Silica-stabilized actinide(IV) colloids at near-neutral pH

Due to their low solubility, tetravalent actinides, An(IV), are usually assumed to be immobile in natural waters. However, it is also well known that insoluble precipitation products can be mobile if they occur as colloids. For An(IV) oxyhydroxides this phenomenon has thoroughly been studied [1]. Here we describe the formation of a new type of An(IV) colloids [2].
Evidence is provided that uranium(IV) and Th(IV) can form silicate-containing colloids in near-neutral solutions containing background chemicals of geogenic nature (carbonate, silicate, sodium ions). These particles remain stable in aqueous suspension over years. A concentration of up to 10-3 M of colloid-borne An(IV) was observed which is a concentration significantly 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. The higher the silicate concentration and the pH, the smaller (and obviously the more stable) are the particles that are formed (however, silicate at the concentrations tested does not form particles in the absence of the actinides). Electrostatic repulsion due to a negative zeta potential caused by the silicate stabilizes the nanoparticles. The mechanism of colloidal stabilization can be regarded as “sequestration” by silicate, a phenomenon well known from trivalent heavy metal ions such as iron(III) [3] or curium(III) [4], but never reported for tetravalent actinides so far. 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. The next-neighbor coordination of U(IV) in the U(IV)-silica colloids is comparable with that of coffinite, USiO4.
The assessment of actinide behavior in the aquatic environment should take the possible existence of An(IV)-silica colloids into consideration. Their occurrence might influence actinide migration in anoxic waters.

[1] Altmaier et al. (2004) Radiochim. Acta 92, 537-543. [2] Dreissig et al. (2011) Geochim. Cosmochim. Acta 75, 352-367. [3] Robinson et al. (1992) J. Am. Water Works Assn. 84, 77-82. [4] Panak et al. (2005) Radiochim. Acta 93, 133-139.