Characterization of HDEHP-lanthanide complexes formed in a non-polar organic phase using 31P NMR and ESI-MS

Abstract

HDEHP (di-2-ethylhexylphosphoric acid) is one of the extractant molecules most intensively used in liquid–liquid extraction systems. Of particular interest in this investigation is its application in the TALSPEAK process, which is among the methods currently considered to be ready for technological deployment for the separation of trivalent actinides (Am^III and Cm^III) from lanthanide (Ln^III) cations. However, several fundamental features of the chemistry of this separation system are not well understood. It has become clear that the lactic acid (LacH), which is employed as a buffer in the aqueous phase, plays a very complex role in the biphasic chemistry of the system. In this study, Nuclear Magnetic Resonance (³¹P NMR) was used to investigate the rate of HDEHP (AH) exchange occurring in the binary complexes Ln(AHA)₃ (Ln = La and Sm), which are usually considered to be the predominant species present in a non-polar organic phase (1,3-diisopropylbenzene). The rate data indicate considerably faster ligand exchange kinetics for La(AHA)₃ than is seen for Sm(AHA)₃, with a corresponding shift from a dissociative interchange to an associative process. With the introduction of lactic acid (LacH) and higher concentrations of lanthanides into the system, ternary complexes (Ln³⁺-HDEHP-lactate) become dominant, as demonstrated using ³¹P NMR and Electrospray Ionization Mass Spectrometry (ESI-MS). Lactate partitioning experiments indicate that the amount of lactate extracted is correlated with the concentration of Ln³⁺. The terminal ternary complex species appears to have the general stoichiometry 1 : 2 : 1 (Ln³⁺ : HDEHP : lactate). The detection of bimetallic ternary complexes (by ESI-MS) with La³⁺ and the observation of multiple phosphorus environments (by NMR) suggest the presence of polymetallic complexes with the general formula (LaA₂Lac)_n. A model is proposed in which DEHP⁻ molecules bridge two metal ions.