Date of Award

5-1-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Mark Dietz

Committee Members

Alexander Arnold, Joseph Aldstadt, Woehl Jorg, Guilherme Indig

Abstract

Nuclear power is one of many energy sources needed to meet current world demand. With certain radioisotopes in the waste generated (e.g. actinides) requiring thousands of years to decay, limiting the amount of waste requiring long-term storage is certainly in the interest of public health. By bombarding the long-lived radioisotopes with neutrons, or transmutation, they can be converted into radioisotopes with significantly shorter (several hundred years) half-lives, making storage a realistic solution for nuclear waste. Transmutation, however, can be negatively impacted by the presence of neutron poisons, particularly lanthanide ions, which have a high neutron capture cross section, preventing efficient bombardment.

The TALSPEAK (Trivalent Actinide Lanthanide Separations by Phosphorus-Reagent Extraction from Aqueous Komplexes) Process was devised to address this problem by providing a means to separate actinides and lanthanides. The process improved upon earlier single extractant systems that performed poorly by incorporating a complexant, diethylenetriaminepentaacetic acid (DTPA) in a buffered aqueous phase that prevents the partition of the actinides into a diisopropylbenzene (DIPB) organic phase containing bis(2-ethyl-hexyl) phosphoric acid (HDEHP). While the separation factors were high (~100), the process used volatile solvents, exhibited modest distribution ratios (~10) and required strict pH control.

Ionic liquids (ILs) have been considered as a replacement for DIPB due to their low volatility, thermal stability and ability to solubilize a wide range of compounds. Initial studies were promising, as ILs exhibited significantly greater distribution ratios (~1000). Unexpectedly, however, ILs have shown limited ability to solubilize common metal ion extractants (e.g. HDEHP). In addition, many ILs are very viscous, complicating or precluding their use in conventional solvent extraction systems. While this problem can be largely overcome by dispersing the IL into a porous support, this process has not been systematically explained for the TALSPEAK system.

In conjunction with efforts to devise an improved IL-based analogue of the TALSPEAK Process for An(III)/Ln(III) separations, a simple thermogravimetric method for the determination of the solubility of extractants in an ionic liquid has been developed. Subsequent application of the method to the determination of the solubility of HDEHP in a variety of 1-alkyl-3-methylimidazolium (Cnmim+) and N-alkyl-pyridinium (N-Pyr+) ILs has unexpectedly revealed that solvent molar volume is the most important factor controlling HDEHP solubility in these solvents. Because ILs exhibiting the high molar volumes are viscous, the utility of solid-supported IL phases was explained as an alternate to conventional liquid-liquid extraction. Measurements of the internal surface are of the porous polymer support employed as a function of support loading for various reagents revealed the presence of a complex network of pores of three distinct size ranges. Additional experiments using HDEHP as the loading reagent showed that the chromatographic performance (e.g., column efficiency, peak tailing) of the material reached it optimum at intermediate levels of extractant loading, after the smallest of the interior pores have been completely filled. This filling can also be accomplished by the use of inert “pore-blocking reagents”, atop which an extractant can be loaded. The performance of the materials too can be explained by consideration of the pore structure of the support. Finally, these pore-blocked supports have been employed in a comparison of HDEHP, both neat and as a solution in either a conventional solvent (i.e., dodecane) or an ionic liquid, to a functionalized ionic liquid (FIL) incorporating the anion of HDEHP, C10mim+DEHP-, as reagents for the separation of Am(III) and Eu(III). The results call into question the utility of the FIL and thus, of the durability of ILs as the basis for an improved TALSPEAK Process.

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