Uranium(V) Incorporation Mechanisms and Stability in Fe(II)/Fe(III) (oxyhydr)Oxides

Understanding 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. Here, we present 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, we show 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.