Characterization of the ALSEP Process: Investigating Equilibrium and Intermediate Complexes of the Scrub Stage

ABSTRACT

A unique combination of real-time and conventional optical spectroscopy, off-line chemical analyses, and equilibrium solvent extraction measurements were employed to monitor progressive changes in the speciation of the organic-phase complexes of neodymium or americium in the biphasic scrub stage of the Actinide-Lanthanide Separation (ALSEP) solvent extraction process proposed for trivalent actinide/lanthanide separations. Consistent with the findings of other researchers, four unique organic-phase species are identified by three separate methods of multivariate analysis. The organic phase initially contains M(TEHDGA)₂(HEH[EHP])₂(NO₃)₃ complexes. As the ALSEP organic phase is scrubbed with an aqueous malonate buffer, the complex loses HNO₃ to form M(TEHDGA)₂(H(EH[EHP])₂)(NO₃)₂ as the first organic-phase intermediate species. Further contact with the scrub aqueous phase forms a second intermediate species, M(TEHDGA)(H(EH[EHP])₂)₂(NO₃), and eventually a complex containing only HEH[EHP], M{H(EH[EHP])₂}₃ (where TEHDGA = N,N,N’,N’-tetra(2-ethylhexyl)diglycolamide and HEH[EHP] = 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester). Similar intermediate species were also observed in equilibrium ALSEP organic phases when lower aqueous concentrations of nitric acid (e.g., 0.5 M HNO₃) were used to extract actinides or lanthanides.

KEYWORDS:

• ALSEP
• chemical speciation
• TEHDGA
• HEH[EHP]
• diglycolamide extractant
• in-situ spectroscopy

Acknowledgments

This material is based upon work supported by the Department of Energy Office of Nuclear Energy under the Award Number DE-NE0008554.

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No potential conflict of interest was reported by the author(s).

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