Thermodynamics of hydration of Na- and K-clinoptilolite to 300 °C

Abstract

The hydration state of Na- and K-exchanged clinoptilolite from Castle Creek (Idaho, U.S.A.) has been measured by a pressure titration method to 300 °C and P _H2O<30 bars. The water content of clinoptilolite can be predicted as a function of water activity and temperature with the equation: a _H2O = [exp[[−ΔH _h ^∘/nRT] + [ΔS _h ^∘/nR] − 1/nRT· [W₁ X _h + W₂ X _{h 2}]− ln(X _a/X _h)]]⁻¹ where T is degrees in Kelvin, ΔH _h ^∘ is the standard molal enthalpy of hydration, ΔS _h ^∘ is the entropy of hydration, X _h and X _a are, respectively, the mole fractions of the hydrous and anhydrous components of the solid solution, W ₁ and W ₂ are interaction parameters, n is the maximum number of moles of H₂O per formula unit (based on 12 oxygens), and R is the gas constant. This equation can be used to locate clinoptilolite-H₂O isohydrons in a _H2O-T space below the liquid-vapor equilibrium curve of water. The standard molal Gibbs free energy of hydration is −47.62 ± 5.52 kJ/mol H₂O and −5.40 ± 2.71 kJ/mol H₂O for the Na- and K-clinoptilolite, respectively. These standard-state thermodynamic properties of clinoptilolite hydration are in good agreement with previous data at low H₂O pressures. The experiments indicate that clinoptilolite progressively dehydrates with increasing temperature at pressures along the liquid-vapor equilibrium curve. Kinetic data above 150 °C show that clinoptilolite dehydration and hydration reactions are fast and reversible and that steady-state hydration states are attained in minutes.