significantly among the three Baltic States (

Table 2. Share of electricity production (%) in the Baltic States in 2020. Fossil fuel, low-carbon sources (solar, wind, hydropower, biomass, waste, geothermal, wave, tidal) [38].

Share	Estonia	Latvia	Lithuania
Fossil fuel	64.79	38.89	42.46
Low-carbon sources (renewables)	35.21	61.11	57.54

) and thus the emissions there are also contrasting. Nevertheless, all three countries are gradually making greater use of renewable resources, especially in electricity production, where such energy sources as solar, wind, hydropower, biomass, and others are put into practice, and Latvia in this respect is the leader with 61.11% share of low-carbon sources in the electricity production.

Furthermore, the Baltic States have ratified the Paris Climate Agreement and have defined their strategic climate targets for emission reduction; this among all includes majorly reducing the use of fossil resources (oil, coal, and gas) in energy production (

Table 3. Energy consumption by source (TWh) in the Baltic States in 2019 [38].

Share	Estonia	Latvia	Lithuania
Oil	18	22	38
Coal	39	<1	2
Gas	4	13	22
Hydropower	<1	5	<1
Wind	2	<1	4
Solar	<1	0	<1
Other renewables	3	2	1

) [38,39].

While the share of energy sources differs amongst the Baltic States (Table 3), all three countries are highly dependent on energy imports. In this regard, Estonia has the lowest share of imports due to its access to domestic oil shale resources [39]. In comparison, in Latvia a major part of the total primary energy supply comes from firewood, but in Lithuania until 2009, more than 70% of primary supply was provided by nuclear energy (zero emission energy). The last Lithuanian nuclear power plant (Ignalina), however, was decommissioned in 2009, due to its similarities to the Chernobyl power plant [39]. Differences in the use of energy sources are reflected in distinct CO₂ emissions amongst Baltic countries. For instance, according to Eurostat (last update on 17 August 2021), in 2019 the Estonian energy sector produced 11,975.96 tCO₂, Latvia 6975.52 tCO₂, but Lithuania caused the release of 11,240.45 tCO₂ into the atmosphere [40].

3. The Technologies for Carbon Dioxide Capture and Storage

The ever-increasing atmospheric CO₂ concentration calls for immediate action to reduce possibilities for global catastrophic risks to the planet and to the very existence of civilization. This is why the development of technologies for CCS are crucial for preserving the future and the quality of life. During the last decades, the development of CCS technologies has been studied widely and intensively to find the balance among efficiency of use, production, and maintenance costs. Notably, the advancement of technologies for carbon capture is equally as important as the new options for carbon storage. The reduction of CO₂ emissions generally relies on its separation from flue gas from the combustion reactions. The technologies for CO₂ separation include six major methods, which are [41]:

• Absorption;
• Adsorption;
• Calcium looping;
• Cryogenic separation;
• Membrane separation;
• Biological separation with microalgae.

The CO₂ absorption technologies generally can be distinguished between chemical and physical absorption. The chemical absorption is then subdivided into formulation and operation. The chemical formulation involves a CO₂ reaction with single amines (derivatives of ammonia), amine blends (solutions of two or more amines in varying compositions), and caustics (strong alkaline chemicals). At the same time, chemical operation is based on the use of a chemical adsorber or stripper and rotating columns to absorb CO₂, where the rotation increases the chemical reactivity and the absorption efficiency [42]. The second absorption type, physical absorption of CO₂, involves the use of units with various solvents and sorbents for gas separation, such as selexol, rectisol, fluor (econamine), purisol, and others [41,43,44,45,46].

Technologies for CO₂ adsorption can be distinguished between adsorbent beds and the use of regeneration cycles. The fixed-bed adsorption generally utilizes activated carbon, alumina (synthetically produced Al₂O₃), silica, zeolites, metal organic frameworks (MOFs), hydrotalcites, amine supported adsorbents, and polymers [41,47,48,49,50,51,52,53,54]. Regeneration cycles, in turn, involve pressure swing adsorption, temperature swing adsorption, steam, or moisture adsorption [41,55,56,57].

The calcium looping technology uses the regenerative calcium cycle, where to separate CO₂ from flue gas, a metal is reversibly reacted between its carbonate form and its oxide form. Within this approach for CO₂ separation, the technology basically involves a loop of calcination and carbonation to remove CO₂ from flue gas [41,58].

The cryogenic technology includes a set of conventional and unconventional methods. Among these, the cryogenic distillation is amidst the most common conventional approaches. In turn, most unconventional methods involve various cryogenic fluids, heat exchangers, and cryogenic packed beds to fix CO₂ [41,59,60,61].

The membrane technology is based on gas separation using specially designed membranes, for instance, a polyphenylene oxide (PPE), poly dimethyl siloxane (PDMS) membrane, or polypropylene (PP) or ceramic bed systems to fix CO₂ [41,62,63,64].

The technology behind the biological CO₂ separation using microalgae involves carbon bio-fixation, for example, through enzyme-catalytic hydrolysis. The idea behind microbial bio-fixation involves CO₂ fixation for microalgae cell growth [41,65,66,67].

Different technologies require divergent approaches; they are also characterized by different use and maintenance costs, as well as the range of applications and scale. Currently, among main technologies used for CO₂ capture is [68,69,70,71,72,73]:

• Cryogenic distillation;
• Membrane purification;
• Electrochemical separation;
• Absorption with liquids;
• Adsorption using solid materials.

Unfortunately, most of these approaches are expensive and high-energy demanding, which therefore limit widespread use of the given technology.

The cryogenic distillation is amongst most used technologies for CO₂ adsorption and gas separation [69]. It is low-temperature CO₂ capture technology, which relies on phase change. The technology can be applied to a range of CO₂ concentrations. Regrettably, it is high-energy demanding and expensive, and thus the use on large industrial scale cannot be economically justifiable. At the same time, this technology can be recommended for smaller-scale applications [41].

The membrane purification involves CO₂ capture in polymeric membranes. The membrane purification technology requires comparatively less energy than cryogenic distillation and is also much easier to operate; however, this technique is limited to minor CO₂ concentrations and thus again cannot be justified for an industrial scale use [70,74].

The electrochemical reduction of CO₂ is among promising future technologies; however, there are numerous challenges for large-scale production [75]. Separation of CO₂ from flue gas using electrochemical cells is complex and energy demanding process. In this context, experiments have shown that this technology can be used to separate CO₂ from flue gas streams produced by coal combustion for electricity generation; however, for this to happen, elevated temperature molten carbonate electrochemical cells would be required and the presence of any kind of contaminants would impact the electrolyte within the cell and thus also the effectiveness of CO₂ separation. At the same time, experiments with low-temperature cells also continue and their applications are continuously advancing, which show comparatively higher work efficiency than the high-temperature cells [76].

Yet another approach—CO₂ adsorption with liquids, in general with alkylamine solution or chilled ammonia, collides with an obstacle—is a complex, expensive, and high-energy demanding technique. Moreover, evaporation of these liquids eventually causes equipment corrosion, and thus this method may not be among long-term solutions [77].

In many aspects, the CO₂ adsorption using solids is among the most reasonable options as this method demands the least energy, can be used in the long-term, and some of the materials can be reused and applied on a large industrial scale. In addition, organic or inorganic sorbents are available with high selectivity in respect to CO₂, high efficiency, and stability during their exploitation [73,78,79]. Use of porous solid materials, such as clays, zeolites, activated carbon, MOFs, metal oxides, layered double hydroxides (LDHs), and porous silica have shown to be very effective for selective CO₂ capture [80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95]. Notably, the majority of CO₂ adsorption studies have been reported in high pressure and elevated temperatures as it is known that with increasing pressure, CO₂ adsorption level would also increase [96]. However, in regard to GHG removal from the atmosphere, CO₂ adsorption at atmospheric pressure also has to be accounted for, and therefore it is crucial to expand on this topic in further studies [97].

The most common technologies for CO₂ capture from large stationary sources are generally based on a selective CO₂ absorption using aqueous solutions of amines, such as monoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine (MDEA), diglycol-amine (DGA), and 2-amino-2-methyl-1-propanol (AMP) [98]. Regrettably, such an approach has several disadvantages over other equivalent solutions for carbon capture; for example, some known disadvantages are equipment corrosion or loss of amines due to evaporation. Moreover, solvent regeneration requires high energy consumption, and thus alternative approaches that are less energy demanding are necessary. In fact, this necessity has promoted the development of new strategies for CO₂ capture that also involve the development of new appropriate solid material sorbents based on cheap substances, for instance, on clay minerals.

4. Materials for Carbon Dioxide Capture Technologies

It is reasonable to consider that several groups of sorbents, both representing solid and liquid phase materials, are suggested for the development of the CCS technologies due to their high efficiency and selectivity to CO₂ capture, long-term applicability, and economic justifiability. In this regard, the materials with high potential for CO₂ capture include a variety of porous materials and liquids of both natural and artificial origin, such as [99,100,101,102,103]:

• Metal organic frameworks (MOFs);
• Graphene organic frameworks (GOFs);
• Covalent organic frameworks (COFs);
• Metal oxides;
• Homogenous porous silica;
• Zeolites;
• Activated carbon;
• Clay minerals;
• Molecular basket sorbents (MBSs);
• Ionic liquids.

It has been reported that MOFs and GOFs form three-dimensional structures with narrow and homogenous pore size distribution, and this allows reversible retention of CO₂ molecules [99,100,101,104,105]. A number of MOFs with designed pore and channel size and large surface area are viewed as materials for CO₂ sorption and storage [72,106].While MOFs and GOFs show significant advantages in gas selectivity and separation in comparison to traditional adsorbents, the main disadvantages of these structures, however, are their relatively low thermal and hydrolytic stability, which are crucial for adsorbent regeneration and low yields. Yet another obstacle is the excessive cost, which is required to prepare these materials on a large industrial scale [99]. At the same time, properties of MOFs can be further improved by several means [107,108]. The main improvements that can be made to MOF composites are increased porosity and sorption capacity, as well as improved special functionality. Moreover, numerous structural modifications and improved kinetics in the synthesis of MOFs can be made, that all together can significantly improve their effectiveness towards CO₂ sorption [109].

Other structures—COFs—are an equal alternative to MOFs and GOFs, as they can be either two- or three-dimensional highly porous organic solids, and they have already been used in the photocatalytic and electrocatalytic systems for large scale CO₂ conversion to CO. However, such photocatalytic systems have poor overall CO₂ selectivity and low effectiveness for CO₂ adsorption [110,111,112,113,114,115]. Moreover, COF applications on an industrial scale are hampered by their low stability in the presence of water [116,117]. Simultaneously, studies show an effective CO₂ adsorption on numerous metal oxides, such as Fe₂O₃, TiO₂, ZrO₂, or Al₂O₃ in the temperature below 0 °C, but unfortunately, the high energy support to maintain the required temperature is high, making this approach high-energy demanding [118]. Notably, the use of such solids as CaO, MgO, or mixed oxides coming from hydrotalcites generally is of low cost; they are highly available and have high overall capacity to capture CO₂. However, use of these oxides is affected by the same complications as other metal oxides [119].

In the last several decades, experimentation with porous silica in regard to CO₂ adsorption has been carried out. Experiments with porous silica with different textural parameters such as mesocellular foams—MCM-41, MCM-48, SBA-15, hexagonal mesoporous silica, and others—have shown that CO₂ capacity is directly proportional to the microporosity [119,120,121]. However, in comparison, MOFs show much better CO₂ adsorption capacity than silica [90,99,120,122,123,124,125].

Zeolites and activated carbon, among all materials, reach by far the highest CO₂ adsorption values. In fact, inorganic zeolites were the first porous materials to be investigated for the ability to sequester CO₂; however, their production is expensive, thus alternatives with similar properties are being sought [99,126,127,128,129]. Zeolites, activated carbon, and other solid porous sorbents are capable to capture CO₂ via physical and chemical adsorption mechanisms [98]. Zeolites can bind CO₂ either by chemical binding or by including both chemical and physical sorption pathways. In comparison, clay minerals are a low-cost alternative with similar CO₂ adsorption capacity to zeolites and activated carbon, thus their use on an industrial scale can be economically justifiable. Clays have high pore and channel size variability, large surface area, a wide variety of structures responsible for CO₂ sorption, and large void volume for carbon storage. Furthermore, clays can be chemically modified to ensure high clay-based sorbent selectivity in respect to other gasses present in flue gas, such as H₂N₂, O₂, CH₄, or CO [72,99]. For instance, Wang et al. (2013) have reported that the support material accounts for over 70% of the total capital cost for sorbent preparation. In this respect, due to the easy availability, excellent thermal and chemical stability, surface properties that can be adjusted for various properties, and low cost, clay minerals can be considered one of the most justifiable materials for CCS [130]. For instance, one of the top sequesters for CO₂ is the exchanged fluorohectorite clay, which belongs to the smectite group [131]. In addition, clay minerals can be used as the base material for other technologies, such as MBSs. The concept of MBS is used to denote sorbents that may selectively capture CO₂ onto a functional basket. Sorbents of this type are typically prepared by immobilizing an amine-functional polymer onto a porous carrier. Recent studies point to the benefits of MBS, such as, superior sorption–desorption characteristics, high sorption capacity, selectivity towards CO₂, high regeneration abilities, and high stability. In addition, MBSs have much less corrosion potential and require much lower energy consumption when compared to the conventional methods, such as amine scrubbing [132]. It is worth mentioning that, while sorption processes have been studied in high detail and general regularities are known, desorption processes are not yet fully understood. At the same time, recent studies show that CO₂ desorption in natural clays and clay modification products, including MBSs, can be controlled both by pore structures and by chemically functionalized groups [133]. However, understanding which factor prevails requires further investigation. Nevertheless, it is believed that desorption can be triggered by changes in clay properties that may endure pressure or temperature change, yet further studies are required.

The last considerable group of materials with a potential for CO₂ fixation are ionic liquids. These are non-volatile liquids with modifiable structure and high CO₂ uptake capacity [134]. Ionic liquids have been explored in various chemical and biological applications; however, use of them comes with a number of potential weaknesses in comparison to clay minerals; this includes weak thermodynamic properties, corrosivity, and toxicity [135]. One of the least substantial options for industrial-scale carbon capture is the precipitation of calcium carbonate in underground reservoirs. Crystallization of calcium carbonate requires a liquid, such as groundwater, thus this method can be viewed within the ionic fluids. However, the crystallization is slow geological process that is sensitive to pressure and temperature changes, and thus this approach is not an effective long-term or industrial-scale solution [136]. Therefore, the use of clay minerals and clay modification to support an increased level of CO₂ fixation are both justifiable and welcomed in regard to reducing GHG emissions.

5. Structural and Property Features of Clay Minerals to Support Carbon Dioxide Capture

The ability of clay minerals to fix CO₂ relies on the structural and property features of a particular clay mineral. While two broad categories of clays can be distinguished (cationic and anionic clays), only one type of these clays can be widely found in nature. Cationic clays are clay minerals found in nature, but anionic clays are LDHs. While cationic clays are widespread in nature, anionic clays are often synthesised due to their rarity and thus their extraction from natural sources cannot be justifiable on a major scale [137]. In addition, the cationic clays often have negatively charged alumina-silicate (Al₂O₃-SiO₄) layers, with small cations in the interlayer space to balance the charge. In comparison, the anionic clays have positively charged brucite (Mg(OH)₂) type metal hydroxide layers with balancing ions and water molecules, which are located interstitially [137,138]. Therefore, there are noticeable structural differences between both clay types and therefore also in their support for CO₂ capture.

Cationic clay minerals are hydrous Al³⁺ or Mg²⁺ phyllosilicates with two-dimensional layered structure [139]. In addition, according to the Association Internationale pour l’Etude des Argiles (AIPEA) and the Clay Minerals Society (CMS) nomenclature committee, clay minerals impart plasticity when wet and harden upon drying or firing [140]. This means that clay minerals generally are water-containing compounds with structure in which silicate tetrahedrons are arranged in sheets. Moreover, clay mineral properties change depending on the water content within. Each layer of clay mineral structure consists of a silica tetrahedral sheet and a metal oxide or hydroxide octahedral sheet, which are linked together in certain proportions [141,142,143]. The above-mentioned silica tetrahedral sheet consists of the central silicon (Si⁴⁺) cation, which is surrounded by four O₂ atoms (Figure 1). This sheet is formed when three out of four O₂ atoms, known as basal oxygens, are corner-linked with three nearest tetrahedrons; this in turn forms an infinite two-dimensional hexagonal mesh pattern [141,144]. The metal oxide or hydroxide octahedral sheet consists of a central cation, which most often is a Al³⁺ or Mg²⁺ cation that is surrounded by six O₂ or hydroxyl (OH) atoms (Figure 1). This sheet is formed when neighbouring octahedrons edge-share two O₂ atoms with each other and the smallest structural unit contains three such octahedrons and is of a hexagonal or pseudohexagonal symmetry [141,142,144,145].

Cationic clay minerals form due to weathering and decomposition of igneous rocks that are in contact with air, water, or steam, and thus they are very common in soils, sedimentary rocks, metamorphic rocks, and volcanoclastic rocks [22,146,147]. While there are numerous clay minerals, they are generally classified into nine groups, which are based on the variations in the chemical composition and atomic structure of these minerals. These groups are [148,149]:

• Kaolin-serpentine (kaolinite, nacrite, dickite etc.);
• Pyrophyllite-talc;
• Mica;
• Vermiculite;
• Smectite (montmorillonite);
• Chlorite;
• Sepiolite-palygorskite;
• Interstratified clay minerals;
• Allophane-imogolite.

The key factor for determining clay mineral type essentially is the SiO₂ ratio in a formula [146]. In addition, clay minerals are also often classified by layer types or tetrahedral and octahedral sheet combinations, which allows to distinguish three general clay mineral types [150]:

• 1:1 layer type, tetrahedral–octahedral sheet combination;
• 2:1 layer type, tetrahedral–octahedral–tetrahedral sheet combination;
• 2:1:1 layer type, tetrahedral–octahedral–tetrahedral sheet combination.

Yet another clay mineral classification considers the charge per formula unit, dioctahedral, or trioctahedral character and chemical composition of the current clay mineral type. Based on both these criteria, there are 10 main clay mineral groups [151,152,153]:

• Serpentine (e.g., chrysolite, lizardite);
• Kaolin (e.g., kaolinite, dickite, halloysite);
• Talc;
• Pyrophyllite;
• Smectite (e.g., saponite, hectorite, montmorillonite, beidellite);
• Vermiculite;
• True mica (e.g., illite, glauconite, phlogopite, biotite);
• Brittle mica (e.g., clintonite, margarite);
• Chlorite (e.g., clinochlore, chamosite, donbassite);
• Mixed layer group (e.g., chlorite-smectite, chlorite-vermiculite, illite-smectite).

Essentially, clay minerals are constructs of two-dimensional tetrahedral stacks of inorganic layers, where atom substitutions generate a negative charge on each layer surface of clay mineral, which is then balanced by exchangeable interlayer cations [28,154]. These interlayer cations determine clay physicochemical characteristics [155,156,157]. The structural and composition features of particular clay minerals give them different physical and chemical properties, e.g., particle size and shape, specific surface area, CEC, plasticity, hydration, swelling, and surface electric charge [28,154,155,156,157,158]. The overall structural arrangement of clay minerals is simple; however, the complexity arises due to the wide variety of isomorphic substitutions that can occur within the aluminosilicate layers, the disordered nature of the interlayer region, stacking defects in the clay layer sequence, or the variable interlayer separation [154].

In regard to the CO₂ capture, minerals such as kaolin, smectite, and brittle mica group have been investigated as prospective materials with necessary properties for carbon capture [99].

The structure of kaolin group minerals (1:1 layer type) is composed of one alumina octahedral sheet and one silica tetrahedral sheet, which are stacked one above other (Figure 2a,b) [141,159]. These sheets are kept together by H₂ bridges between O₂ atoms on the tetrahedral sheet and surface hydroxyl groups on the octahedral sheet [141,142]. The sheet has little or no permanent charge due to the low amount of substitution. As a result, kaolinite has rather low CEC and small surface area [141,142]. In addition, kaolinite is a triclinic (three-dimensional geometrical arrangement) mineral with pseudohexagonal crystals that arrange in book-like aggregates, and their particle size ranges from 0.05 μm to 2 μm [142,144].

The structure of smectite clay minerals (2:1 layer type) is composed of two silica tetrahedral sheets that encompass one octahedral sheet, which contains Al³⁺, Fe³⁺, or Mg²⁺ cations (Figure 2d,e) [143,159,160]. One of the characteristic features of smectite minerals is their isomorphous substitution in the octahedral and tetrahedral sheets. There is a substitution of trivalent for tetravalent ions (Al³⁺ for Si⁴⁺) in the tetrahedral sheet and of divalent for trivalent ions (Mg²⁺ for Al³⁺) in the octahedral sheet [152,158,160]. The substitution results in a charge deficiency, which is then balanced by hydrated cations (K⁺, Na⁺, Ca²⁺ and Mg²⁺) in the interlayer site, which is exchangeable, and thus this leads to high CEC [144,152,159]. The particle size of smectite minerals usually does not exceed 0.5 μm, while the shape varies from subhedral lamellae with irregular outlines to euhedral lamellae with rhombic outlines [144]. The most common smectite minerals are Na⁺ and Ca²⁺ montmorillonites, whose layer charge deficiency is balanced by the interlayer cations, such as sodium or calcium, and water molecules [158].

Fibrous clay minerals, such as palygorskite (attapulgite) and sepiolite, have a unique arrangement of the tetrahedral and octahedral sheets that are distinct from the typical 1:1 and 2:1-layer types. Palygorskite and sepiolite are composed of continuous two-dimensional silica tetrahedral sheets and one Mg²⁺ octahedral sheet, which is continuous in only one dimension. As a result, the tetrahedral sheets are split into ribbons and each of these ribbons are linked to the next ribbon by inversion of silica tetrahedral (Figure 2c). The dimension of channels between the ribbon strips is ranging from 4 to 6 angstroms for palygorskite and from 4 to 9.5 angstroms for sepiolite [141,143,158]. Both minerals are of fibrous morphology with micropores of around 0.54 nm in diameter; at the same time, mineral crystals are arranged in the bundles in an elongated shape. Notably, these properties are highly varied in clay minerals and even more so within different fibrous clay minerals. The morphology of these clay minerals is the most important physical attribute, which determines a high surface area and provides more active sites and thus also high potential for CO₂ sorption [158,161].

The interaction between CO₂ and clay minerals has attracted the interest of the scientific community as such minerals have proven potential to capture carbon [20,24,28]. In addition, clay minerals are a class of potential adsorbents that also possess good stability and economic viability [162]. Moreover, clay minerals are universally available and versatile materials, and due to their physicochemical properties, they can also be used as ion exchangers or catalysts [138]. Therefore, the use cationic or anionic clay minerals, in addition to liquid and supported amines, are amongst options viewed for trapping CO₂ [81,163,164,165,166,167,168]. In this regard, cationic montmorillonite due to its slight basicity seems to be one of the best options for carbon capture [169,170,171,172,173,174,175,176]. In addition, numerous studies suggest that clay minerals can also be used as support for immobilizing enzymes. These enzymes can be immobilized onto clay minerals through non-covalent adsorption and covalent bonding [139]. The non-covalent adsorption involves van der Walls forces, electrostatic interactions, hydrogen bonding, and hydrophobic interactions. The covalent bonding, in turn, can be used to avoid possible desorption of enzymes [139].

In terms of clay mineral parameters with high importance on CO₂ sorption capacity, the moisture may be among the most important ones. This is because there is almost always water in CO₂ containing gas mixtures. Moreover, it is worth noting that the sorption mechanism of CO₂, for example, on amine containing sorbents differs significantly in humid and in dry conditions. For example, in humid conditions between one CO₂ and two amine groups, bicarbonates are formed, but in dry conditions carbamates are forming instead. Thus, sorbed amount of CO₂ in humid conditions is comparatively higher than in dry conditions [177]. This was proven by Irani et al. (2015), who reported that the addition of 1 vol% of water could increase CO₂ sorption capacity from 2.2 mmol g⁻¹ in dry conditions to 3.8 mmol g⁻¹ in humid conditions. Yuan et al. (2018) had similar findings, revealing that the adsorption capacity increased from 1.93 mmol g⁻¹ in dry conditions to 2.21 mmol g⁻¹ in humid conditions. At the same time, there were indications that adsorption–desorption cycles show good regenerability in both dry and humid conditions [177,178].

6. Clay Mineral and Carbon Dioxide Interaction Depending on Mineral Type

Studies of CO₂ sorption onto clay minerals started more than 50 years ago with aims to analyse the surface area of clay minerals and to identify the sorption mechanism [179,180]. Nowadays, clay minerals and CO₂ interactions are studied with two main aims: one is the use of clay minerals as sorbents for CO₂ capture, and another is the geological storage of captured CO₂ by clay minerals or in the underground structures containing clay minerals [96,99,181,182]. Additionally, common clay mineral types have been used to study their potential for gas separation and carbon sequestration processes. In fact, by numerous analytical methods, it has also been suggested to characterise CO₂ sorption process, stability of formed complexes, and desorption processes [24,99,183,184,185]. For example, smectite and CO₂ interaction has been investigated by X-ray diffraction experiments while analysing shifts in basal spacing reflections [183]. Smectite and CO₂ interaction has also been investigated with neutron diffraction and infrared measurements [184,185,186]. Smectites are layered nanoporous materials, and it was found that they can sorb CO₂ in their interlayer space due to gaseous CO₂ intercalation into the interlayer nano-space as confirmed using above mentioned techniques [28]. In addition, experiments with smectites have shown that Na-exchanged montmorillonite can swell up to 9% if it is exposed to high purity CO₂, this however depends on the initial hydration state of the interlayers in smectite [24]. CO₂ adsorption within the smectite interlayer region in Na-exchanged montmorillonite is believed to be the reason behind clay swelling up to the accounted 9% [24]. Moreover, recent studies have shown that the pore size, charge, and solvation energies of the interlayer cations in smectite minerals are greatly affecting the structure, dynamics, and energetics of the intercalated CO₂ molecules [187,188]. At the same time, other studies have shown that, for instance, illite and kaolinite minerals are not known for interlayer expansion under any experimental conditions, whilst they are still able to adsorb considerable amounts of CO₂. It is believed that in these clay minerals, CO₂ sorption occurs on clay platelet surface [186]. Accordingly, it can be concluded that CO₂ adsorption mechanisms vary depending on clay mineral type as it can be either CO₂ adsorption in the interlayers, or it can be surface sorption. These regularities must be further addressed in future experiments.

One of the main factors affecting the physical sorption of CO₂ onto clay minerals is the type of exchangeable ion within; for example, Cs-exchanged and tetramethylammonium-exchanged montmorillonite have sorption capacity for CO₂ up to 1.70 mmol g⁻¹ even at ambient pressure and temperature [189]. At the same time, in case of fibrous clay minerals, such as sepiolite, the sorption takes place due to the presence of nanocavities, which are acting as a molecular sieve and therefore the CO₂ uptake can reach up to 1.48 mmol g⁻¹ [119]. In comparison, in saponite (smectite group), CO₂ physical sorption takes place under ambient conditions on the nanosheet surface at the open spaces on clay mineral [190]. Additionally, a number of other clay minerals have also been studied as potential CO₂ adsorbents, for example, hydrotalcite and hectorite, palygorskite, kerolite, and stevensite, thus demonstrating the high potential of clay minerals for carbon capture [29,119,191,192,193].

7. Prospective for Clay Mineral Modification to Improve Carbon Dioxide Capture

Physicochemical characteristics of clay minerals have been widely studied using numerous techniques and approaches, such as neutron and X-ray scattering, nuclear magnetic resonance (NMR) spectroscopy, tracer experiments, numerical modelling, and others [25,99,155,156,194,195,196,197,198,199,200,201]. According to the data obtained from using these approaches, clay mineral properties, in regard to the improvement of CO₂ adsorption, can be optimized by the insertion of organic, inorganic, or organometallic species between adjacent sheets of clay structure, by functionalization or by acid treatment [27,138,139,193,202]. However, before choosing one or another type of clay mineral for carbon capture, it is important to note that the chemical properties of particular minerals, and consequently also the adsorption of gases on or within them, are significantly affected by the structure of the mineral and the charge of clay layers. Mineral structures with all six octahedral sites occupied are known as trioctahedral structures, while structures with four out of six octahedral sites occupied are referred as dioctahedral structures. For example, minerals such as kaolinite, halloysite (1:1 layer structure), and montmorillonite (2:1 layer structure) are dioctahedral, while serpentine (1:1 layer structure) and vermiculite (2:1 layer structure) are trioctahedral. Furthermore, structures of palygorskite and sepiolite are rather different from above mentioned due to the lack of continuous octahedral sheets; however, there are fibrous clay minerals with dioctahedral and trioctahedral arrangement [203]. At the same time, the structure of a layer, its thickness, and the charge are all interrelated with one another and can therefore affect the particle size and specific surface area of the clay mineral and many other properties [204]. In addition, recent studies indicate that raw clay materials typically show low CO₂ adsorption capacity, and the adsorption properties are generally related to the textural properties of particular minerals [119,121,205,206]. Therefore, clay mineral modification to improve CO₂ capture is a crucial direction of the future research [207,208].

Factors/parameters that are essential for the selection of a potential sorbent material include high CO₂ absorption capacity, high CO₂ selectivity, mechanical, hydrothermal, and chemical stability, rapid adsorption/desorption kinetics, regeneration capacity, low heat capacity, price of raw materials, and costs of synthesis [99,121,209,210]. Therefore, appropriate materials are chosen either based on the highest efficiency within these parameters or materials are modified to improve certain parameters. At the same time, it is important to note that it is almost impossible for a single material to satisfy all of the above-mentioned essential properties of a sorbent. Nonetheless, it is important to assess at least partial compliance and to consider the most important factors.

Efficiency-wise, clay minerals are among the materials with the most essential properties required for CO₂ sorption. In addition, porous clay heterostructures have been evaluated as adsorbents for small hydrocarbons, in their separation and in the encapsulation of organic compounds [192,211,212,213]. There have also been experiments on clay structures as the catalytic support in the NO reduction, partial oxidation of H₂S, preferential oxidation of CO₂, and the esterification reactions to obtain bio-lubricants and all of these experiments have shown high potential of clay applications [214,215,216,217].

Strategies to overcome drawbacks for CO₂ sequestration encourages capturing and storing CO₂ using solid adsorbents, which operate through a repetitive adsorption–desorption cycle, thus using sorbents that display base properties [218]. Solid sorbents usually contain a solid with a large specific surface area, and when a layer of such sorbent passes through a mixture of gases, CO₂ is captured by either a physical sorption or a chemosorption process. These sorbents can be regenerated via pressure or temperature change [219,220,221]. For example, pressure changes have been successfully used to induce adsorption–desorption cycle and to separate CO₂ from N₂ at room temperature. In this regard, montmorillonite clay intercalated with dendrimer polyol silica, goes through a pressure-induced adsorption-desorption cycle and shows a potential for reversible CO₂ capture. Moreover, the hydroxyl groups in montmorillonite are found to act as adsorption sites and interact with CO₂ molecules in the gas phase [29,170,174,218,222,223,224,225,226,227]. Numerous clay mineral modifications, such as clay-graphene hybrids, have also been assessed for CO₂ adsorption [228]. Clay minerals are considered suitable for further modifications involving a simultaneous reduction of basicity and an increase in CO₂ sequestration [218].

One of the options for clay mineral modification to improve the CO₂ sorption capacity is their expansion. For instance, smectites are expanded by incorporation of bulky cations in the interlayer space [229]. Additionally, these modified clays are with significantly improved thermal and mechanical resistance when compared to a raw smectite [229,230]. The reactivity of smectite group minerals can be further improved by replacing mobile and exchangeable cations on their surface by higher valence metal ions (e.g., Mg²⁺, Zn²⁺, Cu²⁺, Al³⁺ or Fe³⁺) [231,232,233,234]. Furthermore, the cation exchange with Ni²⁺ ions can promote formation of a carbonaceous residue [235,236,237]. It can be concluded that amongst the most significant properties of clay minerals to support CO₂ adsorption is the ion exchange. The properties of ion exchange allow for isomorphous substitution of metal cations in the lattice by lower-valent ions. The created negative charge can then be balanced by other cations. The cations can be exchanged when they are brought into the contact with other ions in aqueous solution [137]. Considering cation exchange properties and layered structure of clay minerals, it is possible to introduce molecules of other compounds, which then can be used to change and modify clay mineral properties.

8. Clay Mineral Activation

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 [132].

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 [132]. 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 [132]. 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 [206]. 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 [206]. 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 [132].

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 [99,132]. 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 [119]. 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 [193]. 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 [99,119]. 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 [177]. 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 [119].

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 [238,239].

9. Clay Mineral Functionalization

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 [90,240]. 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 [94,122]. The impregnation of amine-rich polymers is stabilized by hydrogen bonds with the silanol groups of the adsorbent [95,123,241].

Clay minerals can be substantially modified by replacing the natural inorganic interlayer cations with selected organic cations [137]. An example of organic compounds used for intercalation are amines and amino acids [242,243]. 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 [244]. 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 [99,244]. 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 [99,206].

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 [72,99]. 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 [206,244,245]. The tertiary amines interact with the CO₂ and produce bicarbonate [99]. 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.

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 [210]. 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 [178]. The most often used wet impregnation reagents are polyethyleneimine (PEI), tetraethylenepentamine (TEPA), triethylenetetramine (TETA), diethanolamine (DEA), diisopropanolamine, triethanolamine, and diethylenetriamine (DETA) [206,246]. 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 [205]. 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) [247]. 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 [248]. 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 [248].

Several authors have explored the impregnation with amine containing polymers at different loadings [107,178,205,209,246,249]. 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 [107]. 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 [209]. 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 [249]. 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% [246]. Meanwhile, Wang et al. (2013) explored the wet impregnation of bentonite using varied loadings of TEPA [132]. 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 [177]. 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 [250]. 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% [178]. 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 [206].

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 [206]. 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 [251]. 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 [251]. 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 [121]. 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 [206]. 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 [206].

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 [74]. 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 [74]. 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 [119].

Several authors have offered to use the aminosilane-modified clay nanotubes to entrap CO₂ [214,244]. Following this approach, three different aminosilanes—(3-aminopropyl)triethoxysilane, N-[3-(trimethoxysilyl)propyl]-ethylenediamine, and N1-(3-trimethoxysilylpropyl)diethylene-triamine have been used [244]. 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 [244]. 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 [245]. 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 [245]. 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 [119].

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 [119,121]. 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 [121]. 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 [121].

Cecilia et al. (2018) suggested an increase of available amine sites or higher proportion of primary amines obtained after grafting with APTES [119]. 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 [119]. 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 [210].

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 [252]. By intercalating clays with polyol dendrimers, it is possible to obtain organoclays, which recently have been used also for CO₂ capture [99,218]. 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 [218]. 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 [218].

There were several attempts to insert a cationic amine-rich dendrimer in clay material, for example, inserting polyamidoamine (PAMAM) to improve CO₂ sorption capacity [99]. 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 [99]. 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 [251]. 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 [253]. 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 [253]. 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 [252].

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 [107,132]. 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 [254]. 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 [119]. Ouyang et al. (2018) also reported that PEI-loaded MgO-SiO₂ nanofibers from sepiolite reached the highest adsorption capacity of CO₂ at 75 °C [248,249]. 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 [177]. 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 [178].

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 [255,256,257].

10. Clay Mineral Assemblages in the Baltic States

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 this 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 [258,259]. 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 [260].

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 [261]. 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 [261]. 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 [261]. 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 [262].

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 [263]. 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) [264]. 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 [265]. 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 [265]. 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.

11. Discussion

Major climate change is inevitable. However, if action is taken now, then the rapid rise of temperature could be slowed down and the impact on the environment could be less damaging. The climate crisis demands immediate action towards carbon neutrality and carbon negative solutions. Unfortunately, the climate change will also necessitate adaptation to the new climatic conditions. Reducing emissions at this point is critical so that the adaption is gradual.

One of the options to mitigate CO₂ emissions is their capture from emission sources, such as industrial objects. Even though there are numerous media with a potential to capture CO₂, clays or any other amorphous solids seem to be amongst the most reasonable options when considering efficiency, price, maintenance, disposal, and other aspects. In this respect, clays are among the lowest-cost materials. At the same time, it must be considered that different clay minerals have rather different abilities to capture CO₂. For example, research shows that montmorillonite clay mineral has the highest CO₂ adsorption capacity, followed by illite and kaolinite [266]. Notably, that variation in the CO₂ adsorption capacity among different clay minerals is also related to cation exchange capacity [267].

Montmorillonite is the most common smectite group mineral; it is built of several tetrahedral silicate sheets with one octahedral metal oxide or hydroxide sheet in between them. This composition allows montmorillonite to possess large surface area, swelling abilities, numerous functional groups, and ion exchangeability, all of which contribute to the high CO₂ sorption efficiency [139,268,269]. Montmorillonite is composed of octahedral sheets of aluminium or magnesium oxide, which is located between tetrahedral silicone layers. While oxide sheets are negatively charged, silicone layers are positively charged. The charges are balanced by the counter cations located in between the interlayer space [270,271]. The highly porous nature of montmorillonite and substantial number of cations within the structure and high cation exchange capacity make it an important material for CO₂ adsorption. Moreover, high cation density in the interlayers of montmorillonite also makes it viable for modification to improve its adsorption capabilities furthermore [183,247]. Smectite type minerals can sorb CO₂ molecules not only on the surface but also in the interlayer space. However, the content of water molecules and the type of interlaminar cations can play a crucial role in the CO₂ sorption capacity [99]. For example, in palygorskite and sepiolite, the dominant mechanism of CO₂ sorption is related to their morphological properties and fibrous structure with nanochannels. These channels function as a molecular sieve, where the dimension of cavities is in the appropriate size for retention of CO₂ molecules by electrostatic interactions [119,272].

Experiments indicate that illite minerals tend to selectively adsorb CO₂ over CH₄ when potassium cations are present as exchangeable cations, whereas the impact of cation exchange capacity seems to be more pronounced at low pressure, thus indicating a potential for clay modification [119,266,273,274,275].

Kaolinite has one of the lowest sorption capacities on CO₂ capture. CO₂ sorption occurs only on the external surface of kaolinite. This sorption happens through the reaction of hydroxide groups within kaolinite and CO₂ molecules, which is forming bicarbonate ions [162].

The approach for modification depends essentially on the properties and structure of certain minerals, and uniform strategy here is ambiguous. For example, in regard to amino acid immobilization, clays intercalated with arginine and L-histidine adsorb more CO₂ than if they are intercalated with glycine [137]. In addition, amino acid intercalated clay materials are capable of CO₂ retention due to the presence of amine groups in the amino acid structure. Unfortunately, the exact mechanisms leading to the retention of CO₂ inside the clay mineral structure have not been investigated yet. According to one hypothesis, the adsorption of CO₂ can continue through a reaction of a CO₂ molecule with an amine group creating carbamic acid group, which can transform to carbamate due to dissociation [137].

Methods for atmospheric CO₂ emission reduction in addition to direct adsorption by clay minerals include numerous other approaches (

Table 4. Comparison of methods for CO₂ reduction. Costs and scalability [276].

Approach	Costs (Euro/tCO2)	Scalability (GtCO2/y)
Reducing CO2 to its constituent components	−70 to 260	0.3–0.6
Hydrocarbon fuel production	590	1.0–4.2
CO2 fixation with microalgae	200 to 800	0.2–0.9
Use in building materials	−25 to 60	0.1–1.4
Enhanced oil recovery	−35 to 50	0.1–1.8
Bioenergy with carbon capture	50 to 140	0.5–5.0
Forestry	−35 to 10	1.5
Soil carbon sequestration	−80 to 20	0.9–1.9
Biochar	−60	0.2–1.0

) with variable scalability and potential costs.

Methods for CO₂ emission reduction generally include its fixation, and where possible, it also includes its use, such as in building materials. Carbon pricing is among the primary policies to address climate change and approaches for CO₂ emission reduction [277]. Carbon pricing strategies are considered efficient means of reducing carbon emissions [278,279]. However, as of 2019, existing carbon pricing schemes only cover about 20% of global emissions, and more than two-thirds of these have prices below $20 per ton of CO₂ equivalent [277]. Thus, an alternative approach (carbon neutrality) that targets fundamental transformation of existing sociotechnical systems is crucial [280].

12. Conclusions

Carbon capture by clay minerals is among the most reasonable options to date as it requires little energy when compared to other techniques and it has comparatively lower development and maintenance costs. Moreover, clay minerals are low-cost, versatile, and abundant in nature, which allow them to be applied on industrial-scale projects, and moreover their applications in most cases are environmentally friendly. The prospects of clay minerals from the Baltic States for industrial-scale carbon capture are thus promising due to the wide availability of clay in the region. The applicability of clay minerals for selective carbon dioxide adsorption can be improved by their modification; suggestions for clay-based carbon dioxide sorbent designs include immobilization of active sorbents onto clay minerals and clay surface modification. In fact, clay minerals are crucial not only in applications for carbon dioxide capture but also in the caprock of geological structures where they contribute towards the immobilization of carbon dioxide through the pore network and thus minimize possible leakage from a potential storage site. Introduction of industrial-scale CO₂ capture by clay minerals is a step towards a carbon-neutral future.