Role of Valency and Defects in the Incorporation of Uranium into the Goethite [010] Surface: an Embedded Cluster Density Functional Theory Study

Incorporation of actinide species into iron (oxyhydr)oxides could present an environmentally secure method for preventing the release of actinides over an extended period, as would be the case in a number of radioactively contaminated land situations including surface, near-surface, and subsurface disposal and storage. Uranium is known to incorporate into iron (oxyhydr)oxides, including goethite, in a number of valence states, but the atomistic structures of these processes are unclear. In particular, it is increasingly reported that iron-containing minerals can reductively incorporate and stabilize the +V state of uranium, an oxidation state that is known to be unstable with respect to disproportionation. Here, we use density functional theory within the Periodic Electrostatic Embedded Cluster Method to model U(IV), U(V), and U(VI) incorporation into the pristine and iron-vacancy [010] surface and near-surface region of goethite. Solvated and unsolvated surfaces are studied, and the role of electron transfer from the lattice to uranium ions is explored. Comparisons are made with published X-ray absorption spectroscopic data, and we conclude that, based on the expected conditions for surface and near surface storage sites, both U(VI) and U(V) would incorporate into goethite as it transforms from ferrihydrite, forming two distinct structural types. We find that U(VI) incorporated into goethite may be reduced to U(V), where electron transfer occurs from oxygens surrounding iron vacancies and the incorporated uranium, reducing the U(VI) species to U(V). Both U(VI) and U(V) can incorporate into the surface of goethite with an adjacent iron vacancy, or U(V) can uniquely incorporate into the structure within the near-surface region, containing local but not immediately adjacent iron vacancies for charge compensation. Both of these incorporation schemes are little affected by the presence of a monolayer of surface water, suggesting that incorporation into goethite is a viable method to prevent uranium release into the aqueous surroundings.