Selective Media for Actinide Collection and Pre-Concentration: Results of FY 2006 Studies

In this work, we have investigated new materials for potential use in automated radiochemical separations. The work can be divided into three primary tasks: (1) synthesis of new ligands with high affinity for actinide ions, (2) evaluation of new materials for actinide ion affinity, and (3) computational design of advanced ligand architectures for highly selective binding of actinide ions. Ligand Synthesis Work was conducted on synthesizing Kl?ui ligand derivatives containing functionalized pendant groups on the cyclopentadienyl ring. The functionalized pendent groups would allow these ligands to be attached to organic and inorganic solid supports. This work focused on synthesizing the compound Na[Cp?Co(PO(OC2H5)2)3], where Cp?= C5H4C(O)OCH3. Synthesizing this compound is feasible, but the method used in FY 2006 produced an impure material. A modified synthetic scheme has been developed and will be pursued in FY 2007. Work was also initiated on synthesizing bicyclic diamides functionalized for binding to polymeric resins or other surfaces. Researchers at the University of Oregon are collaborators in this work. To date, this effort has focused on synthesizing and characterizing a symmetrically substituted bicyclic diamide ligand with the ?COOH functionality. Again, this synthetic effort will continue into FY 2007. Separations Material Evaluation Work was conducted in FY 2006 to provide a more extensive set of data on the selectivity and affinity of extraction chromatography resins prepared by sorption of Kl?ui ligand onto an inert macroreticular polymeric support. Consistent with previous observations, it was found that these materials strongly bind tetravalent actinides. These materials also adsorb trivalent actinides at low nitric acid concentrations, but the affinity for the trivalent actinides decreases with increasing nitric acid concentration. These materials have relatively low affinity for U(VI), but they do sorb U(VI) to a greater extent than Am(III) at [HNO3] > 0.3 M. Preliminary results suggest that the Kl?ui resins can separate Pu(IV) from sample solutions containing high concentrations of competing ions. Conceptual protocols for recovery of the Pu from the resin for subsequent analysis have been proposed, but further work is needed to perfect these techniques. Work on this subject will be continued in FY 2007. Automated laboratory equipment (in conjunction with Task 3 of the NA-22 Automation Project) will be used in FY 2007 to improve the efficiency of these experiments. The sorption of actinide ions on self-assembled monolayer on mesoporous supports materials containing diphosphonate groups was also investigated. These materials also showed a very high affinity for tetravalent actinides, and they also sorbed U(VI) fairly strongly. Computational Ligand Design An extended MM3 molecular mechanics model was developed for calculating the structures of Kl?ui ligand complexes. This laid the groundwork necessary to perform the computer-aided design of bis-Kl?ui architectures tailored for Pu(IV) complexation. Calculated structures of the Kl?ui ligand complexes [Pu(Kl?ui)2(OH2)2]2+ and [Fe(Kl?ui)2]+ indicate a ''bent'' sandwich arrangement of the Kl?ui ligands in the Pu(IV) complex, whereas the Fe(III) complex prefers a ''linear'' octahedral arrangement of the two Kl?ui ligands. This offers the possibility that two Kl?ui ligands can be tethered together to form a material with very high binding affinity for Pu(IV) over Fe(III). The next step in the design process is to use de novo molecule building software (HostDesigner) to identify potential candidate architectures.