Investigations into the Formation of Intrinsic Colloids and Pseudocolloids of Uranium(IV)

Investigations into the formation of intrinsic colloids and pseudocolloids of uranium(IV) were performed. Coulometric titrations of strongly acidic U(IV) solutions in HClO4/NaClO4 medium were carried out in order to find the pH of first colloid formation. Laser-induced breakdown detection (LIBD) was applied for the detection of traces of uranium colloids as the pH was increased. The pH values at the onset of colloid formation were used for thermodynamic calculations aimed at determining the solubility products of crystalline and amorphous uranium dioxide. Furthermore, the UO2·xH2O(am) colloids were investigated for their colloidal stability. Zeta potentials of ≥ 30 mV were found for pH values of less than 4. The point of zero charge was found at pH ~ 6.9. Colloidal suspensions of 1 mMol/L UO2·xH2O(am) at a pH value of 2.5 proved to be stable over > 1.5 years. The interaction of the forming UO2·xH2O(am) colloids with other colloid-forming species was tested. Therefore, coulometric titrations of U(IV) solutions in the presence of (a) dissolved Al(III) and (b) dissolved silicate were carried out. The presence of Al(III) does not significantly influence the formation of the UO2·xH2O(am) colloids, and the presence of U(IV) does not significantly influence the behaviour of Al(III). Also silicate does not significantly influence the pH of UO2·xH2O(am) colloid formation. However, the presence of U(IV) significantly influences the behaviour of the silicate (formation of pseudocolloids). The nature of the interaction between the silicate and the U(IV) is not yet known (adsorption? formation of a coffinite precursor?). EXAFS experiments to elucidate the local structure around the U(IV) atoms in the pseudocolloids are under preparation.