Erosion Behavior of Different Microplastic Particles in Comparison to Natural Sediments

Microplastic has been detected in marine, limnic, terrestrial and atmospheric environments. However, rivers are often only seen as transport paths for microplastics from inland sources to the oceans, although transport rates in rivers can hardly be determined yet. Microplastic in rivers can either be transported, or it settles to the bottom of the river and either remains there or is remobilised again at higher flow velocities. This remobilisation, also known as erosion, depends on the critical shear stress of a particle and is influenced by the particle properties and the sediment bed. In this study, the critical shear stresses of 14 microplastic particles with different shapes, densities and particle sizes on different sediment beds were experimentally determined and subsequently compared with the basic principles of erosion from sediment transport. Critical shear stresses of the MP particles were between 0.002 N/m² and 0.233 N/m², depending on particle and sediment properties. Furthermore, the hiding-exposure effect was transferred to microplastics and an equation was developed to determine the critical shear stress of different MP particles on natural sediment beds.