Adsorption mechanism of toxic heavy metal ions on oxygen-passivated nanopores in graphene nanoflakes

In this work, using density functional theory, we investigate the adsorption process of toxic metal ions (Hg2+, Cd2+, and Pb2+) on graphene nanoflakes (GNF) comprised of various sized oxygen-passivated nanopores to gain molecular insights into the ability of such surfaces in effectively removing the metal ions from contaminated environments. Thermodynamically, the adsorption of these ions on the oxygen-passivated nanopores is shown to be more energetically favorable than that on the pristine surface. The dispersion corrected adsorption energy calculations indicate Hg2+ ion to have the highest and Pb2+ ion the lowest affinities for interaction with such surfaces (Hg2+ > Cd2+ > Pb2+). The highest adsorption of Hg2+ and Cd2+ ions on the surfaces is seen in the 12-crown-3…Hg2+ (− 382.9 kcal/mol) and 12-crown-3…Cd2+ (− 314.7 kcal/mol) complexes, while Pb2+ ion shows the highest adsorption energy for the 18-crown-6 surface (− 195.0 kcal/mol). Noncovalent interaction plots, Hirshfeld charge analysis and energy decomposition analysis conclude that the role of charge transfer in formation of surface…ion complexes is more important than noncovalent interactions and therefore plays a key role in the adsorption of these ions on the surfaces. The magnitude of charge transfer in the complexes follows the order: surface…Hg2+ > surface…Cd2+ > surface…Pb2+, which is consistent with the adsorption energetic order of these metal ions on the surfaces. We also found that Pb2+ ion is mainly adsorbed on the surfaces via electrostatic interactions, while Hg2+ and Cd2+ ions are adsorbed through van der Waals (vdW) interactions. This could be attributed to the presence of vacant p orbitals on Pb2+ ion, which interact with the π bonds on the GNF and oxygen atoms in the nanopores through electrostatic interactions. No such vacant orbitals are available for Hg2+ and Cd2+ ions thus resulting in such ions adsorbing via vdW interactions. The contribution of ΔEorb, induction energy component of total energy, for the surface…Hg2+ complexes is more than that for the surface…Cd2+ and surface…Pb2+ complexes, following the order: surface…Hg2+ > surface…Cd2+ > surface…Pb2+ again, consistent with the calculated adsorption energy order suggesting that charge transfer indeed plays a major role in formation of such surface…ion complexes. Finally, time-dependent density functional theory calculations show that the absorption spectra of the surfaces undergo significant changes, including peak shifts, peak quenching and appearance of new peaks, upon interaction with Hg2+, Cd2+, and Pb2+ ions, such changes being consistent with binding energetics rank order of ions on these surfaces. These complexes would have potential applications in near-infra-red photonics in addition to their service of effectively remediating of toxic heavy elements from contaminated environments.