Oxidation Of Polycyclic Aromatic Hydrocarbons Research Articles

Oxidation of Polycyclic Aromatic Hydrocarbons (PAHs) Triggered by a Photochemical Synergistic Effect between High- and Low-Molecular-Weight PAHs.

Photooxidation of polycyclic aromatic hydrocarbons (PAHs), which are widely observed in atmospheric particulate matter (PM), largely determines their atmospheric fate. In the environment, PAHs are highly complex in chemical composition, and a great variety of PAHs tend to co-occur. Despite extensive investigation on the photochemical behavior of individual PAH molecules, the photochemical interaction among these coexisting PAHs is still not well understood. Here, we show that during photooxidation, there is a strong photochemical synergistic effect among PAHs extracted from soot particles. We find that neither small PAHs with low molecular weights of 200-350 Da and 4-8 aromatic rings (named PAHsmall) nor large PAHs with high molecular weights of 350-600 Da and 8-14 aromatic rings (named PAHlarge) undergo photooxidation under red-light irradiation (λ = 648 nm), even though PAHlarge can absorb light with this wavelength. Interestingly, when PAHlarge is mixed with PAHsmall, substantial photooxidation is observed for both PAHlarge and PAHsmall. Comparisons of in situ infrared (IR), high-resolution mass spectrometry, and electron paramagnetic resonance analysis indicate that the presence of PAHsmall inhibits the light quenching effect arising from the π-π stacking of PAHlarge. This leads to the formation of singlet oxygen (1O2), which initiates the photooxidation. Our findings reveal a new mechanism for the photooxidation of PAHs and suggest that complex atmospheric PAHs exhibit distinct photoreactivity from simple systems.

The regulating mechanisms of Triton X-100 affected oxidation of PAHs in site soil aggregates using sodium citrate assisted Fe2+-persulfate

Here, we present a first investigation of the inhibition mechanism of surfactant Triton X-100 (TX-100) on the oxidation degradation of polycyclic aromatic hydrocarbons (PAHs) in site soil aggregates using sodium citrate assisted Fe2+-activated persulfate (SC/Fe2+/PS). First, TX-100 was not only competed the adsorption sites of soil aggregates with PS, but also consumed PS, which inhibit the PAHs remediation rate in the TX-100 elution followed by the SC/Fe2+/PS oxidation system from 55.6 % in the oxidation system to 50.3 %. Furthermore, in the oxidation followed by elution system, PAHs was adsorbed on the iron minerals produced during the oxidation, which would be form a bound PAHs that was difficult to react with PS, and then re-eluted to the soil by the TX-100. Additionally, it was found that the oxidative and the elution efficiency of PAHs exhibited negative correlations with aggregate particle sizes. Finally, soil microorganism communities were more strongly changed by SC/Fe2+/PS oxidation and PAHs concentration than that of TX-100 elution, with obvious alterations bacteria than fungi, the effects of SC/Fe2+/PS and PAHs concentration on microorganism communities were opposite. This study provided a proof of regulating mechanisms for the site soil remediation using surfactants combined with the iron-PS system.

Distinct Photochemistry of Odd-Carbon PAHs from the Even-Carbon Ones During the Photoaging and Analysis of Soot.

Polycyclic aromatic hydrocarbons (PAHs) are the primary organic carbons in soot. In addition to PAHs with even carbon numbers (PAHeven), substantial odd-carbon PAHs (PAHodd) have been widely observed in soot and ambient particles. Analyzing and understanding the photoaging of these compounds are essential for assessing their environmental effects. Here, using laser desorption ionization mass spectrometry (LDI-MS), we reveal the substantially different photoreactivity of PAHodd from PAHeven in the aging process and their MS detection through their distinct behaviors in the presence and absence of elemental carbon (EC) in soot. During direct photooxidation of organic carbon (OC) alone, the PAHeven are oxidized more rapidly than the PAHodd. However, the degradation of PAHodd becomes preponderant over PAHeven in the presence of EC during photoaging of the whole soot. All of these observations are proposed to originate from the more rapid hydrogen abstraction reaction from PAHodd in the EC-photosensitized reaction, owing to its unique structure of a single sp3-hybridized carbon site. Our findings reveal the photoreactivity and reaction mechanism of PAHodd for the first time, providing a comprehensive understanding of the oxidation of PAHs at a molecular level during soot aging and highlight the enhanced effect of EC on PAHodd ionization in LDI-MS analysis.

Mechanistic study of OH radical-promoted decomposition of polycyclic aromatic hydrocarbon oxyradicals

Polycyclic aromatic hydrocarbons (PAHs) are widely recognized for their detrimental impact on human health and the atmospheric environment. One of the primary challenges in mitigating these effects is the efficient and rapid decomposition of PAH oxyradicals, a byproduct of PAH oxidation. This study aims to elucidate the promoting mechanism of hydroxyl (OH) radicals on the decomposition of PAH oxyradicals. Employing density functional theory and transition state theory, we systematically investigate the decomposition mechanism of PAH oxyradicals under the influence of OH radicals. Our findings indicate that the interaction between OH radicals and PAH oxyradicals constitutes a pivotal oxidation process within flames. Notably, PAH oxyradicals are more inclined to undergo pyrolysis subsequent to their reaction with OH radicals within a flame, as opposed to direct pyrolysis. This observation underscores the role of OH radicals in facilitating the decomposition of PAH oxyradicals in flame conditions. Further analysis reveals that OH radicals can effectively react with various forms of PAH oxyradicals—zigzag, free, and armchair—leading to the formation of OH-OPAH compounds characterized by HO-C-O functional groups. Importantly, this addition reaction between OH radicals and PAH oxyradicals is found to be independent of the edge types of the PAH oxyradicals. The pyrolysis of OH-OPAH predominantly yields products such as COOH, CO2, and PAHs with five-membered rings, with COOH emerging as the primary oxidation product. This pyrolysis predominantly proceeds via a ring-opening reaction, rather than through cyclization. This study also reveals that the likelihood of PAH oxyradical pyrolysis increases with the number of free edges surrounding the oxygen-containing functional groups. Furthermore, the oxidation activity of the HO-C-O functional group is demonstrated to be more potent than that of the C-O functional group, significantly enhancing the reactivity of PAH oxyradical degradation in the presence of OH radicals. These insights not only contribute to refining the combustion model of PAH oxidation but also advance our understanding of PAH oxyradical decomposition. Such knowledge is crucial for the development and optimization of control technologies targeting PAH pollutants emanating from combustion devices.

Volatility and chemical composition of secondary organic aerosol derived from acenaphthylene and acenaphthene under various oxidant conditions

Acenaphthene (ACE) and acenaphthylene (ACY), distinguished by their unique structures, exhibit high reactivity among polycyclic aromatic hydrocarbons (PAHs). Particularly, the rapid ozonolysis of the carbon-carbon double bond in ACY and the swift oxidation of the fused cyclopenta ring in both ACE and ACY by NO3 underscore their distinct atmospheric fate. Through the utilization of a thermo denuder and high-resolution mass spectrometry, the chemical composition and volatility of four types of secondary organic aerosols (SOA) derived from ACE and ACY are investigated. The chemical composition of SOA formed by ACY ozonolysis is characterized by low O: C ratios and high unsaturation, suggesting the retention of aromatic structures. This leads to measured high SOA volatility and is consistent with the low reactivity of O3 with aromatic compounds. In contrast to ozonolysis, the reaction of NO3 with ACY could yield SOA with very low volatility, characterized by abundant dimers and highly oxidized products. Furthermore, the prevalence of compounds with 11 carbon atoms suggests the presence of complex mechanisms in the ACY oxidation process initiated by NO3, where dealkylation reactions and cleavage reactions may be pivotal. The diverse chemical composition and volatility of SOA from ACE and ACY reacting with NO3 radicals highlight the substantial impact of precursor structure on SOA chemical composition, particularly concerning the presence of active carbon-carbon double bonds. The higher O: C ratio of SOA from OH-initiated oxidation of ACE indicates the significant role of OH in the oxidation of PAHs. These findings offer valuable insights into the atmospheric fate of these PAHs, underscoring the necessity for a comprehensive understanding of their degradation processes and the formation of SOA.

Redox potential model for guiding moderate oxidation of polycyclic aromatic hydrocarbons in soils

In-situ chemical oxidation is an important approach to remediate soils contaminated with persistent organic pollutants, e.g., polycyclic aromatic hydrocarbons (PAHs). However, massive oxidants are added into soils without an explicit model for predicting the redox potential (Eh) during soil remediation, and overdosed oxidants would pose secondary damage by disturbing soil organic matter and acidity. Here, a soil redox potential (Eh) model was first established to quantify the relationship among oxidation parameters, crucial soil properties, and pollutant elimination. The impacts of oxidant types and doses, soil pH, and soil organic carbon contents on soil Eh were systematically clarified in four commonly used oxidation systems (i.e., KMnO4, H2O2, fenton, and persulfate). The relative error of preliminary Eh model was increased from 48–62% to 4–16% after being modified with the soil texture and dissolved organic carbon, and this high accuracy was verified by 12 actual PAHs contaminated soils. Combining the discovered critical oxidation potential (COP) of PAHs, the moderate oxidation process could be regulated by the guidance of the soil Eh model in different soil conditions. Moreover, the product analysis revealed that the hydroxylation of PAHs occurred most frequently when the soil Eh reached their COP, providing a foundation for further microorganism remediation. These results provide a feasible strategy for selecting oxidants and controlling their doses toward moderate oxidation of contaminated soils, which will reduce the consumption of soil organic matter and protect the main structure and function of soil for future utilization. Environmental implicationsThis study provides a novel insight into the moderate chemical oxidation by the Eh model and largely reduces the secondary risks of excessive oxidation and oxidant residual in ISCO. The moderate oxidation of PAHs could be a first step to decrease their toxicity and increase their bioaccessibility, favoring the microbial degradation of PAHs. Controlling the soil Eh with the established model here could be a promising approach to couple moderate oxidation of organic contaminants with microbial degradation. Such an effective and green soil remediation will largely preserve the soil’s functional structure and favor the subsequent utilization of remediated soil.

Advanced oxidation of polycyclic aromatic hydrocarbons in tropical soil: Self-catalytic utilization of natural iron contents in an oxygenation reactor supported with persulfate

The catalysts derived from natural iron minerals in the advanced oxidation process offer several advantages. However, their utilization in soil remediation is restricted due to the presence of soil impurities, which can inhibit the catalytic activity of these minerals. The soils in tropical regions exhibit lower organic matter content, limited cation exchange capacity, and are non-saline, this enhances the efficiency of utilizing natural iron minerals from tropical soil as a catalyst. In this regard, the catalytic potential of naturally iron-bearing tropical soil was investigated to eliminate phenanthrene (PHE), pyrene (PYR), and benzo[α]pyrene (B[α]P) using an oxygenated reactor supported with persulfate (PS). The system showed an efficient performance, and the removal efficiencies under the optimum conditions were 81 %, 73 %, and 86 % for PHE, PYR, and B[α]P, respectively. This indicated that the catalytic activity of iron was working efficiently. However, there were changes in the soil characteristics after the remediation process such as a significant reduction in iron and aluminum contents. The scavenging experiments demonstrated that HO• had a minor role in the oxidation process, SO4•− and O2•− emerged as the primary reactive species responsible for the effective degradation of the PAHs. Moreover, the by-products were monitored after soil remediation to evaluate their toxicity and to propose degradation pathways. The Mutagenicity test showed that two by-products from each PHE and B[α]P had positive results, while only one by-product of PYR showed positive. The toxicity tests of oral rat LD50 and developmental toxicity tests revealed that certain PAHs by-products could be more toxic from the parent pollutant itself. This study represents a notable progression in soil remediation by providing a step forward in the application of the advanced oxidation process (AOP) without requiring additional catalysts to activate oxidants and degrade pollutant PAHs from the soil.

Cardiolipin Membranes Promote Cytochrome c Transformation of Polycyclic Aromatic Hydrocarbons and Their In Vivo Metabolites.

The catalytic properties of cytochrome c (Cc) have captured great interest in respect to mitochondrial physiology and apoptosis, and hold potential for novel enzymatic bioremediation systems. Nevertheless, its contribution to the metabolism of environmental toxicants remains unstudied. Human exposure to polycyclic aromatic hydrocarbons (PAHs) has been associated with impactful diseases, and animal models have unveiled concerning signs of PAHs' toxicity to mitochondria. In this work, a series of eight PAHs with ionization potentials between 7.2 and 8.1 eV were used to challenge the catalytic ability of Cc and to evaluate the effect of vesicles containing cardiolipin mimicking mitochondrial membranes activating the peroxidase activity of Cc. With moderate levels of H2O2 and at pH 7.0, Cc catalyzed the oxidation of toxic PAHs, such as benzo[a]pyrene, anthracene, and benzo[a]anthracene, and the cardiolipin-containing membranes clearly increased the PAH conversions. Our results also demonstrate for the first time that Cc and Cc-cardiolipin complexes efficiently transformed the PAH metabolites 2-hydroxynaphthalene and 1-hydroxypyrene. In comparison to horseradish peroxidase, Cc was shown to reach more potent oxidizing states and react with PAHs with ionization potentials up to 7.70 eV, including pyrene and acenaphthene. Spectral assays indicated that anthracene binds to Cc, and docking simulations proposed possible binding sites positioning anthracene for oxidation. The results give support to the participation of Cc in the metabolism of PAHs, especially in mitochondria, and encourage further investigation of the molecular interaction between PAHs and Cc.

Formation of Polystyrene Coated Persulfate Slow-Release Candles for Polycyclic Aromatic Hydrocarbon Oxidation in Water

Formation of Polystyrene Coated Persulfate Slow-Release Candles for Polycyclic Aromatic Hydrocarbon Oxidation in Water

Insights into secondary organic aerosol formation from the day- and nighttime oxidation of polycyclic aromatic hydrocarbons and furans in an oxidation flow reactor

Abstract. Secondary organic aerosols (SOAs) formed by oxidation of typical precursors largely emitted by biomass burning, such as polycyclic aromatic hydrocarbons (PAHs) and furans, are still poorly characterized. We evaluated and compared the formation yields, effective density (ρeff), absorption Ångström exponent (α), and mass absorption coefficient (MAC) of laboratory-generated SOAs from three furan compounds and four PAHs. SOAs were generated in an oxidation flow reactor under day- (OH radicals) or nighttime (NO3 radicals) conditions. The ρeff, formation yields, α, and MAC of the generated SOAs varied depending on the precursor and oxidant considered. The ρeff of SOAs formed with OH and NO3 tended to increase with particle size before reaching a “plateau”, highlighting potential differences in SOA chemical composition and/or morphology, according to the particle size. Three times lower SOA formation yields were obtained with NO3 compared with OH. The yields of PAH SOAs (18 %–76 %) were five to six times higher than those obtained for furans (3 %–12 %). While furan SOAs showed low or negligible light absorption properties, PAH SOAs had a significant impact in the UV–visible region, implying a significant contribution to atmospheric brown carbon. No increase in the MAC values was observed from OH to NO3 oxidation processes, probably due to a low formation of nitrogen-containing chromophores with NO3 only (without NOx). The results obtained demonstrated that PAHs are significant SOA precursors emitted by biomass burning, through both, day- and nighttime processes, and have a substantial impact on the aerosol light absorption properties.

Degradation of polycyclic aromatic hydrocarbons in oil deodorizer distillates: Kinetics, degradation product identification and toxicity

Polycyclic aromatic hydrocarbons (PAHs) with strong carcinogenicity can cause great threat to human health. However, limited research has focused on the degradation of PAHs in oil deodorizer distillates (ODDs). In this study, E. coli expressing plasmid PodAB were applied in ODD samples to degrade PAHs and compared with white rot fungi (WRF). Prediction model for reactive sites of PAHs and the toxicity of degradation products were developed using density functional theory (DFT) calculations and quantitative structure–activity relationship (QSAR), respectively. The results indicated that the degradation rate of PAHs in ODD reached up to 59.28% using E. coli with the co-expression of pBRCD, which provided the electron carrier. The prediction results using nucleophilic index was consistent with the identification results of GC-MS, which may be more suitable for predicting the PAH oxidation products. Moreover, the R2 of developed QSAR reached 0.8976 for the correlation coefficient between predicted and observed toxicity value, which can be applied to predict the toxicity of degradation products. This work proposes a potential tool for PAH removal in ODD as well as a simple method for predicting PAH oxidation products as well as their toxicity.

Low levels of flavonoids in Arabidopsis thaliana during phenanthrene exposure suggest new roles for a flavanone 3-hydroxylase

Polycyclic aromatic hydrocarbons (PAHs) contamination impacts the well-being of fauna, flora, and the environment. PAHs are a group of compounds consisting of two or more benzene rings. Due to their structure, PAHs are very stable, generally non-reactive and resistant to biodegradation; yet these compounds are cytotoxic in biological systems. In plants, PAHs induce oxidative stress and cell death, and inhibit growth. Previous PAH exposure studies from our laboratory showed an up-regulation of the Arabidopsis thaliana gene At3g51240. At3g51240 encodes a flavanone 3-hydroxylase (F3H), an enzyme catalyzing the hydroxylation of flavanones, key intermediates in the flavonoid biosynthesis pathway. Considering that flavonoids may act as antioxidants and protect cells against oxidative stress, we hypothesized that F3H was important in alleviating PAH-induced stress symptoms. Analysis of flavonoid content by high performance liquid chromatography showed low levels of flavonoids in phenanthrene-treated plants. Analysis of At3g51240 loss-of-function mutant plant lines growing on phenanthrene-containing growth media showed reduced phenotypic stress responses. Interestingly, low levels of H2O2 were observed in the mutant plants, suggesting that this putative flavanone 3-hydroxylase may be involved in PAH oxidation, with the consequent H2O2 generation. Furthermore, in vitro assays with purified recombinant 6xHis tagged AT3G51240 showed enzyme activity on the PAH when phenanthrene was used as a substrate. Together, these data suggest that F3H is involved in PAH detoxification in planta with the subsequent accumulation of toxic metabolites. This research also indicates the potentiality of using specialized metabolism enzymes such as oxidoreductase in the genetic engineering of plants to remediate PAH-contaminated soils.

Occurrence and Sources of Polycyclic Aromatic Hydrocarbons and Factors Influencing Their Accumulation in Surface Sediment of a Deep-Sea Depression, Namely, the Tatar Trough (Tatar Strait, the Sea of Japan)

The concentrations of 14 polycyclic aromatic hydrocarbons (PAHs) in the sediment of the Tatar Trough were studied. Despite the increase in PAH concentrations over recent decades, which is likely the result of the handling and transportation of fossil fuels, PAH levels and ecological risk were found to be low. The spatial pattern revealed that higher PAH concentrations were mainly in the deeper water sites, suggesting that trough slope failure transported the PAHs to the deeper part of the basin. There was no correlation between the PAHs and grain size or the PAHs and organic carbon that is related to the PAH input from a variety of sources and the heterogeneity of organic matter. The PAH composition, isomer ratio, and PCA identified two areas with different PAH sources. The most northern part of the Tatar Trough received petrogenic PAHs that are probably transported downslope from the northern Tatar Strait where fossil fuels are handled in some ports. Another trough part was polluted by the PAHs from the combustion of coal and biomass and the exhaust of marine vehicles. The minor presence of genes responsible for aerobic PAH destruction can be explained by the anaerobic degradation of PAHs or the spontaneous creation of favorable conditions that promote bacterial PAH oxidation.

Oxidation of Polycyclic Aromatic Hydrocarbons with Mono-Oxygen Donors Catalysed by Triaryl-Manganese(III) Corroles

Abstract: The oxidation of polycyclic aromatic hydrocarbons (PAH) with mono-oxygen donors such as iodosylbenzene, hydrogen peroxide and tert-butyl hydroperoxide in the presence of selected 5,10,15-triarylmanganese( III) corroles as catalysts were examined under different reaction conditions. The effect of reaction conditions on triaryl-manganese(III) corroles catalysts and oxidants is discussed. It was observed that in a solvent system consisting of a mixture of dichloromethane and acetonitrile, the oxidation of anthracene with tertbutyl hydroperoxide catalysed by tri(aryl)manganese(III)corroles worked better than with hydrogen peroxide or iodosylbenzene as the oxidants. The oxidation of anthracene with hydroxo-manganese(IV)corroles was also examined in organic solvents under mild conditions. UV-Vis and other spectroscopic data were used to determine the oxidative mechanism for the oxidation of anthracene and other polycyclic aromatic hydrocarbons with mono-oxygen donors catalysed by triaryl-manganese(III)corroles.

Oxidation of Polycyclic Aromatic Hydrocarbons: Evidence of Similarities in Thermochemical Properties and Reaction Paths

ABSTRACT This work elaborates similarities of thermochemical properties and oxidation kinetics among several polycyclic aromatic hydrocarbons. The systems considered are benzene (A1), naphthalene (A2), phenanthrene (A3), pyrene (A4), and the larger ones, benzo[ghi]perylene (A6) and coronene (A7). For each system, thermochemical properties, bond energies and oxidation kinetics are calculated and compared. Interpretation of the results revealed strong similarities and redundancies in behavior among these systems. The results imply that reactions are not dependent on the size of the molecules and, therefore, reaction paths and kinetic barriers determined with high-level calculations for small molecules can be used as estimates in the investigation of larger aromatic hydrocarbons to avoid expensive calculation efforts.

Roles of Natural Phenolic Compounds in Polycyclic Aromatic Hydrocarbons Abiotic Attenuation at Soil-Air Interfaces through Oxidative Coupling Reactions.

Little information is available on the roles of natural phenolic compounds in polycyclic aromatic hydrocarbons (PAHs) attenuation at dry soil-air interfaces. The purpose of this study was to determine the roles of model phenolic constituents of soil organic matter (SOM) on the abiotic attenuation of PAHs. The phenolic compounds can significantly change the attenuation rates of PAHs, among which hydroquinone was the most effective in promoting anthracene and benzo[a]anthracene attenuation. Product identification and sequential extraction experiments revealed hydroquinone enhanced the formation of oxidative coupling products and promoted the incorporation of PAHs into humic analogues, thereby reducing potential risks to humans and ecosystems. Electron paramagnetic resonance spectroscopy analyses showed both PAHs and phenolic compounds could donate electrons to Lewis acid sites of soil minerals, resulting in the generation of persistent free radicals (PFRs). PFRs could promote the generation of ·OH to enhance PAH oxidation and could cross-couple with PAHs, resulting in high-molecular-weight oxidative coupling products. This study revealed for the first time the reaction mechanism between PAHs and phenolic components of SOM under relatively dry conditions and provided new insights into promoting PAHs detoxification in soils but also a potential strategy to increase the organic carbon sequestration.

Kill two birds with one stone: Solubilizing PAHs and activating PMS by photoresponsive surfactants for the cycle remediation of contaminated groundwater

Polycyclic aromatic hydrocarbons (PAHs) represent a dangerous threat to the groundwater environment. PAHs are difficult to be effectively removed from groundwater because of their hydrophobicity. In this study, a combination of a Gemini photosensitive surfactant (N1, N2-bis[4-[4-[(4-butylphenyl) azo] phenoxy] butyl]-N1, N2-tetramethylethane-1,2-diammonium bromide, AzoPBT) with peroxymonosulfate (PMS) is developed to treat PAHs-contaminated groundwater. To investigate the reaction mechanism of the PMS/AzoPBT process, we chose naphthalene (Nap) and phenanthrene (Phe) as target pollutants. The quenching experiments, electron paramagnetic resonance (EPR) spectroscopy and the identified degradation products indicated that singlet oxygen could contribute to the degradation of PAHs. A tentative solution for Phe degradation was proposed based on the degradation products. The role of surfactant micelles in the PMS/AzoPBT process is further explored. The positively charged micelles in the PMS/AzoPBT process escalate the removal of PAHs by electrostatically polymerizing Br- and HSO5- in the solution. Although increasing the number of micelles can enhance the solubilization of PAHs by surfactant solutions, the encapsulation of PAHs by micelles can “protect” PAHs, which hinders the removal of PAHs. The PMS/AzoPBT process can be applied in a broad range of pH values. In the process of recycling AzoPBT to activate PMS to remediate groundwater, PAHs can be completely removed in the first cycle, and 51.4 % of them can be removed after four times of surfactant use. The combination of photoresponsive surfactants and PMS achieves efficient oxidation of PAHs without additional activators while increasing the solubility of PAHs in groundwater. This novel technology of PAHs-contaminated groundwater treatment may provide insights into the degradation mechanism of the PMS/AzoPBT process.

Electrochemistry-enhanced peroxymonosulfate activation by CoAl-LDH@biochar for simultaneous treatment of heavy metals and PAHs

An EC/CoAl-LDH@biochar/PMS system is proposed in this study for activation of PMS and removal of heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs). In this system, over 90% of HMs and PAHs could be effectively removed after 40 min. Compared to other systems, the synergy between CoAl-LDH@biocahr and electrochemistry (EC)-peroxymonosulfate (PMS) had an excellent ability to eliminate the combined pollutants. The precise effect of the experimental parameters, including pH, CoAl-LDH@biochar dose, current density, and PMS concentration, on HMs-PAHs removal was determined, and the system was found to have a broad working pH range. The presence of EC can overcome the inhibition of anions in the degradation of PAHs due to the direct oxidation of PMS and PAHs on the cathode. The scavenging tests confirmed that the free radical and non-radical processes contributed to the degradation of PAHs. It is also revealed that EC can promote the continuous circulation of transition metals, which enhances the activation of PMS. The adsorption on the surface of CoAl-LDH@biochar and electrochemical deposition were the two main mechanisms for HMs removal. Furthermore, the electrode and CoAl-LDH@biochar possessed favorable recyclability in five consecutive cycles. This study has provided a promising and cost-effective approach for treatment of HM-PAH co-contaminated wastewater.

Secondary Organic Aerosol Formation from Semi-Volatile and Intermediate Volatility Organic Compounds in the Fall in Beijing

Intermediate volatility organic compounds (IVOCs) and semi-volatile organic compounds (SVOCs) have recently been proposed as important precursors of secondary organic aerosol (SOA). In the present work, 97 volatile organic compounds (VOCs) and 80 intermediate volatility and semi-volatile organic compounds (IVOCs and SVOCs) were measured by online gas chromatography-mass spectrometer/flame ionization detection (GC-MS/FID), and offline thermal desorption gas chromatography-mass spectrometer (TD-GC-MS), respectively. The average concentration of speciated VOCs, IVOCs, and SVOCs were 22.36 ± 9.02 μg m−3, 1.01 ± 0.32 μg m−3, and 0.10 ± 0.17 μg m−3. Alkanes and polycyclic aromatic hydrocarbons (PAHs) are the main compounds of total S/IVOCs. With the increase in molecular weight, the concentrations decreased in the gas phase, while increasing in the particle phase. Vehicular emission is the most significant source according to the carbon preference index (CPI) and the carbon of the most abundant alkane (Cmax). The yield method was used to estimate SOA from the oxidation of VOCs and S/IVOCs. The estimated SOA mass from IVOCs and SVOCs (0.70 ± 0.57 μg m−3) was comparable to that of VOCs (0.62 ± 0.61 μg m−3), and the oxidation of PAHs and alkanes took up 28.70 ± 8.26% and 51.97 ± 20.77% of the total SOA estimation, respectively. Compared to previous work, our study provided detailed molecular information of ambient S/IVOC species and elucidated their importance on SOA formation. Despite their low concentration, S/IVOCs species are important SOA precursors which shared comparable contribution compared with VOCs.

A ReaxFF molecular dynamics study of polycyclic aromatic hydrocarbon oxidation assisted by nitrogen oxides

Fossil fuel-derived soot poses a persistent problem. A joint reduction is conducted via the reaction between NOx and soot. The underlying reaction mechanisms of large polycyclic aromatic hydrocarbons (PAHs), as well as the interactions between O2, NO and NO2, have been extensively investigated by ReaxFF molecular dynamics simulations for the first time. The pyrolysis and oxidation of coronene are conducted at 2500 K and 50 atm. Coronene in the pyrolysis system first experiences dehydrogenation reactions and subsequently undergoes recombination reactions, leading to the formation of large PAHs. Among the three oxidizers, NO2 is the strongest in coronene oxidation, followed by NO and O2. Meanwhile, the O radical is identified as the key species for PAH oxidation. In the presence of O2, the formation of O radicals requires the assistance of H radicals that are formed by the dehydrogenation of PAHs, which retards the oxidation process. In the presence of NO, O radicals can be directly formed via reactions of NO → N+O and N + NO ⇌ N2 + O. NO2 first undergoes a decomposition reaction: NO2 → NO + O, followed by NO decomposition. O-addition and N-addition, as well as subsequent fragmentation reactions including the generation of COx and (H)CN are important routes in PAH oxidation. Furthermore, compared to NO, NO2 provides more O radicals that significantly accelerate PAH oxidation. This study provides fundamental insight into PAH oxidation that may help to design strategies to inhibit soot and NOx emissions at the source.