A DFT-based analysis of adsorption of Cd2+, Cr3+, Cu2+, Hg2+, Pb2+, and Zn2+, on vanillin monomer: a study of the removal of metal ions from effluents

The density functional theory was used to understand the adsorption process of Cd(II), Cr(III), Cu(II), Hg(II), Pb(II), and Zn(II) ions with the methacrylate monomer derived of vanillin (VMA). Different analyses were carried out: Conformational analysis, molecular electrostatic potential (MEP), adsorption energy, frontier orbitals, hardness, and softness, all of which are necessary to predict the formation of complexes. By means of the molecular electrostatic potential and frontier molecular orbital (FMOs), the best region for adsorption was found, so each metallic ion of the study was placed close to the nitrogen and oxygen atoms of the imine and carboxyl groups of vanillin monomer, respectively. The bond of the metal ions with the nitrogen atom was shown to be stronger than with oxygen atoms, because the charge density of the nitrogen is increased in the formation of the Schiff base with the proximity of the aromatic ring. The monomer showed to be more adsorbent for the Cu(II), Cr(III), and Pb(II) ions because of the high energy values involved. The analysis QTAIM was investigated to understand the character of the interaction between vanillin monomer and metal species, which were shown in almost all cases as covalent partial. Thus, the monomer derived from vanillin has good stability in water and is therefore considered a good material for the remediation of effluents and poisonings.