Formation of Uranium(IV) Colloids

Tetravalent uranium is the typical form of uranium in reducing natural waters such as deep groundwaters or waters of the deeper zones of abandoned uranium mines. The colloidal behaviour of uranium(IV) was investigated. Coulometric titrations of strongly acidic U(IV) solutions in HClO4/NaClO4 medium (I = 0.2 M) 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 (cf. [1]). 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 at pH ~ 6.9. Colloidal suspensions of 10-3 M 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). There is more interaction of the U(IV) with silicate. Silicate, on the one hand, does not significantly influence the pH value of first UO2·xH2O(am) colloid formation, i.e. it does not significantly influence UO2·xH2O(am) solubility. However, the presence of U(IV) significantly influences the precipitation behaviour of the silicate (formation of U(IV)-Si “pseudocolloids” due to co-precipitation). The nature of the interaction between the silicate and the U(IV) is not yet known. Adsorption of the silicate onto the UO2·xH2O(am) colloids and/or formation of a coffinite (USiO4) precursor come into question. Measurements by extended X-ray absorption fine structure (EXAFS) spectroscopy with synchrotron radiation to elucidate the local structure around the U(IV) atoms in the pseudocolloids are underway. Reference: [1] Opel K., Weiß S., Hübener S., Zänker H., Bernhard G. Radiochim. Acta, 95 (2007) 143-149.