Gabriella Rullander: Mitigating Urban Microplastic Pollution: Investigating Nature-Based Filters in Stormwater Management

Abstract

Microplastic (MP) pollution has emerged as a global environmental concern, particularly in urban environments, where stormwater serves as a major transport pathway for MPs into aquatic ecosystems. This thesis investigates MP sources, distribution, and removal strategies in urban settings, with a focus on stormwater filtration solutions. Field studies were conducted in Uppsala, Sweden, to assess MP contamination in road dust and spider webs (Paper I). Results indicated spatial variability, with indoor parking lots exhibiting the highest MP loads, predominantly polyurethane. Spider webs effectively captured airborne MPs, and particularly polyester fibers, demonstrating their potential as passive samplers of atmospheric pollution. To mitigate MP transport via stormwater, laboratory and pilot-scale filtration experiments were conducted using sand, bark, and biochar as filter materials (Papers II-IV). Laboratory-scale horizontal flow filters demonstrated high MP retention across all tested materials. However, results indicated that the retention efficiency was influenced by MP shape and size, with fibers being particularly well retained. Additionally, biochar-beds suggested a polarity-dependent filtration effect, preferentially removing non-polar MPs over polar MPs, emphasizing the role of polymer properties in filtration performance (Paper IV). The pilot-scale biochar-beds were tested with semi-artificial stormwater and with simulated environmental stressors, including prolonged drying and road salt exposure (Paper IV). Results revealed that biochar efficiently removed MPs and particulate-bound metals. Environmental factors influenced filter performance; drying-induced structural changes in biochar resulted in an initial surge of MP release upon rewetting, and pH shifts altered metal speciation and reduced metal removal efficiency. This research advances the understanding of urban MP pollution and its remediation, demonstrating that filtration-based stormwater treatments can effectively capture MPs before they reach aquatic ecosystems. The findings emphasize the need for environmentally robust stormwater management solutions and highlight the importance of considering real-world stressors in filtration system design. Future studies should explore long-term filter performance and the combined effects of multiple environmental stressors to optimize MP removal strategies.