Flocculation of microplastic and cohesive sediment in natural seawater

, ersen Thorbjorn Joest, Rominikan Stiffani, Olsen Ida Stuhr, Skinnebach Kristoffer Hofer, Grube Nynne Zaza, Jedal Soren Roger, Laursen Simon Nyboe, Fruergaard Mikkel.

The flocculation of combinations of microplastic particles (MP) and natural cohesive sediment has been investigated in a laboratory setup using unfiltered seawater. The experiments were setup such that the relative number of MP particles to natural particles were environmentally realistic and both PVC, PET, Nylon and HDPE were examined, ranging in densities from about 1.4 to 0.98 g cm-3. MP particles in the size-range 63 – 125 µm were incubated with suspensions of local untreated seawater and untreated fine-grained sediment (¡ 20µm) collected from a tidal mudflat. Settling experiments were carried out with both a floc-camera video equipment (PCam) and conventional settling tubes. The experiments showed significant increases in settling velocities of the MP due to flocculation with the natural sediment. This was also the case for the buoyant HDPE-particles which was settling due to the flocculation. Analysis of grain size distributions of subsamples from the settling tube experiments showed generally uniform contents of MP in all subsamples, showing that I: the MP was incorporated into flocs and not settling as individual particles and II: the MP did not alter the settling velocity of the natural sediment as these low concentrations. The exact flocculation mechanisms still remains to be revealed but the general cohesiveness of fine-grained natural particles, organic particles as well as particulate and dissolved organic polymers are believed to be responsible for the flocculation. A strong effect of salinity was also observed, confirming the classical concept of increased flocculation of fine-grained particles as they are transported from fresh-water to estuarine and marine waters. The implication of the aggregation is that primary MP from land-based sources are likely to flocculate with other suspended particles, especially in high-turbidity estuarine environments and MP from terrestrial sources are consequently unlikely to escape further offshore.

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