Sedimentation of microplastics due to their aggregation with biogenic particles in turbulent flows*

In this study we investigate the enhancement of sedimentation of microplastics due to their aggregation with biogenic particles in homogeneous and isotropic turbulence using direct numerical simulations and Lagrangian tracking of point particles. The prevalence of microplastics (plastic particles less than 5 mm in size) in oceans and lakes has been a growing environmental concern in the past decade. Only a small fraction of the microplastics that are estimated to enter the ocean, or generated in the oceans through the fragmentation of larger pieces, are found to be floating on the ocean surface. The missing of microplastics from the ocean surface has been attributed to the aggregation of microplastics with heavier biogenic particles. In this regard, we investigate how the combined effects of differential settling/rising of microplastics and biogenic particles and turbulent flow motion contribute to the aggregation of microplastic and biogenic particles for different settling numbers and Stokes numbers. Additionally, we quantify the composition of the aggregates and the rate of settling of microplastics due to the formation of the aggregates.