Dissolution Of Polycyclic Aromatic Hydrocarbons Research Articles

Selective extraction of aromatics from residual oil with subcritical water

Based on theoretical calculations and experimental characterizations, the extraction of heavy oil components using subcritical water (Sub-CW) as an extraction solvent was studied in the temperature range of 250–325 °C. According to theoretical calculations, small-scale polycyclic aromatic hydrocarbons (PAHs) are preferentially dissolved in Sub-CW. Meanwhile, the dissolution of heavy oil molecules with large-scale aromatic structures and heterocyclic rings in Sub-CW is hindered. The increase in temperature reduces the hydrogen bonding force between water molecules and the van der Waals repulsion between Sub-CW and PAHs, promoting the dissolution of PAHs in Sub-CW. Extraction experiments applied in a semi-continuous device confirm that Sub-CW selectively extracts aromatic hydrocarbons from heavy oil. Increasing extraction temperature increases the yield of the extract. The aromatic hydrocarbon content in the extract at 325 °C reaches 80 wt%, which is 27 wt% higher than feedstock value. In addition, the asphaltene content in the extract is reduced from the feedstock value of 14 wt% to 2 wt%.

Polycyclic aromatic hydrocarbons dissolution in supercritical carbon dioxide by molecular dynamics simulation

With the rapid development of industry, the problem of soil contamination has grown increasingly serious, posing a serious threat to the wellness of people. Polycyclic aromatic hydrocarbons (PAHs) are among the most common soil contaminants. Supercritical CO2 extraction has been widely used to treat PAHs in soils. Solute solubility in supercritical fluids is a fundamental thermophysical parameter that must be investigated and modeled. In this work, we employed molecular dynamics simulation to investigate the microscopic dissolution process in supercritical CO2 of four PAHs: naphthalene, anthracene, pyrene, and benzo(α)pyrene. We also looked into the implications of changing the ring number of PAHs on the ability of supercritical CO2 to dissolve PAHs by calculating the interaction energy, radial distribution function, and the coordination number between PAHs and supercritical CO2. The results indicated that the PAHs’ capacity to dissolve in supercritical CO2 decreased gradually with the increase of ring number. The interaction energy between PAHs and supercritical CO2 decreases as the number of PAHs rings increases. Among them, the van der Waals interaction energy plays a decisive role in the dissolution of PAHs in supercritical CO2. As the number of PAHs rings increased, the positions of the first peak and valley of the radial distribution function remained the same, which appeared at about 4.3 Å and 6.1 Å, respectively. However, the magnitudes of the peaks and valleys decreased. As the number of PAHs rings increased, the coordination number of the C atoms of supercritical CO2 around the C atoms of PAHs decreased.

Enhancement of PAHs biodegradation in biosurfactant/phenol system by increasing the bioavailability of PAHs

The poor bioavailability of polycyclic aromatic hydrocarbons (PAHs) is the main limiting factor for their biodegradation in contaminated sites. The addition of biosurfactant is an effective method for enhancing the bioavailability of PAHs. Suitable low molecular weight (LMW) organic matters have been shown to increase the bioavailability of PAHs. Therefore, we investigated the effect of phenol, which often co-exists with PAHs, on the biodegradation of PAHs in biosurfactant solution. The results show that the critical micelle concentration (CMC) of the biosurfactant decreased after phenol was added. The formation of mixed micelles resulted in enhancement of PAHs dissolution. The weight solubilization ratio (WSR) values of biosurfactant for Phe, Pyr and BaP in phenol solution are approximately 1.34, 1.40 and 1.67 times that of the control group, respectively. Phenol, therefore, can assist biosurfactant to increase the availability of PAHs by microbes. The bioavailability of PAHs in sludge increased from 27.7% to 43.1% after the biosurfactant was added, and reached a maximum of 49.2%, following the simultaneous addition of phenol and biosurfactant. Phenol also improved the degradation of PAHs by Stenotrophomonas sp. N5 in biosurfactant solution.

Dissolution of polycyclic aromatic hydrocarbons in supercritical water in hydrogen production process: A molecular dynamics simulation study

Rapid heating of coal particles in supercritical water (SCW) is the prerequisite for complete gasification, while there is a rapid precipitation of volatiles during this process. If the volatiles cannot be dissolved and diffused in time, side reaction is inclined to occur, especially coking. The solubility of the volatiles in SCW has not yet been studied because the composition is not simple and the corresponding interaction with SCW is complex. In this paper, polycyclic aromatic hydrocarbons (PAHs) were used as the volatile model compound. Molecular dynamics (MD) simulations focused on the dissolution of pure PAHs and PAHs mixtures in SCW were carried out. The investigation results indicated that the dissolution behavior of naphthalene was the most excellent in SCW, while heavy PAHs were sensitive to the thermodynamic state of water molecules. The light PAHs were liable to be dissolved, while heavy PAHs were more likely to be remained in the PAH droplet. Moreover, the increase of temperature and density has a positive impact on the dissolution of PAHs in the supercritical solvent. This work is prospective to provide a theoretical basis for ensuring efficient and complete gasification of coal and restraining side effects for hydrogen production.

Dissolution of polycyclic aromatic hydrocarbons in subcritical and supercritical Water: A molecular dynamics simulation study

To get a better understanding of the upgrading of heavy oil under severe hydrothermal environment, a molecular dynamics simulation on the dissolution of oil droplets containing polycyclic aromatic hydrocarbons (PAHs) or PAH mixtures in subcritical or supercritical water (sub-CW/SCW) was applied. Being the representative of light PAHs, naphthalene dissolves readily into sub-CW/SCW. The dissolution of heavy PAHs, like benzo[α]pyrene and benzo[ghi]perylene however is sensitive to the thermodynamic state of water. During the dissolution of PAH mixtures, a preferential dissolution of light PAHs into the water phase occurs, leaving heavy PAHs concentrated in the oil droplets. With the simultaneous increase in water density and temperature, the miscibility of PAHs with sub-CW/SCW is improved by the enhanced attractive electrostatic interaction between PAHs and hydrothermal environment and the weakened interaction between PAHs. By the differences in heavy oil composition and the thermodynamic state of water, the upgrading of heavy oil in sub-CW/SCW could be run in the condensed emulsion or pseudo single-phase structure.

Macondo oil in northern Gulf of Mexico waters – Part 2: Dispersant-accelerated PAH dissolution in the Deepwater Horizon plume

During the Deepwater Horizon blowout, unprecedented volumes of dispersant were applied both on the surface and at depth. Application at depth was intended to disperse the oil into smaller microdroplets that would increase biodegradation and also reduce the volumes buoyantly rising to the surface, thereby reducing surface exposures, recovery efforts, and potential stranding. In forensically examining 5300 offshore water samples for the Natural Resource Damage Assessment (NRDA) effort, profiles of deep-plume oil droplets (from filtered water samples) were compared with those also containing dispersant indicators to reveal a previously hypothesized but undocumented, accelerated dissolution of the polycyclic aromatic hydrocarbons (PAH) in the plume samples. We interpret these data in a fate-and-transport context and conclude that dispersant applications were functionally effective at depth.

Biodegradation and dissolution of polyaromatic hydrocarbons by Stenotrophomonas sp.

The aim of this work was to study the biodegradation capabilities of a locally isolated bacterium, Stenotrophomonas sp. strain IITR87 to degrade the polycyclic aromatic hydrocarbons and also check the preferential biodegradation of polycyclic aromatic hydrocarbons (PAHs). From preferential substrate degradation studies, it was found that Stenotrophomonas sp. strain IITR87 first utilized phenanthrene (three membered ring), followed by pyrene (four membered ring), then benzo[α]pyrene (five membered ring). Dissolution study of PAHs with surfactants, rhamnolipid and tritonX-100 showed that the dissolution of PAHs increased in the presence of surfactants.

A Lagrangian/Eulerian oil spill model for continental waters

The application of the European Water Framework Directive on water quality for human consumption and industrial activities creates a need for water quality assessment and monitoring systems. The MIGR'HYCAR research project was initiated to provide decisional tools for risks connected to oil spills in continental waters. In this paper, the focus is set on a numerical model to simulate oil spill in continental waters and to estimate polycyclic aromatic hydrocarbons (PAHs) toxicity in the aquatic environment. A Lagrangian/Eulerian approach is proposed. The Lagrangian model describes the transport of an oil spill near the free surface. The oil spill model enables to simulate the main processes driving oil plumes: advection, diffusion, evaporation, dissolution, spreading and volatilization. Though generally considered as a minor process, dissolution is important from the point of view of toxicity. To model PAHs dissolution in water, an Eulerian advection–diffusion model is used. Laboratory experiments were conducted to characterize the numerous kinetics of dissolution and volatilization processes. Model validation was carried out with the following test cases: transport processes based on the well-documented seawater spill (advection), weathering processes based on laboratory experiments cited in the literature (spreading and evaporation processes) and artificial river test measurements (dissolution and volatilization).

Dissolution of polycyclic aromatic hydrocarbons from a non-aqueous phase liquid into a surfactant solution using a rotating disk apparatus

The mass transfer of two polycyclic aromatic hydrocarbons (PAHs), naphthalene and phenanthrene from a multicomponent non-aqueous phase liquid (NAPL) into a nonionic surfactant solution, Brij 35 was investigated using a rotating apparatus. Few experimental methods have been applied to the study of solubilization kinetics in organic liquids because in those systems, the interfacial area during mixing is more difficult to maintain and measure. This challenge was overcome by permeating the NAPL through a membrane. Mass transfer experiments were conducted in the absence and presence of surfactant, and the concentrations of naphthalene and phenanthrene in the bulk aqueous phase were determined in samples collected at different time intervals from the time of initial contact of the NAPL phase with the aqueous solution phase. Experiments in pure water demonstrated that the rotating apparatus behaves as in much the same way as the Levich's rotating disk. The mass transfer coefficients and the dissolution of PAHs into the surfactant solution were measured at different doses of Brij 35. As the surfactant concentration increased, the mass transfer coefficients for both PAHs from the NAPL decreased.

Removal of Polycyclic Aromatic Hydrocarbons from Contaminated Soil by Aqueous DNA Solution

An aqueous DNA solution was applied for the extraction of polycyclic aromatic hydrocarbons (PAHs) from a spiked soil. Solubilities of 0.56, 11.78, and 11.24 mg L(-1) for anthracene, phenanthrene, and pyrene, respectively, were achieved after 1 day equilibration in 1% DNA. Using a spiked soil that contained 72 mg kg(-1) anthracene, 102 mg kg(-1) phenanthrene, and 99 mg kg(-1) pyrene, extractions of close to 88, 78, and 94%, respectively, were attained with 5% DNA at a 1:50 soil/extractant ratio. Maximum PAH dissolution occurred after 4-6 h. Comparative tests showed the main advantage of DNA over methyl-beta- and gamma-cyclodextrins and Tween 80 for pyrene removal. An ionic strength of 0.1 M NaCl was found necessary for maximum PAH dissolution and extraction. The performance of hexane regenerated DNA also remained stable after three stages of recycling. These results show the huge potential of DNA as an aqueous washing agent for PAH-contaminated soil.

Single Event–Driven Export of Polycyclic Aromatic Hydrocarbons and Suspended Matter from Coal Tar–Contaminated Soil

Mobile colloidal and suspended matter is likely to affect the mobility of polycyclic aromatic hydrocarbons (PAHs) in the unsaturated soil zone at contaminated sites. We studied the release of mobile particles and dissolved organic matter as a function of variable climatic boundary conditions, and their effect on the export of PAHs at a coal tar–contaminated site using zero‐tension lysimeters. Seepage water samples were analyzed for dissolved organic carbon (DOC), pH, electrical conductivity, turbidity, and particles larger than 0.7 μm. The 16 Environmental Protection Agency PAHs were analyzed in the filtrate <0.7 μm and in the particle fraction. Our results show that extended no‐flow periods that are followed by high‐intensity rain events, such as thunderstorms, promote the mobilization of particles in the size 0.7 to 200 μm. Mobilization is enforced by extended drying during summer. High particle concentrations are also associated with freezing and thawing cycles followed by either rain or snowmelt events. The export of PAHs is strongly connected to the release of particles in the 0.7‐ to 200‐μm size fraction. During the 2‐yr monitoring period, up to 0.418 μg kg−1 PAHs were mobilized in the filtrate (<0.7 μm) while the eightfold mass, 3.36 μg kg−1, was exported with the retentate (0.7–200 μm). Equilibrium dissolution of PAHs and transport in the dissolved phase seem to be of minor importance for the materials studied. Extreme singular‐release events occurred in January 2003 and January 2004, when up to 55 μg L−1 PAHs per one single seepage event were observed within the retentate. Freezing and thawing cycles affect the PAH source materials, that is, the remnants of the nonaqueous phase liquid (NAPL). High mechanical strain during freezing results in the formation of particles. At the onset of the thawing and following rain or snowmelt events, PAHs associated with these particles are then exported from the lysimeter.

Solubilization, solution equilibria, and biodegradation of PAH’s under thermophilic conditions

Biodegradation rates of PAHs are typically low at mesophilic conditions and it is believed that the kinetics of degradation is controlled by PAH solubility and mass transfer rates. Solubility tests were performed on phenanthrene, fluorene and fluoranthene at 20 °C, 40 °C and 60 °C and, as expected, a significant increase in the equilibrium solubility concentration and of the rate of dissolution of these polycyclic aromatic hydrocarbons (PAHs) was observed with increasing temperature. A first-order model was used to describe the PAH dissolution kinetics and the thermodynamic property changes associated with the dissolution process (enthalpy, entropy and Gibb’s free energy of solution) were evaluated. Further, other relevant thermodynamic properties for these PAHs, including the activity coefficients at infinite dilution, Henry’s law constants and octanol–water partition coefficients, were calculated in the temperature range 20–60 °C. In parallel with the dissolution studies, three thermophilic Geobacilli were isolated from compost that grew on phenanthrene at 60 °C and degraded the PAH more rapidly than other reported mesophiles. Our results show that while solubilization rates of PAHs are significantly enhanced at elevated temperatures, the biodegradation of PAHs under thermophilic conditions is likely mass transfer limited due to enhanced degradation rates.

Selective Solubilization of Polycyclic Aromatic Hydrocarbons from Multicomponent Nonaqueous-Phase Liquids into Nonionic Surfactant Micelles

This research investigates the equilibrium solubilization behavior of naphthalene and phenanthrene from multicomponent nonaqueous-phase liquids (NAPLs) by five different polyoxyethylene nonionic surfactants. The overall goal of the study was to achieve an improved understanding of surfactant-aided dissolution of polycyclic aromatic hydrocarbons (PAHs) from multicomponent NAPLs in the context of surfactant-enhanced remediation of contaminated sites. The extent of solubilization of the PAHs in the surfactant micelles increased linearly with the PAH mole fraction in the NAPL. The solubilization extent and micelle-water equilibrium partition coefficient of the PAHs increased with the size of the polar shell region of the micelles rather than the size of the hydrophobic core of the micelle. The presence of both PAHs in the shell region of the micelles was confirmed by 1H NMR analysis. This is an important observation because it is commonly assumed that in multi-solute systems the solutes with relatively greater hydrophobicity are solubilized only in the micellar core. A comparison of the 1H NMR spectra of pure surfactant solutions and solutions contacted with various NAPLs demonstrated that the distribution of PAHs between the shell and the core changed with the concentration of PAHs in the micelles and in the NAPL. Competitive solubilization of the PAHs was observed when both PAHs were present in the NAPL. For example, in surfactant solutions of Brij 35 and Tween 80, the solubilization of phenanthrene was decreased in the presence of naphthalene as compared to systems that contained phenanthrene as the only solute. In contrast, with micellar solutions of Tergitol NP-10 and Triton X-100, phenanthrene solubilization was enhanced in the presence of naphthalene. The activity coefficients of the PAHs in the micellar phase were generally found to increase with PAH concentrations in the micelle.

Oxidation of polycyclic aromatic hydrocarbons (PAH) by the white rot fungus, Phanerochaete chrysosporium

Key factors affecting the oxidation of polycyclic aromatic hydrocarbons (PAH) by the white rot fungus, Phanerochaete chrysosporium, including Mn 2+ concentrations on extracellular enzyme production and PAH source were investigated. P. chrysosporium acted synergistically with soil indigenous microorganisms in the oxidation of low molecular weight PAH (i.e. acenaphthene, fluorene, phenanthrene, fluoranthene and pyrene) in a soil-slurry, where oxidation was enhanced by up to 43% in the presence of fungus. However, limited oxidation occurred for high molecular weight PAH (i.e. chrysene, benzo( a)pyrene, dibenz( ah)anthracene and benzo( ghi)perylene). This was also the case for the oxidation of solid phase PAH (i.e. phenanthrene, pyrene and benzo( a)pyrene) when added in acetone to cultures, where less than 12% of the high molecular weight PAH benzo( a)pyrene was oxidized, compared to up to 84% for relatively soluble phenanthrene. In contrast, surfactant dissolved PAH pyrene and benzo( a)pyrene were efficiently oxidized (i.e. recovery was less than 16.3 and 0.35%, respectively). Results collectively show that PAH dissolution rate is the limiting factor in the oxidation of PAH from contaminated soil and when added in acetone. However, for the surfactant dissolved PAH, it could be the ionisation potential of the PAH, which is the key factor affecting PAH oxidation. The concentration of Mn 2+ strongly regulated manganese peroxidase (MnP) production, and the presence of the lignin peroxidase (LiP) and MnP was detected in cultures in the presence of surfactant dissolved PAH. Although an increase of Mn 2+ concentration in culture medium is beneficial for the oxidation of surfactant dissolved PAH, it is not a pre-requisite. However, the presence of fungal biomass is a pre-requisite for the oxidation of surfactant dissolved pyrene as the biomass-free supernatant did not result in the oxidation of pyrene, despite the presence of extracellular enzyme activity.

Development of a Protocol to Study Aerobic Bacterial Degradation of Polycyclic Aromatic Hydrocarbons: Application to Phenanthrenes

A method was developed and validated to study the degradation of polycyclic aromatic hydrocarbons (PAHs) by bacteria under oxic conditions. Studies of toxicity of acetonitrile used as dissolution solvent have allowed to determine the quantity of acetonitrile necessary to obtain a good PAH dissolution without toxic effects. The degradation of a mixture of phenanthrene (P), 2-methylphenanthrene (2MP) and 9-methylphenanthrene (9MP) by a bacterial community was studied. The results obtained show a difference of degradation rates between the three compounds. P and 2MP were more rapidly degraded than 9MP. Degradation of methylphenanthrenes in petroleum was also investigated. P was more degraded than substituted phenanthrenes and no selectivity of degradation was observed between the methylphenanthrenes. Identification of 2MP metabolites has shown two metabolic pathways. The first step of one way is the hydroxylation of the methylgroup, the second way is initiated by the double hydroxylation of one aromatic ring.

Effect of Nonionic Surfactants on Dissolution of Polycyclic Aromatic Hydrocarbons from Coal Tar

The remediation in the subsurface of low solubility compounds such as polycyclic aromatic hydrocarbons (PAHs) might be limited by their dissolution kinetics. Other studies have already proven that the addition of surfactants will enhance the solubility and dissolution rates of pure single-hydrophobic compounds in water. The purpose of this study is to investigate whether the addition of two nonionic surfactants (Tween 20 and Tween 21) to a complex mixture (coal tar) would similarly enhance the dissolution rates of five PAHs contained within the coal tar. The five PAHs analyzed were naphthalene, phenanthrene, fluoranthene, pyrene, and benzo(a)pyrene. Through a series of partitioning experiments, it was discovered that the partitioning coefficients increased with increasing PAH ring structure. Dissolution experiments showed that the solubilities and dissolution rates of the PAHs also increased in the presence of the two surfactants. Finally, it was found that Tween 21, being the more hydrophobic surfactant, caused a greater enhancement in PAH dissolution rates than did Tween 20.

Studies on the Dissolution of Polycyclic Aromatic Hydrocarbons from Contaminated Materials Using a Novel Dialysis Tubing Experimental Method

Assessment of risk and remediation strategies at contaminated sites requires that both the amounts of contaminants present and their potential for release from materials and soils be evaluated. The release, or dissolution, of polycyclic aromatic hydrocarbons (PAHs) from contaminated materials to water was therefore investigated. To facilitate investigations of PAH dissolution from physically disparate materials such as solid coal tars, creosote, oil, and spent oxide, an experimental method for measuring dissolved PAHs was developed employing dialysis tubing in a batch-type system. This was validated and compared to aqueous-phase PAH concentrations measured using more traditional techniques and also predicted using Raoult's law. The experimental procedure was successfully used to determine ‘near equilibrium' aqueous concentrations of PAHs, but it could only be used to determine relative rates of approach to equilibrium as the dialysis tubing effected the rate constants. It was found that the contaminant materials influenced dissolution, in particular the close to equilibrium concentrations. For materials chemically similar to PAHs, such as nonaqueous-phase liquids (NAPLs), the concentrations could be predicted using Raoult's law. For materials that were chemically dissimilar to PAHs, such as spent oxide, release was more thermodynamically favorable than for NAPLs.

Dissolution of polycyclic aromatic hydrocarbons from weathered contaminated soil

Dissolution of PAHs from soil contaminated with multi-component coal tar was investigated using a polymeric adsorbent, Tenax-TA and a nonionic surfactant, Brij 30. Tenax created maximum concentration gradient at the soil/water interface, thus maximizing interfacial mass transfer. Even in the presence of Tenax, the release of PAHs from the weathered soil was a very slow, noneqilibrium process. The rate-limiting step in the release of PAH was identified as diffusional mass transfer within the contaminant-soil matrix. Based on model analyses, the key parameter controlling dissolution of PAHs from the soil was found to be the diffusivities of PAHs within the soil/tar matrix. Brij 30 solution (5.5 g/L) substantially increased the rate of PAH dissolution from the soil. The enhanced mass transfer rates of PAHs by Brij 30 were attributed to their enhanced matrix diffusivities rather than their increased aqueous solubilities. Matrix diffusivities of PAHs in Brij 30 solution increased by two orders of magnitude compared with those with Tenax. Swelling of the weathered tar matrix of the soil by infiltrating surfactant molecules was suggested as the mechanism responsible for the observed diffusivity enhancement.

Dissolution of polycyclic aromatic hydrocarbons from weathered contaminated soil

Dissolution of PAHs from soil contaminated with multi-component coal tar was investigated using a polymeric adsorbent, Tenax-TA and a nonionic surfactant, Brij 30. Tenax created maximum concentration gradient at the soiVwater interface, thus maximizing interfacial mass transfer. Even in the presence of Tenax, the release of PAHs from the weathered soil was a very slow, noneqilibrium process. The ratelimiting step in the release of PAH was identified as diffusional mass transfer within the contaminant-soil matrix. Based on model analyses, the key parameter controlling dissolution of PAHs from the soil was found to be the diffusivities of PAHs within the soiVtar matrix. Brij 30 solution (5.5 g/L) substantially increased the rate of PAH dissolution from the soil. The enhanced mass transfer rates of PAHs by Brij 30 were attributed to their enhanced matrix diffusivities rather than their increased aqueous solubilities. Matrix diffusivities of PAHs in Brij 30 solution increased by two orders of magnitude compared with those with Tenax. Swelling of the weathered tar matrix of the soil by infiltrating surfactant molecules was suggested as the mechanism responsible for the observed diffusivity enhancement.