Uranium(IV)-Silica colloids at near-neutral pH

Evidence is provided by photon correlation spectroscopy, ultrafiltration and ultracentrifugation that uranium(IV) can form silicate-containing colloids. The particles are generated in near-neutral to slightly alkaline solutions containing background chemicals of geogenic nature (carbonate, silicate, sodium ions). They remain stable in aqueous suspension over years. A concentration of up to 10-3 M of colloid-borne U(IV) was observed which is a concentration much higher than the concentrations of truly dissolved or colloidally suspended waterborne An(IV) species hitherto reported for the near-neutral pH range. The prevailing size of the particles is below 20 nm. Laser Doppler velocimetry reveals that the nanoparticles are stabilized in solution by electrostatic repulsion due to a negative zeta potential caused by the silicate. The isoelectric point of the nanoparticles is shifted toward lower pH values by the silicate. Extended X-ray absorption fine structure (EXAFS) spectroscopy showed that U-O-Si bonds, which increasingly replace the U-O-U bonds of the amorphous uranium(IV) oxyhydroxide with increasing silicate concentrations, make up the internal structure of the colloids.