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Solid-Water Interface Reactions of Actinides and Homologues : Sorption onto Mineral Surfaces

Different approaches to determine and quantify the solid-water interface reactions of actinide ions are discussed with notable examples. Surface complexation modelling of the Eu(III) sorption onto natural hematite is demonstrated to provide a better alternative to the current use of constant Kj-values. The model fits the experimental isotherm data at pH < 6 and even more complex systems, e.g. the presence of anionic ligands as humic acid, can be quantitatively described. Sensitive surface spectroscopic methods give a straightforward insight into sorption reaction mechanisms. The Time Resolved Laser Fluorescence Spectroscopy (TRLFS) is applied to investigate the Cm(III) sorption onto silica, from which different surface-sorbed Cm(III) species can be identified. The removal of almost all H20 from the first coordination sphere of the C m " ion on its sorption at p H > 6 indicates incorporation of Cm(III) into the sorbent structure. This may explain the observed kinetic hindrance of the metal ion desorption reaction at higher pH. As another surface sensitive method, Time Differential Perturbed Angular Correlation spectroscopy (TDPAC) is used for the surface speciation of Hf(IV), which is chosen as a chemical homologue for tetravalent actinides. Apart from the well defined experimental conditions in laboratory, the natural system is characterized by the presence of actinide trace concentrations, highly heterogeneous mineral surfaces and extremely long reaction time. As an approach to validate the laboratory derived sorption data for a given aquifer system, the in-situ K¿ values of U, Th and rare earth elements as chemical homologues for the actinides are compared with the respective K^-values obtained in laboratory. Discussion is made on individual experimental approaches and a concept for the quantitative description of solid-liquid interface reactions of actinides in natural systems is suggested.