Sunlight and tire wear particles - a toxic combination? Evaluating mechanisms for mobilization and degradation of tire particle compounds

Tire wear particles (TWP) are released from abrasion of tires on roadways and are one of the most abundant forms of microplastic pollutants. More than one million metric tons of TWP are flushed into waterways each year, with devastating impacts for fish and other aquatic organisms. While sunlight exposure may break down some of the compounds leached from TWP, some compounds will persist in the environment, and new compounds and more toxic transformation products also will form.

Many of the TWP compounds that leach into water are not yet known, and the effects of sunlight on TWP compound leaching, toxicity, and fate also remain unknown. The goal of this project is to address major gaps in our knowledge of 1) the many organic compounds that leach from TWP, 2) which TWP-derived chemicals should be prioritized for further study based on their toxicity, and 3) how sunlight transforms and degrades different TWP compounds in water.

The research team will identify the diverse compounds leached from tread particles from different tire types under solar exposure and dark conditions, describe their toxicity, and evaluate the persistence and decay of TWP compounds in the presence of simulated sunlight. The successful completion of this project will benefit society through the generation of fundamental knowledge about the toxicity of different tire wear particles which could inform future legislation related to tire manufacture, tire recycling, and green infrastructure for stormwater quality.

Additional benefits to society will be achieved through student education and training including the mentoring of two graduate students at San Diego State University.

The accumulation of tire wear particles (TWP) in watersheds has emerged as major threat for aquatic ecosystems, with TWPs in water being linked directly to urban runoff mortality syndrome for fish and macroinvertebrates. Given that some tire formulations contain as little as 20% natural rubber but have a vast array of synthetic polymers, metals, and additives, many of the TWP-derived compounds and transformation products that may be leached into the water column are not yet known and their toxicity has not been established - especially the ones that form via direct or indirect photolysis.

Some leached compounds and transformation products from TWP will be photo-resistant and persist in the environment while others will decay rapidly, and determining the kinetics of leaching and photochemical degradation on a compound-by-compound basis is a challenge. The overarching goal of the proposed research is to evaluate the fundamental processes that control the production of toxic TWP-derived compounds and transformation products.

The specific objectives are to 1) probe the effects of both direct and indirect photolysis on leaching, transformation, and persistence of toxic micropollutants from TWPs, 2) elucidate the role of reactive intermediates in these processes, and 3) develop kinetic models for decay of TWP-derived compounds. Experiments will be performed using a variety of samples from passenger cars and high-performance tires, which have different chemical compositions and in different conditions, sunlight and dark.

The successful completion of this research has the potential for transformative impact through the mechanistic understanding of the role of sunlight in the formation and destruction of TWP-derived compounds. To implement the education and training goals of the project, graduate students will lead high impact experiences and outreach activities in coordination with the Math Engineering and Science Achievement (MESA) program, including STEM Shadow Day, the “Stormwater Debris Capture Device Competition” for K-12 students, and mentoring undergraduates through the MESA Summer Academy.

In addition, the PIs plan to brief California State policymakers on research findings through the CSU COAST Program network.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.