What question did this study set out to answer?

To explore the coordination properties of humic acid with divalent metals using DFT calculations.

January 23, 2026Scientific Reports0 citationsOpen Access

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

Key Points

To explore the coordination properties of humic acid with divalent metals using DFT calculations.
Developed a humic acid model using DFT at B3LYP/6-31G(d,p) level.
Matched infrared spectrum of the HA model with experimental FTIR results.
Utilized Molecular Electrostatic Potential maps and HOMO/LUMO analysis.
Compared B3LYP method with MP2 and PM6 for accuracy and efficiency in studying R-COOH model.
Investigated coordination of metals Cd, Cu, and Pb, both hydrated and non-hydrated.
Divalent metals were found to coordinate with two R-COOH units in humic acid.
Copper and lead demonstrated higher reactivity compared to cadmium.
Hydrated metals exhibited increased reactivity over non-hydrated counterparts.
General coordination model for hydrated Cu interacting with two full HA units indicated complex dynamics.
Coordination might negatively impact humic acid's stability and contribute to environmental degradation.

Abstract

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.

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