Silica-stabilised actinide(IV) colloids in neutral and slightly alkaline solutions

Due to their low solubility, tetravalent actinides, An(IV), are usually assumed to be immobile in natural waters. However, insoluble precipitation products can also be mobile if they occur as colloids. For An(IV) oxyhydroxides this phenomenon has thoroughly been studied. The formation of a further type of An(IV) colloids is described (cf. [1]): Silica-containing U(IV), Th(IV) and Np(IV) colloids formed in near-neutral to slightly alkaline solutions of background chemicals of geogenic nature (carbonate, silicic acid, Na+). Whereas the silica-containig colloids addressed in the BELBaR project are formed by the adsorption of radionuclides onto pre-existing silica particles, the colloids here under discussion result from a reaction of dissolved An(IV) with dissolved silicic acid (co-precipitation). An-O-Si bonds, which increasingly replace the An-O-An bonds of the amorphous actinide(IV) oxyhydroxide with increasing silica concentration, make up the internal structure of these colloids as was elucidated by EXAFS spectroscopy and further spectroscopic methods. A strong shift of the particles’ isoelectric point by silica incorporation is observed. The particles remain stable in aqueous suspension over years. A concentration of up to 10-3 M of colloid-borne An(IV) was observed. The prevailing size of the particles is below 20 nm (about 10 to 100 kDa). The question if such An(IV) colloids may contribute to the mobility of the actinides in the near-field or the far-field of a nuclear waste repository is discussed.