Effect of Alkyl Group in 1-Alkyl-3-methylimidazolium Hexafluorophosphate Ionic Liquids on the Extraction of Uranium by Tri-n-butylphosphate Diluted with Ionic Liquids

Abstract

Extraction of uranium by 1.1 M TBP dissolved in 1-octyl-3-methylimidazolium hexafluorophosphate (omimPF₆) ionic liquid (IL) as diluent has been studied as a function of concentration of nitric acid and uranium and that of temperature. The results are compared with those of extraction in 1.1 M TBP in 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆). Distribution ratios (D) for cxtraction of uranium in 1.1 M TBP/omimPF₆ were marginally less than those for extraction by 1.1 M TBP/bmimPF₆ at all nitric acid concentrations. The D values increased with increase in the concentration of nitric acid but decreased with increase in the concentration of uranium and temperature. In contrast to the behavior of 1.1 M TBP/n-dodecane, D values for uranium continued to increase even for nitric acid concentrations higher than 5 M, when IL acted as diluent. Solubility of omimPF₆ in nitric acid was 2-3 times less as compared to bmimPF₆. The overall enthalpy change (ΔH_tot) for the extraction of uranium in 1.1 M TBP/omimPF₆ was found to be more exothermic than for extraction by 1.1 M TBP/bmimPF₆. This behavior can be attributed to the lower extent of protonation of TBP in omimPF₆ phase. Viscosity of TBP/IL increased when butyl group was substituted with octyl group in IL.