Clay structure

FIGURE 2.2 The smectite clay structure. (From http //www.pslc.ws/macrog/mpm/composit/nano/stmct3 l. htm, access date 4.11.06.)... 

Dehydration loss of chemically combined water and modification of clay structure... 

During the dehydration stage (between 450°C and 600°C), hydroxyl (OH ) ions in the clay are dislodged from their molecules, combine with each other to form water vapor, and are thus removed from the clay structure and released into the atmosphere. It is during this stage, as a consequence of the displacement of the hydroxyl ions, that the chemical composition and the structure of the clay are irreversibly altered and converted to fired clay. 

Clay that is present in rock and is released when the rock breaks down as a result of physical processes. These clay structures are subject to change once released from rock. 

Figure 3.3. The left structure represents kaolinite, a 1 1 clay mineral, and the right structure, a 2 1 clay mineral. These representations are intended to show surface groups, surface pairs of electrons, unsatisfied bonds, and associations between clay particles. Note that clay structures are three-dimensional and these representations are not intended to accurately represent the three-dimensional nature nor the actual bond lengths. Also, the brackets are not intended to represent crystal unit cells.

Another characteristic of 2 1 clays is isomorphous substitution, where iso means same and morphous means shape. During the formation of clay, cations other than aluminum and silicon become incorporated into the structure. In order for this to work and still ensure a stable clay, the cation must be about the same size as either aluminum or silicon, hence the term isomorphous. There are a limited number of cations that satisfy this requirement. For silicon, aluminum as Al3+ and iron as Fe3+ will tit without causing too much distortion of the clay structure. For aluminum, iron as Fe3+, magnesium as Mg2+, zinc as Zn2+, and iron as Fe2+ will fit without causing too much structural distortion (see Figure 3.4). 

The non-linear response of plastic materials is more challenging in many respects than pseudoplastic materials. While some yield phenomena, such as that seen in clay dispersions of montmorillonite, can be catastrophic in nature and recover very rapidly, others such as polymer particle blends can yield slowly. Not all clay structures catastrophically thin. Clay platelets forming an elastic structure can be deformed by a finite strain such that they align with the deforming field. When the strain... 

The nature of the interfacial structure and dynamics between inorganic solids and liquids is of particular interest because of the influence it exerts on the stabilisation properties of industrially important mineral based systems. One of the most common minerals to have been exploited by the paper and ceramics industry is the clay structure of kaolinite. The behaviour of water-kaolinite systems is important since interlayer water acts as a solvent for intercalated species. Henceforth, an understanding of the factors at the atomic level that control the orientation, translation and rotation of water molecules at the mineral surface has implications for processes such as the preparation of pigment dispersions used in paper coatings. 

Experimental lattice parameters for bulk kaolinite, together with those calculated in the static limit, are listed in Table 1. A difference in the length of parameter b of 2.9% is the largest discrepancy, which is reasonable since our calculation relates to an idealised clay structure. 

The Li ions were introduced in two different ways either before or after Zr intercalation. The montmorillonite (Weston L-Eccagun) was first exchanged with NaCl (IN) and washed. Two montmorillonites with reduced charge were prepared following the Brindley and Ertem method (13). Part of the Na+ montmorillonite was first saturated with LiCl (IN) and washed. The Li+ clay thus obtained and Na+ clay suspension were stirred for 24 hours at 25°C and dried on glass plate. The films were then heated at 220°C for 24 h in order to allow Li diffusion in the clay structure. Two different Li concentrations (F=0.4 and F=0.6) were used. The Na Li+ modified montmorillonite were dispersed in water acetone solution (1/1). The ZrOCla, 8H2O solution was added to the Na+Li+ montmorillonite (0.02g.l l Zr/Clay=5.CEC). The suspension was stirred with NaOH solution (0.1 N) up to a OH/Zr ratio of 0.5. The final pH of the suspension was 1.85. After two hours of reaction at 40°C the Zr pillared clay was washed up to constant conductivity of the solution, freeze-dried and calcined at different temperatures up to 700°C (Eni-02 and EIII-03). 

It has to be noted that the introduction of Li into the structure of the clay before pillaring and a calcination temperature lower than 3(X)°C increase the surface area of the solids. A calcination temperature higher than 5(X)°C gives amorphous solids. The Li clay structure collapses. In addition, these solids treated at 700°C present the same surface area as the Na montmorillonite. 

The Li diffusion in the clay structure slightly enhances the acidity of the Zr pillared montomorillonite as shown by the variation of the amount of desorbed NH3 We also observed a parallel decrease of the Lewis and increase of the Brdnsted sites. 

Figure 7 shows the representative bright field HRTEM images of nanocomposites of NR and unmodified montmorillonite (NR/NA) prepared by different processing and curing techniques. It is apparent that the methodology followed to prepare the nanocomposites by latex blending facilitates the formation of exfoliated clay structure, even with unmodified nanoclays. It has been reported in the literature that hydration of montmorillonite clay leads to extensive delamination and breakdown of silicate layers [94, 95]. It has also been shown that NA disperses fully into the individual layers in its dilute aqueous dispersion (clay concentration <10%)... 

Neither carbonates nor sulfates are considered nor have the various more rarely occurring salts. In general the elements in these minerals either do not enter into clay structures in appreciable quantities (Ca,... 

With small Es/clay loadings, (1/1000-1/100 of the clay s capacity, with the capacity being 10 -10 meq./mg clay) it was difficult to detect changes in the clay after 1-2 weeks by electron microscopy. However, when the clays were loaded to their capacity with Es, extensive destruction of the clay structures were noted in 2-4 days. In Figure 1 are micrographs of kaolin and attapulgite clays with and without exposure to Es. 

The conclusions reached from this preliminary study is that these clays can serve as sorbative material for the trivalent actinides and that even after damage to the clay structures, the actinide will not be released as a soluble species. 

Clays are layered crystalline materials and contain large amounts of water within and between the layers (Keller, 1985). Heating the clays above 100°C can drive out some or all of this water at higher temperatures, the clay structures themselves can undergo complex solid-state reactions. Such behavior makes the chemistry of clays a fascinating field of study in its own right. 

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