A multi-analytical approach to investigate the retention of tyre-road wear particles, tyre-derived chemicals and metals in tunnel wash water

Summary

Environmental context Tunnel wash water contains high concentrations of tyre-road wear particles, tyre-derived chemicals, and metals, which can threaten aquatic ecosystems if untreated. This study applies a multi-analytical approach to assess pollutant retention and transformation during sedimentation, revealing treatment limitations and emphasising the need for advanced strategies to reduce pollution from enclosed road infrastructures. Rationale Tunnel wash water (TWW) accumulates tyre-road wear particles (TRWPs), tyre-derived chemicals (TDCs) and metals at high concentrations, posing risks to aquatic ecosystems if untreated. Understanding pollutant retention and transformation during treatment is essential for effective mitigation. Methodology We investigated TWW from the Vålereng tunnel (Oslo, Norway) using a multi-analytical approach. TRWPs were quantified by pyrolysis–gas chromatography–mass spectrometry (PYR-GC/MS) with a marker mix (M4), a single marker (4-vinylcyclohexene, 4-VCH), and automated scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDX) single particle analysis with machine-learning classification (automated single particle analysis, ASPA, MC2). Metals were analysed by inductively coupled plasma–mass spectrometry (ICP-MS) and TDCs by ultraperformance liquid chromatography–tandem mass spectrometry (UPLC-MS/MS). Samples were collected before, during and after sedimentation treatment over 21 days. Results Zinc (Zn) was the most abundant metal (274–2300 µg L–1), reduced by up to 93% post-treatment. TRWP concentrations ranged from 15–160 mg L–1 (M4), 13–122 mg L–1 (4-VCH) and 13–240 mg L–1 (ASPA MC2), with treatment efficiencies of 95–99.7%. Despite reductions, fine particles (<20 µm) dominated both untreated and treated water (up to 85%). For TDCs, N-(1,3-dimethylbutyl)-N-phenyl-p-phenylenediamine (6PPD) showed 85% reduction, whereas 6PPD-quinone (6PPD-Q) (−26%) and N,N-diphenylguanidine (DPG) (−3.7%) exhibited negative retention, likely due to leaching and transformation. 1-hydroxybenzotriazole hydrate (OHBT) increased by up to 167% during sedimentation. Discussion Sedimentation effectively removed particles and metals but was insufficient for several dissolved TDCs, releasing them at ecotoxicologically relevant concentrations (e.g. 6PPD-Q exceeded Environment Protection Authority freshwater limits). This first cross-validation of three TRWP quantification methods highlights the need for advanced treatment technologies and regulatory thresholds for tyre-derived pollutants.