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This article in SSSAJ

  1. Vol. 44 No. 5, p. 960-964
     
    Received: Feb 5, 1980
    Published: Sept, 1980


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doi:10.2136/sssaj1980.03615995004400050017x

Sodium/Calcium Exchange in Montmorillonite and Illite Suspensions1

  1. I. Shainberg,
  2. J. D. Oster and
  3. J. D. Wood2

Abstract

Abstract

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.

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