Complexation in the System UO22+/PO43-/OH-(aq): potentiometric and spectroscopic investigations at very low ionic strenghts

Sample analysis of seepage waters from old uranium mill tailings in Saxony and Thuringia shows both phosphate and uranium contents up to 10(-5) molar. Therefore the impact of phosphate on the speciation and transport behavior of uranium has to be investigated. At present, available complexation data are based only on few experiments and are not fully consistent. Therefore, the complex stability constants of the various 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 fluorescence spectroscopy (TRLFS). Fluorescence lifetimes were obtained for the free 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 results are compared with the values recommended by NEA [3] and with some recent publications.