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 off 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 degrees C) and enthalpies (25 degrees 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 Ln(3+)/HEDTA complexes, LnL(aq) and Ln(2)(H-1L)(2)(2-) (Ln(3+) refers to La3+, Nd3+, and Eu3+; H-1L4- refers to deprotonation of the hydroxyl group) during titration. The hydroxyl group of HEDTA is coordinated in the Ln(3+)/HEDTA complex. The dinuclear Ln(2)(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 Ln(3+) cations. Complexation of NdLaq is exothermic, while formation of the hydrolytic complex Nd-2(H-1L)(2)(2-) is endothermic. Both NdL aq and Nd-2(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.