Synergistic Enhancement of the Extraction and Separation Efficiencies of Lanthanoid(III) Ions by the Formation of Charged Adducts in an Ionic Liquid

The synergistic ionic-liquid extraction and extraction equilibrium of lanthanoid(III) (Ln(III)) ions have been investigated using 2-thenoyltrifluoroacetone (Htta) and trioctylphosphine oxide (TOPO) in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][Tf2N]). The selective synergistic effect for heavier Ln(III) ions was found using a combination of Htta and TOPO in [C4mim][Tf2N], leading to enhanced separability among Ln(III) ions. The extracted Ln(III) species and the extraction constants in the Htta–TOPO system were determined by three-dimensional extraction equilibrium analysis. In this system, the neutral adduct, Ln(tta)3(TOPO)2, and the cationic charged adducts, Ln(tta)2(TOPO)2+, Ln(tta)2(TOPO)3+, and Ln(tta)(TOPO)32+, were extracted into the [C4mim][Tf2N] phase. The adduct formation constants in [C4mim][Tf2N] were determined to evaluate the synergistic effect of Htta and TOPO on the extraction of Ln(III) ions. The selective synergism for heavier Ln(III) ions in ...