Toward Understanding the Thermodynamics of TALSPEAK Process -- Medium Effects on Actinide Complexation

The ingenious combination of lactate and diethylenetriamine-N,N,N’,N”,N”pentaacetic acid (DTPA) as an aqueous actinide-complexing medium forms the basis of the successful separation of americium and curium from lanthanides known as the TALSPEAK process. While numerous reports in the prior literature have focused on the optimization of this solvent extraction system, considerably less attention has been devoted to the understanding of the basic thermodynamic features of the complex fluids responsible for the separation. The available thermochemical information for both lactate and DTPA protonation and metal complexation reactions are representative of the behavior of these ions under idealized conditions. Significant departures from the speciation predicted based on reported thermodynamic values may be present in the TALSPEAK aqueous environment. Thus, thermodynamic parameters describing the separation chemistry of this process require further examination at conditions significantly removed from conventional ideal systems commonly employed in fundamental solution chemistry. In this report, calorimetric studies of lactate protonation equilibrium illustrate a distinct influence of nitrate, perchlorate, methylsulfonate and triflate on the thermodynamic parameters associated with TALSPEAK aqueous chemistry. Triflate is identified a closer non-complexing representative of lactate solution chemistry. The reported initial two stability constants for the complexation of americium and neodymium by lactate in triflate medium suggest that the identified medium effects do not significantly affect the speciation.