Kaolinite surface area

More than 20 different types of clay can be actually distinguished. Those most appreciated for making ceramics, for example, kaolinite, are built up of combinations of the basic structural units described above. The particles of most consist of platelets (very small, flat sheets) that, when stacked together, form layered arrangements having extensive surface areas, much like the pages of a book. Other common clay particle shapes are fibrous or tubular. 

Limited silica fines stabilization data indicated that increasing copolymer molecular weight from 100,000 to 1,000,000 daltons had, if anything, a negative effect on silica fines stabilization. At a molecular weight of 1,000,000 daltons, this copolymer appeared to be more effective in stabilizing silica fines than silica/kaolinite, calcite, or hematite fines. However, the results may be due in part to the larger particle size and lower surface area of the silica fines (see Table II). 

Adhesive force, non-Brownian particles, 549 Admicelle formation, 277 Adsorption flow rate, 514 mechanism, 646-647 on reservoir rocks, 224 patterns, on kaolinite, 231 process, kinetics, 487 reactions, nonporous surfaces, 646 surface area of sand, 251 surfactant on porous media, 510 Adsorption-desorption equilibria, dynamic, 279-239 Adsorption plateau, calcium concentration, 229... 

Materials. Na-Kaolinite A homoionic sample of kaolinite was prepared from a well-crystallized sample purchased from Source Clays, University of Missouri, using a standardized technique (14) which involved repeated washing with distilled water and by treatment with NaCl solutions to remove exchangeable ions such as Ca, and freeze-drying of the final product. Nitrogen specific surface area of this kaolinite was estimated to be 9.4nr/g and X-ray analysis showed the characteristic pattern of kaolinite. 

In view of the problems associated with the expanding 2 1 clays, the smectites and vermiculites, it seemed desirable to use a different clay mineral system, one in which the interactions of surface adsorbed water are more easily studied. An obvious candidate is the hydrated form of halloysite, but studies of this mineral have shown that halloysites also suffer from an equally intractable set of difficulties (JO.). These are principally the poor crystallinity, the necessity to maintain the clay in liquid water in order to prevent loss of the surface adsorbed (intercalated) water, and the highly variable morphology of the crystallites. It seemed to us preferable to start with a chemically pure, well-crystallized, and well-known clay mineral (kaolinite) and to increase the normally small surface area by inserting water molecules between the layers through chemical treatment. Thus, the water would be in contact with both surfaces of every clay layer in the crystallites resulting in an effective surface area for water adsorption of approximately 1000 tor g. The synthetic kaolinite hydrates that resulted from this work are nearly ideal materials for studies of water adsorbed on silicate surfaces. 

Kaolin Minerals. The 1 1 structures include a group of aluminosilicate minerals which are termed collectively the kaolin minerals specifically these are kaolinite, dickite, nacrite, and halloysite. The basic 1 1 layer for all of these minerals has the composition AlgSigOj-fOHJj, there is a small amount of substitution of iron for aluminum, ana fluoride for hydroxyl ion. All, except halloysite, are normally anhydrous and do not expand (as do the smectites) upon exposure to water and most organic molecules. As a result, they generally have a rather small surface area, on the order of 10 nr... 

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. 

As a function of their structural properties, clays interact differently with organic and inorganic contaminants. Two major groups of clay minerals are selected for discussion here (a) kaolinite, with a 1 1 layered structured aluminosilicate and a surface area ranging from 6 to 39 m g" (Schofield and Samson 1954) and (b) smectites with a 2 1 silicate layer and a total surface area of about 800m g" (Borchardt 1989). 

Fig. 10.12 The effect of increasing amounts of goethite (surface area 51 m g ) on the electrophoretic mobility of kaolinite at various pH. The figures on the curves indicate the amount of goethite added (mg g ) (Venema and Glasauer, unpubl.).

No interaction between ferrihydrite and kaolinite was found at pH 9 because both compounds are negatively charged at this pH (Fig. 16.20b, c). Boiling kaolinite and montmorillonite in a Fe(N03)3 solution for 8 min resulted in clays containing up to ca. 55 mg oxalate soluble Fe/g clay. The BET surface area of kaolinite increased from 18 to 34 m /g and that of montmorillonite from 11 to 62 m g . Whereas kaolinite shows only a small decrease in > 10 pm pores, montmorillonite lost about half of its >10 pm pores even with the lowest Fe oxide content (6.6 mg ECq g clay). It has been speculated that in contrast to kaolinite, the Fe oxide, in the presence of montmorillonite, remained highly disorderd and active due to A1 and Si dissolved from... 

Table 11.2 Adsorption of Nonionic Nitroaromatic Compounds (NACs) to Aluminosilicate Clays (a) Surface Area Factors,/saf, for Different Clays Expressing Maximum Sorption Sites Relative to Kaolinite, and (b) KNAC EDA Values (L- mol 1 sites) Measured for Several NACs on K+-Kaolinite Allowing Estimates of KNACd Values Due to Electron Donor-Acceptor Interactions (Eq. 11-20) ...

Fig.21. The relation of surface area to percent H-O of thirteen kaolinites. (Data from Van Der Marel, 1958.)...

One of the major differences in the reported chemical composition of the kaolinite minerals is in the H20+and H20 values. In part, these variations may be real but many must be due to the presence of halloysite and other impurities, variation in grain size and surface area, and in the methods of dehydration. H20 increases linearly with increase in surface area and with decrease in grain size. 

With kaolinite and non swelling materials, the external surface area obtained from xfe was in good agreement with the BET (N2) surface area. 

The adsorption isotherms in Figure 11.3 are of interest for several reasons. First, it may seem surprising that an assemblage of kaolinite platelets should give a reversible isotherm. The adsorbent had a specific surface area of 17 m2g-1, which would appear to correspond to a platelet thickness of c. 50 nm. The particle rigidity and the house-of-cards packing have probably resulted in the formation of a macro-porous aggregate, which accounts for the appearance of the reversible Type II isotherm. 

In an early study of the effect of calcination on the surface area of kaolinite, Gregg and Stephens (1953) found a small but progressive decline in the BET area over the temperature range 100-800°C. These results were in contrast to a 12% loss of structural water at 450°C. It was concluded that there was no detectable activation and that the crystallite structure was not broken up as a result of theimal dehydroxylation. 

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