GRANTEE: THE PENNSYLVANIA STATE UNIVERSITY
Ore Deposits Research Section
University Park, Pennsylvania 16802
GRANT: DE-FG02-96ER14634
TITLE: Zeolite Thermodynamics and Kinetics
PERSONS IN CHARGE: H. L. Barnes (814-865-7573; Fax 814-863-2001; E-mail barnes@geosc.psu.edu) and R.T. Wilkin (814-865-3565; Fax 814-863-2001; E-mail rwilkin@geosc.psu.edu)
Objectives: The goals of this project are to measure the thermochemical properties and evaluate the kinetic factors that govern dissolution, crystallization, and hydration of zeolites in hydrothermal environments. This research has direct applications to diagenesis, low-grade metamorphism, and repository behavior. The experimental data characterize environments where zeolites are used as sorbtive barriers to migration of contaminants, and industrial settings where zeolites are used as catalysts, cation exchangers and molecular sieves. In each of these applications, there is a need for quantitative data on the kinetic and thermodynamic stabilities of zeolites.
Project Description: The project includes several types of both closed- and open-system experimental investigations on high-purity, natural zeolites. Closed-system experiments involve measuring zeolite solubilities up to about 300ºC and to about 200 bars. Reversible solubility measurements are made on analcime and cation-exchanged varieties of clinoptilolite and mordenite. Resulting thermodynamic data are acquired for Na-, K-, Ca-, and Mg-end-member compositions with fixed water contents and Si/Al ratios. The solubility measurements provide equilibrium concentrations so that rates of dissolution and precipitation can be correlated with states of undersaturation and of supersaturation, respectively.
Rates of congruent zeolite-water reactions are measured using a hydrothermal flow-through system. The flow-through system is advantageous because reaction rates can be measured under conditions of fixed fluid composition, flow rate, temperature, and pressure. An experimental system has been constructed to measure the hydration state of zeolites at high temperatures and at water vapor pressures approaching the liquid-vapor curve of pure water. Zeolite hydration states are determined by evaluating the partitioning of water between water vapor and zeolite hydration water for isovolumetric systems at each temperature and pressure.
In addition to providing basic thermodynamic and kinetic parameters, the experimental data permit kinetic evaluations of reactions among zeolites, for example, clinoptilolite-Analcime-albite. Although the mechanisms of zeolite reactions remain equivocal, such reaction paths define the transition from zeolite to greenschist metamorphic regimes and are characterized by the release of water and silica and large, negative molar volume changes.
Results: During the past year, research efforts centered on (1) the measurement of analcime-water reaction rates, (2) both the design and construction of an experimental system to measure zeolite hydration states and its use in measuring the hydration state of Na- and K-clinoptilolite, (3) solubility measurements of Ca- and Mg-exchanged clinoptilolite, and (4) the collection, preparation, and characterization of mordenite specimens suitable for solubility and kinetic experiments.
(1) Rates of analcime (Mont St. Hilaire) dissolution and precipitation were measured at 125º, 175º, 225º, and 250ºC at pH ~7.7. At each temperature, reaction rates were determined over a wide range of saturation states from supersaturated (0 to 1.2 kcal/mol), to near-equilibrium, to far-from-equilibrium conditions (0 to -8 kcal/mol). Close to equilibrium (0>DGr>-4 k cal/mol), analcime dissolution rates increase with increasing degree of undersaturation. Further from equilibrium (DGr<-4 k cal/mol), dissolution rates appear to be constant and independent of saturation state. Maximum dissolution rates of analcime at 125º, 175º, 225º, and 250ºC are -3.10x10-10, -2.52x10-9, -7.48x10-9, and 1.52x10-8 mol/m2sec, respectively. At constant temperature and saturation state, dissolution rates depend slightly on the ratio of dissolved Si to Al. The temperature dependence of analcime-water reaction rates generally follows Arrhenius behavior. Based on the temperature-dependent rate constants for analcime dissolution, the Arrhenius model gives an apparent activation energy of 11.7±1.5 kcal/mol. An apparent activation energy of 8.8 kcal/mol is obtained for the analcime precipitation reaction based on measurements at 125º and 175ºC.
(2) Three intensive parameters, water vapor pressure, temperature, and water adsorption capacity are necessary to define equilibrium in the clinoptilolite-water system. A model to compute the amount of zeolitic water, at any pressure and temperature, is essential in thermochemical calculations involving these hydrated minerals. Results of experiments indicate that the clinoptilolite hydration/dehydration reaction is reversible and fast. In the presence of liquid water, clinoptilolite progressively dehydrates with increasing temperature. For example, the hydration states of Na- and K-clinoptilolite decrease by about 50% as temperature increases from 25ºC to 250ºC and at water vapor pressures along the water-vapor curve of pure water. The experimental method is promising for other zeolites that contain >10 wt% H2O, such as mordenite, mesolite, thomsonite, laumontite, and phillipsite.
(3) Hydrothermal solubilities have been measured of Ca- and Mg-exchanged clinoptilolite. Solubility experiments were carried out between 75º and 265ºC and at pressures along the liquid-vapor curve of the aqueous solutions. At the measured pH of all solutions sampled, aqueous Al and Si species are dominantly Al(OH)4- and Si(OH)4, respectively. The logarithm of the equilibrium constant, log K, for the reaction:
Ca0.55Al1.1Si4.9O 12(3.8H2O + 8.2H2O]
0.55Ca2+ + 1.1Al(OH)4- + 4.9Si(OH)4
varies from -25.2 at 75ºC to -19.3 at 265ºC, and for the Mg-exchanged clinoptilolite from -21.7 at 125º to -19.0 at 225ºC. These solubility products were obtained with bracketing from both supersaturation and from undersaturation; they are precise to within ±0.3 log units. The largest effects on the free energy of formation of zeolites are contributed from the aluminosilicate framework and the water content of the exchanged clinoptilolite.
GRANTEE: THE PENNSYLVANIA STATE UNIVERSITY
College of Earth and Mineral Sciences
University Park, Pennsylvania 16802
GRANT: DE-FG-02-95 ER 14547.A000
TITLE: Dissolution Rates and Surface Chemistry of Feldspar Glass and Crystal
PERSONS IN CHARGE: Susan L. Brantley (814- 863-1739; Fax 814- 863-7823; E-mail brantley@geosc.psu.edu) and Carlo G. Pantano (814- 863-2071; Fax 814-865-0016; E-mail pantano@ems.psu.edu)
Objectives: This project aims to evaluate the surface chemistry and dissolution behavior of feldspar crystal and glass under laboratory and natural conditions.
Project Description: Feldspar, the most common mineral in the earth's crust, dissolves into soil and subsurface waters during weathering and diagenesis. The rates of feldspar dissolution have been measured in the laboratory, but these rates have been shown to be consistently faster, by several orders of magnitude, than rates measured in the field. In order to understand this discrepancy, better quantification of field and laboratory rates and the surface chemistry of crystals dissolved under field and laboratory conditions is needed. Furthermore, a better understanding of the mechanism of dissolution is needed in order to establish models for feldspar-water reaction. To address these questions, three projects have been pursued. In the first project, the surface chemistry and dissolution rate of albite was measured in a Pennsylvania soil. In the second project, dissolution rates of feldspar from the Cape Cod aquifer were measured in the laboratory and in the field. In the third project, the surface chemistry and rate of dissolution of albite crystal and glass was measured under various conditions of pH in static and flow-through experiments.
Results: In investigating the surface chemistry of albite weathered in a Pennsylvania soil, very little evidence for dissolution was visible using electron microscopy (EM); however, under atomic force microscopy (AFM), etched exsolution lamellae were clearly visible. Using x-ray photoelectron spectroscopy, depletion in Al and Na was documented in the surfaces of the albites weathered 6 months, but after 1 yr the surfaces were enriched in Al (although still depleted in Na). Transmission EM and AFM revealed that the Al-enrichment was due to precipitation of a patchy surface coating containing some kaolinite and some relatively Al-rich amorphous material. Beneath this coating, the feldspar surface was Al-depleted. This set of observations may be the first record that feldspar weathered naturally under mildly acidic conditions develops an Al-depleted surface similar to the Al-depleted surface documented on laboratory-weathered crystals. A similar Al-rich coating was documented on feldspar samples from the Cape Cod aquifer in Massachusetts, where the feldspar dissolution rate was shown to be about a factor of 3 slower in the field than in the lab. However, this discrepancy is not caused by the surface coating, but is probably caused either by differences in solution saturation state, changes in pH along the flow path, or by unconstrained heterogeneities in flow path.
In laboratory experiments investigating crystals and glass of feldspar composition, it was observed that the concentration of nonbridging oxygens (NBOs) in the glasses influenced the leaching and dissolution behavior. Sodium aluminosilicate glass with 5 NBOs per 10 tetrahedra developed an extensive surface layer during both acid and basic dissolution, while a sodium aluminosilicate glass with 1 NBO per 10 tetrahedra showed no surface layer under acid or basic dissolution. The effect of NBOs on dissolution for both glass and crystalline material of feldspar composition will be used to further elucidate the mechanism of dissolution of this crustal mineral.