Chromatography Sheds Light on the Environmental Degradation of Tire and Road Wear Particles

A field study conducted by the Helmholtz Center for Environmental Research (Leipzig, Germany) investigated the long-term exposure of tire and road wear particles (TRWP) and cryo-milled tire tread (CMTT) to environmental conditions. The samples were exposed to sunlight on a rooftop and to water and microorganisms in a sedimentation pond receiving highway runoff. The purpose of this study was to determine the effect of long-term (>1 year) exposure to natural sunlight and in an aqueous system on the physical properties and the chemical composition of CMTT and TRWP, and to determine to what extent do these effects correspond to those found in the laboratory in earlier studies. The extractable concentration of 27 polar and moderately polar compounds was analyzed by liquid-chromatography-mass spectrometry (LC-MS). A paper based on this research was published in Environmental Science: Processes & Impacts (1).

Emitted from driving tires in very high amounts (6 × 10⁹ kg per year globally), TRWP’s primary environmental transport pathway is believed to be washing off because of precipitation and transport by surface water before settling into sediments. The particles are assumed to be altered physically altered and releasing chemicals during this process (2-5).

CMTT is used in most cases as a test material due to it being comparatively inexpensive and easy to obtain and produce (6-9). Recent studies, however, show significant differences between CMTT and TRWP regarding appearance, chemical content, and leaching potential (10,11). During abrasion, tire treads undergo substantial changes in physical appearance, chemical composition, and incorporation of external particulates. Therefore, laboratory-generated TRWP is regarded as a more accurate representation of real-world TRWP than CMTT (10,11).

The aging processes of tire particles is often studied in laboratory experiments. This has the advantage that experiments can be performed easily, aging conditions can be well controlled, and the effect of single factors (such as sunlight or temperature) can be studied separately (10-13). However, the research team chose to conduct their study in the field, because although aging studies in the laboratory are likely to involve optimum (or at least stable) conditions, conditions outside the laboratory are less controlled and not necessarily optimal for degradation processes. Additionally, the research team pointed out that laboratory studies are of relatively short duration—usually a matter of a few weeks, a brief amount of time compared to the anticipated long residence time of TRWP in the environment (1).

The researchers found that, while total quantified extractables (TQE) decreased for about 62-92% within the first sampling period of 8-10 months, concentrations of 100-200 μg g^-1 of TQE remained in TRWP after 17-20 months, mainly benzothiazolesulfonic acid (BTSA) and hydroxy-benzothiazole after sunlight exposure and N-(1,3-dimethylbutyl)-N'-phenyl-1,4-phenylenediamine (6-PPD) after exposure in the sedimentation pond. For the sunlight exposure the results of this long-term field study are well comparable to the results of a previous laboratory study. A laboratory study on (bio) degradation in water with optimized conditions appears to overestimate both leaching and (bio) degradation occurring in the sedimentation pond (1).

Despite these differences, this field study confirms the previous conclusion that, although a substantial part of the polar and moderately polar chemicals is rapidly released, tire particles can be a long-term source of tire-related chemicals. The authors of the study are of the opinion that the prevention of TRWP from entering aqueous environments would substantially reduce the load of polar and moderately polar compounds transported with them (1).

While long-term field and laboratory studies on (bio)degradation in water showed consistent trends for extractable chemicals, degradation processes in CMTT and TRWP proceeded more slowly in the field. This indicates that laboratory tests under optimized conditions may overestimate real-world biodegradation of TRWP. As no data on extractables of TRWP and CMTT after environmental aging could be found in literature, the results obtained in this study, state the team, cannot be compared with other field studies (1).

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References

1. Weyrauch, S.; Seiwert, B.; Voll, M. et al. Environmental Aging of Tire and Road Wear Particles and Tire Additives: A Long-Term Field Study. Environ. Sci. Process. Impacts 2025. DOI: 10.1039/d5em00444f
2. Baensch-Baltruschat, B.; Kocher, B.; Stock, F. et al. Tyre and Road Wear Particles (TRWP) - A Review of Generation, Properties, Emissions, Human Health Risk, Ecotoxicity, and Fate in the Environment. Sci. Total Environ. 2020, 733, 137823. DOI: 10.1016/j.scitotenv.2020.137823
3. Kole, P. J.; Löhr, A. J.; Van Belleghem, F. G. A. J. et al. Wear and Tear of Tyres: A Stealthy Source of Microplastics in the Environment. Int. J. Environ. Res. Public Health 2017, 14 (10), 1265. DOI: 10.3390/ijerph14101265
4. Luo, Z.; Zhou, X.; Su, Y. et al Environmental Occurrence, Fate, Impact, and Potential Solution of Tire Microplastics: Similarities and Differences with Tire Wear Particles. Sci. Total Environ. 2021, 795, 148902. DOI: 10.1016/j.scitotenv.2021.148902
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8. Altenhoff, A.-L.; de Witt, J.; Andler, R.et al. Impact of Additives of Commercial Rubber Compounds on the Microbial and Enzymatic Degradation of Poly(Cis-1,4-Isoprene). Biodegradation 2019, 30, 13-26. DOI: 10.1007/s10532-018-9858-5
9. Christiansson, M.; Stenberg, B.; Holt, O. Toxic Additives - A Problem for Microbial Waste Rubber Desulphurisation, Resour. Environ. Biotechnol., 2000, 3 , 11-21. https://hero.epa.gov/hero/index.cfm/reference/details/reference_id/5740838, (accessed 2024-03-13)
10. Weyrauch, S.; Seiwert, B.; Voll. M. et al. Accelerated Aging of Tire and Road Wear Particles by Elevated Temperature, Artificial Sunlight and Mechanical Stress - A Laboratory Study on Particle Properties, Extractables and Leachables. Sci. Total Environ. 2023, 904, 166679. DOI: 10.1016/j.scitotenv.2023.166679
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