Formation of 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 of a size of 620 nm. A concentration of up to 103 M of colloid-borne U(IV) was observed. 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. Electrostatic repulsion due to a negative zeta potential in the near-neutral to alkaline pH range caused by the silicate stabilizes the U(IV) colloids. The isoelectric point of the nanoparticles is shifted toward lower pH values by the silicate. The mechanism of the colloidal stabilization can be regarded as “sequestration” by silicate, a phenomenon well known from heavy metal ions of high ion potential such as iron(III) or manganese(III,IV), but never reported for uranium(IV) so far. 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) xyhydroxide with increasing silicate concentrations, make up the internal structure of the colloids. The next-neighbor coordination of U(IV) in the U(IV)-silica colloids is comparable with that of coffinite, USiO4. The assessment of uranium behavior in the aquatic environment should take the possible existence of U(IV)-silica colloids into consideration. Their occurrence might influence uranium migration in anoxic waters.