Cation exchange capacity of clays

The methylene blue test can also be used to determine cation exchange capacity of clays and shales. In the test a weighed amount of clay is dispersed into water by a high-speed stirrer. Titration is carried out as for drilling muds, except that hydrogen peroxide is not added. The cation exchange capacity of clays is expressed as milliequivalents of methylene blue per 100 g of clay. 

Acidic solutions tend to dissolve carbonates and clays highly alkaline solutions tend to dissolve silica and clays. Greater pH generally increases cation-exchange capacity of clays. 

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. 

The cation-exchange capacity of clay minerals is in a range of 3 to 150 meq/lOOg (Table 9). These extremely high exchange capacities rely on two physical reasons ... 

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. 

Table 15.1. Cation Exchange Capacities of Clays and Organic Matter Found in Soils (Munson, 1980)...

MERTEL Y.A., DE KIMPE C.R. and LAVERDIERE M.R. 1976. Cation exchange capacity of clay rich soils in relation to organic matter, mineral composition and surface area. Soil Science Society of America Proceedings, 42, 764-767. 

Hedges, R.E.M. and McClennan, M. (1976). On the cation exchange capacity of fired clays and its effect on the chemical and radiometric analysis of pottery. Archaeometry 18 203-207. 

Also, the changes in pH will cause changes in the charges of variable-charge clays and organic matter, thus the cation exchange capacity of acid soils will tend to increase and that of alkaline soils decrease. 

Competitive adsorption on sepiolite clay of a monovalent dye (e.g., methyl green or methyl blue) and of the divalent organo-cationic herbicides diquat and paraquat was studied by Rytwo et al. (2002). To evaluate a possible competitive adsorption between the two organic compounds, separate aqueous solutions of each cation were used and adsorption isotherms were obtained. Fig. 8.27 shows the amount of diquat, paraquat, and methyl green adsorbed on sepiolite as a function of total added divalent cation. It may be observed that, when the added amounts were lower than the cation exchange capacity of the sepiolite (O.Mmol kg ), aU cations were completely adsorbed. 

Cation Exchange Capacity. Various techniques have been used to measure the cation exchange capacity of the clay samples. Unless otherwise noted, in computation of equilibrium quotients, we shall use a value of 0.78 equivalents/kg clay, determined by a column method (14) on the calcium form of Wyoming montmoril-lonite at pH 5. 

At high levels of i (i.e., [M+] )ex[/]), Eq. 11-55 indicates that the organic counterion concentrations must asymptotically approach a constant value set by the total surface charge density (the cation exchange capacity of the clay) as long as CEC Fvic[co-ion], ex ... 

CEC = ffsurfex surf is the cation exchange capacity of the clay (mol kg"1)... 

Figure 11.13 Adsorption isotherms for a series of alkyl ammonium compounds on sodium montmoril-lonite (adapted from Cowan and White. 1958). The horizontal dashed line indicates the cation exchange capacity of the 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). 

Ming, D. W., and Dixon, D. B. (1987). Quantitative determination of clinoptilolite in soils by a cation-exchange capacity method. Clays Clay Min., 35,463-468. 

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). 

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... 

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