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Solvent Extraction and Complexation Studies of Actinyl Cations by Multidentate Schiff Base Ligands for Nuclear Fuel Cycle Applications


Continued research into advanced separations technology for the reprocessing of spent nuclear fuel (SNF) is imperative for a sustainable, zero CO2 emission future. More specifically, increased knowledge in the area of actinide coordination and extraction chemistry can prove beneficial in driving interest towards novel separation processes and strengthening public opinion towards nuclear power. Numerous extraction systems have been developed for the separation of various components present in spent nuclear fuel (SNF). Most notably, the PUREX process (Plutonium Uranium Reduction Extraction) has been used commercially for decades focusing on the extraction and recovery of uranium and plutonium from other components such as neptunium, americium, and other highly radiotoxic elements. The successful separation of these mid-actinides (U, Np, Pu, Am) from other elements is imperative for drastically lowering the long-term radiotoxicity of SNF. The solution chemistry of the mid-actinides permits a wide range of oxidation states (+III, +IV, +V, +VI) where the +V and +VI oxidation states exist in a linear dioxo actinyl cation AnO2n+ (O=An=O)n+, offering the possibility of co-separation of all 4 actinide cations. The actinyl moiety permits ligand coordination in the equatorial plane of the metal ion center, affording a geometry for unique bonding characteristics. Multidentate Schiff base ligands can be prepared with synthetic ease and have rather planar structures that chelate around the equatorial plane of actinyl ions through their nitrogen and oxygen donor groups. This distinct coordination environment may facilitate the selective solvent extraction or aqueous retention of pentavalent and hexavalent actinides.

This work examines the potential use of functionalized multidentate Schiff base ligands for the selective group extraction, or retention, of the actinyl cations in a biphasic liquid-liquid separation process. Solvent extraction studies provided details into the equilibrium, stoichiometry, selectivity, kinetic and thermodynamic parameters of the separation process while complexation chemistry experiments elucidated detail of the actinyl-ligand coordination environment. Solvent extraction studies indicate that the group separation of the mid-actinides is possible although redox stability poses a challenge.

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