Neptunyl (NpO2+) interaction with green rust, GRNa,SO4

Green rust (GR), a member of the Fe(II),Fe(III) layered double hydroxide mineral family, forms in groundwater and during steel corrosion. It has high surface area and is very reactive, especially for redox-sensitive elements such as some actinides. During neutron irradiation of nuclear fuel in a reactor, Np-237 develops. Although the abundance of Np in spent nuclear fuel is only about 0.05% by mass, it has a very long half life, 2.14 x 10(6) years, so there is concern about its mobility in the distant future, when radioactive storage sites might be expected to degrade. Under oxidizing conditions, as are expected from radiolysis in a repository, the very mobile neptunyl-ion, NpO2+, would be the dominant aqueous neptunium species. In this work, we investigated the interaction of NpO2+ with green rust sodium sulphate (GR(Na,SO4)). The aim of the study was to define the processes involved, to determine the final redox speciation of Np, hence its potential mobility, and to characterise changes in the green rust. The GR(Na,SO4) sorbed and reduced NpO2+ within minutes. Reduced Np(IV) was primarily found as precipitated nanoparticles at the edges of the GR(Na,SO4) crystal platelets. The position of the particles at the crystal edges suggests initial sorption of Np(V) and subsequent reduction at GR edge sites. In further experiments, Np-containing GR was allowed to oxidise completely in aqueous suspension. Surprisingly, about 50% of the Np remained associated with the oxidised product and about 75% of the associated Np was still present as Np(IV). (C) 2010 Elsevier Ltd. All rights reserved.