Structural and Thermodynamic Analyses of the Complexation of Pr(III), Nd(III), and Eu(III) with Lactate for...

The complexation of Pr(III), Nd(III), and Eu(III) with lactic acid/lactate (HA/A^-) in aqueous solution was reinvestigated using spectroscopic and potentiometric titrations. In addition to the well-confirmed 1:1, 1:2, and 1:3 complexes (LnA²⁺, LnA₂ ⁺, and LnA₃), more attention was given to the arguable 1:4 species (LnA₄ ^-) previously reported. With spectroscopic methods, the LnA₄ ^- species was clearly identified in aqueous solutions, and the capability and limitation of the potentiometric method were evaluated and discussed. With the confirmed stability constants for the four complexes, a simulation of the TALSPEAK process (Trivalent Actinide Lanthanide Separation with Phosphorus-Reagent Extraction from Aqueous Complexes) was performed, clarifying the error caused by the previously missing 1:4 species. It was found that high concentrations of lactic acid/lactate (over 2 mol/L) slightly affect the extraction of Ln(III) in the TALSPEAK process, but it might not be accountable for the considerable difference between experimental results and predictions from the simulation using currently adapted model.