Thermodynamic Constraints on the Temperature and Pressure Dependence of Ion Exchange in Zeolites

Ion exchange reactions involving zeolites are integral aspects of fluid-rock interaction in many geologic and engineered environments. Although these reactions occur over a range of temperature and pressure, the vast majority of experimental observations of ion exchange equilibria pertain to isothermal, isobaric conditions. In an effort to mitigate this deficiency, published observations of the stoichiometries and thermodynamic properties for common ion-exchanged zeolites were compiled and synthesized. Although most studies ignore the change of water content in zeolites during ion exchange, it has little effect on calculated equilibrium constants. However, in addition to violating the First Law of Thermodynamics, this approach precludes accurate calculation of the thermodynamic properties of ion exchange reactions. Zeolite water contents were found to vary due to both cation number and size. The thermodynamics of ion exchange in zeolites thus includes contributions from both the substitution of cations and the partial de/hydration associated with the ion exchange. Comparison of the thermodynamic properties of ion exchange (enthalpy, entropy, and volume) for both hydrated and dehydrated zeolites indicates that the thermodynamic effects of water content offset those associated with exchange of ions between the aqueous phase and the zeolite. This “offset effect” may explain the generally low temperature dependence of equilibrium constants previously determined for ion exchange in zeolites. Enthalpies of reaction generated in this study were combined with estimated heat capacities of ion-exchanged zeolites to calculate the variation of equilibrium constants for zeolite ion exchange reactions as a function of temperature and pressure. The results suggest that equilibrium constants generally vary less than an order of magnitude between 298.15 and 348.15 K. The effect of temperature and pressure on equilibrium constant is typically larger for cation pairs with different charge (e.g., Ca/K) than that for reactions involving ions of the same charge (e.g., Na/K).