Water-expandable clays

A water-expandable material based on bentonite clay and polyacrylamide is added to the circulating drilling solution [118]. The material expands in water to 30 to 40 times its initial volume. The swelling takes place within 2 to 3 hours. During the circulation of the drilling solution, the material enters the cracks and... 

An understanding of much of aqueous geochemistry requires an accurate description of the water-mineral interface. Water molecules in contact with> or close to, the silicate surface are in a different environment than molecules in bulk water, and it is generally agreed that these adsorbed water molecules have different properties than bulk water. Because this interfacial contact is so important, the adsorbed water has been extensively studied. Specifically, two major questions have been examined 1) how do the properties of surface adsorbed water differ from bulk water, and 2) to what distance is water perturbed by the silicate surface These are difficult questions to answer because the interfacial region normally is a very small portion of the water-mineral system. To increase the proportion of surface to bulk, the expanding clay minerals, with their large specific surface areas, have proved to be useful experimental materials. 

Based on the study of expanding clay minerals, two models of water adsorbed on silicate surfaces have been proposed. One states that only a few layers (<5) of water are perturbed by the silicate surface, the other concludes that many layers (perhaps 10 times that number) are involved. The complexity of the interactions which occur between water molecules, surface adsorbed ions, and the atoms of the silicate mineral make it very difficult to unequivocally determine which is the correct view. Both models agree that the first few water layers are most perturbed, yet neither has presented a clear picture of the structure of the adsorbed water, nor is much known about the bonding of the water molecules to the silicate surface and to each other. 

Our approach has been to study a very simple clay-water system in which the majority of the water present is adsorbed on the clay surfaces. By appropriate chemical treatment, the clay mineral kao-linite will expand and incorporate water molecules between the layers, yielding an effective surface area of approximately 1000 m2 g . Synthetic kaolinite hydrates have several advantages compared to the expanding clays, the smectites and vermiculites they have very few impurity ions in their structure, few, if any, interlayer cations, the structure of the surfaces is reasonably well known, and the majority of the water present is directly adsorbed on the kaolinite surfaces. 

The concept zeolites conventionally served as the synonym for aluminosilicates with microporous host lattice structures. Upon removal of the guest water, zeolites demonstrate adsorptive property at the molecular level as a result they are also referred to as molecular sieves. Crystalline zeosils, AlPO s, SAPO s, MAPO s (M=metal), expanded clay minerals and Werner compounds are also able to adsorb molecules vitally on reproval of any of the guest species they occlude and play an Important role in fields such as separation and catalysis (ref. 1). Inclusion compounds are another kind of crystalline materials with open framework structures. The guest molecules in an inclusion compound are believed to be indispensable to sustaining the framework structure their removal from the host lattice usually results in collapse of the host into a more compact crystal structure or even into an amorphous structure. 

Expanding clays clays that expand or swell on contact with water, e.g., mont-morillonite. 

The material from the Hector area of California is believed to have formed by the action of hot spring waters containing Li and F on clinoptiolite. The Mg was obtained from the alkaline lake waters (Ames and Goldich, 1958). The material from Morocco is associated with marls and is believed to be authigenic. These two types of trioctahedral smectite appear to be the only ones with a relatively pure Si tetrahedral sheet. No analyses were found which indicated tetrahedral Al values between 0.02 and 0.30. Analyses of saponite indicate there is complete isomorphous substitution between the range Si3.70 Al0.3o and Si3.0s Al0.92 (Table XXXIX). Caillere and Henin (1951) reported an analysis of a fibrous expanded clay (diabantite) which had a tetrahedral composition of Si3.i7 Alo.49 Fe3+0.34. There is some question as to whether this should be classified as a smectite regardless, it indicates the possibility of Fe3+ substitution in the tetrahedral sheets of the trioctahedral 2 1 clays. 

Much of the derived expanded clay, even that which resembles montmorillonite (holds two layers of ethylene glycol), will contract to 10 A when exposed to a potassium solution. Weaver (1958) has shown that these clays can obtain sufficient potassium from sea water and readily contract to 10 A. Vermiculite and mixed-layer biotite-vermiculites are rare in marine sedimentary rocks. Weaver (1958) was unable to find any expandable clays in marine sediments that would contract to 10 A when treated with potassium. A few continental shales contained expanded clays that would contract to 10A when saturated with potassium. Most vermiculites derived from micas and illites have high enough charge so that when deposited in sea water they extract potassium and eventually revert to micas and illites. Some layers may be weathered to such an extent that they do not have sufficient charge to afford contraction and mixed-layer illite-montmorillonites form. 

Clays in Water (Penetrating Liquid). The apparent densities of the expanding clays in water are higher than those of the same clays in hydrocarbon liquids. For example, the apparent density of sodium Wyoming bentonite in water is 2.793 grams per cc. (four determinations, standard deviation 0.008 gram per cc.). 

Cations based on substituted silsesquioxanes have also been intercalated [7]. On heating the ion-exchanged clay minerals water is evolved, but oxycation or oxide pillars keep the siliceous layers apart. These materials have enhanced thermal stability compared with clay minerals expanded with organic cations. Expanded clay minerals cover at least as wide a range of accessibilities to the interlamellar micropore spaces as the zeolites, but the pore characteristics of clay minerals with inorganic pillars need more detailed investigation. 

To quantitatively model reaction kinetics of geochemical systems, reliable estimates of the physical and reactive surface areas of the system are needed. The physical surface areas have been measured on the basis of either the macroscopic nature of the surface, i.e. estimates of its bulk geometry, or the microscopic nature, i.e. the areal extent of coverage by atoms or molecules, as in the BET method. In the latter case, comparisons with water sorption isotherms indicate that BET-determincd surface areas produce reliable estimates of the mineral/water interface, except for materials with high microporosity such as expandable clays. 

Studies by Mortland et al. (34) suggested that residual water on the interlayer surfaces of expanding clay minerals is dissociated more... 

Clay adsorbents are modified by combining organic substances, produced synthetically, and these inorgano-organo clays are of vital importance to remove pollutants from industrial waste water. The basic material of inorganic clay phase is varieties of expanding clays, like montmorrilonite, bentonite etc. 

Eikebrokk, B. and Saltnes, T. (2002). NOM removal from drinking water by chitosan coagulation and filtration through lightweight expanded clay aggregate filters, 7. Water Supply Res. Technol. Aqua. 51, 323. 

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