Clay minerals cation exchange capacity

Standard cations used for measuring cation exchange capacity are Na+, NHJ, and Ba2+. NH is often used but it may form inner-sphere complexes with 2 1 layer clays and may substitute for cations in easily weathered primary soil minerals. In other words, one has to adhere to detailed operational laboratory procedures these need to be known to interpret the data and it is difficult to come up with an operationally determined "ion exchange capacity" that can readily be conceptualized unequivocally. 

The clay minerals carried by rivers into the ocean represent a net annual addition of 5.2 X 10 mEq of cation exchange capacity. Most of these exchange sites are occupied by calcivun. Within a few weeks to months following introduction into seawater, sodium, potassium, and magnesium displace most of the calcium. As shown in Table 21.7, this uptake removes a significant fraction of the river input of sodium, magnesium, and potassium. 

CEC, cation exchange capacity A measure of the amount of cations that will adsorb to the negatively charged surface of a clay mineral. It is usually measured in units of meq of charge per lOOg of clay mineral. This adsorption is reversible. 

Montmorillonite An iron-rich clay mineral that has a very high cation exchange capacity. Unlike the other clay minerals, a significant amount of sedimentary montmorillonite is hydrothermal in origin. 

Lim CH, Jackson ML, Koons RD, Helmke PA (1980) Kaolins Sources of differences in cation exchange capacity and cesium retention. Qays Clay Miner 28 223-229 Low PE (1981) The swelling of clay III Dissociation of exchangeable cation. Soil Sci Soc Amer J 45 1074-1078... 

The smectite group of clay minerals is also poorly crystalline but perhaps better known because of their cation exchange capacity and their occurrence in the bentonite clays. A general formula for montmorillonite, which is one of the dioctahedral smectites is... 

Sodium bentonite with a cation exchange capacity (CEC) of 75 meq/100 g of clay, supplied by Commercial Minerals Ltd., Australia, was used as starting clay material, to prepare samples for SCD and surfactant treatments. Besides, sodium montmorillonite (Kunipia G), from Kunimine Industrial Company, Japan, was used as the starting clay for samples of pore opening modification. CEC of this clay is 100 meq/100 g of clay. 

Cation exchange capacity The ability of a solid substance (especially clay minerals) to adsorb cations. The cation exchange capacity of a material represents the total negative charge on the surface of the material and is generally expressed in milliequivalents per 100 g of material (compare with anion exchange capacity). 

Busenberg, E. and Clemency, C.V., 1973. Determination of the Cation Exchange Capacity of clays and soils using an ammonia electrode. Clays, Clay Miner, 21, 213. 

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). 

Garrett, W.G. and Walker, G.F., 1959. The cation exchange capacity of hydrated halloysite and the formation of halloysite salt complexes. Clay Miner., 4 75-80. 

Barium is also adsorbed onto soil and subsoil through electrostatic interactions (Bodek et al. 1988 Singer 1974). The cation exchange capacity of the sorbent largely controls the retention of barium in soils (Bodek et al. 1988). Barium is strongly adsorbed by clay minerals (Kabata-Pendias and Pendias 1984 Lagas et al. 1984). 

---