Uranium dioxide in ionic liquid with a tri-n-butylphosphate–HNO3 complex—dissolution and coordination environment

Abstract

Uranium dioxide can be dissolved directly in an imidazolium-based ionic liquid (IL) at room temperature with a tri-n-butylphosphate(TBP)–HNO₃ complex. The dissolution process follows pseudo first-order kinetics initially. Raman spectroscopic studies show the dissolved uranyl ions are coordinated with TBP in the IL phase with a molar ratio of (UO₂)²⁺ : TBP = 1 : 2. The dissolved uranyl species can be effectively transferred to a supercritical fluid carbon dioxide (sc-CO₂) phase. No aqueous phase is formed in either the IL dissolution or the supercritical fluid extraction process. Absorption spectra of the extracted uranyl species in the sc-CO₂ phase suggests the presence of a UO₂(TBP)₂(NO₃)₂ and HNO₃ adduct probably of the form UO₂(TBP)₂(NO₃)₂·HNO₃. The adduct dissociates in a water–dodecane trap solution during pressure reduction resulting in UO₂(TBP)₂(NO₃)₂ collected in the dodecane phase.