Solvent extraction of U(VI) by trioctylphosphine oxide using a room-temperature ionic liquid

The unique physical and chemical properties of room-temperature ionic liquids (RTILs) have recently received increasing attention as solvent alternatives for possible application in the field of nuclear industry, particularly in liquid-liquid separations of radioactive nuclides. We investigated solvent extraction of U(VI) from aqueous solutions into a commonly used ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₄mim][NTf₂]) using trioctylphosphine oxide (TOPO) as an extractant. The effects of contact time, TOPO concentration, acidity, and nitrate ions on the U(VI) extraction are discussed in detail. The extraction mechanism was proposed based on slope analysis and UV-Vis measurement. The results clearly show that TOPO/[C₄mim][NTf₂] provides a highly efficient extraction of U(VI) from aqueous solution under near-neutral conditions. When the TOPO concentration was 10 mmol/L, the extraction of 1 mmol/L U(VI) was almost complete (> 97%). Both the extraction efficiency and distribution coefficient were much larger than in conventional organic solvents such as dichloromethane. Slope analysis confirmed that three TOPO molecules in [C₄mim][NTf₂] bound with one U(VI) ion and one nitrate ion was also involved in the complexation and formed the final extracted species of [UO₂(NO₃)(TOPO)₃]⁺. Such a complex suggests that extraction occurs by a cation-exchange mode, which was subsequently evidenced by the fact that the concentration of C₄mim⁺ in the aqueous phase increased linearly with the extraction percent of U(VI) recorded by UV-Vis measurement.