Role of Sulfate in Structural Modifications of Sodium Barium Borosilicate Glasses Developed for Nuclear Waste Immobilization

Abstract

A sodium barium borosilicate glass matrix with a higher solubility of sulfate has been developed recently at Bhabha Atomic Research Centre for vitrification of sulfate bearing high‐level nuclear waste. We report here the studies carried out to understand the influence of sulfate ion on the three‐dimensional borosilicate network. Experiments were carried out with sodium barium borosilicate base glass samples loaded with varying amounts of SO 4 2− (0–5 mol%). Phase separation studies on the samples revealed that as much as 3 mol% of SO 4 2− can be loaded within the base glass without any phase separation, however, beyond this limit BaSO 4 (barite) crystallizes within the matrix. Thermal analyses of the samples indicated a shift in glass transition temperature from 534° (0 mol% SO 4 2− ) to 495°C (3 mol% SO 4 2− ) and it remained more or less unaltered afterwards even with high SO 4 2− loading. A similar observation of structure stabilization was obtained from 29 Si MAS–NMR studies also, which showed that with 2 mol% of SO 4 2− loading, the Q 2 :Q 3 ratio changed from 59:41 (for samples with 0 mol% SO 4 2− loading) to 62:38 and it remained almost the same afterwards even with higher SO 4 2− loading. 11 B MAS NMR patterns of the glass samples, however, remained unchanged with SO 4 2− loading ([BO 4 ]:[BO 3 ]=38:62). Based on 29 Si and 11 B MAS NMR studies, the authors propose two different ways of interaction of SO 4 2− ions with the borosilicate network: (i) the network modifying action of SO 4 2− ions with ‐Si–O–Si‐ linkages, at low SO 4 2− ion concentration (<2 mol%) and (ii) the preferential interaction of SO 4 2− with the Ba 2+ ions at high SO 4 2− concentration (>2 mol%).