What are polycyclic aromatic hydrocarbons

PAHs are polycyclic aromatic compounds composed of two or more fused aromatic rings with only carbon and hydrogen atoms.

Structure of 16 PAHs was enlisted as priority pollutants by the United States Environmental Protection Agency (US EPA). Several hundred different combinations of PAHs exist, but only about 28 compounds were labelled hazardous by US EPA in 2008. The two main classes of PAHs are the LMW and HMW PAHs. The LMW PAHs (2–3 ring PAHs) such as naphthalene, fluorene, phenanthrene and anthracene are shown to have significantly less toxicity compared to the HMW PAHs of 4–7 rings (from chrysene to coronenes) which are recalcitrant and carcinogenic to humans[1].

[1]S. Kuppusamy, P. Thavamani, M. Megharaj, R. Naidu. Bioaugmentation with novel microbial formula vs. natural attenuation of a long-term mixed contaminated soil—treatability studies in solid-and slurry-phase microcosms. Water Air Soil Pollut, 227 (2016) 1-15.

Hazards of polycyclic aromatic hydrocarbons

PASs are ubiquitous in the environment and the sources can be divided into natural and anthropogenic sources. Natural sources include volcanic eruptions, forest and grassland fires. PAHs in the environment are mostly derived from human activities, such as the incomplete combustion or pyrolysis of coal, petroleum and other mineral raw materials, biomass fuels, petroleum products and other hydrocarbons. Pahs emitted by natural and human activities enter the atmosphere, water and soil environment. Due to their hydrophobic characteristics, PAHs migrate and transform into sediments and soil. These PAHs dispersed in the air, water and soil are further integrated into human life through skin contact, breathing or eating.

But PAHs are extremely dangerous because they are carcinogenic, teratogenic and mutagenic. They can cause a variety of cancers, and even harm the reproductive system and immune system of organisms. Most PAHs not only have "Three Pathogenic" effects, but also have neurotoxicity.PAHs exhibits deleterious effects to the human health based on the rate of exposure and innate toxicity of different compounds which causes carcinogenicity (ability or tendency to induce cancer), genotoxicity (ability to induce non-transmissible DNA damage), cytotoxicity (the ability of being toxic to cells), nephrotoxicity (damage to the kidney), cardiotoxicity (capacity to cause damage to heart muscles, ocular toxicity (ability to induce eye disorders) and etc [1].Health effects from long-term or chronic exposure to PAHs may include decreased immune function, cataracts, kidney and liver damage (e.g. jaundice), breathing problems, asthma-like symptoms, and lung function abnormalities[2].

Therefore, the harm of polycyclic aromatic hydrocarbons widely distributed in all aspects of nature can not be underestimated. They attack our cells with every breath of air we breathe and every piece of soil we touch.

[1] I.C. Ossai, A. Ahmed, A. Hassan, F.S. Hamid, Remediation of soil and water contaminated with petroleum hydrocarbon: A review, Environmental Technology & Innovation, 17 (2020).

[2]Hussein I. Abdel-Shafy, Mona S.M. Mansour, A review on polycyclic aromatic hydrocarbons: Source, environmental impact, effect on human health and remediation, Egyptian Journal of Petroleum, 25(2016)107-123.

Distribution

Polycyclic aromatic hydrocarbons (PAHs) are a group of chemically related compounds which are environmentally persistent with various structures and varied toxicity. They are of considerable concern primarily due to their ubiquitous presence in the environment and well-recognized carcinogenicity, teratogenicity and genotoxicity. In the environment, PAHs may originate from natural sources such as volcanos and forest fires but are derived mainly from anthropogenic sources such as oil spills and incomplete combustion of biomass and fossil fuels. After emission, PAHs are transported over long distances through water and air and thus end up being widely distributed globally in water, sediment, soil, and the atmosphere [1].

As reported, there was 90% of the environmental PAHs burden in Britain was stored in soil. The emission amounts of PAHs in the different regions depended on fuel consumption, biomass combustion the economic structure and climate. After the dispersion, PAHs concentration in the surface soil are expected to be different. In the urban area of China from 2008 to 2018, PAHs concentration in the soil was about 0.092-4733 ng/g[2].

L. Wang et.al [3] found that, the total concentration of sixteen PAHs (Σ16PAHs) in the urban soil of Xi’an ranged from 390.6 to 10652.8 μg/kg with an average of 2052.6μg/kg, belonging to the moderate level (Table 1). The concentrations of NaP, Phe, Ant, BaA, Chy, BkF, BaP, BghiP, and InP in some soil samples were higher than the Dutch Target Values of Soil Quality, respectively, which were 15, 50, 50, 20, 20, 25, 25, 20 and 25μg/kg,respectively.

Table 1 Concentration of PAHs in urban soil of Xi’an (μg/kg) [3]

Concentrations of PAHs in ambient air vary in a wide range from lower than 50 pg∙m−3 to higher than 1.7 μg∙m−3 (up to 35.02 ng BaP-TEQ m−3), depending on where and when the samples were collected (Table 2). In general, atmospheric PAH concentrations are higher in industrial, urban or residential areas if compared with those measured in remote or rural sites.

Table 2 Total PAH concentrations of atmosphere reported worldwide [4]

X. Wang et.al[5] found that, for the PM10 in the air of Xi’an, the concentration of total PAHs ranged from 144.0 to 640.8 ng∙m-3 (Table 3). For benzene[a]pyrene (BaP), the annual concentration was about 22.4 ng∙m-3 which is higher than 2.5 ng∙m-3 present in the ambient air quality standards of China (GB 3095 — 2012).

Table 3 The mass concentrations of PAHs over four seasons in PM10 of Xi’an [5]

PAHs can reach aquatic ecosystems mainly through dry and wet atmospheric deposition as well as through transport by water, in particular as discharges from sewage systems, industrial and urban effluents, and runoff from agricultural lands and burnt areas [6].

Wei River is the largest tributary of the Yellow River, flowing through some semi-arid cities, some of which are important industrial cities in Northwest China. The section of Shaanxi of Wei River is also called as Weihe/Guanzhong Plain/Basin, which is an important production base of grain and cotton for Shaanxi Province. Wei River and its tributaries make a great contribution to the development of local industry and agriculture. With the rapid development of social economy, a large quantity of pollutants has been discharged into Wei River, posing potential threats to aquatic ecosystem and human health. L. Pang [7] et.al found that, in the sediment of Wei River, the ∑16PAHs ranged from 60.5 to 10,243.1 ng∙g−1 dw, with an average of 2250.4 ng∙g−1 dw, presenting a high contamination level.

[1] Y. Zhang, C. Peng, Z. Guo, X. Xiao, R. Xiao, Polycyclic aromatic hydrocarbons in urban soils of China: Distribution, influencing factors, health risk and regression prediction, Environ Pollut, 254 (2019) 112930.

[2]J. Han, Y. Liang, B. Zhao, Y. Wang, F. Xing, L. Qin, Polycyclic aromatic hydrocarbon (PAHs) geographical distribution in China and their source, risk assessment analysis, Environ Pollut, 251 (2019) 312-327.

[3] L. Wang, S. Zhang, L. Wang, W. Zhang, X. Shi, X. Lu, X. Li, X. Li, Concentration and Risk Evaluation of Polycyclic Aromatic Hydrocarbons in Urban Soil in the Typical Semi-Arid City of Xi'an in Northwest China, Int J Environ Res Public Health, 15 (2018).

[4] N.D. Dat, M.B. Chang, Review on characteristics of PAHs in atmosphere, anthropogenic sources and control technologies, Sci Total Environ, 609 (2017) 682-693.

[5] X. Wang, Z.X. Shen, Y.L. Zeng, F.B. Liu, Q. Zhang, Y.L. Lei, H.M. Xu, J.J. Cao, L. Yang, Day-Night Differences, Seasonal Variations and Source Apportionment of PM10-Bound PAHs over Xi'an, Northwest China, Atmosphere, 9 (2018).

[6] F. Jesus, J.L. Pereira, I. Campos, M. Santos, A. Re, J. Keizer, A. Nogueira, F.J.M. Goncalves, N. Abrantes, D. Serpa, A review on polycyclic aromatic hydrocarbons distribution in freshwater ecosystems and their toxicity to benthic fauna, Sci Total Environ, 820 (2022) 153282.

[7] L. Pang, S.W. Zhang, L.J. Wang, T. Yang, S. Wang, Pollution characteristics and risk assessment of polycyclic aromatic hydrocarbons in the sediment of Wei River, Environ. Earth Sci, 80 (2021) 11.

How to degrade PAHs

The existing remediation methods of PAHS contaminated soil mainly include physical, chemical and biological methods. Among them, the problem with physical method is that the PAHs are still present in their original form, but only in a different location, so they do not fundamentally solve the pollution problem of PAHs. Chemical method is to use chemical oxidation, photocatalytic oxidation, electrochemistry and other technical means of remediation of PAHS contaminated soil. This method has excellent performance in terms of treatment effect, but there are some problems such as high implementation price, high energy consumption and the possibility of secondary pollution. Bioremediation is a green remediation technology that uses microorganisms, plants or enzymes produced by their metabolism to treat contaminated sites [1]. Among them, microbial remediation has attracted much attention due to its low cost and environmentally friendly characteristics.

Several studies have shown that diverse fungi are capable of PAH mineralization. These fungi can be classed into two groups: ligninolytic and nonligninolytic fungi[2].At least two mechanisms are involved in PAH biodegradation: one uses the cytochrome P-450 system and the other uses the soluble extracellular enzymes of lignin catabolism, including lignin peroxidase (LiP), manganese peroxidase (MnP) and laccases [3].Various researches have revealed that the mechanism of oxidation of PAHs by fungi ligninolytic enzymes is similar to the degradation of nonphenolic lignin. A correlation has been found between the ionization potential (IP) of PAHs and the specific activity of MnP and LiP. Aromatic substrates with a lower IP were oxidized by the two ligninolytic enzymes and the lower the IP, the faster the oxidation rate. A threshold value of IP was found for each enzyme: LiP oxidizes PAHs with an IP ≤7.55 eV , while MnP oxidizes PAHs with an IP up to 8.2 eV[4].The effect of laccase on the structural oxidation of non-phenolic lignin is similar to LiP. The laccase gene derived from Bacillus subtilis was successfully expressed in Escherichia coli and the degradation efficiency was verified. These works are of great significance to our laboratory work[5].

[1] M. Megharaj, B. Ramakrishnan, K. Venkateswarlu, N. Sethunathan, R. Naidu, Bioremediation approaches for organic pollutants: a critical perspective, Environ Int, 37 (2011) 1362-1375.

[2] C.E. Cerniglia, Fungal metabolism of polycyclic aromatic hydrocarbons: past, present and future applications in bioremediation, J Ind Microbiol Biotechnol, 19 (1997) 324-333.

[3] G.R. Tortella, M.C. Diez, N. Duran, Fungal diversity and use in decomposition of environmental pollutants, Crit Rev Microbiol, 31 (2005) 197-212.

[4] R.H. Peng, A.S. Xiong, Y. Xue, X.Y. Fu, F. Gao, W. Zhao, Y.S. Tian, Q.H. Yao, Microbial biodegradation of polyaromatic hydrocarbons, FEMS Microbiol Rev, 32 (2008) 927-955.

[5] J. Zeng, Q. Zhu, Y. Wu, X. Lin, Oxidation of polycyclic aromatic hydrocarbons using Bacillus subtilis CotA with high laccase activity and copper independence, Chemosphere, 148 (2016) 1-7.