TRANSFERRING IONS FROM ONE LIQUID PHASE TO ANOTHER: FUNDAMENTAL PRINCIPLES AND THEIR APPLICATION TO NUCLEAR-WASTE SEPARATIONS WITH CROWN ETHERS Bruce A. Moyer , P. V. Bonnesen, J. C. Bryan, R. A. Sachleben, D. J. Presley, and T. J. Haverlock Oak Ridge National Laboratory P.O. Box 2008, Oak Ridge, Tennessee 37831-6119 The field of liquid-liquid extraction, commonly called solvent extraction, has grown extensively in the past half century to become an economically significant family of techniques in industry, analytical chemistry, and research. Precisely because of its usefulness, liquid-liquid extraction has continued to evolve in terms of physical configuration (e.g., solvent extraction, liquid membranes, and extraction chromatography) and chemistry. Some of the major developments have in fact been driven by the needs of pollution prevention and include the use of sophisticated and highly selective extractants. Historically, industrial-scale solvent extraction has its roots in the nuclear industry, as related to the recovery of uranium and thorium from ores and the reprocessing of irradiated nuclear fuels. Even in its early development, solvent extraction was recognized to offer substantial advantages in waste minimization. For example, the replacement of precipitation processes with solvent-extraction processes such as PUREX reduced waste production in nuclear separations by well over an order of magnitude. 1 Now, the legacy of nuclear-weapons production lies before us in the form of stored radioactive wastes and contaminated sites. In the USA, highly radioactive wastes stored in underground storage tanks at Hanford, Idaho Falls, Savannah River, and Oak Ridge await treatment and ultimate safe disposition. As the USDOE turns from Cold War priorities to dealing with such "tank wastes" and other legacy matters, considerable investments are being made in developing new technologies and deciding among treatment options. Whether consciously or not, technologists and decision makers have often been applying green principles in this regard, preferring options that consume less raw materials and produce lower waste volumes, sometimes even when the needed technologies have not yet been proven viable. Tank wastes in particular represent an excellent case in point. At the former plutonium- production site at Hanford, Washington, 55 million gallons of highly alkaline wastes are stored in underground tanks. 2 Although the entire bulk of this waste could be mixed with glass frit and vitrified, the cost of such a massive operation together with subsequent geologic storage of the resulting increased waste volume would be prohibitively expensive. Instead, a more rational approach recognizes that less than 0.1% of the mass of the waste is in the form of harmful radionuclides and that separation of this small, high-level fraction from the waste can greatly reduce the overall cost while concentrating the hazard into a more manageable volume. 3 How to achieve this worthwhile end has been the subject of intensive research for the past 5-10 years at several USDOE facilities. In our own laboratories, we are testing the feasibility of using solvent extraction to remove the key radionuclides 99 Tc, 90 Sr, and 137 Cs from the Hanford waste solutions. 4 This had effectively been proposed in a scheme exploiting largely known processes for removing radionuclides from acid solution. 5 It was our thought, however, that it would be advantageous to develop new solvent-extraction methods capable of removing the contaminants directly from the alkaline waste, thereby obviating the addition of a huge quantity of acid. As discussed widely in the literature, crown ethers hold some promise as selective extractants for the removal of the Cs and Sr from nuclear waste. 6 Figure 1