Milestones And Future Directions In The Solvent Extraction Of Caesium

The remarkable development of solvent-extraction (SX) chemistry for caesium separation over the past half a century as driven by the needs of the nuclear industry now constitutes an instructive case study in exploring the limits of selectivity and cycle efficiency in SX. In this review, key milestones in the pursuit of both fundamentals and applications of caesium extraction will be highlighted along with a look at future prospects. The high-yield fission-product 137Cs constitutes a major fraction of the radioactivity in nuclear wastes, and in view of its heat production, environmental mobility, radiation hazard, and even uses as a radiation source, methods have long been sought for its separation. Toward this end, the evolving science has been challenged by daunting requirements for decontamination in the presence of high concentrations of competing cations, and demands for small footprint, modular design, and high throughput place a premium on selectivity and efficiency. Fortunately, the science has also benefited from the peculiar economics of nuclear separations, which have afforded the development of wonderfully sophisticated reagents. With its location in the lower left side of the periodic table, the Cs+ cation has the distinction of having the lowest charge density of any metal cation except short-lived francium. For practical purposes, Cs+ is thus the least hydrated and, in principle, the most directly extractable metal cation. Technologies employing liquid-liquid cation exchange with very large, durable anions like those from the dicarbollide family have therefore been quite effective based solely on solvation principles. Alternatively, researchers have turned to macrocyclic coordinating extractants, such as calix-crown ethers, following principles of molecular recognition, with dramatic results. Overall, strides continue along these lines, though it is apparent that caesium SX has reached a state of excellent fundamental understanding and technical maturity, evidenced by a suite of highly effective technologies.