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

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 (pH(exp) approximate to 12.5) leads only to a partial (similar to 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 pH(exp) = 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 L-III-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 angstrom and an edge-sharing linkage to Fe-octahedra with Pu-Fe distances of 3.68 angstrom), which is embedded in the magnetite matrix (Pu-Fe distances of 3.93, 5.17, and 5.47 angstrom). 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).