Plutonium Retention Mechanisms by Magnetite under Anoxic Conditions: Entrapment versus Sorption

Abstract

The reliable prediction of possible plutonium migration into the geological environment is crucial for the safety assessment of radioactive waste repositories. Fe­(II)-bearing corrosion products like magnetite, which form on the surface of steel waste containers, can effectively contribute to the retardation of the potential radionuclide release by sorption and redox reactions, eventually followed by formation of secondary precipitates. A retardation process even more efficientespecially when considering the required long time scales for nuclear waste repositionis structural incorporation by magnetite, as has been demonstrated for Tc and U. Here we show that this mechanism might not be as relevant for Pu retention: after a rapid reduction of Pu­(V) to Pu­(III) in acidic Fe­(II)/Fe­(III) solution, base-induced magnetite precipitation (pHexp ≈ 12.5) leads only to a partial (∼50%) incorporation, while the other half remains at the surface by forming tridentate sorption complexes. Neither solid nor sorbed Pu­(IV) species were observed in the starting solution and after precipitation. With Fe­(II)-enforced recrystallization at pHexp = 6.5, a process potentially mimicking long-term, thermodynamically controlled aging, the equilibrium between both Pu species is even further shifted toward the sorption complex. A detailed analysis of the incorporated species by Pu LIII-edge X-ray absorption fine-structure (XAFS) spectroscopy shows a pyrochlore-like coordination environment (split 8-fold oxygen coordination shell with Pu–O distances of 2.22 and 2.45 Å and an edge-sharing linkage to Fe-octahedra with Pu–Fe distances of 3.68 Å), which is embedded in the magnetite matrix (Pu–Fe distances of 3.93, 5.17, and 5.47 Å). This suggests that the reason for the partial incorporation is the structural incompatibility of the large Pu­(III) ion for the octahedral Fe site in magnetite. The adoption of a pyrochlore-like local environment within the magnetite long-range structure might be induced by the rapid coprecipitation rather than being a thermodynamically stable state (kinetic entrapment).

Plutonium(IV) Sorption during Ferrihydrite Nanoparticle Formation

“…However, studies suggest the U­(IV) forms a binuclear complex on magnetite at defect or lattice step/kink sites, which is distinct from the tetradentate Pu­(IV) adsorption species observed in this study. A recent study also observed partial incorporation of Pu as Pu­(III) following coprecipitation with magnetite. Here the Pu had a split Pu–O shell with four O scatterers at 2.22 and 2.45 Å attributed to a pyrochlore-like coordination environment within the structure.…”

Transformation of Ferrihydrite to Goethite and the Fate of Plutonium

“…15 Both the sorption/desorption of Pu to mineral colloid surfaces (pseudocolloids) and formation of Pu oxide colloids (intrinsic colloids) associated with mineral surfaces, as well as Pu coprecipitation with secondary minerals are all likely important environmental processes under a range of geochemical conditions. 11,14,22,57,58 For example, in groundwater at the Mayak site (Russia), colloidal amorphous iron oxides with associated Pu were found up to 4 km away from the contamination source. 59 In contaminated soils at Los Alamos National Laboratory, Batuk et al 60 identified unusual Pu−Fe particles.…”

“…At high Pu concentrations (>10 –8 M), Pu­(IV) intrinsic colloids have been shown to form on the surface of various oxide minerals including iron oxy­(hydroxide) minerals. ,,, On the goethite surface, Pu­(IV) colloids may undergo a lattice distortion, because of epitaxial growth, which leads to a stronger surface binding compared to other mineral phases, such as quartz . Both the sorption/desorption of Pu to mineral colloid surfaces (pseudocolloids) and formation of Pu oxide colloids (intrinsic colloids) associated with mineral surfaces, as well as Pu coprecipitation with secondary minerals are all likely important environmental processes under a range of geochemical conditions. ,,,, For example, in groundwater at the Mayak site (Russia), colloidal amorphous iron oxides with associated Pu were found up to 4 km away from the contamination source . In contaminated soils at Los Alamos National Laboratory, Batuk et al .…”

Environmental Chemistry of Radionuclides : Open Questions and Perspectives

“…With similar spectroscopic tools, Bargar et al described how biogeochemical processes can tune the uranium speciation within domains that are of the order of a few nm. [42,43] At a much larger spatial scale (few cm), the implementation in situ of DGT (Diffusive Gradient in Thin film) and DET (Diffusive Equilibrium in Thin film) in wetlands impacted by former mining activities in the French Massif Central region provides a vertical mapping of Nat U gradient. The depth profiles with fairly good spatial resolution (a few cm) show the correlation with the soil nature, the organic content matter, all factors combined with the history of the site.…”