Photolysis of polycyclic aromatic hydrocarbons on soil surfaces under UV irradiation

Photodegradation enhances the toxic effect of anthracene on skin

Anthracene, a polycyclic aromatic hydrocarbon (PAH), is a widespread environmental pollutant that poses potential risks to human health. Exposure to anthracene can result in various adverse health effects, including skin-related disorders. Photo exposure sufficiently removes the anthracene from the environment but also generates more degradation products which can be more toxic. The goal of this study was to assess the change in anthracene dermotoxicity caused by photodegradation and understand the mechanism of this change. In the present study, over 99.99% of anthracene was degraded within 24 h of sunlight exposure, while producing many intermediate products including 9,10-anthraquinone and phthalic acid. The anthracene products with different durations of photo exposure were applied to 2D and 3D human keratinocyte cultures. Although the non-degraded anthracene significantly delayed the cell migration, the cell viability and differentiation decreased dramatically in the presence of the photodegraded anthracene. Anthracene photodegradation products also altered the expression patterns of a number of inflammation-related genes in comparison to the control cells. Among these genes, il1a, il1b, il8, cxcl2, s100a9, and mmp1 were upregulated whereas the tlr4 and mmp3 were downregulated by the photodegraded anthracene. Topical deliveries of the photodegraded and non-degraded anthracene to the dorsal skin of hairless mice showed more toxic effects by the photodegraded anthracene. The 4-hour photodegradation products of anthracene thickened the epidermal layer, increased the dermal cellularity, and induced the upregulation of inflammatory markers, il1a, il1b, s100a9, and mmp1. In addition, it also prevented the production of a gap junction protein, Connexin-43. All the evidence suggested that photodegradation enhanced the toxicities of anthracene to the skin. The 4-hour photodegradation products of anthracene led to clinical signs similar to acute inflammatory skin diseases, such as atopic and contact dermatitis, eczema, and psoriasis. Therefore, the potential risk of skin irritation by anthracene should be also considered when an individual is exposed to PAHs, especially in environments with strong sunlight.

Photodegradation of 2-chlorodibenzo-p-dioxin (2-CDD) on the surface of municipal solid waste incineration fly ash: Kinetics and product analysis

Considering that waste incineration fly ash is the main carrier of dioxins and can migrate over long distances in the atmosphere, it is of great significance to study the photochemical transformation behavior of dioxins on the surface of fly ash. In this work, 2-chlorodibenzo-p-dioxin (2-CDD) was selected to conduct a systematic photochemical study. The influence of various factors on the photodegradation of 2-CDD were first explored, and the results showed that small particle size of fly ash, low concentration of 2-CDD and appropriate level of humidity were more conducive to photodegradation, with the highest degradation percentage reaching 76%-84%. The components of fly ash (Zn (Ⅱ), Al (Ⅲ), Cu (Ⅱ) and SiO2) also had a certain promoting effect on the degradation of 2-CDD, which increases the degradation efficiency by 10%-20%, because they could act as effective photocatalysts to produce free radicals for reaction. With a higher total light exposure intensity, natural light environments led to a more complete degradation of 2-CDD than laboratory Xe lamp irradiation (90% degradation Vs. 79% degradation). Based on chemical probe and radical quenching experiment, hydroxyl radical also contributed to 2-CDD photodegradation on fly ash. A total of 16 intermediate products were detected by mass spectrometry analysis, and four initial reaction pathways of 2-CDD were speculated in the process, including dechlorination, ether bond cleavage, hydroxyl substitution, and hydroxyl addition. According to the results of density functional theory calculation, the reaction channels of ether bond cleavage and •OH attack were determined. The toxicity assessment software tool (TEST) was used to assess the toxicity and bioconcentration coefficient of reaction products, and it was found that the overall toxicity of the photodegradation products was reduced. This study would provide new insights into the environmental fate of dioxins during long-range atmospheric migration process.

Photochemical transformation of anthracene (ANT) in surface soil: Chlorination and hydroxylation

To reveal the fate of anthracene (ANT) in soil, the photodegradation behavior of ANT was systematically studied using SiO₂ to simulate a soil environment. Under xenon lamp irradiation, more than 90% of ANT loaded on SiO₂ could be removed after 240 min. Moreover, the effects of water content, chloride ions (Cl^-) and humic acid (HA) were examined. It was found that the presence of water and HA can significantly inhibit the photolysis of ANT on SiO₂, while the addition of chloride alone has no obvious effect. However, when water is present, the inhibition effect of chloride became more obvious. According to radical quenching experiments and electron paramagnetic resonance (EPR) spectra, hydroxyl radicals (•OH) and chlorine radicals (Cl•) were formed in the system. Possible reaction pathways were speculated based on products identified by mass spectrometry. ANT was attacked by •OH to form hydroxylated products, which can be further hydroxylated and oxidized with the final formation of ring-opening products. ANT directly excited by light may also react with Cl• to produce chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs). Finally, the experimental results were verified on real soil. This study provides important information for understanding the photochemical transformation mechanism of ANT at the soil/air interface.

Iron redox cycling in layered clay minerals and its impact on contaminant dynamics: A review

A majority of clay minerals contain Fe, and the redox cycling of Fe(III)/Fe(II) in clay minerals has been extensively studied as it may fuel the biogeochemical cycles of nutrients and govern the mobility, toxicity and bioavailability of a number of environmental contaminants. There are three types of Fe in clay minerals, including structural Fe sandwiched in the lattice of clays, Fe species in interlayer space and adsorbed on the external surface of clays. They exhibit distinct reactivity towards contaminants due to their differences in redox properties and accessibility to contaminant species. In natural environments, microbially driven Fe(III)/Fe(II) redox cycling in clay minerals is thought to be important, whereas reductants (e.g., dithionite and Fe(II)) or oxidants (e.g., peroxygens) are capable of enhancing the rates and extents of redox dynamics in engineered systems. Fe(III)-containing clay minerals can directly react with oxidizable pollutants (e.g., phenols and polycyclic aromatic hydrocarbons (PAHs)), whereas structural Fe(II) is able to react with reducible pollutants, such as nitrate, nitroaromatic compounds, chlorinated aliphatic compounds. Also structural Fe(II) can transfer electrons to oxygen (O₂), peroxymonosulfate (PMS), or hydrogen peroxide (H₂O₂), yielding reactive radicals that can promote the oxidative transformation of contaminants. This review summarizes the recent discoveries on redox reactivity of Fe in clay minerals and its links to fates of environmental contaminants. The biological and chemical reduction mechanisms of Fe(III)-clay minerals, as well as the interaction mechanism between Fe(III) or Fe(II)-containing clay minerals and contaminants are elaborated. Some knowledge gaps are identified for better understanding and modelling of clay-associated contaminant behavior and effective design of remediation solutions.

Potential Toxicity Risk Assessment and Priority Control Strategy for PAHs Metabolism and Transformation Behaviors in the Environment

In this study, 16 PAHs were selected as the priority control pollutants to summarize their environmental metabolism and transformation processes, including photolysis, plant degradation, bacterial degradation, fungal degradation, microalgae degradation, and human metabolic transformation. Meanwhile, a total of 473 PAHs by-products generated during their transformation and degradation in different environmental media were considered. Then, a comprehensive system was established for evaluating the PAHs by-products' neurotoxicity, immunotoxicity, phytotoxicity, developmental toxicity, genotoxicity, carcinogenicity, and endocrine-disrupting effect through molecular docking, molecular dynamics simulation, 3D-QSAR model, TOPKAT method, and VEGA platform. Finally, the potential environmental risk (phytotoxicity) and human health risks (neurotoxicity, immunotoxicity, genotoxicity, carcinogenicity, developmental toxicity, and endocrine-disrupting toxicity) during PAHs metabolism and transformation were comprehensively evaluated. Among the 473 PAH's metabolized and transformed products, all PAHs by-products excluding ACY, CHR, and DahA had higher neurotoxicity, 152 PAHs by-products had higher immunotoxicity, and 222 PAHs by-products had higher phytotoxicity than their precursors during biological metabolism and environmental transformation. Based on the TOPKAT model, 152 PAH by-products possessed potential developmental toxicity, and 138 PAH by-products had higher genotoxicity than their precursors. VEGA predicted that 247 kinds of PAH derivatives had carcinogenic activity, and only the natural transformation products of ACY did not have carcinogenicity. In addition to ACY, 15 PAHs produced 123 endocrine-disrupting substances during metabolism and transformation. Finally, the potential environmental and human health risks of PAHs metabolism and transformation products were evaluated using metabolic and transformation pathway probability and degree of toxic risk as indicators. Accordingly, the priority control strategy for PAHs was constructed based on the risk entropy method by screening the priority control pathways. This paper assesses the potential human health and environmental risks of PAHs in different environmental media with the help of models and toxicological modules for the toxicity prediction of PAHs by-products, and thus designs a risk priority control evaluation system for PAHs.

Environmental Risk Assessment of Polycyclic Aromatic Hydrocarbons in Farmland Soils near Highways: A Case Study of Guangzhou, China

Recently, the rapid growth in vehicle activity in rapidly urbanized areas has led to the discharge of large amounts of polycyclic aromatic hydrocarbons (PAHs) into roadside soils and these compounds have gradually accumulated in the soil, which poses a serious threat to national food security and public health. However, previous studies did not clearly investigate the seasonal differences in PAH pollution of roadside soil by different highways. Therefore, based on field investigations, this study collected 84 soil surface samples to compare the pollution characteristics of 16 PAHs in farmland soils located near different roads in different seasons in Guangzhou, China. The results showed that the concentration of Σ16PAHs in farmland soils in spring (with a mean value of 258.604 μg/kg) was much higher than that in autumn (with a mean value of 157.531 μg/kg). There are differences in the PAH compositions in spring (4 ring > 3 ring > 5 ring > 6 ring) and autumn (4 ring > 5 ring > 6 ring > 3 ring). The proportion of 4−6 ring PAHs was much higher than 2−3 ring PAHs in both seasons. The spatial differences were significant. The sampling areas with higher concentrations of 16 PAHs were Tanbu Town, Huadu District (TB), Shitan Town, Zengcheng District (ST), and Huashan Town, Huadu District (HS), while the lowest concentration was in Lanhe Town, Nansha District (LH). The results of the diagnostic ratios showed that the main source of soil PAHs consists of a mixed source from petroleum and biomass combustion. The results from the total pollution assessment method and Nemerow index method indicated that the pollution levels of PAHs in the farmland soils indicated weak contamination. Our study provides a scientific basis for the prevention and control of soil pollution in farmlands near highways.

White Light-Photolysis for the Removal of Polycyclic Aromatic Hydrocarbons from Proximity Firefighting Protective Clothing

The presence of polycyclic aromatic hydrocarbons (PAHs) on firefighters' personal protective equipment is a concern. One form of preventing from these compounds is to decontaminate proximity firefighting protective clothing (PFPC). Traditional decontamination methods do not promote total removal of pollutants and alter the properties of PFPC. The objective of this work was to evaluate the effectiveness of white light-photolysis (WLP), an advanced oxidation process (AOP), for removing PAHs from PFPC, while maintaining the integrity of the fabric fibers. Experiments were carried out, varying reaction time and concentration of H₂O₂. With WLP (without H₂O₂), it was possible to remove more than 73% of the PAHs tested from the outer layer of PFPC in 3 days. The WLP provided the greatest removal of PAHs, compared with the most common mechanical decontamination techniques (laundering and wet-soap brushing). The fibers' integrity after exposure to the white light was evaluated with infrared spectroscopy and scanning electron microscopy/energy dispersive X-ray spectrometry. In addition, a tearing strength test was performed. No remarkable fabric degradation was observed, indicating a possible, routine-compatible, simple, and inexpensive method of decontamination of PFPC, based on photolysis, which is effective in the degradation of PAHs and maintains the integrity of fabric fibers.

Unexpectedly High Indoor HONO Concentrations Associated with Photochemical NO2 Transformation on Glass Windows

Nitrous acid (HONO) is an important gaseous pollutant contributing to indoor air pollution because it causes adverse health effects and is the main source of hydroxyl radicals (OH). Here, we present direct measurements of HONO produced through light-induced heterogeneous reactions of NO₂ with grime adsorbed on glass window. The uptake coefficients of NO₂ [γ(NO₂)] on the glass plates from the kitchen increased markedly from (2.3 ± 0.1) × 10^-6 at 0% RH to (4.1 ± 0.5) × 10^-6 at 90% RH. We report a significant quantity of daytime HONO produced in the kitchen, compared to the living room and bedroom. Kinetic modeling suggests that phase state and bulk diffusivity play important roles in the NO₂ uptake; the best fit to the measured uptake coefficients is obtained with fixed diffusion coefficients. Photon scattering may be occurring at the surface of the films, leading to enhanced photon-excitation rates of polycyclic aromatic hydrocarbons. By taking these effects into account, the results from this study indicate that the HONO yields obtained in this study can explain the missing HONO in the photochemical models describing the indoor air chemistry.

Removal of 1,4-Naphthoquinone by Birnessite-Catalyzed Oxidation: Effect of Phenolic Mediators and the Reaction Pathway

This study investigated the birnessite (δ-MnO₂) catalyzed oxidative removal of 1,4-naphthoquinone (1,4-NPQ) in the presence of phenolic mediators; specifically, the kinetics of 1,4-NPQ removal under various conditions was examined, and the reaction pathway of 1,4-NPQ was verified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The removal rate of 1,4-NPQ by birnessite-catalyzed oxidation (pH = 5) was faster in the presence of phenolic mediators with electron-donating substituents (pseudo-first-order initial stage rate constant (k₁) = 0.380-0.733 h^-1) than with electron-withdrawing substituents (k₁ = 0.071-0.244 h^-1), and the effect on the substituents showed a positive correlation with the Hammett constant (Σσ) (r² = 0.85, p < 0.001). The rate constants obtained using variable birnessite loadings (0.1-1.0 g L^-1), catechol concentrations (0.1-1.0 mM), and reaction sequences indicate that phenolic mediators are the major limiting factor for the cross-coupling reaction of 1,4-NPQ in the initial reaction stages, whereas the birnessite-catalyzed surface reaction acts as the major limiting factor in the later reaction stages. This was explained by the operation of two different reaction mechanisms and reaction products identified by LC-MS/MS.

Transformation and degradation of polycyclic aromatic hydrocarbons (PAHs) in urban road surfaces: Influential factors, implications and recommendations

Polycyclic aromatic hydrocarbons (PAHs) are prone to post-emission transformation and degradation to yield transformed PAH products (TPPs) that are potentially more hazardous than parent PAHs. This review provides a comprehensive evaluation of the potential environmental processes of PAHs such as sorption, volatilisation, photo- and bio-transformation and degradation on road surfaces, a significant accumulation point of PAHs. The review primarily evaluates key influential factors, toxicity implications, PAHs and TPPs fate and viable options for mitigating environmental and human health impacts. Photolysis was identified as the most significant transformation and degradation process due to the light absorption capacity of most PAHs. Climate conditions, physicochemical properties of road dust (sorbent), PAHs and TPPs and the existence of heavy metals such as Fe (III) are notable underlying factors for photolysis. Available data points to the predominance of carbonyl TPPs than other products such as nitro and hydroxyl TPPs with decreasing concentration trend of 9-fluorenone > 9,10-anthraquinone > benzo[a]fluorenone on road surfaces. The review recommends conducting future investigations targeting the influential factors pertaining to the fate of road deposited PAHs and TPPs. Furthermore, development of cost and time effective modern analytical methods is needed to quantify PAHs and TPPs present in minute quantities of samples. The review also identified that the unavailability of toxicity equivalency factors (TEF) for the most critical TPPs can be addressed using quantitative structure-activity relationship (QSAR) models and bioassays simultaneously. The content of this review is significant to the future work of researchers across various fields including analytical and environmental chemistry, stormwater pollution and toxicology.

Innovative combination of tracing methods to differentiate between legacy and contemporary PAH sources in the atmosphere-soil-river continuum in an urban catchment (Orge River, France)

Polycyclic aromatic hydrocarbons (PAH) have been released by human activities during more than a century, contaminating the entire atmosphere - soil - river continuum. Due to their ubiquity in the environment and their potential severe biological impacts, PAH became priority pollutants and were targeted by environmental public agencies. To better manage PAH pollution, it is necessary to identify unambiguously the sources and pathways of those compounds at the catchment scale, and to evaluate the persistence of historical PAH pollution in the environment especially in those urban contexts concentrating multiple PAH sources. Accordingly, the current research monitored the contamination in atmospheric fallout, soils and rivers of a 950-km² catchment (Orge River) characterized by an increasing urban gradient in downstream direction, and located in the Seine River basin characterized by a high level of PAH legacy contamination. A combination of various approaches was used, including the widely used PAH diagnostic ratios, together with innovative methods such as PAH correlations and sediment fingerprinting using fallout radionuclides to clearly identify both the origin of PAH and their main PAH pathways to the river. The results demonstrated the persistence of legacy PAH contamination in the catchment, responsible for the signature of the suspended particulate matter currently transiting in the Orge River. They underlined the conservation of PAH through the soil - river continuum. Finally, urban runoff was demonstrated to provide the main PAH source to the river in the densely urbanized area by both PAH correlations and sediment fingerprinting. These results were used to model PAH concentrations in those particles supplied from urban areas to the river.

Assessing the Fate of an Aromatic Hydrocarbon Fluid in Agricultural Spray Applications Using the Three-Stage ADVOCATE Model Framework

Components of emulsifiable concentrates (ECs) used in pesticide formulations may be emitted to air following application in agricultural use and contribute to ozone formation. A key consideration is the fraction of the ECs that is volatilized. This study is designed to provide a mechanistic model framework for estimating emissions of an aromatic hydrocarbon fluid used in ECs based on the results of spray chamber experiments that simulate fate as the fluids become subject to volatilization, sorption to soil, and biodegradation. The results indicate the need to treat the volatilization losses in three stages: (i) losses during spraying, (ii) losses up to 12 h after spraying in which the soil is coated with the ECs, and (iii) subsequent longer term losses in which the ECs become increasingly sorbed and subject to biodegradation. A mass balance model, the agrochemical derived volatile organic compound air transfer evaluation (ADVOCATE) tool, is developed, treating the ECs as seven hydrocarbon component groups, to estimate the volatilization and biodegradation losses using parameters fitted to empirical data. This enables losses to be estimated for each hydrocarbon component under field conditions, thereby providing a basis for improved estimation of ozone formation potential and for designing ECs that have lower emissions.

Photolysis degradation of polyaromatic hydrocarbons (PAHs) on surface sandy soil

Polycyclic aromatic hydrocarbons (PAHs) are potent environmental pollutants, and some of them have been identified as carcinogenic and mutagenic. To advance the knowledge of the environmental fate of PAHs, we systematically investigated the influence of different UV wavelengths irradiation on photolysis of PAHs on sandy soil under tow wavelengths (254 and 306 nm) UV irradiation for six PAHs. In addition, kinetic model and influence of several parameters on PAHs photolysis have been studied. The results obtained indicated that UV radiation with a wavelength of 306 nm was more efficient in the photolysis of the polycyclic aromatic hydrocarbons. Our results showed that fluoranthene (Flt) was the fastest in decomposition, has the greatest value for the coefficient of photolysis (7.4 × 10(-3) h(-1)), and has less half-life, reaching 94 h when using a wavelength of 254 nm. The results indicated that the pyrene (Pyr) was more resistant to photolysis in comparison with indeno(1,2,3-cd) pyrene (IP) and fluoranthene (Flt). The results indicate that photolysis is a successful way to remediate the six studied PAHs compounds.

Sorption and degradation of petroleum hydrocarbons, polycyclic aromatic hydrocarbons, alkylphenols, bisphenol A and phthalates in landfill leachate using sand, activated carbon and peat filters

Landfill leachates are repeatedly found contaminated with organic pollutants, such as alkylphenols (APs), phthalates and polycyclic aromatic hydrocarbons (PAHs) at levels exceeding water quality standards. It has been shown that these pollutants may be present in the colloidal and truly dissolved phase in contaminated water, making particle separation an inefficient removal method. The aim of this study was to investigate sorption and degradation of petroleum hydrocarbons (PHCs), selected APs, bisphenol A (BPA), phthalates and PAHs from landfill leachate using sand, granulated activated carbon (GAC) and peat moss filters. A pilot plant was installed at an inactive landfill with mixed industrial and household waste and samples were collected before and after each filter during two years. Leachate pre-treated in oil separator and sedimentation pond failed to meet water quality standards in most samples and little improvement was seen after the sand filter. These techniques are based on particle removal, whereas the analysed pollutants are found, to varying degrees, bound to colloids or dissolved. However, even highly hydrophobic compounds expected to be particle-bound, such as the PHCs and high-molecular weight PAHs, were poorly removed in the sand filter. The APs and BPA were completely removed by the GAC filter, while mass balance calculations indicate that 50-80% of the investigated phenols were removed in the peat filter. Results suggest possible AP degradation in peat filters. No evidence of phthalate degradation in the landfill, pond or the filters was found. The PHCs were completely removed in 50% and 35% of the measured occasions in the GAC and peat filters, respectively. The opposite trend was seen for removal of PAHs in GAC (50%) and peat (63%). Oxygenated PAHs with high toxicity were found in the leachates but not in the pond sediment. These compounds are likely formed in the pond water, which is alarming because sedimentation ponds are commonly used treatment techniques. The oxy-PAHs were effectively removed in the GAC, and especially the peat filter. It was hypothesized that dissolved compounds would adsorb equally well to the peat and GAC filters. This was not completely supported as the GAC filter was in general more efficient than peat.