TEM study of Green Rust Sodium Sulphate (GR_(Na,SO4)) Interacted with Neptunyl Ions (NpO_2~+)

摘要

Green rust (GR) as sodium sulphate form, NaFe(II)_6Fe(III)_3(OH)_(18)(SO_4)_2 ? 12H_2O, is reacted with an aqueous solution of neptunyl ions (NpO_2~+) 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 NpO_2 crystallizing in a fluorite-type structure. Furthermore, the Np-O_(4,5) edges recorded by EELS at the Np(IV) phase are presented, expanding the EELS-data set currently available in the literature for Np.