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 SO₄^2- (0–5 mol%). Phase separation studies on the samples revealed that as much as 3 mol% of SO₄^2- can be loaded within the base glass without any phase separation, however, beyond this limit BaSO₄ (barite) crystallizes within the matrix. Thermal analyses of the samples indicated a shift in glass transition temperature from 534° (0 mol% SO₄^2-) to 495°C (3 mol% SO₄^2-) and it remained more or less unaltered afterwards even with high SO₄^2- loading. A similar observation of structure stabilization was obtained from ²⁹Si MAS–NMR studies also, which showed that with 2 mol% of SO₄^2- loading, the Q²:Q³ ratio changed from 59:41 (for samples with 0 mol% SO₄^2- loading) to 62:38 and it remained almost the same afterwards even with higher SO₄^2- loading. 11B MAS NMR patterns of the glass samples, however, remained unchanged with SO₄^2- loading ([BO₄]:[BO₃]=38:62). Based on ²⁹Si and 11B MAS NMR studies, the authors propose two different ways of interaction of SO₄^2- ions with the borosilicate network: (i) the network modifying action of SO₄^2- ions with -Si–O–Si- linkages, at low SO₄^2- ion concentration (<2 mol%) and (ii) the preferential interaction of SO₄^2- with the Ba²⁺ ions at high SO₄^2- concentration (>2 mol%).