Sulphate incorporation and glass formation in phosphate systems for nuclear and toxic waste immobilization

Glassformation and the capacity for sulphur incorporation have been investigated within the systems P2O5–Al2O3–Na2O–Fe2O3 and P2O5–Fe2O3–RyO (R = Li, Na, K, Mg, Ca, Ba, Pb). Linear relationships have been identified linking log [SO3] with the oxide ion activity, which can be represented to varying degrees by normalized cation field strength index ∑(z/a2), theoretical optical basicity Λth, oxygen to phosphorus molar ratio [O]/[P], or P2O5 content. When prepared under oxidizing conditions only basic, highly depolymerized phosphate glasses for which ∑(z/a2) < ∼1.2; Λth > ∼0.5; [O]/[P] > ∼3.8; or P2O5 content <∼40 mol% can incorporate levels of sulphur that could be considered useful from a waste immobilization perspective. Data for several phosphate and borosilicate glasses suggest that the ∑(z/a2) scale provides the best residual fitting error and that if glasses are considered within their familial limits, this can provide a useful methodology for predicting sulphur capacity.