Settling behaviour of irregular-shaped polystyrene microplastics

Abstract

Microplastics are emerging pollutants that can pose threats to aquatic ecosystems by carrying toxic additives to the food web. Their abundance has been reported in many waterways globally, including the Sai Gon River, Vietnam. A good understanding of microplastic sinking behaviours can help predict the distribution and removal of these pollutants in water. Microplastic dynamics are still mostly assumed to be similar to that of suspended sediment. However, MPs possess many artificial properties that can make them move distinctly in the water from suspended sediment like densities, structures, shapes, and surface properties. This research, therefore, aims to investigate the correlation of the polystyrene microplastic size and irregular shapes with its vertical movement. The experiments employed the Particle Tracking Velocimetry (PTV) method and an automatic image processing algorithm to simultaneously measure MP geometrical properties and settling velocity while freely settling in a still water column. We found that the circumscribed diameter is the most appropriate geometrical parameter to represent MP size. The settling velocity of irregular-shaped polystyrene microplastics in size of 0.2 to 0.9 mm is mostly between 0.5 and 2 mm/s, half of the sediment but four-fold that of biological aggregates at the same size. The settling velocity increases two-fold when MP size enlarges from 0.2 to 0.9 mm. The shape irregularity can slow down the sinking of large microplastics but do not affect the vertical movement of microplastics smaller than 0.32 mm. The ratio between microplastic projected area and the area of the smallest circumscribed circle was found to be the two-dimensional representation of three-dimensional shapes in the correlation between size and settling velocity. Therefore, the fitting curve equations suggested herein can be used as a simple tool to estimate the settling velocity of irregular-shaped polystyrene microplastics found in many in-situ sampling campaigns.