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

Green rust (GR) as sodium sulphate form, NaFe(II) (6) Fe(III) (3) (OH) (18) (SO 4) (2) center dot 12H(2)O, 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), highangle 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 mechanismforNp(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 andmore particularly theHRTEMfindings 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.