Complexation of Light Trivalent Lanthanides with N-(2-Hydroxyethyl)ethylenediamine- N, N', N'-triacetic Acid in Aqueous Solutions: Thermodynamic Analysis and Coordination Modes

N-(2-Hydroxyethyl)ethylenediamine-N,N',N'-triacetic acid (HEDTA, denoted as H₃L) is a strong chelating ligand that is widely used in the separation of f elements as relevant to the nuclear fuel cycle. There is much to be known about the structure and composition of the coordination sphere of the complexes of HEDTA with lanthanides. The complexation of HEDTA with light lanthanides (La³⁺, Nd³⁺, and Eu³⁺) was investigated thermodynamically and structurally in aqueous solutions. Potentiometry and microcalorimetry were performed to acquire the complexation constants (25-70 °C) and enthalpies (25 °C), respectively, at I = 1.0 mol·L^-1 NaClO₄. Coordination modes of the complexes were analyzed by luminescence spectroscopy and NMR spectroscopy. The results indicate that there are two successive Ln³⁺/HEDTA complexes, LnL_aq and Ln₂(H_-1L)₂^2- (Ln³⁺ refers to La³⁺, Nd³⁺, and Eu³⁺; H_-1L^4- refers to deprotonation of the hydroxyl group) during titration. The hydroxyl group of HEDTA is coordinated in the Ln³⁺/HEDTA complex. The dinuclear Ln₂(H_-1L)₂^2- complex is present as a carboxyl-bridged centrosymmetric dimer, and two carboxyl groups in bridging positions are coordinated to two adjacent Ln³⁺ cations. Complexation of NdL_aq is exothermic, while formation of the hydrolytic complex Nd₂(H_-1L)₂^2- is endothermic. Both NdL_aq and Nd₂(H_-1L)₂^2- complexes are driven by entropic force. These data will help to predict the behavior of lanthanides in the separation process, where HEDTA is used as the aqueous complexant.