Lanthanide-Polyaminopolycarboxylate Complexation Kinetics in High Lactate Media: Investigating the Aqueous Phase of TALSPEAK
Brigham, Derek Mackenzie
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In advanced nuclear fuel reprocessing schemes, the TALSPEAK (Trivalent Actinide Lanthanide Separation with Phosphorus-reagent Extraction from Aqueous Komplexes) process has been proposed as a means to separate Am and Cm from the lanthanides. One significant limitation of the TALSPEAK process is slow phase transfer kinetics of the lanthanides to the organic phase. Increasing the lactic acid buffer concentration is found to improve the solvent extraction kinetics. However, concentrations of greater than 1 M are necessary to achieve rates of mass transfer fast enough for TALSPEAK to be applicable on an industrial scale. The TALSPEAK process employs diethylenetriaminepentaacetic acid (DTPA) as an aqueous phase complexant to selectively bind to the actinides and prevent their extraction, however, DTPA also binds with the lanthanides. Understanding the mechanism of the interaction between DTPA and the lanthanides in high total lactate will help to explain the accelerative effect of increased total lactate on TALSPEAK mass transfer rates.This dissertation describes the homogeneous aqueous complexation kinetics of the lanthanides Pr3+, Nd3+, Sm3+-Lu3+ and the polyaminopolycarboxylate ligands DTPA, ethylenediaminetetraacetic acid (EDTA), and (hydroxyethyl)ethylenediaminetriacetic acid (HEDTA) in a 1 M total lactate aqueous media similar to that found in the aqueous phase of a TALSPEAK separation system. Temperature studies on the interactions of select lanthanides with DTPA, EDTA, and HEDTA were performed to obtain activation parameters associated with the complex formation reaction. Additional studies on the interaction of Eu3+ with DTPA were performed under varying total lactate conditions at several different constant concentrations of lactate ion and pH values to determine the mechanistic role of the species in the lactate/lactic acid buffer system. Kinetic data was obtained using the method of equilibrium perturbation by ligand displacement via stopped-flow spectrophotometry employing the colorimetric dye arsenazo III.This work provides an increased understanding of lanthanide interactions with polyaminopolycarboxylate ligands in TALSPEAK-like aqueous media. From the insights gained in these studies, a possible explanation for the accelerative effect of lactate on TALSPEAK phase transfer rates is proposed. The overall conclusion of this work is that under high concentrations of total lactate the lactate ion governs the aqueous phase complexation kinetics.