Molecular dynamics study of the exchange processes of heavy metals into montmorillonite: Characterization of hydrated edge surfaces and dynamic exchange mechanism

Molecular-level understanding of the exchange processes of heavy metals into the interlayer region of clay minerals help to predict the geochemical cycle of these elements. In this study, we conducted molecular dy-namics by using the newly developed CLAYFF-MOH parameters to elucidate the hydration characteristics of edge surfaces. On this basis, we explored the exchange processes of a series of heavy metal cations. Results showed that Mg for Al isomorphous substitution in the secondary outermost positions would reduce the coordination number (CN) of edge Al on both edge surfaces; water films were formed onto the interface owing to H-bonding interaction. Moreover, the cation exchange rates were Pb2+ > Cr3+ > Cd2+ > Ag+ > Ni2+ > Cu2+; Ag+ showed the largest exchange amount, followed by Ni2+, Cd2+, Pb2+, Cu2+, and Cr3+. A small part of Cl- was exchanged along with multivalent cations. The simulations of Ni2+-Cd2+-Pb2+ system indicated that coexistence of divalent cations had no significant effect on the exchange, while the exchange process in Ag+-Cd2+-Cr3+ mixing system was more complicated. Electrostatic interaction was the dominant driving force responsible for exchange. Additionally, Na+ and Ag+ were complexed on the octahedral vacancy, Si-OH, and Al-OH sites. The mobilities of cations decreased with increased ionic valence increased, and overall the diffusion in the interlayer was more constrained than in external solution. This study provides fundamental molecular-level knowledge of the fate of heavy metals in natural environment.

Automated summary

The molecular dynamics simulation reveals the molecular mechanisms of the exchange processes of heavy metals in the interlayer region of clay minerals, which is crucial for understanding the geochemical cycle of these elements. The results demonstrate that isomorphous substitution of Mg for Al can reduce the coordination number of edge Al and promote the formation of water films through H-bonding interaction. The rates of cation exchange follow the order of Pb2+ > Cr3+ > Cd2+ > Ag+ > Ni2+ > Cu2+, with Ag+ showing the highest exchange amount. A small amount of Cl- is exchanged along with the multivalent cations. The study also suggests that electrostatic interaction is the dominant driving force for cation exchange.