Heavy metals enhance the deposition of clay colloids on silica surfaces via heterogeneous pathways

Abstract

Clay colloids, abundant in aquatic environments, can affect the migration of heavy metals, but knowledge on their deposition and release on environmental solid surfaces is incomplete. Here, the deposition and release of montmorillonite colloids (MONTs) on the silica surface was investigated in the presence of heavy metal (HM) ions. Quartz crystal microbalance with dissipation (QCM-D) test showed that MONT deposition onto silica is affected by the positive charge of MONT edges, whereby a low pH and a high ionic strength are conducive to this deposition. Deposition mainly occurred in the face-plane mode and this was reversible. HMs promoted deposition more strongly than Na⁺, Mg²⁺, or Ca²⁺. The bonding stability between MONT edges and the silica surface was so strongly enhanced by HM bridging that the deposition was irreversible. Density functional theory simulation revealed that the MONT edge (010) was more easily attached to the silica surface than the MONT surface (001). The adsorption energy of MONT edges on the silica surface increase with HM bridging and increased with heavy metal electronegative values. This work revealed how HMs favor the deposition of clay colloids onto silica and highlights the importance of heavy metal properties in the retention, thereby contributing to a comprehensive understanding of the migration and fate of HMs related to clay colloids.