Uranium is a significant contaminant in the global legacy of radioactive waste and contaminated land, with higher activity wastes destined for deep underground geological disposal, and radioactively contaminated land now being decommissioned. Iron (oxyhydr)oxide phases are expected to be ubiquitous in and around these engineered and natural environments and, given the numerous reports of uranium-incorporated iron (oxyhydr)oxides, these mineral phases may be an additional barrier to the migration of uranium (and other radionuclides) in the environment. However, the long-term stability of these phases under environmental perturbations is largely unknown. Specifically, U-incorporated iron (oxyhydr)oxide phases may interact with aqueous Fe(II) and aqueous sulfide species in the sub-surface. Significant electron transfer may occur between adsorbed Fe(II) and stable iron oxyhydroxides (e.g. goethite), with associated incorporated species released and/or reduced. On exposure to aqueous sulfide, iron (oxyhydr)oxides may also undergo extensive reductive dissolution. Here, U(VI)-goethite was synthesized, reacted with either aqueous Fe(II) or S(-II), and the system monitored over time using geochemical analysis and X-ray absorption spectroscopy (XAS). Reaction with aqueous Fe(II) resulted in significant electron transfer between Fe(II) and U(VI)-goethite, with up to 52% U(VI) reduced to U(V). XAS analysis revealed that electron transfer only occurred within the outermost atomic layers of goethite, with an enriched U(V)-goethite outer-layer and an unchanged core of U(VI)-goethite formed on reaction with Fe(II). Minimal recrystallisation of goethite (2%) occurred following electron transfer, with both U(VI) and U(V) retained within the mineral structure. Conversely, rapid reductive dissolution of U(VI)-goethite occurred on reaction with aqueous sulfide. Over the first day of sulfidation, a transient release of aqueous U was observed, followed by a progressive reductive scavenging to U(IV), and with solid phase U retained in the long-term as nanocrystalline UO<sub>2</sub>. Overall, these results further understanding of the long-term behavior of U-incorporated iron (oxyhydr)oxides under environmental perturbations.

Exploring the fate of U(VI)-incorporated goethite: Impact of environmental perturbations

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.