Transport processes of tyre wear particles, tyre-derived chemicals, and metals in road ditch soil with and without the addition of biochar

Abstract This study investigates the transport of tyre wear particles (TWP), a significant source of microplastics, together with metals and tyre-derived chemicals (TDC) in soil mixtures that could be used in road ditches. The aim was to identify soil mixtures with high pollutant retention. In a ten-month column experiment, soil mixtures were tested to evaluate their retention efficiency (RE) for TWP, metals, and TDC from polluted stormwater used for irrigation at different frequencies. The soil mixtures consisted of duplicates both with and without grass, made from clay, sand, and biochar added as a carbon sink and sorbent. The samples were analysed by Pyr-GC/MS for TWP in soils 0.3–500 μm and water 1.6–500 μm. The stormwater used for column irrigation was collected from a sedimentation facility next to a highway with an AADT of 105,000 vehicles, including 11% heavy-duty trucks. It contained concentrations of TWP ranging from 1.8 to 3.2 µg/L. In addition, stormwater spiked with sediments from the same facility was used for irrigation, resulting in concentrations of TWP of 230–1500 µg/L. After the experiment, the top layer contained 91–96% of retained TWP mass (0.3–500 μm) in soil mixtures, with decreasing amounts at greater depths. The removal efficiencies for TWP (1.6–500 μm) from stormwater were high and ranged from 93% in clay with grass and up to 99% in sandy loam–clay without grass. Biochar-amended soils generally exhibited similarly high retention of TWP as the corresponding soils without biochar. The concentrations of tyre-derived chemicals were highest in the upper soil layers and followed the order: 6PPD > BTSA > DPG > DPPD > 6PPD-Q > OHBT > TMQ > MBT > MTBT > PhBT > IPPD > HMMM > 7PPD > CPPD. Grass enhanced TWP transport following drought conditions, suggesting that vegetation may increase particle mobility within road ditch soils under certain climatic conditions. Nevertheless, TWP mainly accumulated in the upper soil layers, and all investigated soil mixtures showed high retention efficiencies. Overall, the results indicate that sandy loam-clay, sandy, and biochar-amended road ditch soils all have a high capacity to retain TWP. While biochar did not provide a clear additional retention benefit under the conditions tested, vegetation may influence TWP mobility and should be considered in the design and management of road ditch systems.