Neptunium Speciation in Humic Acid-rich Clay Water upon Interaction With Radioactive Waste Glass Samples
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Neptunium Speciation in Humic Acid-rich Clay Water upon Interaction With Radioactive Waste Glass Samples Pirlet V., Van Iseghem P. Waste and Disposal Department, SCK•CEN (Belgian Nuclear Research Center) Mol, Belgium ABSTRACT Organic complexes of actinides are known to occur upon interaction of high level waste glass and Boom Clay which is a potential host rock formation for disposal of high level waste in Belgium. The solubility and mobility of 237Np, one of the most critical radionuclides, can be affected by the high dissolved organic carbon content of the Boom Clay porewater through complexation with the humic substances. The influence of humic substances on the Np behaviour is considered through dissolution tests of Np-doped glasses in Boom Clay water and through fundamental study of the specific interaction between Np(IV) and the humic acids using spectroscopic techniques. High Np(IV) concentrations are found in the glass dissolution tests. These concentrations are higher than what we should expect from the solubility of Np(OH)4, the solubility limiting solid phase predicted under the reducing conditions and pH prevailing in Boom Clay. Studying the specific interaction of Np(IV) with humic acids in Boom Clay porewater, high soluble Np concentrations are also measured and two main tetravalent Np-humate species are observed by UV-Vis spectroscopy. The two species are interpreted in terms of mixed hydroxo-humate complexes, Np(OH)xHA with x = 3 or 4. These species are the most likely species that can form according to the pH working conditions. Using thermodynamic simplified approaches, high complexation constants, i.e. log β 131 and log β 141 respectively equal to 46 and 51.6, are calculated for these species under the Boom Clay conditions. Comparing the spectroscopic results of the dissolution tests with the study of the interaction of Np(IV) with humic substances, we can conclude that the complexation of Np(IV) with the humic acids may occur and increases the solubility of Np(OH)4 upon interaction of a Npdoped glass and the Boom Clay porewater. INTRODUCTION 237
Np with its half-life of two millions years and its relatively large abundance in nuclear waste is recognised to be of large importance for the long-term safety of the disposal of vitrified nuclear waste forms in accordance with the present disposal concept in Belgium [1]. Boom Clay is the potential host rock formation for disposal of high level waste in Belgium. The interstitial solution in Boom Clay contains a high dissolved organic carbon (DOC) concentration (up to 200 ppm) which can have an impact on the Np solubility and mobility due to complexation by the organic molecules and mostly by the humic substances. Humic substances are naturally occuring polyelectrolytes containing a large number of complexing functional groups which appear to be responsible for the complexation of radionuclides. The complexation of the oxidised form of Np, Np(V), with humic substances is often reported in the literature [2-10]. The neptunyl cation has a low affi
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