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Equilibrium and kinetic characterization of the ALSEP process: probing organic phase metal complex speciation & defining the role of buffer in the strip step
Picayo, Gabriela A.
Picayo, Gabriela A.
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2021
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Abstract
Implementation of advanced nuclear fuel cycles is critical to a sustainable future for nuclear energy production. Closed nuclear fuel cycles proposing to transmute long-lived minor actinide elements, such as americium, into shorter-lived nuclides using advanced reactors hinge on the separation of americium from the lanthanides. Though many separation schemes have been proposed, the physicochemical properties of the trivalent actinides and lanthanides pose a challenge to refining their efficiency for industrial application.
The ALSEP (Actinide-Lanthanide SEParation) process is a relatively new separations protocol whereby the An3+ and Ln3+ cations are (1) co-extracted from 3-4 M nitric acid into a mixture of the extractants HEH[EHP] (2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester) and TEHDGA (N,N,N’,N’-tetra(2-ethylhexyl)diglycolamide) in n-dodecane, (2) scrubbed of excess nitric acid with a buffered aqueous solution, and (3) selectively stripped of americium by a buffered aqueous phase containing polyaminopolycarboxylate ligand, commonly DTPA (diethylenetriaminepentaacetic acid) or HEDTA (N-(2-hydroxyethyl)ethylenediamine-N,N’,N’-triacetic acid). Though ALSEP has demonstrated many advantages over its predecessors, the fundamental chemistry of the strip step remains a critical bottleneck to the process’ implementation.
The body of work available for understanding equilibrium complexation and the kinetic barriers in ALSEP is limited. Accurate interpretation of kinetic data begins with defining the speciation of the metals, ligands, and extractants in each phase and step. The equilibrium characterization work presented here is the first to provide a definitive stoichiometry for the metal extracting complex in ALSEP. In addition, changes in complexation were probed in both buffered and un-buffered systems, over a broad range of process relevant acidities. A speciation model of the metal-extractant complexes that describes the speciation transitions from start to finish are also defined for the first time.
Finally, the kinetics of the strip step was investigated using buffered and self-buffered EDTA (ethylenediaminetetraacetic acid) solutions. Rate laws were attained, and reactions contributing to the rate-limiting mechanism were suggested. Additionally, EDTA was replaced with the self-buffering picolinic acid-substituted derivative EDTA-Mpic (N-2-methylpicolinate-ethylenediamine-N,N’,N’-triacetic acid), to access the stripping kinetics under lower pH conditions. The evidence so far suggests that picolinate functionalization improves mass transfer rates through the solubility of the picolinic acid substituted arm and provides effective self-buffering for similar kinetic benefits to that seen in the standard ALSEP system.
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