Ionic liquids (ILs) are salts with low melting points composed of an organic cation and an anion of various forms (Binnemans 2007). The properties of ILs with respect to miscibility with water, solubility of metal salts, polarity, viscosity, etc., can be changed by choice of the anion and the cation. One type of room temperature ILs widely used for current studies is based on the 1-alkyl-3-methylimidazolium cation (bmin) with different forms of anion. With a fluorinated anion, the bmim-based ILs can be water immiscible (hydrophobic). The bmin-based ionic liquid with bis(trifluoromethylsulfonyl)imide anion (Tf2N-) is of particular interest for extraction of metal ions due to their water stability, relative low viscosity, high conductivity, good electrochemical and thermal stability (Mekki et al., 2006). The chemical structure of the ionic liquid [bmin][Tf2N-] is given in Fig. 14.9. Aqueous metal ions are usually not soluble in this type of IL but with the aid of hydrophobic ligands or chelating agents, metal ions may become soluble in the IL phase. Extraction of uranyl ions, trivalent lan­thanides and actinides from nitric acid solutions into the ILs has been investigated using a variety of ligands including TBP, octyl(phenyl)-N, N — diisobutylcarbamoylmethyl phosphine oxide (CMPO), P-diketone, and acidic dialkylorganophosphorous (Binnemans 2007). In general, cation exchange processes have been attributed to the observed metal extraction mechanism for IL systems, in which transfer of positively charged metal ions into the IL phase is accompanied by a simultaneous loss of cations to the aqueous phase (Visser et al. 2003, Jensen et al. 2003). After extraction of metal ions into the ILs, recovering the dissolved metal can be accom­plished by back-extraction with an organic solvent. Using electrochemical methods for recovering metal species in ILs has also been investigated (Rao et al. 2008).

It is known that sc-CO2 dissolves effectively in ILs whereas the solubility of the latter in the former is negligible. Therefore, sc-CO2 provides an effec­tive medium for removing solutes from ILs. Mekki et al. have recently demonstrated that Cu2+ ions and trivalent lanthanides (La3+ and Eu3+) can be extracted from aqueous solutions into an imidazolium-based ionic liquid (1-butyl-3-me thy limidazolium, or bmin) with Tf2N — counter anions using P-diketones as extractants (Mekki et al. 2005, 2006). The metal-P-diketonates in the ionic liquid phase can be effectively transferred to sc-CO2 at 50 °C and 150 atm. The efficiency of extracting lanthanide-P-diketonates from the IL to sc-CO2 is typically greater than 98% under the specified conditions. The reports by Mekki et al. (2005, 2006) suggest that sc-CO2 may be an effective medium for stripping metal species dissolved in an ionic liquid phase without carrying over the ionic liquid to the CO2 phase. Therefore, a two-step extraction process involving three phases (water, ionic liquid, and sc-CO2) may provide an alternative for removing radioactive materials from aqueous solutions to the CO2 phase (Fig. 14.9). The advantages of this new IL-sc-CO2 coupled extraction technique include: (1) radionuclides from the aqueous wastes can be transferred to and concentrated in an ionic liquid under ambient temperature and pressure, and (2) back extraction of the radionuclides from the IL phase to the sc-CO2 phase may be selective because the solvation strength of sc-CO2 is tunable, and (3) no loss of the IL occurs in the back-extraction process and no organic solvent is intro­duced into the ionic liquid phase. Actually, due to its self cleaning nature, sc-CO2 could help removing undesirable organic residues from the IL phase. After the back-extraction step, the IL can be reused and the CO2 recycled after precipitation of the solutes by pressure reduction.

Transport of uranyl ions (UO2)2+ from aqueous nitric acid solutions to [bmin][Tf2N-] using TBP as a complexing agent followed by sc-CO2 strip­ping of the uranyl complex from the ionic liquid phase to the supercritical

[bmin][Tf2N] =

14.9 Extraction of uranium from nitric acid solution using ionic liquid and supercritical CO2 in conjunction — a two-loop extraction system involving three phases.

fluid phase has been reported recently (Wang et al. 2009). The uranyl species involved in this three-phase system were studied using spectroscopic tech­niques. The efficiency of extracting (UO2)2+ in 3 M nitric acid to [bmin] [Tf2N-] with 1.1 M TBP is greater than 95% at room temperature under ambient pressure at an acid to IL phase ratio of 1:1 by volume. The uranyl species extracted into the IL phase showed a UV-Vis absorption spectrum similar to that of UO2(NO3)2(TBP)2 but with some noticeable differences. The uranyl species in the IL phase can be effectively extracted into sc-CO2 phase (>98%) at 40 °C and 150 atm. The rate of supercritical fluid extraction of UO2(NO3)2(TBP)2 from the IL phase appears to follow a zero-order rate equation. The absorption spectrum of the uranyl species extracted into the supercritical fluid phase is similar to that of UO2(NO3)2(TBP)2 known in the literature. The uranyl compound in sc-CO2 was trapped in a hexane solution after depressurizing the system and the absorption spectrum is identical to that of UO2(NO3)2(TBP)2 observed from a standard solution prepared from synthesized UO2(NO3)2(TBP)2 crystals. These studies have provided a basis for developing a new process of extraction and separation of lanthanides and actinides from acidic solutions using a hyphenated IL-sc-CO2 method. Separation of lanthanides and actinides after the sc-CO2 extraction step could be achieved using the counter-current method described in the Areva NP project in the previous section. Separation of uranium from transuranic elements may also be possible using the counter­current method although no experimental results have been reported in the literature yet.

The hyphenated IL-sc-CO2 extraction/separation technique may find applications in reprocessing spent fuel. In a recent report, Billard et al. demonstrated that uranium dioxide can be dissolved directly in [bmin] [Tf2N-] containing nitric acid (Billard et al. 2007). The IL can dissolve about 12,000 ppm of water without forming a separate aqueous phase (Billard and Gaillard, 2009). Our recent study also shows that uranium dioxide and lanthanide oxides can be dissolved directly in [bmin][Tf2N-] containing the TBP(HNO3)18(H2O)06 without forming a second aqueous phase. These new developments suggest the possibility of dissolving spent fuel directly in the IL without an aqueous nitric acid dissolution step. Therefore no aqueous waste would be produced and the dissolution can be done at room tem­perature under ambient pressure. Recovery of uranium from the IL may be achieved using the sc-CO2 back-extraction method or by other tech­niques. For example, using electrochemical techniques for recovering uranium from the IL phase is another option.