Dilution of concentrations of PAHs from atmospheric particles, bulk deposition to soil: a review

Machine learning models reveal how polycyclic aromatic hydrocarbons influence environmental bacterial communities

Polycyclic aromatic hydrocarbons (PAHs) are harmful and widespread pollutants in the environment, posing an ecological threat. However, exploring the influence of PAHs on environmental bacterial communities in different habitats (soil, water, and sediment) remains a major challenge. We collected and reanalyzed 1924 16S rRNA sequencing samples to determine the effects of PAHs on bacterial communities in different habitats and used machine learning to predict potential degrading bacteria. It was found that PAHs had substantial effects on the bacterial community, and that the bacterial community structure changed differently in different habitats. PAH contamination decreased the relative abundance of Proteobacteria in the soil (16.3 %) and sediment (10.1 %), whereas the abundance of Proteobacteria in water increased by 20.2 %. Among the tested models, the random forest model best identified the effects of PAHs on bacterial groups, with an accuracy of 99.51 % for soil, 97.72 % for sediment, and 100 % for water at the genus level. Using the random forest model, we identified 70 biomarkers that respond to PAHs, including potentially degrading microorganisms such as A4b, Bacillus, Flavobacterium and Polynucleobacter. Furthermore, PAH contamination did not significantly alter the functions of bacterial communities in the environment. This study provides a candidate strain set for future screening of PAH-degrading bacteria and contributes to the study of the adaptability of engineered PAH-degrading bacteria to the environment.

Distribution of carcinogenic polycyclic aromatic hydrocarbons in urban soil across major cities of Bihar, India: seasonal variation, source apportionment, and health risk assessment

This study investigates the distribution of sixteen priority polycyclic aromatic hydrocarbons (PAHs), various sources of PAHs, and their probable cancer-causing risks in the soil samples collected from urban cities in Bihar, India. During the winter season, the ∑16 PAH concentration was dominant and ranged from 979.36 to 5149.37 ngg-1 with a mean value of 2684.79 ngg-1, while for the summer season, it ranged from 690.06 to 4539.55 ngg-1 with a mean value of 2194.31 ngg-1. The (4-ring) PAH compounds were the major contributors, accounting for 40% and 37% in the winter and summer seasons, respectively followed by (5- and 6-ring) PAHs at 30% and 32%, and (2- and 3-ring) PAHs at 29% and 30% in the respective seasons. Carcinogenic PAHs constituted ~ 50% of the ∑16 PAHs, with mean values of 1353.97 ngg-1 and 1098.09 ngg-1 for the winter and summer seasons, respectively. Positive matrix factorization (PMF) confirmed the dominance of fossil fuel burning and biomass burning as a primary source in the urban soil of Bihar. Total mean benzo(a)pyrene equivalent (BaPeq) values for the ∑16 PAHs were 312.04 ngg-1 for the winter season and 262.83 ngg-1 for the summer season. These values were higher in current study sites as compared with other studies. However, the concentration range fell within the limit set by the Canadian soil quality standard (700.00 ngg-1) and exceeded the limit of the Dutch target value (32.96 ngg-1). The Incremental Lifetime Cancer Risk (ILCRs) from dermal and ingestion pathways were approximately 104 to 105 times lower than the inhalation pathway, suggesting greater risk. The study revealed higher mean cancer risk values for children (1.16 × 10-5) and adults (1.03 × 10-5) in the winter season, falling within the unacceptable range (10-6 and 10-4) of carcinogenic risk that might lead to human health risk in the study sites.

Increasing contamination of polycyclic aromatic hydrocarbons in Chinese soils

China is facing a serious threat PAHs contaminated soil. To better understand the current state of soil PAH pollution in China and contribute to the development of feasible prevention and control measures and policies in the future. This study examines the spatiotemporal distributions of soil Polycyclic Aromatic Hydrocarbons (PAHs) pollution in China since 2000, and investigates the key factors influencing changes in levels of soil PAHs. The results of the survey on soil PAHs concentration levels in 716 areas were analyzed by visualization of ArcGIS pro data, correlation analysis and linear regression analysis, it was found that the increase in soil PAH pollution in China is concerning. The analysis indicates significant regional disparities, with pollution levels in the north being higher than in the south. Over the 20-year period, the median level of PAHs in soil increased by 476.8 μg/kg. Construction land areas that heavily rely on fossil fuels and industrial activities exhibit significantly higher concentrations of polycyclic aromatic hydrocarbons (PAHs) compared to other land use types. The study identifies key socio-economic factors linked to rising PAH levels, including energy consumption (notably coal and oil), industrial and domestic waste production. Coal consumption is highlighted as the leading factor in PAH concentration changes in 18 provinces, followed by industrial waste in 6 provinces. Future projections up to 2030 suggest continued influence of these factors on soil PAH levels. The research emphasizes the urgent necessity for comprehensive soil management policies to address the growing PAH pollution, offering insights into its dynamics and contributing factors in China.

Environmental contamination with polycyclic aromatic hydrocarbons and contribution from biomonitoring studies to the surveillance of global health

This work presents an integrated overview of polycyclic aromatic hydrocarbons' (PAHs) ubiquity comprising environmental contamination in the air, aquatic ecosystems, and soils; characterizes the contamination in biota; and identifies main biomonitors and human exposure to PAHs and associated health risks. Urban centers and industrial areas present increased concentrations in the air (1344.4-12,300 versus 0.03-0.60 ng/m3 in industrial/urban and rural zones) and soils (0.14-1.77 × 106 versus 2.00-9.04 × 103 versus 1.59-5.87 × 103 ng/g in urban, forest, and rural soils), respectively. Increased concentrations were found in coastal zones and superficial waters as well as in sediments (7.00 × 104-1.00 × 109 ng/g). Benzo(a)pyrene, a carcinogenic PAH, was found in all environmental media. Mosses, lichens, tree leaves, bivalves, cephalopods, terrestrials' snails, and honeybees are good biomonitors of biota contamination. More studies are needed to improve characterization of PAHs' levels, distribution, and bioaccumulation in the environmental media and assess the associated risks for biota and human health. Actions and strategies to mitigate and prevent the bioaccumulation of PAHs in the environment and trophic chains toward the WHO's One-Health Perspective to promote the health of all ecosystems and human life are urgently needed.

Air-soil cycling of oxygenated, nitrated and parent polycyclic aromatic hydrocarbons in source and receptor areas

Polycyclic aromatic hydrocarbons (PAHs) and their oxygenated and nitrated derivatives, OPAHs and NPAHs, are semivolatile air pollutants which are distributed and cycling regionally. Subsequent to atmospheric deposition to and accumulation in soils they may re-volatilise, a secondary source which is understudied. We studied the direction of air-soil mass exchange fluxes of 12 OPAHs, 17 NPAHs, 25 PAHs and one alkylated PAH in two rural environments being influenced by the pollutant concentrations in soil and air, by season, and by land cover. The OPAHs and NPAHs in samples of topsoil, of ambient air particulate and gas phases and in the gas-phase equilibrated with soil were analysed by GC-APCI-MS/MS. The pollutants soil burdens show a pronounced seasonality, a winter maximum for NPAHs and PAHs and a summer maximum for OPAHs. One order of magnitude more OPAH and parent PAH are found stored in forest soil than in nearby grassland soil. Among a number of 3-4 ring PAHs, the OPAHs benzanthrone and 6H-benzo(c,d)pyren-6-one, and the NPAHs 1- and 2-nitronaphthalene, 9-nitrophenanthrene and 7-nitrobenz(a)anthracene are found to re-volatilise from soils at a rural background site in central Europe in summer. At a receptor site in northern Europe, net deposition of polycyclic aromatic compounds (PACs) prevails and re-volatilisation occurs only sporadic. Re-volatilisation of a number of PACs, including strong mutagens, from soils in summer and even in winter indicates that long-range atmospheric transport of primary PAC emissions from central Europe to receptor areas might be enhanced by secondary emissions from soils.

Preferential association of polycyclic aromatic hydrocarbons (PAHs) with soil colloids at an e-waste recycling site: Implications for risk of PAH migration to subsurface environment

Polycyclic aromatic hydrocarbon (PAH) contamination at e-waste recycling sites poses high ecological and human-health risks. Of note, PAHs in surface soils can be mobilized through colloid-facilitated transport, and may migrate into the subsurface and pollute groundwater. Here, we show that the colloids released from the soil samples at an e-waste recycling site in Tianjin, China contain high concentrations of PAHs, with total concentrations of 16 PAHs as high as 1520 ng/g dw. Preferential association of the PAHs with the colloids is observed, with the distribution coefficients of PAHs between colloids and bulk soil often above 10. Source diagnostic ratios show that soot-like particles are the main source of PAHs at the site, due to the incomplete combustion of fossil fuels, biomass, and electronic wastes during the e-waste dismantling practices. Due to their small sizes, a large fraction of these soot-like particles can be remobilized as colloids, and this explains the preferential association of PAHs with colloids. Moreover, the colloids-soil distribution coefficients are higher for the low-molecular-weight PAHs than for the high-molecular-weight ones, possibly attributable to the different binding routes/modes of these two groups of PAHs to the particles during combustion. Notably, the preferential association of PAHs with colloids is even more pronounced for the subsurface soils, corroborating that the presence of PAHs in the deeper soils is primarily the results of downward migration of PAH-bearing colloids. The findings highlight the important role of colloids as a vector for the subsurface transport of PAHs at e-waste recycling sites, and call for further understanding of colloid-facilitated transport of PAHs at e-waste recycling sites.

Regioselective oxidation of heterocyclic aromatic hydrocarbons catalyzed by cytochrome P450: A case study of carbazole

Recently there are increasing interests in accurately evaluating the health effects of heterocyclic PAHs. However, the activation mechanism and possible metabolites of heterocyclic PAHs catalyzed by human CYP1A1 is still elusive to a great extent. Here, leveraged to high level QM/MM calculations, the corresponding activation pathways of a representative heterocyclic PAHs, carbazole, were systematically explored. The first stage is electrophilic addition or hydrogen abstraction from N-H group. Electrophilic addition was evidenced to be more feasible and regioselectivity at C3 and C4 sites were identified. Correlations between energy barriers and key structural/electrostatic parameters reveal that O-Cα distance and Fe-O-Cα angle are the main origin for the catalytic regioselectivity. Electrophilic addition was determined as the rate-determining step and the subsequent possible reactions include epoxidation, NIH shift (the hydrogen migration from the site of hydroxylation to the adjacent carbon) and proton shuttle. The corresponding products are epoxides, ketones and hydroxylated carbazoles, respectively. The main metabolites (hydroxylated carbazoles) are estimated to be more toxic than carbazole. The regioselectivity of carbazole activated by CYP1A1 is different from the environmental processes (gas and aqueous phase). Collectively, these results will inform the in-depth understanding the metabolic processes of heterocyclic PAHs and aid the accurate evaluation of their health effects.

Endophytic, extremophilic and entomophilic fungi strains biodegrade anthracene showing potential for bioremediation

Anthropogenic activities have been increasing Polycyclic Aromatic Hydrocarbons (PAHs) release, promoting an urgent need for decontamination methods. Therefore, anthracene biodegradation by endophytic, extremophilic, and entomophilic fungi was studied. Moreover, a salting-out extraction methodology with the renewable solvent ethanol and the innocuous salt K₂HPO₄ was employed. Nine of the ten employed strains biodegraded anthracene in liquid medium (19-56% biodegradation) after 14 days at 30 °C, 130 rpm, and 100 mg L^-1. The most efficient strain Didymellaceae sp. LaBioMMi 155, an entomophilic strain, was employed for optimized biodegradation, aiming at a better understanding of how factors like pollutant initial concentration, pH, and temperature affected this process. Biodegradation reached 90 ± 11% at 22 °C, pH 9.0, and 50 mg L^-1. Futhermore, 8 different PAHs were biodegraded and metabolites were identified. Then, experiments with anthracene in soil ex situ were performed and bioaugmentation with Didymellaceae sp. LaBioMMi 155 presented better results than natural attenuation by the native microbiome and biostimulation by the addition of liquid nutrient medium into soil. Therefore, an expanded knowledge about PAHs biodegradation processes was achieved with emphasis to the action of Didymellaceae sp. LaBioMMi 155, which can be further employed for in situ biodegradation (after strain security test), or for enzyme identification and isolation aiming at oxygenases with optimal activity under alkaline conditions.

Atmospheric Deposition of Benzo[a]pyrene: Developing a Spatial Pattern at a National Scale

Benzo[a]pyrene (BaP), an indicator of polycyclic aromatic hydrocarbons (PAHs) in the atmosphere, is an important ambient air pollutant with significant human health and environmental effects. In the Czech Republic (CR), BaP, together with aerosol and ambient ozone, ranks (with respect to limit value exceedances and resulting population exposure) among the most problematic air pollutants. The aim of this study is to develop atmospheric deposition patterns of BaP in three years, namely 2012, 2015 and 2019, reflecting different BaP ambient levels. With respect to the available measurements, we accounted for dry deposition fluxes, neglecting wet contribution. We assumed, nevertheless, that the real atmospheric deposition is dominated by dry pathways in our conditions, which is supported by measurements from the rural site of Košetice. The dry deposition spatial pattern was constructed using an inferential approach, with two input layers, i.e., annual mean ambient air BaP concentrations, and deposition velocity of 0.89 cm·s−1. Though our results show an overall decrease in BaP loads over the years, the BaP deposition fluxes, in particular in the broader Ostrava region, remain very high. The presented maps can be considered an acceptable approximation of total BaP deposition and are useful for further detailed analysis of airborne BaP impacts on the environment.