Ca-K-Na exchange equilibria on a smectitic soil: Modeling the variation of selectivity coefficient

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 ^cK_Mg/Ca, slightly scattered around an average of −0.553. The values of In ^cK_K/Caand In ^cK_Na/_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, ^cK_K/Cavaried in quite a similar manner as in the binary system whereas In ^cK_K/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 ^cK_K/Ca=−4.271.N_K,+4.220, In ^cK_Na/K=2.417.N_K,−2.742, and In ^cK_K/Ca−In ^cK_Na/Ca+ 2ln ^cK_Na/K= 0, where N_Kis the mole fraction of K in the exchanger phase.