Most effective separations are based on the phase transfer of the species of interest away from a diverse mixture. Distillation and precipitation are two examples of practical applications of phase transfer in separations. In solvent extraction, partitioning of the species of interest between two immiscible liquids is employed to accomplish the desired separation. Commonly used in organic chemistry, the difference in solubility of the selected species between the immiscible fluids forms the basis for the separation (Nernst partitioning). For metal ion separations, more complex interactions are typically required. The specific features of the application of both solvent extraction and pyrometallurgy (molten salts/molten metals) to the processing of used nuclear fuel have been discussed in detail elsewhere. [10]

Owing to the substantial differences in the polarity of the aqueous and organic solutions, there is, in general, minimal tendency for metal ions or their electroneutral salts to spontaneously partition into nonpolar media (like kerosene), particularly for polyvalent metal ions, which tend to have substantial hydration energies. For the transfer of metal ions or their polar salts into non-polar medium to occur, the presence of amphiphilic (featur­ing polar and non-polar regions, like surfactants) molecules is necessary. Such molecules can (and often do) aggregate in the bulk organic solutions to improve their compatibility with the typically low dielectric constant medium.

These same molecules tend, at the same time, to arrange themselves at the polar-non-polar fluid interface with the polar end penetrating signifi­cantly into the aqueous solution while the lipophilic portion of the molecule remains firmly in the organic side of the interface. If the interaction of the polar end of the extractant molecule with the cation is strong enough, it promotes dehydration of the metal cation, resulting in a substantial increase in entropy for the biphasic system, which can drive the phase transfer process. The extractant molecule can either simultaneously transfer a more weakly hydrated cation to the organic phase or facilitate the re-solvation of the anion needed to accompany the cation into the organic solution. Typically, once this interfacial transfer reaction is completed, the nascent metal complex completes the transfer to the bulk by completing the dehy­dration through selective solvation (or chelation) by lipophilic molecules present in the organic phase.

Two primary procedures for cation partitioning are possible: cation exchange or solvation of metal salts. The latter process can also exhibit considerable ability to support the partitioning of mineral acids into the organic phase. Under most circumstances in the processing of dissolved used fuel solutions, acidic conditions are maintained (to minimize the possibility for hydrolysis and precipitation of metal hydroxides). In some systems, the application of salting out agents has been required for efficient phase transfer. Under these conditions, the primary cation exchange reaction will involve H+ exchange. A general example of the cation exchange reaction is:

M3+aq + 3 HLorg = ML3org + 3 H+aq

The alternative reaction of partitioning an electroneutral salt could be defined as,

M3+aq + 3 X-aq + 2 Yorg = MX3Y2org

where X is the supporting anion in the aqueous phase and Y is the solvating extractant. A third process related to the ion solvation method arises in extraction systems based on the use of lipophilic tertiary amine or quater­nary ammonium compounds, in which an anionic metal solvate coordina­tion compound is extracted,

M3+aq + 4 X-aq + HY+org = MXYHY+org

The energetic details and kinetics of these systems can be quite different and selectivity can arise from a variety of different interactions.

Selectivity of solvent extraction processes can be altered through the introduction of water-soluble chelating agents. Complex equilibria can control these processes and multiple interactions between species in each phase are possible. Selectivity can arise from the nature of the extracting agent, the anions co-extracted by solvating or anion exchange reagents, the

chelating agents present in the aqueous phase or by changing the oxidation state of the extracted species relative to that of competing matrix ions. In many cases, classical solution chemistry provides useful guidance to the prediction of process efficiency, though supramolecular organization of solute or solvent molecules and the mutual miscibility of aqueous and organic phases can further alter system performance.

Solid-liquid separations based on organic or inorganic ion exchange materials can also be (and have been) employed as an alternative or com­plement to solvent extraction. In some instances, anion exchange has also proven an acceptable mode for conducting separations of nuclear materials. An advantage that accrues from the application of chromatographic methods like ion exchange is the large numbers of theoretical plates that can be achieved in such separations when conducted in column mode. If solvent extraction reagents are immobilized on solid support materials (extraction chromatography), the selectivity advantages and flexibility of solvent extraction can be augmented with multiplicity of re-equilibrations of chromatography. These solid-liquid separation methods have one par­ticular limitation in the context of operating closed-loop fuel cycles in their batch-wise mode of operation, which differs from the continuous (or nearly so) operation that is possible in solvent extraction. Devices that employ easily replaceable ion exchange cartridges have been developed, though their application in a processing canyon (where remote operation and main­tenance is required) might offer unique challenges in operations. In prin­ciple, solvent impregnated membranes could also be operated in a semi-continuous mode, though research to date on such methods applied to metal ion separations processes has revealed issues with the stability for membranes and the kinetics of mass transfer.