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

Xingliang Li; Zhicheng Zhang; Bijun Liu; Wanjun Mu; Yuchuan Yang; Leigh R Martin; Shunzhong Luo; Linfeng Rao

doi:10.1021/acs.inorgchem.9b02706

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

Inorg Chem. 2019 Nov 18;58(22):15618-15628. doi: 10.1021/acs.inorgchem.9b02706. Epub 2019 Nov 1.

Authors

Xingliang Li¹, Zhicheng Zhang², Bijun Liu¹, Wanjun Mu¹, Yuchuan Yang¹, Leigh R Martin³, Shunzhong Luo¹, Linfeng Rao²

Affiliations

¹ Institute of Nuclear Physics and Chemistry , China Academy of Engineering Physics , Mianyang , Sichuan 621999 , China.
² Chemical Sciences Division , Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States.
³ Nuclear Security & Isotope Technology Division , Oak Ridge National Laboratory , Oak Ridge , Tennessee 37831 , United States.

PMID: 31674777
DOI: 10.1021/acs.inorgchem.9b02706

Abstract

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.