Clay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried or fired. Clay deposits are mostly composed of clay minerals (phyllosilicate minerals) and variable amounts of water trapped in the mineral structure by polar attraction. Organic materials which do not impart plasticity may also be a part of clay deposits. Clay is a widely distributed, abundant mineral resource of major industrial importance for an enormous variety of uses (Ampian, 1985). In both value and amount of annual production, it is one of the leading minerals worldwide. In common with many geological terms, the term “clay” is ambiguous and has multiple meanings: a group of fine-grained minerals which show plasticity through a variable range of water content, and which can be hardened when dried or fired i.e., the clay minerals; a particle size (smaller than silt); and a type of rock i.e., a sedimentary deposit of fine-grained material usually composed largely of clay minerals (Patterson & Murray, 1983; Bates & Jackson, 1987). Clays find wide range of applications, in various areas of science, due to their natural abundance and the propensity with which they can be chemically and physically modified to suit practical technological needs (Xi et al., 2005).
Clays are distinguished from other fine-grained soils by various differences in composition. Silts, which are fine grained soils which do not include clay minerals tend to have large particle sizes than clays but there is some overlap in both particle size and other physical properties, and there are many naturally occurring deposits which include both silts and clays. The distinction between silts and clay varies by discipline.Geologists and soil scientists usually consider the separation to occur at a particle size of 2µm (clays being finer than silts), sedimentologists often use 4-5µm, and colloid chemists use 1um. Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, ISO 14688 grades; clay particles as being smaller than 0.063mm and silts one larger.
There are three or four main groups of clays; kaolinite, montmorillonite-smecite, illite and chlorite. Chlorites are not always considered clay, sometimes being classified as a separate group within the phyllosilicates. There are approximately thirty different types of “pure” clays in these categories but most “natural” clays are mixtures of these different types along with other weathered minerals (Lagaly, 1984).
Clay minerals likely are the most utilized minerals not just as the soils that grow plants for foods and garment, but a great range of applications, including oil absorbants, iron casting, animal feeds, pottery, china, pharmaceuticals, drilling fluids, waste water treatment, food preparation, paint e.t.c.
Clay minerals are hydrous aluminium phyllosilicates sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and other cations. Clays form flat hexagonal sheets similar to themicals. Clay minerals are common weathering products (including weathering of felspar) and low temperature hydrothermal alteration products. Clay minerals are very common in fine grained sedimentary rocks such as shale, mudstone and siltstone and in fine grained metamorphic slate and phyllite. Clay minerals are usually (but not necessarily) ultrafine-grained (normally considered to be less than 2µm in size on standard particle size classifications) and so may require special analytical techniques for their identification/study. These include x-ray diffraction, electron diffraction methods, various spectroscopic methods such as Mossbauer spectroscopy, infrared spectroscopy and SEM-EDX or automated minerology solutions. These methods can be enlarged by polarized light microscopy, a traditional technique establishing fundamental occurrences or petrologic relationships.
Clay minerals can be classified as 1:1 or 2:1clays; this originates from the fact that they are fundamentally built of tetrahedral silicate sheets and octahedral hydroxide sheets, as described in Figure 1 below. A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, for example, kaolinite and serpentine. A2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, for example, talc, vermiculite and montmorillonite.
Clay minerals include the following groups:
· Kaolin group which includes the minerals kaolin, dickite, halloysite and nacrite (polymorphs of Al2Si2O5(OH)4). Some sources include the kaolinite-serpentine group due to structural similarities (Bailey 1980).
· Smectite group which includes dioctahedral smectites such as montmorillonite and nontronite and trioctahedral smectites, for example sapronite.
· Illite group which includes the clay-micas. Illite is the only common mineral.
· Chlorite group includes a wide variety of similar minerals with considerable chemical variation.
Other 2:1 clay types exist such as seriolite or attapulgite which areclays with long water channels internal to their structure.
Typically, the structural formula for kaolinite is Al4Si4O10(OH)8 and the theoretical chemical composition given in Table 1.
Table 1: Theoretical Chemical Composition of Kaolinite (J.Fafardet al; 2012)
Percentage Composition (%)
Mixed layer clay variations exist for most of the above groups. Ordering is described as random or regular ordering, and is further described by the term reichweite, which is German for range or reach.
1.1.1 Structureof Clay Minerals
Like all phyllosilicates, clay minerals are characterized by two-dimensional sheets of corner sharing SiO4tetrahedral and/or AlO4 octahedral. The sheet units have the chemical composition (Al,Si)3O4. Each silica tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedral forming a hexagonal array in two-dimensions. The fourth vertex is not shared with another tetrahedron and all of the tetrahedral "point" in the same direction; that is, all of the unshared vertices are on the same side of the sheet. In clays, the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminum or magnesium, and coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also forms part of one side of the octahedral sheet, but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedral. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorized depending on the way that tetrahedral and octahedral sheets are packaged into layers.
Fig 1. Structure of kaolinite( Papke,Keith1970)
1.1.2 Cation Exchange Capacity (CEC)
The ion-exchange capability of clay minerals, in particular, kaolinites, influences their unique physical properties, such as the cation retention and diffusion processes of charged and uncharged molecules. These processes influence cation and molecule migration through clay-rich barriers in nature. The numerical value of this property is described by the cation exchange capacity (CEC). Methods for determining CEC involve the complete exchange of the naturally occurring cations by a cationic species, such as ammonium, K, Na, methylene blue, Co(III) hexamine complex (RadmyandOrsini, 1976), Ba, Ag thiourea complex, and Cu(II) ethylenediamine complex. Exchange with organic cations, such as alkylammonium, provides an indirect method for the determination of CEC.
CEC may be defined as the quality of exchangeable cations expressed in milliequivalents per 100g of ignited weight of clay (Newman 1987).The cation exchange capacity (CEC) of a clay is a measure of the quantity of negatively charged sites on clay surfaces that can retain positively charged ions (cations) such as calcium (Ca2+), magnesium (Mg2+), and potassium (K+), by electrostatic forces. Cations retained electrostatically are easily exchangeable with cations in the clay solution, so clay with a higher CEC has a greater capacity to maintain adequate quantities of Ca2+, Mg2+ and K+ than clay with a low CEC. It is also a very important tool in the preparation of organoclay. CEC is a good indicator of clay quality and productivity. It is normally expressed in one of two numerically equivalent sets of units: meq/100 g (milliequivalents of charge per 100 g of dry clay) or cmolc/kg (centimoles of charge per kilogram of dry clay).Because of the differing methods to estimate CEC, it is important to know the intended use of the data. For clay classification purposes, clay’s CEC is often measured at a standard pH value. Examples are the ammonium acetate method of Schollenberger and Dreibelbis (1930) which is buffered at pH 7, and the barium chloride-triethanolamine method of Mehlich (1938) which is buffered at pH 8.2 (Rhoades,1982.) . This procedure involves determination of the expansion of the layers and calculations involve charge density (Oliset et al., 1990; Lagaly, 1981). Depending on the method utilized, the excess of the exchanged cations is removed in a subsequent step and the amount retained on the clay is determined. However, the determined CEC values are dependent on the method used. Although time consuming, the exchange with ammonium acetate is the standard method for CEC determination (Mackenzie, 1951).
To obtain complete ion exchange and to obtain reliable values of CEC, either a high surplus of an exchanging cation or a cation with a high affinity for the clay mineral must be employed.
Surface modifications of clay minerals have received attention because it allows the creation of new materials and new applications. The main focus of surface modification of clays is materials science, because organoclays are essential to develop polymer nanocomposites. Nanocomposites constitute one of the most developed areas of nanotechnology. It is reportedTheng (1974), that the adsorption capacity of organoclays is improved over and above untreated clays for the removal of various organic contaminants. Besides, organoclays are more cost effective compared with other adsorbents, such as activated carbon and have been shown to be potentially effective for the uptake of water contaminants in aqueous solution.
Clay minerals have been found to be ineffective adsorbents in removing organic compounds because the hydration of inorganic cations on the surface of the clay makes them hydrophilic (Y. Xi et al; 2011). However, with the use of quaternary ammonium compounds (QACs), the surface properties of clay minerals have been greatly improved by replacing the natural inorganic interlayer cations with the organic cations present in the QACs (A.R. McLauchlin and N.L. Thomas;2008) to produce organoclays that are highly effective as adsorbents used in organic contaminant attenuation. The intercalation of a cationic surfactant between the clay layers renders the clay mineral hydrophobic at the surface while also increasing its wettability and thermodynamically favorable interactions with organic molecules (Y. Xi et al; 2011).
The studies on the interaction between clay minerals and organic compounds have been conducted from the beginning of the 20th century increasing in number and in topics. The research of intercalation of organic molecules into the interlayer space of clay minerals started in the 1920s, after the introduction of X-ray diffraction in 1913 (Merinska et al., 2002). One of the earliest papers was from Smith in 1934 on interactions, Gieseking (1939) found methylene blue to be very effective in replacing interlayer cations. These results suggested the possibility of using ammonium ions of the NHR3+, NH2R2 +, NHR3+, and NR4+ types to throw more light on the mechanism of cation exchange in clay minerals. Kaolinites, beidellite and nontronite types of clay minerals were treated with solutions of the hydrochlorides or hydroiodides of the various amines. The clay minerals adsorbed the organic ions, giving rise to basal spacing greater than those of the same clay minerals saturated with smaller cations such as calcium or hydrogen.
McEwan (1944) reported that the identification of montmorillonite was notoriously difficult. For this reason, he developed an unambiguous method based on the intercalation of glycerol into the interlayer space of the clay mineral. He observed that when montmorillonite was treated with glycerol, a very sharp and intense first-order basal reflexion was obtained at 1.77 nm, and the method is very suitable for identification.
Bradley (1945) studied the molecular association between montmorillonite and organic liquids aliphatic di- and polyamines and glycols, polyglycols and polyglycol ethers. Analysis of the complexes established that the amines are active in base exchange, while glycerol and glycol enter into the interlayer space without displacing cations.
Studies of interactions between clay minerals and organic compounds have been presented, among others, by Theng (1974), Lagaly (1984), and Yariv and Cross (2002). The countless clay–organic complexes of great industrial importance are the organoclays prepared from smectites and quaternary ammonium salts.
Hauser (1950) in his patent (US 2,531,427) described procedures for obtaining organoclays that swell and disperse forming gels in organic liquids in the same way as sodium smectites usually swell in water. Jordan first developed a research group on those organophilic clays (Beneke and Lagaly, 2002) and published important papers on their properties (Jordan, 1949; Jordan et al., 1950; Jordan, 1954). Jordan (1949) investigated some of the factors involved with the swelling of organoclays and the extent of the conversion of the clay from hydrophilic to hydrophobic.
Organophilic kaolinites were prepared by the reaction of kaolinite with various aliphatic ammonium salts. The swelling of the organoclays was studied in several organic liquids and liquid mixtures. Jordan concluded that the degree of solvation depends on at least three factors:
i. The extent of the surface coating of the clay particles by organic matter;
ii. The degree of saturation of the exchange capacity of the clay mineral by organic cations; and
iii. The nature of the organic liquid.
In 1950 Jordan et al. investigated the formation of gels of organoclays in several organic liquids and liquid mixtures and an optimum gelation occurred. Intercalation of organic guest species into kaolinite is a way of constructing ordered inorganic–organic assemblies with unique microstructures controlled by host–guest and guest–guest interactions (Kakegawa and Ogawa, 2002). Currently, an important application of the organoclays is in the polymer nanocomposites.
Organoclays are the most dominant commercial nanomaterial to prepare polymer nanocomposites, accounting for nearly 70% of the volume used (Markarian, 2005). Proper organophilization procedure is a key step for successful exfoliation of clay minerals particles in the polymeric matrix. The organophilic feature reduces the energy of the clay mineral and makes it more compatible with the organic polymers. The addition of organoclays into polymeric matrices improves mechanical, physical (thermal and barrier) and chemical properties of the matrices and reduces cost in some cases. Typically, organoclays replace talc or glass fillers at a 3:1 ratio. For example, 5% of an organoclay can replace 15–50% of a filler like calcium carbonate reducing cost and improving mechanical properties (Markarian, 2005). Organoclays also have been used in other applications. These applications include adsorbents, rheological control agents, paints, grease, cosmetics, personal care products, oil well drilling fluids, etc. (Santos,1989; Beall and Goss, 2004; Xi et al., 2005; Araújoet al., 2005). These clay minerals swell in water into a manner similar to smectites, and have interlayer charge densities higher than that of smectites. Unlike natural clay minerals, they have high crystallinity, controllable composition and fewer impurities. For this reasons, the use of such micas as host materials is expected to be more advantageous than the use of natural clay minerals. However, there are few studies about the chemical intercalation of hectorite, sepiolite and synthetic fluoro-micas.