Aggregation behavior of polyethylene microplastics in the nearshore environment: The role of particle size, environmental condition and turbulent flow

Estuary and coastal waters are hotspot areas for microplastics (MPs) pollution. MPs of varying sizes converge in this complex nearshore environment. Aggregation is an important process that affects the transport and fate of MPs in the aqueous environment. Nevertheless, the influence of different factors on the aggregation behavior and the aggregates structure of MPs is unclear. In this study, the aggregation behavior and the aggregates structure of polyethylene microplastics (PEs) of different sizes under the impact of nearshore environmental conditions (i.e., salinity gradient, dissolved organic matter-DOM, turbulent flow) were investigated. The results show that particle size was the dominant factor affecting the stability of PEs in the aqueous environment, and the critical coagulation concentration (CCC) of PEs shifts to the right with increasing size. It was also found that the size of PEs stable aggregates is negatively correlated with the turbulent kinetic energy dissipation rate. The particle size of PEs can significantly affect the fractal dimension (FD) of stable aggregates, and the smaller the particle size, the more compact the aggregates formed. Moreover, salinity and DOM control the size and FD of PEs stable aggregates through different mechanisms. The findings of this study will be helpful for the prediction of the transport and fate of MPs in the aqueous environment.