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Discovery of Synergism Changes Course of Research on Crown Ethers for Extraction of Metal Ions

Although the discovery of crown ethers by Pedersen in 1967 opened up many opportunities for selective separations of metal ions, it wasn't until the discovery of synergistic extraction by crown ethers at Oak Ridge National Laboratory that many practical industrial and analytical applications were made possible. To pursue these possibilities, researchers in numerous countries have published approximately 140 papers in the chemical literature, and at least one firm, East Tennessee Radiometric Analytical Chemicals, Inc. (ETRAC), uses the principle of synergistic extraction by crown ethers in its products and services. Below is a depiction of synergistic extraction of Cs+ ion from a mixture by a crown ether in synergistic combination with a sulfonic acid extractant. 137Cesium is an important fission product found as a contaminant in wastes, soils, and groundwater at many USDOE sites, and a system like that shown in the figure is in use for radiochemical analysis. The role of the crown ether entails selective binding of the metal ion, rendering it extractable. The problem arises in that the extraction by the crown ether used alone is often prohibitively weak, because this requires the co-extraction of a negative ion (called an anion) and anions present in most solutions of interest are inextractable. By offering an exchangeable hydrogen ion, the sulfonic acid circumvents this problem completely. In addition, it has the advantage that the extracted metal may be back-extracted by raising the acidity of the aqueous phase to reverse the hydrogen-ion exchange. It is also tolerant of wide variation in matrix composition. Different combinations of crown ethers and organic acids are possible, making a scheme like that shown for cesium applicable to many ions and to many types of aqueous solutions. Research at ORNL spanning two decades from the mid 1970s has demonstrated the generality of the approach and has elucidated the underlying principles in representative systems.

Extraction Synergism

Chemical Separations Group R & D Projects

Provided by Oak Ridge National Laboratory's Chemical Sciences Division
Rev:   October 20, 2005