ABSTRACT
Complexants .................................................................................. 619 11.2.3 Silicon-Functionalized Ligands as Supercritical
Fluid-Soluble Complexants ........................................................... 620 11.2.4 Supercritical CO2 Solubility of “Conventional”
Extractants-Revisited .................................................................. 623 11.2.5 Supercritical Fluid-based Approaches to Nuclear Fuel
Reprocessing ..................................................................................624 11.3 Ionic Liquids (ILs) ...................................................................................... 627
11.3.1 Background .................................................................................... 627 11.3.2 Fundamental Aspects of Metal Ion Partitioning into
Ionic Liquids (ILs) ......................................................................... 629 11.3.3 Overcoming the Limitations of ILs as Extraction Solvents .......... 630 11.3.4 Acidic Extractant-IL Systems ........................................................ 633 11.3.5 Task-Specic Ionic Liquids (TSILs) .............................................. 634
11.4 Conclusions ................................................................................................. 635 References .............................................................................................................. 635
Despite the demonstrated effectiveness of solvent-extraction (SX) processes employing conventional organic diluents in nuclear-fuel reprocessing and
nuclear-waste treatment,1,2 recently there has been growing interest in the possibilities afforded by less conventional alternatives. In large measure, this interest has been driven by the increasing appeal of processes exhibiting both high efciency and selectivity and minimal environmental impact, a combination of characteristics not readily obtained with traditional extraction systems, which are frequently characterized by the use of toxic, volatile, or ammable diluents. Nash3 and others4 have noted that any of several “unconventional” systems, in particular aqueous biphases,5 solid sorbents incorporating supported extractants,6 supercritical uids (SCFs) (particularly supercritical carbon dioxide, SC-CO2),7 or ionic liquids (ILs),8 may ultimately provide a viable alternative to traditional, aqueous-based actinide (An) and ssion-product (FP) separations. At present, however, the need for high salt concentrations to generate a biphase, while acceptable in analytical applications, argues against the utility of aqueous two-phase systems for many large-scale separations.9 Similarly, despite attempts to improve the physical stability of solid-supported extractants,10-12 their suitability for process-scale applications remains questionable. In contrast, both supercritical carbon dioxide and ILs, certain drawbacks notwithstanding, appear to offer promise as the basis for new separation schemes for Ans and FPs. In this chapter, we provide an overview of progress in the application of these two classes of so-called “neoteric”13 solvents to the development of such schemes. The objective here is not to provide an exhaustive review of all potentially relevant IL and SCF literature, but rather to highlight those studies that have the most direct bearing on the issue of the utility of these novel solvents for large-scale An and FP separations, such as are encountered in nuclear fuel reprocessing and waste management.
