Thermodynamics and solubility of (U x Np1- x )O2(am) solid solution in the carbonate system

Journal Article
Radiochimica Acta, vol. 92, iss. 9-11, pp. 527-535, 2007
Dhanpat Rai, Nancy J. Hess, M. Yui, Andrew R. Felmy, D. A. Moore
Summary 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 x Np1- 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.