Features of Clay Minerals Supporting Carbon Dioxide Capture: History
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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.

  • carbon sequestration
  • clay minerals

1. 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 [1]. 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 [1][2]. 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 [3]. 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 [4]. 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 [5][6][7]. 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 [5][8]. 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 [5][6][8][9].
Figure 1. A schematic representation of silica tetrahedron and magnesium or aluminium octahedron.
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 [10][11][12]. 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 [13][14]:
  • 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 [11]. 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 [15]:
  • 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 [16][17][18]:
  • 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 [19][20]. These interlayer cations determine clay physicochemical characteristics [21][22][23]. 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 [19][20][21][22][23][24]. 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 [20].
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 [25].
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) [5][26]. These sheets are kept together by H2 bridges between O2 atoms on the tetrahedral sheet and surface hydroxyl groups on the octahedral sheet [5][6]. 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 [5][6]. 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 [6][8].
Figure 2. Structural schemes for some of the clay minerals: (a) kaolinite; (b) halloysite; (c) sepiolite; (d) vermiculite, and (e) smectite (adapted from Schulze 2005) [5].
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) [7][26][27]. 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 [17][24][27]. 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 [8][17][26]. 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 [8]. 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 [24].
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 [5][7][24]. 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 [24][28].
The interaction between CO2 and clay minerals has attracted the interest of the scientific community as such minerals have proven potential to capture carbon [29][30][19]. In addition, clay minerals are a class of potential adsorbents that also possess good stability and economic viability [31]. 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 [2]. Therefore, the use cationic or anionic clay minerals, in addition to liquid and supported amines, are amongst options viewed for trapping CO2 [32][33][34][35][36][37][38]. In this regard, cationic montmorillonite due to its slight basicity seems to be one of the best options for carbon capture [39][40][41][42][43][44][45][46]. 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 [3]. 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 [3].
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 [47]. 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 [47][48].

2. 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 [49][50]. 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 [51][25][52][53]. 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 [30][25][54][55][56]. For example, smectite and CO2 interaction has been investigated by X-ray diffraction experiments while analysing shifts in basal spacing reflections [54]. Smectite and CO2 interaction has also been investigated with neutron diffraction and infrared measurements [55][56][57]. 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 [19]. 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 [30]. 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% [30]. 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 [58][59]. 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 [57]. 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 [60]. 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 [61]. 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 [62]. 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 [63][61][64][65][66].

3. 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 [67][25][21][22][68][69][70][71][72][73][74][75]. 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 [76][2][3][66][77]. 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 [78]. 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 [79]. 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 [61][80][81][82]. Therefore, clay mineral modification to improve CO2 capture is a crucial direction of the future research [83][84].
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 [25][80][85][86]. 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 [65][87][88][89]. 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 [90][91][92][93].
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 [94]. 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 [95][96][97]. 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 [63][40][44][94][98][99][100][101][102][103]. Numerous clay mineral modifications, such as clay-graphene hybrids, have also been assessed for CO2 adsorption [104]. Clay minerals are considered suitable for further modifications involving a simultaneous reduction of basicity and an increase in CO2 sequestration [94].
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 [105]. Additionally, these modified clays are with significantly improved thermal and mechanical resistance when compared to a raw smectite [105][106]. 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+) [107][108][109][110]. Furthermore, the cation exchange with Ni2+ ions can promote formation of a carbonaceous residue [111][112][113]. 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 [1]. 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.

This entry is adapted from the peer-reviewed paper 10.3390/min12030349

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