Sodium/Calcium Exchange in Montmorillonite and Illite Suspensions1
- I. Shainberg,
- J. D. Oster and
- J. D. Wood2
The effect of salt concentration and exchanger composition on Na/Ca exchange in montmorillonite and illite suspensions (0.02 to 0.08 g clay/g water) was studied, where the equivalent fraction of exchangeable Ca, ECa, ranged from 0.6 to 1. A theoretical evaluation based on double-layer theory for montmorillonite indicates that the formation of tactoids and internal surfaces at the Ca3+ end of the exchange isotherm can cause the Vanselow selectivity coefficient, KV, to depend on exchangeable cation composition and total electrolyte concentration. Whereas for montmorillonite, KV increased with ECa (0.6 < ECa < 1), as predicted for internal surfaces, the reverse was true for illite, where external surfaces predominate. Likewise, for a given Na adsorption ratio, dilution of the equilibrium solution for illite (0.6 < ECa < 1) and for montmorillonite (ECa ≃ 0.6) decreased KV as predicted for external surfaces.
The average Gapon selectivity coefficient for the montmorillonite (0.011) and illite clays (0.023) with ECa ≤ 0.8, suggests that the value for soils, (0.0148) reflects their mixed mineralogy.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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