Microplastics in the urban environment : Concentrations and composition in stormwater sediments and UV-degradation of common litter

Sammanfattning: Microplastics (MPs) are small pieces of plastic material including tyre wear particles (TWPs) smaller than 5000 µm. One of the primary pathways through which MPs including TWPs enter aquatic and marine environments is stormwater runoff from urban areas. The aim of this thesis is to advance the knowledge of MPs found in sediments from gully pots and stormwater ponds, as well as increase the understanding of and assess one of the sources of secondary MPs, i.e. UV-degradation of four commonly found plastic litters. To advance the knowledge, an experimental approach was used where results from a laboratory study and two field studies where synthesized. UV-degradation of macroplastic litter (a bag, wrapper, cup lid, and bottle) into MP was studied in a laboratory experiment, highlighting one of the sources of MPs in urban environments. MPs generated using accelerated UV-degradation with three exposure times (corresponding to ¼, 1 and 2 years outdoors in Sweden) were quantified with micro-Fourier Transform Infrared Spectroscopy (µFTIR). In the field studies sediment samples were collected form 29 gully pots and six stormwater ponds in urban areas in Sweden, in order to investigate MP composition and concentrations. All sediment samples were analysed with regards to MPs (non-carbon black) using µ-FTIR, enabling the assessment of MP concentrations by number, mass as well as particle size. Using the same method (µFTIR) enabled comparison of the results between the laboratory and field studies. As a complement carbon-black MPs were analysed in gully pot sediments and TWPs in pond sediments using Attenuated Total Reflection (ATR)-FTIR and Pyrolysis-GC-MS respectively.  MPs were detected in all sediment samples, indicating that both gully pots and ponds temporarily trap MPs through sedimentation, thus they were not transported further downstream. Polypropylene (PP), EPDM (gully pots) and TWPs (ponds) were among the most commonly detected polymer type in sediment samples. The laboratory study showed that common plastic litters degrade due to UV- exposure (with the PP- wrapper being most prone to degradation), highlighting PP as a possible source of MPs in stormwater. The majority of (non-black) MPs, by number, were in the size range 126 - 250 µm in gully pot sediments, in contrast to pond sediments the majority were smaller than 100 µm. These results indicates that larger MP particles tend to settle early in the urban drainage system for example in gully pots, while smaller particles may be transported further downstream via stormwater, for example reaching ponds and mainly settle in pond sediment Evaluating MP concentrations and composition with regards to polymer type and particle size in gully pot and stormwater pond sediments and common sources one of the source of MPs in the urban environment has advanced the understanding for example a commonly detected type (PP) and what MP sizes that tend to settle in the different studied storm water management facilities i.e., gully pots and ponds.