Extraction Behavior of Lanthanoids(III) with Bis(2-ethylhexyl)phosphoric Acid into Ionic Liquid 1-Butyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)imide

Abstract

The extraction behavior of 14 trivalent lanthanoids (Lns) with bis(2-ethylhexyl)phosphoric acid (HR) into a typical hydrophobic ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, was investigated. The extraction is so slow that a shaking time of more than 36 hours is needed to reach extraction equilibrium. Compared to the extraction system using toluene as an extraction solvent, the IL extraction system has higher extraction ability for all Lns and higher selectivity for some couples of Lns (e.g., Ce/La and Tb/Gd). The dependences of the distribution ratio on the aqueous pH and the HR concentration suggest that the main extracted species are neutral LnR₃(HR)₃ for light and middle Lns and Tb, but anionic LnR₄(HR)₂^– and LnR₅(HR)^2– for heavy Lns (except for Tb). The addition of 1-octanol as a phase stabilizer decreases the extractability of Lns, which can be explained in terms of interaction between 1-octanol and HR dimer.