Phase transfer kinetics and mechanisms of advanced TALSPEAK solvent extraction systems

Forrest  Dominick Heller

doi:10.7273/000004020

The increasing need to manage used nuclear fuel necessitates development of new separation methods to provide pathways for the ultimate disposal or reuse of the fuel. Closing of the nuclear fuel cycle requires a number of chemical separation processes. At the tail end of this series of processes, only the lanthanides and minor actinides remain. The minor actinides (Am and Cm) are the major dose contributors and the lanthanides are neutron poisons which greatly affect any effort to transmute the minor actinides. To separate these very chemically similar metals, the solvent extraction process known as Advanced TALSPEAK (Trivalent Actinide Lanthanide Separation with Phosphorus-reagent Extraction from Aqueous Komplexes) was developed. Prior to this work the phase distribution kinetics of the trivalent metal ions in this process were poorly understood. As the Advanced TALSPEAK process relied upon rapid and efficient extraction of fission product lanthanides, the kinetics of the lanthanide transport between the aqueous phase and organic phase are of significance. In order to investigate this process, a Lewis cell was built and modified to allow spectroscopic monitoring of both aqueous and organic extraction phases simultaneously. This new Lewis cell was tested and shown to be as functional as any previous design and was named COSMOFLEX (COntinuous Spectroscopic MOnitoring of Forrest's Liquid-liquid EXtraction cell). The kinetics of extraction of praseodymium, neodymium, samarium, europium and holmium was investigated in situ using the UV-Vis monitoring probes. This continuous spectroscopic monitoring has revealed an unexpected reaction mechanism in the organic phase, where initially formed interfacial Ln•organic complex rearranges to a spectroscopically distinct complex at equilibrium. Time Resolved Fluorescence measurements have shown a change in fluorescence decay lifetimes of the extracted lanthanide species. This work has resulted in a new type of instrumental setup which can give unique mechanistic data as well as fresh insight into the kinetics of Advanced TALSPEAK or other suitable solvent extraction system. Some of the mechanistic details are resolved, but many more details remain mysteries. Of particular interest is the newly found observation of multiple transient metal-extractant species in the organic phase.

Phase transfer kinetics and mechanisms of advanced TALSPEAK solvent extraction systems

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