Thermodynamic and Structural Studies on the Ln(III)/An(III) Malate Complexation

Area of Science:

• Coordination Chemistry
• Radiochemistry
• Spectroscopy

Background:

• Lanthanide and actinide complexation with organic ligands is crucial for understanding their environmental behavior and separation processes.
• Malate is a common organic ligand found in natural environments, influencing the speciation and mobility of trivalent metals.

Purpose of the Study:

• To investigate the complexation of trivalent lanthanides (Nd(III), Eu(III)) and the actinide (Am(III)) with malate.
• To determine the stoichiometry, structure, and thermodynamic properties of these metal-malate complexes.
• To explore the homology between lanthanide and actinide complexation behavior.

Main Methods:

• Multi-method approach combining structural and thermodynamic techniques.
• Structure-sensitive methods: Vibrational spectroscopy, Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy.
• Thermodynamic methods: UV-vis, laser fluorescence, isothermal titration calorimetry (ITC).
• Computational methods: Quantum-mechanical ab initio molecular dynamics.
• Data analysis: Parallel factor analysis, iterative transformation factor analysis, specific ion interaction theory (SIT).

Main Results:

• A tridentate ring structure was revealed for Nd(III), Eu(III), and Am(III) malate complexes, with metal coordination by two carboxylate groups and one hydroxyl group.
• Two complex species with 1:1 and 1:2 (metal/malate) stoichiometry were consistently identified.
• Stability constants (log β⁰) for Nd(III)-malate complexes were determined to be approximately 6 (1:1) and 9 (1:2) at zero ionic strength.
• Complexation enthalpies (Δ_r H⁰_m) were around 5 kJ·mol^-1, typical for small organic molecules.
• The malate binding motif, speciation, and thermodynamics were found to be transferable from lanthanides(III) to actinides(III).

Conclusions:

• The study provides a profound molecular understanding of lanthanide and actinide malate complexation.
• The observed 4f/5f element homology is supported by the consistent complexation behavior of Nd(III) and Am(III) with malate.
• These findings have implications for predicting the behavior of these elements in geological and nuclear waste management contexts.