Plastic waste in the oceans and other bodies of water is a serious environmental issue that is currently being researched. Almost every month, new reports are released. Tires, or particles from tire and road construction materials, have been identified in some publications as one of the major sources of microplastics ending up in oceans and other bodies of water.
Tire and road wear particles are one of the major sources of microplastics released into the environment in Switzerland, but the chemical compounds contained in those particles—and their effects—remain largely unknown. To fill that knowledge gap, researchers at EPFL and two other Swiss research institutes are investigating the toxicity of tire-particle compounds and how easily they are absorbed by living organisms. The first phase of the study has just been completed.
The study, which kicked off in April 2020, is being sponsored by a consortium of leading tire manufacturers. The two other research institutes working with EPFL are the Swiss Centre for Applied Ecotoxicology (Ecotox Centre), which is coordinating the project, and the Swiss Federal Institute of Aquatic Science and Technology (Eawag). The first phase concluded with two papers published in Environmental Science & Technology (in late November 2021 and late October 2022). Thibault Masset, a postdoc at CEL, is the lead author of both papers. These articles look specifically at the solubilization and bioaccessibility of tire-particle compounds in the digestive systems of rainbow trout.
These compounds are more complicated than standard polymers like polystyrene and PET—and there are probably hundreds of them. Most research today focuses on microplastic pollution from packaging and waste, but microplastics from tires account for 30–40% of plastic pollution in the environment. So this kind of pollution is worth studying too.
Florian Breider
Impact of food coingestion
The scientists used an innovative in vitro approach based on simulated gastric and intestinal fluids to carry out their research. They studied eleven compounds and discovered that the rate of solubilization in gastrointestinal fluids ranges from 0.06% to 44.1%, and that this rate varies depending on whether food is consumed.
6PPD-quinone (6PPD-Q) is a toxic byproduct of the oxidation of 6PPD, an antioxidant widely used in the tire industry. When 6PPD-Q-containing tire particles are consumed alongside amphipods, the compound solubilized in the fish gut increases. With other chemical compounds, however, coingestion has the opposite effect. More research is being conducted to determine the toxicity of various tire-particle substances.
“These compounds are more complicated than standard polymers like polystyrene and PET—and there are probably hundreds of them,” says Florian Breider, the head of EPFL’s Central Environmental Laboratory (CEL) and the corresponding author of the two papers.
“Most research today focuses on microplastic pollution from packaging and waste, but microplastics from tires account for 30–40% of plastic pollution in the environment. So this kind of pollution is worth studying too.”
Byproducts and how they age
The ultimate goal of the study is to determine the bioaccessibility, bioaccumulation and toxicity of tire-particle compounds and related additives. With phase 1 now completed, the scientists will begin phase 2, which will involve examining how the compounds are passed up the food chain—for instance, from insect larva to rainbow trout.
The researchers also intend to investigate the byproducts of tire-particle compounds and how the compounds degrade over time. 6PPD-Q is one example, which is formed when 6PPD (added to the rubber mixture during tire manufacturing) reacts with ambient O2 and O3. A 2021 study published in Science found that 6PPD-Q was to blame for acute salmon mortality events in Seattle’s Elliott Bay. “Tire manufacturers must take a more holistic approach to their products and consider the unstable nature of some of the chemicals they use,” Breider says. “These chemicals can decompose and produce unintended byproducts, which can be toxic in some cases.”
Rubber, soot, and heavy metals
A Swiss federal report on plastics in the environment, published, discovered that tire and road wear is one of the country’s leading causes of microplastic pollution. The resulting particles are composed of 60% rubber, 30% soot, and 10% heavy metals. Every year, over 13,500 metric tons of these particles are produced in Switzerland, with approximately 8,900 metric tons of that total being released into our air, soil, and water.
An EPFL study published in TrAC Trends in Analytical Chemistry in 2018 estimated that tire and road wear accounted for up to 61% of microplastics entering Lake Geneva. Scientists don’t yet have a firm grasp of all the chemical compounds contained in these particles, nor of what the consequences may be.
