Occurrence and Pathways of Microplastics in Bioretention Filters

The spread of microplastics in urban environments is an increasingly highlighted environmental problem. To prevent their spread, the treatment of stormwater from urban surfaces in bioretention filters may be a possible solution. This research project aimed to identify the occurrence and pathways for microplastics in bioretention filters that receive contaminated stormwater from impervious surfaces in a city. Sampling was conducted in selected bioretention filters, incorporating soil sampling at different depths and flow-proportional stormwater sampling during various rain events. Eleven different polymers on particles >10 µm were analysed in the soil samples and stormwater with Thermal Extraction and Desorption Gas Chromatography-Mass Spectrometry (TED-GC/MS). Additionally, some of the stormwater samples were analysed by pyrolysis PYR-GC/MS on particles >27 µm. The polymers most prevalent in all samples were polyvinyl chloride (PVC), polyethylene terephthalate (PET), and polystyrene (PS). The study revealed variations in polymer composition between water and soil samples, with a tendency for more polar polymers, such as polyethylene terephthalate (PET), to be more prevalent in water samples. This suggests differential transport and retention mechanisms for various polymer types, with more polar polymers perhaps being more soluble or less likely to adhere to soil particles, thus remaining more abundant in stormwater runoff. Further investigation is needed to fully understand the implications of these findings for the design and optimization of bioretention systems in capturing a broad spectrum of microplastic pollutants. Furthermore, the highest microplastic concentrations in soil samples were found in the upper layers of the bioretention filters, demonstrating effective microplastic reduction through their accumulation in soil. However, different outcomes were observed between the analytical methods and some uncertainties in the analysis, necessitating further studies with simultaneous sampling and analysis by different analytical methods of both soil and stormwater. Overall, the investigated bioretention filters proved to be efficient measures for reducing the spread of microplastics from impervious surfaces, preventing their release to recipient surface waters.