In all geodisposal scenarios it is key to understand the interaction of radionuclides with mineral particles during their formation/recrystallisation. Studying processes at the molecular scale provides insight into long-term radionuclide behaviour. Uranium is a significant radionuclide in higher activity wastes destined for geological disposal, and iron (oxyhydr) oxides (e.g. goethite, 􀟙-FeOOH). are ubiquitous in and around these systems, formed via processes including metal corrosion and microbially induced reactions. There are numerous reports of uranium-incorporation into iron (oxyhydr) oxides, therefore it has been suggested that they may be a barrier to uranium migration in geodisposal systems. However, long-term stability of these phases during environmental perturbations are unexplored. Specifically, U-incorporated iron (oxyhydr) oxide phases may interact with Fe(II) and sulphide from biological or geological origin. Firstly, electron transfer occurs between adsorbed Fe(II) and iron oxyhydroxides, with potential for changes in the speciation of incorporated uranium e.g. oxidation state changes and/or release. Secondly, on exposure to aqueous sulfide, iron (oxyhydr) oxides undergo reductive dissolution and recrystallisation to iron sulphides. Understanding the fate of incorporated uranium during these process in key to understanding its long term behaviour in subsurface systems. A series of experimental studies were undertaken where U(VI)-goethite was synthesized then reacted with either aqueous Fe(II) or S(-II), and the system monitored over time using geochemical analysis and X-ray absorption spectroscopy (XAS) techniques e.g. U LIII-edge and MIV-edge HERFD-XANES. Reaction with aqueous Fe(II) resulted in electron transfer between Fe(II) and U(VI)-goethite, with > 50% U(VI) reduced to U(V). XAS analysis revealed that U remained within the goethite structure, and electron transfer only occurred within the outermost atomic layers of goethite. which led to U reduction. Rapid reductive dissolution of U(VI)-goethite occurred on reaction with sulfide at pH7. A transient release of aqueous U was observed during the first day, likely due to uranyl(VI)-persulfide species. However, U was retained in the solid phase in the longer term. In contrast, the sulfidation of U adsorbed to ferrihydrite at pH 12.2 led to the immediate release of U (< 10% Utotal) associated with a colloidal erdite (NaFeS2·2H2O) phase. Moreover, in the bulk phase the surface of ferrihydrite was passivated by sulfide, and U was found to have been trapped within surface associated erdite-like fibres. Overall, these studies further understanding of the long-term behaviour of U-incorporated iron (oxyhydr)oxides supporting the overarching concept of iron (oxyhydr) oxides acting as a barrier to U migration.

60+ Years of nuclear power generation has led to a significant legacy of radioactively contaminated land at a number of nuclear licenced “mega sites” around the world. The safe management and remediation of these sites is key to ensuring there environmental stewardship in the long term. Bioremediation utilizes a variety of microbially mediated processes such as, enzymatically driven metal reduction or biominerialisation, to sequester radioactive contaminants from the subsurface limiting their migration through the geosphere. Additionally, some of these process can provide environmentally stable sinks for radioactive contaminants, through formation of highly insoluble mineral phases such as calcium phosphates and carbonates, which can incorporate a range of radionuclides into their structure. Bioremediation options have been considered and deployed in preference to conventional remediation techniques at a number of nuclear “mega” sites. Here, we review the applications of bioremediation technologies at three key nuclear licenced sites; Rifle and Hanford, USA and Sellafield, UK, in the remediation of radioactively contaminated land.