Microplastic-sediment aggregation increases microplastic settling velocity to values comparable to those of pure sediment flocs.

Understanding the deposition of microplastics in riverine and estuarine environments is essential for predicting their environmental fate and ecological impacts. Aggregation with sediment flocs may enhance microplastic deposition because microplastics incorporated into sediment flocs might settle faster than individual microplastic particles, highlighting the need to quantify this effect. In laboratory experiments, microplastic-sediment flocs were generated by mixing illite clay, a synthetic flocculant, and microplastics of varying shapes and sizes under low shear. Floc size and settling velocity were quantified using a settling column combined with video microscopy, while a second camera was used to detect microplastics within flocs. In addition, the settling velocity of individual microplastics (without illite) was measured. In total, 4586 flocs were analyzed. Incorporated microplastics settled approximately one order of magnitude faster than non-incorporated microplastics, with settling velocities comparable to those of sediment flocs. Fibers were incorporated across the full floc size range, whereas larger fragments and spheres were primarily found in larger flocs. Across the full floc population, the presence of microplastics slightly increased both floc size and settling velocity. These findings suggest that microplastic-sediment aggregation may represent an important mechanism influencing the transport and deposition of microplastics in systems with cohesive sediments. Once incorporated into sediment flocs, microplastics are likely to be deposited in a manner similar to natural sediments, as settling velocities are similar to those of natural flocs. Consequently, areas of high sediment accumulation may also act as potential hotspots for microplastic deposition.