Kaolinite exchange capacity

Halloysite has a chemical composition similar to kaolinite, but with a higher water content. The layers of halloysite are like those in kaolinite, but they are stacked with highly random displacements parallel to the layers, as opposed to the regular stacking found in kaolinite. The interlayer distance is greater in halloysite, allowing for the presence of a sheet of water molecules. A small ion-exchange capacity is measurable in kaolinite and halloysite minerals, which arises from a small amount of iso-morphous replacement of Si4+ or Al3+ in the framework 234). 

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.)...

In a study of Georgia kaolinites Bundy et al. (1965) stated that they believed much of the exchange capacity of kaolinite to be due to the presence of minor amounts of montmorillonite (identified by Mg content). Approximately 0.8 mequiv./lOO g was the highest exchange capacity measured for kaolinites which they believed contained no montmorillonite. This could be accounted for entirely by edge charge. 

It is assumed that some exchange is due to isomorphous substitution but this has not been proven. Schofield and Samson (1953) calculated that only one Al3+ need replace one Si4+ in 400 unit cells to afford an exchange capacity of 2 mequiv./lOOg. There is enough excess Al3+ in most kaolinites to account for 10 times this exchange capacity. Thus it appears likely that most of the excess Al3+ does not substitute in the tetrahedral sheet. The iron-rich kaolinite described by Kunze and Bradley (1964)has an exchange capacity of 60 mequiv./lOO g however, it is likely that much of this is due to the presence of iron oxides. 

Dehydrated halloysites have C.E.C. in the range of 6—10 mequiv./lOO g (Van der Marel, 1958 Garrett and Walker, 1959). Garrett and Walker have shown that the exchangable cations are located on the external surfaces of the crystals and not in the interlayer position of halloysite. Until it is possible to obtain accurate chemical analyses of the kaolinite minerals, it will be difficult to determine their exchange capacity and the source of the charge. 

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. 

There are inconsistencies in the model for the calculation of activity products for the "clays. Exchangeable cations are disregarded for the low exchange capacity kaolinite, halloysite, chlorite, and moderate capacity illite. For certain expansible layer silicates and two zeolites, the logjo of the activity of selected cations is added into the sum of the activity products. 

Of special significance with respect to their properties as sorbents are the clay minerals (e.g. kaolinite, montmorillonite, vermiculite, illite, chlorite), mainly due to their high exchange capacity. 

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. 

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