Investigation of 6PPD-Quinone in Rubberized Asphalt Concrete Mixtures

N-(1,3-dimethylbutyl)-N & PRIME;-phenyl-p-phenylenediamine(6PPD)-quinone(6PPD-Q), a transformation byproduct of 6PPD used in tires as an antiozonantand antioxidant, was recently discovered as the chemical primarilyresponsible for the acute lethal toxicity of urban storm runoff tocoho salmon. The asphalt concrete (AC) surface layer is the primarymedium to contact 6PPD-Q immediately upon its release from tires,and the addition of recycled tire rubber (RTR) to the asphalt binderand mixture is a widely accepted practice in asphalt production. Therefore,it is urgent to understand the fate of 6PPD-Q at the asphalt concretesurface layer-water interface. This study analyzed the sorptionand desorption of 6PPD-Q by compacted and crushed loose (loose particles,& SIM;5 mm) rubberized asphalt mixtures and their mobilization fromcompacted asphalt mixtures during simulated rainfall events. It shouldbe noted that the crushed loose asphalt mixtures demonstrated thephysicochemical properties of the asphalt materials, while the compactedasphalt mixtures represent in-service AC layers. Sorption of 6PPD-Qby crushed loose and compacted asphalt mixtures reached equilibriumwithin 12 days, with a sorption coefficient of 151.57-257.51L/kg for compacted asphalt mixtures. Within 12 days, desorption of6PPD-Q from crushed loose and compacted rubberized asphalt mixtures(20 g particles/L) to the double deionized (DDI) water and syntheticstormwater was 0.01-0.09 and 0.025-0.05 & mu;g/L,respectively. Through the rainfall simulation experiments, 0.0015-0.0049 & mu;g/L 6PPD-Q was detected in the runoff water, much lower thanthe lethal concentration (LC50) of 6PPD-Q of 0.095 & mu;g/Land 308.67 & mu;g/L for coho salmon and zebrafish larvae. Our resultsindicate that, while the release of 6PPD-Q from compacted rubberizedasphalt mixtures is minor, the mixtures can serve as sorbents fortire-derived 6PPD-Q and retain this emerging contaminant.