Removal of Heavy Metals From Aqueous System Using 4‐Hydroxybenzaldehyde Macrocycles: A DFT Study

ABSTRACT Heavy metals in water pose a potential threat to the biosphere. To address this issue, we investigated the interactions of Pb 2+ , Cd 2+ , and Hg 2+ with macrocycles ( 1‐ to‐ 3 ) based on 4‐hydroxybenzaldehyde using DFT calculations. The interaction energy shows that Pb 2+ and Cd 2+ interact with all macrocycles, while Hg 2+ interacts only with macrocycle‐ 1 . In macrocycle‐ 1, Cd 2+ effectively interacts with nitrogen atoms, Pb 2+ with C≡C having interaction energies −11.28 and −6.92 eV, respectively. In macrocycles 2 and 3, Pb 2+ effectively interacts with nitrogen (N) atoms, O═C, and C≡C, and their interaction energies are −6.97, −5.92, −10.62, −5.18, and −5.09 eV, respectively. The negative value of Δ G reveals that all processes are spontaneous. Frontier‐Molecular‐Orbitals (FMOs) analysis shows an increase in HOMO–LUMO energies and a decrease in the energy gap of macrocycles after metal cation interactions, which confirms the binding of metal cations with the macrocycles. The charge transfer analysis reveals that the highest amount of charge transferred during the interaction of Pb to N‐site ( Mac‐3 ), Pb to O═C ( Mac‐2 ), and Pb to N ( Mac‐1 ), which are 0.96, 0.91, and 0.81 electrons, respectively. The QTAIM analysis shows that the interaction nature of Pb 2+ with N, Cd 2+ with C≡C(macrocycle‐ 1 ), Pb 2+ with O═C, N, Cd 2+ with O═C(macrocycle‐ 2 ), and Pb 2+ with N(macrocycle‐ 3 ) are covalent, while all other are noncovalent bonds.