Microplastics in coastal North Sea sediments - Analyzed using Fourier Transform Infrared Spectroscopy

Anthropogenic litter, especially highly persistent plastic litter, has become a global problem. It is present in almost all marine habitats and freshwater ecosystems. Plastic production volume has continuously risen throughout the last 100 years, making its future monitoring and removal one of the greatest challenges for environmental protection. In this context, microplastics, which are defined as synthetic polymer particles smaller than 5 mm in diameter or length, are way more difficult to handle than larger plastic debris. Moreover, microplastic particles pose a threat to a huge spectrum of organisms. Animals ingest particles, which then accumulate along the food chain via biomagnification. Microplastics’ potential adverse effects stem from the obstruction of different Lumina and their chemical properties, i.e. due to sorption and transfer of Persistent Organic Pollutants or leaching of toxic additives or residual monomers. According to their formation, microplastics are divided into two categories: primary and secondary microplastics. Primary microplastics are produced in micrometer dimensions for domestic applications, brasives (e.g. shot blasting) or industrial processing (e.g. virgin pellets for injection molding), whereas secondary microplastics result from the fragmentation of larger items. Regrettably, degradation rates of plastics in general are very slow and depend on the presence of UV radiation, weathering, physical stress and biological factors such as biofilm formation. Once microplastics reach the seafloor, degradation can come to a nearly complete halt or at least be reduced by several orders of magnitude which means that marine sediments can become an ultimate repository for microplastics. In the present study, microplastics in 14 sediment samples from locations close to the Frisian Island, from the English Channel and offshore locations were isolated, quantified, measured and assigned to polymer clusters by state-of-the-art methods. In contrast to studies that solely use visual identification, this study employed µ-FTIR imaging to detect microplastics. Density separation with the MicroPlastic Sediment Separator in combination with ZnCl2 solution (ρ = 1.7 g mL-1) was used to separate the microplastics from sediment. Subsequently, the samples were subdivided into a fraction ≥ 500 µm and one < 500 µm. The first fraction was visually sorted and manually analyzed using ATR-FTIR spectroscopy whereas the latter was enzymatically and chemically purified using recently developed microplastic-reactors. Afterwards, the samples were enriched on inorganic membrane filters and automatically analyzed using µ-FTIR imaging. The concentrations of microplastics for the different stations ranged between 34 and 1457 particles per kg dry weight. All particles had a size ≤ 300 µm. Most particles (69%) were between 11 µm and 25 µm in size which indicates a high risk for ingestion, e.g. by filter feeding marine organisms. The study provides a substantial contribution to the assessment of the microplastic contamination status of the North Sea which the Marine Strategy Framework Directive targets. To date, data on microplastic burden of North Sea sediments are very scarce as only three studies exist with inter-study comparability being hampered by the lack of a standard operation procedure.