An "ionic liquid chelate extraction system", which is a novel liquid-liquid extraction method for the extractive separation of metal cations into the ionic liquid extraction phase with a chelate extractant, was investigated mainly from the point of difference from a normal chelate extraction system using a volatile organic solvent as the extraction phase. In this system, the extracted metals are easily recoverd by using back-extraction into an acidic solution. In the extraction of divalent metal cations (M
2+) with 2-thenoyltrifluoroacetone (Htta) (or other β-diketone having trifluoromethyl group), some metals were extracted into the 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF
6]) extraction phase as coordination-unsaturated (hydrated) neutral complexes (M(tta)
2(H
2O)
n), whereas others were extracted as coordination-saturated anionic complexes (M(tta)
3−) with anion-exchange reactions. The extractability of the former species was reduced by using a more hydrophilic ionic liquid as the extraction phase, whereas that of the latter was enhanced. On the contrary, the use of 8-hydroxyquinoline (Hq) (or its derivative) as the extractant resulted in the extraction of M
2+ into [bmim][PF
6] as neutral M(q)
2(H
2O)
n. Furthermore, a comparison of the extraction behavior of M
2+ with 8-(trifluoromethanesulfonamido) quinoline (H(CF
3)sq) having a trifluoromethyl group with that with 8-(methanesulfonamido) quinoline (H(CH
3)sq) suggested that the existence of a trifluoromethyl group in an chelate extractant contributes to an enhancement of the extractability for an anionic complex into the ionic liquid extraction phase. In addition, a fundamental study was performed on an ionic liquid synergistic cation-exchange extraction system with co-using Htta and 18-crown-6 (18C6) as extractants for the selective extraction of trivalent light lanthanoids, such as lanthanum (La). In this system, La
3+ was extracted into 1-butyl-3-methylimidazolium bis (trifluoromethanesulfonyl) imide as cationic ternary complexes, such as La(tta)
2(18C6)
+ and La(tta)(18C6)
2+.
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