Solubility and stability of zeolites in aqueous solution; I, Analcime, Na-, and K-clinoptilolite

The solubilities of analcime and clinoptilolite were determined in dilute, weakly alkaline, aqueous solutions below 300°C and at vapor-saturated pressures. Analcimes used in this study were from Mont St. Hilaire, Quebec (an1, Si/Al = 2.02) and Wikieup, Arizona (an2, Si/Al = 2.55); clinoptilolite samples were from Castle Creek, Idaho (Si/Al = 4.50). The effects of alkali content (Na,K) on clinoptilolite solubility were determined by using cation-exchanged varieties of the Castle Creek material (cp1, cp2). In neutral to weakly alkaline solutions, the dominant solubility-controlling reactions of these zeolites are NaAlSi2O6·H2O(an1) + 5H2O(1) ⇌ Na+ + Al(OH)-4 + 2Si(OH)4(aq), Na0.85Al0.85Si2.15O6 · H2O(an2) + 5H2O(1) ⇌ 0.85Na+ + 0.85Al(OH)-4 + 2.15Si(OH)4(aq), Na1.1Al1.1Si4.9O12·3.5H2O(cp1) + 8.5H2O(1) ⇌ 1.1Na+ + 1.1A1(OH)-4 + 4.9Si(OH)4(aq), and K1.1Al1.1Si4.9O12·2.7H2O(cp2) + 9.3H2O(1) ⇌ 1.1K+ + 1.1Al(OH)-4 + 4.9Si(OH)4(aq). The logarithm of the equilibrium constants of these reactions were fit to the function: log K = A + BT + C/T + D log T. At 25°C, log K25 values for the Mont St. Hilaire analcime, Wikieup analcime, Na-clinoptilolite, and K-clinoptilolite are -16.1, -15.0, -26.5, and -28.1, respectively. These data were combined with the thermodynamic properties of the aqueous (aq) species Si(OH)4, Al(OH)-4, Na+, K+, and liquid water (1) to determine standard Gibbs free energies of formation as a function of temperature. Values of ΔG0f at 25°C and 1 bar for the Mont St. Hilaire analcime and Wikieup analcime are -3089.2 and -3044.4 kJ/mol, respectively. The ΔG0f values for hydrous Na-clinoptilolite and K-clinoptilolite, respectively, are -6267.9 and -6107.4 kJ/mol at 25°C and 1 bar. The solubility data reported here, and results obtained from previous calorimetric studies, indicate that the aluminosilicate frameworks of analcime and clinoptilolite are stabilized by an increase in Al content.