Liquid/liquid extraction of metal ions in room temperature ionic liquids

The search for more environmentally-friendly reaction media has prompted the development of a wide array of alternative systems that will sustain biphasic separations with aqueous solutions without the use of volatile organic compounds (VOCs). We have begun to employ Room Temperature Ionic Liquids (RTIL), specifically 1-alkyl-3-methylimidazolium hexafluorophosphate ([C(n)mim][PF6]), as VOC replacements in liquid/liquid separations of metal ions from aqueous solutions. Here we show that the partitioning of metal ions in these novel biphasic systems is consistent with traditional liquid/liquid separations: the metal ion affinity for the hydrophobic phase necessitates the presence of an extractant. In this report we explore the application of well-known organic (1-(pyridylazo)-2-napthol, PAN, and (1-thiazolylazo)-2-napthol, TAN) and inorganic (CN-, OCN-, SCN-, and halides) extractants for partitioning a variety of metal cations between [C(4)mim][PF6] or [C(6)mim][PF6] and an aqueous phase. PAN and TAN show pH dependent extraction of Cd2+, Co2+ Ni2+, and Fe3+ where their partitioning to the RTIL increases at least 2 orders of magnitude from pH 1 to 13. The effect of the halides on the partitioning of Hg2+ complexes increases F- < Cl- < Br- < I-. Pseudohalides, especially SCN-, had the greatest effect on enhancing the partitioning of Hg2+ to the RTIL, whereas CN- and OCN- provided little benefit for the extraction of any of the metal ions examined.