Investigations on microplastic infiltration within natural riverbed sediments.

Several studies focused on the role of rivers as vectors of microplastics (MPs) towards the sea. It is well known that during their path through the fluvial environment, MPs interact with riverbed sediments; however, the main factors impacting the mobility of MPs within the upper part of the hyporheic zone are not clear yet. The present work investigates the role of different sediment size layers in affecting the mobility of the most common MP (Polyethylene terephthalate - PET - spheres, PET 3D-ellipsoids, polystyrene - PS - fragments and polyamide - PA - fibers) within sediment porous media under different hydraulic loads (HL) and time scales (t) conditions. Results indicated the relationship between the characteristic MP diameter and that of the grains as the main parameter for the MP infiltration into the sediment layer. The maximum infiltration depth was found to not depend on HL and t. However, HL was able to influence the percentage of MPs penetrating the superficial layer and their distribution within the first 10-15 cm of the sediment layer. None of the MPs were found at depths >20-25 cm, where only PET spheres were detected. Starting from the suffusion theory, a model able to predict the MP maximum infiltration depth in the range of parameter values was provided. The outcome indicates the importance of considering geometrical and hydrodynamic aspects of the riverbed sediment layer to better characterize the spatial and temporal scales of MP transport in freshwater environments.

Suspended sediments mediate microplastic sedimentation in unidirectional flows.

Microplastic particles (MP) are an emerging contaminant threatening many aquatic systems. Because of the sharp increase 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.

On the prediction of settling velocity for plastic particles of different shapes.

Transport processes of plastic particles in freshwater and marine environments are one of the relevant advances of knowledge in predicting the fate of plastic in the environment. Here, we investigated the effect of different shapes on the settling velocity, finding a representative reference diameter which encompasses three-dimensional shapes like pellets or spherules, two-dimensional shapes like fragments or disks, and one-dimensional shapes like filaments or fibers. The new method is able to predict the settling velocity of plastic and natural particles given the representative size and the Corey shape factor coefficient, over the entire range of viscous to turbulent flow regime. The calibration of the method with experimental data, and the validation with an independent dataset, support its application in a wide range of hydraulic conditions.