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

Tire-additive chemicals and their derivatives in urban road dust: Spatial distributions, exposures, and associations with tire and road wear particles

Tires, as a globally utilized product, emit pollutants including various tire-additive chemicals (TACs), and tire and road wear particles (TRWPs) containing substantial microplastics. Several TACs have been proven to have aquatic toxicity, induce health risks including endocrine disruption, allergic effects, and cardiovascular problems, while TRWPs are also very strongly suspected. With the emergence of multiple novel TAC derivatives recently, yet knowledge of their occurrence, spatial distribution, and potential associations remains largely insufficient. In this work, we have systematically investigated 25 TACs, including para-phenylenediamine (PPDs), several newly identified quinones (PPD-Qs) first reported in urban dust, and TRWPs across road dust from Hong Kong and Berlin. By coupling UPLC-MS/MS and pyrolysis-GC/MS analyses, we achieved concurrent quantification of trace TAC derivatives and TRWPs. Distinct spatial patterns of these tire-derived contaminants were identified, in which Hong Kong exhibited higher TACs linked to dense traffic, while the TRWP levels reached 9420 μg/g near auto factory in Berlin. Highway dust contained maximum contaminant loads of both TACs (5.0-fold higher) and TRWPs (5.3-fold higher) compared to other road types. Significantly linear associations were found between emerging TAC derivatives and TRWPs, implying their potential as candidate markers. Exposure assessments revealed PPDs and PPD-Qs intake doses surpassed benzothiazole and its derivatives, highlighting non-negligible risks caused by emerging TAC derivatives. This work establishes a framework for evaluating region-specific emission drivers and provides critical baseline data of multiple tire-related contaminants across different compartments. Potential markers for TRWPs were identified, which may be further utilized for their identification and determination.

Low-dose tire wear chemical 6PPD-Q exposure elicit fatty liver via promoting fatty acid biosynthesis in ICR mice

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q) as a major metabolite of tire wear chemical 6PPD has been demonstrated to be an emerging burden of exposure in human populations, via contamination from drinking water, air particulate matter and food sources. Whilst increasing attention has been moved toward its adverse effect, the potential hepatotoxicity of 6PPD-Q in mammals at realistic dose remains unknown. Here, the toxic effects of 6PPD-Q at environmentally relevant dose on the liver of adult mice and its underlying mechanism were investigated through an integrative approach combining transcriptomic and lipidomic analyses. We found that 6PPD-Q exposure induced excessive lipid deposition following three weeks of exposure, ultimately contributing to the pathogenesis of fatty liver disease. Mechanistically, 6PPD-Q exposure caused a remarkable increase in the contents of fatty acids within the hepatic tissue of mice by enhancing their biosynthesis, thereby facilitating lipid deposition. In summary, this study provides a new understanding on the endocrine disrupting effects of 6PPD-Q on hepatic lipid metabolism and how it may contribute to elevated risk of fatty liver disease. Our findings call for a potential public health attention on the risk assessment of 6PPD-Q, particularly towards the risk of chronic metabolic diseases.

Analysis of 6PPD-Q in finfish, shellfish, and marine mammal tissues

6PPD-quinone (6PPD-Q), a transformation product of tire rubber anti-oxidant 6PPD, has been identified as the primary causal toxicant for the urban runoff mortality syndrome observed in coho salmon (Oncorhynchus kisutch) in the Pacific Northwest, USA. Several other fish species are also vulnerable to 6PPD-Q. However, monitoring efforts on 6PPD-Q have been focused on water, particulate matter, soils, and sediments, while that in tissues remains scarce. This study presents a workflow for extraction and quantitative analysis of 6PPD-Q in complex tissues from shellfish, finfish, and marine mammals. A multi-residue extraction protocol was developed for quantitative analysis of 6PPD-Q and persistent organic pollutants (PCBs, PBDEs, organochlorine pesticides) and PAHs in tissues in a single extraction. A GC-MS/MS based 6PPD-Q measurement was also developed. The protocol was evaluated in tissues including fish fillets, whole fish homogenates, mussels, and whale blubber. Limits of quantification of 6PPD-Q were between 0.03 and 0.12 ng/g ww and the surrogate (6PPD-Q-d5) recoveries were ∼60-100 % among matrices. We also conducted an initial biomonitoring study using caged mussels (Mytilus trossulus) and juvenile Chinook salmon (Oncorhynchus tshawytscha) from Puget Sound, WA. 6PPD-Q detection rates were at least 50 % but the concentrations were mostly <1 ng/g ww. Our protocol will aid 6PPD-Q biomonitoring in aquatic environments and also exposure assessments for improved understanding of 6PPD-Q bioaccumulation potential in these food webs.

Occurrence, fate and chiral signatures of p-phenylenediamines and their quinones in wastewater treatment plants, China

p-Phenylenediamines (PPDs) and their derived quinones (PPD-Qs) are emerging pollutants. Information on their occurrence, fate and chiral signatures in the wastewater treatment plant (WWTP) and its receiving waters is scarce. This study explored the distribution, removal efficiency, mass balance, environmental emission and enantiomeric fractions of these pollutants in two WWTPs in Guangzhou, China. It also examined the impacts of WWTP effluents on the distribution of these contaminants in the receiving rivers. The concentrations of Σ(PPDs+PPD-Qs) ranged from 3.95 to 20.4 ng/L in influent, 0.69-3.94 ng/L in effluent, and 2.14-19.8 ng/g dry weight in sludge, respectively. PPDs (81.6 %) and PPD-Qs (78.6 %) were effectively removed in the WWTPs primarily through biotransformation, but sludge adsorption and separation also contributed to the removal. One of the WWTPs could increase the levels of these pollutants in the downstream of receiving river, suggesting that WWTP effluents are significant vectors of PPDs and PPD-Qs to the aquatic environment. The results also highlight different nonracemic chiral signatures of 6PPD and 6PPD-Q between the two WWTPs and the receiving waters, which merit further investigation for mechanism. Future studies should elucidate the environmental risks that PPDs and PPD-Qs may pose to receiving rivers and accurately assess such effects at the enantiomeric level.

p-Phenylenediamines and their derived quinones: A review of their environmental fate, human exposure, and biological toxicity

p-Phenylenediamines (PPDs) are widely used as antioxidants in numerous rubber products to prevent or delay oxidation and corrosion. However, their derived quinones (PPD-Qs), generated through reactions with ozone, are ubiquitous in the environment and raise significant health and toxicity concerns. This review summarizes the current state of knowledge on environmental distribution and fate, human exposure, and biological toxicity of PPDs and PPD-Qs, and makes recommendations for future research directions. Although PPDs and PPD-Qs have been monitored in a variety of environmental matrices, studies on soil, sediment, and organisms remain limited. This shortcoming hinders our understanding of their distribution patterns and migration mechanisms in these specific environments. These contaminants can enter the human body through various exposure routes, but toxicological studies have not yielded sufficient results to derive risk thresholds for the assessment of human health. Most studies examining biological and toxicological effects have focused on acute exposure scenarios, which do not accurately reflect the long-term interactions that occur in natural settings. The toxic effects of PPDs and PPD-Qs on zebrafish, nematodes, and mammals include neurobehavioral changes, reproductive dysfunction, and digestive damage, which are linked to mitochondrial stress, DNA adduct formation, and disrupted lipid metabolism, respectively. However, the underlying toxicological mechanisms remain poorly understood. Future research should prioritize the investigation of the impacts of PPDs and PPD-Qs on various organizational levels within biota to provide a scientific basis for developing effective risk management measures.

Correlation between 6PPD-Q and immune along with metabolic dysregulation induced liver lesions in outdoor workers

Outdoor workers who are exposed to traffic-derived pollutants often suffer from a range of diseases, with liver disease being particularly notable. Recently, a rubber stabilizing additive antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its transformed-quinone product 6PPD-quinone (6PPD-Q) attracted attention. However, their implication for human health remains inadequately elucidated. In this study, outdoor and indoor workers were recruited to analyze 6PPD and 6PPD-Q distribution in their serum and urine. Simultaneously, blood cell counts, liver function, renal function, blood glucose level, and lipid profile were evaluated by 23 physiological parameters. For the first time, we found that the concentrations of 6PPD (0.54 - 1.66 μg L-1) and 6PPD-Q (0.58 - 4.04 μg L-1) in outdoor group serum were two- and three-fold in the indoor group, respectively. Compared with indoor workers, 18 biochemical parameters, notably total bilirubin and indirect bilirubin, were elevated in outdoor workers (p < 0.05). A computed tomography scan showed liver lesions in 60% of the outdoor group, whereas only 30% of the indoor group. The statistical analysis exhibited that significant positive correlations exist between the serum 6PPD-Q and immune cell counts, total bilirubin, indirect bilirubin, and triglycerides in human beings (p < 0.05). The logistic regression implied that for each 1 μg L-1 increase of 6PPD-Q in serum, the risk of human liver lesions increased by 2.31 times. Our results suggest that outdoor exposure is associated with increased concentrations of 6PPD-Q in serum, which could potentially influence glucose and lipid metabolism, immune cell regulation, and liver health.

Distribution of synthetic antioxidants in outdoor and educational indoor dusts and assessment of human exposure

Antioxidants have been received recognized as contaminants in the environmental field due to the reports of adverse effects. In this study, a total of 35 outdoor dusts and 20 educational indoor dusts were sampled to investigated the occurrence and spatial distribution of antioxidants, and eight antioxidants were positively found in dusts. For the indoor dusts, the total antioxidant concentrations (Σ8Ant) were in the range of 15.7-5282 ng/g, 2,6-di-tert-butyl-4-methylphenol (BHT) was the dominate compound which constituted of 46.4% in the total concentrations of detected antioxidants. The composition profiles of antioxidants (67.9%) in outdoor dusts were different from that in indoor dusts, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) was the main component of all the identified antioxidants. Various microenvironments exhibited different contamination characteristics of antioxidants, the rank order of Σ8Ant was following as: campus roads (average concentration: 35.4 ng/g) < pedestrian streets (59.1 ng/g) < urban roads (92.9 ng/g) < teaching buildings (105 ng/g) < laboratory buildings (530 ng/g) < dormitories (1652 ng/g). Based on the measured Σ8Ant in dusts, we estimated daily intake via dust ingestion to be 3.90e-2 to 9.55e-2 ng/kg BW/day for adults under high exposure scenario. Overall, the occurrence and spatial distribution of antioxidants in outdoor and educational indoor spaces were investigated and the potential risks of detected antioxidants exposure in toddlers and adults were assessed in the present study.

Rubber-derived chemicals in urban sewer networks and receiving waters: Fingerprints, driving factors and ecological impacts

Rubber-derived chemicals (RDCs), which include rubber additives (RAs) and their transformation products (TPs), can be released into aquatic environments when rubber products, such as vehicle tires, are in use or discarded. However, RDCs and associated ecological risks have not been thoroughly investigated inside urban sewer networks and their receiving water bodies. To address these issues, we investigated the RDCs in Hong Kong's municipal sewer networks, including sewage and stormwater, as well as their receiving waters, such as rivers and coastal water. Among 45 target RDCs, the vulcanizing agents and corrosion inhibitors were found to be predominant in the water samples, accounting for 26-66 % and 29-72 % of total concentrations of 45 RDCs (∑45RDC), respectively, while antioxidants and their TPs presented in smaller quantities, accounting for 0.21-26 % and 0-15 % of ∑45RDC, respectively. Ten RAs from five classes were additionally identified by suspect screening. An estimated mass load of ∑45RDC amounting to 1690 kg/month is discharged into the coastal marine environment of Hong Kong, with sewage effluent being the primary source. Population density and vehicle-related factors (e.g., traffic load) were the major drivers shaping the spatial distribution of RDCs in surface water. Based on the ecological risk assessment outcomes, 16 out of 45 RDCs exhibited medium to high risks, and lists of candidate contaminants for various water bodies were proposed to support future risk management in water quality. These findings suggest that RDCs in stormwater and rivers should be carefully monitored, and management strategies should be developed to mitigate their risks.

Elucidating the size distribution of p‑Phenylenediamine-Derived quinones in atmospheric particles

Transformed from p-phenylenediamines (PPDs) antioxidant, PPD-derived quinones (PPD-Qs) have recently been recognized as emerging contaminants due to their potential negative impacts on the environment and human health. While there have been measurements of airborne PPD-Qs, the size distribution of PPD-Qs and the impact of particle size on PPD transformation chemistry remain largely unknown. Here, through the measurements of atmospheric particles in three megacities in China (Beijing, Xi'an, and Hefei), we find that PPD-Qs are widely distributed in these cities. Further analysis of the size-fractioned particles in Hefei indicates that 48 % of PPD-Qs reside in coarse particles. Given that previous studies mainly focus on the measurement of PPD-Qs in fine particles, the previously reported PPD-Q concentrations and the corresponding human exposure dosages are likely to be significantly underestimated. Furthermore, the ratio of PPD-Q to PPD concentration (PPD-Q/PPD) for particles with size range of 0.056 - 0.1 μm is up to 3 times higher than that with size range of 10 - 18 μm, highlighting the key role of particle size in determining the atmospheric oxidation reactivity of PPDs. Model simulations reveal a size-dependent pattern for the estimated concentration of particulate PPD-Qs in human body. In addition, we also demonstrate that PPD-Qs can induce the formation of cellular reactive oxygen species, suggesting that they may pose risks to human health. Overall, our results emphasize the importance of considering the particle size effect when evaluating the reaction potential and exposure risk of airborne PPD-Qs.

Insights into the impact of 6PPD-Q and 6PPD on nitrogen metabolism and microbial community in the anammox system

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is an antioxidant commonly used in tire manufacturing, and its release into the environment has significantly increased due to rapid urbanization. When subjected to ozonation, 6PPD converts into the harmful pollutant 6PPD quinone (6PPDQ). These substances enter wastewater treatment plants (WWTPs) via stormwater runoff and pipelines, posing significant risks to the functional microorganisms. Anammox, a strictly controlled and sensitive microbial nitrogen removal process, is especially susceptible to the effects of the pollutants. This study investigates the comprehensive impact of 6PPD-Q and 6PPD on anammox communities based on characterization analysis and metagenomics. At environmental concentrations, 6PPD-Q at 200 ng/L-1000 ng/L led to the disintegration of anammox granules. Extended exposure to both 6PPD-Q and 6PPD significantly reduces the population of anammox bacteria (AnAOB). By utilizing organic matter from dead cells and incoming carbonate as a carbon source, the system evolved into a nitrogen metabolism network primarily focused on denitrification and dissimilatory nitrate reduction to ammonium (DNRA). This transformation was accompanied by a reshuffling of the microbial community and associated genes, resulting in an accumulation of NH4+-N. These findings underscore the toxicity of 6PPD-Q and 6PPD to anammox and stress the importance of incorporating 6PPD into regulatory and preventive strategies.

Hydrolysis of p-Phenylenediamine Antioxidants: The Reaction Mechanism, Prediction Model, and Potential Impact on Aquatic Toxicity

While p-phenylenediamine antioxidants (PPDs) pose potential risks to aquatic ecosystems, their environmental persistence and transformation remain ambiguous due to the undefined nature of PPD C-N bond hydrolysis. Here, we investigated the hydrolysis patterns of PPDs by analyzing their hydrolysis half-lives, hydrolysis products around neutral pH (pH 6.0-7.7), and the role of atoms within the C-N bonds in PPDs. Hydrolysis preferentially targets the aromatic secondary amine N with the strongest proton affinity and the C atom of C-N with the highest nucleophilic-attack reactivity. The hydrolysis half-life (t1/2) shortens when the maximum proton affinity of N increases. These results are supported by theoretical calculations, demonstrating a hydrolysis reaction propelled by proton transfer from water to N and complemented by aromatic nucleophilic substitution of N in C-N by water hydroxyl. With the experimental results and the atom reactivity-based predictive model, the t1/2 around neutral pH for 60 PPDs (monitored in environment, commercially available, or under investigation) is determined, showing variations ranging from 2.2 h to 47 days. The model prediction of primary C-N hydrolysis is confirmed through typical PPDs. With the elucidated mechanism and developed model, this research provides new insights into PPD hydrolysis, underscoring its significance in delineating environmental impacts.

Environmental occurrence, fate, human exposure, and human health risks of p-phenylenediamines and their quinones

P-phenylenediamine antioxidants (PPDs) are widely used in the rubber industry and their release and transformation in the environment has become one of the current environmental research hotspots. PPDs are readily oxidized in the environment to form quinone transformation products (PPD-Qs), some of which (e.g. 6PPD-Q) have been shown to be highly toxic and persistent in the environment, posing a potential threat to aquatic organisms and ecosystems. The present study provides an overview of the physicochemical properties, environmental distribution, and potential human exposure and toxicological effects of PPDs and PPD-Qs. PPDs and PPD-Qs are found in water, air, dust and soil around the world, and humans are inevitably exposed to them by inhaling, ingesting and through dermal contact. There is growing evidence indicates that PPDs and PPD-Qs are present in human body fluids and tissues, where they are subject to metabolic and transformational processes in the liver and blood. Furthermore, PPDs and PPD-Qs have the potential to induce adverse health effects, including digestive, respiratory, neurotoxic and reproductive toxicity. Nevertheless, there is a paucity of evidence concerning the direct effects of PPDs and PPD-Qs on human health. Consequently, future research should concentrate on this area in order to provide quantitative support for the assessment of the risk posed by PPDs and PPD-Qs to human health.

Rubber additives and relevant oxidation products in groundwater in a central China region: Levels, influencing factors and exposure

This study investigated the presence of rubber additives and relevant oxidation products (RAROPs) in groundwater in central China's aboveground river region. Seven RAROPs were detected, and their levels in shallow groundwater showed a mild decreasing trend from the area near the Yellow River (Avg: 8.49 ng L-1) to the area on the far bank of the Yellow River (Avg: 5.01 ng L-1). In contrast, deep groundwater's RAROPs contents showed a dramatic decrease to only 0.26 ng L-1. The dominant contaminant was found to be N-(1, 3-dimethylbutyl) -N'-phenyl -p-phenylenediamine (6PPD). The vicinity of the garages and car parks was often characterized as contamination hotspots. Correlation analyses further indicated that aquaculture was likely to be a potential pathway for shallow groundwater contaminant inputs. The amount of RAROPs intake by humans through groundwater is nearly 30 times different due to the imbalanced development between urban and rural areas. Children were the most vulnerable to RAROPs. Therefore, human activities (transportation, waste tire storage, water resource allocation and utilization patterns, diversion of Yellow River water to aquaculture ponds) may exacerbate RAROPs pollution in groundwater by leaching contaminants through the surface soil. These results are important for developing appropriate utilization and protection strategies for groundwater resources in developing countries.

p-phenylenediamines (PPDs) and PPD-quinones (PPD-Qs) in human urine and breast milk samples: Urgent need for focus on PPD-Qs and the establishment of health threshold criteria

PPDs and their oxidation products, PPD-Qs, are emerging environmental contaminants arising from the addition and oxidation of rubber products. Although numerous studies have been conducted to elucidate their risks, the primary focus has been on 6PPD and 6PPD-Q, with limited attention given to other PPDs and especially other PPD-Qs. This study comprehensively examines the occurrences of frequently used PPDs and their degradation products, PPD-Qs, in human urine and breast milk samples. The average concentrations of ΣPPDs and ΣPPD-Qs in urine were 27 ± 78 ng/mL and 16 ± 12 ng/mL, respectively. IPPD and DNPD were the predominant PPDs, while DPPD-Q, CPPD-Q, and IPPD-Q were the predominant PPD-Qs. Notably, the concentrations of 6PPD, CPPD, and DPPD were significantly lower than their oxidized quinone products. Weak or no correlations were observed between most PPDs and their corresponding PPD-Qs, suggesting that PPD-Qs in the human body are primarily derived from direct environmental intake rather than in vivo conversion of PPDs. PPDs and PPD-Qs were widely detected in breast milk, exhibiting concentrations and patterns similar to those found in urine, with comparable major pollutants. Estimated daily intakes of PPDs + PPD-Qs for infants were several μg/(kg·day), with the 95th percentile intake approaching 10 μg/(kg·day).

Stress of soil moisture and temperature exacerbates the toxicity of tire wear particles to soil fauna: Tracking the role of additives through host microbiota

Tire wear particles (TWPs) are considered as an emerging threat to soil fauna. However, how TWP toxicity to soil fauna responds to the stress of soil moisture and temperature remains unclear. We assessed the toxicity of environmentally relevant TWPs to the soil model species Enchytraeus crypticus under three soil moisture and two temperature gradients. Typical thermoplastic polypropylene (PP) was selected for comparison. Results showed that compared with PP, TWPs exerted stronger toxicity, including decreasing the worm growth, survival and reproduction rates, disturbing the soil and worm gut microbiota, and leaching more diverse and higher contents of additives. Stress of soil moisture and temperature exacerbated TWP toxicity mainly through affecting the leaching and transformation of additives. Fourteen mediated additives significantly contributed to the shift of the gut microbiota under soil moisture and temperature stress, among which 1,3-diphenylguanidine, N,N'-bis(methylphenyl)-1,4-benzenediamine quinone, N-tert-butyl-2-benzothiazolesulfenamide, and 2-aminobenzothiazole were identified as the main drivers. In addition, this study provided the first clear evidence that increased soil moisture and temperature promoted the transformation of additives in the soil. Our study revealed the non-negligible aggravated toxicity of TWPs to soil fauna under stress of soil moisture and temperature, providing novel insights into the environmental behavior of additives.

Unveiling spatiotemporal distribution, partitioning, and transport mechanisms of tire additives and their transformation products in a highly urbanized estuarine region

Numerous tire additives are high-production volume chemicals that are used extensively worldwide. However, their presence and partitioning behavior remain largely unknown, particularly in marine environments. This study is the first to reveal the spatiotemporal distribution, multimedia partitioning, and transport processing of 22 tire additives and their transformation products (TATPs) in a highly urbanized estuary (n = 166). Nineteen, 18, and 20 TATPs were detectable in water, suspended particulate matter (SPM), and sediments, respectively, with total levels of 59.7-2021 ng/L, 164-6935 ng/g, and 4.66-58.4 ng/g, respectively. The multimedia partitioning mechanisms of TATPs are governed by their molecular weight, hydrophobicity, and biodegradation rate. Mass inventories coupled with model simulations have revealed that substantial quantities of TATPs accumulate within estuarine environments, and these compounds can be continuously transported into the ocean, particularly during the wet season. According to the multi-criteria evaluation approach, four and three TATPs were identified as high-priority pollutants during the dry and wet seasons, respectively. Unexpectedly, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone was only listed as a medium-priority pollutant. This study underscores the importance of marine surveillance and advocates for particular attention to these ubiquitous but underexplored TATPs in future studies.

Occurrence and risk assessment of p-phenylenediamines and their quinones in aquatic environment: From city wastewater to deep sea

p-Phenylenediamines (PPDs) and PPD-derived quinones (PPD-Qs) have been considered emerging pollutants recently. Their available data on sediment and sewage sludge are limited, especially the ecological risks. Here, typical PPDs and PPD-Qs were measured in the sludge of wastewater treatment plants and surface sediment of a developed river basin (including reservoirs, estuaries, and rivers) and deep-sea troughs. The total concentrations of PPDs (∑PPD) were highest in sludge (range: 9.06-248 ng g-1), followed by surface sediment of the Dongjiang River basin, China (3.33-85.3 ng g-1), and lowest in sediment of the Okinawa Trough (0.01-7.46 ng g-1). The median value of ∑PPD in surface sediment of rivers (9.54 ng g-1) was higher than those in reservoirs (4.28 ng g-1) and estuaries (5.26 ng g-1). N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) was the major congener in all samples, accounting for over 60 % of ∑PPD. For quinones, 6PPD-Q and IPPD-Q were frequently detected in sludge, only trace 6PPD-Q was detected in the sediment of estuaries (nd-0.62 ng g-1) and rivers (nd-5.24 ng g-1), and both of them were absent from the sediment of the Okinawa Trough. The occurrence of PPDs in the trough may be the in-situ release of microplastics, and due to the low-light and weak alkaline conditions of deep-sea water, quinones may hardly photodegrade from PPDs. The PPD concentrations in sludge were positively correlated with local GDP, and the annual PPD emission from sludge will exceed 1370 kg in China. The results of ecological risk assessments indicated low risks for PPDs in sludge-amended soil, median risks for several PPDs in river sediment, but median to high risks for 6PPD-Q contamination sludge-amended soil. For the first time, we explored the potential environmental risk of PPDs and related quinones in sludge used as a soil conditioner.

Unpuzzling spatio-vertical and multi-media patterns of aniline accelerators/antioxidants in an urban estuary

Aniline accelerators and antioxidants (AAs) are high-production-volume industrial additives that have recently attracted emerging concern given their ubiquity in environmental compartments and the associated (eco)toxic effects. Nonetheless, available information on the multi-media behavior of AAs and their transformation products (TPs) remains scarce. Therefore, we determined the residues of twenty-four AA(TP)s in paired dissolved phases (i.e., filtered water), suspended particulate matter (SPM), and sediment samples collected from the Yangtze River Estuary (YRE), a highly urbanized estuary in the East China. The median total concentrations of targeted compounds were 0.73 ng/g dw, 34.4 ng/L, and 39.6 ng/L in sediments, surface and bottom water, respectively. Diphenylamine (DPA) was the most abundant congener in SPM, while 1,3-diphenylguanidine (DPG) and dicyclohexylamine (DChA) dominated in the dissolved phases and sediments. Various anthropogenic emissions and (a)biotic degradation may collectively shape the matrix-specific accumulation patterns and spatial trends of these compounds across the YRE. However, the vertical patterns of AA(TP)s were obscure, probably due to the estuarine hydrodynamics and/or the modest sample size. The SPM fractions of AA(TP)s in water (Ф: 7.9-100%) and the sediment sorption coefficients (KOC: 0.01-6.56) both positively correlated with their hydrophobicity as indicated by the octanol-water partition coefficient (KOW). Moreover, risk quotients implied moderate to high aquatic toxicity posed by several AA(TP)s at certain YRE sites. The estimated total annual fluxes of our analytes transported via water and sediments towards the East China Sea were 5.90-365.5 tons and 4.23-1,100 kg, respectively. This work provides a systematic investigation of multi-media processes and ecological risks of AA(TP)s in a highly-urbanized estuary, contributing to holistic comprehension of these emerging contaminants in estuarine environments.

Development and application of diffusive gradients in thin-films for in-situ monitoring of 6PPD-Quinone in urban waters

The occurrence and risk of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q), derived from the oxidation of the tire antidegradant 6PPD, has raised significant concern since it was found to cause acute mortality in coho salmon when exposed to urban runoff. Given the short half-life period and low solubility of 6PPD-Q, reliable in situ measurement techniques are required to accurately understand its occurrence and behaviour in aquatic environments. Here, using the diffusive gradients in thin-films (DGT) method with HLB as a binding agent, we developed a new methodology to measure 6PPD-Q in urban waters. 6PPD-Q was rapidly and strongly adsorbed on the HLB-binding gel and was efficiently extracted using organic solvents. The HLB-DGT accumulated 6PPD-Q linearly for >7 d and its performance was not significantly affected by pH (6.5-8.5), ionic strength (0.0001-0.5 M) or dissolved organic matter (0-20 mg L-1). Field evaluation of the DGT method demonstrated its effectiveness in urban runoff, detecting 6PPD-Q levels of 15.8-39.5 ng L-1 in rivers. In snowmelt, DGT detected 6PPD-Q levels of 210 ng L-1 which is two times higher than the value obtained by grab sampling. 6PPD-Q levels were much higher in snowmelt than those in rivers. This indicates that snowfall constitutes an important transport pathway for 6PPD-Q and that DGT effectively captured the fraction continuously released from dust particles in the snow samples. 6PPD-Q posed a substantial risk to migratory fish in urban waters, and its release from tire wear particles requires further investigation. This study is the first to develop a DGT-based method for 6PPD-Q determination in urban waters, and the method can ensure an accurate measurement of the release of 6PPD-Q to the environment, particularly in rainfall or snowmelt, important pathways for its entry into the aquatic environment.

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.

Potential human health risk of the emerging environmental contaminant 6-PPD quinone

The tire antioxidant 6-PPD has been widely used to enhance tire performance and extend tire lifespan. 6-PPD quinone (6-PPDQ), a quinone derivative derived from 6-PPD in the presence of ozone, has been recognized an emerging environmental contaminant. In addition to causing acute lethality to coho salmon, 6-PPDQ exhibits toxic effects on other aquatic species and mammals. Based on the existing evidence, we provide a critical overview on the human internal exposure, potential adverse effects on health, and prediction of human health risk of 6-PPDQ. 6-PPDQ could be detected in human samples, including human urine, blood, and cerebrospinal fluid. Human exposure to 6-PPDQ in the environment is inevitable and may lead to adverse health effects, including hepatotoxicity, enterotoxicity, pulmonary toxicity, neurotoxicity, reproductive toxicity, and cardiotoxicity. Additionally, potential human health risk to 6-PPDQ through exposure routes and human samples were predicted. This review is helpful to identify the existing knowledge gaps and future research directions regarding the human health effects of 6-PPDQ.

Nationwide occurrence and prioritization of tire additives and their transformation products in lake sediments of China

As a group of emerging contaminants of global concern, tire additives and their transformation products (TATPs) are causing a severe threat to aquatic ecosystems, particularly the highly lethal effects of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q) on certain fish species. Yet, the contamination status of TATPs in the lake ecosystems remains largely uncharacterized. This study conducted the first nationwide monitoring of the distribution characteristics of TATPs in 208 lake sediments collected from five lake regions across China. All the 13 TATPs were identified in lake sediments, with the total levels varying between 1.4 and 1355 ng/g, and 4-hydroxydiphenylamine (4-OH-PPD) as the most dominant. The total levels of TATPs decreased in the following order: Yunnan-Guizhou Plateau > Inner Mongolia-Xinjiang Region, Eastern Plain > Qinghai-Tibet Plateau, and Northeast Plain (p < 0.05). The geographical distribution of TATPs in lake sediments was significantly driven by total organic carbon content, temperature, and population density. N,N'-di-2-naphthyl-p-phenylenediamine, 6PPD-Q, N,N'-diphenyl-p-phenylenediamine, and 4-OH-PPD belonged to high-priority contaminants. Our study emphasizes that emerging pollutant TATPs place significant pressure on lake ecosystems and deserve urgent attention.

Typical Tire Additives in River Water: Leaching, Transformation, and Environmental Risk Assessment

Tire wear particles (TWPs) released during vehicle driving can enter water bodies, leading to leaching of tire additives (TAs) in aquatic environments. However, the transformation behavior and related ecological impacts of TAs and their transformation products (TPs) remain unclear. In this study, laboratory-based simulation experiments and field investigations were conducted to explore the transformation mechanisms and ecological risks of TAs. After being placed in river water for 24 h, about 7-95% of 12 investigated TAs in TWPs were leached. Forty-eight TPs from eight TAs were tentatively identified along with different transformation pathways via suspect screening by high-resolution mass spectrometry. Semiquantitative results indicated that TPs derived from N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylene-diamine (6PPD) were predominant in leachates, while aryl hydrolysis and quinone pathways were the main transformation pathways. Field investigations on urban surface water samples from 16 sites in Hong Kong revealed the occurrence of 17 TAs and 1 TP, with concentrations ranging from 13.9 to 2230 ng/L (median ± standard deviation: 226 ± 534 ng/L). Sixteen TPs from six TAs were additionally identified via suspect screening. It is estimated that 6PPD-quinone and seven TAs could pose medium to high ecological risk, while N-(1,3-dimethylbutyl)-N'-phenyl-p-quinonediimine, a frequently detected TP, was identified as a persistent-bioaccumulative-toxic substance.

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.

Protective role of ghrelin against 6PPD-quinone-induced neurotoxicity in zebrafish larvae (Danio rerio) via the GHSR pathway

The toxicity mechanisms of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q), an antioxidant derivative of 6PPD via ozone reaction commonly used in rubber and tire industries, were investigated in zebrafish larvae with concentrations ranging from 0 to 50 μg/L. Despite normal hatchability, 6PPD-Q exposure led to reduced body length and swimming distance in 120 hours post-fertilization (hpf) larvae. At the highest concentration (50 μg/L), 6PPD-Q significantly impaired dopaminergic neuron development and neurotransmitter levels, including dopamine, 5-hydroxytryptamine, and glutamate. Transcriptome profiling unveiled perturbations in growth and developmental gene expression, such as upregulation of runx2a, runx2b, and ghrl (ghrelin and obestatin prepropeptide), and downregulation of stat1b, auto1, and cidea. Notably, anamorelin, a growth hormone secretagogue receptor (GHSR) agonist, recovered the behavioral deficits induced by 6PPD-Q, implying a neuroprotective role of ghrelin possibly mediated via the ghrelin/GHSR pathway. Collectively, our findings indicate that ghrelin upregulation may counteract 6PPD-Q toxicity in zebrafish larvae, shedding light on potential therapeutic avenues for mitigating the adverse effects of this antioxidant byproduct.

Precipitation contributes to alleviating pollution of rubber-derived chemicals in receiving watersheds: Combining confluent stormwater runoff from different functional areas

The release of rubber-derived chemicals (RDCs) in road surface runoff has received significant attention. Urban surface runoff is often the confluence of stormwater runoff from specific areas. However, the impact of precipitation on RDCs contamination in confluent stormwater runoff and receiving watersheds remains poorly understood. Herein, we investigated the profiles of RDCs and their transformation products in confluent stormwater runoff and receiving rivers affected by precipitation events. The results showed that 34 RDCs are ubiquitously present in confluent stormwater runoff and surface water, with mean concentrations of 1.03-2749 and 0.28-436 ng/L, respectively. The most dominant target compounds in each category were N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), 6PPD-quinone, 2-benzothiazolol, and 1,3-diphenylguanidine. Total RDCs concentrations in confluent stormwater runoff decreased spatially from industrial areas to business districts to college towns. A significant decrease in RDCs levels in surface water after rainfall was observed (P < 0.01), indicating that precipitation contributes to alleviating RDCs pollution in receiving watersheds. To our knowledge, this is the first report of N,N'-ditolyl-p-phenylenediamine quinone (DTPD-Q) levels in surface waters in China. The annual mass load of ∑RDCs reached 72,818 kg/y in confluent stormwater runoff, while 38,799 kg/y in surface water. The monitoring of confluent stormwater runoff is an efficient measure for predicting contamination loads from RDCs in rivers. Risk assessment suggested that most RDCs posed at least medium risks to aquatic organisms, especially 6PPD-quinone. The findings help to understand the environmental fate and risks of RDCs in the confluent stormwater runoff and receiving environments after precipitation events.

Nanospray Laser-Induced Plasma Ionization Mass Spectrometry for Sensitive and High-Coverage Analysis of Broad Polarity Organic Pollutants

Accurate, sensitive, and high-coverage analysis of various organic pollutants in complex samples and single cells is significant in investigating their environmental behaviors and toxic effects. Here we proposed a nanospray laser-induced plasma ionization mass spectrometry (nLIPI-MS) method for sensitive and high-coverage analysis of broad polarity organic pollutants under ambient and open-air conditions. The nLIPI-MS method combines nanospray with laser-induced plasma ionization, enabling direct analysis without sophisticated sample pretreatment. For the analysis of nonpolar and very weakly polar organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) and alkyl-PAHs, the nLIPI-MS method showed desirable analytical performance, with limits of detection of as low as the μg/L level. For moderately polar organic pollutants such as p-phenylenediamine quinones (PPD-Qs), nLIPI-MS displayed significant enhanced sensitivity by 1 to 2 orders of magnitude in comparison to nanoelectrospray ionization (nESI)-MS. For more polar organic pollutants such as per- and polyfluoroalkyl substances (PFASs), nLIPI-MS also showed good analytical performance, with a sensitivity improvement of 1.3- to 2.7-fold over that of nESI-MS. The sensitivity of nLIPI-MS for the analysis of PPD-Qs and PFASs reached as low as the ng/L level, enabling the analysis of ultratrace levels of these pollutants in complex samples and even single cells. Our experimental results demonstrated that nLIPI-MS was an extensive ambient MS method, showing desirable analytical performance for the analysis of organic pollutants with broad polarity ranges.

Neurotoxicity from long-term exposure to 6-PPDQ: Recent advances

The recent acceleration of industrialization and urbanization has brought significant attention to N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ), an emerging environmental pollutant from tire wear, due to its long-term effects on the environment and organisms. Recent studies suggest that 6-PPDQ can disrupt neurotransmitter synthesis and release, impact receptor function, and alter signaling pathways, potentially causing oxidative stress, inflammation, and apoptosis. This review investigates the potential neurotoxic effects of prolonged 6-PPDQ exposure, the mechanisms underlying its cytotoxicity, and the associated health risks. We emphasize the need for future research, including precise exposure assessments, identification of individual differences, and development of risk assessments and intervention strategies. This article provides a comprehensive overview of 6-PPDQ's behavior, impact, and neurotoxicity in the environment, highlighting key areas and challenges for future research.

Tire-derived contaminants 6PPD and 6PPD-Q: Analysis, sample handling, and reconnaissance of United States stream exposures

The environmental ubiquity of tire and road wear particles (TRWP) underscores the need to understand the occurrence, persistence, and environmental effects of tire-related chemicals in aquatic ecosystems. One such chemical is 6PPD-quinone (6PPD-Q), a transformation product of the tire antioxidant 6PPD. In urban stormwater runoff 6PPD-Q can exceed acute toxicity thresholds for several salmonid species and is being implicated in significant coho salmon losses in the Pacific Northwest. There is a critical need to understand the prevalence of 6PPD-Q across watersheds to identify habitats heavily affected by TRWPs. We conducted a reconnaissance of 6PPD and 6PPD-Q in surface waters across the United States from sites (N = 94) with varying land use (urban, agricultural, and forested) and streamflow to better understand stream exposures. A rapid, low-volume direct-inject, liquid chromatography mass spectrometry method was developed for the quantitation of 6PPD-Q and screening for 6PPD. Laboratory holding times, bottle material, headspace, and filter materials were investigated to inform best practices for 6PPD-Q sampling and analysis. Glass bottles with PTFE-lined caps minimized sorption and borosilicate glass fiber filters provided the highest recovery. 6PPD-Q was stable for at least 5 months in pure laboratory solutions and for 75 days at 5 °C with minimal headspace in the investigated surface water and stormwaters. Results also indicated samples can be frozen to extend holding times. 6PPD was not detected in any of the 526 analyzed samples and there were no detections of 6PPD-Q at agricultural or forested sites. 6PPD-Q was frequently detected in stormwater (57%, N = 90) and from urban impacted sites (45%, N = 276) with concentrations ranging from 0.002 to 0.29 μg/L. The highest concentrations, above the lethal level for coho salmon, occurred during stormwater runoff events. This highlights the importance of capturing episodic runoff events in urban areas near ecologically relevant habitat or nursery grounds for sensitive species.

Unraveling the fate of 6PPD-Q in aquatic environment: Insights into formation, dissipation, and transformation under natural conditions

The widespread occurrence of N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q) in aquatic environments and its hazards to aquatic species underscore the necessity of comprehending its environmental fate. Here, we investigated the transformation from 6PPD to 6PPD-Q and the attenuation of 6PPD-Q in surface water under natural conditions. Contrary to prior findings, this work revealed that 6PPD-Q and its precursor 6PPD-OH/6PPD-(OH)2, were not detected through target analysis and suspect screening during 6PPD transformation in the surface water under the natural conditions. 6PPD-Q predominantly accumulated in TWPs in ambient atmosphere with 1.28 % mass yield from the 6PPD dissipation. Subsequently, 6PPD-Q was eluted from TWPs and released to the water environment. The investigation on the natural attenuation of 6PPD-Q in the surface water demonstrated that direct and indirect photolysis facilitated the rapid dissipation of 6PPD-Q with a half-life of 2.57 h. Utilizing the liquid chromatography high resolution mass spectrometry (LC-HRMS), including both time of flight (TOF) MS and Orbitrap MS, twelve novel transformation products (TPs) of 6PPD-Q were identified by using a comprehensive non-targeted screening strategy. The results from two dimensions gas chromatography (GC×GC) TOF-MS revealed additional two TPs. Based on the molecular structure of TPs, four major pathways of 6PPD-Q attenuation were proposed, including bond cleavage, hydroxylation, quinone cleavage and rearrangement. All TPs were predicted to exhibit lower toxicity, indicating the natural attenuation of 6PPD-Q reduced its toxicity and potential environmental risks. This study provides crucial insights into the environmental fate of 6PPD-Q, highlighting the significance of understanding both its formation from 6PPD and its subsequent attenuation processes under natural conditions.

Synthesis of a cysteine functional covalent organic framework via facile click reaction for the efficient solid phase extraction of substituted p-phenylenediamine-derived quinones

N,N'-Substituted p-phenylenediamine quinones (PPD-Qs) are the emerging toxicant, which transform from the rubber tire antioxidant N,N'-substituted p-phenylenediamines (PPDs). Because of their potential toxic and widespread occurrence in the environment, PPD-Qs have received great attention. However, efficiently extracting PPD-Qs from complex samples is still a challenge. Herein, a cysteine functional covalent organic framework (Cys-COF) designed according to the "donor-acceptor" sites of hydrogen bonding of PPD-Qs was synthesized via click reaction and then used as solid-phase extraction (SPE) adsorbent. Cys-COF can form the seven-member ring adsorption structure with PPD-Qs via hydrogen bonding. The adsorption mechanism was tentatively revealed by density functional theory (DFT). After optimizing the Cys-COF-SPE parameters, PPD-Qs were efficiently extracted from water, soil, sediment, and fish, followed by detection using ultra-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). The Cys-COF-SPE-UHPLC-MS/MS method exhibited ideal linearity (R2 ≥ 0.9932), high relative recoveries (80.4-111 %), and low limits of detection (0.0001-0.0013 ng mL-1). In addition, the bioconcentration kinetics in goldfish provides a feasible platform to investigate the toxicity and accumulated ability of PPD-Qs.

Neurotoxicity and accumulation of CPPD quinone at environmentally relevant concentrations in Caenorhabditis elegans

CPPD quinone (CPPDQ) is a member of PPDQs, which was widely distributed in different environments. Using Caenorhabditis elegans as an animal model, we here examined neurotoxicity and accumulation of CPPDQ and the underlying mechanism. After exposure to 0.01-10 μg/L CPPDQ, obvious body accumulation of CPDDQ was detected. Meanwhile, exposure to CPPDQ (0.01-10 μg/L) decreased head thrash, body bend, and forward turn, and increased backward turn. Nevertheless, only exposure to 10 μg/L CPPDQ induced neurodegeneration in GABAergic system. Exposure to CPPDQ (0.01-10 μg/L) further decreased expressions of daf-7 encoding TGF-β ligand, jnk-1 encoding JNK MAPK, and mpk-1 encoding ERK MAPK. Additionally, among examined G protein-coupled receptor (GPCR) genes, exposure to CPPDQ (0.01-10 μg/L) decreased dcar-1 expression and increased npr-8 expression. RNAi of daf-7, jnk-1, mpk-1, and dcar-1 resulted in susceptibility, and nhr-8 RNAi caused resistance to CPPDQ neurotoxicity and accumulation. Moreover, in CPPDQ exposed nematodes, RNAi of dcar-1 decreased jnk-1 and mpk-1 expressions, and RNAi of npr-8 increased mpk-1 expression. Therefore, exposure to CPPDQ potentially resulted in neurotoxicity by inhibiting TGF-β, JNK MAPK, and ERK MAPK signals. The inhibition in JNK MAPK and ERK MAPKs signals in CPPDQ exposed nematodes was further related to alteration in GPCRs of DCAR-1 and NHR-8 in nematodes.

Comparison of intestinal toxicity in enhancing intestinal permeability and in causing ROS production of six PPD quinones in Caenorhabditis elegans

As the derivatives of p-phenylenediamines (PPDs), PPD quinones (PPDQs) have received increasing attention due to their possible exposure risk. We compared the intestinal toxicity of six PPDQs (6-PPDQ, 77PDQ, CPPDQ, DPPDQ, DTPDQ and IPPDQ) in Caenorhabditis elegans. In the range of 0.01-10 μg/L, only 77PDQ (10 μg/L) moderately induced the lethality. All the examined PPDQs at 0.01-10 μg/L did not affect intestinal morphology. Different from this, exposure to 6-PPDQ (1-10 μg/L), 77PDQ (0.1-10 μg/L), CPPDQ (1-10 μg/L), DPPDQ (1-10 μg/L), DTPDQ (1-10 μg/L), and IPPDQ (10 μg/L) enhanced intestinal permeability to different degrees. Meanwhile, exposure to 6-PPDQ (0.1-10 μg/L), 77PDQ (0.01-10 μg/L), CPPDQ (0.1-10 μg/L), DPPDQ (0.1-10 μg/L), DTPDQ (1-10 μg/L), and IPPDQ (1-10 μg/L) resulted in intestinal reactive oxygen species (ROS) production and activation of both SOD-3::GFP and GST-4::GFP. In 6-PPDQ, 77PDQ, CPPDQ, DPPDQ, DTPDQ, and/or IPPDQ exposed nematodes, the ROS production was strengthened by RNAi of genes (acs-22, erm-1, hmp-2, and pkc-3) governing functional state of intestinal barrier. Additionally, expressions of acs-22, erm-1, hmp-2, and pkc-3 were negatively correlated with intestinal ROS production in nematodes exposed to 6-PPDQ, 77PDQ, CPPDQ, DPPDQ, DTPDQ, and/or IPPDQ. Therefore, exposure to different PPDQs differentially induced the intestinal toxicity on nematodes. Our data highlighted potential exposure risk of PPDQs at low concentrations to organisms by inducing intestinal toxicity.

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.

In Vitro and In Vivo Biotransformation Profiling of 6PPD-Quinone toward Their Detection in Human Urine

The antioxidant N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its oxidized quinone product 6PPD-quinone (6PPD-Q) in rubber have attracted attention due to the ecological risk that they pose. Both 6PPD and 6PPD-Q have been detected in various environments that humans cohabit. However, to date, a clear understanding of the biotransformation of 6PPD-Q and a potential biomarker for exposure in humans are lacking. To address this issue, this study presents a comprehensive analysis of the extensive biotransformation of 6PPD-Q across species, encompassing both in vitro and in vivo models. We have tentatively identified 17 biotransformation metabolites in vitro, 15 in mice in vivo, and confirmed the presence of two metabolites in human urine samples. Interestingly, different biotransformation patterns were observed across species. Through semiquantitative analysis based on peak areas, we found that almost all 6PPD-Q underwent biotransformation within 24 h of exposure in mice, primarily via hydroxylation and subsequent glucuronidation. This suggests a rapid metabolic processing of 6PPD-Q in mammals, underscoring the importance of identifying effective biomarkers for exposure. Notably, monohydroxy 6PPD-Q and 6PPD-Q-O-glucuronide were consistently the most predominant metabolites across our studies, highlighting monohydroxy 6PPD-Q as a potential key biomarker for epidemiological research. These findings represent the first comprehensive data set on 6PPD-Q biotransformation in mammalian systems, offering insights into the metabolic pathways involved and possible exposure biomarkers.

Tracking Aromatic Amines from Sources to Surface Waters

This review examines the environmental occurrence and fate of aromatic amines (AAs), a group of environmental contaminants with possible carcinogenic and mutagenic effects. AAs are known to be partially responsible for the genotoxic traits of industrial wastewater (WW), and AA antioxidants are acutely toxic to some aquatic organisms. Still, there are gaps in the available data on sources, occurrence, transport, and fate in domestic WW and indoor environments, which complicate the prevention of adverse effects in aquatic ecosystems. We review key domestic sources of these compounds, including cigarette smoke and grilled protein-rich foods, and their presence indoors and in aquatic matrices. This provides a basis to evaluate the importance of nonindustrial sources to the overall environmental burden of AAs. Appropriate sampling techniques for AAs are described, including copper-phthalocyanine trisulfonate materials, XAD resins in solid-phase extraction, and solid-phase microextraction methods, which can offer insights into AA sources, transport, and fate. Further discussion is provided on potential progress in the research of AAs and their behavior in an aim to support the development of a more comprehensive understanding of their effects and potential environmental risks.

Environmental profiles, hazard identification, and toxicological hallmarks of emerging tire rubber-related contaminants 6PPD and 6PPD-quinone

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is commonly used in rubber compounds as antioxidants to protect against degradation from heat, oxygen, and ozone exposure. This practice extends the lifespan of rubber products, including tires, by preventing cracking, aging, and deterioration. However, the environmental consequences of waste generated during rubber product use, particularly the formation of 6PPD-quinone (6PPD-Q) through the reaction of 6PPD with ozone, have raised significant concerns due to their detrimental effects on ecosystems. Extensive research has revealed the widespread occurrence of 6PPD and its derivate 6PPD-Q in various environmental compartments, including air, water, and soil. The emerging substance of 6PPD-Q has been shown to pose acute mortality and long-term hazards to aquatic and terrestrial organisms at concentrations below environmentally relevant levels. Studies have demonstrated toxic effects of 6PPD-Q on a range of organisms, including zebrafish, nematodes, and mammals. These effects include neurobehavioral changes, reproductive dysfunction, and digestive damage through various exposure pathways. Mechanistic insights suggest that mitochondrial stress, DNA adduct formation, and disruption of lipid metabolism contribute to the toxicity induced by 6PPD-Q. Recent findings of 6PPD-Q in human samples, such as blood, urine, and cerebrospinal fluid, underscore the importance of further research on the public health and toxicological implications of these compounds. The distribution, fate, biological effects, and underlying mechanisms of 6PPD-Q in the environment highlight the urgent need for additional research to understand and address the environmental and health impacts of these compounds.

Widespread occurrence of two typical N, N'-substituted p-phenylenediamines and their quinones in humans: Association with oxidative stress and liver damage

While N, N'-substituted p-phenylenediamines (PPDs) and their quinone derivatives (PPDQs) have been widely detected in the environment, there is currently limited data on their occurrence in humans. In this study, we conducted the first serum analysis of two PPDs and PPDQs in the healthy and secondary nonalcoholic fatty liver disease (S-NAFLD) cohorts in South China. The concentrations of four oxidative stress biomarkers (OSBs), namely, 8-iso-prostaglandin F2α (8-PGF2α), 11β-prostaglandin F2α (11-PGF2α), 15(R)-prostaglandin F2α (15-PGF2α), and 8-hydroxy-2'-deoxyguanosine in serum samples were also measured. Results showed that N-(1,3-dimethybutyl)-N'-phenyl-p-phenylenediamine (6PPD) quinone was the predominant target analytes both in the healthy and S-NAFLD cohorts, with the median concentrations of 0.13 and 0.20 ng/mL, respectively. Significant (p < 0.05) and positive correlations were found between 6PPD concentration and 8-PGF2α, 11-PGF2α, and 15-PGF2α in both the healthy and S-NAFLD cohorts, indicating that 6PPD may be associated with lipid oxidative damage. In addition, concentrations of 6PPD in serum were associated significantly linked with total bilirubin (β = 0.180 μmol/L, 95%CI: 0.036-0.396) and direct bilirubin (DBIL, β = 0.321 μmol/L, 95%CI: 0.035-0.677) related to hepatotoxicity. Furthermore, 8-PGF2α, 11-PGF2α, and 15-PGF2α mediated 17.1%, 24.5%, and 16.6% of 6PPD-associated DBIL elevations, respectively. Conclusively, this study provides novel insights into human exposure to and hepatotoxicity assessment of PPDs and PPDQs.

First Evidence of the Bioaccumulation and Trophic Transfer of Tire Additives and Their Transformation Products in an Estuarine Food Web

The discovery of the significant lethal impacts of the tire additive transformation product N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q) on coho salmon has garnered global attention. However, the bioaccumulation and trophic transfer of tire additives and their transformation products (TATPs) within food webs remain obscure. This study first characterized the levels and compositions of 15 TATPs in the Pearl River Estuary, estimated their bioaccumulation and trophic transfer potential in 21 estuarine species, and identified priority contaminants. Our observations indicated that TATPs were prevalent in the estuarine environment. Eight, six, seven, and 10 TATPs were first quantified in the shrimp, sea cucumber, snail, and fish samples, with total mean levels of 45, 56, 64, and 67 ng/g (wet weight), respectively. N,N'-Diphenyl-p-phenylenediamine (DPPD) and N,N'-bis(2-methylphenyl)-1,4-benzenediamine (DTPD) exhibited high bioaccumulation. Significant biodilution was only identified for benzothiazole, while DPPD and DTPD displayed biomagnification trends based on Monte Carlo simulations. The mechanisms of bioaccumulation and trophodynamics of TATPs could be explained by their chemical hydrophobicity, molecular mass, and metabolic rates. Based on a multicriteria scoring technique, DPPD, DTPD, and 6PPD-Q were characterized as priority contaminants. This work emphasizes the importance of biomonitoring, particularly for specific hydrophobic tire additives.