Towards a simplified TALSPEAK process using DTPA-amino acid conjugates

The TALSPEAK process [1] was developed at Oak Ridge National Laboratory in the 1960’s to separate trivalent actinide ions, An(III), from the chemically similar lanthanide ions, Ln(III), by solvent extraction. The idea was to use an extracting agent unselective for An(III) over Ln(III) and add a complexing agent to the aqueous phase to infer selectivity [2]. Despite the simplicity of this concept, TALSPEAK is actually a rather complex system, with numerous equilibria involved. The original TALSPEAK system consisted of HDEHP (bis-2-ethylhexyl phosphoric acid) as the extracting agent, DTPA (diethylenetriamine pentaacetic acid) as the An(III)-selective complexing agent and lactic acid as buffering agent. Constantly improving TALSPEAK by substituting the components of the original system, ALSEP [3] and Advanced TALSPEAK [4, 5] have been developed.
We have pursued a concept which differs in two aspects, (a) using amino acids (AA) as buffering agent [6] and (b) linking them to the complexing agent. This way, only a single molecule is added to the aqueous phase and the system is expected to behave more simply. Several DTPA-AA conjugates were synthesised, DTPA-Arg, DTPA-His, DTPA-Ser and DTPA-Ala. These were tested in solvent extraction experiments. Organic phase was 0.2 mol/L HDEHP in kerosene, aqueous phases were solutions containing 50 mmol/L DTPA-AA conjugate, Am(III) and Ln(III) (Y; La–Lu). pH was adjusted by adding HNO3 or NaOH.
The lowest pH at which separation was achieved (i.e., Am(III) distribution ratio < 1 and all Ln(III) distribution ratios > 1) was in the range of 1.1–1.5. Separation factors for Eu(III) over Am(III) were ≈ 100 and the minimum separation factors between Am(III) and the least extracted Ln(III), being Nd(III) in all cases, were 30–40.
The coordination chemistry was studied by luminescence and NMR. Photophysical studies of the Eu(III)-DTPA-AA complexes demonstrated characteristic Eu(III) emission, with associated lifetime decay profiles confirming 1:1 coordination. 1H NMR studies were performed with Am(III), confirming successful complexation.
In conclusion, an improved TALSPEAK system is reported which compares favourably to known TALSPEAK systems.
1. Weaver, B.; Kappelmann, F. A., USAEC report ORNL-3559, Oak Ridge National Laboratory, USA: 1964.
2. Nilsson, M.; Nash, K. L., Solvent Extr. Ion Exch. 2007, 25 (6), 665–701.
3. Gelis, A. V.; Lumetta, G. J., Ind. Eng. Chem. Res. 2014, 53 (4), 1624–1631.
4. Braley, J. C.; Grimes, T. S.; Nash, K. L., Ind. Eng. Chem. Res. 2010, 51 (2), 629–638.
5. Lumetta, G. J.; Casella, A. J.; Rapko, B. M.; Levitskaia, T. G.; Pence, N. K.; Carter, J. C.; Niver, C. M.; Smoot, M. R., Solvent Extr. Ion Exch. 2015, 33 (3), 211–223.
6. Grimes, T. S.; Tillotson, R. D.; Martin, L. R., Solvent Extr. Ion Exch. 2014, 32 (4), 378–390.