Direct spectroscopic speciation of actinide ions at the water/mineral interface

The reactions of actinides at the water/mineral interface constitute the most important mechanism for their retention in the geosphere. A well-founded description of the sorption mechanism necessitates identification of interface reactions and their quantification by a thermodynamic model, e.g. a surface complexation model. However, surface models are usually parameterized by fitting experimental sorption data, without any confirmation of the species postulated. Whereas a number of high-vacuum methods are available for the speciation of sorbed metal ions, only few methods are applicable for metal ion speciation at surfaces in contact with the aqueous phase. EXAFS has recently been applied to various surface sorbed metal ions; however, its lack of sensitivity prohibits studies at low sorption densities. For speciation at trace concentrations, more sensitive methods such as EPR, Time Differential Perturbed Angular Correlation Spectroscopy (TDPAC), and Time Resolved Laser Fluorescence Spectroscopy (TRLFS) are necessary. Applicability of these methods is restricted to only few elements. We have used TDPAC in combination with EXAFS for the speciation of Hf(IV) (as a chemical homologue for tetravalent actinides) sorbed onto silica. TRLFS was used for the speciation of U(VI), Eu(III) and Cm(III) at various water/mineral interfaces. Advantages and disadvantages of these methods are shortly reviewed. Specific examples are presented for the speciation of the interface equilibrium reaction of Cm(III) by TRLFS with colloidal mineral phases such as silica, alumina, titania and hydrous ferric oxide. (author)