Improvement of the cloud point extraction of uranyl ions by the addition of ionic liquids

The cloud point extraction (CPE) of uranyl ions by different kinds of extractants in Triton X-114 (TX-114) micellar solution was investigated upon the addition of ionic liquids (ILs) with various anions, i.e., bromide (Br-), tetrafluoroborate (BF4-), hexafluorophosphate (PF6-) and bis[(trifluoromethyl)sulfonyl]imide (NTf2-). A significant increase of the extraction efficiency was found on the addition of NTf2- based ILs when using neutral extractant tri-octylphosphine oxide (TOPO), and the extraction efficiency kept high at both nearly neutral and high acidity. However, the CPE with acidic extractants, e.g., bis(2-ethylhexyl) phosphoric acid (HDEHP) and 8-hydroxyquinoline (8-HQ) which are only effective at nearly neutral condition, was not improved by ILs. The results of zeta potential and 19F NMR measurements indicated that the anion NTf2- penetrated into the TX-114 micelles and was enriched in the surfactant-rich phase during the CPE process. Meanwhile, NTf2- may act as a counterion in the CPE of UO22+ by TOPO. Furthermore, the addition of IL increased the separation factor of UO22+ and La3+, which implied that in the micelle TOPO, NTf2- and NO3- established a soft template for UO22+. Therefore, the combination of CPE and IL provided a supramolecular recognition to concentrate UO22+ efficiently and selectively. (C) 2013 Elsevier B.V. All rights reserved.