We conducted binary and ternary cation exchange experiments on a smectitic soil and calculated the Vanselow selectivity coefficients. The values of the natural logarithm of the Vanselow selectivity coefficient for Ca→Mg exchange, In cKMg/Ca, slightly scattered around an average of −0.553. The values of In cKK/Caand In cKNa/Cain respective binary systems, on the other hand, significantly varied as the mole fraction of K in the exchanger phase increased. In the Ca-K-Na ternary system, cKK/Cavaried in quite a similar manner as in the binary system whereas In cKK/Cabehaved differently. Based on these observations, we modeled the variation of the selectivity coefficients using the data from ternary exchange experiments. The obtained equations are: In cKK/Ca=−4.271.NK,+4.220, In cKNa/K=2.417.NK,−2.742, and In cKK/Ca−In cKNa/Ca+ 2ln cKNa/K= 0, where NKis the mole fraction of K in the exchanger phase.
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