Polymer Model of Zeolite Thermochemical Stability

Abstract

The polymer model provides a relatively simple and robust basis for estimating the standard Gibbs free energies of formation (ΔGfo) and standard enthalpies of formation (ΔHfo) of clay minerals and other aluminosilicates with an accuracy that is comparable to or better than can be obtained using alternative techniques. The model developed in the present study for zeolites entailed the selection of internally consistent standard thermodynamic properties for model components, calibration of adjustable model parameters using a linear regression technique constrained by ΔGfo and ΔHfo values retrieved from calorimetric, solubility, and phase-equilibrium experiments, and assessments of model accuracy based on comparisons of predicted values with experimental counterparts not included in the calibration dataset. The ΔGfo and ΔHfo predictions were found to average within ±0.2% and ±0.3%, respectively, of experimental values at 298.15 K and 1 bar. The latter result is comparable to the good accuracy that has been obtained by others using a more rigorous electronegativity-based model for ΔHfo that accounts explicitly for differences in zeolite structure based on differences in framework density and unit-cell volume. This observation is consistent with recent calorimetric studies indicating that enthalpies of transition from quartz to various pure-silica zeolite frameworks (zeosils) are small and only weakly dependent on framework type, and suggests that the effects on ΔHfo of differences in framework topology can be ignored for estimation purposes without incurring a significant loss of accuracy. The relative simplicity of the polymer model, together with its applicability to both zeolites and clay minerals, is based on a common set of experimentally determined and internally consistent thermodynamic properties for model components. These attributes are particularly well suited for studies of the effects of water-rock-barrier interactions on the long-term safety of geologic repositories for high-level nuclear waste (HLW).

Thermodynamic modelling of alkali-activated slag cements

“…Thermodynamic data are available for a large number of zeolites (see the compilation by Arthur et al (2011) for example), but the large variety of possible chemical compositions and the availability of only a few data for each zeolite framework type (often only one datum), mean that more thermochemical or solubility data are needed to better define these phases in thermodynamic databases.…”

zeo19: A thermodynamic database for assessing zeolite stability during the corrosion of nuclear waste immobilization glasses

“…Chipera and Apps 10 observed a relative difference between predicted and measured thermodynamic properties, namely, of 0.5% for the Gibbs free energy and enthalpy of formation (using data from Chermak and Rimstidt 38 ) and 15% for the entropy (using data from Holland 40 ). The method of Arthur et al 36 (i.e., based on a summation of hydroxides) typically offers Gibbs free energy and enthalpy of formation values that are within 0.2% and 0.3% of experimental data, respectively. On the other hand, the method of Vieillard, 41 which is based on a summation of fictive oxides, offers entropy and heat capacity predictions that are within 3% of experimental data.…”

“…On the other hand, the method of Vieillard, 41 which is based on a summation of fictive oxides, offers entropy and heat capacity predictions that are within 3% of experimental data. 36,41 Importantly, however, differences in molecular water content (i.e., water of crystallization) of zeolites can also affect their thermodynamic properties. For example, for analcime, we note that a 10% change in the number of zeolitic water molecules yields a variation of 0.75% in the Gibbs free energy of formation of the solid, which, in turn, yields a variation of 4 log K units in the predicted solubility constant.…”

“…We review the zeolite phases included in the database and detail the method(s) that were used to estimate their thermodynamic properties. In general, thermodynamic data for the zeolite phases considered herein were sourced from published experimental data and, when such data was not available, were calculated via the predictive methods developed by: (a) Arthur et al 36 (for the enthalpy of formation), (b) Vieillard 41 (for the heat capacity and entropy), and/or (c) volume-based correlations based on knowledge of the molar volume of the compound. 75 Since predicted solubility data are very sensitive to small variations in thermodynamic inputs, the solubility constants at standard conditions (p = 1 bar, T = 25°C) were calculated and assessed using reaction data from both the Gibbs free energy of formation (obtained from calorimetric or solubility measurements) and the enthalpy of formation and entropy (obtained by calorimetric measurements).…”

“…28,29 However, the use of such platforms relies on the availability of comprehensive and accurate thermodynamic data. 10 Although calorimetric data for some zeolites [30][31][32][33][34][35] are available and empirical methods exist for estimating their thermodynamic properties, [36][37][38][39][40][41] broadly speaking, available thermodynamic data remain sparse. This partially arises from the fact that the stability of some zeolites is limited to small domains of solution composition and temperature.…”