A glass dissolution model for the effects of S/V on leachate pH

Abstract

The observed relationship between the ratio of the glass surface area to the volume of the leachant (S/V) and the leachate pH is discussed in terms of a simple model of the glass dissolution process. Data from leach tests on several nuclear waste glass compositions at different S/V ratios show that the leachate pH increases with time and stabilizes at a nearly constant value beyond about 28 days. This stabilized pH increases systematically with the S/V ratio of the test. The model developed here reproduces the essential features of the data and suggests that a single parameter describing the intrinsic rate of alkali diffusion and ion exchange from the glass is sufficient to represent the major glass composition dependence. Interestingly, the results are essentially independent of the rate constant for matrix dissolution. This study suggests that the diffusion-ion exchange process is central in determining the solution pH and its dependence on S/V. Under static or low-flow-rate conditions, the rate of the matrix hydrolysis reaction slows to a long-term value that is determined by the (diffusional) rate of alkali release.