Settling velocities of microplastics with different shapes in sediment-water mixtures

The widespread distribution of microplastics (MP) in aquatic systems highlights the need for a clear understanding of how they are transported and accumulate on the bottom of water bodies. Developing predictive models for MP dispersion, sedimentation, and bioaccumulation is crucial for informing regulatory decisions and mitigating the impact of MP and related pollutants. Among the key parameters, MP settling velocity is considered the most critical for predicting their behavior in aquatic environments. Recent studies suggest an intricate and not fully understood relationship between MP settling and sediment dynamics. To date, none of the current models can predict how sediment modifies MP settling velocity. Therefore, previous understanding on MP settling does not fully account for the sedimentation of MP in aquatic ecosystems, where sediment suspension is present. This study provides further evidence that the presence of sediment alters the sedimentation rate of MP based on their shape, offering a quantitative estimate of this interaction. For the first time, the effects sediment interaction has on MP sinking trajectories and inclinations are presented. A preliminary, modified version of an existing formula is proposed to estimate MP settling velocity in the presence of sediment, laying the groundwork for more accurate predictive models of MP transport in aquatic environments.