Award Date
12-1-2024
Degree Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry and Biochemistry
First Committee Member
Artem Gelis
Second Committee Member
Frederic Poineau
Third Committee Member
Kenneth Czerwinski
Fourth Committee Member
Alexander Barzilov
Number of Pages
198
Abstract
Developing sustainable green energy systems is a continually growing concern as the world is faced with urgent issues of climate change and global warming. Nuclear energy provides one of the most efficient solutions to supplying power to the electrical grid without causing more carbon pollution. There are nearly a hundred thousand metric tons of used nuclear fuel (UNF) stored on-site at decommissioned nuclear power plants for an indefinite period of time. Achieving a finalized closed fuel cycle is a crucial part in developing the nuclear energy necessary for combating the issues of pollution and greener energy. Understanding actinide speciation especially in the organic phase of any separation process is a critical factor for developing and optimizing advanced nuclear fuel reprocessing flowsheets. The extractability of actinide metals by various separation processes is important for reducing heat loading and radiotoxicity in used nuclear fuel, however the mechanisms of extraction are poorly understood, not to mention that high actinide concentration solutions can be self-radiolyzing due to alpha activity from the actinides further complicating the chemistry. To get to a better understanding of these processes, the individual (and combined) role of each of the used extractant species should be understood. This dissertation examines the speciation of two actinides, Th(IV) and U(VI), in organic phases containing two extractants: the organophosphorus ligand, 2-ethyhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]), and/or the diglycolamide ligand, N,N,N′,N′-tetra-2-ethylhexyldiglycolamide (T2EHDGA). Spectroscopic techniques used in this work include FT-IR, low-temperature 31P NMR, XAFS, UV-Vis, ICP-MS/AES, and LSC.
In the 31P NMR study, the extraction of Th(IV) and U(VI) from nitric acid media using HEH[EHP] in heptane was examined. The distribution ratio as a function of nitric acid concentration was quantified using UV-Vis spectroscopy, and then the speciation of HEH[EHP]-metal complexes in the organic phase was investigated using FT-IR and low-temperature 31P NMR. In addition to perturbation of the vibrational modes proximal to the phosphonic moiety in HEH[EHP] in the FT-IR spectra, the appearance of a nitrate signal was found in the organic phase following extraction from the highest acidity conditions (8 M) for U(VI). The 31P NMR spectra of the organic phase at -70 C exhibited a surprising number (n) of resonances (n ≥ 7 for Th(IV) and n ≥ 11 for U(VI)), with the distribution between these resonances changing with the initial concentration of nitric acid in the aqueous phase. The possible Th(IV) species proposed include Th(HL2)4(H2L2)m(HL)n, Th(NO3)(HL2)3(H2L2)m(HL)n, Th(NO3)2(HL2)2(H2L2)m(HL)n, or Th(NO3)3(HL2) (H2L2)m(HL)n. The proposed U(VI) species are UO2(HL2)2, UO2(NO3)(HL2), UO2(NO3)2(H2L2)m(HL)n, as well as UO2(HL2)2(H2L2)m(HL)n & UO2(NO3)(HL2)(H2L2)m(HL)n.
In the EXAFS study, U(VI) in HNO3 media was extracted by HEH[EHP] and/or T2EHDGA in dodecane. In the U(VI)-HNO3-HEH[EHP]-dodecane system, the EXAFS data suggested a mixture of UO2(A4) (30%) and UO2(NO3)2(H2A2) (70%) (A= EH[EHP]) where UO2(A4) collectively represents the species UO2(HA2)2, [UO2•2H2A2][NO3]2 and a UO2(HA2)2/[UO2•2H2A2][NO3]2 mixture. In the U(VI)-HNO3-T2EHDGA-dodecane system, EXAFS results pointed to the species UO2(NO3)2(T2EHDGA). In the U(VI)-HNO3-HEH[EHP]-T2EHDGA-dodecane system (ALSEP mixed solvent) EXAFS results indicated the presence of 60% UO2(NO3)2•H2A2 and 40% UO2(A4).
Lastly, a Job plot analysis indicated a majority 3:2 or 2:1 HEH[EHP] to T2DEHDGA ratio in the organic phase stoichiometry, although a range of relative fractions of HEH[EHP] were actually implied from 40-85% HEH[EHP]. The most probable ratios from this range of HEH[EHP] fractions are 1:1 and 2:1, corresponding to UO2(HEH[EHP])(T2EHDGA) or UO2(HEH[EHP])2(T2EHDGA) species.
Keywords
diglycolamide; exafs; nmr; organophosphorus reagent; speciation; uranium
Disciplines
Chemistry | Radiochemistry
File Format
File Size
4100 KB
Degree Grantor
University of Nevada, Las Vegas
Language
English
Repository Citation
Castillo, Joel, "Uranium(VI) & Thorium(IV) Speciation In Organic Media Containing Organophosphorus & Diglycolamide Extractants" (2024). UNLV Theses, Dissertations, Professional Papers, and Capstones. 5165.
http://dx.doi.org/10.34917/38330374
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