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Dhanpat Rai, Nancy J. Hess, Mikazu Yui, Andrew R. Felmy, Dean A. Moore

Thermodynamics and solubility of (U_xNp_1-x)O₂(am) solid solution in the carbonate system

The formation of a solid solution can significantly affect the solubility of a minor component. The objectives of this study were to determine the nature of U(IV) and Np(IV) solid solutions and their thermodynamic properties. For this purpose, (U_xNp_1-x)O₂(am) precipitates with values of x varying from 0.05 to 0.95 were prepared, characterized, and used to determine their solubility in dilute to concentrated K₂CO₃ and KHCO₃ solutions. The solubility data show that the equilibrium in these systems is reached in < 7 days, the solubility-controlling solids are amorphous, and that we were successful in maintaining U and Np in the tetravalent state. A scanning electron microscope equipped with energy dispersive spectrometry and X-ray absorption spectroscopic analyses of the solids indicated the quilibrated co-precipitates to be solid solutions. A close agreement between the thermodynamic predictions, assuming an ideal solid solution, and the experimental solubilities shows that U(IV) and Np(IV) form an ideal solid solution and that non-ideal solid solution behavior for this system can be ruled out.

Radiochimica Acta, Oldenbourg Wissenschaftsverlag

Print ISSN: 0033-8230
Volume: 92, 09/2004
Pages: 527 - 535

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