Structural compactness governs the environmental fate of polystyrene Nanoplastics: Reaggregation mechanisms in laboratory-scale aquatic systems

Abstract

Most previous studies have been largely been limited to simple stability assessments based on critical coagulation concentration, providing insufficient understanding of the particle size distribution and dynamic behavior of polystyrene nanoplastics (PS NPs, <1 mu m) under realistic environmental conditions such as salinity gradients in estuaries or flow-induced disturbances. To bridge this knowledge gap, we investigated the aggregation-disaggregation-reaggregation mechanisms of environmentally representative, non-functionalized PS NPs under various environmental conditions (pH, ionic strength, and dissolved organic matter) and physical perturbations. Real-time monitoring using a quartz crystal microbalance with dissipation enabled quantitative assessment of structural compactness and viscoelastic changes in PS NP aggregates. Our findings revealed that the final aggregate size increased by approximately 21.5% as the ionic strength rose from 100 to 600 mM. The coexistence of DOM significantly reduced the structural stiffness of the aggregates, resulting in a disaggregation degree of approximately 38.5% under physical disturbance and facilitating the redispersion of aggregates into nanometer-sized particles. Consequently, PS NPs are likely to persist as submicrometer dispersions over extended periods, highlighting their potential for long-range transport and widespread distribution within aquatic ecosystems.