Spectroscoptic and Chemical Characterization of Rare f-Element Parent/Daughter Pairs for Separations Applications

Purifying the f-elements is necessary for nuclear waste remediation, diagnostic or therapeuticmedical applications, and to enable fundamental chemistry with these relatively rare elements. These separations are challenging because the lanthanides (Ln) and minor actinides (An) often share an oxidation state (M(III)) and have similar ionic radii, precluding common techniques like size exclusion chromatography. This dissertation considers the separations and fundamental chemistry of specific f-elements in two decay chains - 227Ac and 254Es - as well as lower-Z analogs Gd, Ce and Cs. Ac is interesting to study because it has the largest ionic radius of all the f-elements and is more difficult to chelate, but an effective chelator in biological systems is necessary to facilitate 225Ac targeted alpha therapies. A series of solution thermodynamic experiments using liquid/liquid extraction (LLE) probes the efficacy of eleven linear molecules as Ac chelators and compares those results to Gd as an analogous lanthanide. In general, the ligands chelates Gd more strongly than Ac, although the two phosphonic acids studied effectively retain Ac in the aqueous phase at lower concentrations than necessary for Gd retention. The octadentate hydroxypyridinone-based ligand 3,4,3-LI(1,2-HOPO) (343HOPO), which has been widely used for separations and fundamental investigations of the actinides, demonstrates similar behavior to diethylenetriamine pentaacetate (DTPA), a commonly used ligand in nuclear waste LLE systems. 343HOPO outperforms three catecholamide-containing ligands with the same backbone, and also outperforms the other carboxylic acids evaluated. In addition to this Ac solution chemistry, fundamental properties of the last two Group 1 metals, Cs and Fr, were explored using 137Cs and 223Fr via a different LLE system. Three crown ethers with increasing cavity sizes were compared for each element, and the results indicate a surprisingly similar size match between the two metals. These experiments present the first set of solution thermodynamic measurements conducted with a pure stream of Fr, and the first with 223Fr (t1/2 = 22 min); previous experiments have used 225Ac as an in situ generator of the short-lived 221Fr (t1/2 = 4.7 min). To produce this pure stream, a column-based scheme for rapid separation of 227Ac from its daughters 227Th, 223Fr, and 223Ra was developed, which can be used to obtain radio- and chemically-pure 223Fr as frequently as every 3 hours for at least 14 days. In addition, the procedure provides clean streams of the other 227Ac daughters, 227Th and 223Ra, both of which could be used as other targeted alpha therapy agents. A final set of column separation experiments consider schemes to separate Es/Bk/Cf, including a novel ionic exchange resin-based separation of Bk from Cf using 343HOPO's unique oxidizing power to change the overall charge of the Bk(343HOPO) complex. Furthering characterization of f-element-343HOPO complexes, 254Es(343HOPO) luminescence was measured, corroborating previous measurements of a blue shift upon Es complexation in contrast with the other actinides, which are red-shifted upon complexation. X-ray absorption spectroscopy (XAS) measurements of 254Es(343HOPO) and Ce(IV/III)(343HOPO) are also presented. The 254Es(343HOPO) XAS measurements include the first measurement of the elemental white line (closely corresponding to theory) and unexpectedly short M-O bond lengths, indicating a potential break in the An series. The Ce(343HOPO) measurements show beam-induced reduction of the complexed metal, demonstrating 343HOPO's remarkable oxidizing power, and providing a potential explanation for the challenges experienced in measuring Bk(IV)(343HOPO) via XAS despite solutions-based measurements that clearly demonstrate that oxidation state. Some initial forays into other ligands that mimic 343HOPO's oxidation of Ce are also presented, again with an eye toward biologically-relevant ligands and/or systems where Ce can be replaced with Bk. A final chapter considers the history of nuclear isotope production, evaluating the extent to which super heavy element researchers and research facilities can be considered science diplomacy, rather than 'mere' international science. While the common taxonomies of science diplomacy emphasize scientists' actions instead of state participation, a state-centered analysis that evaluates the dichotomy of competition and cooperation in the field concludes that some nations pursue the discovery of new elements for the science while others have international agendas.

Advisor: Abergel, Rebecca J