STRUCTURAL ENVIRONMENT OF URANIUM (VI) AND EUROPIUM (III) SPECIES SORBED ONTO PHOSPHATE SURFACES - XPS AND OPTICAL SPECTROSCOPY STUDIES

In order to characterize the structure of the surface complexes formed by interaction between uranyl and europium (III) ions and the surface of solid matrices, optical and X-ray photoelectron spectroscopies exp eriments on uranyl/europium loaded phosphate solids (Th4P2O7(PO4)(4), ZrP2O7, and Zr2O(PO4)(2)) have been performed. The use of complimentar y spectroscopic techniques allows an identification of the sorption me chanism and a structural characterization of the sorption sites and th e sorbed species on phosphate surfaces. The samples were prepared from aqueous uranyl or europium solutions in the pH range from 1.5 to 6.0. The surface coverage was varied from 1 to 40 % of a monolayer. The di fferences between the emission spectra of europium ions either sorbed on the surface of phosphate samples or doped inside the solid unambigu ously indicates that these sorbed ions are exclusively located on the surface and that they do not migrate inside the matrix, which shows cl early that surface complexation is involved during the sorption proces s. The U4f XPS spectrum of uranyl ions sorbed on zirconium diphosphate exhibits only one component, while the spectrum corresponding to uran ium on thorium matrix shows two different unresolved peaks attributed to two different chemical environments. These results, corroborated by the uranyl emission spectra and the associated decay times and those obtained by optical spectroscopy of europium sorbed on the same solids , have been interpreted in terms of two sorption sites probably formed by the oxygens of the PO4 and P2O7 surface groups. (C) 1998 Academic Press