Do Aqueous Ternary Complexes Influence the TALSPEAK Process?

The aqueous speciation of trivalent lanthanide and actinide cations in solutions containing DTPA (diethylenetriamine-N,N,N',N',N ''-pentaacetic acid) and lactic acid were studied under conditions representative of the TALSPEAK process. Spectrophotometric titrations, fluorescence spectroscopy, and thermometric titrations were used to search for indications of ternary metal-DTPA-lactate complexes. The addition of lactate anions to metal-DTPA complexes was undetectable by any of these techniques, even at free lactate concentrations of 0.75 M. Although lactic acid is necessary for the optimal performance of the TALSPEAK process, we find that the fractions of aqueous ternary Ln(3+)/An(3+)-DTPA-lactate complexes are far too low to account for the observed acid dependence of TALSPEAK metal extraction.