Aquatic Aggregates as “Vector” for Microplastics

Easily transportable microplastics (plastic particles < 5 mm) have become an increasingly important component of suspended particulate matter (SPM) in the aquatic environment, and their fate is significantly influenced by aggregation and flocculation. Aggregation modifies particle properties (e.g., size) controlling the hydrodynamics of SPM in the aquatic environment. Hence, understanding and quantifying aggregation is key to predicting the behaviour of both SPM and associated microplastics. However, quantifying the aggregation degree of microplastics with complex parameters in various water environments is very difficult.Here, an extensive range of microplastics including 8 polymer types, 3 shapes, different weathering conditions and different sizes (10-300 µm for fragments and microbeads, and 10-1500 µm in length for microfibers, respectively), were used to explore the aggregation dynamics of microplastics. Over 4000 measurements of incorporated microplastics were collected, and we found microplastic size (MinFeret diameter of fragments, diameter of microfibers) is the key parameter to determining the aggregation behavior. Our results simplified the aggregation of microplastics with a wide range of properties in various water ecosystems into two parameters, the size of microplastics and the size of aggregates. A boundary curve for microplastics was fitted based on size relationships between microplastics and aggregates to divide microplastics into aggregable and un-aggregable groups. This study can aid better understanding the fate of microplastics in various aquatic environments at multiple scales.