Multicomponent ion exchange in zeolites. Part 1.—Equilibrium properties of the sodium/calcium/magnesium–zeolite A system

Abstract

The results of a detailed study of the ternary exchange equilibrium system Na/Ca/Mg in zeolite A are discussed. A complete ternary isotherm (25 °C) is presented, together with isotherms for the three conjugate binary systems (Na/Ca-A, Ca/Mg-A and Na/Mg-A). The Na/Ca-A isotherm is compared with previously published data, and reasons for the discrepancies in the corresponding standard free energies are discussed in terms of both experimental uncertainty and interference through partial hydronium exchange. The exchange isotherm for the Ca/Mg-A system shows enormous preference for calcium over magnesium; it was only found possible to replace 1.8% of the calcium by magnesium at 25 °C. Despite many attempts, it was also found impossible to replace sodium completely by magnesium in zeolite A: a maximum level of exchange of 85% was achieved. This observation contradicts previous reports in the literature. The selectivity trends observed in the ternary Na/Ca/Mg-A system are next described quantitatively in terms of ternary and pseudo-binary separation factors, and some features of the ternary system which could not have been predicted from the binary data are highlighted. It is emphasised that separation-factor data should not be used to infer details regarding the sitings of the ions within the zeolite.