Speciation and Structural Study of U(IV) and -(VI) in Perchloric and Nitric Acid Solutions

In order to elucidate the uranium solution chemistry at the high HNO₃ concentrations typically employed for the reprocessing of spent nuclear fuels, speciation and complex structures of U^IV and U^VI are studied in aqueous HNO₃ solutions, as well as in HClO₄ solutions, by means of UV-visible-near-infrared and X-ray absorption spectroscopies and density functional theory calculations. In 1.0 M HClO₄, U^IV exists as a spherical cation of U⁴⁺, which is surrounded by 9-10 water molecules in the primary coordination sphere, while it forms a colloidal hydrous oxide, U^IVO₂ nH₂O, at a lower acidic concentration of 0.1 M HClO₄. U^VI exists as a transdioxo uranyl cation, UO₂ ²⁺, and forms a 5-fold pure hydrate complex of [U^VIO₂(H₂O)₅]²⁺ in 1.0M HClO₄. With increasing HNO₃concentration, the water molecules of the U^IV and U^VI hydrate complexes are successively replaced by planar bidentate coordinating nitrate ions (NO₃ ^-), forming dominant species of [U^IV(H₂O)_x(NO₃)₅]^- in 9.0 M HNO₃ and [U^VIO₂(NO₃)₃]^- in 14.5 M HNO₃, respectively. The present multitechnique approach also suggests the formation of two intermediate U^VI species, a 5-fold mononitrato complex ([U^VIO₂(H₂O)₃(η₂-NO₃)]⁺) and a 6-fold dinitrato complex ([U^VIO₂(H₂O)₂(η₂-NO₃)₂]⁰), involving an increase in the total coordination number on the uranyl(VI) equatorial plane from 5 to 6 with increasing HNO₃ concentration. The presence of unidentate coordinate nitrato complexes or tetranitrato U^VI complexes is less probable in the present HNO₃ system.