Kinetics of dissolution of β-uranium trioxide in acid and carbonate solutions

Abstract

The kinetics of dissolution of β-UO₃ in acid and carbonate solutions have been studied as functions of stirring rate, hydrogen-ion concentration, carbonate concentration, various other cations and anions in solution, and temperature. The dissolution rates are: (i) independent of stirring rate, (ii) dependent on the 0.5 order in [H⁺] in acid solutrons and a substantially lower order in carbonate solutions; (iii) inhibited by the presence of cations in solution in the order Mg²⁺ > Li⁺ > Na⁺ > [NH₄]⁺ > K⁺ Cs⁺ in acid solutions and Na⁺ > [NH₄]⁺ > Cs⁺ K⁺ in carbonate solutions, the relative effects of the cations in the acid and carbonate solutions being very similar; (iv) increased by sulphate but unaffected by chloride, nitrate, and perchlorate ions in acid solutions; and (v) characterised by activation energies of ca. 12 kcal mol^–1 in acid solutions and 15 kcal mol^–1 in carbonate solutions. The observed kinetic data have been rationalised in terms of a charge-transfer (c.t.) mechanism, in which the transfer of charged complexes of the uranyl and oxide ions of the β-UO₃ lattice across the electrochemical-potential barrier at the solid–solution phase boundary is the rate determining step during the dissolution reaction. In HCl, HClO₄, and HNO₃ the c.t. species are the uranyl and protonated oxide ions of the lattice, while in H₂SO₄ sulphatouranate complexes are also involved. In carbonate solution the dicarbonatouranate ion and the protonated oxide ion are the c.t. species. The β-UO₃ dissolution reactions are interesting examples of c.t. kinetics since the oxide anions and uranyl cations of the β-UO₃ lattice form complexes with the cations and anions in the solutions in the process of dissolution and these complex-formation reactions are mirrored in the rate expressions for dissolution.