A Preliminary Study on Pollution of Polycyclic Aromatic Hydrocarbons in Soil around a Thermal Power Plant of Xi'an City

. To understand the status and risk of polycyclic aromatic hydrocarbons (PAHs) pollution in industrial areas in China. The total of 4 surface soil samples were collected from a thermal power plant of Xi'an. The concentrations of 16 priority polycyclic aromatic hydrocarbons (PAHs) were analyzed by high performance liquid chromatography (HPLC). In addition, the composition, source, pollution level and risk assessment of PAHs in surface soil of thermal power plant were studied. The results showed that the total concentrations of 16 PAHs ranged from 3.28 to 8.88 μg•g -1 , with a mean of 5.52 μg•g -1 . The concentration of 7 carcinogenic PAHs ( Σ 7 CPAHs) ranged from 1.52 to 4.82 μg•g -1 , with a mean of 2.60 μg•g -1 . The ∑PAHs in around thermal power plant surface soils of Xi'an belonged to serious pollution level. The PAHs in present study were mainly composed of medium molecular weight PAHs and high molecular weight PAHs, which have strong three effects. The results of source analysis showed that the PAHs in surface soil were mainly originated from the combustion of fossil oil, coal, wood and other biomass. The results of ecological risks of PAHs in the surface soil showed that all samples were polluted generally. The potential ecological risk of PAHs belonged to the serious level in individual PAHs and samples. There are two samples value of TEQ Bap exceed the security value 0.600 μg•g -1 . Therefore, the environment of industrial areas should be attention and controlled.

Polycyclic aromatic hydrocarbons (PAHs) is a kind of common persistent organic pollutants widely existing in the environment, which are composed of two or more benzene rings [1] . Because of its strong "three effects" (carcinogenic, teratogenic and mutagenic), it is listed as the priority control pollutant by the United States Environmental Protection Agency (USEPA). China has also listed seven PAHs in the "blacklist of environmental priority pollutants" [2] . The sources of PAHs include both natural sources and anthropogenic processes. Such as forest fires and volcanic eruptions. Man-made pollution includes incomplete combustion of fossil and biomass fuels, industrial emissions, etc. PAHs enter the soil directly or indirectly through diffusion, sedimentation, adsorption and other processes, and more than 90% of PAHs remain in the soil, making the soil become the main environmental load of PAHs [3] . High concentration of PAHs have been found in soils. PAHs in soil can enter human body through respiratory tract, skin, digestive tract and food chain, which increases the risk of cancer [4] .With the rapid development of industrialization and urbanization in China, PAHs pollution in soil is becoming more and more serious, which has a potential negative impact on human health and ecological environment. Therefore, several studies have focused on the PAHs contamination status of soils in different regions.For instance, Tarragona [5] , Isfahan [6] , Kocaeli [7] and Germany [8] ,etc. And many cities in China, such as Beijing [9] , Shanghai [10] , Shenyang [1] , Shenzhen [11] , Nanjing [12] , Tianjin [13] , etc.
Xi'an is located in the Midwest of China. It is an important hub city connecting the northwest and southwest of China.
It is inevitable to cause environmental problems by the rapid development of the city. The accumulation and migration of pollutants in the soil medium will inevitably threaten the soil ecological environment. As a special functional area of the city, the industrial zone, as a result of the impact of its energy structure, discharges a large number of pollutants into the soil through various ways, resulting in soil pollution. In order to understand the environmental problems caused by rapid industrialization, this research study takes the surrounding soil of a thermal power plant in Xi'an as the research object, preliminarily determine the contamination level, pollution status and evaluate the health risk of 16 kinds of PAHs in the soil. It can provide theoretical guidance for the environmental management of the surrounding soil in Xi'an industrial Park.

Sample collection and pretreatment
5-10 sampling sites are respectively set in four directions around the thermal power plant, each sampling point adopts diagonal sampling method to collect 0-20cm topsoil for uniform mixing, and finally uses quartering method to form a mixed sample,a total of 4 soil samples, specific information and numbers are: east(I1), west(I2), south(I3), north(I4), respectively. Sample pretreatment method reference the preliminary study [14] .

Pollution classification standard
The PAHs pollution degree of soil around the thermal power plant in Xi'an was classified according to the standards of PAHs pollution grade proposed by maliszewska kordybach [15] . Pollution classification standards of PAHs are given in Table 1.

Ecological risk assessment
There is no unified PAHs ecological risk assessment method at home and abroad. Most of the studies use the quality standard method and quality standard method. In this study, the quality standard method and quality standard method are used to evaluate the soil PAHs ecological risk. The two commonly used in quality benchmark method are effects range low (ERL) and effects range (ERM), ERL and ERM indexes divide the ecological risk of PAHs into three levels: if the concentration of pollutants is less than ERL, there is little negative ecological effect and the probability of ecological risk is low (less than 10%); if ERL ≤ the concentration of pollutants≤ ERM, there will be occasional negative ecological effect on the surrounding ecological environment; if the concentration of pollutants is greater than ERM, there will be often negative ecological effect and ecological riskThe probability of birth is higher (more than 50%) [16] . The specific reference values of ERL and ERM see Table 2.  Generally, the quality standard method adopts the method issued by Quebec in 2006, which sets five evaluation indexes to evaluate the pollution degree and ecological risk of PAHs. They are rare effect concentration (REL), critical effect concentration (TEL), accidental effect concentration (OEL), possible effect concentration (PEL) and frequent effect concentration (FEL) [17] .See Table 3 for specific reference values.

Risk assessment of toxic equivalent
According to the Netherlands soil quality standard, the over standard rate of the site was judged.In order to further determine the ecological risk of PAHs, the toxicity risk assessment proposed by Nisbet was used. BaP was used as the toxicity standard substance. The toxicity equivalent concentration (BEQ i ,ug·g -1 ) and total toxicity equivalent concentration (TEQ,ug·g -1 ) of each PAHs monomer were calculated by comparing other PAHs with BaP [15] .The calculation formulas are as follows.
The C i refers to the concentration of components, and TEF i refers to the toxic equivalent factor of components. The TEF i specific values are given in Table 4.  [15] , the surrounding soil of thermal power plant has reached the serious pollution level, which indicates that the surrounding soil has been generally polluted by PAHs. Therefore, the relevant departments should attach great importance to the pollution prevention and control. And adjust the energy structure and the treatment of relevant environmental pollution prevention and control equipment in the plant.

PAHs composition in soil
According to the properties and molecular weight of PAHs, 16 kinds of monomer PAHs are generally divided into three types: small molecule 2 -3 ring (Nap, Acy, Ace, Flu, Phe, Ant) low molecular weight PAHs (LMW PAHs), 4 ring (Fla, Pyr, BaA, Chy) medium molecular weight PAHs (MMW PAHs) and high molecular weight PAHs (HMW PAHs) of 5-6 rings (BbF, BkF, BaP, DbahA, BghiP,IcdP).The toxicity and carcinogenicity of PAHs are also different due to their different nature and environmental behavior [17] .Among them, low ring PAHs are volatile and toxic to aquatic organisms, while PAHs with high ring have "three effects" [18] , so it is of great significance to study the composition characteristics of PAHs in soil. See Fig.1 for the relative abundance of PAHs in the surface soil around the thermal power plant.It can be seen from the figure that PAHs in the surrounding soil of thermal power plant are mainly composed of PAHs in the middle ring and PAHs in the high ring, and the distribution of PAHs in the whole shows the law of PAHs in the high ring (43.16%) > PAHs in the middle ring (35.54%) > PAHs in the low ring (22.29%), which is consistent with the characteristics of PAHs pollution in typical industrial areas studied by Ran Zongxin [18] . The content of PAHs in low ring is 11.31% -38.17%, that in middle ring is 26.41% -39.58%, and that in high ring is 26.93% -57.99%. However, the relative abundance of PAHs with E3S Web of Conferences 204, 01012 (2020) ICRTEG 2020 https://doi.org/10.1051/e3sconf/202020401012 different ring numbers can reflect the status of pollution sources. The proportion of 4 rings and above PAHs in the soil around the thermal power plant is 61.83% -88.69%, significantly higher than the PAHs content in the low ring, indicating that PAHs in the surrounding soil of the study area may have a strong "three effects", which should be highly valued by the surrounding residents and relevant departments. Most of the pollution sources come from the combustion of various fossil fuels.  [19,20] .
These characteristic ratios are used to analyze the PAHs source in the soil around the thermal power plant, and the results are shown in Figure 2.The ratio of LMW / HMW is in the range of 0.1-0.7, which is less than one, indicating that PAHs in the soil around the thermal power plant mainly comes from incomplete combustion process.It can be seen from Figure 2 that the ratios of BaA/(BaA+Chy) and Icdp/(Icdp+BghiP) indicate that PAHs in the soil around the thermal power plant are mainly from petroleum combustion, the ratio of Fla / (Pyr + FLA) indicates that PAHs in the soil are from biomass combustion sources such as coal and wood, and Ant/(Ant+Phe ) indicates that PAHs in the soil are from mixed sources, with fossil fuel combustion and petroleum pollution. Based on the results of the above five common characteristic ratio methods, it can be concluded that PAHs in the soil around the thermal power plant comes from the combustion process of biomass such as oil, coal and wood, accompanied by the mixed sources of oil pollution, among which fossil fuel is the main combustion source. The source of soil PAHs in this study is consistent with the source of soil PAHs around Shanxi Iron and Steel Industrial Park studied by Bai Xinyue [21] and the source of soil PAHs in a coal-fired power plant in southwest China studied by Zou Yiping [2] . This is consistent with the energy structure of industrial zones and cities in China. Multivariate statistical analysis: Considering that the ratio method is easy to be affected by environmental factors and the ratio may be unstable, this study further carried out principal component analysis on the surrounding soil of a thermal power plant in Xi 'an, and extracts two factors (PC1 and PC2) with characteristic values greater than 1, with cumulative variance contribution rate of 96.05%, with specific results aare shown in Table 6.The variance contribution rate of PC1 is 64.23%, which is mainly composed of PAHs with 3-6 rings. The PAHs with higher load are Ant, Fla, Pyr, BaA, Chy, BbF, BkF, BaP, BghiP and IcdP. Which Ant, Fla, Pyr, BaA, BbF, BkF and BaP are generally regarded as indicators of coal combustion [3] .In addition, Fla and Pyr are also the main polycyclic aromatic hydrocarbons emitted during wood combustion [22] .Bghip and IcdP are considered to be the main pollutants emitted by automobile engines [22] .Therefore, the definition factor 1 represents the combustion mixture source, mainly including PAHs generated by coal, gasoline, wood and other combustion. Factor 2 accounted for 31.82% of the total variance, among which, Nap, Acy, Ace, Flu and Phe were heavily loaded on PC2.And Nap, Acy and Ace indicated the leakage of petrochemical products or the low-temperature transformation of organic matters.Therefore, it is further confirmed that the PAHs source of the soil around the thermal power plant is a mixed source, in which the combustion mixed source is dominant.

PAHs ecological risk assessment
Ecological risk assessment of PAHs in soil around thermal power plants was carried out by using quality benchmark methods (ERL and ERM). From the average concentrations of the four sites, only the average concentrations of Ant, Pyr, BaA, BkF and BaP did not exceed the ERL limit, indicating that these five PAHs have low ecological risk level and low probability of ecological risk (10%). The average value of DBahA is greater than ERM limit, which indicates that the probability of ecological risk is extremely high and there will be frequent negative ecological effects. The average concentrations of the remaining PAHs are between ERL and ERM, indicating that ecological risks will occasionally occur to the surrounding ecological environment. The maximum concentration of Ant and Pyr did not exceed ERL, which indicated that the probability of ecological risk of Ant and Pyr in surrounding soil was extremely low. The maximum concentrations of Ace and DBahA exceed ERM values, resulting in extremely high probability of ecological risks. The maximum concentrations of the remaining PAHs exceeded ERL and were located between ERL and ERM, indicating that the PAHs in the surrounding soil will generate occasional ecological risk.
The ecological risk assessment of PAHs in the surrounding soil of thermal power plant was carried out by using the quality standard method. The reference criteria are: there is high ecological risk if it is greater than FEL. Between PEL and FEL, the potential risk probability is higher. There is medium potential risk if it is between OEL and PEL. Between TEL and OEL, the potential risk probability is low; Between REL and TEL, the potential risk probability is the lowest. Less than REL, no potential risk. Therefore, it can be seen that the concentrations of Nap and Chy is between OEL and PEL, with moderate potential risk. Acy, Phe and Bap concentrations are between OEL and FEL, with potential risks ranging from medium to high. Flu, Fla and Pyr concentrations are between TEL and PEL, with potential risks ranging from low to medium. The concentration of Ant is low and the probability of potential risks is low. Ace, BaA and DBahA concentrations are distributed, so the potential risks are also different. Generally speaking, PAHs in the soil around the thermal power plant has certain potential ecological risks at individual points, and relevant departments should take measures to actively control and control the emission of pollutants.

PAHs health risk assessment
See Table 7 for the calculation results of total toxic equivalent concentration (TEQ Bap ) of BaP in the study area.The results showed that the total TEQ Bap of 16 PAHs was between 0.443-1.886 μg·g -1 , and the average value was 0.939 μg·g -1 .It is much higher than the TEQ Bap (0.138 μg·g -1 ) of the surface soil in Jinan [23] , the TEQ Bap of the wetland soil in the Pearl River Delta [15] , the TEQ Bap (0.428 μg·g -1 ) of the urban soil in Shanghai [24] and the TEQ Bap (0.013μg·g -1 ) of the surface soil in the Middle East of the Qinghai Tibet Plateau [20] . The TEQ Bap of 7 kinds of carcinogenic PAHs (Σ 7 CPAHs) ranged from 0.438 to 1.873 μg·g -1 , with an average value of 0.939 μg·g -1 . The TEQ Bap of Σ 7 CPAHs accounted for 99.3% of the total TEQ Bap of ΣPAHs, indicating that 7 kinds of carcinogenic PAHs were the main contributors of the total TEQ Bap . According to the Canadian standard for the protection of human health on risk-based soil (CC-ME, 2010) [15] , there are two sample sites in the region with TEQ Bap greater than 0.600 μg·g -1 , indicating that the soil around the thermal power plant has a high risk and potential carcinogenicity, which should be focused on management and control, especially the TEQ Bap of PAHs at I2 site is as high as 1.886μg·g -1 should be paid special attention to and controlled by the surrounding residents and relevant departments.

Conclusion
1) The content of soil around the thermal power plant is 3.28-8.88 μg·g -1 , which is polluted by PAHs and reaches the level of serious pollution. The PAHs content of 4 ring and above is the main advantage, which indicates that PAHs in the surrounding soil may have strong "three effects".
2) Two kinds of source analysis methods show that PAHs in the soil around the thermal power plant comes from the combustion process of biomass such as oil, coal and wood, accompanied by the mixed source of oil pollution, in which fossil fuel combustion is the main source.
3) The risk assessment shows that PAHs at individual points and individual PAHs are at risk, and there are two sample points with teqbap greater than 0.600 μg·g -1 safety level, indicating that the soil around the thermal power plant is at high risk and has potential carcinogenicity, so it is necessary to carry out key control.