Zeolite RHO. Part 2.—Cation exchange equilibria and kinetics

Abstract

Exchange kinetics were measured in zeolite RHO for 2NH⁺₄→ Ca²⁺, → Ca²⁺→ 2NH⁺₄, 2NH⁺₄→ Ba² and Ba²⁺→ 2NH⁺₄, all at 23°C; and for 2NH⁺₄→ Ca²⁺ at 80°C. Exchange isotherms were determined at 80°C for the pairs NH⁺₄⇌ Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺; Na⁺⇌ Cs⁺ and Ag⁺; and 2NH⁺₄⇌ Ca²⁺, Sr²⁺ and Ba²⁺. In addition, the isotherms for NH⁺₄⇌ Na⁺ and Cs⁺ were measured at 23°C.

From the kinetic measurements diffusion coefficients were obtained and for all reversible isotherms thermodynamic equilibrium constants were evaluated. From the triangle rule equilibrium constants were then obtained for other combinations of the reversible exchange pairs. Kielland plots were determined and activity coefficients of the end members of the exchange (i.e., the homoionic forms) when admixed as the zeolitic solid solutions were evaluated as functions of the cationic composition. Trends in these curves for uni-univalent exchanges could be related to the differences between the ionic radii of the exchanging pairs. The thermodynamic affinity sequence for ions entering the zeolite was Cs⁺ > Rb⁺ > K⁺, Ag⁺ > NH⁺₄ > Na⁺ > Li⁺ and Ba²⁺ > Sr²⁺ > Ca²⁺ for uni- and divalent ions, respectively.