Experimentally determined dissolution kinetics of Na-rich borosilicate glass at far from equilibrium conditions: Implications for Transition State Theory Jonathan P. Icenhower a, * , B. Peter McGrail a , Wendy J. Shaw a , Eric M. Pierce a , P. Nachimuthu b,c , David K. Shuh b , Elsa A. Rodriguez a , Jackie L. Steele a a Pacific Northwest National Laboratory, Applied Geology and Geochemistry Group, 902 Battelle Boulevard, MSIN: K6-81, Richland, WA 99352, USA b Lawrence Berkeley National Laboratory, Actinide Chemistry Group, 70A-1150, Berkeley, CA 94720, USA c Department of Chemistry, University of Nevada Las Vegas, Las Vegas, NV 89154, USA Received 30 March 2007; accepted in revised form 27 February 2008; available online 14 April 2008 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 (1–100 mL/d) in a dynamic single-pass flow-through (SPFT) apparatus. The chemically complex borosilicate glasses are representative of pro- spective 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 iv B to iii B (N 4 ) var- ies from 0.66 to 0.70. Despite large differences in bulk chemistry, values of d 29 Si peak shift determined by MAS-NMR varies only by about 7 ppm (d 29 Si = 94 to 87 ppm), indicating small differences in polymerization state for the glasses. Forward rates of reaction measured in dynamic experiments converge (average log 10 rate [40 °C, pH 9] = 1.87 ± 0.79 [g/(m 2 d)]) 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 mod- el 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 (NaBSi 3 O 8 ), 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 29 Si chemical shifts. The chemical shift of 29 Si 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. Ó 2008 Elsevier Ltd. All rights reserved. 1. INTRODUCTION The reactivity of silicate minerals in low temperature aqueous environments has been the subject of a burgeoning set of investigations (e.g., Schott et al., 1981; Chou and Wollast, 1985; Mast and Drever, 1987; Sverdrup, 1990; Manning et al., 1991; Huang and Longo, 1992; Knauss et al., 1993; Lasaga et al., 1994; Brantley and Chen, 1995; Blum and Stillings, 1995; Dove, 1999). One motivation for these studies is to elucidate a link between the structural chemistry and the corrosion resistance of the solid. These data establish the foundation for ab-initio and computer 0016-7037/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.02.026 * Corresponding author. E-mail address: jonathan.icenhower@pnl.gov (J.P. Icenhower). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 72 (2008) 2767–2788