Affinity Rate Law Failure to Describe Sodium Borosilicate Glass Alteration Kinetics
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$IILQLW\5DWH/DZ)DLOXUHWR'HVFULEH6RGLXP%RURVLOLFDWH*ODVV$OWHUDWLRQ.LQHWLFV P. Frugier, S. Gin and C. Jégou Commissariat à l’Énergie Atomique – CEA Valrhô DIEC/SESC, BP 17171, 30207 Bagnols-sur-Cèze Cedex, France $%675$&7 Simplified glass compositions were chosen to improve our knowledge of the alteration kinetics of complex glasses dedicated to the confinement of high-level waste. Since 1998, the sodium borosilicate glass system is at the center of a passionate debate between an affinity-based kinetic rate law and a protective surface layer theory. All the authors who have investigated ternary 68/14/18 SiO2–B2O3–Na2O glass agree on the fact that the affinity law cannot satisfactorily account for its alteration kinetics. Some authors explained that these discrepancies between classical kinetic rate law and experimental findings could be due to macromolecular amorphous separation in the 68/14/18 sodium borosilicate system and that this simplified glass could be divided into 90% reedmergnerite (NaBSi3O8) and 10% diborate (Na2O–2B2O3). This article provides evidence of the homogeneity of ternary 68/18/14 SiO2–B2O3–Na2O glass at nanometric scale and shows that even phase separation at less than nanometric scale could not explain the inability of hydrated glass-solution affinity laws to describe its alteration. The relative simplicity of the SiO2–B2O3–Na2O chemical system allows a critical examination of the macroscopic alteration laws developed over the last twenty years based only on the hydrated glass-solution chemical affinity without taking into account the formation and reactivity of the gel or its passivating properties. ,1752'8&7,21 A model describing reactivity is required to predict the long-term rate of waste glass alteration in the subsurface over geologic time scales. Aagaard and Helgeson in the early 1980s [1] fashioned a general equation to describe the dissolution of minerals in aqueous solution. & − (D 4 U = N D+− η+ exp 1− 57 .
σ
∏ DL − QL , M L
Initially, the ion activity product 4 and the solubility constant . were assumed to contain all the elements of the considered phase, with their molar fractions in the solid as the exponent. Grambow [2] was the first to limit the number of elements taken into account: considering that it was difficult from a conceptual standpoint to assume a thermodynamic equilibrium between the glass and solution, he applied this concept to a hydrated glass in which only silicon was taken into account in the affinity function; other authors later included in the affinity term additional sparingly soluble glass elements such as aluminum or iron. [3]. This affinity equation was subjected to critical analysis by French researchers, who demonstrated its theoretical and experimental limits [4–7]. They considered it unreasonable at low temperatures in aqueous media to postulate a reverse reaction forming a glass or even a hydrated glass, and therefore that an equilibrium cannot be defined between the glass and solution. Conv
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