Reactive oxygen species and chromophoric dissolved organic matter drive the aquatic photochemical pathways and photoproducts of 6PPD-Quinone under simulated high-latitude conditions

Abstract

The photochemical degradation pathways of 6PPD-quinone (6PPDQ), a toxic transformation product of the tire antiozonant 6PPD, were determined under simulated sunlight conditions typical of high-latitude surface waters. Direct photochemical degradation resulted in 6PPDQ half-lives ranging from 17.5h at 20°C to no observable degradation over 48h at 4 °C. Sensitization of excited triplet state pathways demonstrated that 6PPDQ does not decompose significantly from a triplet state relative to a singlet state. However, assessment of processes involving reactive oxygen species (ROS) quenchers and sensitizers indicated that singlet oxygen and hydroxyl radical do significantly contribute to the degradation of 6PPDQ. Investigation of these processes in natural lake waters indicated no difference in attenuation rates for direct photochemical processes. This suggests that the predominant photochemical pathway is through ROS reactivity, which itself is mediated by chromophoric dissolved organic matter (CDOM), and this pathway is strongly affected by environmental factors such as light screening and temperature. Transformation products were identified via UHPLC-Orbitrap mass spectrometry, revealing four major processes: 1) oxidation and cleavage of the quinone ring in the presence of ROS, 2) dealkylation, 3) rearrangement, and 4) deamination. These data indicate that 6-PPDQ can photodegrade in cool, sunlit waters under the appropriate conditions.