Extended X-ray absorption fine structure study on gel/liquid extraction of f-block elements

Gel/liquid extraction was investigated to achieve selective recognition and separation of trivalent actinides over lanthanides. N,N,N',N'-tetraallylpyridine-2,6-dicarboxamine (tetraallyl-PDA) was synthesized and polymerized with N-isopropylacrylamide (NIPA) and N,N'-methylenebisacrylamide (BIS) to produce a thermosensitive hydrogel. Introducing ligands into a three-dimensional network of thermosensitive hydrogel is expected to allow the complex structure and coordination behavior to be controlled. The PDA is an N,O hybrid donor ligand and typical PDA derivatives are O-donor-driven and show higher extractability for heavier lanthanides. Tetraallyl-PDA showed moderate extractability under weakly acidic conditions with a series of lanthanides, and its extractability for heavier lanthanides was higher owing to the stronger surface charge of the heavier lanthanides. NIPA gel swells and shrinks more effectively under weakly acidic conditions. Hence, tetraallyl-PDA was applicable for gel/liquid extraction. To understand the difference in complexation between the solution and gel, the radial structural function (RSF) was obtained by extended X-ray absorption fine structure (EXAFS) experiments at Super Photon Ring-8 GeV (SPring-8). The RSF of solution with gels with and without ligands at different temperatures showed slight differences. As the temperature increased, the RSF decreased owing to the increase in thermal vibration, but PDA-NIPA-BIS gel showed a shift in the first peak toward the shorter direction. This shift was attributed to the change in stoichiometry of PDA-Ln(III) complexes or coordination number of water molecules affected by the change of conformation of polymer chains and the hydrophilic/hydrophobic properties in the hydrogel. Theoretical fitting was difficult because various chemical species coexist in gel system. Future work will include further fitting, optimizing the gel synthesis, and modifying the ligands for practical use. (author)