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This article in SSSAJ

  1. Vol. 44 No. 5, p. 960-964
    Received: Feb 5, 1980
    Accepted: June 19, 1980

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Sodium/Calcium Exchange in Montmorillonite and Illite Suspensions1

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

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