Dual-Host Combinations: Using Tripodal Amides to Enhance Cesium Nitrate Extraction by Crown Ethers

Abstract

Extraction of alkali metal salts by designed cation hosts, such as crown ethers 1 and calixarenes,2 has been widely investigated in recent years, as driven, to an extent, by the importance of separations of certain cationic contaminants such as Cs+ and Sr2+ in nuclear-fuel reprocessing and waste remediation. 3 New cation hosts used as extractants for these and other metals have reached impressive levels of selectivity for such demanding “needle-in-the-haystack” applications. The success of neutral cation hosts used in ion-pair extraction systems stems in part from the practical advantage of stripping extracted salts with water, resulting in processes that yield a pure salt product stream with little secondary waste. Examples of such practical ion-pair extraction applications include processes for nuclear-waste treatment that produce aqueous streams of relatively pure cesium nitrate2h or sodium pertechnetate,4 suitable for vitrification and geologic disposal. These processes employ calix-crown and crown ether extractants, such as those shown in Figure 1 for cesium. Although ion-pair extraction involves anion co-extraction, ligand design has focused primarily on the design of the cation host, typically with little or no attention to accommodating the anion.