EVALUATION OF SOLUTION COMPOSITION ALUMINOBOROSILICATE WASTE GLASS DISSOLUTION AT 40°C

摘要

Successful application of Transition State Theory (TST) to the study of glass-water interactions and the prediction of the behavior of glass waste forms over geological time-scales requires an understanding of the fundamental chemical reactions that occur at the glass-water interface. These processes play an important role in determining the overall rate because, in accordance with TST, the overall rate is governed by the slowest elementary reaction. A TST-based chemical affinity rate law that relates the effect of solution composition on the dissolution rate of minerals has also been applied to glasses. But there is considerable debate concerning whether or not the chemical affinity based rate law or the protective layer model best describes the majority of the experimental results collected on the glass-water reaction. Glass is a thermodynamically unstable solid, as such, the glass-water reaction can never achieve a condition where the rate of dissolution and precipitation are equal (i.e., equilibrium). Because thermodynamic equilibrium can never be achieved, the ability to define or describe the observed decrease in the dissolution rate as the solution concentration of glass components increases poses an unique problem. Therefore, the debate over whether or not the observed decrease in the rate of glass dissolution can be attributed to the buildup of aqueous glass components (i.e., chemical affinity effect) or the formation of a protective gel layer must be addressed. The protective gel layer is an amorphous layer that forms as a result of aluminum and silicon condensation reactions that occur at the glass-water interface and results in the build up of reaction products on the glass surface. A series of single-pass flow-through (SPFT) experiments have been conducted on three aluminoborosilicate low-activity waste (LAW) glasses under conditions that vary from dilute to near-saturated with respect to glass components by altering the flow-through rate, q, to sample surface area, S. All experiments discussed in this paper were conducted at pH(23°C) = 9.0 and 40°C.