Aquatic chemistry and solubility phenomena of actinide oxides/hydroxides

The present paper gives an overview of the solubility behavior of actinide oxides/hydroxides, taking into account hydrolysis reactions up to colloid formation. The analogies, systematic trends, and differences in the hydrolysis and solubility constants of actinides in the oxidation states An(III) to An(VI) correlate with the charge and size of the actinide ions. The formation of amorphous and crystalline solids and the discrepancies between the corresponding experimental solubility data may be explained as an effect of particle size. However, using thermodynamic data for the stable crystalline solids, the predicted solubilities are often significantly lower than experimental data (even in long-time experiments), indicating that the solubility is controlled by the surface properties. Typical examples are the known U(VI) solids schoepite and sodium diuranate. The most striking example is provided by the tetravalent actinides. Above the threshold of hydrolysis, the dissolution of microcrystalline or crystalline An(IV) oxides is found to be irreversible. The measured concentrations approach those of the amorphous hydroxides or hydrous oxides.