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Calcium-Magnesium Exchange on Illite in the Presence of Adsorbed Sodium¹

ABSTRACT

Binary and ternary cation exchange experiments were performed with Na⁺, Ca²⁺, and Mg²⁺ on Silver Hill illite and on a soil illite suspended in 50 mol m^–3 perchlorate at pH 7. The binary experiments indicated no preference for Ca over Mg on the specimen illite, but selectivity for bivalent cations over Na⁺ and for Ca²⁺ over Mg²⁺ on the soil illite. The ternary exchange data showed that, for both illites, there was no effect of adsorbed Na on the Ca-Mg exchange isotherm at exchangeable sodium percentages from 0 to 30%, either for freshly-prepared clay suspensions or for those aged up to 270 d in NaClO₄. No significant differences in the total adsorbed metal charge (Q_o) were observed among the binary and ternary exchange experiments, and Q_o was independent of the degree of bivalent cation saturation (E_Ca or E_Mg) in the Na Ca and Na Mg exchange experiments on the soil illite. The Vanselow selectivity coefficient for Mg Ca exchange on the soil illite decreased as E_Ca increased, showing a plateau near E_Ca 0.6 and dropping to 1.0 as E_Ca became equal to 1.0. This behavior was interpreted as evidence for at least two classes of surface complex involving Ca and Mg.

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

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

Received for publication August 12, 1985.

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