Abstract
Technetium incorporation into magnetite and its behavior during subsequent oxidation has been investigated at high pH to determine the technetium retention mechanism(s) on formation and oxidative perturbation of magnetite in systems relevant to radioactive waste disposal. Ferrihydrite was exposed to Tc(VII)(aq) containing cement leachates (pH 10.5-13.1), and crystallization of magnetite was induced via addition of Fe(II)aq. A combination of X-ray diffraction (XRD), chemical extraction, and X-ray absorption spectroscopy (XAS) techniques provided direct evidence that Tc(VII) was reduced and incorporated into the magnetite structure. Subsequent air oxidation of the magnetite particles for up to 152 days resulted in only limited remobilization of the incorporated Tc(IV). Analysis of both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data indicated that the Tc(IV) was predominantly incorporated into the magnetite octahedral site in all systems studied. On reoxidation in air, the incorporated Tc(IV) was recalcitrant to oxidative dissolution with less than 40% remobilization to solution despite significant oxidation of the magnetite to maghemite/goethite: All solid associated Tc remained as Tc(IV). The results of this study provide the first direct evidence for significant Tc(IV) incorporation into the magnetite structure and confirm that magnetite incorporated Tc(IV) is recalcitrant to oxidative dissolution. Immobilization of Tc(VII) by reduction and incorporation into magnetite at high pH and with significant stability upon reoxidation has clear and important implications for limiting technetium migration under conditions where magnetite is formed including in geological disposal of radioactive wastes.
Highlights
Technetium-99 (Tc-99) is a long-lived radioactive fission product present in many spent fuel and reprocessing derived radioactive wastes
Oxidative maghematisation was evident from a shift in the magnetite (511) and (440) peaks in the X-ray diffraction (XRD) pattern to higher 2θ in the 152 days of oxidation in both the intermediate cement leachate (ICL) and old cement leachate (OCL) experiments, in agreement with the quantitative powder X-ray diffraction (QXRD) data (Figure SI-2, Supporting Information)
In the 152 day samples, a shortening of the unit cell length was observed in all three systems indicating a decrease in the Fe(II)/Fe(III) ratio consistent with oxidation (Table 1)
Summary
Technetium-99 (Tc-99) is a long-lived radioactive fission product (half-life 2.1 × 105 years) present in many spent fuel and reprocessing derived radioactive wastes. Under reducing conditions, Tc(IV) is dominant and generally precipitates as sparingly soluble Tc(IV)-hydroxide phases (10−8−10−9 M)[2−4] or adsorbs to mineral surfaces.[5−8] Due to its high mobility under oxic conditions and its long half-life, Tc-99 is a significant risk driving radionuclide for geological disposal of radioactive wastes, as well as presenting a significant challenge to remediation of radioactively contaminated land[9] in the UK,[10,11] the USA,[12−14] and Russia.[15] Past work has shown that Fe(II) bearing oxides[16−18] and sulfides[19−21] are effective in removing Tc(VII) from solution via reductive adsorption or precipitation. Transport of Tc-99 in the environment will be significantly controlled by the Tc-99 interactions with Fe(II) bearing solid minerals (e.g., iron oxides, sulfides) through either sorption[8,28] or surface mediated reduction and precipitation.[29,30]
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