Loading...
Speciation and kinetics of actinides and lanthanides in the stripping step of the actinide lanthanide separation (ALSEP) process
Eddy, Madeleine Anne
Eddy, Madeleine Anne
Citations
Altmetric:
Advisor
Editor
Date
Date Issued
2022
Date Submitted
Keywords
Collections
Research Projects
Organizational Units
Journal Issue
Embargo Expires
2023-09-30 00:00:00
Abstract
The chemistry of trivalent actinide/lanthanide separations has been of interest since the dawn of the modern nuclear era. These separations, which target the transplutonium actinides generated in nuclear reactors during power production, have the potential to greatly reduce the radiochemical and thermal toxicity of used nuclear waste, alleviating the strain on future nuclear waste storage facilities. The Actinide Lanthanide Separation (ALSEP) process is a recently developed solvent extraction approach to this chemical challenge that is designed to separate the trivalent actinides from the lanthanides through coextraction followed by selective stripping of the actinides via an aminopolycarboxylate aqueous ligand. The selective stripping of the actinides in this final step is kinetically limited by slow chemical reactions and it can take several minutes to reach equilibrium. Effective implementation of ALSEP on an industrial scale will require equilibration times on the order of 30 - 60 seconds.
Until now few efforts have been made to analyze and define the mechanisms of metal transfer in the ALSEP process. Existing literature on the kinetics of similar separations processes is sparse and focuses on systems which operate in the extraction direction (that is aqueous to organic phase transfer) rather than the stripping direction (organic to aqueous) of ALSEP. This work highlights a series of investigations into the ALSEP stripping step, in order to further our understanding of the metal phase-transfer processes. First, an overview of the literature surrounding single-phase aminopolycarboxylate complexation kinetics is provided in an effort to better understand the importance of these complexation reactions in biphasic solvent extraction systems. Next, a spectroscopic investigation into the equilibrium aqueous complexes present in an ALSEP system containing N-(2-hydroxyethyl) ethylenediamine-N,N’,N’-triacetic acid (HEDTA) and citrate buffer is presented, with a novel ternary Nd(HEDTA)(Hcitrate) complex identified and characterized. Finally, a series of kinetic investigations employing microfluidic techniques using neodymium and americium, and biphasic absorption spectroscopy of neodymium, were used to measure the effects of component concentration on the solvent extraction kinetics of the phase transfer reactions in the HEDTA/citrate, diethylentriamine- N,N,N’,N”,N”-pentaacetic acid (DTPA)/citrate, and DTPA/malonate systems. Reaction orders obtained through these kinetic measurements were used to define the rate laws for metal extraction and metal stripping in these different ALSEP systems. In all systems the rates of metal extraction were limited by the dissociation of the aqueous metal complexes, either M-HEDTA-citrate or M-DTPA. The rates of metal stripping also were limited by the rates of metal-HEHEHP complex dissociation. The nature of both the aqueous and organic complexes in the ALSEP system allowed for acid catalyzed complexation pathways that were able to increase the reaction rates as acidity was increased. Because the citrate buffer takes on the role of a co-complexant in the HEDTA system, it has a negative effect on the kinetics of the system compared to the DTPA systems where buffer concentration is shown to modestly facilitate the metal phase-transfer reactions. Defining the roles of these components in the chemical mechanisms of different configurations of the ALSEP process improves our understanding of the potential pathways for system modification to achieve faster kinetics of metal phase-transfer in the development of the next generation of trivalent actinide/lanthanide separations processes.
Associated Publications
Rights
Copyright of the original work is retained by the author.