Polycyclic aromatic hydrocarbons (PAHs) are a class of diverse organic compounds containing two or more fused aromatic rings made up of carbon and hydrogen atoms [1
]. Generally, they are produced from incomplete combustion of organic materials, fossil fuels, petroleum product spillage and various domestic and industrial activities [2
]. Once emitted, PAHs can be widely dispersed in air, water, soil and sediment. Due to the hydrophobicity and lipophilicity of PAHs, soil is the most important sink for PAHs in natural environment [4
]. It has been reported that soil can store approximately 90% of PAHs [6
]. PAHs in soils can be carried into surface/ground water through precipitation and surface runoff, emitted into atmosphere by volatilization, and transported into crops from polluted soil and air via root and leaf adsorption, which may further accumulate in human and other organisms via food chains [7
]. Thus, monitoring the concentration of PAHs in soils is important for understanding its environmental fate.
Many PAHs are mutagenic and some are carcinogenic, raising concerns over their occurrence in the environment [8
]. Based on their potential toxicity, the United States Environmental Protection Agency (USEPA) has identified 16 PAHs as priority pollutants [10
]. Meanwhile, the USEPA and the International Agency for Research on Cancer (IARC) have also considered 7 of 16 priority PAHs as probable/possible human carcinogens. In addition, they are considered as candidates of persistent organic pollutants (POPs) that merit further investigation for possible early listing in the Stockholm Convention on POPs. Thus, more attention has been paid to PAHs in recent years [11
]. However, there has been less research on the concentration, distribution and possible sources of PAHs in petroleum-contaminated soils, as compared to urban and agricultural soils.
In recent years, China has arduously implemented the One Belt One Road initiative, and constructed what is known as the New Silk Road. Because the countries and terrains along this route are rich in oil and gas resources, it is expected to very soon become an ‘Energy Road’. The Loess Plateau is one such main terrain area along the Silk Road Economic Belt, and it is also the key energy base in China. The Loess Plateau has abundant oil and gas resources. The most abundant oil resources on the Loess Plateau are specifically distributed in Yulin, Yan’an and Qingyang. With the large-scale exploitation of these petroleum resources, the ecological environment has become severely polluted. Even though this region has large geological reserves, with wide distribution petroleum-rich areas, yet the peculiar geographical structures limit the reservoir scale within a relatively small area. Oil wells are plentiful, yet not well connected. This makes it very difficult to systematically monitor petroleum contamination. Consequently, our research on petroleum-contaminated soils addresses an urgent need.
Furthermore, petroleum is a complex mixture of alkanes, aromatics, resins, asphaltenes, and other organic matter [14
]. Of all petroleum components, PAHs are considered the most important. Petroleum and its derivatives are easily released into the environment during petroleum extraction, storage and transportation. These processes not only entail wastage of precious petroleum resources, but also pollute and destroy the ecological environment, and endanger human health. Therefore, it is imperative to conduct research on petroleum-contaminated soils.
On the whole, these three representative areas (Yulin, Yan’an and Qingyang) with petroleum pollution on the Loess Plateau are the research subjects of this study. The main objective of the present study was to determine the concentration levels of PAHs in petroleum-contaminated soils, and to assess the probable sources of PAHs contamination. Additionally, the ecological risk and toxicity of PAHs in soils were evaluated. The results obtained may significantly provide basic theoretical data for the PAHs remediation.
2. Materials and Methods
2.1. Study Area Description
The Loess Plateau is located in the north-central part of China and is one of the four major highlands in China. It extends over 8 latitudes (34–41° N) and 14 longitudes (101–114° E), with a total area of 640,000 km2. It covers almost all of the provinces of Shaanxi and Shanxi and extends into parts of Gansu, Ningxia, and Inner Mongolia. It has a semi-arid climate, with extensive monsoonal influence. The average annual temperature ranges from 6 to 14 °C. The soil type is classified as typical loessal soil, which is easily eroded, causing nutrient deficiency. Yulin, Yan’an and Qingyang, as key research areas, are distributed from north to south on the Loess Plateau.
2.2. Sample Collection
The sampling sites (35°28′44′′ N–37°30′41′′ N, 107°42′12′′ E–109°53′5′′ E) are located on the Loess Plateau. A total of 60 petroleum-contaminated soil samples were collected from 20 sampling sites in July 2017. Soil samples (0–10 cm depth, 10–30 cm depth and 30–50 cm depth) were taken with a stainless steel soil auger after removal of the uppermost cover. Five samples were gathered over an area of 100 m2
, mixed to form a composite sample [15
]. During the whole sampling process a global position system (GPS) was used to accurately provide the location of each sampling point as shown in Figure 1
. The basic information of the sampling sites in details is given in Table 1
. After transport to the laboratory, the soil samples were air dried, ground, passed through a 60-mesh screen, homogenized, and stored at 4 °C until analysis.
2.3. Reagents and Standards
A standard mixture containing 16 PAHs: naphthalene (NAP), acenaphthylene (ACY), acenaphthene (ACE), fluorene (FLU), phenanthrene (PHE), anthracene (ANT), fluoranthene (FLA), pyrene (PYR), benz(а)anthracene (BаA), chrysene (CHR), benzo(b)fluoranthene (BbF), benzo(k)fluoranthene (BkF), benzo(а)pyrsne (BаP), indeno(1,2,3-c,d)pyrene (InP), dibenz(а, h)anthracene (DBA), benzo(g,h,i)perylene (BgP), was purchased from AccuStandard Inc. (New Haven, CT, USA). High Performance Liquid Chromatography (HPLC) grade dichloromethane was purchased from Waters Company (Milford, MA, USA). The other reagents were all analytical grade. QuEChERS extraction kits containing 50 mg C18, 150 mg PSA and 900 mg Na2SO4 were provided by Agilent Technologies Inc. (Santa Clara, CA, USA). Milli-Q water was used to perform the analytical procedures.
2.4. Sample Extraction
In the laboratory, the samples were air-dried at room temperature and stones, roots and other debris were removed. The samples were then ground and sieved through a 60-mesh screen. Soil samples (2.0 g) were mixed with anhydrous sodium sulfate (3.0 g), and extracted with dichloromethane (20 mL) for 30 min under ultrasoound. After centrifuging the tubes at 9500 r/min for 10 min, a 2-mL supernatant sample was transferred to a single-use centrifuge tube containing 150 mg of PSA, 50 mg of C18, and 900 mg of anhydrous Na2SO4. The mixtures were shaken vigorously for 1 min using a vortex mixer to ensure that the solvent contacted the entire sample. Subsequently, the samples were centrifuged at approximately 9500 r/min for 10 min. Then, the upper layer of the prepared sample was filtered through a 0.22 μm syringe filter and transferred to an autosampler vial for injection.
2.5. Instrumental Analysis
The determination of PAHs was performed on GCMS-TQ8040 (Shimadzu (China) Co., Ltd., Xi’an, China) with splitless injection, MRM acquisition mode. The capillary column Rxi-5Sil Ms (30 m × 0.25 mm × 0.25 μm) was used for separations. Helium (99.999%) was used as the carrier gas. The oven temperature program was as follows: initial temperature of 50 °C was held for 2 min, then increased to 250 °C at a rate of 20 °C/min and held for 3 min, and finally increased to 300 °C at a rate of 5 °C/min and held for 5 min.
2.6. Quality Control
All analytical procedures were monitored with strict quality assurance and quality control measures. Quantitation was performed using an external standard calibration method (seven-point calibration: 2, 10, 50, 100, 200, 500 and 1000 μg/L), and correlation coefficients (R2) for the calibration curves that were all greater than 0.996. The limit of detection (LOD) was calculated as three times of standard deviation of the blank. The LODs of NAP, ACY, ACE, FLU, PHE, ANT, FLA and PYR were 0.02, 0.80, 0.60, 0.12, 0.04, 0.16, 0.12 and 0.16 μg/kg dw, and those of BaA, CHR, BbF, BkF, BaP, InP, DBA and BgP were 0.18, 0.08, 0.16, 0.20, 0.20, 0.06, 0.06 and 0.10 μg/kg dw, respectively. The recoveries of NAP, ACY, ACE, FLU, PHE, ANT, FLA and PYR were 118 ± 1.7%, 117 ± 0.5%, 119 ± 3.5%, 112 ± 0.8%, 109 ± 6.7%, 94 ± 6.4%, 109 ± 1.1% and 110 ± 5.9%, and those of BaA, CHR, BbF, BkF, BaP, InP, DBA and BgP were 95 ± 2.8%, 96 ± 3.5%, 98 ± 9.1%, 93 ± 7.6%, 65 ± 8.9%, 93 ± 6.3%, 94 ± 3.9% and 80 ± 9.5%, respectively.
2.7. Ecological Risk of PAHs in Soils
PAHs accumulated in soils may enter water bodies and plants, posing a potential ecological risk. A risk quotient (RQ) was used to assess ecological risk of some organic substances. The negligible concentrations (NCs) and the maximum permissible concentrations (MPCs) of PAHs in soils were used as the quality values in the medium [16
]. Therefore, RQ(NCs)
were defined as follows:
was the quality values of the NCs of PAHs in the medium and CQV(MPCs)
was the quality values of the MPCs of PAHs in the medium. The RQ∑PAHs
were defined as follows:
Based on the ecosystem risk assessment of 16 individual PAHs, RQ(NCs) and RQ(MPCs) of individual PAHs which were not less than 1 were added to calculate the RQ∑PAHs(NCs) and RQ∑PAHs(MPCs) of ∑PAHs. RQ(NCs) < 1.0 indicated that the single PAHs might be of negligible concern, RQ(MPCs) > 1.0 would indicate that the contamination of the single PAHs posed high risk, and RQ(NCs) > 1.0 and RQ(MPCs) < 1.0 indicated that the contamination of the single PAHs was of moderate risk.
2.8. Toxicity Assessment of PAHs in Soils
PAHs can be absorbed by humans through the skin and respiratory tract, and they may cause skin cancer, lung cancer and other diseases. Exposure to PAHs in the environment for a long time may cause chronic poisoning. Toxic equivalency factors (TEFs) were used to estimate the exposure risks posed by individual and total PAHs to human health. The toxicities of PAHs in sampling sites were evaluated BaP equivalent concentration (BaPeq). The TEFs for the 16 PAHs were calculated according to USEPA and Nisbet and LaGoy [10
]. The total toxicity equivalency concentrations (BaPeq) were calculated using the following equation:
is the concentration of individual PAHs and TEFi
is the corresponding toxic equivalency factor.
2.9. Properties Analysis
Soil pH was measured (soil: water 1:2.5 w
) by using a pH-meter (pHS-3B, Leici, Shanghai, China) and the soil organic carbon contents were determined by the Walkey-Black method [18