Experimental determination of thermodynamic properties of ion-exchange in heulandite: Binary ion-exchange experiments at 55 and 85 C involving Ca2+, Sr2+, Na+, and K+

Abstract

Heulandite is a common rock-forming zeolite that exhibits wide solid solution of extraframework cations, presumably due to ready ion exchange with aqueous solutions. In order to provide a quantitative basis for interpreting and predicting the distribution of aqueous species between heulandite and aqueous solutions, ion exchange equilibrium between heulandite and aqueous solutions with respect to the binary cation pairs Ca2+ –K+, Ca2+ –Na+, K+ –Na+, K+ –Sr2+, Na+ –Sr2+, and Ca2+ –Sr2+ was investigated. Homoionic Ca-, K-, and Na-heulandites prepared from natural heulandite were equilibrated with 0.1 N Cl− solutions containing various proportions of the cations in a given binary pair at 55 and 85 °C to define isotherms describing partitioning of the cations over a wide range of heulandite and solution composition with respect to the cations in each pair. In general, the experiments equilibrated rapidly, within 11 to 15 weeks at 55 °C and 3 to 4 weeks at 85 °C. The exception was the Ca2+ –Sr2+ binary exchange, which did not equilibrate even after 3 months at 55 °C and 4 weeks at 85 °C. Slow exchange of Sr2+ for Ca2+ also prohibited preparation of homoionic Sr-heulandite from the natural (Ca-rich) heulandite within 10 weeks in 2N SrCl2 solution at 90 °C, although near homoionic Sr-heulandite was produced by exchange of K- and Na-heulandite. Experimentally determined isotherms were used to derive equilibrium constants for the ion exchange reactions and asymmetric Margules models describing the extent of non-ideality in extraframework solid solutions in heulandite. Regressed equilibrium constants for Ca2+-Na+, Ca2+-K+, and K+-Na+ binary cation pairs at 55 °C are internally consistent among each other (complying with the triangle rule), indicating good accuracy of these data. The maximum departure from internal consistency among the equilibrium constants for three binary pairs was 900 J per mole of charge equivalents (eq) for the 55 °C experiments and 2300 J eq−1 for the 85 °C experiments. The applicability of the present experimental results and thermodynamic models was assessed by calculating the composition of heulandite in Icelandic geothermal systems from known compositions using the regressed thermodynamic properties of Ca2+-Na+ exchange at 85 °C. Calculations predict an average Ca mole fraction [defined as Ca/(Ca+Na)] in heulandite of 0.74, in excellent agreement with observed compositions of heulandite from geothermal and metamorphic systems in Iceland (0.75). Thermodynamic data for heulandite ion-exchange derived in this study can be used to predict partitioning of Ca, K, Na, and Sr between heulandite and aqueous solutions in geologic systems. Because heulandite is the most effective sink for Sr in basaltic aquifers that have undergone zeolite facies metamorphism, the experimental results of this study will provide essential data for modeling Sr transport in aquifers in low-grade metabasalts.