Protactinium (Pa) is an actinide with chemical properties that are unique among the actinide elements. While the properties of other actinides are to a large extent understood, much of the chemistry of Pa remains a mystery. This thesis aims to illuminate new understanding of Pa chemistry through behavioral analysis using analytical techniques including liquid-liquid extraction (LL); extraction chromatography (ExC); and spectroscopic studies.
Applications of radioanalytical chemistry and Pa: Through the research presented in this dissertation, we have developed a new way to separate uranium (U), thorium (Th), and Pa from complex environmental samples. The approach has been demonstrated for U-series dating of materials by alpha spectrometry. The method can be applied to geochronology, as well as to nuclear-forensic analysis of uranium-containing materials. In studies presented here, samples from a Paleolithic lake (Lake Bonneville, Utah USA) were analyzed for the radioactivity concentration of 230Th, 231Pa, 234U, 235U, and 238U by isotope dilution alpha spectrometry. Radioactivities were used to estimate of the time period of formation of the deposit from which the samples were collected. Ages were determined from the isotopics ratios; i.e., 231Pa/235U (40 ka); and 230Th/238U (39.5 ka) we found to be concordant with radiocarbon-14 dates (37 ka) obtained by collaborators at Brigham Young University. These studies inspired the development of a novel ExC resin to facilitate preparation of highly pure tracer isotope (233Pa) from a neptunium-237 (237Np) source. The material used for this development comprised 1-octanol adsorbed to a semi-porous resin material. The new approach greatly improved the yield and purity of 233Pa used for these chronometric analyses
Developing an understanding of the chemistry of Pa at trace concentrations: The new-improved analytical described above led to the hypothesis that analytical separations approaches could be used to develop a more detailed understanding of Pa chemistry. Toward this goal, experiments were conducted to understand how the extraction of Pa is impacted by solution acidity [H+], anion concentration [A-; Cl-, NO3-], and extractant concentration ([2,6-dimethyl-4-heptanol, DIBC]). A full-factorial experimental design was employed to create a model that would allow for predictions in Pa behavior, as well as describe the nature of the observations. This model generated a multivariate equation that relates the distribution coefficient ([Pa] organic phase/ [Pa] aqueous phase) to each of the parameters ([H+], [A-], and [DIBC]). Further studies expanded to other alcohols (ROH) used as extractants (1-octanol, (2,6)-dimthyl-4-heptanol, and 2-ethyl-hexanol); and the results were analyzed using the slope analysis and comparative extraction studies using the model and compared to other actinide elements (Th, U, Np, americium (Am)) by both LL and ExC systems. These experiments revealed unique chemical behavior of Pa with respect to the other actinides. For example, it was found that Pa was the only actinide element to be extracted into the organic phase under acidic conditions (HCl and HNO3). Slope analysis experiments elucidated the stoichiometric identity of Pa species, with respect to the anion and extractant. Future studies will aim to identify the oxygen stoichiometry and species by X-ray absorption techniques.
Investigations of the organic phase: In the final sections of this thesis, experiments are presented that are intended to determine if aggregation plays a key role in the extraction of Pa in systems containing 1-octanol and 2-ethyl-hexanol. This work is done in the absence of metal ions to control the dynamics of the organic phase, and are analyzed by tensiometry and Karl Fisher titrations with small angle X-ray scattering and molecular dynamic simulations. A key novel finding of these studies in that ROH molecules arrange in nanoscale aggregates that decrease the interfacial tension between the phases and extract a significant amount of water into the aggregates stabilized by a network of H-bonding. These studies lead to the hypothesis for future studies that Pa extraction is likely facilitated by solvation into the organic phase via ROH aggregates.
The sum of the findings and observations of this dissertation provide insight into the chemical nature of Pa: (1) Novel extraction methods to obtain radiochemically pure fractions show that Pa can be efficiently extracted and separated from complex matrices to aid in chronometric analysis for geochronology or nuclear forensics; (2) Statistical modeling to develop a better understanding of the main effects of solvent extraction parameters; (3) Equilibrium analysis to improve our understanding of chemistry of Pa and how it is unique to the actinides; (4) Aggregation analysis to demonstrate a solvent centric understanding of extraction studies, these results lead to future experiments to investigate how organic phase aggregation can influence solvent extraction selectivity.