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<SUB>4</SUB><SUP>2-</SUP> (0–5 mol%). Phase separation studies on the samples revealed that as much as 3 mol% of SO<SUB>4</SUB><SUP>2-</SUP> can be loaded within the base glass without any phase separation, however, beyond this limit BaSO<SUB>4</SUB> (barite) crystallizes within the matrix. Thermal analyses of the samples indicated a shift in glass transition temperature from 534° (0 mol% SO<SUB>4</SUB><SUP>2-</SUP>) to 495°C (3 mol% SO<SUB>4</SUB><SUP>2-</SUP>) and it remained more or less unaltered afterwards even with high SO<SUB>4</SUB><SUP>2-</SUP> loading. A similar observation of structure stabilization was obtained from <SUP>29</SUP>Si MAS–NMR studies also, which showed that with 2 mol% of SO<SUB>4</SUB><SUP>2-</SUP> loading, the Q<SUP>2</SUP>:Q<SUP>3</SUP> ratio changed from 59:41 (for samples with 0 mol% SO<SUB>4</SUB><SUP>2-</SUP> loading) to 62:38 and it remained almost the same afterwards even with higher SO<SUB>4</SUB><SUP>2-</SUP> loading. 11B MAS NMR patterns of the glass samples, however, remained unchanged with SO<SUB>4</SUB><SUP>2-</SUP> loading ([BO<SUB>4</SUB>]:[BO<SUB>3</SUB>]=38:62). Based on <SUP>29</SUP>Si and 11B MAS NMR studies, the authors propose two different ways of interaction of SO<SUB>4</SUB><SUP>2-</SUP> ions with the borosilicate network: (i) the network modifying action of SO<SUB>4</SUB><SUP>2-</SUP> ions with -Si–O–Si- linkages, at low SO<SUB>4</SUB><SUP>2-</SUP> ion concentration (&#60;2 mol%) and (ii) the preferential interaction of SO<SUB>4</SUB><SUP>2-</SUP> with the Ba<SUP>2+</SUP> ions at high SO<SUB>4</SUB><SUP>2-</SUP> concentration (>2 mol%)