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

Abstract

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.