Actinides, including uranium, neptunium and plutonium, are significant in subsurface contaminated land sites impacted by nuclear fuel cycle operations, and within radioactive wastes associated with geological disposal facilities. In both systems, the speciation and mobility of actinides is controlled, to a significant extent, by interactions with mineral particle, including iron (oxyhydr)oxides. In addition, the decontamination of radioactively contaminated effluents often relies on effective sorption of radionuclides to iron (oxyhydr)oxide nanoparticules. In these natural and engineered systems complex pathways of particle formation and transformation occur with time and changing biogeochemical conditions. Key to understanding the sequestration of actinides by these phases is quantifying their lability and speciation during particle nucleation, growth, crystallisation and transformation. This information then provides detailed insights into how the mobility of radionuclides could evolve over time. We have utilised a range of techniques including in situ small angle X-ray scattering (SAXS), X-ray photoemission spectroscopy (XPS) and transmission electron microscopy (TEM) to characterize the formation of ferrihydrite, and its crystallisation (hematite) and transformation (mackenawite) at the nanoscale. In conjunction with this we have utilised X-ray absorption spectroscopy (XAS) and nanoscale chemical mapping to determine the sorption mechanism of Pu, Np and U during these mineral reactions. During ferrihydrite formation Pu4+and U6+adsorb to ferrihydrite, with evidence of direct interaction of Pu with the Fe13Keggin precursor. During crystallisation to hematite 60-75% of adsorbed U(VI) becomes incorporated within hematite across 6 orders of magnitude of U:Fe molar ratios (6.3x10-9to 5.9x10-3). These data indicate that incorporation proceeds via a particle mediated growth mechanism, such as oriented attachment. In this mechanism, a constant proportion of the nanoparticle surface, and hence initially adsorbed U, is incorporated during hematite formation. Overall, these results provide a mechanistic understanding of the long term behaviour of actinides associated with iron (oxyhydr)oxide mineral phases.

Adsorption and incorporation of actinides during iron oxyhydroxide formation, crystallisation and transformation

2019 Spring National Meeting

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