Extraction Mechanism of Metal Ion from Aqueous Solution to the Hydrophobic Ionic Liquid, 1-Butyl-3-methylimidazolium Nonafluorobutanesulfonate

Summary. We studied the extraction behavior of metal ionic species in aqueous solutions into the hydrophobic ionic liquid, 1-butyl-3-methylimidazolium nonafluorobutanesulfonate ([bmi][NfO]). The extraction ratios (E/%) of Li(I), Na(I), Cs(I), Ca(II), Sr(II), and La(III) species were found to be 39, 24, 5.0, 81, 79, and 98. This result is similar to the phenomena that the metal ions with larger charge are more easily adsorbed onto cation exchange resins. In order to examine the extraction mechanism, we studied the extraction behavior of La(III) species from the aqueous to the [bmi][NfO] phase. As a result, it was found that the E values of La(III) decrease remarkably with an increase in concentrations of HNO₃ (0–1 M) in the aqueous phase and that the amount of La(III) transferred into [bmi][NfO] phase increases linearly with an increase in the amount of [bmi] transferred into the aqueous phase. Furthermore, we investigated the extraction behavior of La(III) species using 1-pentyl- and 1-hexyl-3-methylimidazolium nonafluorobutansulfonate ([pmi][NfO], [hmi][NfO]), which are more hydrophobic than [bmi][NfO], and found out that the E values of La(III) decrease in order of [bmi][NfO] > [pmi][NfO] > [hmi][NfO]. From these results, it was proposed that the extraction of metal ionic species from the aqueous to the [bmi][NfO], [pmi][NfO], or [hmi][NfO] phase is mainly ascribed to the cation exchanges between two phases.