Critical Role of Coexistence Order and Interfacial Forces in the Aggregation of Polystyrene Nanoplastics Induced by Nano-SiO2 and Metal Cations

This study investigated the aggregation of polystyrene (PS) NPs by kinetics experiments, considering single, simultaneous, and sequential addition of silica (SiO2) nanoparticles and metal cations. Results demonstrated that ion concentration and strength played a crucial role in determining PS stability across all systems. Surface interaction forces, particularly van der Waals forces, were fundamental mechanisms for PS aggregation. In simultaneous addition system, SiO2 improved PS stability. The higher the concentration of SiO2, the more significant its stabilizing effect on PS, as SiO2 competed with PS for metal cations. DLVO calculations and zeta potential analyses suggested that electric double layer repulsion was primary factor behind PS stabilization by SiO2. Additionally, other non-DLVO forces, such as hydrogen bonding and π-π interactions, might also affect the stabilization. SiO2 was found to promote PS aggregation in the sequential addition system, contrasting with its stabilizing effect in the simultaneous addition system. The increase in aggregate size was primarily attributed to the bridging effects, though the enhanced electrostatic repulsion prevented a substantial rise in aggregate size. This study highlights how the presence of non-plastic particles influences the environmental behavior of NPs and deepens our understanding of the interactions between PS NPs and SiO2 in complex and realistic aqueous environments.