Beyond Substituted p-Phenylenediamine Antioxidants: Prevalence of Their Quinone Derivatives in PM_2.5

Abstract

Substituted para -phenylenediamine (PPD) antioxidants have been extensively used to retard oxidative degradation of tire rubber and were found to pervade multiple environmental compartments. However, there is a paucity of research on the environmental occurrences of their transformation products. In this study, we revealed the co-occurrence of six PPD-derived quinones (PPD-Qs) along with eight PPDs in fine particulate matter (PM 2.5 ) from two Chinese megacities, in which N , N ′-bis(1,4-dimethylpentyl)- p -phenylenediamine quinone (77PD-Q) was identified and quantified for the first time. Prevalent occurrences of these emerging PPD-Qs were found in Taiyuan (5.59–8480 pg/m 3 ) and Guangzhou (3.61–4490 pg/m 3 ). Significantly higher levels of PPDs/PPD-Qs were observed at a roadside site, implying the possible contribution of vehicle emissions. Correlation analysis implied potential consistencies in the fate of these PPD-Qs and suggested that most of them were originated from the transformation of their parent PPDs. For different subpopulation groups under different exposure scenarios, the estimated daily intakes of PPD-Qs (0.16–1.25 ng kg bw –1 day –1 ) were comparable to those of their parent PPDs (0.19–1.41 ng kg bw –1 day –1 ), suggesting an important but overlooked exposure caused by novel PPD-Qs. Given the prolonged exposure of these antioxidants and their quinone derivatives to traffic-relevant occupations, further investigations on their toxicological and epidemiological effects are necessary.

Rubber Antioxidants and Their Transformation Products: Environmental Occurrence and Potential Impact

“…The TPs of rubber antioxidants have been observed in some studies under environmental conditions. As one of the widespread rubber antioxidants, amine antioxidants (PPDs: TMPPD, DPPD, 6PPD, and 6PPDTZ) could react with O 3 (in parts per billion volume levels) in the environment and produce PPD-quinone [ 31 ]. A previous study clarified the early stage of oxidation reaction for various PPDs as a reaction between free radical cations and amine groups in PPDs in the presence of O 3 [ 32 ].…”

“…The concentrations of DPPD-quinone had a particularly high proportion (75.9%) in the total PPD-quinones in air particles [ 73 ]. Eight PPDs (6PPD, 7PPD, DPPD, IPPD, 77PD, CPPD, DTPD, DNPD) and six PPD-quinones (IPPD-quinone, CPPD-quinone, 6PPD-quinone, 77PD-quinone, DPPD-quinone, DTPD-quinone) were also detectable in PM 2.5 collected from three sites located in Taiyuan and Guangzhou, China [ 31 ]. This indicated that atmospheric PPDs and PPD-quinones are ubiquitous in China.…”

“…It was observed that PPD-derived quinones in urban PM 2.5 samples exhibited significant positive correlations with their parent compounds and atmospheric concentrations of O 3 , suggesting that PPD-derived quinones in the air should not be overlooked [ 73 , 74 ]. It was notable that traffic emission was significantly related to the distribution of PPDs and PPD-quinones in air, and their concentrations in the roadside air samples were obviously higher ( Table 1 ) [ 31 , 74 ]. This may not only reveal the traffic impact on the occurrence of antioxidants and their TPs in air, but also further confirm that road runoff is a main pathway of antioxidants into the urban surface water.…”

Computational studies of rubber ozonation explain the effectiveness of 6PPD as an antidegradant and the mechanism of its quinone formation

“…Historically, work on PPD ozonation has emphasized formation of N−O oxides, analogous to ozonation of amines, resulting in dinitrone products for PPD ozonation. 34,37,[69][70][71] In the case of 6PPD, this assignment was recently repudiated on the basis of two-dimensional nuclear magnetic resonance spectroscopy, 12 and quinones are now understood to form instead, 39,72 representing about 10 % of the product distribution for 6PPD. 73 Beyond this, a great number of reaction products have been determined experimentally, 34,[37][38][39]69 though mechanistic details of these transformations are not always clear.…”

p-Phenylenediamine-Derived Quinones as New Contributors to the Oxidative Potential of Fine Particulate Matter

“…Emerging evidence has indicated the occurrence of tire rubber-added p -phenylenediamine-derived quinones (PPD-quinones) in PM 2.5 . − One of these PPD-quinones, N -(1,3-dimethylbutyl)- N ′-phenyl- p -phenylenediamine quinone (6PPD-Q), which originates from the oxidation of a tire rubber antioxidant, was proven to be highly toxic to aquatic organisms, including coho salmon, rainbow trout, and brook trout. , In addition, a toxicological study indicated that exposure to 6PPD-Q can cause reddening in the lumen of the intestine of zebrafish, a typical symptom of inflammation . However, to the best of our knowledge, the detailed toxicity mechanism of 6PPD-Q is still unknown.…”