Erosion Behavior of Different Microplastic Particles in Comparison to Natural Sediments

Microplastic (MP) has been detected in marine, limnic, terrestrial, and atmospheric environments. However, rivers are often only seen as transport paths for MPs from inland sources to the oceans, although transport rates in rivers can hardly be determined yet. MP in rivers can either be transported, or it settles to the bottom of the river and either remains there or is remobilized again at higher flow velocities. This remobilization, 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 MP 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 and 0.233 N/m2, depending on particle and sediment properties. Furthermore, the hiding-exposure effect was transferred to MPs and an equation was developed to determine the critical shear stress of different MP particles on natural sediment beds.