Transport of polyamide microplastics at the sediment-water interface – First results from mesocosm studies

The occurrence of microplastics in the environment has become a major research interest in the last years. Aside from the marine environment, microplastic particles in various amounts, forms and compositions have now also been identified in freshwater bodies, groundwater, soils and interfaces connecting these compartments such as the hyporheic zone. Their transport and fate in these compartments has thus become the focus of recent field and laboratory studies.

Here we present first results from mesocosm studies performed at the Environmental Change Outdoor Laboratory (ECOLAB) facility of the University of Birmingham, UK. In a series of experiments conducted in recirculating flumes (12 setups in total, flume dimensions are 200 by 42 by 15 cm) we studied the behaviour of polyamide fragments and fibers in two different types of sediments and under different flow conditions. Polyamide fragments (diameter < 600 μm) were obtained from crushing larger pellets using a ball mill and liquid nitrogen while fibers (Flock Depot, Germany, fiber length of 500 μm, 1.7 dtex) were obtained commercially. Sand and gravel of known particle diameter ranges were used to represent natural sediments. Flow experiments were conducted over several days in duplicate by injecting a known concentration of microplastic particles into the flumes and taking samples (20 mL) at three flume locations at predefined intervals. Target particles were stained with Nile red before injection for better visibility and further analyzed using a stereomicroscope after filtering. First results show significant differences in fiber and fragment particle concentrations suspended in the water column/retained in the sediments as well as between different fragment sizes.

Further studies are planned as to the long-term behaviour of these microplastics in freshwater sediments experiencing biofilm growth as well as regarding possible chemical additives. The information generated with these flume experiments improves our understanding of microplastic distribution and immobilization at the sediment-water interface.