Uranium(V) incorporation mechanisms and stability in Fe(II)/Fe(III) iron (oxyhydr)oxides

Understanding the interactions between radionuclides and mineral phases underpins site environmental clean-up and waste management in the nuclear industry. The transport and fate of radionuclides in many subsurface environmental systems are controlled by adsorption, redox and mineral incorporation processes. Interactions of iron (oxyhydr)oxides with uranium have been extensively studied due to both 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 where U(VI) was reacted with environmentally relevant iron(II/III) (oxyhydr)oxide mineral phases. Based on diffraction, microscopic and spectroscopic evidence, we propose the reduction of U(VI) via a one electron transfer to U(V) and stabilisation of the U(V) by incorporation 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 behaviour of uranium within engineered and natural environments.