Quaternary Cation Exchange on Silver Hill Illite

The simultaneous exchange of Na, K, Ca, and Mg on Silver Hill illite suspended in 5 mol m −3 perchlorate background at pH 7 was investigated systematically. Composition data for both the exchanger and the equilibrium aqueous solution phase were obtained directly. The data showed that the total adsorbed metal charge was independent of exchanger composition provided that all four cations were considered. Analysis of the composition data with exchange isotherms and calculated Vanselow selectivity coefficients showed that: (i) K + was preferred over Na + , Ca 2+ , and Mg 2+ , and the preference decreased as E K decreased ( E = charge fraction on the clay mineral); (ii) bivalent cations were preferred only slightly over Na + for E Na > 0.01, whereas for E Na ≤ 0.01, the reverse was true; (iii) no preference was shown in Ca‐Mg exchange at E Na values between 0 and 0.13.