Organic-solvent/Water/Ionic-liquid Triphasic System for the Fractional Extraction of Divalent Metal Cations
Solvent extraction is a traditional separation technique using an organic-solvent/water biphasic system. This method, however, has only one extraction (organic) phase, and is not suitable for the simultaneous fractional separation of various components. Recently, various extraction systems using several hydrophobic ionic liquids (ILs), such as 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([bmim][Tf2N]), instead
... m][Tf2N]), instead of organic solvents as an extraction phase have been investigated for the extraction of metal cations and other species. 1,2 These hydrophobic ILs are not only immiscible to water, but also immiscible to various non-polar aliphatic solvents. Namely, a non-polar-aliphatic-solvent/water/hydrophobic-IL three-component system can form three separate phases. 3 Previously, one of the authors reported that several divalent metal cations (M 2+ ) including Cu 2+ , Ni 2+ , Zn 2+ and Cd 2+ can be extracted effectively into [bmim][PF6] with 8-quinolinol (HQ) as hydrated neutral complexes (MQ2(H2O)n) and that the coexistence of tri-n-octylphosphine oxide (TOPO) results in the interfering extraction of Zn 2+ and Cd 2+ . 4,5 These facts have suggested that the co-use of HQ and TOPO as extractants in the triphasic system can actualize the fractional extraction of metal cations. In this communication, we report on a fundamental study concerning a novel triphasic fractional extraction system for divalent metals, consisting of cyclohexane and [bmim][PF6] as competitive extraction phases and HQ and TOPO as competitive extractants. In addition, this is the first report on use of an IL-containing triphasic system in analytical chemistry. Experimental Chemicals were of reagent-grade, and were used without further purification. Ionic liquid [bmim][PF6] was synthesized according to a reported procedure. 6 Distilled and deionized water was used throughout. An extraction study was performed as follows. In a centrifuge tube, an aliquot (1 cm 3 ) of cyclohexane containing 1 × 10 -2 mol dm -3 of HQ and 1 × 10 -1 mol dm -3 of TOPO, 1 cm 3 of [bmim][PF6] and 5 cm 3 of an aqueous phase containing 1 × 10 -5 to 1 × 10 -4 mol dm -3 of M 2+ (M = Mn, Ni, Cu, Zn, Cd or Pb), 1 × 10 -1 mol dm -3 of potassium nitrate and 1 × 10 -2 mol dm -3 of buffer (chloroacetic acid, acetic acid or Good's buffers) were shaken mechanically at 25 ± 1 C for 45 min. After the three phases were separated by centrifugation, the pH in the aqueous phase was determined. The metal concentration in the aqueous phase and those in the two extraction phases (after back-extraction into 10-fold volume of 1 mol dm -3 nitric acid) were determined by flame atomic absorption spectrophotometry. Results and Discussion In the cyclohexane/water/[bmim][PF6] triphasic system, the extracted ratios of M 2+ into the cyclohexane phase (%Eorg) and those into the [bmim][PF6] phase (%EIL) were plotted as a function of the aqueous-phase pH. The results co-using HQ and TOPO as extractants are shown in Fig. 1 along with plots obtained when using HQ alone. Upon using HQ alone, all metals were extracted only into the [bmim][PF6] phase, and the extraction behavior was almost the same as that in the water/[bmim][PF6] biphasic system. The results are consistent with our opinion that an IL as the extraction phase can stabilize extracted hydrated neutral metal complexes. 7 Upon co-using HQ and TOPO, on the contrary, Mn 2+ , Zn 2+ , Cd 2+ and Pb 2+ were extracted mainly to the cyclohexane phase, whereas Ni was extracted quantitatively extracted into the [bmim][PF6] phase. At pH 5, the %Eorg values for Mn 2+ , Zn 2+ , Cd 2+ and Pb 2+ were 73, 83, 76 and 87%, respectively, and these 2009 A novel cyclohexane/water/ionic-liquid (1-butyl-3-methylimidazolium hexafluorophosphate, [bmim] [PF6]) triphasic extraction system was studied for a possible fractional extraction of divalent metal cations with co-using 8-quinolinol (HQ) and tri-n-octylphosphine oxide (TOPO) as competitive extractants. In this system, Ni 2+ was extracted into the [bmim][PF6] phase as a Q --complex, whereas each of Mn 2+ , Zn 2+ , Cd 2+ and Pb 2+ was extracted into the cyclohexane phase as an ion-pair of a cationic TOPO-complex and PF6 -. A suitable selection of extractants and ionic liquid can result in developing a powerful triphasic fractional extraction system.