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Sodium-Calcium-Magnesium Exchange on Silver Hill Illite¹

ABSTRACT

Binary and ternary cation exchange reactions involving sodium (Na⁺), calcium (Ca²⁺), and magnesium (Mg²⁺) on Silver Hill illite suspended at pH 7 in a 50 mol m^–3 perchlorate background were investigated. The binary exchange experiments indicated a preference of the clay mineral for Ca and Mg over Na and for Ca over Mg. The principal ternary exchange data, on the other hand, indicated no selectivity difference between Ca and Mg in the presence of adsorbed Na at a nominal exchangeable sodium percentage (ESP) of 25. No differences among the values of the total adsorbed metal charge (Q_o) were observed in a set of binary and ternary NaM (M = Ca and/or Mg) exchange experiments carried out at the same time. There was, however, a systematic increase in the value of Q_o on a time-scale of months in Na-illite suspended in NaClO₄. The increase in Q_o was produced by adsorbed Na⁺ that could be displaced by ammonium (NH⁺₄) but not by Ca²⁺ or Mg²⁺ ions. The quantity of this "quasi-exchangeable" Na present in the principal ternary exchange experiments was determined to be about 13% of the value of Q_o, an amount concluded to be sufficient to cover the high-energy exchange sites on the illite and thereby relegate Ca-Mg exchange to sites having relatively low selectivity differences, as observed experimentally.

¹ Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521.

² Professor of Soil Science and Staff Research Associate III, respectively.

Received for publication December 17, 1984. Accepted for publication May 8, 1985.

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