Long-Term Leaching of 6PPD-Quinone from Recycled Rubber Mulch and Crumb in a Cold-Region Climate

The antiozonant 6PPD is commonly added to rubber tires to protect the rubber from ozone attack. Recent studies have illustrated the acute toxicity of its transformation product 6PPD-quinone (6PPD-Q) toward various salmonid fishes. Most studies measuring environmental levels of 6PPD-Q have focused on release from tire wear particles generated on paved roads, with less emphasis on recycled tires at the end of service life. Recycled tires are often converted into landscaping materials, resulting in 6PPD-containing rubber being exposed to ozone and rain, potentially leaching 6PPD-Q. However, the magnitude and duration of these additional releases are presently not known. To address this, we designed a long-term outdoor leaching study to quantify 6PPD-Q release from rubber tire crumb and rubber tire mulch under environmental conditions using liquid chromatography high-resolution mass spectrometry. Measured concentrations ranged from 1.81 to 34.5 μg/L, with a median concentration of 10.6 μg/L. Environmental factors potentially affecting 6PPD-Q concentrations in the leachate were identified using multiple linear regression models. In this way, we could demonstrate that unintended leaching of 6PPD-Q can occur under environmental conditions and highlight multiple factors that influence those outcomes. Ultimately, this paper contributes much-needed data on the sources of 6PPD-Q beyond paved roads.