Group Separation of Hexavalent Actinides from Lanthanides through Selective Extraction by Sterically Hindered 2-Ethylhexyl Phosphonic Acid Mono-2-ethylhexyl Ester

Group separation of actinides (An, including U, Np, Pu, and Am) from lanthanides (Ln) is of great significance for nuclear resources recycling and long-term radiotoxicity reduction. But this task represents a big challenge due to the chemical complexity of the actinides. In the present work, we proposed a strategy for group An/Ln separation by oxidizing the actinides to hexavalent oxidation states and then selectively extracting them from lanthanides by a sterically hindered ligand, 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (EHEHPA). The intrinsic selectivity of EHEHPA to hexavalent actinides over trivalent/tetravalent lanthanides affords single-stage An/Ln separation factors in the range of 102–105, depending on the specific actinides or lanthanides. The selective mechanism of An(VI)/Ln(III/IV) was analyzed by means of UV–vis absorption and mass spectroscopies, which suggests that the Ln(III/IV)/EHEHPA and An(VI)/EHEHPA complexes possess similar metal/ligand stoichiometries but the former one may consist of more hydrophilic nitrate ion or water molecules, thus leading to high selectivity to An over Ln by EHEHPA.