The effects of silicic acid, aluminate ion activity and hydrosilicate gel development on the dissolution rate of a simul
- PDF / 565,274 Bytes
- 8 Pages / 381.06 x 606.06 pts Page_size
- 4 Downloads / 166 Views
Abstract. The dissolution rate of a simulated Magnox waste glass has been investigated in single-pass flow-through experiments designed to investigate the role of Al and Si in the dissolution process. The results indicate that both Al and Si species suppress the rate of dissolution. These effects may be modelled using a combined Al/Si affinity term in a conventional glass dissolution rate law. Aluminium species may also play an inhibitory role when present at relatively high solution activities. In Si-rich alkaline media, the concentration of aluminium is controlled to very low levels by the development of secondary aluminosilicate phases. Removal of Al by secondary phase precipitation results in dissolved Al activities below that required to reach 'saturation' with respect to the glass.
Introduction. Rates of silicate glass and mineral dissolution in aqueous systems have been widely modelled using a rate law based on transition state theory (TST) [1,2]. Such models include terms to account for kinetic effects (such as catalysis and inhibition) and thermodynamic effects (the rate
dependence on the reaction free energy). A generalised form of a TST rate law for a heterogeneous surface reaction (such as silicate dissolution) has been given [3]:
Rate = koSe -ha/RT (AG )g(I)l-J al
(1).
i
Here, k, is the intrinsic rate constant, S is the reactive surface area, Ea is the activation energy, RT is the product of the gas constant and the absolute temperature, thefAG, term is a free energy function describing the reaction rate dependence on the deviation from equilibrium (a reaction affinity term), g(I) is a term accounting for the ionic strength dependence of the reaction and the product term (Hnain') describes the net catalytic or inhibitory effects of rate influencing species. An important prediction of TST rate laws (such as (1)) is that the rate is dependent on the activity of catalytic species (such as H+(aq)) and on the deviation from thermodynamic equilibrium. The pH dependence of glass dissolution has been widely investigated, but the form of the reaction free energy dependence and the potential catalytic or inhibitory role of species other than HW(aq) has received little attention. There is continuing debate as to the form of appropriate affinity terms in glass dissolution rate laws [4-8]. Grambow [4] proposed the first application of a TST model to glass dissolution, in
401
Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
which the free energy dependence of the rate was described by a term which includes only the activity of silicic acid, that is : (2), fAGr = (1 -exp(AGr/RT)) = (1-(aH 4SiO 4/aHnSiO4 (,at))) where, aHnSiO 4 is the activity of silicic acid and aHnSiO 4(sAt)is the silicic acid activity at saturation at the glass/solution interface. More recently, rates laws have been proposed which include elements other than Si. Bourcier et al. [5] proposed that the affinity term relates to the saturation state with respect to a secondary amorphous gel containing Si, Fe, Al, Ca and Mg
Data Loading...
