Abstract
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/Ca and In cKNa/Ca in 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/Ca varied in quite a similar manner as in the binary system whereas In cKK/Ca behaved 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 NK is the mole fraction of K in the exchanger phase.