Ternary Complex Formation and Extraction Modeling in Malonate-Buffered Trivalent Actinide–Lanthanide Separations

Abstract

The formation of an aqueous ternary complex was investigated in the malonate-based Advanced TALSPEAK system by solvent extraction and optical spectroscopy. Features of the spectroscopy confirmed the presence of a ternary complex between malonate, HEDTA, and the trivalent americium or lanthanide ion. The ternary complex was then incorporated into a solvent extraction model to refine stability constants under Advanced TALSPEAK conditions. Good agreement was found in the equilibrium constants determined by both methods. These values were then used to model metal speciation and predict extraction behavior at different concentrations of malonate and HEDTA. Calculated metal distribution ratios show good correlation with experimental values. The apparent simplicity of the chemical interactions and predictability of malonate-buffered Advanced TALSPEAK provides a significant advantage over the less ideal interactions of conventional TALSPEAK.

Characterization of a Ternary Neodymium-HEDTA-Citrate Complex in the Actinide Lanthanide Separation Process

“…Unlike malonate, citric acid, a triprotic, polydentate ligand, can complex through two of its three carboxylate functionalities, while the third remains protonated, and it will still exhibit the added stability afforded by the chelation effect allowing for a complex with a stability constant of the same order of magnitude as that of the metal-malonate complex. 16 , 17 …”

“…One example of this phenomenon is observed in actinide/lanthanide separation systems that incorporate an aqueous phase containing both a carboxylic acid buffer and an aminopolycarboxylate ligand. 15 , 16 As such, deprotonated carboxylic acids are also well-known ligands for the lanthanide and actinide elements. They are smaller ligands that generally form mono-, di-, or tri-dentate complexes with actinide and lanthanide cations.…”

“… 8 However, one consequence of using HEDTA in this separation process is the potential formation of ternary metal-HEDTA-buffer complexes that could alter the baseline kinetics of the metal-HEDTA complexes. 15 , 16 The predecessors of the ALSEP process generally opted to use the bulkier DTPA ligand, which is octadentate and will occupy the entire metal coordination sphere leaving no room for the formation of a ternary species. 20 Though the formation of ternary f-element complexes with HEDTA and a secondary ligand is known, reports of investigations into the thermodynamics of these complexes are relatively sparse.…”

“… 20 Though the formation of ternary f-element complexes with HEDTA and a secondary ligand is known, reports of investigations into the thermodynamics of these complexes are relatively sparse. 15 , 16 Lanthanide-HEDTA-lactate and lanthanide-HEDTA-malonate complexes have been identified, but no literature exists examining the metal-HEDTA-citrate complexes that are likely to form within the ALSEP process. Studies of such ternary complexes are important because speciation models of the ALSEP process currently omit what we find to be the predominant aqueous metal-containing species under the conditions used for An/Ln separation in the ALSEP process.…”

Americium Recovery from Highly Active PUREX Raffinate by Solvent Extraction: The EXAm Process. A Review of 10 Years of R&D

“…[43,44] A ternary complex between trivalent f-elements, HEDTA and malonate was recently found under Advanced TALSPEAK conditions using spectroscopic and solvent extraction methods. [45] Stability constants of the different complexes formed with Am(III) and Eu(III) including the mixed complex have been determined by distribution measurements and are reported in Table 2.…”