Tire Wear and Microrubber Particles: From Problems to Solutions (2021)
Pieter Jan Kole, Open University; Farhan R. Khan, University of Oslo; and Frank G. A. J. Van Belleghem, Open University
The emissions of tire wear particles (TWP) per capita are estimated to be around 0.8 kg/year globally, and recent reports have shown that microsized tire rubber, termed as tire road wear particles (TRWP), TWP and microrubber contribute significantly to the plastic pollution in our oceans. Tire rubber is a complex mixture of various rubbers, including styrene-butadiene (SBR), polybutadiene and natural rubber (NR), as well as substances such as carbon black, silica, zinc, sulphur, polycyclic aromatic hydrocarbons (PAHs), trace elements and antioxidants. Several questions regarding TWP’s presence in the environment and its ecotoxicological impact remain unanswered. Particle detection is analytically challenging, and it is difficult to distinguish between the toxicological effects of the particle itself and those caused by the leached chemicals.
This session included seven platform presentations and three posters covering two broad topics: (i) environmental presence and detection and (ii) biotic interactions and effects.
Environmental Presence and Detection
Peter Tromp, TNO, compared analytical techniques for determining TWP levels in air, road runoff and wastewater. After analyzing 30 car tires, the amount of total rubber, NR and SBR was found to be the most consistent substance. The best results were obtained using thermal extraction and desorption-GC/MS, as well as the markers 4-vinylcyclohexene for SBR and dipentene for NR. A 14-stage Dekati impactor was used to measure the size distribution from 30 nm to 10 μm in air. Particulate matter (PM) was measured using a Harvard impactor. Tire content was found to be 3–5% in urban regions and 0.4% in rural environment. Part of the smaller TWP was formed by the condensation of tire material that evaporated due to heat generated in road contact. This demonstrates the importance of research into TWP generation for toxicological research.
Yoonah Jeong, Korea Institute of Civil Engineering and Building Technology, studied benzathiazole, 2-hydroxyl benzothiazole and zinc as markers. All markers demonstrated linear trends, with organic markers displaying a higher correlation than zinc. Particles from the same brand of tires with tread wear factors of 250, 500 and 700 were tested. The tread wear factor is used as an indicator under the Uniform Tire Quality Grading (UTQG) system. Tires with a higher tread wear factor emit less TWP. The differences between tread wear factors 250 and 500 were minor, but TWP tread wear factor 700 released more than three times as many chemicals. This implies that the risk assessment for TWP must consider both the amount and the leaching tendency.
Sirajum Monira, Royal Melbourne Institute of Technology, studied microplastics in Australian road dust from an industrial and a residential area. Using optical microscopy, the particles were classified as fragments, fibers, films or beads based on their morphology. Fibers accounted for 48% of total microplastics in the residential area, while beads, accounting for 31%, were dominant in the industrial area. Fourier-transform infrared spectroscopy (FTIR) was used to identify polyethylene, polyester, polypropylene, polyethylene terephthalate, rubber and polyvinyl chloride. The predominance of fibers in the residential area was explained by the origin, clothing and home furniture. In the industrial area, 460 tire particles per 100 grams of road dust were counted, whereas only 90 particles were counted in the residential area.
The discussion started with the question of whether a standardized method for producing and analyzing tire wear samples is needed. In toxicological studies, people often use their own methods to acquire tire particles and to handle their samples. Tire wear from a road simulator, for example, is used by some, while cryomilled lumps cut from a new or used tire are used by others. Tromp concurs there is lack of standardization. It isn’t just the variation in the tire’s composition that should be considered, it’s also the variation in the road wear adhering to the particles, such as asphalt. Then there’s the difference between nano- and microsized particles. Nanoparticles are formed in a different way; heat from tire road contact causes evaporation, which is followed by condensation into sub-micron-sized particles with different polymer properties than the abraded microsized particles. Jackie Lang, University of California, Davis, mentioned that particles generated from a tire’s tread differ from particles generated from a tire’s side wall. According to Andy Booth, SINTEF Ocean, wear from road contact should be considered the same as wear from a new surface. As a tire wears, a new surface emerges, which then wears and a new surface appears, and so on. Particles generated in the laboratory from material located a few millimetres from the tread surface should therefore equal wear from the surface. Reaching detection limits also poses challenges, such as distinguishing signal from background and preparing samples without losing anything. Barbara Scholz-Böttcher, University of Oldenburg, proposed defining a tire wear material standard to facilitate data comparison between studies. To account for combined effects, studies should focus on tire wear rather than isolated markers.
The discussion then moved on to standardizing concentrations, with the question of whether concentration should be expressed numerically or by mass. When comparing with results from other contaminants, Samreen Siddiqui, Oregon State University, suggested that using the mass would be preferable. However, in the case of microplastics, it would be advantageous to use both number and mass. Craig Davis, Exxon Mobil Biomedical Sciences, proposed a hybrid approach. He pointed out that people who study sediment use mass, whereas those who study water use numbers. According to Scholz-Böttcher, the entire composition of the particles must be considered when studying toxicological effects due to the inhomogeneity of the wear. Booth emphasized the significance of surface area as a metric because that’s what an organism will interact with. Moreover, leaching will also be related to the surface area.
Biotic Interactions and Effects
Soyoung Lee, University of Tokyo, studied the effects of benzopyrene (BaP), fluoranthene (FLU), and 2-mercaptobenzothiazole (2-MBT) on the marine amphipod Grandidierella japonica. They estimated the LC50 by concentrations in spiked-sediment to be 200, 15 and 12 mg/kg for BaP, FLU and 2-MBT, respectively. These figures were compared with data in the existing literature on the LC50 of Daphnia magna in water, after conversion to LC50 in sediment using Equilibrium Partitioning (EqP) theory. This resulted in LC50 in sediment equivalents of 27, 0.56 and 0.094 mg/kg for BaP, FLU and 2-MBT, respectively. The LC50 from the spiked sediment test was higher than the estimated LC50 calculated from the EqP theory using the water-only test, implying that the model overestimated toxicity.
Ula Rozman, University of Ljubljana, studied the effects of tire particles and leachate on the water flea Daphnia magna and duckweed Lemna minor when exposed to 100 mg/L particles or their equivalent leachate. After 48 hours, the tire particles and the leachate had no significant effect on the water flea. There was also no negative impact on duckweed’s growth rate or chlorophyll content. However, after seven days, duckweed exposed to tire particles showed a 37% reduction in root length, while there were no differences between the weed exposed to leachate and the control. Rozman concluded the negative effect on root lengths was caused by mechanical abrasion by the tire particles.
Victor Carrasco Navarro, University of Eastern Finland, exposed midges (Chironomus riparius) to tire particles with size ranges of 2.5-260 µm and 0.5-3 mm, representing both tire wear and artificial turf infill. Sediment from Lake Höytiäinen was spiked with the particles at 1, 5 and 10% dry weight. Midget larvae were added, and Carrasco Navarro monitored mortality, growth and emergence rates. The results for both size ranges showed no differences between the midges exposed to tire particles and the control. In addition, they exposed the blackworm Lumbriculus variegatus to 2.5-260 µm tire particles mixed into tributyltin (TBT)-contaminated sediment from Huruslahti bay in Finland. These tests on joint toxicity revealed no differences between exposed blackworm and control.
Brittany Cunningham, Oregon State University, investigated the ecotoxicity of microtire particles (1-20 µm), nanotire particles (<1 µm) and leachate on zebrafish Danio rerio and water flea Daphnia magna. Exposure of zebrafish to microparticles resulted in greater mortality compared with nanoparticle and leachate exposure. However, exposure to nanoparticles resulted in hatch delay, which was not observed under microparticle or leachate exposure. Differing results may be explained by particle characteristics and exposure setup. For instance, nanoplarticles would be small enough to penetrate the chorion membrane of the embryo and may cause hatch delay. Water flea mortality could be attributed to the leachate since no particle effects were observed in this study. Zebrafish and water fleas studies may show differential susceptibility to TWP in the environment. Thus, TWP toxicity tests should not be limited to a single species.
Siddiqui studied inland silverside Menidia beryllina and mysid shrimp Americamysis bahia using a DanioVision video tracking system. The species were subjected to a variety of environmentally relevant tire particle concentrations, leachate, salinities and particles sizes of <1 µm and 1–20 µm. Particle exposure affected the behavior of both silversides and shrimps, even at the lowest concentrations. Also, their growth was reduced. These adverse effects indicate that even at current environmental levels, which will rise due to future accumulation, tire particles have an impact on aquatic ecosystems. The observed behavioral changes may increase the risk of predation and foraging difficulties. This could, in the long term, have an impact on trophic transfer.
Andraz Dolar, University of Ljubljana, studied the effect of tire particles on woodlice, Porcellio scaber. The woodlice were exposed to Lufa 2.2 standard loamy sand, mixed with 1.5% (w/w) wood dust, silica powder or tire particles. The tire particles were obtained by cryomilling used tires to an average size of 13 mm. The total haemocytes count (THC) was measured for three weeks. The woodlice exposed to tire particles showed a decrease in THC in the first week before returning to control levels in the following weeks. When the different haemocytes were examined, the granulocytes count increased while the semi-granulocytes count decreased. Woodlice were unaffected by either wood dust or silica powder.
Thibault Masset, École Polytechnique Fédérale de Lausanne, studied the bioavailability of heavy metals and polyaromatic hydrocarbons (PAH) in rainbow trout, Oncorhynchus mykiss, in vitro. They tested cryomilled tire tread particles (CMTT) with a size of 150 mm and lab generated TRWP with a size of 29 mm. First, the particles were added in a concentration of 10 g/l to a simulated gastric fluid pH 2 for 3 hours. The particles were then transferred to a simulated intestinal fluid with a pH of 7.4 for 26 hours. The CMTT had lost 10% of their zinc after the test, while the TRWP had lost 23%. Due to their smaller size and rough skin, TRWP have a larger surface, which stimulates leaching. This demonstrates the significance of using standardized particles once more.
The second part of the discussion began with the particle–leachate dilemma. Cunningham noted that different particle sizes and leachate had different effects on zebrafish, whereas effects on daphnia were less differentiated. Siddiqui stated that exposure to microparticles caused more behavioral changes than exposure to nanoparticles. In the experiment, leachate was found to be more toxic to shrimp than fish. Both studies show that the toxicity of particles and leachate varies depending on the organism. Masset found smaller particles to leach their zinc faster. Ingestion has an effect on toxicity as well. Carrasco Navarro found that particles in the gut leach faster than in water. The amount of toxicants leached will also be affected by particle heterogeneity. Cunningham emphasized the significance of the methods used. Her zebrafish were studied in multiple ways, while the daphnia were only studied for mortality and immobility. This could have an impact on the conclusion. Not only the organism but also the method will have an influence on the outcome. Farhan Khan, University of Oslo, proposed the use of a matrix-related indicator organism. According to Kees van Gestel, Vrije Universiteit Amsterdam, different species will be required; there is no most sensitive species. Different species that are representative of the exposed compartments should be studied. In order to reveal toxicological modes-of-action (MoA), we may also need in vitro tests. Susanne Brander, Oregon State University, brought up the study by Zhenyu Tian, which linked sudden death in coho salmon after rainstorms in the U.S. Pacific Northwest to tire wear. The salmons died as a result of 6PPD, an antiozonant that leached from tire rubber in the runoff and reacted with ozone to form the toxic 6PPD-quinon. When compared to particles, leachate could be more toxic to fish and invertebrates.
Frank Van Belleghem, Open Universiteit, inquired whether particles had been detected in tissues. TWP was found inside the gut of silverside larvae and early life shrimp by Brander and Siddiqui using confocal microscopy. Cunningham used confocal microscopy to visualize microparticles in the gut of daphnia; nanoparticles, however, were too small to detect. Van Belleghem mentioned the heterogeneity of tire wear as an advantage. Because of the presence of zinc, TWP can possibly be detected using electron microscopy with energy dispersive x-ray analysis (EDX).
The discussion came to a close with the question, “Are tire particles really microplastics?” Participants agreed that there is a general framework for microplastic research that is robust, critical and appropriate for TWP. However, the framework was built without the current understanding. The discussion also focused on the often confusing and overlapping terminology used for TWP/TWRP and the relationship to “other” microplastics. Carrasco Navarro suggested the term “anthropogenic particles” as proposed by Louise Halle, Roskilde University, may add clarity. Similarly, Scholz-Böttcher believes that we should change the term “microplastics” to “anthropogenic polymers” to better define them.
Session Overview
The topic of TWP has advanced significantly in the last year, but there is still much work to be done. Little is known about the specific environmental presence of the particles, but progress has been made in detecting tire particles in the environment. Due to the large number of substances in road dust and their complex interactions, knowledge of the individual toxicants is still limited. Moreover, the combined effects of these chemicals in leachate, as well as the physical effects of the particles, make studying ecotoxicological impact challenging. Furthermore, accumulation processes make predicting future effects even more difficult. However, we are making progress in understanding these complex processes, which will eventually allow us to assess human and environmental risks. The knowledge that is currently being developed should be transferred to the manufacturers so that tires can be manufactured in a safer, mitigation-friendly manner. This will necessarily require a global effort and contributions from all stakeholders, including consumers, regulators, industry and researchers from various disciplines.
Authors’ contact information: [email protected], [email protected] and [email protected]