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TEM study of Green Rust Sodium Sulphate (GRNa,SO4) Interacted with Neptunyl Ions (NpO2+)

Abstract Green rust (GR) as sodium sulphate form, NaFe(II)6Fe(III)3(OH)18(SO4)2 · 12H2O, is reacted with an aqueous solution of neptunyl ions (NpO2+) and the resulting Np(IV) solid phase is investigated at a nanometer scale by different transmission electron microscopy (TEM) techniques, including high-resolution TEM (HRTEM), high-angle annular dark-field scanning TEM (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDXS), and electron energy-loss spectroscopy (EELS). The aim of the analyses is to achieve insight into the potential immobilization mechanism for Np(V) in the context of safety assessment of a nuclear-waste repository. The neptunium is found to be immobilized at the edge of the green rust platelets, in a rim composed of nanocrystallites about 2–3.5 nm in size. The EELS results and more particularly the HRTEM findings are consistent with NpO2 crystallizing in a fluorite-type structure. Furthermore, the Np-O4,5 edges recorded by EELS at the Np(IV) phase are presented, expanding the EELS-data set currently available in the literature for Np.