Extraction of metal ions from aqueous solutions of mineral acids with crown ether-ionic liquid systems

Abstract

The extraction of U(VI), Sr, and Cs from solutions of mineral acids (HNO3, HCl) with a crown ether, cis-syn-cis-dicyclohexyl-18-crown-6 (DCH18C6-A), dissolved in ionic liquids (ILs), 1-butyl-3-methyl-imidazolium derivatives (bmimPF6, bmimBF4, bmimTf2N), was studied. The best physicochemical characteristics (solubility in the aqueous phase, viscosity, hydrophobicity, etc.) are exhibited by a solution of DCH18C6-A in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (bmimTf2N). The metal distribution ratios (D M) in the extraction with a 0.01 M DCH18C6-A solution are 80 for CsNO3, 78 for CsCl, and 162 for Sr(NO3)2. With an increase in the HNO3 and HCl concentrations, D M decreases, and with 1 M acids it does not exceed 1. The U(VI) extraction from nitric acid solutions with a 1 M solution of DCH18C6-A in bmimTf2N initially increases with an increase in the aqueous phase acidity, with D U(VI) reaching 5.4 in 4 M HNO3, but then decreases in the interval 4–8 M HNO3, whereas in the extraction with a 1 M solution of tributyl phosphate (TBP) in bmimTf2N D U(VI) monotonically increases with an increase in the HNO3 concentration to 8 M. From hydrochloric acid solutions, U(VI) is extracted with solutions of DCH18C6-A in bmimTf2N with the D U(VI) values characteristic of solutions of DCH18C6-A in nonpolar organic diluents. On the whole, ILs as solvents exhibit unusual properties in the extraction of alkali and alkaline-earth elements from neutral solutions and of U(VI) with TBP from concentrated nitric acid solutions.