Aquatic chemistry and solubility phenomena of
actinide oxides/hydroxides*
Th. Fanghänel and V. Neck
Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung, Postfach 3640,
D-76021 Karlsruhe, Germany
Abstract: 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.
* Lectures presented at the 10th International Symposium
on Solubility Phenomena, Varna, Bulgaria, 22 -26 July 2002. Other
lectures are published in this issue, pp. 1785-1920.
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