Theoretical investigation of the chloride effect on aqueous Hg(II) adsorption on the kaolinite(001) surface

Abstract

The pivotal role of chloride in complexation has prompted research into the adsorption of metal complexes on mineral surfaces in water. However, understanding the adsorption behavior of aqueous Hg(II) by kaolinite in the presence of chloride remains challenging. In this study, the structure and electronic properties of typical [HgCl]+ adsorption on the kaolinite(001) surface at the atomic scale were explored from first principles. The results indicated that the existing chloride could impede the spatial occupancy of aqueous ligands, leading to the decline in mercury coordination number. The formation energies decreased by a factor of 2.34 to 2.56 (top-site: from −111.25 to −47.54 kcal/mol; bridge-site: from −94.18 to −36.77 kcal/mol) in the presence of chloride. The lower stability of the adsorption complexes was attributed to the overlaps of Hg-6s and Os-2p (surface oxygen) anti-bonding orbitals in the range of +2.0 eV ~ +3.0 eV, accompanied with less charge transfer (no more than 0.81 electrons) at the interfaces. By comparative analysis of Cl-free and Cl-containing systems, we demonstrated the adverse effect of chloride and concluded that Hg(II) desorption should be the dominate process on the kaolinite(001) surface under the influence of chloride, which was further validated through molecular dynamics simulations. This theoretical investigation provides guidance for future aqueous mercury research in the field of environmental remediation.