Facile incorporation of technetium into magnetite, magnesioferrite, and hematite by formation of ferrous nitrate in situ: precursors to iron oxide nuclear waste forms

Abstract

The fission product, ⁹⁹Tc, presents significant challenges to the long-term disposal of nuclear waste due to its long half-life, high fission yield, and to the environmental mobility of pertechnetate (TcO₄⁻), the stable Tc species in aerobic environments. Migration of ⁹⁹Tc from disposal sites can potentially be prevented by incorporating it into durable waste forms based on environmentally stable minerals. Since Tc(IV) and Fe(III) have the same ionic radius, Tc(IV) can replace Fe(III) in iron oxides. Environmentally durable iron oxides include goethite (α-FeOOH), hematite (α-Fe₂O₃), and magnesioferrite (MgFe₂O₄). The incorporation of Tc into two of these, hematite and magnesioferrite, as well as magnetite (Fe₃O₄) by means of simple, aqueous chemistry is presented starting from TcO₄⁻ in 5 M nitric acid. A combination of X-ray diffraction and X-ray absorption fine structure spectroscopy reveals that Tc(IV) replaces Fe(III) within the iron oxide structures. Following incorporation, Tc doped samples were suspended in deionized water under aerobic conditions, and the release rates of Tc were determined. The results of this work show that Tc leaches more quickly from Fe₃O₄ than from α-Fe₂O₃ or MgFe₂O₄. Modeling the leach rates and comparison with the leach rate of Tc from TiO₂ indicate that release of Tc is controlled by solid state diffusion.