Three-layer clays

Sorption depends on Sorption Sites. The sorption of alkaline and earth-alkaline cations on expandable three layer clays - smectites (montmorillonites) - can usually be interpreted as stoichiometric exchange of interlayer ions. Heavy metals however are sorbed by surface complex formation to the OH-functional groups of the outer surface (the so-called broken bonds). The non-swellable three-layer silicates, micas such as illite, can usually not exchange their interlayer ions but the outside of these minerals and the weathered crystal edges ("frayed edges") participate in ion exchange reactions. 

In terms of composition, the simplest of the marine clay minerals is kaolinite in which tetrahedral and octahedral layers alternate (Figure 14.5a) creating a two-layer repeating imit. In three-layer clays, the repeating unit is composed of an octahedral... 

The CEC of clay minerals is partly the result of adsorption in the interlayer space between repeating layer units. This effect is greatest in the three-layer clays. In the case of montmorillonite, the interlayer space can expand to accommodate a variety of cations and water. This causes montmorillonite to have a very high CEC and to swell when wetted. This process is reversible the removal of the water molecules causes these clays to contract. In illite, some exchangeable potassium is present in the interlayer space. Because the interlayer potassium ions are rather tightly held, the CEC of this illite is similar to that of kaolinite, which has no interlayer space. Chlorite s CEC is similar to that of kaolinite and illite because the brucite layer restricts adsorption between the three-layer sandwiches. 

Fig. 77. The arrangement of layers of water molecules (dashed lines) in hydrated montmor-illonites and the corresponding interlayer spacings in (a) a one-layer clay (b) a two-layer clay (c) a three-layer clay. The spacing adopted by a particular montmorillonite depends on the nature of the silicate layer and on the exchangeable cation as well as on certain other conditions of intercalation (e.g., relative humidity and temperature) (470).

The meaning of one-, two-, and three-layer clays is best illustrated diagrammatically (see Fig. 77). Layers in this context refer to the interlamellar water, though the precise chemical nature of this entrained water is not easily established and is, in any case, a function of the parent silicate. In some sheet silicates the water is believed to take up an ice-like monolayer. Recent studies reveal that the interlamellar ion and associated water are rather mobile above room temperature. Such water is readily, but not always... 

The three-layered clay mineral montmorillonite (bentonite) is characterised by a low-hydraulic conductivity and a capacity to bind water molecules and positively charged ions (cations). As such, water-saturated compacted bentonite powder is used as a hydrological barrier in areas such as waste disposal, for example around land-fill sites where the desire is to prevent leakage of contaminants from the land-... 

Three-layer clays (2 1 clays, TOT) An A10(0H) octahedral sheet is between two tetrahedral Si04 layers. The distance of the layers can be varied (—0—0—bonds), so the layers can be expanded. 

Montmorillonite has some important characteristics that justify its use as a model substance for the study of the interfacial processes of rocks and soils. It is a dioctahedral three-layer clay (2 1 clays, TOT) an A10(0H) octahedral sheet is between two tetrahedral Si04 layers (Chapter 1, Table 1.2). The distance between the layers is not fixed (—O—O-bonds) the layers can be expanded. Because of the layered structure, it has two surface types external and internal surfaces. The external surface is the surface of the particles (edge surface), and its size depends on particle size distribution. Its area can be measured by the BET method, usually by the adsorption of nitrogen gas at the temperature of liquid nitrogen (Chapter 1, Section 1.1.3). The internal surface is the surface between the layers (interlayer surface), and its size can be determined by introducing substances into the interlayer space (e.g., water) (Chapter 1, Section 1.1.3). The internal surface area is independent of particle size distribution. 

Clay Colloids. Three clay minerals are important components of the clay colloid fraction of soils, namely, montmorillonite, illite, and kaolinite (Adams, 1973). Mont-morillonite consists of one layer of aluminum oxide between two layers of silicon oxide (Figure 11.2). An important feature of this mineral is its multilayer arrangement, which permits smaller molecules such as pesticides to penetrate between them. This is referred to as an "expanding lattice" clay. Illite is also a three-layer clay but it does not form multilayers. Kaolinite is a two-layer mineral of aluminum oxide and silicon oxide. 

Lippmann, F. The solubility products of complex minerals, mixed crystals, and three-layer clay minerals. N. Jb. Miner. Abs. 130, 243-263 (1977). 

Although no detailed theory on the sorption mechanism of heavy metals on three-layer clays is available, many inferences imply the specific coordinative interaction of end-standing hydroxo groups with nonhydrolyzed metal ions. 

As mixed-layer I/S clays become smectite-rich, their ion exchange and swelling properties approach those of the pure smectites. Other three-layer clays, including the chlorites and vermiculites, also commonly occur in soils in mixed-layer form (e.g., mixed-layer chlorite-smectite [-vermicu-lite]) (cf. Wilson and Nadeau 1985 Drever 1988). 

Cation substitutions frequently occur within the sheet structure. For example, silicon in the tetrahedral sheet can be replaced with aluminum and the aluminum in the octahedral sheet can be replaced with magnesium or ferrous ion. These isomorphic substitutions cause a charge imbalance within the sheet that is balanced by adsorption of cations between the layers. This is the origin of most charge density in three-layer clays. 

The primary rock-forming silicate minerals in terrestrial rocks can react with liqnid water to form various two-layer and three-layer clays (Faure 1998). For example, Na-plagioclase (i.e., albite) is transformed into kaoUnte by the reaction ... 

A schematic presentation of the atom arrangements in a unit cell for three-layer clay such as MMT is shown in Figure 15.1. 

Structurally, clays can be classified as either two-layer clays in which oxygen atoms are shared between a tetrahedral sheet and an adjacent octahedral sheet, and three-layer clays in which an octahedral sheet shares ojg gen atoms with tetrahedral sheets on either side. These layers composed of either two or three sheets are called unit layers. A unit layer of a two-layer clay typically is around 0.7 nanometers (nm) thick, whereas that of a three-layer clay exceeds 0.9 nm in thickness. The structure of the two-layer clay kaohnite is represented in Figure 17.4. Some clays, particularly... 

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