Interspecies Differences in 6PPD-Quinone Toxicity Across Seven Fish Species: Metabolite Identification and Semiquantification.

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q) is a recently identified contaminant that originates from the oxidation of the tire antidegradant 6PPD. 6PPD-Q is acutely toxic to select salmonids at environmentally relevant concentrations, while other fish species display tolerance to concentrations that surpass those measured in the environment. The reasons for these marked differences in sensitivity are presently unknown. The objective of this research was to explore potential toxicokinetic drivers of species sensitivity by characterizing biliary metabolites of 6PPD-Q in sensitive and tolerant fishes. For the first time, we identified an O-glucuronide metabolite of 6PPD-Q using high-resolution mass spectrometry. The semiquantified levels of this metabolite in tolerant species or life stages, including white sturgeon (Acipenser transmontanus), chinook salmon (Oncorhynchus tshawytscha), westslope cutthroat trout (Oncorhynchus clarkii lewisi), and nonfry life stages of Atlantic salmon (Salmo salar), were greater than those in sensitive species, including coho salmon (Oncorhynchus kisutch), brook trout (Salvelinus fontinalis), and rainbow trout (Oncorhynchus mykiss), suggesting that tolerant species might detoxify 6PPD-Q more effectively. Thus, we hypothesize that differences in species sensitivity are a result of differences in basal expression of biotransformation enzyme across various fish species. Moreover, the semiquantification of 6PPD-Q metabolites in bile extracted from wild-caught fish might be a useful biomarker of exposure to 6PPD-Q, thereby being valuable to environmental monitoring and risk assessment.

Toxic Tire Wear Compounds (6PPD-Q and 4-ADPA) Detected in Airborne Particulate Matter Along a Highway in Mississippi, USA.

Tire wear particles (TWPs) are a major category of microplastic pollution produced by friction between tires and road surfaces. This non-exhaust particulate matter (PM) containing leachable toxic compounds is transported through the air and with stormwater runoff, leading to environmental pollution and human health concerns. In the present study, we collected airborne PM at varying distances (5, 15 and 30 m) along US Highway 278 in Oxford, Mississippi, USA, for ten consecutive days using Sigma-2 passive samplers. Particles (~ 1-80 µm) were passively collected directly into small (60 mL) wide-mouth separatory funnels placed inside the samplers. Particles were subsequently subjected to solvent extraction, and extracts were analyzed for TWP compounds by high resolution orbitrap mass spectrometry. This pilot study was focused solely on qualitative analyses to determine whether TWP compounds were present in this fraction of airborne PM. The abundance of airborne TWPs increased with proximity to the road with deposition rates (TWPs cm-2 day-1) of 23, 47, and 63 at 30 m, 15 m, and 5 m from the highway, respectively. Two common TWP compounds (6PPD-Q and 4-ADPA) were detected in all samples, except the field blank, at levels above their limits of detection, estimated at 2.90 and 1.14 ng L-1, respectively. Overall, this work suggests airborne TWPs may be a potential inhalation hazard, particularly for individuals and wildlife who spend extended periods outdoors along busy roadways. Research on the bioavailability of TWP compounds from inhaled TWPs is needed to address exposure risk.

Uptake and Biotransformation of the Tire Rubber-derived Contaminants 6-PPD and 6-PPD Quinone in the Zebrafish Embryo (Danio rerio).

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PPD) is a widely used antioxidant in tire rubber known to enter the aquatic environment via road runoff. The associated transformation product (TP) 6-PPD quinone (6-PPDQ) causes extreme acute toxicity in some fish species (e.g., coho salmon). To interpret the species-specific toxicity, information about biotransformation products of 6-PPDQ would be relevant. This study investigated toxicokinetics of 6-PPD and 6-PPDQ in the zebrafish embryo (ZFE) model. Over 96 h of exposure, 6-PPD and 6-PPDQ accumulated in the ZFE with concentration factors ranging from 140 to 2500 for 6-PPD and 70 to 220 for 6-PPDQ. A total of 22 TPs of 6-PPD and 12 TPs of 6-PPDQ were tentatively identified using liquid chromatography coupled to high-resolution mass spectrometry. After 96 h of exposure to 6-PPD, the TPs of 6-PPD comprised 47% of the total peak area (TPA), with 4-hydroxydiphenylamine being the most prominent in the ZFE. Upon 6-PPDQ exposure, >95% of 6-PPDQ taken up in the ZFE was biotransformed, with 6-PPDQ + O + glucuronide dominating (>80% of the TPA). Among other TPs of 6-PPD, a reactive N-phenyl-p-benzoquinone imine was found. The knowledge of TPs of 6-PPD and 6-PPDQ from this study may support biotransformation studies in other organisms.

Occurrence and Fate of Substituted p-Phenylenediamine-Derived Quinones in Hong Kong Wastewater Treatment Plants.

para-Phenylenediamine quinones (PPD-Qs) are a newly discovered class of transformation products derived from para-phenylenediamine (PPD) antioxidants. These compounds are prevalent in runoff, roadside soil, and particulate matter. One compound among these, N-1,3-dimethylbutyl-n'-phenyl-p-phenylenediamine quinone (6PPD-Q), was found to induce acute mortality of coho salmon, rainbow trout, and brook trout, with the median lethal concentrations even lower than its appearance in the surface and receiving water system. However, there was limited knowledge about the occurrence and fate of these emerging environmental contaminants in wastewater treatment plants (WWTPs), which is crucial for effective pollutant removal via municipal wastewater networks. In the current study, we performed a comprehensive investigation of a suite of PPD-Qs along with their parent compounds across the influent, effluent, and biosolids during each processing unit in four typical WWTPs in Hong Kong. The total concentrations of PPDs and PPD-Qs in the influent were determined to be 2.7-90 and 14-830 ng/L. In the effluent, their concentrations decreased to 0.59-40 and 2.8-140 ng/L, respectively. The median removal efficiency for PPD-Qs varied between 53.0 and 91.0% across the WWTPs, indicating that a considerable proportion of these contaminants may not be fully eliminated through the current processing technology. Mass flow analyses revealed that relatively higher levels of PPD-Qs were retained in the sewage sludge (20.0%) rather than in the wastewater (16.9%). In comparison to PPDs, PPD-Qs with higher half-lives exhibited higher release levels via effluent wastewater, which raises particular concerns about their environmental consequences to aquatic ecosystems.

Toxicity of 6PPD-quinone to four freshwater invertebrate species.

The antioxidant N-(1,3-Dimethylbutyl)-N'-phenyl-p- phenylenediamine (6PPD) is used to protect the rubber in tires from oxidation, which extends the life of the tire. When oxidized, 6PPD is transformed into 6PPD-quinone (6PPDQ). 6PPDQ, along with other tire ingredients, can enter aquatic ecosystems through the transport of tire wear particles in runoff during a precipitation event. The mass mortality of coho salmon following precipitation events in urban areas lead to the discovery that 6PPDQ is the likely cause due to coho salmon's relatively high sensitivity to 6PPDQ. The assessment of 6PPDQ toxicity to other aquatic species has expanded, but it has focused on fish. This study investigated the toxicity of 6PPDQ to four freshwater invertebrate species, larval burrowing mayfly (Hexagenia spp.), juvenile cladoceran (Daphnia magna), file ramshorn snail embryo (Planorbella pilsbryi), and adult washboard mussel (Megalonaias nervosa). For all four species, the highest concentration of 6PPDQ tested did not result in significant mortality. This translated into the determination of the highest concentration that did not cause significant mortality (NOEC) for Hexagenia spp., D. magna, P. pilsbryi, and M. nervosa of 232.0, 42.0, 11.7, and 17.9 µg/L, respectively. The data from this study indicate that freshwater invertebrates are not as sensitive to 6PPDQ as some salmonid species (e.g., coho salmon Oncorhynchus kisutch). This study also analyzed 6PPDQ in road runoff from around the city of Guelph in Ontario, Canada. 6PPQ was detected in all samples but the concentration was two orders of magnitude lower than the NOECs for the four tested species of freshwater invertebrate.

H/D Exchange Coupled with 2H-labeled Stable Isotope-Assisted Metabolomics Discover Transformation Products of Contaminants of Emerging Concern.

Stable isotope-assisted metabolomics (SIAM) is a powerful tool for discovering transformation products (TPs) of contaminants. Nevertheless, the high cost or lack of isotope-labeled analytes limits its application. In-house H/D (hydrogen/deuterium) exchange reactions enable direct 2H labeling to target analytes with favorable reaction conditions, providing intuitive and easy-to-handle approaches for environmentally relevant laboratories to obtain cost-effective 2H-labeled contaminants of emerging concern (CECs). We first combined the use of in-house H/D exchange and 2H-SIAM to discover potential TPs of 6PPD (N-1,3-dimethylbutyl-N'-phenyl-p-phenylenediamine), providing a new strategy for finding TPs of CECs. 6PPD-d9 was obtained by in-house H/D exchange with favorable reaction conditions, and the impurities were carefully studied. Incomplete deuteride, for instance, 6PPD-d8 in this study, constitutes a major part of the impurities. Nevertheless, it has few adverse effects on the 2H-SIAM pipeline in discovering TPs of 6PPD. The 2H-SIAM pipeline annotated 9 TPs of 6PPD, and commercial standards further confirmed the annotated 6PPDQ (2-anilino-5-(4-methylpentan-2-ylamino)cyclohexa-2,5-diene-1,4-dione) and PPPD (N-phenyl-p-phenylenediamine). Additionally, a possible new formation mechanism for 6PPDQ was proposed, highlighting the performance of the strategy. In summary, this study highlighted a new strategy for discovering the TPs of CECs and broadening the application of SIAM in environmental studies.

Tire-rubber related pollutant 6-PPD quinone: A review of its transformation, environmental distribution, bioavailability, and toxicity.

The antioxidant 6-PPD has been widely used to prevent cracking and thermal oxidative degradation and to extend the service life of tire rubber. 6-PPD quinone (6-PPDQ) is formed via the reaction of 6-PPD with O3. Due to its acute lethality in coho salmon, 6-PPDQ has become an emerging pollutant of increasing concern. In this review, we provide a critical overview of the generation, environmental distribution, bioavailability, and potential toxicity of 6-PPDQ. The transformation pathways from 6-PPD to 6-PPDQ include the N-1,3-dimethylbutyl-N-phenyl quinone diamine (QDI), intermediate phenol, and semiquinone radical pathways. 6-PPDQ has been frequently detected in water, dust, air particles, soil, and sediments, indicating its large-scale and potentially global pollution trend. 6-PPDQ is bioavailable to both aquatic animals and mammals and acute exposure to 6-PPDQ can be lethal to some organisms. Exposure to 6-PPDQ at environmentally relevant concentrations could induce several types of toxicity, including neurotoxicity, intestinal toxicity, and reproductive toxicity. This review also identifies and discusses knowledge gaps and research needs for the study of 6-PPDQ. This review facilitates a better understanding of the environmental occurrence and exposure risk of 6-PPDQ.

Micron-size tire tread particles leach organic compounds at higher rates than centimeter-size particles: Compound identification and profile comparison.

Tire tread particles (TTP) are environmentally prevalent microplastics and generate toxic aqueous leachate. We determined the total carbon and nitrogen leachate concentrations and chemical profiles from micron (∼32 µm) and centimeter (∼1 cm) TTP leachate over 12 days. Dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) were used to measure the concentration of leached compounds. Nontargeted chemical analysis by comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC/TOF-MS) was used to compare the chemical profiles of leachates. After leaching for 12 days, DOC was 4.0 times higher in the micron TTP leachate than in the centimeter TTP leachate, and TDN was 2.6 times higher. The total GC×GC/TOF-MS chromatographic feature peak area was 2.9 times greater in the micron TTP leachate than the centimeter TTP leachate, and similarly, the total relative abundance of 54 tentatively identified compounds was 3.3 times greater. We identified frequently measured tire-related chemicals, such as 6PPD, N-cyclohexyl-N'-phenylurea (CPU), and hexa(methoxymethyl)melamine (HMMM), but nearly 50% of detected chemicals were not previously reported in tire literature or lacked toxicity information. Overall, the results demonstrate that smaller TTP have a greater potential to leach chemicals into aquatic systems, but a significant portion of these chemicals are not well-studied and require further risk assessment.

Environmental concentrations of tire rubber-derived 6PPD-quinone alter CNS function in zebrafish larvae.

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-quinone) is a degradation product of 6PPD, an antioxidant widely used in rubber tires. 6PPD-quinone enters aquatic ecosystems through urban stormwater runoff and has been identified as the chemical behind the urban runoff mortality syndrome in coho salmon. However, the available data suggest that the acute effects of 6PPD-quinone are restricted to a few salmonid species and that the environmental levels of this chemical should be safe for most fish. In this study, larvae of a "tolerant" fish species, Danio rerio, were exposed to three environmental concentrations of 6PPD-quinone for only 24 h, and the effects on exploratory behavior, escape response, nonassociative learning (habituation), neurotransmitter profile, wake/sleep cycle, circadian rhythm, heart rate and oxygen consumption rate were analyzed. Exposure to the two lowest concentrations of 6PPD-quinone resulted in altered exploratory behavior and habituation, an effect consistent with some of the observed changes in the neurotransmitter profile, including increased levels of acetylcholine, norepinephrine, epinephrine and serotonin. Moreover, exposure to the highest concentration tested altered the wake/sleep cycle and the expression of per1a, per3 and cry3a, circadian clock genes involved in the negative feedback loop. Finally, a positive chronotropic effect of 6PPD-quinone was observed in the hearts of the exposed fish. The results of this study emphasize the need for further studies analyzing the effects of 6PPD-quinone in "tolerant" fish species.

Ubiquity of Amino Accelerators and Antioxidants in Road Dust from Multiple Land Types: Targeted and Nontargeted Analysis.

Amino accelerators and antioxidants (AAL/Os), as well as their degradation derivatives, are industrial additives of emerging concern due to their massive production and use (particularly in rubber tires), pervasiveness in the environment, and documented adverse effects. This study delineated their inter-regional variations in road dust collected from urban/suburb, agricultural, and forest areas, and screened for less-studied AAL/O analogues with high-resolution mass spectrometry. 1,3-Diphenylguanidine (DPG; median concentration: 121 ng/g) and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q; 9.75 ng/g) are the most abundant congeners, constituting 69.7% and 41.4% of the total concentrations of AAL/Os (192 ng/g) and those of AAO transformation products (22.3 ng/g), respectively. The spatial distribution across the studied sites suggests evident human impacts, reflected by the pronounced urban signature and vehicle-originated pollution. Our nontargeted analysis of the most-contaminated road dust identified 16 AAL/O-related chemicals, many of which have received little investigation. Particularly, environmental and toxicological information remains extremely scarce for five out of the 10 most concerning compounds prioritized in terms of their dusty residues and toxicity including 1,2-diphenyl-3-cyclohexylguanidine (DPCG), N,N''-bis[2-(propan-2-yl)phenyl]guanidine (BPPG), and N-(4-anilinophenyl)formamide (PPD-CHO). Additionally, dicyclohexylamine (DChA), broadly applied as an antioxidant in automobile products, had an even greater median level than DPG. Therefore, future research on their health risks and (eco)toxic potential is of high importance.

Bioretention filtration prevents acute mortality and reduces chronic toxicity for early life stage coho salmon (Oncorhynchus kisutch) episodically exposed to urban stormwater runoff.

As the human population of western North America continues to expand, widespread patterns of urban growth pose increasingly existential threats to certain wild stocks of Pacific salmon and steelhead (Oncorhynchus sp.). Rainfall previously absorbed into the soils of forests and grasslands falls instead on pavement and other hardened surfaces. This creates stormwater runoff that carries toxic metals, oil, and many other contaminants into salmon-bearing habitats. These include freshwater streams where coho salmon (O. kisutch) spawn in gravel beds. Coho salmon embryos develop within a thick eggshell (chorion) for weeks to months before hatching as alevins and ultimately emerging from the gravel as fry. Untreated urban runoff is highly toxic to older coho salmon (freshwater-resident juveniles and adult spawners), but the vulnerability of the earliest life stages remains poorly understood. To address this uncertainty, we fertilized eggs and raised them under an episodic stormwater exposure regimen, using runoff collected from a high-traffic arterial roadway from 15 discrete storm events. We monitored survival and morphological development, as well as molecular markers for contaminant exposure and cardiovascular stress. We also evaluated the benefit of treating runoff with green infrastructure (bioretention filtration) on coho salmon health and survival. Untreated runoff caused subtle sublethal toxicity in pre-hatch embryos with no mortality, followed by high rates of mortality from exposure at hatch. Bioretention filtration removed most measured contaminants (bacteria, dissolved metals, and polycyclic aromatic hydrocarbons), and the treated effluent was considerably less toxic - notably preventing mortality at the alevin stage. Our findings indicate that untreated urban runoff poses an important threat to early life stage coho salmon, in terms of both acute and delayed-in-time mortality. Moreover, while inexpensive management strategies involving bioinfiltration are promising, future green infrastructure effectiveness research should emphasize sublethal metrics for contaminant exposure and adverse health outcomes in salmonids.

Tire-Derived Transformation Product 6PPD-Quinone Induces Mortality and Transcriptionally Disrupts Vascular Permeability Pathways in Developing Coho Salmon.

Urban stormwater runoff frequently contains the car tire transformation product 6PPD-quinone, which is highly toxic to juvenile and adult coho salmon (Onchorychus kisutch). However, it is currently unclear if embryonic stages are impacted. We addressed this by exposing developing coho salmon embryos starting at the eyed stage to three concentrations of 6PPD-quinone twice weekly until hatch. Impacts on survival and growth were assessed. Further, whole-transcriptome sequencing was performed on recently hatched alevin to address the potential mechanism of 6PPD-quinone-induced toxicity. Acute mortality was not elicited in developing coho salmon embryos at environmentally measured concentrations lethal to juveniles and adults, however, growth was inhibited. Immediately after hatching, coho salmon were sensitive to 6PPD-quinone mortality, implicating a large window of juvenile vulnerability prior to smoltification. Molecularly, 6PPD-quinone induced dose-dependent effects that implicated broad dysregulation of genomic pathways governing cell-cell contacts and endothelial permeability. These pathways are consistent with previous observations of macromolecule accumulation in the brains of coho salmon exposed to 6PPD-quinone, implicating blood-brain barrier disruption as a potential pathway for toxicity. Overall, our data suggests that developing coho salmon exposed to 6PPD-quinone are at risk for adverse health events upon hatching while indicating potential mechanism(s) of action for this highly toxic chemical.

Contamination Pattern and Risk Assessment of Polar Compounds in Snow Melt: An Integrative Proxy of Road Runoffs.

To assess the contamination and potential risk of snow melt with polar compounds, road and background snow was sampled during a melting event at 23 sites at the city of Leipzig and screened for 489 chemicals using liquid chromatography high-resolution mass spectrometry with target screening. Additionally, six 24 h composite samples were taken from the influent and effluent of the Leipzig wastewater treatment plant (WWTP) during the snow melt event. 207 compounds were at least detected once (concentrations between 0.80 ng/L and 75 µg/L). Consistent patterns of traffic-related compounds dominated the chemical profile (58 compounds in concentrations from 1.3 ng/L to 75 µg/L) and among them were 2-benzothiazole sulfonic acid and 1-cyclohexyl-3-phenylurea from tire wear and denatonium used as a bittern in vehicle fluids. Besides, the analysis unveiled the presence of the rubber additive 6-PPD and its transformation product N-(1.3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ) at concentrations known to cause acute toxicity in sensitive fish species. The analysis also detected 149 other compounds such as food additives, pharmaceuticals, and pesticides. Several biocides were identified as major risk contributors, with a more site-specific occurrence, to acute toxic risks to algae (five samples) and invertebrates (six samples). Ametryn, flumioxazin, and 1,2-cyclohexane dicarboxylic acid diisononyl ester are the main compounds contributing to toxic risk for algae, while etofenprox and bendiocarb are found as the main contributors for crustacean risk. Correlations between concentrations in the WWTP influent and flow rate allowed us to discriminate compounds with snow melt and urban runoff as major sources from other compounds with other dominant sources. Removal rates in the WWTP showed that some traffic-related compounds were largely eliminated (removal rate higher than 80%) during wastewater treatment and among them was 6-PPDQ, while others persisted in the WWTP.

Analysis, environmental occurrence, fate and potential toxicity of tire wear compounds 6PPD and 6PPD-quinone.

Tire wear compounds N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its derivative 6PPD-quinone have been considered as emerging pollutants and attracted much attention recently. As an antioxidant and antiozonant widely used, 6PPD would be released during the production or use of rubber-related products. Because of the mass production and wide use of rubber-related products, 6PPD and 6PPD-quinone have been identified to be ubiquitous in the environment. In this study, we firstly reviewed the current available literature on the analytical procedures, concentrations and distribution of 6PPD and 6PPD-quinone, and then investigated the potential toxic effects of these two compounds on aquatic organisms. Current studies have been mainly focused on the occurrence of 6PPD and 6PPD-quinone in dust and water, while available information on atmosphere, soil, sediments and organisms is limited. The fate and distribution of 6PPD and 6PPD-quinone would be influenced by environmental factors such as temperature, illumination, and storm events, etc. Although 6PPD and 6PPD-quinone have potential adverse effects on aquatic organisms, and 6PPD-quinone has species-specific toxicity, toxicological mechanisms of these compounds are still unclear. Based on the review and analysis of current studies, some suggestions for future research of 6PPD and 6PPD-quinone are given.

Screening p-Phenylenediamine Antioxidants, Their Transformation Products, and Industrial Chemical Additives in Crumb Rubber and Elastomeric Consumer Products.

Recently, roadway releases of N,N'-substituted p-phenylenediamine (PPD) antioxidants and their transformation products (TPs) received significant attention due to the highly toxic 6PPD-quinone. However, the occurrence of PPDs and TPs in recycled tire rubber products remains uncharacterized. Here, we analyzed tire wear particles (TWPs), recycled rubber doormats, and turf-field crumb rubbers for seven PPD antioxidants, five PPD-quinones (PPDQs), and five other 6PPD TPs using liquid chromatography-tandem mass spectrometry. PPD antioxidants, PPDQs, and other TPs were present in all samples with chemical profiles dominated by 6PPD, DTPD, DPPD, and their corresponding PPDQs. Interestingly, the individual [PPDQ]/[PPD] and [TP]/[PPD] ratios significantly increased as total concentrations of the PPD-derived chemical decreased, indicating that TPs (including PPDQs) dominated the PPD-derived compounds with increased environmental weathering. Furthermore, we quantified 15 other industrial rubber additives (including bonding agents, vulcanization accelerators, benzotriazole and benzothiazole derivatives, and diphenylamine antioxidants), observing that PPD-derived chemical concentrations were 0.5-6 times higher than these often-studied additives. We also screened various other elastomeric consumer products, consistently detecting PPD-derived compounds in lab stoppers, sneaker soles, and rubber garden hose samples. These data emphasize that PPD antioxidants, PPDQs, and related TPs are important, previously overlooked contaminant classes in tire rubbers and elastomeric consumer products.

Amino antioxidants: A review of their environmental behavior, human exposure, and aquatic toxicity.

Amino antioxidants (AAOs), a suite of emerging organic contaminants, have been widely used in numerous industrial and commercial products to inhibit oxidation and corrosion. Recently, their environmental ubiquity, health risks, bioaccumulative and toxic potential have led to mounting public concern. This review summarizes the current state of knowledge on the production and usage, environmental occurrence, bioavailability, human exposure, and aquatic toxicity of representative AAOs, and provides suggestions for future research directions. Previous studies have revealed widespread distribution of many AAOs in various environmental matrixes, including air, water, sediment, dust, and biota. In addition to parent compounds, their degradation products, such as 2-anilino-5-(1,3-dimethylbutylamino)-1,4-benzoquinone (6PPD-Q) and 4-nitrodiphenylamine (4-NO2-DPA), have also been detected at high levels in multiple compartments. Dust ingestion and air inhalation are the two most well-investigated routes for human exposure to AAOs and their transformation products, while studies on other pathways (e.g., skin contact and dietary intake) still remain extremely limited. Moreover, AAO burdens in human tissue have been poorly documented. Toxicological data have shown that a few AAOs may cause teratogenic, developmental, reproductive, endocrinic, neuronic, and genetic toxicity to aquatic organisms, and the toxic capacities of degradation products differ from their precursors. Future studies should focus on elucidating AAO exposure for humans and associated health risks. Additionally, more attention should be given to AAO transformation products (particularly those quinoid derivatives possessing substantial affinity with DNA) and to the effects of complex mixtures of these chemicals.

Widespread Occurrence and Transport of p-Phenylenediamines and Their Quinones in Sediments across Urban Rivers, Estuaries, Coasts, and Deep-Sea Regions.

p-Phenylenediamines (PPDs) are widely used as antioxidants in tire rubber, and their derived quinone transformation products (PPD-Qs) may pose a threat to marine ecosystems. A compelling example is N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD)-derived quinone, called 6PPD-Q, as the causal toxicant for stormwater-linked acute mortality toward coho salmon. However, the knowledge of the co-occurrences of PPDs and PPD-Qs and their transport from freshwater to oceanic waterbodies on a large geographical scale remains unknown. Herein, we performed the first large-scale survey of these chemicals in sediments across urban rivers, estuaries, coasts, and deep-sea regions. Our results demonstrated that seven PPDs and four PPD-Qs are ubiquitously present in riverine, estuarine, and coastal sediments, and most of them also occur in deep-sea sediments. The most dominant chemicals of concern were identified as 6PPD and 6PPD-Q. Total sedimentary concentrations of PPDs and PPD-Qs presented a clear spatial trend with decreasing levels from urban rivers (medians: 39.7 and 15.2 ng/g) to estuaries (14.0 and 5.85 ng/g) and then toward coasts (9.47 and 2.97 ng/g) and deep-sea regions (5.24 and 3.96 ng/g). Interestingly, spatial variation in the ratios of 6PPD to 6PPD-Q (R6PPD/6PPD-Q) also presented a clear decreasing trend. Our field measurements implied that riverine outflows of PPDs and PPD-Qs may be an important route to transport these tire rubber-derived chemicals to coastal and open oceans.

Uptake, Metabolism, and Accumulation of Tire Wear Particle-Derived Compounds in Lettuce.

Tire wear particle (TWP)-derived compounds may be of high concern to consumers when released in the root zone of edible plants. We exposed lettuce plants to the TWP-derived compounds diphenylguanidine (DPG), hexamethoxymethylmelamine (HMMM), benzothiazole (BTZ), N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine (6PPD), and its quinone transformation product (6PPD-q) at concentrations of 1 mg L-1 in hydroponic solutions over 14 days to analyze if they are taken up and metabolized by the plants. Assuming that TWP may be a long-term source of TWP-derived compounds to plants, we further investigated the effect of leaching from TWP on the concentration of leachate compounds in lettuce leaves by adding constantly leaching TWP to the hydroponic solutions. Concentrations in leaves, roots, and nutrient solution were quantified by triple quadrupole mass spectrometry, and metabolites in the leaves were identified by Orbitrap high resolution mass spectrometry. This study demonstrates that TWP-derived compounds are readily taken up by lettuce with measured maximum leaf concentrations between ∼0.75 (6PPD) and 20 µg g-1 (HMMM). Although these compounds were metabolized in the plant, we identified several transformation products, most of which proved to be more stable in the lettuce leaves than the parent compounds. Furthermore, continuous leaching from TWP led to a resupply and replenishment of the metabolized compounds in the lettuce leaves. The stability of metabolized TWP-derived compounds with largely unknown toxicities is particularly concerning and is an important new aspect for the impact assessment of TWP in the environment.

Occurrence and risks of 23 tire additives and their transformation products in an urban water system.

Tire wear particles (TWPs) enter road surface with the friction between tires and road surfaces. Under the volatilization, leaching, and transformation action on TWPs by sunlight and rain, tire additives are released into urban water systems, such as surface rainfall runoff, wastewater treatment plants (WWTPs), receiving surface waters, and drinking water treatment plant (DWTP). In this study, we investigated the occurrence of 23 tire additives and their transformation products in the urban water system of the Pearl River Delta region, South China. Nineteen target compounds were detected in the surface runoff, with 1,3-Diphenylguanidine (DPG) showing highest maximum concentration of 58780 ng/L. Benzothiazole and its transformation products are detected at the frequency of 100 % with the total concentrations of 480-42160 ng/L. The antioxidant derivative N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q) was also detected up to 1562 ng/L, which was considerably higher than that of the parent compound 6PPD (the maximum concentration of 7.52 ng/L). Eleven and 8 compounds were detected in WWTPs influents and effluents, respectively, with removal rates of - 62-100 %. Seventeen compounds were detected in the receiving Zhujiang and Dongjiang rivers, while 9 compounds were detected in drinking water sources and DWTP samples. Road runoff, with total concentrations of target compounds up to 79200 ng/L, is suggested as the main non-point source for receiving rivers, while WWTPs effluents are the point sources due to incomplete removal of target compounds after accepting the initial runoff. 6PPD-Q and other 10 compounds displayed median to high ecological risks in surface waters, and the human daily intake of tire additives was estimated to be 2.63 × 10-8-3.16 × 10-5 mg/(kg d) via drinking water. This is the first report of the 6PPD-Q and 1,3-Diphenylurea levels in surface waters in China.

Time-concentration profiles of tire particle additives and transformation products under natural and artificial aging.

Tire and road wear particles (TRWP) are polymer-based microparticles that are emitted into the environment during tire usage. Growing efforts are currently being made to quantify these emissions, characterize the leachates and assess their environmental impact. This study aimed to investigate the effect of aging on TRWP composition. Cryomilled tire tread particles (CMTTP) and TRWP were exposed for different durations to three aging conditions: accelerated thermal and photochemical aging and natural outdoor aging. Particles were then extracted with cyclohexane/ethanol. The time-concentration profiles of 23 additives and transformation products present in these extracts were determined by UHPLC-HRMS. Several chemicals, such as N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PPD) or 1,3-diphenylguanidine (DPG), decayed exponentially under all aging conditions, with half-lives of a few days under artificial photoaging versus dozens of days under pure thermal aging at 60 °C. The natural aging profiles lie between those 2 laboratory aging conditions. Other chemicals, such as 6PPD-quinone, presented bell-shaped concentration profiles within CMTTP when particles were exposed to UV light. 6PPD-quinone reached a maximal concentration within a month under natural aging. For TRWP, the initial load of 6PPD-quinone had already reached a maximum prior to the aging experiments and decreased exponentially under natural aging with a half-life below one month. Pure thermal aging induced a significantly slower decay of 6PPD-quinone within TRWP (half-life of half a year), emphasizing a greater stability and persistence in environmental compartments without light. This study highlighted that the more readily accessible CMTTP could be considered a reasonable proxy of TRWP to investigate the fate of chemicals within rubber particles, at least from a qualitative standpoint. Overall, the concentrations of 20 of the evaluated chemicals decreased by >50 % within 50 days under natural aging.