Suspended sediments mediate microplastic sedimentation in unidirectional flows

Microplastic particles (MP) are an emerging contaminant threatening many aquatic systems. Because of the sharp in-crease in plastic manufacture, the concentration of MP in natural ecosystems has grown dramatically. While it is known that when MP enter aquatic ecosystems they are transported and dispersed via different mechanisms (currents, waves, turbulence), the processes involved are still poorly understood. In the current study, the transport of MP by a unidirectional flow has been investigated in a laboratory flume. MP enter the system through a plume that can (or not) have suspended sediment. The interaction between MP and sediment was studied for three different MP particle types (Polyamide (PA) and Polyvinyl Chloride (PVC) fragments, and Polyethylene Terephthalate (PET) fibers), and four different sediment concentrations (0 g/l, 15 g/l, 30 g/l and 45 g/l). In all cases, sediment increased the vertical transport of MP to the bottom. The greater the sediment concentration, the greater the downward flux of MP. Sediment particles scavenged PA fragments downwards at the highest rate, followed by PET fibers and finally PVC fragments. These results indicate that a sediment particle-laden plume carrying MP may induce a differential settling of MP as they are advected. The scavenging of MP by sediments may result in sedimentation segregated patterns, with MP being found at shorter distances than expected for the case without sediment, therefore increasing the presence of MP near their contaminant sources.

Automated summary

Microplastic particles (MP) are a growing concern for aquatic ecosystems due to increased plastic manufacture. The transport and dispersion mechanisms of MP in aquatic environments are not well understood. This study investigated the transport of MP under a unidirectional flow in a laboratory setting. The presence of sediment increased the vertical transport of MP to the bottom, with higher sediment concentrations resulting in greater downward flux of MP. The scavenging of MP by sediment particles led to differential settling patterns, potentially increasing the presence of MP near their sources.