Incorporation and speciation of sulphur in glasses for waste immobilisation

The incorporation and speciation of sulphur in glass compositions relevant to the immobilisation of nuclear and toxic wastes has been investigated; and for the first time we present a detailed analysis of S-bearing phosphate glasses. Sulphur K edge x-ray absorption near edge structure (XANES) has provided detailed information on the oxidation state of sulphur. It confirms that in phosphate glasses prepared under oxidising conditions, sulphur is present as sulphate, SO₄^2-. Linear relationships have been identified linking log[SO₃] (mol%) with the oxygen ion activity, which can be represented by the normalised cation field strength index Σ(z/a ²). When prepared under oxidising conditions only basic, highly depolymerised phosphate glasses for which Σ(z/a²) <<∼1.2 can incorporate levels of sulphur that would be considered useful from a waste immobilisation perspective. When glasses are evaluated within familial limits, this methodology provides a useful tool for predicting sulphur capacity and designing new glass compositions that can safely incorporate greater levels of sulphur.