On the Interaction between uranyl carbonate and UO2(s) in anaerobic solution

It was repeatedly observed that in nearly neutral anoxic solutions (10 mM NaCl, 2 mM NaHCO3-, pH ~ 8.5) U(VI) concentration decreases from 10-5 M to 10-7 M after 24 days interaction with UO2(s) surfaces. The formation of a layer of hyper-stoichiometric uranium oxide UO2+x(s) on fresh UO2(s) surfaces is proposed as a possible reaction mechanism. Gibbs free energy calculations and other batch experiments were carried out to test the validity of this hypothesis. The calculated negative AG values indicate that the formation of a layer of UO2+x (x=0.25 or 033) is thermodynamically possible. The U(VI)/U(IV) ratio of the reacted uranium oxide surface is 0375, corresponding to UO2.27(s). The average U(VI)/U(IV) ratio obtained for the suspended particles worn off from the reacted UG2(s) surface on an ultrasonic bath resulted 0.162. Our preliminary results from this work support the above hypothesis, which might be also an explanation for some of the results obtained during spent fuel leaching experiments under ana...