The effect of sediment on microplastic detection, settling and environmental distribution

Microplastic (MP) pollution is widespread in the natural environment and local distribution of MP particles in sediments can be highly variable. Sediment is thought to act as an eventual MP sink. However, the factors affecting MP transport behaviour and eventual distribution in natural sediments are complex. This study adds to the literature regarding the influence of sediment characteristics such as grain size and mineralogy on the detection, transport, and distribution of MPs using a variety of novel methods. Initially, method development was performed to identify appropriate protocols for density separation and core division for both the extraction of MPs and the preservation of sedimentary facies data. In this process, vertical distribution of MPs in three cores from the River Thames, London, UK was examined, identifying high MP concentrations ranging from 617 – 2549 MP/10kg-1 dw. Glass microspheres were found in high concentration in the Lambeth Bridge core, with the abrasion of road paint from the upstream Lambeth Bridge considered a possible source. Methods of core division for facies analysis were trialled and the Surrey Docks core was selected for facies analysis. A Bray-Curtis dissimilarity test was performed as part of this analysis. Results showed low to moderate levels of dissimilarity of MP morphologies between different facies associated with point bar environments. This suggests that sedimentological processes are not the dominant driver of MP morphological distribution, and that variation in local input of plastic pollution is likely to overprint dispersion of MPs in facies at this site. Vertical settling experiments mixing sediment of varying grades with microplastic fibres and fragments in water of varying salinity were performed in an attempt to elucidate the role of variation in sedimentological factors on MP distribution. MPs generally formed a layer at the top of the sediment layer due to slower settling rates, with a smaller number retained in the resulting deposit. However, a higher concentration were retained in finer grades of sediment, particularly in the silt and clay experiment runs. Results also suggested that MP fibres are more likely to become entrained in sediment during vertical settling than MP fragments, a higher proportion of which appeared to sit on top of the resulting deposit. Changes in salinity appeared to have a minimal effect on the settling behaviour of polyethylene terephthalate (PET) in conjunction with sediments, at least in the ranges tested in this study. Finally, detection limits for MPs in bulk samples using pyrolysis GC/MS (Py-GM/MS) in association with glass microbeads of varying sediment grades was investigated and compared to bentonite clay. Detection limits of PET were found to be between 0.05 and 0.1% by volume for sediment grades ranging from silt – coarse sand. Sediment grade was found to have a minimal effect on the peak area and relative abundance of PET-identifying pyrolyzates, which suggests sediment grade has a minimal effect on the ability to detect PET compounds using pyrolysis. However, use of bentonite clay appears to significantly increase the detection limit of PET in sediment. This implies that PET detection methods using pyrolysis may be less successful when samples are made up of clay minerals. The work presented in this thesis helps inform future work examining relationships between sediment grade and MPs by exploring and developing novel methodologies for MP detection, facies association, and experimental study.