Tyre Wear Particles: Emissions and Distribution in Soil and Stormwater Systems in Near Road Environments

Doktorsavhandling, 2025

Tyre wear particles (TWP) are a major source of microplastic pollution in road environments, yet their emissions, distribution, and environmental fate remain insufficiently characterised. This thesis presents a multi-scale investigation of TWP emissions in Sweden, their occurrence and distribution in two roadside ditches and one stormwater system, and the effectiveness of selected mitigation strategies. A detailed methodology was developed to estimate national TWP emissions using vehicle-specific emission factors and mileage data. Passenger cars were identified as the largest contributors (55%), followed by heavy-duty vehicles (31%), with rural roads accounting for the highest emissions. Field studies confirmed the occurrence of TWP in stormwater components such as gully pots, wells, and receiving waters. Fine particles 1.6–20 µm constituted a substantial portion of the total TWP load in both water and sediment samples. In roadside soils, TWP <500 µm were quantified at all depths, with concentrations decreasing with distance from the road. Laboratory column experiments demonstrated high retention efficiencies (93–99%) for TWP, particularly in fine-grained soils amended with biochar. Vegetation influenced TWP mobility by creating preferential flow paths, especially during dry periods, while drought conditions reduced retention. Co-transport of TWP with metals and tyre-derived chemicals (TDC) was observed primarily in the water phase. Strong correlations between TWP and total metals were found in effluent samples, but not in soil mixtures. Fourteen TDC were analysed, with particulate-bound forms such as 6PPD and 6PPD-Q showing high retention (>97%), while more water-soluble compounds (e.g., HMMM, OHBT, MTBT, BTSA) exhibited variable leaching. The addition of biochar improved the retention of TWP, TDC and metals in fine, non-vegetative soils, and may contribute to climate mitigation by offsetting carbon emissions from construction materials such as concrete and steel. The thesis also evaluated mitigation strategies, including stormwater systems, road ditches, and bioretention filters. Bioretention filters with the addition of sorption materials demonstrated high removal efficiencies (97–100%) for TWP even under cold and high-flow conditions. The findings in this thesis contribute to improved understanding of TWP transport and retention and support the development of targeted measures to reduce microplastic pollution from road traffic.