The joint effects of salt and 6PPD contamination on a freshwater herbivore

Phytotoxicity of 6PPD and its uptake by Myriophyllum verticillatum: Oxidative stress and metabolic processes

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a widely utilized antioxidant in automobile tires and rubber goods, is frequently detected in aquatic ecosystems and poses a potential threat to aquatic organisms. However, research on the impact of 6PPD on aquatic plants is still scarce. Here, we investigated the bioaccumulation of 6PPD in Myriophyllum verticillatum (M. verticillatum) (watermilfoil), and its impacts on biochemical characteristics and metabolomics. 6PPD (10,100 mg/L) significantly inhibited the growth and photosynthetic pigment content of M. verticillatum. After 14 days of exposure to 100 μg/L 6PPD, accumulation levels of 6PPD and its metabolite 6PPDQ in M. verticillatum reached 0.52 mg/kg and 0.09 mg/kg, respectively. Moreover, 6PPD significantly induced the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) enzymes and glutathione (GSH), reducing the levels of reactive oxygen species (ROS) and malondialdehyde (MDA), thereby mitigating oxidative damage in M. verticillatum. Furthermore, metabolic pathway analysis revealed that 6PPD has remarkable effects on amino acid and sugar metabolism. This study provides data support for understanding the toxic effects of 6PPD on aquatic plants and evaluating its potential risks.

Environmental concentrations of 6PPD and 6PPD-Q cause oxidative damage and alter metabolism in Eichhornia crassipes

N-(1,3-dimethylbutyl)-N '-phenyl-p-phenylenediamine (6PPD) and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q) are ubiquitous in the environment and can cause toxicity to aquatic animals. However, research on the toxicological effects of 6PPD and 6PPD-Q on aquatic plants remains limited. The present study investigated the physiological, biochemical, and metabolic responses of the floating aquatic plant Eichhornia crassipes (E. crassipes) to environmentally relevant concentrations (0.1, 1, and 10 μg·L-1) of 6PPD and 6PPD-Q. We found that 6PPD and 6PPD-Q elicited minimal effects on plant growth, but 6PPD induced a concentration-dependent decrease in the content of photosynthetic pigments. Low doses (0.1 μg·L-1 and 1 μg·L-1) of 6PPD-Q significantly elevated Reactive Oxygen Species (ROS) content in E. crassipes roots, indicating oxidative damage. Furthermore, 6PPD-Q induced a more pronounced osmotic stress compared to 6PPD. Metabolic analyses revealed that carbohydrates were significantly altered under 6PPD and 6PPD-Q treatments. The findings of this study enhance the understanding of the environmental risks posed by 6PPD and 6PPD-Q to plants and reveal the potential mechanisms of phytotoxicity.

Comparative toxic effect of tire wear particle-derived compounds 6PPD and 6PPD-quinone to Chlorella vulgaris

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a widely used antioxidant in rubber products, and its corresponding ozone photolysis product N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q), have raised public concerns due to their environmental toxicity. However, there is an existing knowledge gap on the toxicity of 6PPD and 6PPD-Q to aquatic plants. A model aquatic plant, Chlorella vulgaris (C. vulgaris), was subjected to 6PPD and 6PPD-Q at concentrations of 50, 100, 200, and 400 μg/L to investigate their effects on plant growth, photosynthetic, antioxidant system, and metabolic behavior. The results showed that 6PPD-Q enhanced the photosynthetic efficiency of C. vulgaris, promoting growth of C. vulgaris at low concentrations (50, 100, and 200 μg/L) while inhibiting growth at high concentration (400 μg/L). 6PPD-Q induced more oxidative stress than 6PPD, disrupting cell permeability and mitochondrial membrane potential stability. C. vulgaris responded to contaminant-induced oxidative stress by altering antioxidant enzyme activities and active substance levels. Metabolomics further identified fatty acids as the most significantly altered metabolites following exposure to both contaminants. In conclusion, this study compares the toxicity of 6PPD and 6PPD-Q to C. vulgaris, with 6PPD-Q demonstrating higher toxicity. This study provides valuable insight into the risk assessment of tire wear particles (TWPs) derived chemicals in aquatic habitats and plants.

Response of Ceratophyllum demersum L. and its epiphytic biofilms to 6PPD and 6PPD-Q exposure: Based on metabolomics and microbial community analysis

The emerging contaminant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its ozone conversion product N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q) pose a threat to aquatic ecosystems. Aquatic animals and plants exhibit vigorous responses at very low ambient concentrations. However, studies of submerged macrophytes, key producers in aquatic ecosystems, are limited and the full extent of their toxic effects and feedback mechanisms is unknown. To investigate the phytotoxicity of 6PPD and 6PPD-Q, we modeled plant responses to abiotic stress using Ceratophyllum demersum L. (C. demersum) as a representative submerged plant. Our findings indicate that 6PPD and 6PPD-Q disrupt physiological and biochemical processes in C. demersum, encompassing growth inhibition, reduction in photosynthetic pigments, induction of oxidative damage, and metabolic alterations. Moreover, unfavorable modifications to biofilms induced were also discernible supported by confocal laser scanning microscopy (CLSM) images and microbial community profiling. More importantly, we found a robust correlation between differentially expressed metabolites (DEMs) and dominant genera, and 6PPD and 6PPD-Q significantly altered their correlation. Overall, our results imply that even though C. demersum is a resilient submerged macrophyte, the toxic effects of 6PPD and 6PPD-Q cannot be disregarded.

Photolysis of p-phenylenediamine rubber antioxidants in aqueous environment: Kinetics, pathways and their photo-induced toxicity

The widespread use of rubber antioxidants, especially p-phenylenediamines (PPDs), has raised increasing concerns about their risk assessment. However, there is a notable lack of research on their transformation products (TPs). Photolysis, influenced by active components, plays a significant role in the environmental fates of PPDs. This study investigated four emerging PPDs (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), N, N'-diphenyl-p-phenylenediamine (DPPD), N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD), and N-cyclohexyl-N'-phenyl-p-phenylenediamine (CPPD)) through a combination of experiments (photolysis kinetics, quenching experiments, acute toxicity test to Vibrio Fischeri (V. fischeri) and identification of photolytic products) and theoretical calculations. The results revealed different pathways for indirect photolysis mediated by the hydroxyl radicals (•OH) and singlet oxygen (1O2) of DPPD and IPPD under simulated sunlight irradiation. The effects of dissolved organic matter (DOM) and fulvic acid (FA) on the rates of photolysis of PPDs highlighted the complex interactions among the molecular structure, light absorption properties, and environmental variables. Quenching for reactive oxygen species (ROS) reduced photo-induced toxicity, whereas the addition of DOM and FA increased it, suggesting the crucial role of ROS in the formation of more toxic photolytic products. The study of photolysis pathways and the evaluation of the health risks provide a comprehensive understanding of the environmental effects of these pollutants.

P-phenylenediamine antioxidants and their quinone derivatives: A review of their environmental occurrence, accessibility, potential toxicity, and human exposure

Substituted p-phenylenediamines (PPDs), a class of antioxidants, have been widely used to extend the lifespan of rubber products, such as tires and pipes. During use, PPDs will generate their quinone derivatives (PPD-Qs). In recent years, PPDs and PPD-Qs have been detected in the global environment. Among them, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q), the oxidation product of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), has been identified as highly toxic to coho salmon, with the lethal concentration of 50 % (LC50) being 95 ng/L, highlighting it as an emerging pollutant of great concern. This review summarizes the physicochemical properties, global environmental distribution, bioaccessibility, potential toxicity, human exposure risk, and green measures of PPDs and PPD-Qs. These chemicals exhibit lipophilicity, bioaccumulation potential, and poor aqueous stability. They have been found in water, air, dust, soil, and sediment worldwide, indicating their significance as emerging pollutants. Notably, current studies have identified electronic waste (e-waste), such as discarded wires and cables, as a non-negligible source of PPDs and PPD-Qs, in addition to tire wear. PPDs and PPD-Qs exhibit strong bioaccumulation in aquatic organisms and mammals, with a tendency for biomagnification within the food web, posing health threats to humans. Available toxicity data indicate that PPDs and PPD-Qs have negative effects on aquatic organisms, mammals, and invertebrates. Acute exposure leads to death and acute damage, and long-term exposure can cause a series of adverse effects, including growth and development toxicity, reproductive toxicity, neurotoxicity, intestinal toxicity, and multi-organ damage. This paper discusses current research gaps and offers recommendations to understand better the occurrence, behavior, toxicity, and environmental exposure risks of PPDs and PPD-Qs.

The spatio-temporal accumulation of 6 PPD-Q in greenbelt soils and its effects on soil microbial communities

6 PPD-Q (6 PPD-Quinone) is an ozone-induced byproduct derived from the degradation of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6 PPD), commonly found in road dust resulting from tire wear. However, the extent of 6 PPD-Q pollution in urban soil remains unclear. This study investigates the spatial and temporal accumulation patterns of 6 PPD-Q in greenbelt soils in Ningbo, and explores the correlation between 6 PPD-Q accumulation and soil microbial community composition and functions. Our findings indicate that 6 PPD-Q is present (ranging from 0.85 to 12.58 μg/kg) in soil samples collected from both sides of urban traffic arteries. Soil fungi exhibit higher sensitivity to 6 PPD-Q accumulation compared to bacteria, and associated fungi (Basidiomycota) may be potential biomarkers for environmental 6 PPD-Q contamination. Co-occurrence network analysis reveals that the bacterial microbial network in summer exhibits greater stability and resilience in response to 6 PPD-Q inputs than in winter. However, 6 PPD-Q accumulation disrupts the network structure of fungal communities to some extent, leading to reduced diversity in fungal microbial communities. Long-term accumulation of 6 PPD-Q weakens the nitrogen and phosphorus cycling potential within urban soil, while the enhancement of carbon cycling may further promote 6 PPD-Q degradation in urban soil. Taken together, this study provides new insights into the ecological risks of 6 PPD-Q in urban soils.

p-Phenylenediamines and p-phenylenediamine quinone derivatives in rubber consumer products and typical urban dust: Sources, transformation profiles, and health risks

N,N'-Substituted p-phenylenediamines (PPDs) are widely used as antioxidants in the rubber industry and are released into the environment in large quantities during the production and use of rubber products. We quantified PPDs and PPD quinone derivatives (PPD-Qs) in rubber consumer products, including car tires, rubber belts, rubber gloves, rubber cables, and rubber hoses, to obtain information on the degree of weathering over time during their use. Additionally, we investigated the occurrences and sources of PPDs and PPD-Qs in dust samples collected from four typical urban environments (roads, parking lots, automotive repair shops, and residences). The detected compounds included the highly toxic N-(1,3-dimethylbutyl)-N'-phenyl-1,4-phenylenediamine quinone, which can cause acute mortality of coho salmon (Oncorhynchus kisutch). Concentrations of PPDs in the automotive repair shops reached 56.0 μg/g, and were much higher than in the other environments, while the residential samples had the lowest contaminant concentrations. In road and residential samples, N,N'-di-2-naphthyl-p-phenylenediamine accounted for 17 %-30 % of the PPDs, and may have originated from different sources. We preliminarily identified 32 transformation products, and 11 of these were N,N'-di-2-naphthyl-p-phenylenediamine transformation products. The average daily intakes of PPDs and PPD-Qs were calculated to assess the health risks of dust exposure in each environment. Workers had high total intakes of PPD [60.3 ng/(kg day)] and PPD-Qs [20.1 ng/(kg day)], and were at some risk of occupational exposure. These results improve our understanding of the environmental occurrences, sources, transformation, and health risks of PPDs and PPD-Qs.

Presence of N, N'-Substituted p-Phenylenediamine Quinones in Tap Water: Implication for Human Exposure

Monitoring studies have demonstrated the wide presence of N, N'-substituted p-phenylenediamine-derived quinones (PPDQs) in environmental matrices. The general population may be potentially exposed to PPDQs through the consumption of tap water. While, the existence of PPDQs in tap water has not been well examined. To fill this gap, in this study we collected tap water samples from Hangzhou, China, and examined seven homologues of PPDQs in collected samples. All target PPDQs were identified in the collected tap water samples, with distinct detection frequencies (38-89%). PPDQs detected in tap water was dominated by N-(1, 3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPDQ; mean 0.56 ng/L, < LOD-4.0 ng/L). The profiles of PPDQs concentrations in tap water from the four districts of Hangzhou city were slightly different. The daily intake (DI) was found highest for 6PPDQ (mean 14-22 pg/kg bw/day, median 10-15 pg/kg bw/day) through tap water intake. The relatively higher DIs of various PPDQs were displayed for infants (mean 10-22 pg/kg bw/day, median 6.5-15 pg/kg bw/day), relative to the children (8.0-18 pg/kg bw/day, 5.4-12 pg/kg bw/day) and adults (6.7-14 pg/kg bw/day, 4.5-10 pg/kg bw/day). These data are crucial for assessing the overall human exposure to PPDQs. This study first, to our knowledge, reveals the concentrations and profiles of PPDQs in tap water.

Distinct Species-Specific and Toxigenic Metabolic Profiles for 6PPD and 6PPD Quinone by P450 Enzymes: Insights from In Vitro and In Silico Studies

The tire rubber antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its quinone product (6PPDQ) are prevalent emerging contaminants, yet their biotransformation profiles remain poorly understood, hampering the assessment of environmental and health risks. This study investigated the phase-I metabolism of 6PPD and 6PPDQ across aquatic and mammalian species through in vitro liver microsome (LM) incubations and in silico simulations. A total of 40 metabolites from seven pathways were identified using the highly sensitive nano-electrospray ionization mass spectrometry. Notably, 6PPDQ was consistently detected as a 6PPD metabolite with an approximate 2% yield, highlighting biotransformation as a neglected indirect exposure pathway for 6PPDQ in organisms. 6PPDQ was calculated to form through a facile two-step phenyl hydroxylation of 6PPD, catalyzed by cytochrome P450 enzymes. Distinct species-specific metabolic kinetics were observed, with fish LM demonstrating retarded biotransformation rates for 6PPD and 6PPDQ compared to mammalian LM, suggesting the vulnerability of aquatic vertebrates to these contaminants. Intriguingly, two novel coupled metabolites were identified for 6PPD, which were predicted to exhibit elevated toxicity compared to 6PPDQ and result from C-N oxidative coupling by P450s. These unveiled metabolic profiles offer valuable insights for the risk assessment of 6PPD and 6PPDQ, which may inform future studies and regulatory actions.

Effects of environmental concentrations of 6PPD and its quinone metabolite on the growth and reproduction of freshwater cladoceran

The widespread occurrence and accumulation of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its quinone metabolite, 6PPD quinone (6PPD-Q), have been globally recognized as a critical environmental issue. However, knowledge on the adverse effects of 6PPD and 6PPD-Q on freshwater invertebrates is limited. This study investigated the effects of 6PPD and its oxidative byproduct, 6PPD-Q, on the growth and reproduction of Daphnia pulex. Through 21-day exposure experiments, we measured the uptake of 0.1, 1, and 10 μg/L 6PPD and 6PPD-Q by D. pulex and assessed the effects on growth and fecundity of D. pulex. While 6PPD and 6PPD-Q did not affect the mortality rate of D. pulex, 6PPD-Q exposure inhibited the growth of D. pulex, indicating potential ecological risks. In particular, the reproductive capacity of D. pulex remained unaffected across the tested concentrations of 6PPD and 6PPD-Q, suggesting specific toxicological pathways that warrant further investigation. This study underscored the importance of evaluating the sublethal effects of emerging contaminants such as 6PPD and 6PPD-Q on aquatic invertebrates, and highlighted the need for comprehensive risk assessments to better understand their environmental impacts.

6PPD induced cardiac dysfunction in zebrafish associated with mitochondrial damage and inhibition of autophagy processes

The compound 6PPD is widely acknowledged for its antioxidative properties; however, concerns regarding its impact on aquatic organisms have spurred comprehensive investigations. In our study, we advanced our comprehension by revealing that exposure to 6PPD could induce cardiac dysfunction, myocardial injury and DNA damage in adult zebrafish. Furthermore, our exploration unveiled that the exposure of cardiomyocytes to 6PPD resulted in apoptosis and mitochondrial injury, as corroborated by analyses using transmission electron microscopy and flow cytometry. Significantly, our study demonstrated the activation of the autophagy pathway in both the heart of zebrafish and cardiomyocytes, as substantiated by transmission electron microscopy and immunofluorescent techniques. Importantly, the increased the expression of P62 in the heart and cardiomyocytes suggested an inhibition of the autophagic process. The reduction in autophagy flux was also verified through in vivo experiments involving the infection of mCherry-GFP-LC3. We further identified that the fusion of autophagosomes and lysosomes was impaired in the 6PPD treatment group. In summary, our findings indicated that the impaired fusion of autophagosomes and lysosomes hampered the autophagic degradation process, leading to apoptosis and ultimately resulting in cardiac dysfunction and myocardial injury. This study discovered the crucial role of the autophagy pathway in regulating 6PPD-induced cardiotoxicity. SYNOPSIS: 6PPD exposure inhibited the autophagic degradation process and induced mitochondrial injury and apoptosis in the heart of adult zebrafish.

Acute Toxicity Testing of Pink Salmon (Oncorhynchus gorbuscha) with the Tire Rubber-Derived Chemical 6PPD-Quinone

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-quinone) is a widespread contaminant of emerging concern resulting from oxidation of 6PPD, which is an antidegradant substance added to tires. The recent identification of 6PPD-quinone as the cause of acute mortality in coho salmon has quickly motivated studies on 6PPD-quinone toxicity to other species. Subsequent findings have shown that 6PPD-quinone toxicity is highly species specific. Closely related species can differ widely in response to 6PPD-quinone from extremely sensitive to tolerant. Hence toxicity testing is currently the only way to establish whether a species exhibits 6PPD-quinone toxicity. We investigated the acute toxicity of 6PPD-quinone in pink salmon alevins (sac fry). This species has is the only Pacific salmon that so far has not been tested for 6PPD-quinone sensitivity. Fish were exposed in static water in eight treatments with initial concentrations ranging from 0.1 to 12.8 μg/L. Fish were observed for 48 h, and changes in concentrations of 6PPD-quinone were monitored throughout the experiment. No mortalities or substantial changes in behavior were recorded. Environ Toxicol Chem 2024;43:1332-1338. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Where the rubber meets the road: Emerging environmental impacts of tire wear particles and their chemical cocktails

About 3 billion new tires are produced each year and about 800 million tires become waste annually. Global dependence upon tires produced from natural rubber and petroleum-based compounds represents a persistent and complex environmental problem with only partial and often-times, ineffective solutions. Tire emissions may be in the form of whole tires, tire particles, and chemical compounds, each of which is transported through various atmospheric, terrestrial, and aquatic routes in the natural and built environments. Production and use of tires generates multiple heavy metals, plastics, PAH's, and other compounds that can be toxic alone or as chemical cocktails. Used tires require storage space, are energy intensive to recycle, and generally have few post-wear uses that are not also potential sources of pollutants (e.g., crumb rubber, pavements, burning). Tire particles emitted during use are a major component of microplastics in urban runoff and a source of unique and highly potent toxic substances. Thus, tires represent a ubiquitous and complex pollutant that requires a comprehensive examination to develop effective management and remediation. We approach the issue of tire pollution holistically by examining the life cycle of tires across production, emissions, recycling, and disposal. In this paper, we synthesize recent research and data about the environmental and human health risks associated with the production, use, and disposal of tires and discuss gaps in our knowledge about fate and transport, as well as the toxicology of tire particles and chemical leachates. We examine potential management and remediation approaches for addressing exposure risks across the life cycle of tires. We consider tires as pollutants across three levels: tires in their whole state, as particulates, and as a mixture of chemical cocktails. Finally, we discuss information gaps in our understanding of tires as a pollutant and outline key questions to improve our knowledge and ability to manage and remediate tire pollution.

A Review of N-(1,3-Dimethylbutyl)-N'-phenyl-p-Phenylenediamine (6PPD) and Its Derivative 6PPD-Quinone in the Environment

As an antioxidant and antiozonant, N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is predominantly used in the rubber industry to prevent degradation. However, 6PPD can be ozonated to generate a highly toxic transformation product called N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-quinone), which is toxic to aquatic and terrestrial organisms. Thus, 6PPD and 6PPD-quinone, two emerging contaminants, have attracted extensive attention recently. This review discussed the levels and distribution of 6PPD and 6PPD-quinone in the environment and investigated their toxic effects on a series of organisms. 6PPD and 6PPD-quinone have been widely found in air, water, and dust, while data on soil, sediment, and biota are scarce. 6PPD-quinone can cause teratogenic, developmental, reproductive, neuronal, and genetic toxicity for organisms, at environmentally relevant concentrations. Future research should pay more attention to the bioaccumulation, biomagnification, transformation, and toxic mechanisms of 6PPD and 6PPD-quinone.

Mixture toxicity of 6PPD-quinone and polystyrene nanoplastics in zebrafish

Plastic pollution, including micro- and nanoplastics, is a growing concern. Tyre-wear particles (TWPs) are the second largest source of microplastics in the ocean following abrasion of synthetic fibres. In addition to the particles themselves, TWPs contain many harmful chemicals, including 6PPD. This chemical reacts with atmospheric ozone and forms the toxic compound 6PPD-quinone (6PPDq), which poses a danger to aquatic life. There is a knowledge gap in understanding risks associated with the combined toxicity of nanoplastics (NPs) and 6PPDq. The present study aimed to investigate the toxicity of NPs and 6PPDq on adult zebrafish using phenotypic (behaviour, histology) and transcriptomic endpoints. Zebrafish were exposed to four treatments: control (contaminant-free), 50 μg/L 6PPDq, 3 mg/L polystyrene (PS)-NPs, and a combination of 50 μg/L 6PPDq and 3 mg/L PS-NPs. We did not observe locomotory dysregulation in zebrafish exposed to NPs. However, we found significant hyperlocomotion in zebrafish exposed to 6PPDq and this effect was even more substantial after co-exposure with PS-NPs. This study explores the molecular mechanisms behind these effects, identifying genes associated with neurotransmitters and fatty acid metabolism that were dysregulated by the co-exposure. Transcriptomic analysis further showed that both 6PPDq and PS-NPs impacted cellular processes associated with sterol biosynthesis, cholesterol metabolism, and muscle tissue development. The effects on these mechanisms were stronger in co-exposed zebrafish, indicating a heightened risk to cellular integrity and mitochondrial dysfunction. These results highlight the significance of mixture toxicity when studying the effects of NPs and associated chemicals like 6PPDq.

Salinity-dependent top-down effect of rotifer Brachionus plicatilis on removing harmful alga Phaeocystis globosa

Using appropriate zooplankton to transfer the primary productivity of harmful algae to higher trophic levels through food chain is an eco-friendly mode to remove harmful algae. To assess the top-down efficiency of rotifer removing Phaeocystis and the salinity effect, we adopted a series of salinities to carry out Phaeocystis-rotifer population dynamics and rotifer life-history experiments. Results showed that the time for rotifers to remove Phaeocystis population was the shortest when the salinity was ≤20 ‰. With salinity rising to above 25 ‰, although the clearance time of Phaeocystis population by rotifer was significantly prolonged, ultimately the Phaeocystis population were almost completely eliminated at all salinities. Additionally, rotifer matured and reproduced earlier at low salinity, while high salinity significantly delayed first reproductive time and decreased the total offspring. The above findings are helpful to assess the impacts of external environmental factors on the application of zooplankton to control harmful algae.

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.

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.

Urban runoff mortality syndrome in zooplankton caused by tire wear particles

Stormwater runoff from roadways is a global threat to water quality, aquatic organisms, and ecosystems. Tire tread wear particles (TWP) from roadway runoff may lead to urban runoff mortality syndrome (URMS) in some aquatic organisms. We tested the hypothesis that urban runoff from roadways can kill zooplankton. Both roadway runoff and TWP leachate were acutely lethal to a model species, the water flea Daphnia pulex. Life table experiments further revealed the lowered survival rates, intrinsic rate of increase, average life span, and net productive rate of D. pulex when exposed to roadway runoff and TWP leachate. The tire rubber antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) mainly contributed to the TWP toxicity. The toxicity of TWP and 6PPD extracted varied with time in nature. Cladocerans and rotifers were more sensitive to TWP and 6PPD than copepods. These results demonstrate the presence of URMS in zooplankton, which may cascade through food webs and affect aquatic ecosystems.

Toxicological effects of tire rubber-derived 6PPD-quinone, a species-specific toxicant, and dithiobisbenzanilide (DTBBA) in the marine rotifer Brachionus koreanus

The ingredients of tire-rubber products include a complex range of chemicals additives, most of which are leached into surrounding water as unmeasured toxicants with unexplored ecotoxicological impacts. The present study summarizes the reported species-specific acute toxicity of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q), the ozonation product of anti-oxidant 6PPD used in tire rubber. Also, chronic toxicity and oxidative response of 6PPD-Q and another tire-rubber derivative, 2',2'''-dithiobisbenzanilide (DTBBA), in rotifer Brachionus koreanus were investigated. Although 6PPD-Q has been reported to be highly toxic to several species of salmonids, only moderate chronic toxicity was observed in B. koreanus. In contrast, DTBBA significantly retarded the population growth and fecundity. The varying toxicity of 6PPD-Q and DTBBA was linked to the level of reactive oxygen species in which DTBBA exposure caused a significant concentration-dependent increase. Our results imply unanticipated risks to aquatic species posed by chemical additives in tire-rubber which may be considered emerging contaminants of toxicological concern.

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.

Exploration of emerging environmental pollutants 6PPD and 6PPDQ in honey and fish samples

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPDQ) can pose a threat to human health through the food chain because of their ubiquitous presence in the environment and the biotoxicity on organisms. In this work, we developed modified QuEChERS methods coupled with high performance liquid chromatography tandem mass spectrometry (HPLC/MS-MS) to explore whether 6PPD and 6PPDQ are present in fish and honey. The proposed methods showed acceptable limits of quantification (0.00043-0.001 mg/kg), linearity (R2 > 0.99), recovery (73.3%-108.3%), matrix effect (70.4%-95.6%) and repeatability (RSD < 8.4%). Accordingly, 6PPD and 6PPDQ have been discovered in snakehead, weever and Spanish mackerel fish, while none of which have been detected in the honey samples. The results of our work contributed to increasing public attention to 6PPD and 6PPDQ in agricultural products and provided important reference for the analysis of them.

Acute Toxicity Testing of the Tire Rubber-Derived Chemical 6PPD-quinone on Atlantic Salmon (Salmo salar) and Brown Trout (Salmo trutta)

Recent identification of 6PPD-quinone as the chemical causing acute toxicity in coho salmon has led to substantial concern regarding the toxicity of this contaminant for other aquatic species. Environmental occurrence of 6PPD-quinone is probably high, because it is an oxidation product of a common tire rubber additive. Research on 6PPD-quinone toxicity in fish has revealed a rather unusual pattern, with closely related species exhibiting responses ranging from extreme sensitivity to no effect. Of 11 previously studied fish species, 6PPD-quinone was toxic to four. The species-specific toxicity of 6PPD-quinone complicates urgently needed environmental risk assessment. We investigated the acute toxicity of 6PPD-quinone in Atlantic salmon and brown trout alevins (sac fry). These species have previously not been tested for sensitivity to 6PPD-quinone. The fish were exposed in static conditions in eight treatments with initial concentrations ranging from 0.095 to 12.16 µg/L. Fish were observed for 48 h, and changes in concentrations of 6PPD-quinone were monitored throughout the experiment. No mortalities or substantial changes in behavior were recorded in either Atlantic salmon or brown trout. This provides an important first step in assessing effects of 6PPD-quinone on these economically and culturally highly important species. Environ Toxicol Chem 2022;41:3041-3045. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Negative Effects of Cyanotoxins and Adaptative Responses of Daphnia

The plethora of cyanobacterial toxins are an enormous threat to whole ecosystems and humans. Due to eutrophication and increases in lake temperatures from global warming, changes in the distribution of cyanobacterial toxins and selection of few highly toxic species/ strains are likely. Globally, one of the most important grazers that controls cyanobacterial blooms is Daphnia, a freshwater model organism in ecology and (eco)toxicology. Daphnia-cyanobacteria interactions have been studied extensively, often focusing on the interference of filamentous cyanobacteria with Daphnia's filtering apparatus, or on different nutritional constraints (the lack of essential amino acids or lipids) and grazer toxicity. For a long time, this toxicity only referred to microcystins. Currently, the focus shifts toward other deleterious cyanotoxins. Still, less than 10% of the total scientific output deals with cyanotoxins that are not microcystins; although these other cyanotoxins can occur just as frequently and at similar concentrations as microcystins in surface water. This review discusses the effects of different cyanobacterial toxins (hepatotoxins, digestive inhibitors, neurotoxins, and cytotoxins) on Daphnia and provides an elaborate and up-to-date overview of specific responses and adaptations of Daphnia. Furthermore, scenarios of what we can expect for the future of Daphnia-cyanobacteria interactions are described by comprising anthropogenic threats that might further increase toxin stress in Daphnia.

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

Antioxidants are prevalently used during rubber production to improve rubber performance, delay aging, and extend service life. However, recent studies have revealed that their transformation products (TPs) could adversely affect environmental organisms and even lead to environmental events, which led to great public concern about environmental occurrence and potential impacts of rubber antioxidants and their TPs. In this review, we first summarize the category and application of rubber antioxidants in the world, and then demonstrate the formation mechanism of their TPs in the environment, emphasizing their influence on the ozone oxidative degradation. The potential toxic effects of antioxidants and their TPs are further reviewed to improve understanding of their biological health impact and environmental risks. Finally, the environmental occurrences of antioxidants and their TPs are summarized and their environmental impacts are demonstrated based on the recent studies. Due to the currently limited understanding on the toxic and biological effects of these compounds, further studies are required in order to better assess various TPs of these antioxidants and their environmental impact. To our knowledge, this is the first review on antioxidants and their TPs in the environment, which may elevate the environmental risk awareness of rubber products and their TPs in the near future.

The need for environmental regulation of tires: Challenges and recommendations

The interest in tire wear particles (TWPs), generated from abrasion of tires, have gained traction over the past few years, both in regards to quantifying particulate emissions, leaching of different compounds, toxicity, and analytical methods. The life of a tire, from cradle to end-of-life, crosses over different scenarios during its lifetime and transcends environmental compartments and legislative areas, underlining the need for a collective approach. Sustainability for a tire encompasses the use of raw materials, recycling of raw materials, circular economy and material sourcing. The tire industry is currently making significant efforts towards a greener and more sustainable production considering reduction of CO₂-emissions, recycling, material sources and implementing the use of biomass from plants rather than oil-derived alternatives. In this paper, we aim to analyze and discuss the need for environmental regulation of tires in order to provide a series of targeted recommendations for future legislation. Our study shows that the numerous regulations related to tires focus on chemicals, manufacturing, raw materials, use of tires on roads, waste handling, safety and polycyclic aromatic hydrocarbons (PAHs) in different life cycle stages of a tire. However, none directly addresses the contribution of TWPs to the environment. Despite the overall good intentions of the existing regulations, there is a lack of focus on the compounds that partition from the tire and disperse in the environment, their mixture effects, and the transformative products from the parent compounds in the environment. Therefore, a renewed focus is needed on risk assessment of complex mixtures like TWPs. Thus, transparency in regard to use of chemicals in TWP, mixtures, minimization of emissions, and capture of particulate pollution should be a priority.

Multiview behavior and neurotransmitter analysis of zebrafish dyskinesia induced by 6PPD and its metabolites

The typical tire manufacturing additive 6PPD, its metabolites 6PPDQ and 4-Hydroxy should be monitored because of their ubiquitous presence in the environment and the high toxicity of 6PPDQ to coho salmon. The toxic effect of 6PPD and its metabolites have been revealed superficially, especially on behavioral characteristics. However, the behavioral indicators explored so far are relatively simple and the toxic causes are poorly understood. With this in mind, our work investigated the toxic effects of 6PPD, 6PPDQ and 4-Hydroxy on adult zebrafish penetratingly through machine vision, and the meandering, body angle, top time and 3D trajectory are used for the first time to show the abnormal behaviors induced by 6PPD and its metabolites. Moreover, neurotransmitter changes in the zebrafish brain were measured to explore the causes of abnormal behavior. The results showed that high-dose treatment of 6PPD reduced the velocity by 42.4% and decreased the time at the top of the tank by 91.0%, suggesting significant activity inhibition and anxiety. In addition, γ-aminobutyric acid and acetylcholine were significantly impacted by 6PPD, while dopamine exhibited a slight variation, which can explain the bradykinesia, unbalance and anxiety of zebrafish and presented similar symptoms as Huntingdon's disease. Our study explored new abnormal behaviors of zebrafish induced by 6PPD and its metabolites in detail, and the toxic causes were revealed for the first time by studying the changes of neurotransmitters, thus providing an important reference for further studies of the biological toxicity of 6PPD and its metabolites.

Chiral perspective evaluations: Enantioselective hydrolysis of 6PPD and 6PPD-quinone in water and enantioselective toxicity to Gobiocypris rarus and Oncorhynchus mykiss

As a ubiquitous tire antidegradant, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is persistently released into the environment. It is highly toxic to aquatic organisms, and its transformation product 6PPD-quinone (6PPD-Q), is "very highly toxic" to Oncorhynchus kisutch at a median lethal concentration (LC50) of < 0.1 ng/mL. Notably, 6PPD and 6PPD-Q are chiral compounds. Here, enantioselective evaluations, including hydrolysis and acute toxicity were conducted after preparing the enantiomer, confirming the enantiomer absolute configuration and establishing enantioseparation methods. In the 6PPD hydrolysis experiments, the products 6PPD-Q, phenol, 4-[(1,3-dimethylbutyl)amino]- (4-DBAP) and 4-hydroxydiphenylamine (4-HDPA) were detected. In different water solutions, the hydrolysis of 4-DBAP and 4-HDPA was very fast (0.87-107 h), while the 6PPD-Q hydrolysis half-lives (12.8-16.3 d) were significantly longer than 6PPD (4.83-64.1 h). At the enantiomeric level, no enantioselective hydrolysis and conversion occurred. R-6PPD generated R-6PPD-Q, and S-6PPD generated S-6PPD-Q, and the formation rate of S-6PPD-Q was 1.77 times faster than R-6PPD-Q. In terms of the enantioselective toxicity, the 6PPD enantiomer was highly toxic to China-specific Gobiocypris rarus (LC50, 162-201 ng/mL), and it had no enantioselective difference. 6PPD-Q was "very highly toxic" (LC50, 1.66-4.31 ng/mL) to Oncorhynchus mykiss, which is of commercial importance, and the toxicities of rac-6PPD-Q and S-6PPD-Q were 1.9 and 2.6 times higher than R-6PPD-Q. Furthermore, the formation concentrations of S-6PPD-Q and R-6PPD-Q in 6PPD water solutions were higher than the LC50 values of O. kisutch and O. mykiss, and the toxicity of 6PPD-Q was highly species-specific, which should raise concern. These results provide important information for environmental risk assessments of 6PPD and 6PPD-Q, especially from the perspective of enantiomers.