Liquid–liquid extraction of neodymium(iii) by dialkylphosphate ionic liquids from acidic medium: the importance of the ionic liquid cation

Abstract

The ionic liquids 1-hexyl-3-methylimidazolium bis(2-ethylhexyl)phosphate, [C₆mim][DEHP], 1-hexyl-1-methylpyrrolidinium bis(2-ethylhexyl)phosphate, [C₆mpyr][DEHP], and tetrabutylammonium bis(2-ethylhexyl)phosphate, [N₄₄₄₄][DEHP], were prepared and characterized using ¹H and ¹³C NMR spectroscopy. The extraction behavior of neodymium(III) from nitrate medium by these ionic liquids, diluted with the room temperature ionic liquids 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C₆mim][NTf₂], 1-hexyl-3-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [C₆mpyr][NTf₂], and tributylmethylammonium bis(trifluoromethylsulfonyl)imide, [N₁₄₄₄][NTf₂], was studied. The distribution ratio of neodymium(III) was measured as a function of various parameters, such as pH, concentration of the ionic liquid extractant, nature of diluents, concentration of ionic liquid cations and nitrate anions in the aqueous phase. The extraction behavior was compared with that obtained for a solution of the molecular extractant bis(2-ethylhexyl)phosphoric acid (DEHPA) in an ionic liquid diluent. The extraction of neodymium(III) in the ionic liquids [C₆mim][DEHP] and [C₆mpyr][DEHP] showed markedly different extraction properties in comparison with that of the quaternary ammonium analogue [N₄₄₄₄][DEHP], especially concerning the pH dependence of the extraction process. These results show that the extraction process can be tuned by the selection of the ionic liquid cation. The extraction experiments also included the trivalent rare-earth ions lanthanum(III), cerium(III), praseodymium(III), ytterbium(III) and yttrium(III). Studies of the stripping behavior and the reusability of the ionic liquids were carried out, which indicate that the ionic liquids can be reused with no loss in activity.