PNNL

Abstract

Single-pass flow-through tests were conducted to determine the pH (7 - 12) and temperature (23- 90°C) dependence of kinetic rate-law parameters; ko, ?, and Ea, for the dissolution of glass in aqueous solution. Experiments were performed with three prototypic nuclear waste glasses that span a wide compositional range, which covers, with high probability, the expected processing composition range for candidate immobilized low-activity waste (ILAW) glasses. Comparison of the B to Na release rates for one glass was incongruent at 23 and 40°C, and pH(23°C) = 7.0 and 8.0, suggesting two distinct mechanisms are responsible for the Na+ release, namely Na+-H+ ion exchange and matrix dissolution. Matrix dissolution became the dominant dissolution mechanism for all glasses at pH values greater than 9.0 as evident by the congruent release of Al, B, Na, and Si to solution. By combining the results collected for each ILAW glass at pH values greater than 9.0, pH and temperature dependent rate law parameters were determined for Al, B, Na, and Si release. A comparison of the pH power-law coefficient for Al, B, Na, and Si at each temperature suggest that ? does not depend on temperature within experimental error and suggests the release of these elements into solution is controlled by the same dissolution mechanism at the forward rate of reaction. The activation energy (Ea), based on B release, range from 52 ±4 to 56 ±6 kJ/mol which suggest that dissolution is a surface-controlled reaction mechanism. The data presented in this manuscript suggest that for these three ILAW glasses the chemical durability for each glass is relatively the same under these test conditions. A lack of compositional dependence on the dissolution rate is observed even though there is as much as a 39 kJ/mol difference in the free energy of hydration (?Ghyd) among the borosilicate waste glasses tested. This similarity in dissolution rate despite the large ?Ghyd difference is almost certainly the result of the structural similarities between these glasses, which suggests that ILAW glasses have similar, if not identical, polymerization states. This is evident from their almost identical 29Si chemical shifts and molar ratio of the alkali elements, S(Na+ K + Li), to the network former elements, S(AlIV + BIV + Fe + Si), contained in each glass. The polymerization state is an indication of the number of framework SiO4 linkages contained in the glass network. In general, the greater the number of framework SiO4 linkages the more durable the glass. Finally, in agreement with previous work, these results suggest breakage of the Si-O bond is the rate-determining dissolution mechanism under alkaline conditions [pH(23°C) > 9.0] far from saturation with respect to a alteration phase or phases.