Roberts, and 6 co-authors, reported in 2017 that an understanding of the interactions between radionuclides and mineral phases underpins site environmental cleanup and waste management in the nuclear industry. The transport and fate of radionuclides in many subsurface environments are controlled by adsorption, redox, and mineral incorporation processes. Interactions of iron (oxyhydr)oxides with uranium have been extensively studied because of the abundance of uranium as an environmental
contaminant and the ubiquity of iron (oxyhydr)oxides in engineered and natural environments. Despite this, detailed mechanistic information regarding the incorporation of uranium into Fe(II)-bearing magnetite and green rust is sparse.
The authors presented a co-precipitation study in which U(VI) was reacted with environmentally relevant iron(II/III) (oxyhydr)oxide mineral phases. On the basis of diffraction, microscopic, dissolution, and spectroscopic evidence, the authors show that the reduction of U(VI) to U(V) and stabilization of the U(V) by incorporation within the near surface and bulk of the particles during co-precipitation with iron (oxyhydr)oxides. U(V) was stable in both magnetite and green-rust structures and incorporated via substitution for octahedrally coordinated Fe in a uranate-like coordination environment. As the Fe(II)/Fe(III) ratio increased, a proportion of U(IV) was also precipitated as surface-associated UO2. These novel observations have significant implications for the behavior of uranium within engineered and natural environments.
The authors indicated that U(V) incorporation during magnetite and green rust formation occurs via a two-step process (see abstract graphic). First, the authors suggested that U(VI) is adsorbed to the surface of the growing Fe(II)/Fe(III) (oxyhydr)oxide nanoparticles and undergoes reduction to U(V) via one-electron transfer. Second, during rapid crystal growth, the U(V), which is compatible with the Fe octahedral site in both magnetite and green rust, becomes incorporated within the structure. These steps are consistent with those proposed for U(V) incorporation during goethite formation; however, further research is required to determine the exact nature of each step in the process.
Recent work has highlighted that disproportion of U(V) can occur on the magnetite surface, implying that stabilization of structural U(V) may occur only during rapid crystal growth. Under these conditions, isolation of U(V) within the mineral structure may prevent its disproportionation to U(IV) and U(VI). Indeed, U(V) is reportedly stable in magnetite for up to 550 days, suggesting that incorporated U(V) may be stable over extended time periods. Furthermore, steric constraints seem likely to favor U(V) stabilization in these systems as U(IV) strongly prefers larger coordination environments (e.g., n = 8). Indeed, steric constraints may be the most crucial factor in stabilizing U(V) as both magnetite and green rust are electrically conductive, and therefore, physical isolation of U(V) alone seems unlikely to prevent disproportionation.
The authors concluded that the reduction of U(VI) to U(V) followed by incorporation into both magnetite and green rust is the dominant process during the direct precipitation of these mineral phases. The potential for U(V) incorporation processes to be dominant in these environmentally relevant phases is certainly highlighted in this and other very recent work and has important implications, which have not been fully explored, for understanding and predicting U mobility in engineered and natural environments.
Clearly, the extent of incorporation of U(V) into Fe(II)/Fe(III) (oxyhydr) oxides forming under contaminated land and geodisposal conditions needs to be fully quantified. Incorporation of U(V) into mineral phases commonly present in contaminated environments may offer a more resilient species for oxidative remobilization compared to uraninite and adsorbed U(IV), which are readily oxidized to soluble U(VI) in many environmental systems. The incorporation of U(V) into magnetite and green rust offers a promising prospect for optimizing its incorporation in a range of engineered settings, while the stability of U(V) in these systems clearly warrants further investigation.
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