While Purified Cyanex301 (HC301) has shown great potential in accomplishing the challenging trivalent actinide/lanthanide (An/Ln) separation, the extraction mechanism of f-elements by HC301 has remained ambiguous for over two decades. Herein, systematic spectroscopic study on this extraction system suggests the formation of different extractive species and the speciation dependence on the equilibrium pH and the type of elements contribute to the high selectivity. At low pH, HC301 and C301- bind directly to the metal cation in inner-sphere mode via sulfur atoms. As the pH increases, the portion of outer-sphere species, where M3+ is partly solvated by water molecules, increases. In comparison with An3+, this speciation evolution occurs more easily for Ln3+. As a result, trivalent actinides and lanthanides forms different extractive species with Cyanex301 under the same extraction condition, and thus leading to the extraordinary separation factor (SF). This speciation evolution mechanism as well as the results from DFT calculation also explains the unique relationship between SF and equilibrium pH, i.e., the SF increases with equilibrium pH at the beginning and decreases with the further increase of equilibrium pH. The variation in extraction mechanism presented in this work depicts generally the complicated nature of f-element extraction by dithiophosphinic acid ligands.

Speciation evolution underlying the unusual An3+/Ln3+ selectivity by purified cyanex301

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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