The structural properties of iron in vitrified toxic waste ashes

A statistical approach has been utilised to characterise the structural and chemical properties of vitrified model sewage sludge combustion ashes. A d-optimal design has been applied to determine the effects of the compositional variability of vitrified wastes on iron redox and coordination as measured by 57Fe Mössbauer spectroscopy. Several Fe sites have been observed through the deconvolution of Mössbauer spectra, representing the Fe2+ oxidation state and tetrahedrally and octahedrally coordinated Fe3+ ions. The interdependency of iron redox and iron coordination is demonstrated, and suggests that conversion of tetrahedral Fe3+ to octahedral Fe2+ preferentially occurs upon increasing Fe2+/ΣFe. For Fe2+/ΣFe=0·30-0·89, the abundance of octahedrally coordinated Fe3+ remains approximately constant whilst tetrahedrally coordinated Fe3+ exhibits a linear decrease in its abundance with increasing Fe2+/ΣFe ratio.