The complexation of HDEHP, HEH[EHP], T2EHDGA, and TODGA and mixtures thereof in n-dodecane with Nd(III), Pm(III), and Am(III), extracted from pH 3 aqueous media, were analyzed by distribution analysis, IR, UV−vis, and X-ray spectroscopies. Appreciable ternary complex formation is seen among HDEHP with DGA components and to a lesser extent with HEH[EHP]. Ternary complex formation constants were determined by absorption spectroscopy and slope analysis. The effect of the adduct formation on the process performance and on the Ln/An selectivity is discussed. It has been shown that the ternary complexes have the tendency to extract larger trivalent cations as Am and Nd over Eu, thus diminishing the overall Ln/Am selectivity of the process. Also, previously unreported extraction constants were derived for HEH [EHP] with Am and Pm as well as coordination information for Nd-HEH[EHP] and Nd-T2EHDGA complexes.

Complexes of the trivalent lanthanides and Am with di-2-ethylhexylphosphoric acid (HDEHP) dissolved in an aliphatic diluent were probed with UV-vis, X-ray absorption fine structure, and time-resolved fluorescence spectroscopy while the water concentration was determined by Karl Fischer titrations. In particular, our work focuses on the Nd-hypersensitive UV-vis absorbance region to identify the cause of changing absorbance values at 570 and 583 nm in relation to the pseudooctahedral Nd environment when coordinated with three HDEHP dimers. In contrast to recently reported interpretations, we establish that while impurities have an effect on this electronic transition band, a high water content can cause distortion of the pseudooctahedral symmetry of the six-coordinate Nd, resembling the reported spectra of the seven-coordinate Nd compounds. Extended X-ray absorption fine structure analysis of the Nd in high-concentration HDEHP solutions also points to an increase in the coordination number from 6 to 7. The spectral behavior of other lanthanides (Pr, Ho, Sm, and Er) and Am as a function of the HDEHP concentration suggests that water coordination with the metal likely depends on the metal's effective charge. Fluorescence data using lifetime studies and excitation and emission spectra support the inclusion of water in the Eu coordination sphere. Further, the role of the effective charge was confirmed by a comparison of the Gibbs free energies of six- and seven-coordinate La-HDEHP-HO and Lu-HDEHP-HO complexes using density functional theory. In contrast, HEH[EHP], the phosphonic acid analogue of HDEHP, exhibits a smaller capacity for water, and the electronic absorption spectra of Nd or Am appear to be unchanged, although the Pr spectra show a noticeable change in intensity as a function of the water content. Electronic absorption extinction coefficients of Am, Nd, Pr, Sm, Er, and Ho as a function of the HDEHP concentration are reported for the first time.