Facile incorporation of technetium into magnetite, magnesioferrite, and hematite by formation of ferrous nitratein 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.

Kinetics of Co-Mingled 99Tc and Cr Removal during Mineral Transformation of Ferrous Hydroxide

“…In addition to rapid removal of Tc and Cr, the amount of Fe released into the solution during the initial 0.08 h of reaction is only a small fraction (≤4%) of the available Fe in the system, suggesting that, (i) if more Fe­(II) aq had been released, these ions have since oxidized and precipitated likely as Fe oxyhydroxides and (ii) reaction products are being formed (Supporting Information, Table S1). Following reduction of Cr­(VI) and Tc­(VII) (confirmed by XANES/EXAFS in later sections), several potential pathways are expected to contribute to their removal and stabilization, including Cr­(III) and Tc­(IV) incorporation into iron spinels (e.g., magnetite [Fe 3 O 4 ] and chromite [FeCr 2 O 4 ]), ,,− ,− iron oxyhydroxides (e.g., goethite [α-FeOOH] and feroxyhyte [δ-FeOOH]), ,,,,, or the formation of other oxide and hydroxide phases (e.g., TcO 2 · x H 2 O and Cr­(OH) 3 ). ,,− However, formation of the final solid product is highly dependent on the simulant chemistry ,,− and the nature of the final product is critical to assessing the fate of Tc and Cr. X-ray diffraction (XRD) patterns collected from select solids after their reaction in Tc-free simulants were used to quantify the distribution of mineral phases and identify mineralogical differences as a function of simulant chemistry (Figure S1 and Table ).…”

Spontaneous redox continuum reveals sequestered technetium clusters and retarded mineral transformation of iron

“…The majority of these studies investigated reductive removal of Tc from solution via structural incorporation into in situ formed mineral iron oxide/oxyhydroxite phases, i.e., ferrihydrite, magnetite, hematite, and goethite, starting with soluble homogeneous Fe 2+ precursors 20 . Immobilization of Tc within the crystalline structure of a host helps to achieve recalcitrant oxidative leaching of Tc 4+ , and synthesized Tc 4+ -doped magnetite, hematite and goethite that have been evaluated as durable waste forms for long-term geological disposal demonstrated relatively low Tc release rates 13 , 14 , 21 . When structurally incorporated into in situ formed magnetite, Tc 4+ was prone to remain in the reduced state even upon oxidation of magnetite 15 .…”

Pentavalent Uranium Incorporated in the Structure of Proterozoic Hematite

“…This discovery explains why hematite can be a robust host for U over a billion years, which affirms the use of hematite for accurate U/Pb dating as implemented by Courtney-Davies et al (2019, 2020), , and demonstrates that hematite can be a viable waste form for the long-term storage of U, either tailored for the task or incidental because of the corrosion of steel canisters. This also raises the profile of hematite as a potential host for other anthropogenic radioactive elements − that are critical risk drivers for the geologic sequestration of nuclear waste.…”