Citric and Tartaric Acids as Green O-Donor Ligands for Seawater Cation Capture: DFT-Based Thermodynamic and Electronic-Structure Insights

Understanding how environmentally benign organic ligands bind aqueous metal ions is important for developing sustainable ion-capture materials for water treatment. In this work, density functional theory calculations were used to investigate the adsorption of Na⁺, K⁺, Ca 2 ⁺, Mg 2 ⁺, and Al 3 ⁺ ions on citric acid (CA) and tartaric acid (TA), two biodegradable oxygen-rich ligands containing carboxyl and hydroxyl donor groups. Geometry optimizations, frequency calculations, and solvation corrections were performed at the B3LYP-D3/6–311 + + G(d,p) level using the IEF-PCM water model. Six adsorption motifs were examined for each ligand, and the resulting complexes were analyzed using interaction energies, Gibbs free energies, M–O bond distances, BSSE corrections, NBO second-order perturbation analysis, molecular electrostatic potential maps, NCI-RDG analysis, ELF/LOL maps, and global reactivity descriptors. All complexes exhibit negative interaction energies and Gibbs free energies under the adopted computational model, indicating favorable adsorption. The preferred adsorption sites are Site 5 for monovalent ions, Site 2 for divalent ions, and Site 3 for Al 3 ⁺. The overall binding strength follows the order Al 3 ⁺ ≫ Mg 2 ⁺ > Ca 2 ⁺ > Na⁺ ≈ K⁺, consistent with increasing cation charge density, shorter M–O distances, and stronger donor–acceptor stabilization. BSSE corrections are small, confirming that basis-set superposition error has a limited effect on the reported trends. NBO, MEP, NCI, and ELF/LOL analyses collectively show that carboxyl oxygen atoms dominate ion coordination, while hydroxyl groups provide secondary stabilization. CA is favored for Al 3 ⁺ due to its higher denticity, whereas TA shows slightly stronger local coordination for mono- and divalent ions. These findings provide molecular-level guidance for designing biodegradable organic-acid-based motifs for selective aqueous ion capture.