Microplastics Settling in Turbid Water: Impacts of Sediments-Induced Flow Patterns on Particle Deposition Rates

When microplastics (MPs) enter water bodies, they undergo various transport processes, including sedimentation, which can be influenced by factors such as particle size, density, and interactions with other particles. Surface waters contain suspended natural particles (e.g., clay and silt), which may impact MP settling rates. Here, we investigated how the presence of suspended sediments (SS) influenced the deposition patterns and rates of MPs in turbid waters. We systematically analyzed the settling velocities of particles, including different MP sizes and SS concentrations, in a plexiglass column with a camera array. For each experimental variant, we collected data on thousands of individual MPs, strengthening the statistical analysis of the particles' velocities. Simultaneous measurements of the SS flow and MPs trajectories revealed that the SS induced complex flow patterns, with MPs spending more time in downwelling flow regions, thereby accelerating MPs sedimentation. This effect was more pronounced when SS were aggregated. Additionally, we found that smaller MP fragments were more affected by the fluctuations than spheres or larger fragments. Collectively, our results provide valuable data for future MP fate models and help to understand the sedimentation processes of MPs in natural waters, which is crucial for assessing their environmental transport and impact.