Non-Linearity in Glass Composition Dependence of Dissolution Rates: Effect of Solution pH

Abstract

ABSTRACTIn our previous studies on the optimization of glass compositions for high-level nuclear waste vitrification it was found that, over certain composition ranges, PCT leachate concentrations increased dramatically with very small changes in glass composition. The large differences that are observed between the leachate pH values for the “durable” and the “less-durable” glasses is one possible cause for this strongly non-linear glass composition effect; conversely, the pH difference may be merely another symptom. In this study, four simulated nuclear waste glasses (two of the less-durable and two of the durable types), were leached in both zwitterionic and inorganic buffer solutions, at fixed pH-values in the ranges of 7 to 12. The very different leaching behaviors of the two types of glasses persisted and, furthermore, different pH-dependence was found despite their very similar glass composition. This study suggests that the leachate pH difference observed between the less-durable and the durable glasses under uncontrolled pH conditions is not the major cause of the large difference of leaching behavior between those glasses. The normalized release ratios of soluble components (B, Li, Na) to Si show significant differences for the two types of glasses.