Stability and Metastability in Zeolite Ion Exchange Reactions: The Case of Clinoptilolite and Heulandite

Ready ion exchange exhibited by many zeolites enjoys widespread application in a number of passive and engineered applications for the treatment and modification of aqueous solutions. Often, the range of exchangeable ion contents achievable experimentally exceeds that observed in nature. A salient example of this phenomenon is manifested by the isostructural zeolite species heulandite and clinoptilolite. Although both are readily exchanged over the whole compositional range between homoionic Na-, K-, and Ca-forms, in nature the more silica-rich species clinoptilolite exhibits a much wider range of exchangeable cation contents than does the more alumina-rich species heulandite. Natural heulandites are restricted to Ca-rich compositions, rarely exhibiting mole fractions of monovalent cations greater than 0.5, whereas clinoptilolites have mole fractions of monovalent cations that range from essentially 0 to 1. In order to elucidate the cause of this phenomenon, thermodynamic calculations of the stability of heulandite-clinoptilolite solid solutions were conducted. These calculations were based on the published solubility, calorimetric, and ion exchange experiments along with estimated thermodynamic properties of Si-Al exchange based on observations in other zeolite systems. The calculations indicate that Na-and K-rich heulandites are metastable with respect to aqueous solutions in equilibrium with albite-quartz and K-feldspar-quartz bearing assemblages, respectively. This indicates that experimentally-obtained monovalent cation rich heulandite compositions are themselves metastable and a consequence of the more ready kinetics of ion exchange relative to phase transformation. At elevated silica activities, Na- and K-rich clinoptilolite is stable relative to alkali feldspars and may persist as a metastable phase provided quartz saturation is not achieved. These results explain the discrepancy between experimentally-obtained compositions and observations of natural parageneses. Application of zeolite ion exchange for long-term applications such as radioactive waste repositories requires consideration of exchanged zeolite stability relative to phase assemblages buffered by diagenetic silicates containing ions involved in proposed exchange reactions.