Investigation of 6PPD-Quinone in Rubberized Asphalt Concrete Mixtures

Abstract

N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD)-quinone (6PPD-Q), a transformation byproduct of 6PPD used in tires as an antiozonant and antioxidant, was recently discovered as the chemical primarily responsible for the acute lethal toxicity of urban storm runoff to coho salmon. The asphalt concrete (AC) surface layer is the primary medium to contact 6PPD-Q immediately upon its release from tires, and the addition of recycled tire rubber (RTR) to the asphalt binder and mixture is a widely accepted practice in asphalt production. Therefore, it is urgent to understand the fate of 6PPD-Q at the asphalt concrete surface layer–water interface. This study analyzed the sorption and desorption of 6PPD-Q by compacted and crushed loose (loose particles, ∼5 mm) rubberized asphalt mixtures and their mobilization from compacted asphalt mixtures during simulated rainfall events. It should be noted that the crushed loose asphalt mixtures demonstrated the physicochemical properties of the asphalt materials, while the compacted asphalt mixtures represent in-service AC layers. Sorption of 6PPD-Q by crushed loose and compacted asphalt mixtures reached equilibrium within 12 days, with a sorption coefficient of 151.57–257.51 L/kg for compacted asphalt mixtures. Within 12 days, desorption of 6PPD-Q from crushed loose and compacted rubberized asphalt mixtures (20 g particles/L) to the double deionized (DDI) water and synthetic stormwater was 0.01–0.09 and 0.025–0.05 μg/L, respectively. Through the rainfall simulation experiments, 0.0015–0.0049 μg/L 6PPD-Q was detected in the runoff water, much lower than the lethal concentration (LC50) of 6PPD-Q of 0.095 μg/L and 308.67 μg/L for coho salmon and zebrafish larvae. Our results indicate that, while the release of 6PPD-Q from compacted rubberized asphalt mixtures is minor, the mixtures can serve as sorbents for tire-derived 6PPD-Q and retain this emerging contaminant.

Uptake of Tire-Derived Compounds in Leafy Vegetables and Implications for Human Dietary Exposure

“…PPD-quinones are also hydrophobic (log KOW of 2.58 to 4.30 (14,77)), and could exhibit specific binding at the quinone moiety. This results in a high sorption potential for 6PPD-quinone (77,78), the same is presumably true for other PPD-quinones. Thus it is likely that PPDs and PPD-quinones in the soil environment would sorb to soil organic matter, which could limit the availability for plant uptake (50,51).…”

Uptake of tire-derived compounds in leafy vegetables and implications for human dietary exposure

“…PPD-quinones are also hydrophobic [log K OW of 2.58-4.30 (14,79)], and could exhibit specific binding at the quinone moiety. This results in a high sorption potential for 6PPD-quinone (Hu et al, 2023;Lokesh et al, 2023), the same is presumably true for other PPD-quinones. Hydrophobic compounds are also known to accumulate in plant roots, as has been previously shown for 6PPD and 6PPD-quinone (Castan et al, 2023), so future work should investigate the occurrence of these tire-derived compounds in root vegetables, such as carrots or potatoes.…”