Thermodynamics of binary and ternary ion exchange in zeolites: the exchange of sodium, ammonium and potassium ions in mordenite

Abstract

In many ion exchanges that involve zeolites, more than two types of exchanging ions are involved. Most work to date has been concerned with equilibria involving two different types of ion only. In this paper, a recently devised thermodynamic treatment for exchange equilibria involving three ions is tested experimentally. In § 1 the most important aspects of this thermodynamic model are discussed, and after this the experimental techniques necessary to obtain a ternary exchange isotherm are summarized. Data for three binary exchanges in synthetic mordenite are presented in §3 (the ion pairs Na-NH 4 , NA-K and K-NH 4 ) together with a ternary isotherm obtained for the simultaneous exchange of Na+, NH +4 and K + ions in the same zeolite, all data being measured at a temperature of 25 °C. After some general considerations in §4 on the nature of non-ideality in the zeolite phase and its effects on the variation of observed ion selectivities as a function of exchanger composition, the experimental data are next used to test, first, the validity, and secondly the range of applicability of numerous models found in the literature, all of which attempt to predict multicomponent exchange equilibria from binary exchange data alone. In §5 models that were developed primarily with ion exchange resins in mind are discussed (Soldatov & Bychkova 1980). In §6 approaches that attempt to predict, from binary activity data, the coefficients of ternary exchanger systems are considered, especially those that have been used with some success on clays (Elprince & Babcock 1975; Chu & Sposito 1981). All of these models are shown to be, at best, only partially successful in predicting experimental data for the comparatively simple Na-NH 4 -K-mordenite exchange system. The use of zeolites seems, therefore, to introduce additional complications. The recently developed model of Fletcher & Townsend (1981 b is next used to determine directly the exchanger phase activity coefficients 0 Na, 0 NH 4 and 0 K , and these are indeed found to vary with the exchanger phase composition in a complicated manner. Finally, in §8, the observed non-ideality of the ternary exchange system is rationalized in terms of site heterogeneity within the zeolite framework, and the consequences of this phenomenon are discussed, particularly with respect to the problems this presents in the successful prediction of multicomponent exchange phenomena, when only a limited quantity of data are available.