The widespread presence of microplastics (MPs) in freshwater environments underscores the need to better understand their temporal and spatial dynamics. Investigating the settling velocity (W) of MPs in the water column is crucial for comprehending their transport mechanisms within river systems. Several models have been proposed to estimate the W of this type of pollutant. However, to date, none of them account for the simultaneous presence of suspended sediments.

This study aims to address this knowledge gap by conducting laboratory experiments to analyze the W of 12 different types of MPs with various shapes, under both clear and turbid water conditions in a still water tank. For each experimental run trajectories are captured by using high resolution camera and UV lighting to enhance the visibility of MPs. Both vertical and horizontal W components, tilt angles, oscillation frequencies and trajectory angles have been calculated. Appropriate non-dimensional parameter (i.e. Reynolds number (Re), Galileo Number (Ga), Stability Number (I*), Strouhal number (St)) have been used to better describe the MPs hydrodynamics. Results have shown, for the first time, that suspended sediments influence the MPs falling behaviour by inducing secondary motions that increases MPs settling velocity.

Particularly, the more elongated the MPs the greater the increasing rate of W. Findings have also shown a Gaussian probability distribution of the particle’s lateral position along the water column (with respect to the vertical axis of the tank) suggesting a Fickian-type diffusion of MPs throughout vertical water profile with several implications for their accumulation in calm water environment.

The above findings highlight the importance of including suspended sediment as a key factor in developing MP transport models, due to its significant impact on the mass balance of MPs in aquatic ecosystems.