Kaolinite, cation exchange capacity

Most laboratory experiments demonstrating the utility of EO transport of organic compounds were conducted with kaolinite as the model clay-rich soil medium. Shapiro et al. (1989) used EO to transport phenol in kaolinite. Bruell et al. (1992) have shown that TCE can be transported down a slurry column by electroosmotic fluid flow, and more recently, Ho et al. (1995) demonstrated electroosmotic movement of p-nitrophenol in kaolinite. Kaolinite is a pure clay mineral, which has a very low cation exchange capacity and is generally a minor component of the silicate clay mineral fraction present in most natural soils. It is not, therefore, representative of most natural soil types, particularly those which are common in the midwestem United States. The clay content can impact the optimization and effectiveness of electroosmosis in field-scale applications, as has recently been discussed by Chen et al. (1999). 

Robertson et al. (1954) analyzed two kaolinites in detail and concluded that Fe was present in the octahedral sheet and that there was sufficient isomorphous substitution to account for the cation exchange capacity (Table LXIV). These clays were not pure and it was necessary to make a number of assumptions in order to obtain these results. 

Worral and Cooper (1966) analyzed a pure, poorly crystallized kaolinite from Jamaica (Table LXVII). The cation exchange capacity is 24.4 rrfequiv./lOO g. They suggest that substitution in the octahedral sheet is the cause of the high cation exchange capacity and may be the cause of the disorder. 

The cation exchange capacity of the kaolinite minerals is relatively low but due to... 

Fig.23 Cation exchange capacity (C.E.C.) in mequiv./lOO g of various kaolinite samples compared with their surface in m2/g. (After Van Der Marel, 1958.)...

Aluminosilicate clays (kaolinite) with a cation exchange capacity of 2.2meq/100g were blended with calcium oxide and starch prior to spray addition of the epoxide. The reaction proceeded at ambient temperature without mixing. Greater reaction efficiencies are claimed.43... 

Kahr, G., and F. T. Madsen. 1995. Determination of cation exchange capacity and surface area of bentonite, illite, and kaolinite by methylene blue adsorption. Appl. Clay Sci. 

The ideal constitution of the kaolin layer represents an electrically neutral unit, with rarely any isomorphous substitution of cations of different charges within the lattice. Consequently, kaolinite and related minerals would not be expected to show a large cation exchange capacity, and indeed this is usually the case. That a small but varying exchange capacity does occur may be attributed to two principal causes. 

Table 1.2 Cation exchange capacity of kaolinite in relation...

Aggregation of dissolved humic substances can also occur with particulate materials in the estuarine water column. Preston and Riley (1982) showed that the adsorption of riverine humic substances onto kaolinite, montmorillonite, and illite increased with increasing salinity and dissolved humic substance concentration. Adsorption increased in the order kaolinite < illite < montmorillonite, which they ascribed to increasing cation-exchange capacity of the clays. They found considerable quantitative differences between the extent of adsorption of riverine versus extracted sedimentary humic substances, indicating the importance of using materials of proper origin in experiments of this type. 

The cation exchange capacity of clays results from lattice imperfections or defects, isomorphous substitutions, and/or broken bonds on clay particle surfaces. Explain how the CEC s of kaolinite, the smectites, and illite, and their variation with pH, reflect these sources of their surface charge. 

Sakurai, K., Teshima, A., and Kyuma, K., Changes in zero point of charge (ZPC), specific surface area (SSA), and cation exchange capacity (CEC) of kaolinite and montmorillonite, and strongly weathered soils caused by Fe and Al coatings. Soil Sci. Plant Nutr, 36, 73, 1990. 

The unit formula for kaolinite has a Si/Al ratio of 1. This ratio is matched by its chemical composition, which suggests that soil kaolinites have little or no isomorphic substitution. Any differences from 1 could be due to surface coatings that were not removed during preparation of the sample. Most of the 10- to 100-mmol(+) kg-1 cation exchange capacity of kaolinite has been attributed to dissociation of OH groups at clay edges. However, if only one Si4+ of every 200 in the silica sheet were... 

Chemical Properties. An important chemical property of clays, which directly affects fines migration is the cation exchange capacity (CEC) (6-9). CEC is a measure of the capacity of a clay to exchange cations. It is usually reported in units of milliequivalents per 100 g of clay (meq/100 g). The CEC depends on the concentration of exchangeable cations in the diffuse Gouy-Chapman layer (see later). This concentration depends on the total particle charge, which may vary with pH. Unless stated otherwise, the reported values of CEC are measured at neutral pH. CEC values (meq/lOOg) of common clay minerals are as follows smectites, 80-150 vermiculites, 120-200 illites, 10-40 kaolinite, 1-10 and chlorite, <10 (10). 

Representative data for soil minerals are presented in Table 2.Ill and 2.IV, taken from the publication of Greenland and Quirk (1962). Montmorillonite has the highest surface area, ranging up to 800 m /g. This means that a 10-g sample has an area of approximately two acres or four-fifths of a hectare. This is certainly an impressive value. Kaolinites have surface areas ranging between 10 and 40 m /g, whereas illites have values intermediate between montmorillonite and kaolin. Cation exchange capacities tend to vary directly with surface areas. Of course the importance of the surface area depends upon the activity of the surface, particularly toward water. 

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