Solution Thermodynamics for the Thorium Complexation with N‑(2-Hydroxyethyl) Iminodiacetic Acid

N-(2-Hydroxyethyl)­iminodiacetic acid (HEIDA-H₂) is one of the major chelating agents present in the mixed nuclear waste of the Hanford site. The complexation of thorium with HEIDA-H₂ in aqueous medium was investigated to understand the capability of HEIDA-H₂ to affect the migration of actinides from the waste storage sites. The present studies aimed at determining the thermodynamic parameters of Th­(IV)–HEIDA complexes to know the speciation, stability (log K) and strength of bond formations (enthalpy and entropy changes) by potentiometric and calorimetric titrations, respectively. Thorium forms ML, ML₂ and ML₃ complexes with HEIDA. The obtained log K values 11.25 ± 0.10, 9.24 ± 0.10, and 8.1 ± 0.10, are higher than the complexes of thorium with monodenatate ligands (simple carboxylates), bidentate ligands (aminomonocarboxylates, dicarboxylates), and heterodicarboxylates. The very high entropy values of the Th­(IV)–HEIDA complexes in comparison to the Th­(IV) complexes with structurally similar ligands reflects the stronger complexation of Th with HEIDA over others. HEIDA acts as a tridentate ligand forming two five-membered chelates through the two carboxylate oxygens and the nitrogen with thorium, while the hydroxyethl group was found to enhance the stabilization by indirect participation. Density functional theory was used to optimize the geometries and to determine the thermodynamic parameters, bond distances and partial charges on individual atoms for the experimentally predicted complexes. The theoretical predictions are found to be in agreement with the experimental observations.