Rapid generation of aged tire-wear particles using dry-, wet-, and cryo-milling for ecotoxicity testing

Strict environmental laws have been enacted to regulate the emission of exhaust particulate matter (PM), which is one of the most hazardous pollutants that reduce air quality and pose a serious risk to the human health. In addition, non-exhaust PM, such as road wear, tire wear, and brake wear debris, is a significant source of airborne pollutants. Road dust less than 100 μm in size may include tire wear particles (TWPs), which are broken down into finer particles with sizes on the order of tens of micrometers because of weathering. TWPs can be transported to water bodies via runoff, potentially contaminating water systems and negatively affecting aquatic ecosystems. Therefore, ecotoxicity tests using reference TWPs are required to investigate the impact of TWPs on the human health and environment. In this study, aged TWPs were produced using dry-, wet-, and cryo-milling methods, and the dispersion stability of TWPs in dechlorinated water was evaluated. Aged TWPs prepared by dry- and wet-milling had an average particle size of 20 μm, whereas pristine TWPs had an irregular shape and average particle size of 100 μm. The capacity of the ball-milling cylinder and excessively long 28-d generation time constrain the amount of aged TWPs that can be produced through conventional milling. In contrast, cryo-milling reduces the particle size of TWPs at the rate of -275.0 μm/d, which is nine times higher than that upon dry- and wet-milling. Dispersed cryo-milled TWPs had a hydrodiameter of 2.02 μm and were more stable in the aqueous phase in relation to the other aged TWPs. The results of this study suggest that cryo-milled TWPs can be used for aquatic exposure assessments as controls for real-world TWPs.