Uranium redox transition pathways in acetate-amended sediments

Redox transitions of uranium [from U(VI) to U(IV)] in low-temperature sediments govern the mobility of uranium in the environment and the accumulation of uranium in ore bodies, and inform our understanding of Earth's geochemical history. The molecular-scale mechanistic pathways of these transitions determine the U(IV) products formed, thus influencing uranium isotope fractionation, reoxidation, and transport in sediments. Studies that improve our understanding of these pathways have the potential to substantially advance process understanding across a number of earth sciences disciplines. Detailed mechanistic information regarding uranium redox transitions in field sediments is largely nonexistent, owing to the difficulty of directly observing molecular-scale processes in the subsurface and the compositional/physical complexity of subsurface systems. Here, we present results from an in situ study of uranium redox transitions occurring in aquifer sediments under sulfate-reducing conditions. Based on molecular-scale spectroscopic, pore-scale geochemical, and macroscale aqueous evidence, we propose a biotic-abiotic transition pathway in which biomass-hosted mackinawite (FeS) is an electron source to reduce U(VI) to U(IV), which subsequently reacts with biomass to produce monomeric U(IV) species. A species resembling nanoscale uraninite is also present, implying the operation of at least two redox transition pathways. The presence of multiple pathways in low-temperature sediments unifies apparently contrasting prior observations and helps to explain sustained uranium reduction under disparate biogeochemical conditions. These findings have direct implications for our understanding of uranium bioremediation, ore formation, and global geochemical processes

Formation of stable uranium(VI) colloidal nanoparticles in conditions relevant to radioactive waste disposal

The favored pathway for disposal of higher activity radioactive wastes is via deep geological disposal. Many geological disposal facility designs include cement in their engineering design. Over the long term, interaction of groundwater with the cement and waste will form a plume of a hyperalkaline leachate (pH 10-13), and the behavior of radionuclides needs to be constrained under these extreme conditions to minimize the environmental hazard from the wastes. For uranium, a key component of many radioactive wastes, thermodynamic modeling predicts that, at high pH, U(VI) solubility will be very low (nM or lower) and controlled by equilibrium with solid phase alkali and alkaline-earth uranates. However, the formation of U(VI) colloids could potentially enhance the mobility of U(VI) under these conditions, and characterizing the potential for formation and medium-term stability of U(VI) colloids is important in underpinning our understanding of U behavior in waste disposal. Reflecting this, we applied conventional geochemical and microscopy techniques combined with synchrotron based in situ and ex situ X-ray techniques (small-angle X-ray scattering and X-ray adsorption spectroscopy (XAS)) to characterize colloidal U(VI) nanoparticles in a synthetic cement leachate (pH > 13) containing 4.2-252 μM U(VI). The results show that in cement leachates with 42 μM U(VI), colloids formed within hours and remained stable for several years. The colloids consisted of 1.5-1.8 nm nanoparticles with a proportion forming 20-60 nm aggregates. Using XAS and electron microscopy, we were able to determine that the colloidal nanoparticles had a clarkeite (sodium-uranate)-type crystallographic structure. The presented results have clear and hitherto unrecognized implications for the mobility of U(VI) in cementitious environments, in particular those associated with the geological disposal of nuclear waste