Atomic-Scale Insights into Methylmercury Complexation on Clay Mineral: A Density Functional Study

Clay minerals play a vital role in methylmercury (MeHg) retention via surface complexation, significantly affecting its bioavailability and mobility. However, the structural complexity of clay mineral interfaces, especially at the edge surface and interlayer, limits experimental methods in precisely elucidating MeHg adsorption mechanisms at the molecular level. The first-principles calculations based on density functional theory (DFT) were conducted to investigate the microstructural dependence and molecular adsorption mechanisms of MeHg onto kaolinite (Kln) and montmorillonite (Mnt). The preferred adsorption configurations, charge transfer, and bonding mechanisms were determined using adsorption energy, electron density difference, Bader charge, projected density of states, crystal orbital bond index, and Wannier functions analyses. The adsorption capacity of MeHg follows the order of Mnt interlayer > Mnt (010) surface > Mnt (001) surface > Kln (010) surface > Kln (001) surface. Mnt has a higher active site density than Kln, contributing to its higher MeHg adsorption potential. The most stable structure on the Mnt interlayer is a monodentate SiO–Hg complex, and a bidentate SiO(AlOH)–Hg complex was found to be the most stable configuration on the Kln (010) surface. The Hg atom in MeHg gains charge from the surface O atom of Mnt and Kln, forming Hg–O polar covalent bonds. The bonding arises from the overlap and hybridization of Hg-5d and O-2p orbitals, with Mnt bonding driven by Hg-5dz2 orbital, and Kln bonding involving Hg-5dyz, Hg-5dz2, and Hg-5dx2-y2 orbitals. These findings provide deeper molecular-level insights into MeHg-mineral interactions, offering significant implications for MeHg risk assessment and environmental remediation strategies.