A Mössbauer study of iron in vitrified wastes

57Fe Mössbauer spectroscopy has been used to study the environment and oxidation state of iron in a series of vitrified sewage sludge ash (SSA) wastes, which are broadly similar in composition and variability to some intermediate-level legacy wastes (ILLW). The SSA wastes studied here are incapable of forming an homogeneous glass when melted at 1450 °C although increasing additions of CaO reduce the crystalline content, which consists of Ca3(PO4)2 and Ca3Mg3(PO4)4. Bulk glass transition temperatures of 670–850 °C have been measured, the value decreasing with increasing CaO content. Tetrahedrally coordinated Fe3+ appears to exist in vitrified SSA only at high CaO contents (> ∼ 29 mol. %) and whilst broadly similar to our previous results for simulated vitrified SSA, behavioural differences have been noted between fitted Mossbauer parameters for real and simulated vitrified SSA. We suggest that these could be attributable to the buffering action of carbon and other reducing agents in the real SSA wastes.