DOI: 10.1002/chem.200500559 Extraction of Lanthanides from Aqueous Solution by Using Room- Temperature Ionic Liquid and Supercritical Carbon Dioxide in Conjunction Soufiane Mekki,* [a, b, d] Chien M. Wai,* [c] Isabelle Billard,* [a] Gilles Moutiers, [d] James Burt, [c] Byunghoon Yoon, [c] Joanna S. Wang, [c] Clotilde Gaillard, [a] Ali Ouadi, [a] and Peter Hesemann [e] Introduction Supercritical fluids and room-temperature ionic liquids (RTILs) are increasingly used in numerous fields of chemis- try owing to their “green” characteristics. [1–7] Carbon dioxide is the most widely used gas for supercritical fluid extraction due to the fact that it has moderate critical constants and is commercially available with high purity at a low cost. The main “green” advantages of using this fluid are the inertness of carbon dioxide and the absence of waste-solvent genera- tion. In the specific field of metal extraction, as the usual or- ganic phases (mostly volatile organic compounds) can be re- placed by supercritical carbon dioxide (Sc-CO 2 ), procedures using Sc-CO 2 can be regarded as “green processes”. The po- tential of Sc-CO 2 for metal extraction has been extensively studied during the last decade. Numerous reports in the lit- erature have demonstrated that (quantitative) extraction of metal species, from solid or liquid samples, can be accom- plished by using proper chelating agents as extractants. [2,8] For example, actinides, lanthanides, and transition metals are easily extracted by using highly soluble fluorinated b-di- ketones in Sc-CO 2 . [9,10] Adding an organophosphorus re- agent such as tri(n-butyl)phosphate (TBP) to the b-dike- tone/Sc-CO 2 system could create a positive synergistic effect, further enhancing the metal-extraction efficiencies for both solid and liquid samples. [11,12] RTILs also show unique “green” properties such as near- zero vapor pressure and nonflammability, in addition to their interesting physicochemical properties [4] (in particular, polarity, polarizability, and molar volume) including wide liquidus and high solubilities for a variety of compounds (both neutral and charged). These properties make RTILs good candidates to replace traditional volatile organic sol- vents in current extraction processes. [13–18] In the case of their potential nuclear fuel reprocessing applications, RTILs display the additional advantage of good radiation stabili- ty. [19] However, few studies have focused on the extraction abilities of water/RTIL systems for lanthanides and acti- Abstract: For the first time, the study of a three-step extraction system of water/ionic liquid/supercritical CO 2 has been performed. Extraction of trivalent lanthanum and europium from an aqueous nitric acid solution to a super- critical CO 2 phase via an imidazolium- based ionic liquid phase is demonstrat- ed, and extraction efficiencies higher than 87 % were achieved. The quantita- tive extraction is obtained by using dif- ferent fluorinated b-diketones with and without the addition of tri(n-butyl)- phosphate. The complexation phenom- enon occurring in the room-tempera- ture ionic-liquid (RTIL) phase was evi- denced by using luminescence spectros- copy. Keywords: carbon dioxide · ionic liquids · lanthanides · metal extraction · supercritical fluids [a] Dr. S. Mekki, Dr. I. Billard, Dr. C. Gaillard, Dr. A. Ouadi IReS, CNRS/IN2P3-ULP Chimie NuclØaire B.P. 28, 67037 Strasbourg Cedex 2 (France) Fax: (+ 33) 388-106-431 E-mail: soufiane.mekki@ires.in2p3.fr isabelle.billard@ires.in2p3.fr [b] Dr. S. Mekki CEA Saclay, INSTN/UECCC 91191 Gif-sur-Yvette Cedex (France) [c] Prof. C. M. Wai, J. Burt, B. Yoon, Dr. J. S. Wang Department of Chemistry, University of Idaho Moscow, Idaho, 83844-2343 (USA) Fax: (+ 1) 208-885-6173 E-mail: cwai@uidaho.edu [d] Dr. S. Mekki, Dr. G. Moutiers CEA Saclay, DEN/DPC/SCP/DIR 91191 Gif Sur Yvette Cedex (France) [e] Dr. P. Hesemann CNRS, HØtØrochimie MolØculaire et MacromolØculaire Laboratoire de Chimie OrganomØtallique 8, rue de l)Ecole Normale, 34296 Montpellier Cedex 05 (France) # 2006 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim Chem. Eur. J. 2006, 12, 1760 – 1766 1760