DFT insights into humic acid coordination of Cd(II) Cu(II) and Pb(II)

Abstract This study used Density Functional Theory (DFT) at the B3LYP/6-31G(d, p) level to develop a model for humic acid (HA), characterizing it as an organic molecule with functional groups featuring hydrogen bonding. The calculated Infrared (IR) spectrum of the HA model matched experimental results from Fourier-Transform Infrared (FTIR) spectroscopy. Analysis using Molecular Electrostatic Potential (MESP) maps and Highest Occupied/Lowest Unoccupied Molecular Orbitals (HOMO/LUMO) suggested that HA could be simplified to a reactive R-COOH (carboxylic acid) model. A computational comparison between B3LYP/6-31G(d, p), MP2, and the semi-empirical PM6 method found that PM6 was suitable for studying the R-COOH model, offering a balance of accuracy and computational efficiency. The coordinating ability of the divalent metals Cd, Cu, and Pb was investigated, showing they can coordinate with two R-COOH units. Cu and Pb were found to be more reactive than the coordinated Cd. Further simulations, where each metal was hydrated with four water molecules, revealed that the hydrated coordinated metals were more reactive than their non-hydrated counterparts. A general model for metal/HA coordination was explored by interacting hydrated Cu with two full HA units. It was concluded that HA is a useful agent for coordinating divalent metals in aquatic environments. However, the Quantum Theory of Atoms in Molecules (QTAIM) analysis indicated that this coordination process might negatively affect the HA’s stability and environmental degradation.