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Transport Processes of Microplastic Particles in the Fluvial Environment: Erosion, Transport and Deposition
(PhD Thesis)

December, 2020

Microplastic enters the environment in different ways and accumulates there due to the persistence of the material. For a long time, microplastic was exclusively considered and studied in the marine environment: From the first environmental studies, to ecotoxicological studies with marine organisms, to hydro-numeric models that were intended to describe the distribution of microplastic in the seas and oceans. However, studies have gradually concluded that most of the microplastic is discharged into the oceans by land and therefore by rivers, and the focus has widened to include the fluvial environment. Initially, rivers were considered to be only transport pathways for microplastic from the land-based sources to the open sea. However, it soon became clear that microplastic can also be retained and deposited in rivers and that the concentrations in the fluvial environment are as high asin some hot spots in the marine environment. Due to the limited knowledge about the transport behavior of microplastic in the aquatic environment, the basics of classical sediment transport were simply adapted to the properties of microplastic. However, whether this transfer is appropriate was not examined. The differences between microplastic and classic sediment are undeniable: While sediment has an average density of 2.65 kg/cm³, microplastic can be both lighter and heavier than water, but it is always significantly lighter than natural sediment. Moreover, microplastic has very variable shapes, so it can appear either as pellets or microbeads, but also as fragments, fibers or films. Sediment, on the other hand, consists mainly of granular grains. Finally, the different trends of mean grain diameters along the course of the river are also to be mentioned. While classical sediment is ground smaller and smaller along the course of the river, microplastic is introduced via numerous sources along the course, so that no trend in grain sizes can be formed. Based on these fundamentals, a transferability of the theoretical principles from sediment transport must therefore at least be questioned. Thus, in this thesis the behavior of microplastic is compared with the theoretical calculations from classical sediment transport by using physical model experiments. The transport process is herein divided up into erosion, sedimentation and rise as well as infiltration into the river bed. A special focus was layed on the effects of particle properties such as density, diameter and shape of the microplastic on the transport mechanisms. The sedimentation and rise behavior was examined by experiments in a sedimentation column and thus the terminal settling and rise velocities of different microplastic particles were determined. These velocities could not be represented sufficiently by the typical formulas from sediment transport (e.g. Stokes settling formula), so that new theoretical approaches based on the physical model experiments were determined. The erosion behavior was investigated in the annular flume of the IWW by applying single microplastic particles to different sediment beds and then slowly increasing the shear stress on the bottom of the channel until the particle started to move. Based on these experiments, the critical shear stresses of the different microplastic particles were determined as a function of their particle properties and the sediment bed and compared with the calculation methods from classical sediment transport, namely Shields diagram and hiding-exposure effect. In the comparison it became clear that microplastic moves earlier than determined by the theoretical approaches so that a greater mobility of the microplastic than previously thought is to be expected. Finally, new approaches were developed todescribe the erosion behavior of microplastic more accurately. For investigating the infiltration behavior of microplastic into the river bed, an infiltration column with glass spheres of different diameters (1.5 - 11 mm) was used, on which water was evenly sprinkled from above. Different microplastic particles were applied to the surface of the glass spheres and then their infiltration depth was determined as a function of their shape, density, and size and the grain size of the glass spheres. The subsequent comparison with the basic principles of fine sediment infiltration showed that these could be transferred so that on this basis the ideal sampling depth of fluvial sediment could be determined. This work therefore offers a first investigation of the transport mechanisms of microplastic in the fluvial environment. When examinating the transferability of theoretical principles from classical sediment transport to microplastic transport, it became clear that the application of these principles produces only insufficient results. Therefore, new approaches were developed, which can be used in the future for the simulation of the transport behavior of microplastic.

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