What Are Polycyclic Aromatic Hydrocarbons?

Polycyclic aromatic hydrocarbons (Polycyclic Aromatic Hydrocarbons PAHs) are volatile hydrocarbons produced when coal, petroleum, wood, tobacco, organic polymer compounds and other organic substances are not completely burned. So far, more than 200 types of PAHs have been found, and a considerable part of them are carcinogenic, such as benzopyrene and benzoanthracene. PAHs are widely distributed in the environment and can be found in every corner of our lives. Polycyclic aromatic hydrocarbons may be produced in any place where organic matter is processed, discarded, burned or used.

Chinese name: PAHs
Foreign name: Polycyclic Aromatic Hydrocarbons PAHs
Features: Important environmental and food pollutants

Harm: Carcinogenicity
To source: combustion
Way of being: Atmosphere, water, soil

Introduction to PAHs

Hydrocarbons containing more than two benzene rings in the molecule, including more than 150 compounds such as naphthalene, anthracene, phenanthrene, and fluorene. Some PAHs also contain nitrogen, sulfur, and cyclopentane. Common PAHs with carcinogenic effects are mostly four to six ring fused ring compounds. ^[1]

The International Cancer Research Center (IARC) (1976) listed 94 carcinogenic compounds in laboratory animals, 15 of which are polycyclic aromatic hydrocarbons. Since benzo [] pyrene was the first environmental chemical carcinogen to be discovered, And carcinogenicity is very strong, so benzo [] pyrene is often used as a representative of polycyclic aromatics, which accounts for 1% -20% of all carcinogenic polycyclic aromatic hydrocarbons.

Polycyclic aromatic hydrocarbons

PAHs mainly include 18 kinds of similar substances:

18 common PAHs

NAP Naphthalene (NAP)

2 .ANY Acenaphthylene (ANY)

3.Acenaphthene pentane (ANA)

FLU Fluorene F (FLU)

PHE Phenanthrene (PHE)

ANT Anthracene (ANT)

FLT Fluoranthene (FLT)

PYR Pyrene (PYR)

BaA Benzo (a) anthracene Benzo (a) anthracene (BaA)

CHR Chrysene (CHR)

BbF Benzo (b) fluoranthene benzo (b) fluoranthene (BbF)

BKF Benzo (k) fluoranthene benzo (k) fluoranthene (BkF)

BaP Benzo (a) pyrene benzo (a) pyrene (BaP)

IPY Indeno 1,2,3-cd) pyrene Indeno 1,2,3-cd (IPY)

DBA Dibenzo (a, h) anthracene

BPE Benzo (g, hi) perylene benzo (ghi) (perylene)

17.Benzp (e) pyrene Benzo (e) pyrene (BeP)

18.Benzo (j) fluoranthene benzo (j) fluoranthene (BjF)

Source of PAHs

In nature, such compounds exist in ways of elimination such as biodegradation, hydrolysis, photolysis, etc., so that the PAHs content in the environment always has a dynamic balance, and thus remains at a lower concentration level. With the intensification of human production activities, its dynamic balance in the environment has been disrupted, and PAHs in the environment have increased significantly.

PAHs Natural Source

It mainly includes combustion (forest fires and volcanic eruption) and biosynthesis (sediment diagenesis, biotransformation, and gas in tar pits). Unmined coal and petroleum also contain large amounts of polycyclic aromatic hydrocarbons.

Anthropogenic PAHs

The anthropogenic sources of PAHs come from industrial processes, anoxic combustion, waste incineration and landfilling, food production and direct transportation emissions, and the accompanying asphalt wear and tyre wear and pavement wear and road dust generated by road wear. The development of oil has increased greatly, accounting for the vast majority of the total PAHs in the environment; oil spills have also become an anthropogenic source of PAHs. Therefore, how to speed up the elimination of PAHs in the environment and reduce the pollution of PAHs to the environment has attracted increasing attention.

Changes in PAHs

PAHs

PAHs are widely found in the natural environment in which humans live, such as the atmosphere, water bodies, soil, crops, and food. As of April 2013, there are about 200 known polycyclic aromatic hydrocarbons. Atmospheric PAHs exist in two forms: gas and solid. Among them, 2-3 ring PAHs with small molecular weight mainly exist in gaseous form, 4-ring PAHs have basically the same distribution in gaseous and granular states, and 5-7 ring large molecular weight PAHs. Most of them exist in the form of particles. Polycyclic aromatic hydrocarbons in water can be in three states: adsorbed on suspended solids; dissolved in water; and emulsified. There are more than 20 types of polycyclic aromatic hydrocarbons known in surface water. Polycyclic aromatic hydrocarbons were also detected in groundwater and seawater. The PAHs concentration in the soil is generally in the range of 10-10 & micro; g / Kg, and the PAHs concentration in the suburban soil is higher, reaching 10-10 & micro; g / Kg. Soil pollution will inevitably affect the growth of crops. The content of BPa in vegetables is most leafy vegetables, followed by root vegetables and fruit vegetables.

PAHs migration

Most PAHs exist in the adsorption and emulsified state in the environment. Once they enter the environment, they are affected by various natural processes and change. Through complex physical migration, chemical and biological transformation reactions, it constantly changes in the atmosphere, water, soil, organism and other systems to change the distribution. PAHs in different states and systems show different behaviors. After entering the atmosphere, PAHs can enter the soil and water bodies through chemical reactions, dust reduction, rainfall, and snowfall.

Polycyclic aromatic hydrocarbons

PAHs Physical Properties

Most PAHs are colorless or pale yellow crystals, some of which are dark, with high melting point and boiling point, low vapor pressure, mostly insoluble in water, easily soluble in benzene aromatic solvents, and slightly soluble in other organic solvents. In the solvent, the octanol-water partition coefficient is relatively high. Most PAHs have large conjugated systems, so their solutions have some fluorescence. Generally speaking, as the molecular weight of PAHs increases, the melting point increases and the vapor pressure decreases. The color, fluorescence and solubility of polycyclic aromatic hydrocarbons are mainly related to the conjugate system of polycyclic aromatic hydrocarbons and the arrangement of molecular benzene rings. As the number of p electrons increases and the delocalization of p electrons increases, the color deepens and the fluorescence increases. , The maximum absorption wavelength in the ultraviolet absorption spectrum also shifted significantly to the long wave direction; for linear polycyclic aromatic hydrocarbons, the number of benzene rings increased, the octanol-water partition coefficient increased, and the number of polycyclic aromatic hydrocarbons with the same number of benzene rings, benzene ring structure The more "clusters" the larger the octanol-water partition coefficient.

PAHs Chemical Properties

Polycyclic aromatic hydrocarbons are chemically stable. When they react, they tend to retain their conjugated ring systems. Generally, derivatives are formed by electrophilic substitution reactions and metabolized into the active form of the final carcinogen. Its basic unit is a benzene ring, but its chemical properties are not completely similar to benzene. Divided into the following categories:

Compounds with fused polybenzene structure

Such as triphenylene, dibenzo [e, i] pyrene, tetrabenzo [a, c, h, j] anthracene, etc., have similar chemical stability with benzene, indicating that the distribution of electrons in these PAHs is Similar to benzene. As shown in Figure 1:

Polycyclic aromatic hydrocarbons with x-electron distribution similar to benzene

Figure 1x PAHs with electron distribution similar to benzene

Polycyclic aromatic hydrocarbons arranged in a straight line

Such as anthracene, Ding province, E province, etc., much more active than benzene's chemical properties. Its reactivity becomes stronger as the number of rings increases. The number of rings reaches 7 in Geng, and its chemical properties are extremely lively, making it almost impossible to obtain pure products. The characteristic of this polycyclic aromatic hydrocarbon chemical reaction is that it usually occurs at the relative carbon position (referred to as the middle anthracene position) of the middle benzene ring corresponding to anthracene. as shown in picture 2:

Figure 2 Linear PAHs

Polycyclic aromatic hydrocarbons arranged in an angular shape

Such as phenanthrene, benzo [a] Ci, tza [2,3-a] anthracene, pyrene, [2,3-a] pyrene, etc., its chemical activity is generally less than the corresponding linear isomers. In the addition reaction, it is usually carried out at the middle double bond site corresponding to phenanthrene, that is, the phenanthrene 9,10 bond (referred to as the middle phenanthrene bond). The electrons are largely confined to the phenanthrene bond, so the chemistry of the phenanthrene bond is very close to the olefin bond. Angular PAHs containing more than 4 rings, in addition to the more active Sino-Philippine bonds, often contain linear PAHs that are similar to the active para --- the cis position, such as the benzo [a] onion 8,15 Bit. However, the degree of activity is lower than that of the corresponding linear isomer, and it is basically enhanced with the increase of the number of rings as shown in Figure 3:

Figure 3.Angular arrangement of PAHs

More complex fused ring hydrocarbons

Such as benzo [a] flower, dibenzo [a, i] pyrene, etc., have a lively Philippine bond, but no active para. Many of these polycyclic aromatic hydrocarbons are carcinogenic. For example, benzo [a] pyrene is the most carcinogenic polycyclic aromatic hydrocarbon. Their structure is shown in Figure 4:

Figure 4 Complex PAHs ("*" indicates Sino-Philippine bonds)

Polycyclic aromatic hydrocarbon analysis method

With the continuous advancement of science and technology, the detection methods of polycyclic aromatic hydrocarbons are also constantly changing, from column adsorption chromatography, paper chromatography, thin layer chromatography (TLC) and gel permeation chromatography (GPC) to the current gas phase. Chromatography (GC), reversed-phase high-performance liquid chromatography (RP-HPLC), as well as ultraviolet absorption (UV) and emission spectra (including fluorescence, phosphorescence, and low-temperature luminescence, etc.), as well as mass spectrometry, nuclear magnetic resonance, and infrared spectroscopy techniques , And the combination of techniques between various analytical methods. More commonly used are spectrophotometry and reversed-phase high-performance liquid chromatography. In recent years, the analysis methods of PAHs have developed rapidly, and new analytical techniques such as microwave-assisted solvent extraction, solid-phase microextraction, and supercritical fluids have appeared.

Polycyclic aromatic hydrocarbon spectrophotometry

Spectrophotometry includes ultraviolet spectrophotometry, fluorescence spectroscopy, phosphorescence, low-temperature luminescence spectroscopy, and some new luminescence analysis methods. The analysis of polycyclic aromatic hydrocarbon samples of RAHs by luminescence technology has the advantages of high sensitivity and specificity compared with absorption spectrophotometry. The sensitivity of the luminescence method is 10-100 times higher than that of the absorption method, and its detection amount is about 10.6 to 10.8 g. Ultraviolet spectrophotometry is also common due to its simplicity and versatility. Generally, the molar absorption coefficient () of PAHs is about 10-10, and the detection sensitivity is about & micro; g. Table 1 below shows the maximum UV absorption wavelength of some PAHs.

Table 1 Maximum UV absorption waves of some PAHs

Substance maximum absorption wavelength (nm)

Naphthalene 275

Anthracene 370

Ding460

Low-temperature fluorescence analysis is a new PAH analysis method that has appeared in recent years. Yu Lijun and others used a fiber-conducting low-temperature device coupled with a fluorescence spectrophotometer to perform low-temperature excitation and emission spectrum scanning of polycyclic aromatic hydrocarbons. The Shpolskii low-temperature fluorescence spectrum of polycyclic aromatic hydrocarbons was obtained. Cycloaromatic hydrocarbons have good discrimination ability.

PAHs Physical Method

heating method

The concentration of benzo [a] pyrene in water can be reduced by heating and boiling. When heated to boiling, its content can be reduced by 37% -57%. If heating is continued, the content will no longer decrease, and some benzo [a] Rhenium has been transferred to the scale formed by heating and boiling. Other PAHs can also be partially removed during heat boiling.

Coagulation

Using this method and chlorination, 15% to 85% of benzo [a] pyrene can be removed. If synthetic flocculants and filtration through activated carbon are used, the content of polycyclic aromatic hydrocarbons in the treated water can reach the standard of drinking water. .

Adsorption method

The fluoranthene, benzo [al pyrene, indene pyrene, 3,4-benzofluoranthene, 11,12-benzofluoranthene, pyrene and pyrene in the wastewater discharged from the petrochemical plant can be removed by adsorption with activated carbon. Although the use of powdered activated carbon can reduce its odor, it is quite difficult to reach the standard of drinking water, namely 0.2 & micro; g / L. Activated carbon has a poor adsorption effect on polycyclic aromatic hydrocarbons with large molecular weight, and it has a better adsorption effect using macroporous resin. In general, it is difficult to treat PAHs only by physical methods, and it should be used in combination with biochemical and chemical methods.

Polycyclic aromatic hydrocarbon chemical method

There are two main types of chemical treatment of PAHs: photooxidation and chemical oxidation.

In the process of photooxidation, PAHs in water are oxidatively degraded under the action of singlet oxygen, ozone or light radicals induced by light. Benzo [a] pyrene can be removed by 56% due to photooxidation and form benzo [a] pyrene-3,6-dione or other diketone compounds, as well as some unknown compounds.

In chemical oxidation, there are mainly two types of ozone oxidation and chlorination. Ozone removal of PAHs is better than other oxidation methods. 4 & micro; g / L benzo [a] pyrene in the aqueous solution is treated with 2.5 & micro; g / L ozone for 3 minutes, and its residual amount is 0.06 & micro; g / L; treated with 0.45mg / L ozone for 5 minutes, The residual amount is 0.04 & micro; g / L. Increasing the ozone concentration and prolonging the action time can improve the removal rate, but the residual amount will not be less than 0.02 & micro; g / L Wuxikang.

Polycyclic aromatic hydrocarbon degradation method

Degradation of polycyclic aromatic hydrocarbons by microbial treatment methods has been studied more because of low operating cost and wide application scope, and has a high degree of industrialization. It has been put into use by many organic pollutant wastewater treatment plants.

Microorganisms have strong catabolic capabilities, variety diversity and high metabolic rates. Many bacteria, fungi, and algae have the ability to degrade polycyclic aromatic hydrocarbons. Microbial degradation of PAHs generally uses PAHs as the sole carbon source, energy source, and co-metabolism of PAHs with other organic matter. For low-molecular-weight polycyclic aromatic hydrocarbon compounds of less than 3 rings in soil, microorganisms generally adopt the first metabolism method; for soil 4-ring or polycyclic polycyclic aromatic hydrocarbons in soil generally use co-metabolism.

Microorganisms produce oxygenases to degrade polycyclic aromatic hydrocarbons. Monooxygenases can add an oxygen atom to the substrate to form aromatic compounds, which are then oxidized to trans-dihydroethanol and phenols. Bacteria produce dioxygenases. It adds two oxygen atoms to the substrate to form dioxane, which is further oxidized to cis-dihydroethanol. Both produce many intermediate products that are used to synthesize their own cellular proteins and energy. The initial oxidation of PAHs, that is, the oxygenation of benzene rings, is a key step in controlling the rate of PAHs biodegradation. After that, the degradation process is accelerated, and there is no or little accumulation of intermediate metabolites. However, it is reported that The parent compound (polycyclic aromatic hydrocarbons) is equally carcinogenic and mutagenic.

PAHs can undergo anaerobic degradation under denitrification conditions, and nitrate is used as an electron acceptor. In the sulfate-reducing environment, microbial degradation of polycyclic aromatic hydrocarbons can also occur. With sulfate as an electron acceptor, it can degrade Cai, phenanthrene, fluoranthene, and so on.

In short, the conventional physical methods for removing PAHs are heating method, coagulation precipitation method, adsorption method, chemical methods include photo-oxidation and chemical agent oxidation, and biochemical treatment methods. The physical method can only remove 50 0%, and it cannot completely degrade PAHs; the conventional chemical methods can not completely degrade PAHs; the biochemical treatment time is too long, and the removal rate is only 30-40%.

Polycyclic aromatic hydrocarbon thin layer chromatography

Thin layer chromatography, also known as thin layer chromatography, includes adsorption and distribution of thin layers. Its advantages are short development time, corrosive developer can be used, spot density is large and easy to detect, samples in spots after development can be extracted for determination by spectrophotometry, and can be used as a pre-experiment method for preparative chromatography.

Adsorption thin layer

There are adsorption activities on the surface of various adsorbents, which show different degrees of adsorption capacity for organic compounds. It is precisely the use of different interactions between molecules of different components in the mixture, solvent molecules and the molecules on the surface of the adsorbent. The solvent system (mobile phase) is unfolded. As the solvent and the components in the mixture compete for the active surface of the adsorbent, a reversible process of adsorption and desorption occurs. As the mobile phase moves, this process continues to make each component The two phases move at different speeds and move different distances to achieve the purpose of separation.

Due to the differences in structural properties of different compounds, the eluting ability of the developing agent and their adsorption and desorption performance on the adsorbent are also different. Therefore, the distance moved on the adsorbent will not be the same, and various components will be separated to different degrees from each other. The greater the difference in properties, the better the separation effect.

Distribute thin layer

Partitioning a thin layer is a method to achieve separation by using materials with different partition coefficients in two immiscible solvents. Its basic principle comes from two-phase countercurrent extraction. It uses a porous substance as a support (this is different from the adsorbent that adsorbs a thin layer). The polar solvent is always fixed on the support during the chromatography process. The stationary phase, which is eluted with a solvent that is immiscible with the stationary phase, is called the mobile phase. The separated materials are continuously and dynamically allocated between the stationary phase and the mobile phase, and different components have different distribution coefficients between the two phases to achieve the purpose of separation.

Reversed-phase high-performance liquid chromatography (RP.HPPLC)

High performance liquid chromatography (HPLC) can work at room temperature, high resolution and sensitivity to PAHs, easy collection of post-column fractions, and suitable for fluorescence detector analysis. It has been widely used in the separation and quantification of PAHs, especially for many Rings and high relative molecular weight PAHs have advantages. The US EPA recommends the use of acetonitrile and water as the mobile phase for HPLC, but acetonitrile is more expensive and toxic. Jia Ruibao uses methanol and water as the mobile phase for gradient elution. The 16 PAHs spiked recoveries are 79% -104% The relative standard deviation is 5.2% -19.5%, which is suitable for the detection of PAHs. Lin Lin et al. Used microwave extraction and high performance liquid chromatography to determine polycyclic aromatic hydrocarbons in soil. The detection limit was 0.10-0.80 & micro; g / kg, the relative standard deviation was 0.60% -4.60%, and the recovery was 58.1% -97.8%. Tao Jingqi et al. Used solid phase microextraction and high performance liquid chromatography to determine eight polycyclic aromatic hydrocarbons in water samples. By optimizing the extraction and desorption conditions, the detection limit of the method was 0.002-0.180 & micro; g / L, relative standard deviation. It is 4.4% -2.2% and the recovery rate is 91.1% -115.8%. It is a fast method for the analysis of trace polycyclic aromatic hydrocarbons in environmental water samples.

Other PAHs

PAH Research

The biological removal of difficult-to-degrade organics in the environment, especially PAHs, is a common concern for environmental scientists. Although microbial remediation is the most effective way to remove PAHs pollution, the technology is still limited by many factors in order to be successfully applied in practice. The special structure of PAHs and their low water solubility limit their degradation by indigenous microorganisms. The following aspects are worthy of further study:

Isolate highly efficient degrading strains that can use high molecular weight polycyclic aromatic hydrocarbons with more than four rings as the sole carbon source, study the factors affecting biodegradation, and improve the biodegradability of existing strains.

Study on the microbial degradation pathway and mechanism of PAHs and the co-metabolism mechanism in the degradation process, the structural properties of the intermediate steps of the degradation process and the intermediate products accumulated during the degradation process, and the toxicity of some degradation products are studied in depth.

(3) Through genetic engineering technology, the plasmid or gene encoding the enzyme that degrades pollutants is integrated into the DNA of indigenous microorganisms that can grow and survive in polluted environments, so that they have a strong ability to degrade pollutants and give full play to the role of biological repair .

Study on the rhizosphere mechanism of microbial and plant joint repair. Phytoremediation of PAH-contaminated soil is in its infancy, and microbial and plant joint repair will be a promising new type of repair technology.

In-depth study of the mechanism of biosurfactant production and its application in practical treatment, the impact of environmental factors on the biodegradation of PAHs, and a series of issues related to actual pollution of soil and groundwater bioremediation projects.

Carcinogenic toxicity of PAHs

Polycyclic aromatic hydrocarbons (PAHs) are strong carcinogens that can cause carcinogenesis in humans through contact. Of the more than 500 known carcinogens, more than 200 are related to polycyclic aromatic hydrocarbons and have become synonymous with cancer.

Polycyclic aromatic hydrocarbons (PAHs) refer to a class of organic compounds having two or more benzene rings. Polycyclic aromatic hydrocarbons are hydrocarbons containing more than two benzene rings in the molecule, including more than 150 compounds such as naphthalene, anthracene, phenanthrene, and fluorene. English full name is polycyclic aromatic hydrocarbon, referred to as PAHs. The International Cancer Research Center (IARC) (1976) lists 94 compounds that are carcinogenic to laboratory animals. 15 of them belong to polycyclic aromatic hydrocarbons. Since benzoa pyrene is the first environmental chemical carcinogen to be discovered and has strong carcinogenicity, benzo (a) pyrene is often used as a representative of polycyclic aromatic hydrocarbons, which accounts for all carcinogenicity. Polycyclic aromatic hydrocarbons 1% -20%. ^[2]

PAHs Related News

In March 2013, according to the CCTV Quality Report program, the three major luxury car brands exposed the use of asphalt dampers, causing serious air pollution in the car. Owners frequently complained of no results. Experts estimate that in the whole year of 2012, the sales volume of cars of the three major brands of Beijing Benz, BMW Brilliance and FAW Audi was about 650,000. Using only asphalt dampers alone, a total of 97 million to 1 year can be generated. A profit of 30,000 million yuan.

Zhou Guangya, a senior engineer of the Material Technology Institute of China National Heavy Duty Truck Technology Center, introduced that the damping sheet is the steel plate on the inner surface of the car body. It is attached with a layer of viscoelastic material, which reduces noise and vibration. Said to play a damping role.

If the damping fins on Mercedes, BMW and Audi cars have an odor, they are likely to be made of asphalt, as auto parts manufacturers say. Asphalt is a residue produced after coal tar or petroleum refining. Because it contains polycyclic aromatic hydrocarbons, sulfur, phenol and other harmful substances, the International Cancer Research Center (IARC) of the World Health Organization has coal tar pitches as early as 1976. Listed as a class of carcinogens. Due to the sun exposure and engine heat dissipation, the asphalt damping sheet close to the steel plate is easily decomposed due to heat and releases toxic polycyclic aromatic hydrocarbon gases, and this is a long-term slow release process.

CCTV previously reported that 3.15 reported the use of asphalt dampers by cars such as Beijing Benz. A few days ago, the reporter selected three brands, such as Mercedes-Benz E300, BMW 520, and Audi Q5, to conduct sampling tests and showed that the samples exhibited the same thermal weight loss law as No. 70 asphalt. This means that, in addition to domestic cars, similar-level cars exported to China also use asphalt damping plates.

According to experts, PAHs are organic compounds that are highly carcinogenic and can cause carcinogenesis in the human body through breathing or direct skin contact. Among the polycyclic aromatic hydrocarbons, benzopyrene is the most influential on the human body. It is a mutant, a carcinogen, and a relatively fat-soluble substance. It can be inhaled into the body, enter the alveoli and even the blood, causing lung cancer and heart Vascular disease. ^[3]