The curious case of microplastic settling velocity within suspended sediment

The great abundance of microplastics (MPs) in freshwater environment poses the need for better understanding the time and spatial scales of these persistent pollutants. As floating particles in the water column, the settling velocity is crucial to determine the transport of these particles in the water. Suspended sediments (SS) are also suspected to enhance the settling rate of MP in the water column, even if their influence was not yet carefully investigated. This study aims to provide further information on this mechanism through a series of laboratory experiments. Five MP were considered to verify the role of the particle shape on the settling velocity in calm water with a predetermined SS concentration. Results suggest the importance of taking SS concentration into account as a fundamental parameter in the development of MP transport models due to its capacity of significantly modifying the mass balance of MPs discharged into aquatic ecosystems.