Photolysis Of Polycyclic Aromatic Hydrocarbons Research Articles

In situ investigation of N deposit effects on polycyclic aromatic hydrocarbons (PAHs) photolysis in snow

As snow is an ideal scavenger of polycyclic aromatic hydrocarbons (PAHs) for both particulate- and gas-phase PAHs, a large amount of PAHs are accumulated in snowpacks. These PAHs could be released from seasonal snowpacks into environment over a short time during the snow melting period, causing a flush of contaminants. Understanding the fate of PAHs in snow therefore is important to evaluate the contaminant flush during snow melting. However, as an important natural process, PAH photolysis in snow is still unclear, especially in the context of global climate change. Therefore, photolysis of the 16 PAHs, which are listed as priority controlled pollutants by United States Environmental Protection Agency (US EPA), was investigated with N deposit in field in snow in this study. A mixture of the 16 US EPA priority PAHs and nitrate/ammonia were spiked into sealed quartzose bottles containing snow samples in field. The bottles were then exposed to solar radiation in field for 6 h. Experimental results showed that >50% of PAHs were lost during 6 h photolysis, mostly occurred in the first hour. Generally, HMW PAHs were photolyzed faster than LMW PAHs, though the photolysis rate did not strictly increase with molecular weight. Low concentrations of nitrate can enhance photolysis of Ant and Pyr, while photolysis of investigated PAHs was inhibited with nitrate concentration increasing. In contrast, both low and high concentration of ammonium can inhibit PAH photolysis. The inhibiting effect of both nitrate and ammonia on photolysis of HMW PAH is stronger than that of LMW PAHs, indicating direct photolysis of PAHs is the main pathway of PAH degradation in snow in situ. This study firstly showed the effect of N deposition on PAH photolysis in snow. Under current increasing N deposition in northeast China, this study implied that more PAHs in snow could be preserved during winter and released shortly during snowmelt, resulting in high ecological risk in spring.

Hydroxyl radical (OH) formation during the photooxidation of anthracene and its oxidized derivatives

Polycyclic aromatic hydrocarbons (PAH) are a ubiquitous atmospheric pollutant, comprising between 2% and 27% of PM2.5 by mass. They undergo rapid photooxidation forming oxidized products (oxPAH), and humic-like substances (HULIS). HULIS has known climate-forcing potential, and oxPAH have been reported to be more toxic than their parent PAH. The hydroxyl radical (OH) is associated with the photolysis of PAH. Anthracene and a selection of oxPAH (1-naphthol, 2-naphthol, 1,4-naphthoquinone, 1,4-anthraquinone, and 9,10-anthraquinone) were exposed to simulated sunlight for 16 h and OH concentration was measured. Formation rates (ROH) were calculated for the first 30 min of the reaction. The ROH for 1,4-naphthoquinone was initially very high and declined logarithmically over the 30-min interval, while anthracene had a low initial ROH which then increased linearly over the 30-min interval. It was determined that the production of OH is not a sole predictor of HULIS formation: photolysis of both 1-naphthol and 2-naphthol result in HULIS formation, but 2-naphthol does not result in a substantial OH concentration. Conversely, neither 1,4-anthraquinone or 9,10-anthraquinone result in HULIS, but OH was seen with 9,10- anthraquinone photolysis. This suggests that, while OH is important, pathways resulting in HULIS from PAH which do not include OH are also significant.

Photolysis of polycyclic aromatic hydrocarbons adsorbed on polyethylene microplastics

Contaminants adsorbed on microplastics (MPs) are a potential risk to aquatic environments. Several studies have demonstrated that polycyclic aromatic hydrocarbons (PAHs), which adsorb on MPs, can be photolyzed in aqueous solutions. We investigated photolysis of PAHs on MPs under sunlight conditions to estimate their environmental fate for the first time. The PAHs (25 ng each) were added to polyethylene powder, which was used as the MP sample. The MP sample was agitated in water with sunlight irradiation; thereafter, the concentration of the PAHs on the MP sample was determined by high-pressure liquid chromatography with a fluorescence detector. The half-life values of the PAHs were estimated between 3.4 × 102 (pyrene) and 3.2 × 104 min (benz[j]fluoranthene). These values are 0.5 (fluoranthene) to 25 (benzo[a]pyrene) times larger than those in the aqueous phase. Additionally, the degradation of PAHs by OH radicals produced by the photolysis of nitrate was observed.

Influence of photolysis on source characterization and health risk of polycyclic aromatic hydrocarbons (PAHs), and carbonyl-, nitro-, hydroxy- PAHs in urban road dust.

In this study, PAHs and their transformed PAH products (TPPs) in road dust were subjected to UV driven photolysis, and then extracted using simultaneous pressurized fluid extraction, and analysed using Shimadzu Triple Quadrupole GC/MS. The results of the analysis were used to investigate the robustness and reliability of 14 existing diagnostic ratios (DRs) and two newly proposed molecular DRs that are relevant for characterizing the sources of PAHs and TPPs. The influence of photolysis on the carcinogenic health risk posed to humans by these hazardous pollutants was then assessed. The findings indicated that the DRs segregated into stable, moderately stable and non-stable classes of source characteristics under the influence of photolysis. Only two of the existing DRs, namely, benzo(a)pyrene/benzo(ghi)perylene (BaP/BghiP) and total index exhibited consistent stability to photolysis, whilst fluoranthene/(fluoranthene+pyrene) (FRT/(FRT+PYR)) showed moderate stability. The two newly proposed DRs, naphthalene/1-nitronaphthalene (NAP/NNAP) and pyrene/(1-nitropyrene+1-hydroxypyrene) (PYR/(1NPY+HPY)) were found to be highly reliable in post-emission source characterization. The cross-plots of the most stable DRs showed that traffic emissions is the primary source of PAHs, whilst post-emission photolysis is the secondary source of nitro-PAH (NPAH) TPPs. The percent resonance energy thermodynamic stability of the PAH pollutants does not exert any direct influence on the source characteristics of the DRs. Adults are more vulnerable to potential carcinogenic risks as a result of PAH and TPPs photolysis whereas negligible risk exist for children. This study contributes to a more reliable diagnosis of PAH and TPP sources and thus, to the regulatory mitigation of these hazardous pollutants thereby, promoting enhanced protection of human health and the environment.

Photolysis of polycyclic aromatic hydrocarbons (PAHs) on Fe3+-montmorillonite surface under visible light: Degradation kinetics, mechanism, and toxicity assessments

Photochemical behavior of various polycyclic aromatic hydrocarbons (PAHs) on Fe3+-modified montmorillonite was explored to determine their potential kinetics, pathways, and mechanism under visible light. Depending on the type of PAH molecules, the transformation rate follows the order of benzo[a]pyrene ≈ anthracene > benzo[a]anthracene > phenanthrene. Quantum simulation results confirm the crucial role of “cation-π” interaction between Fe3+ and PAHs on their transformation kinetics. Primary intermediates, including quinones, ring-opening products and benzene derivatives, were identified by gas chromatography-mass spectrometer (GC-MS), and the possible photodegradation pathway of benzo[a]pyrene was proposed. Meanwhile, radical intermediates, such as reactive oxygen species (ROS) and free organic radicals, were detected by electron paramagnetic resonance (EPR) technique. The photolysis of selected PAHs, such as anthracene and benzo[a]pyrene, on clay surface firstly occurs by electron transfer from PAHs to Fe3+-montmorillonite, followed by degradation involving photo-induced ROS such as ·OH and ·O2−. To investigate the acute toxicity of photolysis products, the Microtox® toxicity test was performed during the photodegradation processes of various PAHs. As a result, the photo-irradiation initially induces increased toxicity by generating reactive intermediates, such as free organic radicals, and then the toxicity gradually decreases with increasing of reaction time. Overall, the present study provides useful information to understand the fate and photo-transformation of PAHs in contaminated soils.

Anthracene and pyrene photolysis kinetics in aqueous, organic, and mixed aqueous-organic phases

Condensed phases in the atmosphere, such as cloud droplets and aerosols, often contain both water and organic matter (OM). Reactivity can differ significantly between aqueous and organic phases. We have measured photolysis kinetics of the polycyclic aromatic hydrocarbons (PAHs) anthracene and pyrene in several organic solvents and in water, as well as in miscible and phase-separated aqueous-organic mixtures at atmospherically-relevant wavelengths. Photolysis rate constants generally increased with increasing solvent polarity; photolysis of both PAHs was more than ten times faster in water than in octanol. Local polarity had a much greater effect on PAH photolysis kinetics than changes in PAH absorptivity or singlet oxygen concentrations. Photolysis kinetics in homogeneous aqueous-organic mixtures varied monotonically with2 OM volume fraction. Kinetics in immiscible (phase-separated) solutions were more complex, with different dependences on OM content observed in stagnant and turbulent solutions. Our results suggest that OM could greatly affect the photochemical lifetimes of PAHs in atmospheric condensed phases such as aerosols, even if the OM does not itself absorb photons.

Effect of low-molecular-weight organic acids on photo-degradation of phenanthrene catalyzed by Fe(III)–smectite under visible light

The photolysis of polycyclic aromatic hydrocarbons (PAHs) is potentially an important process for its transformation and fate on contaminated soil surfaces. In this study, phenanthrene is employed as a model to explore PAH photodegradation with the assistance of Fe(III)–smectite under visible-light while focusing on roles played by five low-molecular-weight organic acids (LMWOAs), i.e., malic acid, oxalic acid, citric acid, ethylenediaminetetraacetic acid (EDTA), and nitrilotriacetic acid. Our results show that oxalic acid is most effective in promoting the photodegradation of phenanthrene, while only a slight increase in the rate of phenanthrene photodegradation is observed in the presence of malic acid. Electron paramagnetic resonance experiments confirm the formation of CO2− radicals in the presence of malic and oxalic acid, which provides strong evidence for generating OH and subsequent photoreaction pathways. The presence of EDTA or nitrilotriacetic acid significantly inhibits both Fe(II) formation and phenanthrene photodegradation because these organic anions tend to chelate with Fe(III), leading to decreases in the electron-accepting potential of Fe(III)–smectite and a weakened interaction between phenanthrene and Fe(III)–smectite. These observations provide valuable insights into the transformation and fate of PAHs in the natural soil environment and demonstrate the potential for using some LMWOAs as additives for the remediation of contaminated soil.

Exchangeable cations-mediated photodegradation of polycyclic aromatic hydrocarbons (PAHs) on smectite surface under visible light

Clay minerals saturated with different exchangeable cations are expected to play various roles in photodegradation of polycyclic aromatic hydrocarbons (PAHs) via direct and/or indirect pathways on clay surfaces. In the present study, anthracene and phenanthrene were selected as molecule probes to investigate the roles of exchangeable cations on their photodegradation under visible light irradiation. For five types of cation-modified smectite clays, the photodegradation rate of anthracene and phenanthrene follows the order: Fe3+>Al3+>Cu2+>>Ca2+>K+>Na+, which is consistent with the binding energy of cation–π interactions between PAHs and exchangeable cations. The result suggests that PAHs photolysis rate depends on cation–π interactions on clay surfaces. Meanwhile, the deposition of anthracene at the Na+-smectite and K+-smectite surface favors solar light absorption, resulting in enhanced direct photodecomposition of PAHs. On the other hand, smectite clays saturated with Fe3+, Al3+, and Cu2+ are highly photoreactive and can act as potential catalysts giving rise to oxidative radicals such as O2− , which initiate the transformation of PAHs. The present work provides valuable insights into understanding the transformation and fate of PAHs in the natural soil environment and sheds light on the development of technologies for contaminated land remediation.

Nonchromophoric Organic Matter Suppresses Polycyclic Aromatic Hydrocarbon Photolysis in Ice and at Ice Surfaces

We have investigated the effects of organic matter (OM) that does not absorb sunlight ("nonchromophoric") on the reactive environment presented by bulk ice and ice surfaces. Fluorescence spectroscopy showed that the presence of as little as 2.5 × 10(-4) M octanol or decanol reduces the extent to which naphthalene self-associates at ice surfaces, which indicates that naphthalene partitions between ice and organic phases present there. We also measured photolysis kinetics of the polycyclic aromatic hydrocarbons (PAHs) anthracene, pyrene, and phenanthrene in bulk ice and at ice surfaces containing 2.5 × 10(-5) M to 7.5 × 10(-3) M OM. In bulk ice, even the lowest concentrations of OM reduced photolysis kinetics to below our detection limits. Organic matter also reduced measured photolysis kinetics of PAHs at ice surfaces, but generally to a lesser extent than in bulk ice. Our results support previous reports that bulk ice and ice surfaces present distinct reaction environments, and show for the first time that OM can affect PAH photolysis kinetics by altering the physical environment within bulk ice and at ice surfaces.

Photolysis of polycyclic aromatic hydrocarbons on soil surfaces under UV irradiation

Photolysis of some polycyclic aromatic hydrocarbons (PAHs) on soil surfaces may play an important role in the fate of PAHs in the environment. Photolysis of PAHs on soil surfaces under UV irradiation was investigated. The effects of oxygen, irradiation intensity and soil moisture on the degradation of the three PAHs were observed. The results showed that oxygen, soil moisture and irradiation intensity enhanced the photolysis of the three PAHs on soil surfaces. The degradation of the three PAHs on soil surfaces is related to their absorption spectra and the oxidation-half-wave potential. The photolysis of PAHs on soil surfaces in the presence of oxygen followed pseudo first-order kinetics. The photolysis half-lives ranged from 37.87 days for benzo[a]pyrene to 58.73 days for phenanthrene. The results indicate that photolysis is a successful way to remediate PAHs-contaminated soils.

Direct photolysis of polycyclic aromatic hydrocarbons in drinking water sources

The widely used low pressure lamps were tested in terms of their efficiency to degrade polycyclic aromatic hydrocarbons listed as priority pollutants by the European Water Framework Directive and the U.S. Environmental Protection Agency, in water matrices with very different compositions (laboratory grade water, groundwater, and surface water). Using a UV fluence of 1500 mJ/cm 2, anthracene and benzo(a)pyrene were efficiently degraded, with much higher percent removals obtained when present in groundwater (83–93%) compared to surface water (36–48%). The removal percentages obtained for fluoranthene were lower and ranged from 13 to 54% in the different water matrices tested. Several parameters that influence the direct photolysis of polycyclic aromatic hydrocarbons were determined and their photolysis by-products were identified by mass spectrometry. The formation of photolysis by-products was found to be highly dependent on the source waters tested.

Anthracene Photolysis in Aqueous Solution and Ice: Photon Flux Dependence and Comparison of Kinetics in Bulk Ice and at the Air−Ice Interface

We report an investigation of the photolysis kinetics of polycyclic aromatic hydrocarbons (PAHs) in aqueous solution, frozen in ice, and at air-ice interfaces. Measurements of PAH photolysis rates in aqueous solution and at air-ice interfaces as a function of lamp power show that the kinetics depend nonlinearly on photon flux. In both media, the rates do not increase when lamp powers are above 0.1 W, which corresponds to total photon fluxes lower than 10(13) photon cm(-2) s(-1) in the actinic region. This suggests that extrapolating laboratory-measured rates to expected atmospheric photon fluxes may not yield accurate lifetimes for some species. In the plateau region of the photon flux dependence, anthracene located within the ice matrix (or in liquid pockets or veins in the ice) undergoes photolysis at a similar rate to that in room temperature aqueous solution, but the rate of anthracene photolysis at air-ice interfaces is five times greater. This indicates that in order to accurately predict the lifetimes of aromatic pollutants in snow and ice, the quasi-liquid layer (QLL) must be treated separately from bulk ice.

Photolysis of Polycyclic Aromatic Hydrocarbons on Water and Ice Surfaces

Laser-induced fluorescence detection was used to measure photolysis rates of anthracene and naphthalene at the air-ice interface, and the kinetics were compared to those observed in water solution and at the air-water interface. Direct photolysis proceeds much more quickly at the air-ice interface than at the air-water interface, whereas indirect photolysis due to the presence of nitrate or hydrogen peroxide appears to be suppressed at the ice surface with respect to the liquid water surface. Both naphthalene and anthracene self-associate readily on the ice surface, but not on the water surface. The increase in photolysis rates observed on ice surfaces is not due to this self-association, however. The wavelength dependence of the photolysis indicates that it is due to absorption by the PAH. No dependence of the rate on temperature is seen, either at the liquid water surface or at the ice surface. Molecular oxygen appears to play a complex role in the photolytic loss mechanism, increasing or decreasing the photolysis rate depending on its concentration.

Photolysis of polycyclic aromatic hydrocarbons associated with fly ash particles under simulated sunlight irradiation

Fly ash particle-associated polycyclic aromatic hydrocarbons (PAHs) were produced by combustion of polyvinyl chloride (PVC), wood, high-density polyethylene (HDPE), and polystyrene in a stove. Fly ash particles were deposited on the glass slides (76 mm × 26 mm) placed in the exposure chamber. The concentration of 16 PAHs ranged from 10.1 pg/dm 2 fly ash for acenaphthene to 2348.7 pg/dm 2 fly ash for fluoranthene. Medium to large molecular weight PAH concentrations were much higher compared to low molecular weight PAHs on fly ash. Photolysis of PAHs associated with fly ash particles deposited on glass slides was investigated under simulated sunlight irradiation for about 550 h. Generally, the photolysis rates of the 16 PAHs follow first-order kinetics. Under simulated sunlight containing UV-B, the photolysis of PAHs includes two phases: fast (0–50 h) and slow phase (50–550 h). The photolysis rate constants of PAHs in the initial fast phase were far greater than those in the slow phase. When UV-B was removed from the simulated sunlight irradiation, the photolysis rate constants of PAHs were significantly lower compared to those observed in the fast phase with UV-B exposure, suggesting UV-B radiation could accelerate photolysis of PAHs adsorbed on the fly ash particles. In addition, partial least squares (PLS) analysis models were applied to explain the molecular structural characteristics that affect the PAH photolysis rates. The results suggested that the photolysis rate constants of PAHs decrease with the increasing of molecular weight ( M w), the energy of the lowest unoccupied molecular orbital ( E LUMO), E LUMO + E HOMO, and the standard heat of formation (Δ H f).

Disappearance of polycyclic aromatic hydrocarbons sorbed on surfaces of pine [Pinua thunbergii] needles under irradiation of sunlight: Volatilization and photolysis

The solar photodegradation of 16 polycyclic aromatic hydrocarbons (PAHs), sorbed on surfaces of pine [ Pinua thunbergii ] needles was investigated. The PAHs were produced by combustion of polystyrene and exposed onto the surfaces of pine needles. The disappearance of PAHs sorbed on the pine needle surfaces is mainly caused by volatilization and photolysis , with photolysis playing a major role. The volatilization rates correlate with PAH molecular weight significantly. The photolysis of the 16 PAHs follows first-order kinetics and their photolysis half-lives ( t 1/2,P ) range from 12.9 h for naphthalene to 65.4 h for fluorene. The PAHs have similar half-lives whether they are sorbed on spruce or pine needles. Compared with water, the cuticular waxes of pine needles can stabilize photolysis of PAHs and facilitate accumulation of PAHs. t 1/2,P for selected PAHs correlate with semi-empirically calculated energy of the highest occupied orbital ( E HOMO ). Photochemical behaviors of PAHs are dependent not only on their molecular structures but also the physical–chemical properties of the substrate on which they are adsorbed.

Photolysis of polycyclic aromatic hydrocarbons adsorbed on spruce [Picea abies (L.) Karst.] needles under sunlight irradiation

Photolysis of polycyclic aromatic hydrocarbons (PAHs) sorbed on surfaces of spruce [Picea abies (L.) Karst.] needles under sunlight irradiation was investigated. PAHs were produced by combustion of polyvinyl chloride (PVC), wood, high-density polyethylene (HDPE), and styrene in a stove. The factors of sunlight irradiation on the surfaces of spruce needles were taken into consideration when investigating the kinetic parameters. The photolysis of the 18 PAHs under study follows first-order kinetics. The photolysis half-lives range from 15 h for dibenzo(a,h)anthracene to 75 h for phenanthrene. Photolysis of some PAHs on surfaces of spruce needles may play an important role on the fate of PAHs in the environment.

PAH degradation by UV/H 2O 2 in perfluorinated surfactant solutions

Polycyclic aromatic hydrocarbons (PAHs) solubilized in perfluorinated surfactant (PFS) solutions were degraded by direct photolysis and UV/H 2O 2 process. The subsequent recovery and reuse of these surfactant solutions were also demonstrated. Phenanthrene and pyrene were selected as representative PAHs and an anionic PFS; ammonium perfluorooctanoate (APFO) was used. In our experiments, micellar APFO solutions retarded the phenanthrene photolysis and enhanced the pyrene photolysis. The results indicate that the photochemical reactivity of compound in micelles is strongly dependent on specific properties of the solubilizate, possibly due to the different excited state behaviors of compound. UV/H 2O 2 process exhibits a greatly enhanced rate of PAH photolysis in both water and APFO compared to direct photolysis, indicating that hydroxyl radicals may be generated or penetrated at the sites of PAHs solubilized in the micelles. Additionally, a smaller rate enhancement by UV/H 2O 2 in micelles than in water suggests that micelles provide some degree of protection from hydroxyl radical attack. The possibility of recovery and reuse of PFS has been demonstrated by measuring the solubilizing capacity of APFO after direct photolysis and UV/H 2O 2 process. Overall, this study demonstrates UV/H 2O 2 process can be an effective treatment method for not only PAH degradation but also surfactant recovery and reuse.

Photolysis of PAHs in aqueous phase by UV irradiation

The photooxidation of polycyclic aromatic hydrocarbons (PAHs) was investigated in an aqueous ethanolic solution irradiated with a medium-pressure mercury lamp in laboratory photoreactors equipped with a quartz immersion well. Degradation photolysis of fluorene was more efficient than sensitized photolytic oxidation in the presence of TiO 2 suspensions. Photolysis kinetics was dependent on molecular weight and the presence and type of substituents. During the photolytic degradation of fluorene and its derivatives, 9-fluorenone and its corresponding derivatives, which were more resistant to photolysis, were formed.

Is it possible to develop a QSPR model for direct photolysis half-lives of PAHs under irradiation of sunlight?

By the use of the partial least squares method and 11 fundamental quantum chemical descriptors computed from the PM3 Hamiltonian, a Quantitative Structure–Property Relationship model was obtained for direct photolysis half-lives of selected polycyclic aromatic hydrocarbons (PAHs) under irradiation of sunlight. Direct photolysis half-lives for some other PAHs without reported values were predicted. It was concluded from the model that the main factors affecting photolysis half-lives of PAHs under irradiation of sunlight are PAH absolute hardness and electronegativity, which are related to the energy difference between the lowest unoccupied molecular orbital and the highest occupied molecular orbital, ( E lumo− E homo) and ( E lumo+ E homo), respectively. Increasing E homo and the average molecular polarizability ( α) values of the PAHs leads to decrease of photolysis half-lives. Increasing ( E lumo- E homo) and E lumo values of the PAHs leads to an increase of the PAH photolysis half-lives.

Photolysis of polycyclic aromatic hydrocarbons in water

The decomposition of benzo[a]pyrene (BAP), chrysene (CHR) and fluorene (FLU) in an aqueous solution by means of photolysis has been studied. The influence of initial polycyclic aromatic hydrocarbons’ (PAHs) concentration, pH of the reaction mixture, temperature, presence of oxygen and tert-butyl alcohol ( t-BuOH) on the degradation rate has been observed. BAP and CHR are decomposed by a mechanism different than FLU. Quantum yields of the photolytic decomposition of BAP, CHR and FLU were determined and equal 0.014, 0.0031 and 0.0038, respectively.