COMPLEXATION IN THE SYSTEM UO22+ PO43-/OH-(AQ) - PATENTIOMETRIC AND SPECTROSCOPIC INVESTIGATIONS AT VERY-LOW IONIC STRENGTHS/

Sample analysis of seepage waters from old uranium mill tailings in Sa xony and Thuringia shows both phosphate and uranium contents up to 10( -5) molar. Therefore the impact of phosphate on the speciation and tra nsport behavior of uranium has to be investigated. At present, availab le complexation data are based only on few experiments and are not ful ly consistent. Therefore, the complex stability constants of the vario us uranyl(if) phosphate complexes were determined Over a pH range from 2 to 5 for uranyl and phosphate concentrations to 10(-6) molar, using potentiometric titrations and time-resolved laser-induced fluorescenc e spectroscopy (TRLFS). Fluorescence lifetimes were obtained for the f ree uranyl cation and uranyl phosphate complexes. Stability constants were determined for the complexes UO2(H2PO4)(2)(aq), UO2(H2PO4)(+) and UO2HPO4(aq), using the software C-LETAGROP [1] and SQUAD [2]. The res ults are compared with the values recommended by NEA [3] and with some recent publications.