Jean-Marie Lehn (Nobel laureate, 1987) suggested ligand thickness to be an important consideration in the design of host molecules for cation recognition. We have recently expanded the role of this simple ligand property by demonstrating a case in which ligand thickness contributes significantly to anion discrimination. It was found that in the extraction of sodium nitrate and perchlorate by a simple crown ether, bis(t-octylbenzo)-14-crown-4 (BOB 14C4), the normal preference for perchlorate is almost completely lost when the complex cation has the open-face sandwich vs. the sandwich structure.

This paper summarizes progress at three collaborating US national laboratories on the extraction of the fission products {sup 99}Tc and {sup 137}Cs from alkaline high-level wastes (HLW). Efficient, economical processes for Tc and Cs extraction (SRTALK and alkaline-side CSEX, respectively) have been developed, and testing has progressed through batch tests on actual wastes and continuous countercurrent centrifugal-contactor tests on simulants.