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 Fe-57 Mossbauer spectroscopy. Several Fe sites have been observed through the deconvolution of Mossbauer 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+/Sigma Fe. For Fe2+/Sigma Fe=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+/Sigma Fe ratio.