Modeling of Neptunium(V) Sorption Behavior onto Iron-Containing Minerals
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MODELING OF NEPTUNIUM (V) SORPTION BEHAVIOR ONTO IRON-CONTAINING MINERALS T. FUJITA, M. TSUKAMOTO, and T. OHE Central Research Institute of Electric Power Industry (CRIEPI) 2-11-1, Iwado-Kita, Komae, Tokyo 201, JAPAN S. NAKAYAMA and Y. SAKAMOTO Japan Atomic Energy Research Institute (JAERI) Tokai, Naka, Ibaraki 319-11, JAPAN ABSTRACT Sorption behaviors of neptunium(V) on naturally-occurring magnetite (Fe3 O4 ) and goethite (a-FeOOH) in 0.1M NaNO 3 electrolyte solution under aerobic conditions were interpreted using the surface complexation model (SCM). The surface properties of these materials were experimentally investigated by CO2 -ree potentiometric titration, and SCM parameters for the constant capacitance model, such as protonation/deprotonation constants of the surface hydroxyl group, were determined. The number of negatively charged sorption sites of goethite rapidly increased with the increase of the bulk solution pH compared with that of magnetite and this tendency was similar to the pH dependence of neptunium sorption. This implies that the neptunyl cation, NpO 2รท,plays a dominant role in possible sorption reactions. Assuming that the dominant surface complex is XO-NpO 2, modeling by means of SCM was carried out, and the results were found to agree with experimental data. INTRODUCTION Neptunium-237 is included in high-level nuclear waste, although the amount is relatively small compared to other nuclides [11. However, due to its long half-life and the potential hazard to humans, Np-237 is considered to be one of the most hazardous radionuclides in the time range from 10 to 10' years after disposal [21. An understanding of Np-237 sorption behavior is thus required to quantitatively describe the transport of this actinide in surface and groundwater systems.
It is known that some metal oxides have sorption ability for ions of transition elements. Potential corrosion products of the iron-based overpack, such as magnetite and goethite, are
therefore expected as effective sorbents for some radioisotopes released from waste glass. Although long-term corrosion analyses of overpacks have been carried out 13], they are still insufficient due to the uncertainty of surrounding environmental conditions and difficulties of long-term experiments. However if corrosion products have sorption ability for some radionuclides released from waste glass, it may compensate the insufficiency of long-term performances for radionuclide isolation by the overpack. A large amount of iron may be brought into a repository as an overpack. This means that corrosion products of an iron-based overpack may become a new barrier to radioisotope migration. For alkaline and transition elements, surface complexation models of sorption have been successfully used to describe both ions and aqueous complexes on several oxides [4]. This model has also been applied in recent studies of actinide sorption on iron oxides [51, clays [61 and rocks [7], but rarely for neptunium [8]. In this study, a surface complexation model was applied to the interpreta
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