Abstract
Tyre-derived pollutants (TDPs), including tyre wear particles, are contaminants of emerging concerns in aquatic environments. Despite increasing concern, their environmental fate, biological effects, and formulation-specific risks remain poorly understood. This research aimed to investigate the distribution, chemical composition, and ecological impact of TDPs across multiple trophic levels. The study used both acute and sublethal chronic exposure assays. It also sought to improve experimental and predictive methods for assessing their environmental risks. The study began with an international expert horizon scan (solicitation survey) of international researchers, which identified ten research priorities grouped into four themes: detection and distribution, chemical composition, biological effects, and mitigation. These priorities highlighted the lack of data on the transport and toxicity of TDPs, as well as regulatory and methodological gaps. To address these, fieldwork was conducted in two southern UK estuaries (Langstone Harbour and the River Hamble). A total of 14 to 136 TDPs were detected in sediment samples, 37 to 104 in seaweed, and 11 to 79 in surface water. Concentrations of several polycyclic aromatic hydrocarbons (PAHs) in water samples exceeded environmental quality standards. They include fluoranthene (0.012 µg/L), benzo(a)pyrene (0.270 µg/L), benzo(b)fluoranthene and benzo(k)fluoranthene (0.017 µg/L), and benzo(g,h,i)perylene (0.008 µg/L). TDPs concentrations in sediment and seaweed were moderately correlated (Pearson’s R = 0.407, p < 0.001). No significant correlations were observed between sediment and surface‐water TDPs (Pearson’s R = 0.004, p = 0.982) or between seaweed and surface‐water TDPs (Pearson’s R = 0.111, p= 0.407). Wilcoxon tests found no significant changes in TDP concentrations before and after rainfall in Langstone Harbour: surface water (W = 1540.5, p = 0.631), sediment (W = 9249.5, p = 0.092), and seaweed (W = 18208, p = 0.978). These results suggest that tidal mixing, rather than storm runoff, controls contaminant persistence. Laboratory studies assessed the toxicity of three TDPs (6PPD- quinone, diphenylguanidine (DPG), and mercaptobenzothiazole (MBT)). Experiments were carried out on freshwater (Gammarus pulex (G. pulex)) and marine (Marinogammarus marinus) amphipods, as well as the marine diatom Phaeodactylum tricornutum. In the amphipod tests, M. marinus showed significant behavioural inhibition of up to 75% at MBT 0.01 µg/L by Day 14, and 64% inhibition from DPG at the same concentration by Day 7. In contrast, G. pulex exhibited early hyperactivity (42% increase at MBT 100 µg/L on Day 4), followed by 47% inhibition at MBT 0.01 µg/L by Day 14. 6PPD- quinone caused inconsistent effects in G. pulex, but significantly reduced activity in M. marinus at 1 µg/L during the second light phase. The algal growth inhibition assay revealed that DPG was the most toxic to P. tricornutum, with an EC₅₀ of 0.101 µg/L, followed by MBT with an EC₅₀ of 0.212 µg/L. 6PPD- quinone showed lower acute toxicity, with an EC₅₀ of 4.072 µg/L. No-observed-effect concentration (NOEC) and lowest-observed-effect concentration (LOEC) values were 0.020 µg/L and 0.062 µg/L for DPG, and 0.095 µg/L and 0.299 µg/L for MBT, respectively. All three compounds showed statistically significant dose–response relationships (one-way ANOVA, p < 0.001). These results suggest that marine diatoms are sensitive to TDPs, even at environmentally relevant concentrations. The final experiment compared the effects of leachates from two tyre brands. They were selected based on their predicted toxicity scores from chemical profiling by Emissions Analytics. Gammarus pulex exposed to leachates from the higher-toxic rated tyre (L-tyre, 1 mg/L) showed significantly reduced swimming activity compared to controls (p < 0.05). This was frequently observed under dark-phase conditions on Day 4 and Day 14. Three-way ANOVA confirmed a significant phase-treatment interaction (p < 0.05). In contrast, C-tyre exposures (0.5 and 1 mg/L) showed weaker and less consistent effects, with partial behavioural recovery over time. Particle exposures at 0.3 mg/L did not significantly affect activity (p > 0.05). Building on the expert survey’s mitigation priorities, we outline three potential solutions: (1) development and regulatory adoption of low-toxicity tyre additives, (2) deployment of green-infrastructure stormwater treatments (e.g., biofilters, activated-carbon systems) at runoff hotspots, and (3) optimized street-sweeping and road-surface maintenance to capture tyre particles before they reach waterways. Overall, this research contributes to the emerging body of multi-species, sublethal ecotoxicological data on key TDP compounds. The findings suggest that TDPs could impair behaviour and primary productivity at concentrations that fall within, or below, current regulatory thresholds (e.g., NORMANs database). This highlights the need for improved testing frameworks, greater chemical transparency in tyre manufacturing, and internationally coordinated regulation of non-exhaust vehicle emissions. The thesis also supports the use of predictive chemical screening to guide risk-based assessments and the development of lower-impact tyre formulations
| Date of Award | 8 Sept 2025 |
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| Original language | English |
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| Supervisor | Alex Ford (Supervisor), Fay Couceiro (Supervisor) & Fran Cabada-Blanco (Supervisor) |