Published research

Para-phenylenediamines (PPDs) are antioxidants added to tires to protect the rubber. They are released from tire and road wear particles (TRWP) but the extent of their aerobic microbial degradation and the transformation products (TPs) formed are not known. Therefore, aerobic microbial degradation of seven tire-related PPDs, parent compounds as well as known transformation products, was studied for up to 28 days. Half-lives ranged from 0.2 ± 0.1 days (N-(1,3-dimethylbutyl)-N'-phenyl-1,4-benzenediamine, 6-PPD) and 0.6 ± 0.1 days (N-isopropyl-N’-phenyl-1,4-phenylenediamine, IPPD) to 3 ± 0.1 days (N-(1,3-dimethylbutyl)-N'-phenyl-1,4-benzenediamine quinone, 6-PPDQ). A total number of 48 TPs was tentatively identified by liquid chromatography-high resolution-mass spectrometry for the seven study compounds. Of these TPs, only four did not decrease in concentration when the parent compounds were degraded completely. Biotransformation in aqueous solution forms several TPs not known for abiotic, photolytic or oxidative transformation. For the PPDs with aliphatic substituents (6-PPD, IPPD) hydrolysis to 4-HDPA was the major initial transformation. Formation of 6-PPDQ from 6-PPD was not detectable. For the fully aromatic DPPD aerobic microbial transformation, likely, proceeded via a quinone diimine intermediate, leading to products different to those of the aliphatic PPDs. From 6-PPDQ, 26 TPs were detected. A suspect screening for the TPs detected from the biodegradation experiments was performed in data of a soil degradation study over 23 months with TRWP and cryo-milled tire tread (CMTT) and in data from the influent and effluent of a municipal wastewater treatment plant during a rain event. In total, 10 TPs were found in those data with variable intensities, most of which originated from 6-PPDQ. While all seven test compounds were (primary) degraded under aerobic conditions, mineralization was not studied. A number of TPs remain as suspects to search for in the environment.

This work, “Biodegradation pathways and products of tire-related phenylenediamines and phenylenediamine quinones in solution – a laboratory study” by "Han et al." was originally published in Water Research, and is licensed under the Creative Commons Attribution 4.0 International License. You may view the original publication here.

Tire and road wear particles (TRWP) are continuously formed by automotive traffic on roads. This study reports effects of long-term degradation over 2 years in water and in soil in the presence of microbes on TRWP and on cryo-milled tire tread (CMTT). Degradation in water had little measurable effect on physical properties of TRWP; a shift towards larger particle sizes was mainly due to the mechanical stress from stirring. The total quantified extractables (TQE) of 27 chemicals and transformation products determined from tire particles were reduced by 90 % from TRWP and CMTT in water and by 85 % in soil. Most of this decrease occurs within the first months. For both materials, however, the speed of loss of TQE in water and in soil decreased drastically over time. Its kinetics was approximated by two phases of 1st order kinetics, resulting in half-lives from 17 days for diphenylguanidine (DPG) in phase 1 to 520 days for 6-PPD-quinone (6-PPDQ) in phase 2 of TRWP biodegradation in water. For soil, half-lives tend to be clearly longer in phase 2 compared to water but remained <1000 days for chemicals such as benzothiazole sulfonic acid (BTSA), N,N′-diphenyl-p-phenylendiamine (DPPD) and hydroxybenzothiazole (OH-BT). For N-(1,3-dimethylbutyl)-N′-phenyl-1,4-phenylenediamine (6-PPD) and 6-PPDQ they exceeded 2000 days from TRWP. Only 1–15 % of TQE lost from the tire materials remained detectable at the end of the experimental period in the supernatant of the suspension or in leachates of the soil. Mostly benzothiazoles were determined from solution. The biodegradation experiments show an effective reduction of a large part of the chemical burden of TRWP of polar and moderately polar compounds. Despite that, TRWP may serve as a long-term reservoir for some of the tire related chemicals or their transformation products in the environment.

This work, “Long term biodegradation study on tire and road wear particles and chemicals thereof” by "Weyrauch et al." was originally published in Science of the Total Environment, and is licensed under the Creative Commons Attribution 4.0 International License. You may view the original publication here.

There is a growing interest in the development of reliable analytical methods for characterizing tire and road wear particles (TRWP). The current research extends the use of single particle analysis techniques to various experimental biota samples. TRWP and cryogenically milled tire tread (CMTT) were identified using a weight of evidence framework including density separation, optical microscopy, and chemical mapping (scanning electron microscopy coupled with energy dispersive X-ray spectroscopy). Our techniques successfully identified CMTT particles in laboratory earthworms exposed to soil spiked with CMTT. A river biota sample (bivalves) collected from the Seine with no detectable TRWP was spiked with road dust containing TRWP. Particle identification was performed after a biota digestion protocol and density separation of particles > 1.5 g/cm³ and < 2.2 g/cm³ which resulted in sufficient TRWP for identification and characterization. The average TRWP particle size from the road dust spiked biota sample was 126 μm by number and 220 μm by volume (range: 9 –572 μm). The size distribution overlay of TRWP identified from spiked biota were consistent with TRWP identified from the original road dust sample suggesting that the current method for biota digestion, dual density separation, and TRWP characterization is feasible for similar samples.

This work, “Characterization of tire and road wear particles in experimental biota samples” by "Kovochich et al." was originally published in Nature, and is licensed under the Creative Commons Attribution 4.0 International License. You may view the original publication here

The ongoing energy transition, marked by notable advancements in electric vehicles, presents new challenges related to tire emissions. In addition, these emissions and their distribution may be affected by other future trends like prolonged heat periods and an increase in stormwater events, which are both related to the ongoing climate change. An understanding of future trends and robust data on tire emissions during the use phase that inform these trends is essential for evaluating the potential environmental impact and implementing effective mitigation strategies even today. In this structured literature review current and future environmental exposure pathways of tire emissions during vehicle use including particulate tire wear, leachables and volatiles are discussed. A total of 502 publications between 1985 and 2024 were reviewed, resulting in a conceptual exposure model (CEM) for tire emissions during the use phase. Analytical tools are discussed and a proposal for a fit-for-purpose analytical methodology is adapted from microplastic research to inform the CEM of tire emissions. This concept follows a tiered approach covering exploratory, screening, mass, single particle, and chemical analysis of environmental samples with dedicated analytical methods and quality assessment criteria for each tier. Further, the current state of knowledge on factors controlling tire emissions is assessed to determine whether sufficient information is currently available to predict future emissions from tires during use. In conclusion, knowledge needs that need to be solved for a predictive environmental assessment of tire emissions during the use phase are identified.

Tread wear emission inventories, uncertainty about the future development of the emissions and observed adverse effects of tire constituents in the environment have raised the need for an environmental risk assessment of tire wear emissions. While progress has been made in exposure and hazard assessment of tire wear emissions in the environment, the complexity of tire wear emissions creates some challenges which are not yet overcome. For instance, there is no universal agreed risk assessment framework for tire wear emissions. It was proposed that existing frameworks, for example for microplastics, be adapted to tire wear emissions because there are similarities between particulate tire wear emissions and microplastics, e.g. particulate material with a polymer backbone. The review discusses whether these are applicable for tire wear emissions and proposes adaptations. It provides a comprehensive assessment of exposure and hazard data for tire wear emission and reveals needs and data gaps for environmental risk assessment of tire wear. Based on the available exposure and hazard data sets a low risk prioritization of particulate tire wear emissions in aquatic and terrestrial environments was estimated. Risk prioritization of leachables from tire emissions is not yet possible due to inconsistent hazard data sets. It was found that for environmental risk assessment, insufficient consistent exposure and hazard data is available. It is suggested to develop clear harmonization guidelines how exposure and hazard studies should be designed. Such guidelines should be developed between all relevant stakeholders covering the entire product life cycle.

Pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) enables quantification of tire and road wear particles (TRWP) in environmental matrices, but method refinements are needed to account for elastomer subunit variations. Environmentally distributed elastomers are a composite of formulations from many tire manufacturers, which can be represented by specially prepared reference materials. Thus, this study analyzed cryogenically milled tire tread (CMTT) composite materials blended from United States and European Union market-representative tire mixtures to determine operationally defined styrene (S_t) and 1,4-butadiene (B_t) monomer subunit content fractions in synthetic rubber (SR) fractions. Bootstrap resampling with least squares optimization revealed similar B_t (0.64–0.73) and S_t (0.06–0.09) contents across market composites, though precision decreased with larger particle sizes. Calibration polymer solutions stored at 4 °C maintained stability for three months, with response ratio changes below 15 %. Although alternative internal standards were evaluated, structural similarity between target and calibration polymers proved essential for complex environmental matrices, with up to 20 % quantitation differences observed when using dissimilar standards. This study demonstrates that market-representative composite CMTT materials provide operationally-defined elastomer subunit profiles that account for commercial variability in tire formulations. This approach enables accurate environmental TRWP quantification without requiring individual elastomer characterization.

This work, “Pyrolysis-GC/MS calibration for environmental quantification of tire tread: Standards and marketplace averaged elastomer subunit profiles” by "Thornton et al." was originally published in Chemosphere, and is licensed under the Creative Commons Attribution 4.0 International License. You may view the original publication here.