Mechanism insights into Hg(II) adsorption on kaolinite(001) surface: A density functional study

Environmental mercury contaminations pose a great threat to public health. The research of efficient adsorbents for mercury removal has brought unprecedented challenges. Theoretical studies employing density functional theory (DFT) can provide an understanding of adsorption structures and bonding mechanisms on the atomic level. Here, the adsorption behavior of Hg(II) on kaolinite(001) surface in an aqueous environment was explored using DFT calculations. The effective coordination number and preferred adsorption positions inclusive of two top-site and four bridge-site models were established. Adsorption complexes with 3-4 aqua ligands preserve coordination number of 5 in the stabilized structures. Energy calculations indicate that Hg(II) has substantially strong interactions on partially deprotonated surface due to the existing chemical bonding. Mulliken bond population, charge density analysis and partial density of states (PDOS) methods were investigated to explore the bonding nature. The results reveal that Hg-O-s (surface-O) bonds exhibit covalent characteristic which is ascribed to the Hg-5d and O-s-2p orbital overlap, accompanying with charge transfer from the substrate to adsorbate. This finding provides intuitive views for Hg(II) adsorption mechanisms on kaolinite surface and further encourages experimental studies on environmental remediation.