Molecular simulation of adsorption properties of thiol-functionalized titanium dioxide (TiO2) nanostructure for heavy metal ions removal from aqueous solution

We investigated the adsorption of Cd ion as a primary contaminant heavy metal in drinking water on titanium dioxide (TiO2) and thiol-functionalized TiO2 (thiol-TiO2). The dispersion corrected density functional theory (DFT-D2) calculation is used for structural relaxation and second-order MP2 (second-order Møller-Plesset) calculations for interaction energy (Eint) estimation. The results reveal that the Cd bound very strongly (Eint: −4.566 eV) to the TiO2 and strongly to the thiol-TiO2 (Eint: −1.882 eV). Our findings show that the H2O molecule is adsorbed very strongly on the TiO2 surface (Eint −4.535 eV) while it is attached weakly to the thiol-TiO2 (Eint −0.076 eV). The DFT-based molecular dynamic (MD) simulations at ambient conditions reveal that water molecules are first adsorbed on the nano-TiO2 surface, and then the Cd ion is co-adsorbed on the surface. Meanwhile, for thiol-nanoTiO2, a new chemical bond was formed between the Cd ion and S atom of the thiol group at 1 ps. The analysis of the AIM (Atom-In-Molecule) theory demonstrated that the types of interactions are typical for the electrostatics accompanied with partial covalent bonding between Cd ion and both O and S atoms. Our molecular simulations offer a well-founded understanding of the adsorption mechanisms of heavy metals on TiO2 and thiol-TiO2 at ambient conditions.