Speciation Change of Uranyl in Lithium Borate Glasses

Determining the uranyl(VI) UO₂²⁺ reactivity in crystalline and amorphous oxides is necessary to control its mobility. The intrinsic versatility of borate structural units containing both triangular BO₃ and tetrahedral BO₄ makes them original and rich hosts for uranyl. As part of the effort to determine the uranium stability in borate oxides, we have determined the speciation of uranium(VI) in two lithium borate glasses containing, respectively, 10 mol % and 30 mol % Li₂O using a combined structural and spectroscopic approach based on X-ray absorption spectroscopy (XAS). M₄- and L₃-edge high-resolution XAS demonstrates the speciation of U(VI) as uranyl in both glasses. Comparison of uranyl bond distances obtained by EXAFS with distances found in borate crystals reveals that in the low alkali borate glass, uranyl is present as hexagonal bipyramids with six equatorial oxygen ligands. This local environment was never observed in any other oxide glass. We show that the increase of the lithium content induces the decrease of the equatorial coordination number. The associated uranyl bond elongation suggests the influence of the alkali cations in relation with drastic changes in the structure of the borate network. The spectroscopic evidence of this speciation change is discussed in terms of change in the uranyl electronic structure and covalency.