Influence of ionic strength, pH and silicate on colloidal UO2 formation

The solubility of uranium in aqueous solutions under anaerobic conditions is low and the probability of solid UO2 formation increases with the U(IV) concentration. Only in solutions with low pH and high ionic strength the U(IV) hydrate itself is stable even at higher U(IV) concentrations. With increasing pH several U(IV) hydroxides becomes dominant. Below their solubility limits two solid phases can be observed, i.e. the microcrystalline UO2 and the amorphous UO2•xH2O/U(OH)4 which both can occur as colloids in solution. Also the presence of silicate influence the formation and structure of U(IV) nanoparticles. However, even in solutions with equimolar ratio of UO2 and SiO2 the precipitation do not result in the formation of a USiO4-like phase, which can be observed easily under hydrothermal conditions. Some structural aspects of the resulting solid phases have been investigated with x-ray diffraction, EXAFS and wide angle x-ray scattering techniques and will be briefly discussed.