Colloid Generation and Stability in Simulated Reduced Groundwaters

The focus was on the investigation of U(IV) colloids during this phase of the project. First, a technique to reduce U(VI) to U(IV) was developed. Colloid formation and solubility of U(IV) in acidic HClO4/NaClO4 solutions were investigated by coulometric titration. Quantification of traces of U(VI) by laser fluorescence spectroscopy proved that the tetravalent state of uranium had been maintained. 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 dioxides. In acidic solutions, the UO2·xH2O(am) colloids showed colloidal stability over a period of at least 10 months during which they did not aggregate and precipitate. Relatively high zeta potentials were found for the UO2 colloids in the acidic and in the alkaline pH regions; the point of zero charge was at 6.9. No indications of the formation of mixed U(IV)-Al(III) complexes were found. There was also no influence of Al(III) on the UO2(cr) solubility.