Incorporation and Retention of 99-Tc(IV) in Magnetite under High pH Conditions

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.

Uranium fate during crystallization of magnetite from ferrihydrite in conditions relevant to the disposal of radioactive waste

“…The Fe(II)-induced crystallization of ferrihydrite to magnetite was studied in U(VI)-amended synthetic cement leachates chosen to simulate early-(pH 13.1) and late-( pH 10.5) stage evolution of a GDF (Berner, 1992;Marshall et al, 2014a;Moyce et al, 2014 2-line ferrihydrite was synthesized (Cornell and Schwertmann, 2003) and the Fe(III) content was determined by dissolution in 1 M HCl and analysis (Viollier et al, 2000). Ferrihydrite was equilibrated with the cement leachates at 4 g l -1 for 1 hour at room temperature.…”

Weathering of ancient and medieval glasses—potential proxy for nuclear fuel waste glasses. A perennial challenge revisited

“…Hence, it is suggested that alteration of the glasses under thoroughly reducing conditions sustained over very long time will keep the Tc ions confined to an insoluble secondary phase akin to pyrolusite, thus preventing their hazardous accumulation in the biosphere. Recent research suggests that Tc(IV) ions can be sorbed onto FeS 2 surfaces or incorporated in iron oxides such as hematite (α‐Fe 2 O 3 ) or magnetite (Fe 3 O 4 ) as well as iron oxihydroxides such as goethite (α‐FeOOH) . The mechanism relies on direct substitution of Tc(IV) onto Fe(III) lattice sites that may be retained stably under environmental conditions to be expected in a nuclear waste disposal vault.…”

“…Schematics of the sequence of major interaction, leaching and precipitation reactions determining the corrosion mechanisms of glass (redrawn after Wicks)…”

Incorporating technetium in minerals and other solids: A review

“…In all spinel, oxygen is arranged in a face-centered-cubic lattice, wherein these sites are situated. [36,37]. The short-lived 99m Tc isotope has also been added to magnetite particles for use in medical imaging [38].…”

“…Sintering oxides at pressures of GPa has been shown to create MgFe 2 O 4 spinel with up to 0.35 mass% rhenium, [54]. Rhenium is a non-radioactive chemical surrogate for Tc, with similar volatility and chemical speciation [6,36].This is meant to mimic the geological conditions that yield Re-bearing spinel, with concentrations on the order of fractions of ~1 ng Re/g of spinel [55].…”