Clay minerals exchange capacity

The clay mineral bentonite (sodium montmorillonite) has an excellent ion exchange and adsorption capacity. Films can be applied to electrode surfaces from colloidal clay solutions by simple dip or spin coating that become electroactive after incorporation of electroactive cations or metal particles 136-143)... 

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... 

The ion-exchange capacities discussed above (and the identification of the principal desorbing species) appear consistent with capacities of about O.U to 0.8 mequiv./gm (principal desorbing species being sodium, potassium, calcium and magnesium) reported for a related pacific red clay (7). The sorption capacities given above also appear reasonably consistent with capacities of about 0.8 to 1.5 mequiv./gm, which have been reported for related clay minerals found within the continental United States (8,9>10>12,lU). 

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