Experimentally determined dissolution kinetics of Na-rich borosilicate glass at far from equilibrium conditions: Implications for Transition State Theory

Abstract

The dissolution kinetics of five chemically complex and five chemically simple sodium silicate glass compositions (Na–Si±Al±B) were determined over a range of solution saturation values by varying the flow-through rates (1–100mL/d) in a dynamic single-pass flow-through (SPFT) apparatus. The chemically complex borosilicate glasses are representative of prospective hosts for radioactive waste disposal and are characterized by relatively high molar Si/(Si+Al) and Na/(Al+B) ratios (>0.7 and >1.0, respectively). Analysis by X-ray absorption spectroscopy (XAS) indicates that the fraction of ivB to iiiB (N4) varies from 0.66 to 0.70. Despite large differences in bulk chemistry, values of δ29Si peak shift determined by MAS-NMR varies only by about 7ppm (δ29Si=−94 to −87ppm), indicating small differences in polymerization state for the glasses. Forward rates of reaction measured in dynamic experiments converge (average log10 rate [40°C, pH 9]=−1.87±0.79 [g/(m2d)]) at high values of flow-rate (q) to sample surface area (S). Dissolution rates are independent of total Free Energy of Hydration (FEH) and this model appears to overestimate the impact of excess Na on chemical durability. For borosilicate glass compositions in which molar Na>Al+B, further addition of Na appears to stabilize the glass structure with respect to hydrolysis and dissolution. Compared to other borosilicate and aluminosilicate glasses, the glass specimens from this study dissolve at nearly the same rate (0–∼56×) as the more polymerized glasses, such as vitreous reedmergnerite (NaBSi3O8), albite, and silica. Dissolution of glass follows the order: boroaluminosilicate glass>vitreous reedmergnerite>vitreous albite>silica glass, which is roughly the same order of increasingly negative 29Si chemical shifts. The chemical shift of 29Si is a measure of the extent of bond overlap between Si and O and correlates with the forward rate of reaction. Thus, dissolution appears to be rate-limited by rupture of the Si–O bond, which is consistent with the tenants of Transition State Theory (TST). Therefore, dissolution at far from equilibrium conditions is dependent upon the speed of the rate-controlling elementary reaction and not on the sum of the free energies of hydration of the constituents of boroaluminosilicate glass.