The application of novel hydrophobic ionic liquids to the extraction of uranium(vi) from nitric acid medium and a determination of the uranyl complexes formed

Abstract

Novel ammonium based hydrophobic ionic liquids (ILs) have been synthesised and characterised, and their use in the liquid–liquid extraction of uranium(VI) from an aqueous nitric acid solution using tri-n-butyl phosphate (TBP), studied. On varying the nitric acid concentration, each IL was found to give markedly different results. Relatively hydrophilic ILs showed high uranium(VI) extractability at 0.01 M nitric acid solution which progressively decreased from 0.01 to 2 M HNO₃ and then increased again as the nitric acid concentration was increased to 6 M. An analysis of the mechanisms involved for one such IL, pointed to cationic-exchange being the predominant route at low nitric acid concentrations whilst at high nitric acid concentrations, anionic-exchange predominated. Strongly hydrophobic ILs showed low extractability for nitric acid concentrations below 0.1 M but increasing extractability from 0.1 M to 6 M nitric acid. The predominant mechanism in this case involved the partitioning of a neutral uranyl complex. The uranyl complexes were found to be UO₂²⁺·(TBP)₃ for the cationic exchange mechanism, UO₂(NO₃)₂(TBP)₂ for the neutral mechanism and UO₂(NO₃)₃⁻·(TBP) for the anionic exchange mechanism.