The influence of silica to stabilize actinide(IV) colloids at neutral pH

The solubility and environmental mobility of the actinides under reducing conditions is a widely discussed issue. Tetravalent actinides show a strong tendency towards hydrolysis followed by the formation of oligomers and colloids. However, An(IV) oxohydroxide colloids easily precipitate already at low pH values. We found that silica is able to stabilize such colloids at neutral pH through modification of the inner structure and by influencing the surface charge [1]. The internal structure of the particles can be characterized by An-O(H)-Si bonds that replace the An-O(H)-An bonds of the oxyhydroxide structure. The structure and the formation process were investigated with EXAFS, LIBD, XPS, HEXS, UV-Vis and NMR. The results suggest that the assessment of actinide behaviour in the aquatic environment should take the possible existence of An(IV)-silica colloids into consideration.