Uranium(IV) colloid in near-neutral solutions: Influences on particle size

Uranium(IV) is only sparingly soluble in near-neutral solutions [1]. As a consequence, uranium is expected to be immobile (deposited) under reducing conditions. However, we found out that under certain conditions U(IV) formes stable colloids which leads to a mobilization of the uranium. The colloids were generated by galvanostatic reduction of a 20 mM UO2(ClO4)2 solution in 1 M NaHCO3. The stock solutions were diluted to 1 mM uranium using solutions of dissolved silicic acid (2 – 5 mM). Then they were neutralized with perchloric acid. Measurement of the scattered light intensity (SLI), photon correlation spectroscopy (PCS) and ultracentrifugation (UC) were carried out.
Typically, the colloids produced did not reach their final particle size immediately but showed a certain growth phase of about one week. This could be seen from both PCS and simple SLI measurements.
It turned out that the highest SLIs occurred in the solutions of the lowest silicate content and vice versa. All samples exhibited an increase in SLI with decreasing pH. Obviously, very small colloidal particles are generated at high pH and/or high silicate concentration and larger ones at low pH and/or low silicate concentration. At the highest initial silicate concentration applied, the colloids were still stable at pH values below 7.
This silicate and pH dependency of U(IV) colloid particle size was also confirmed by our UC results. The smaller the colloids, the longer the time they needed for separation at the given centrifugal acceleration of 170000 x g. After 1 h of UC the supernatants of samples with higher silicate content still contained significantly higher U concentrations than samples with low silicate content. After 5 h UC time the uranium was almost completely removed from all samples, indicating that truly dissolved uranium did not play a significant part in the system, i.e. almost all the U was colloidal.