Thermokinetic Model of Borosilicate Glass Dissolution: Contextual Affinity
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THERMOKINETIC MODEL OF BOROSILICATE GLASS DISSOLUTION: CONTEXTUAL AFFINITY T. ADVOCAT*, J.L. CROVISIER**, B. FRITZ" and E. VERNAZ* *CEN-Valrhi, SDHA, BP 171, 30205 Bagnols-sur-Chze Cedex, France **CGS (CNRS), 1 rue Blessig 67000 Strasbourg,France.
ABSTRACT Short and long-term geochemical interactions of R7T7 nuclear glass with water at 100°C were simulated with the DISSOL thermokinetic computer code. Both the dissolved glass quantity and the resulting water composition, saturation states and mineral quantities produced were calculated as a function of time. The rate equation used in the simulation was first proposed by Aagaard and Hegelson: v = k+.S-a(H+)--(l - e-(A/Ir)). It simulates a gradually diminishing dissolution rate as the reaction affinity diminishes. The best agreement with 1-year experimental data was obtained with a reaction affinity calculated from silica activity (Grambow's hypothesis) rather than taking into account the activity of all the glass components as proposed by Jantzen and Plodinec. The concept of residual affinity was introduced by Grambow to express the fact that the glass dissolution rate does not cease. We prefer to replace the term "residual affinity" by "contextual affinity", which expresses the influence on the dissolution rate of three factors: the solution chemistry, the metastability of SiO 2(m), and the possible precipitation of certain aluminosilicates such as zeolites. INTRODUCTION The explicit expression of time in a predictive dissolution model requires the knowledge of an initial rate value and of a rate equation by which rate variations can be expressed as a function of extrinsic factors irrespective of the progress of the reaction. A general rate equation was proposed by Aagaard and Hegelson[ll, Hegelson et aL.12 and Lasaga[31 for hydrolysis of silicates, and was applied by Grambow[41 to nuclear glass dissolution. It requires a choice of activated complex (the desorption of which is a limiting factor) and knowledge of the pH and ion activity dependence. The activity of the determining ions is used at each step of the reaction to define the chemical affinity: A = RT In K/Q = -AG(T,P) in which Gr is the Gibbs free energy, K the equilibrium constant, Q the ionic activity product, R the gas constant and T the temperature. One of the main difficulties is to evaluate K for a glassy material, which we consider a turning point in kinetic modeling. A second difficulty is to define a realistic value for Q; in fact, Q is easily computed by standard geochemical codes, but is strongly dependent on the potential sequence of secondary products (e.g. code data bank), and on the equilibrium between solution and atmosphere (02 and CO 2 fugacity). The purpose of this investigation was to study the influence of the choice of likely secondary products on the Q values, and its consequences on the reaction rate via the affinity. The DISSOL code can calculate a large composition field for clay minerals, one of the main secondary products in natural analogs, by a regular solid solution. GENERAL
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