Trans-Lanthanide Extraction Studies in the TALSPEAK System: Investigating the Effect of Acidity and Temperature

A recent review of the TALSPEAK (Trivalent Actinide - Lanthanide Separation by Phosphorous reagent Extraction from Aqueous Komplexes) process summarized previous research characterizing the performance of this system. Attempts were made in that report to develop correlations between the previous separations data and the thermodynamic data that describes the interactions among the components of the separation system. However, the diversity of observations and the breadth of conditions employed in the many studies from the literature make comparisons between the TALSPEAK system performance and existing thermodynamic data challenging. To better test the adequacy of the available thermodynamic database for modeling this system, an internally consistent set of observations of lanthanide extraction under TALSPEAK conditions is offered in this report. The extraction of the lanthanides from La through Lu (excluding Pm) plus Y, and that of Am3+ from buffered lactic acid solutions containing diethylenetriamine-N,N,N',N'',N''-pentaacetic acid (DTPA) into organic solutions of bis-(2-ethylhexyl)phosphoric acid (HDEHP) solutions in 1,4-diisopropylbenzene has been investigated as a function of temperature, pH, and ionic strength. Extraction of both lanthanides and Am3+ decreases with increasing pH and with temperature. At the same time, separation factors remain approximately constant. Calculations based on an internally consistent thermodynamic data set confirm the prior conclusion that the existing thermodynamic data do not predict the observed decline in partitioning with increased pH, though the data do support the constancy of the separation factors. The relationship between the basic thermodynamic data, calculated speciation, and the performance of the separation system is discussed.