Abstract
Polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants of global concern, mainly originating from industrial activities, biomass combustion, and vehicle emissions. With the acceleration of industrialization, the surrounding environment of the industrial area may have potential health effects on the population. Therefore, we focused on the occurrence, source, and health risk assessment of PAHs in soil, plants, and water near industrial areas in southern Xinjiang, China. The occurrence of PAHs in different soil layers (0–10, 10–30, and 30–50 cm) was studied, with PAHs mainly concentrated in the topsoil (11.50–34.68 ng/g). In plants, PAHs varied from 56.63 to 597.28 ng/g, with the highest concentrations in reed (267.29–597.28 ng/g). Total PAHs in water ranged from 4183.85 to 24803.45 ng/L, with an average of 10,240 ng/L. 3-ring PAHs were the dominant species in soil, plants, and water with 55, 69, and 59%, respectively. PAHs isomer ratio results indicated that PAHs in soil, plants, and water mainly came from fossil fuels and biomass combustion. Incremental lifetime cancer risk estimation results demonstrated that adults might face higher potential health risks than children. Adults’ dermal contact was the dominant route of exposure, while oral ingestion was the dominant exposure pathway for children. The total carcinogenic risk value of corn is much higher than that of walnuts and red dates, indicating that PAHs pollution in corn must be taken seriously. The results can clarify the local pollution situation, and provide suggestions for improving pollution prevention and control measures.
Graphical abstract
1 Introduction
Polycyclic aromatic hydrocarbons (PAHs) are volatile hydrocarbons produced by the incomplete combustion of organic compounds, for instance, coal, oil, wood, and tobacco [1,2,3]. The low molecular weight (LMW) PAHs (containing 2–3 aromatic rings) are regarded as highly toxic substances [4], while high molecular weight (HMW) PAHs (containing 4–6 aromatic rings) are structurally stable and difficult to degrade, and are generally regarded as genotoxic substances [2]. PAHs are persistent organic pollutants with properties of high toxicity, bioaccumulation, and long-distance migration [5]. PAHs exist in the ambient air in the form of gas phase [6] and migrate to the soil [7], water [1], and other environmental media through the dry and wet deposition process. The hydrophobicity of PAHs makes them easily absorbed by soil particles [8], and soil systems are considered to be the main storage and transfer stations of PAHs in nature [9]. However, PAHs in plants mainly come from contaminated soil and irrigation water, and atmospheric precipitation [10,11]. Therefore, plants will be affected by PAHs pollution in the surrounding environment. For example, PAHs accumulated in the soil will enter the plants through absorption by the root, and move from the root to the aboveground part with the growth of plants [10]. Various environmental media are closely related to human daily production and life, and PAHs have potential carcinogenic, teratogenic, and mutagenic effects [9]. Consequently, PAHs pollution not only harms the entire ecosystem but also poses a great threat to human health [12].
Studies of PAHs pollution, source, and health risk contribute to the knowledge of the process of identifying PAHs contamination and maintaining ecological balance and human health [13,14]. Compared to urban and rural soils, agricultural soils near industrial areas have attracted less attention. With the acceleration of industrialization, the burning of fossil fuels in the production process of the factory produces a large amount of PAHs. Therefore, PAHs contamination in agricultural soil and plants in the proximity of industrial areas is quite serious [15,16]. During recent decades, multiple studies have largely focused on this including Shanghai [17,18], Hangzhou [7], and Changchun in China [19], Uzbekistan [20], South Korea [21], and Australia [22]. However, there are few reports on the pollution of PAHs in southern Xinjiang. The current research on PAHs in Xinjiang shows that the total amount of 15 PAHs in eastern Pamirs varied from 2.1 to 34.0 ng/g [23], and the total amount of 16 PAHs in Urumqi varied from 331 to 15,799 ng/g [24], pollution degree of PAHs varies in different regions. Previous studies have contributed to a better understanding of PAHs, revealing PAH pollution levels, inspiring the development of more precise PAHs pollution treatments, and providing useful guidance for assessing potential health risks to human health.
PAHs pollution is closely related to human activities and economic development. With the acceleration of China's industrialization process and the population growth driven by organizational strategies and policies in the past decades, it is expected that PAHs pollution near the industrial area will be a key problem in the future. At present, few studies have investigated the occurrence of PAHs in farmland near the industrial areas in southern Xinjiang. Most studies focus on the pollution of PAHs in a single environmental medium, and whether there is a certain rule for the occurrence of PAHs in soil layers at different depths, as well as the difference in the content of PAHs in different plants remains unclear. Based on the above analysis, we (1) investigated the pollution of 16 PAHs in farmland soil, plants, and water near the industrial parks in Kuqa city and Shaya county of Aksu district, in southern Xinjiang, China; (2) determined the source of PAHs in these three media through specific diagnostic ratio and principal component analysis-multiple linear regression (PCA-MLR) method; (3) assessed ecological and incremental lifetime cancer risks associated with PAHs in farmland soils and different types of plants. Ultimately, based on the results of the study, recommendations are made for local PAH pollution prevention and control measures.
2 Materials and methods
2.1 Sampling
From September 29 to October 2, 2020, soil, plant, and water samples were collected near the industrial parks of Kuqa city and Shaya county. Near the sampling site is a new ecological industrial park, which mainly produces chemical products, petroleum additives, and fertilizers. The distribution of the sampling points is shown in Figure 1. The P14 sampling point is the control sample far away from the industrial park. In this study, soil samples were collected from three depths (0–10, 10–30, and 30–50 cm) to analyze the vertical distribution of PAHs in the soil. Three soil samples were collected at different depths (Range: 5 m × 5 m) at every sampling area and mixed as one sample. A total of 42 soil samples were collected. According to the plant cultivation in the study area, the typical plant types growing locally were collected in the corresponding soil sampling area, a total of 35 plant samples were collected including cotton (n = 17), corn (n = 6), walnuts (n = 4), red dates (n = 3), Tamarix ramosissima Lcdcb (n = 3), and reeds (n = 2). Water samples were collected from the nearest irrigation water of plants in each soil sample collection area, and a total of 22 water samples were collected, including canal water (n = 10), well water (n = 7), and sewage (n = 5). Collected 1,500 ml of each water sample, and stored it in a clean amber glass bottle. Soil and plant samples were placed in self-sealing polyethylene bags, and brought back to the laboratory, and spread in a dark basement to dry at room temperature. Prepared 250 g soil samples from which impurities have been removed, grounded, and sieved through 100 μm stainless steel mesh, sealed and stored, and refrigerated at −20℃ for standby until experimental analysis. Plant samples were divided into root, leaf, and flower parts, grounded in mortar respectively, sieved through 100 μm stainless steel mesh. Collected 10 g of each plant sample, stored in the seal, and refrigerated at −20℃ until experimental analysis.
Map of the study area and sampling points.
2.2 Preparation and analysis
15 g soil samples (accurate to 0.01 g) were weighed and placed in a 250 mL centrifuge tube, to which 30 mL (v:v = 1/1) of dichloromethane acetone solution was added and left to stand for 2 h, and then homogenized for 1 min. And the samples were oscillated in a water bath thermostatic oscillator (SHA-C) at room temperature for 30 min. Then, the mixture was centrifuged in a high-speed centrifuge (CT18RT) at 1,000 rpm for 5 min, and 5 mL of supernatant was extracted and concentrated to near dryness, dilute with n-hexane to 2.5 mL, vortexed for 15 s, and filtered using a 0.22 μm microporous membrane into an injection bottle for loading. 1 g plant samples were weighed (accurate to 0.01 g) into a 50 mL centrifuge tube, and 30 mL (v:v = 1/1) dichloromethane acetone solution was added and left to stand for 2 h and homogenized for 1 min. The rest of the steps are similar to soil samples. 150 mL water samples were weighed (accurate to 0.1 mL), and 30 mL (v:v = 1/1) dichloromethane acetone solution was added to stand for 2 h and homogenized for 1 min. The samples were oscillated in a thermostatic oscillator (SHA-C) for 30 min, then the organic phase was collected in a 50 mL centrifuge tube, and sodium chloride was added to make them supersaturated and precipitated. The precipitation was centrifuged for 5 min at 10,000 rpm, took 5 mL of the organic phase, concentrated to nearly dry, added 5 mL (v:v = 1/1) dichloromethane acetone solution, the extraction was repeated 3 times. 15 mL of the liquid were collected after the column, concentrated to nearly dry, dilute with n-hexane to 2.5 mL, the steps of vortex and filtration are similar to the previous process. The experiments were performed with Agilent GC7000 gas chromatography-tandem mass spectrometry (GC-MS). The detector system is equipped with an HP-5MS capillary column (30 m × 250 µm × 0.25 µm). The initial temperature was set at 80°C for 2 min, increased to 235℃ at speed of 10 °C/min, then raised to 300℃ at 4 °C/min, and lasted for 4 min. The injection volume was 1 µL, injected in non-divergent mode. The inlet temperature was 300℃, and it operated in electron impact mode of 70 eV. The temperature of interface was 280℃, and the mass scanning varied from 50 to 500 m/z. The data were obtained in selective ion detection mode, and the content of measured components was determined by internal standard method.
2.3 Quality control
Internal standards were naphthalene-d8, acenaphthene-d10, phenanthrene-d10, chrysene-d12, and perylene-d12. The procedural blanks, spiked blanks, and sample duplicates were determined, and no interferences were detected. Recoveries varied from 75.6 to 105.9%, method detection limits were between 0.01 and 0.64 ng/g. The method detection limits of different samples are shown in supplemental data (Table S1).
2.4 Risk assessment
Benzo[a]pyrene (BaP) is assumed to be a potential carcinogen according to the classification standard of carcinogenic chemical substances in the United States Environmental Protection Agency (USEPA) integrated risk information system. In 1992, Nisbet and LaGoy measured the carcinogenic toxicity ratio of PAHs to BaP through toxicological test, and proposed the concept of toxic equivalence factor (TEF) to evaluate the ecological risk of PAHs in soil [25]. The equivalent concentration (TEQ) was computed by the following formula:
where C i is the content of PAH congener i [26].
Incremental lifetime cancer risks (ILCRs) were used to estimate the potential health risks of PAHs in soil to humans [27], which is calculated by the following equations:
where CS (mg/kg) is the sum of 16 PAHs TEQ; CSF is the carcinogenic slope factor (mg/(kg day)), BW is the body weight (kg), AT is the average lifespan (days), EF is the exposure frequency (day/annum), ED is the exposure duration (a), IRIngestion and IRInhalation are soil intake (mg/day) and inhalation (m3/day) rate, respectively, SA is the skin exposure area (cm2), AF is the skin adherence fraction (mg/cm2), and ABS is the skin absorption efficiency. The CSF of ingestion, dermal, and inhalation were 7.3, 25, and 3.85 (mg/(kg day)), individually, and PEF is the particle emission factor (m3/kg). Specific parameters are in the supplemental data (Table S2) [7].
The carcinogenic risk value of PAHs caused by dietary intake was calculated according to the formula provided by the USEPA.
where IRcrop is the dietary intake (g/day). The carcinogenic risk assessment has been carried out on the edible parts of corn, red dates, and walnuts in the measured plant samples. The per capita daily intakes of corn, red dates, and walnuts were taken according to the values in the Xinjiang Statistical Yearbook 2021 (427.37, 160.47, and 10.14 g/day, respectively) [28].
3 Results
3.1 PAH concentration in soils
Statistics of PAH concentration detected in agricultural soils near industrial areas of southern Xinjiang are shown in Table 1. PAH contamination in soil varied from 6.47 to 56.08 ng/g. Naphthalene (Nap) has the highest concentration (1.00–20.15 ng/g), followed by phenanthrene (Phe) (0.61–14.21 ng/g). The average concentration of PAHs in top layer soil was 21.53 ng/g, of which Phe was the highest, accounting for 4.25–41.03% of the total PAHs, followed by chrysene (Chr) (3.4–24.1%). In the middle layer soil, the average PAHs concentration was 19.20 ng/g, and the highest concentration was of Phe (11.9–42.4%), followed by fluoranthene (Flu) (5.1–19.9%). PAHs in bottom layer soil was 17.83 ng/g in average, and the content of Phe was the highest, accounting for 25.63–41.91% of the total PAHs, then Flu (7.3–15.3%). The proportion of PAHs in soil is Phe (32.9%) > fluorene (Fl) (10.8%) > Nap (9.4%) > Flu (9.2%) > anthracene (Ant) (8.2%), which is consistent with the previous research showing that LMW-PAHs such as Nap and Phe were the most abundant components in soil [29]. The sum of seven carcinogenic PAHs (benz[a]anthracene (BaA), BaP, benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), Chr, dibenz[a,h]anthracene (DahA), and indeno[1,2,3-cd]pyrene (IcdP)) varied from 0.60 to 23.56 ng/g, accounting for 20% of total PAHs, which were observed below the maximum permissible limit (4,230 ng/g) formulated by the Ministry of Environmental Protection, China (2007). In addition, PAHs in soils were decreasing gradually from the surface to the deep layer (Table 2), this is probably because topsoil is most directly affected by atmospheric sedimentation. Previous studies have found that the total PAHs concentration decreases with the increase in soil depth, and the bacterial community structure also changes with the soil depth and the degree of PAHs contamination [30].
Detection of PAHs in soil samples (ng/g)
| PAHs | Ring | Min | Max | Mean value | SD |
|---|---|---|---|---|---|
| Nap | 2 | 1.00 | 20.15 | 2.37 | 3.89 |
| Acy | 3 | 0.06 | 0.39 | 0.18 | 0.08 |
| Ace | 3 | 0.12 | 0.60 | 0.32 | 0.12 |
| Fl | 3 | 0.73 | 3.09 | 1.88 | 0.55 |
| Phe | 3 | 0.61 | 14.21 | 6.17 | 2.75 |
| Ant | 3 | 0.52 | 4.12 | 1.52 | 0.72 |
| Flu | 4 | 0.48 | 4.37 | 1.77 | 0.83 |
| Pyr | 4 | 0.31 | 4.63 | 1.35 | 0.85 |
| BaA | 4 | 0.10 | 4.49 | 0.73 | 0.83 |
| Chr | 4 | 0.28 | 9.03 | 1.86 | 1.94 |
| BbF | 5 | 0.04 | 2.12 | 0.37 | 0.41 |
| BkF | 5 | 0.06 | 3.05 | 0.74 | 0.73 |
| DahA | 5 | 0.05 | 2.30 | 0.47 | 0.51 |
| BaP | 5 | 0.05 | 2.41 | 0.52 | 0.61 |
| IcdP | 6 | 0.01 | 0.71 | 0.12 | 0.16 |
| BghiP | 6 | ND | ND | ND | ND |
| ∑16PAHs | — | 6.47 | 56.08 | 19.56 | 9.68 |
| Σ7CPAHs | — | 0.60 | 23.56 | 4.80 | 5.04 |
ND: not detectable; SD: standard deviation; ∑16PAHs: concentration of the total 16 PAHs; ∑7CPAHs: concentration of the total 7 carcinogenic PAHs (BaA, Chr, BbF, BkF, BaP, IcdP, and DahA).
Concentration of total PAHs in soil samples at different depths (ng/g)
| Soil samples | 0–10 cm | 10–30 cm | 30–50 cm |
|---|---|---|---|
| P1 | 56.08 | 39.06 | 25.13 |
| P2 | 34.68 | 38.39 | 29.99 |
| P3 | 24.41 | 28.78 | 19.88 |
| P4 | 25.00 | 19.42 | 28.81 |
| P5 | 19.12 | 17.67 | 20.23 |
| P6 | 18.08 | 16.70 | 13.05 |
| P7 | 21.59 | 20.94 | 19.90 |
| P8 | 11.50 | 12.82 | 11.43 |
| P9 | 15.05 | 9.58 | 19.58 |
| P10 | 19.01 | 16.19 | 13.90 |
| P11 | 17.87 | 14.50 | 14.30 |
| P12 | 13.08 | 12.34 | 11.93 |
| P13 | 9.69 | 6.47 | 6.61 |
| P14 | 16.20 | 15.89 | 14.91 |
| Mean value | 21.53 | 19.20 | 17.83 |
| SD | 11.36 | 9.45 | 6.55 |
The highest PAHs contamination in soils was found at site P1 (56.08 ng/g) since it was located near a chemical plant and has the highest elevation. The lowest contamination was sample P13 (6.47 ng/g). As presented in Figure 2, 3–4 rings PAHs were dominant compounds in 14 sampling points, 3-ring PAHs accounts for the largest proportion (55%), which was consistent with that of farmland soils in Shandong Province [31]. 4-ring PAHs accounted for 27%, and 2-ring PAHs accounted for 9% (Figure 3).
Concentration percentages of PAHs with different ring numbers in 14 soil samples.
Compositional circle patterns of composition profiles of Σ16 PAHs.
Compared with other Chinese regions, the soil PAH contamination in our research was lower. The average PAHs level in soil of China was 730 ng/g, which was at a low or medium level compared with that of other countries. Some scholars have found that the contamination of PAHs in soil was primarily associated with the level of economic development, population density, traffic volume, and natural climatic conditions [8]. PAHs pollution in southern Xinjiang was much lower than that in most of the study areas, such as Shanghai (790–6,200 ng/g) [15], Beijing (314.7–1618.3 ng/g) [32], Shenyang (2,370 ng/g on average) [3], and other first-tier cities. It was lower than that of Shandong Province (152.2–1317.7 ng/g) [31] and Zhejiang Province (150.2–83,096 ng/g) [7]. There are only few studies on the PAH contamination of soil in Xinjiang, the concentration of PAHs in the topsoil of the Issyk-Kul Lake Basin in Xinjiang was 68.58–475.95 ng/g, which was higher than our study. The PAHs content in our research was similar to that in the western Qinghai–Tibet Plateau (14.40–59.50 ng/g) [33] and the central part of the Qinghai–Tibet Plateau (0.43–26.66 ng/g) [34]. Previous studies have indicated that the total concentration of PAHs in soil of China decreases in the order of northeast > north > east > south > west, and the differentiation of economic development is the main reason for the distribution of PAHs in the soil of China [8].
3.2 PAH concentration in plants
The total PAHs in plants ranged from 56.63 to 597.28 ng/g (mean value 225.29 ng/g), and carcinogenic PAHs varied from 1.74 to 34.06 ng/g (Table 3). Among all the cotton samples, the content of PAHs in sample P1 was the highest, since it is located on the east side of a chemical plant, surrounded by farmland. Walnuts in the P7 sample contain more PAHs because the sample site is close to the fertilizer plant, and surrounded by farmland. The content of Nap is the highest in these two samples, as it may be used in agricultural crops as the main component of insecticide. Nap is the byproduct of low-temperature combustion of organic compounds, and is produced by the spillage of crude oil and petroleum products. It will easily spread to the surrounding environment because of its volatility. By comparing the occurrence of PAHs in different plants, it can be found that the content of PAHs is higher in reed, Tamarix ramosissima Lcdcb, and walnuts. The contamination of PAHs in different plants were as follows: leaves (176.27–273.08 ng/g) > roots (115.35–232.56 ng/g) > flowers (56.63–299.95 ng/g). But in walnuts, it was leaves (382.45–522.85 ng/g) > fruits (69.59–111.08 ng/g). The results showed that the concentration of PAHs in aboveground part of plants were higher than those in underground part. This is consistent with previous studies that showed PAHs concentration in plant leaves were higher than in roots. In addition, they found that plants accumulate more PAHs from the air through their leaves than from the soil through their roots [35]. However, it cannot be ignored that the concentration of PAHs in plants will also increase with the increase in soil pollution [36].
Detection of PAHs in plant (ng/g) and water samples (ng/L)
| Max | Min | Mean value | |
|---|---|---|---|
| Plants | |||
| Cotton (n = 17) | 299.95 | 56.63 | 179.25 |
| Corn (n = 6) | 193.96 | 162.60 | 178.28 |
| Walnuts (n = 4) | 522.85 | 69.59 | 271.49 |
| Red dates (n = 3) | 295.89 | 98.23 | 173.66 |
| Tamarix ramosissima Lcdcb (n = 3) | 538.98 | 203.09 | 384.05 |
| Reeds (n = 2) | 597.28 | 267.29 | 432.28 |
| ∑16PAHs (n = 35) | 597.28 | 56.63 | 225.29 |
| ∑7CPAHs (n = 35) | 34.06 | 1.74 | 9.43 |
| Water | |||
| Canal water (n = 10) | 24803.45 | 4520.96 | 11958.92 |
| Well water (n = 7) | 11032.34 | 4183.85 | 8092.92 |
| Sewage (n = 5) | 22750.29 | 5273.67 | 9896.98 |
| ∑16PAHs | 24803.45 | 5273.67 | 10236.78 |
| Σ7CPAHs | 1069.95 | 228.67 | 419.76 |
∑16PAHs: concentration of the total 16 PAHs.
∑7CPAHs: concentration of the total 7 carcinogenic PAHs (BaA, Chr, BbF, BkF, BaP, IcdP, and DahA).
Dominant compounds in plant samples were 3–4 rings PAHs (Figure 3), 3-ring PAHs, including acenaphthylene (Acy), acenaphthene (Ace), fluorene (Fl), Phe, Ant, accounted for the largest proportion (69%). LMW-PAHs dominate the total PAHs, largely consistent with prior research [17,35]. In addition, the composition of PAHs in plants were similar to that in soil, which is dominated by 3–4 rings PAHs (Figure 3). LMW-PAHs in soils were more tended to be assimilated by plants, consequently the HMW-PAHs were rarely enriched in plants.
3.3 PAH concentration in water
PAHs in water varied from 4183.85 to 24803.45 ng/L (Table 3). Highest pollution was by Nap, accounting for 9.3–74.7% of the total PAHs, followed by Phe (7.6–40.8%). The dominant PAHs in water samples were LMW-PAHs, accounting for 96.5% of the total PAHs, which were consistent with those in plants and soils (Figure 3). PAHs obtained from canal water and sewage samples were higher than that in well water samples, which may be due to the exposure of canal water and sewage to air and the influence of PAHs discharged from upstream industrial parks. However, research shows that PAHs produced by human activities in the surface environment are easy to enter the groundwater quickly [37]. Even though the level of PAHs in this area is relatively low, it will still pose a threat to the local water resources security, and our report was intended to draw public attention.
3.4 Source identification and contributions of PAHs
Most PAHs are produced in the process of human production and living, and research works have indicated that the primary causes of PAHs are pyrogenic and crude oil production and use [38]. The purpose of pollutant source identification is to control and reduce pollutant emission. In this study, the PAH isomer ratios and PCA-MLR methods were used to analyze the sources of PAHs, and the specific parameters of PAHs diagnostic ratios are listed in Table S3.
In soil samples ∑LMW-PAHs/∑HMW-PAHs ratios were greater than 1 (Figure 4a), which suggested petroleum combustion as the main contribution [3]. The Flu/(Flu + pyrene (Pyr)) in all samples were higher than 0.5, representing the biomass and coal combustion [7]. The Ant/(Ant + Phe) were around 0.11–0.72, greater than 0.1, indicating that it was mainly pyrogenic source [18]. Most BaA/(BaA + Chr) ranged from 0.2 to 0.35, which indicated mix origins of petroleum, biomass, and coal combustion [27]. These results indicated that soil PAHs were mostly produced from fossil fuel and biomass burning, which can be ascribed to the reality that the sampling points are comparatively near the factories and heavy traffic.
In order to quantitatively evaluate the source of PAHs, PCA-MLR (SPSS 23.0 software) was employed to distinguish sources by quantifying the loadings of factors (Table S4). The first two factors (PC1–PC2) represent 83.5% of total variance. PC1 accounted for 72.4%, dominated by Chr (0.95), BbF (0.94), BaP (0.93), BkF (0.92), IcdP (0.90), DahA (0.90), BaA (0.87), and Pyr (0.86), these substances are generally associated with combustion. Thereinto, Chr, BaP, Pyr, BaA, and BkF were coal burning index [39,40,41], and IcdP, DahA, and BaP were related to traffic emissions [42,43]. Accordingly, PC1 reflected traffic emissions and coal combustion source. PC2 achieved 11.1%, took the initiative by Ace (0.84), Fl (0.84), and Nap (0.83). Ace indicate an oil source, primarily produced by the spillage of crude oil and petroleum products [44]. Fl represents coke burns and vehicle sources, Nap could arise from coke oven emission [43]. Therefore, PC2 reflected coke and petroleum burning hybrid sources. This allows the conclusion that primary origin of soil PAHs was coal and petroleum combustion with traffic emission. MLR analytical regression equation is as follows:
In plant samples the ratios of ∑LMW-PAHs/∑HMW-PAHs were all greater than 1 (Figure 4b), and primary source of PAHs was petroleum [3]. Flu/(Flu + Pyr) were around 0.05 to 0.67, and 91% of Flu/(Flu + Pyr) were more than 0.5, indicating that PAHs in plants in study area came from combustion of herbs, firewood, and coal [7]. As it was farmland, human activities such as straw burning cannot be ruled out. Only 3% of Flu/(Flu + Pyr) ratios were below 0.4, indicating that was less affected by the oil combustion. Ant/(Ant + Phe) were around 0.03 to 0.10, less than 0.1, which was mainly petroleum contamination [18]. And 49% of BaA/(BaA + Chr) ratios were between 0.20 and 0.35, which is mainly affected by the mix origins of coal, biomass, and petroleum combustion sources [27]. Study area was surrounded by industrial parks, which cannot avoid the oil consumption in the process of industrial activities and transportation. 40% of BaA/(BaA + Chr) were greater than 0.35, revealed burning source. In summary, combustion of biomass and petroleum were the dominant source of PAHs in plants.
In water samples ∑LMW-PAHs/∑HMW-PAHs ratios were greater than 1 (Figure 4c), which suggested petroleum combustion as the main contributions [3]. All Flu/(Flu + Pyr) were above 0.5, representing biomass and coal burning [7]. Ant/(Ant + Phe) were around 0.10 to 0.21, greater than 0.1, indicating that it was mainly pyrogenic source. And 86.4% of BaA/(BaA + Chr) were above 0.35, which indicated biomass and coal combustion [27]. Summarized the results, the dominant origin of PAHs in water were combustion of biomass, fossil fuel and petroleum, which was identical with the origin results of PAHs in plant and soil samples.
Cross-plots for the isomeric ratios of Ant/(Ant + Phe) vs Flu/(Flu + Pyr) in soil samples (a), plant samples (b), and water samples (c).
3.5 Risk assessment of PAHs in soil and plants
3.5.1 Risk assessment of PAHs in soil
According to the non-pollution soil standard proposed by Netherlands [32], it is found that PAHs in our study except Nap are within the safe range of standard values. Although the minimum safety values were not specified for Acy, Ace, Flu, Pyr, BbF and DahA, the detection levels of these 6 PAHs were lower. Soil in study area were mainly polluted by Nap, which is easy to volatilize in the air and is the main component of insecticide. It is speculated that the application of insecticides in the farmland soils around the research area causes the accumulation of Nap in the environment. According to the method of soil PAHs pollution classification adopted by some scholars [33,45], the total PAHs content in our research is less than 200 ng/g, which is in pollution free state. Furthermore, we compared the PAHs contents in study area with the first standard limit of soil environmental quality in China, results showed that the soil in the study area had been polluted to different degrees, and the content of Nap in some samples exceeded the standard limit (15 ng/g), and the carcinogenic BaA and Chr were close to the standard limit (5 and 10 ng/g, dividually). Because the ecosystem of Xinjiang arid area is fragile and the biodiversity is less, the existence of trace pollution elements may have some side effects and toxicity to the extreme drought environment, so the potential ecological risks of the environment in the arid region cannot be ignored. The TEQ values were used to assess the ecological risk of PAHs in local soil, the BaP toxicity was the highest, followed by DahA (Table 4). However, the TEQ values of soil samples were much lower than the safe value of 700 ng/g set by the Canadian government [13].
TEQ value of PAHs in soil samples
| TEF | Concentration | TEQ | |
|---|---|---|---|
| Nap | 0.0010 | 64.18 | 0.06 |
| Acy | 0.0010 | 7.59 | 0.01 |
| Ace | 0.0010 | 13.63 | 0.01 |
| Fl | 0.0010 | 78.88 | 0.08 |
| Phe | 0.0010 | 259.10 | 0.26 |
| Ant | 0.0010 | 64.04 | 0.06 |
| Flu | 0.0010 | 74.16 | 0.07 |
| Pyr | 0.0010 | 56.55 | 0.06 |
| BaA | 0.1000 | 30.56 | 3.06 |
| Chr | 0.0100 | 77.98 | 0.78 |
| BbF | 0.1000 | 15.73 | 1.57 |
| BkF | 0.1000 | 30.95 | 3.09 |
| DahA | 1.0000 | 19.62 | 19.62 |
| BaP | 1.0000 | 21.83 | 21.83 |
| IcdP | 0.1000 | 5.07 | 0.51 |
| BghiP | 0.0100 | ND | — |
| ∑16PAHs | — | 819.85 | 51.08 |
Comprehensive comparison of several soil quality standards shows that the study area is in a low ecological risk level, but the increasingly frequent industrial production, living, tourism, transportation, and other activities have caused certain pollution to the arid area of Southern Xinjiang. PAHs decompose slowly in soils and are persistent for a long time, their potential toxicity cannot be ignored.
The ILCRs due to human exposure to PAHs in soil ranges from 3.78 × 10−7 to 9.10 × 10−5 (Table 5). Comparison of the 4 standard ILCR values (Table S5) showed that 71% of the local soil samples reached level I, and 29% reached level II. The ILCR outcome authenticated that dermal contact was the most effective element among the three routes of exposure in adults, accounting 64% for both female and male adults. Oral ingestion was the dominant exposure pathway for children, with female and male accounting for 99 and 94% of the aggregate risk, individually, demonstrating that adults might face higher potential health risk than children, and the health risk of females was higher than males.
Carcinogenic risk of PAHs in soil to different populations
| Children | ||||||||
|---|---|---|---|---|---|---|---|---|
| Female | Male | |||||||
| Ingestion | Dermal | Inhalation | ILCRs | Ingestion | Dermal | Inhalation | ILCRs | |
| Mean | 3.48 × 10–06 | 4.34 × 10–10 | 5.98 × 10–11 | 3.48 × 10–06 | 3.46 × 10–06 | 4.32 × 10–10 | 5.94 × 10–11 | 3.46 × 10–06 |
| Min | 3.81 × 10–07 | 4.75 × 10–11 | 6.55 × 10–12 | 3.81 × 10–07 | 3.78 × 10–07 | 4.73 × 10–11 | 6.50 × 10–12 | 3.78 × 10–07 |
| Max | 1.51 × 10–05 | 1.88 × 10–09 | 2.60 × 10–10 | 1.51 × 10–05 | 1.50 × 10–05 | 1.50 × 10–05 | 2.58 × 10–10 | 3.00 × 10–05 |
| Adult | ||||||||
| Mean | 7.51 × 10–06 | 1.34 × 10–05 | 4.66 × 10–10 | 2.09 × 10–05 | 7.17 × 10–06 | 1.27 × 10–05 | 4.44 × 10–10 | 1.99 × 10–05 |
| Min | 8.23 × 10–07 | 1.47 × 10–06 | 5.10 × 10–11 | 2.29 × 10–06 | 7.85 × 10–07 | 1.39 × 10–06 | 4.86 × 10–11 | 2.18 × 10–06 |
| Max | 3.27 × 10–05 | 5.83 × 10–05 | 2.03 × 10–09 | 9.10 × 10–05 | 3.12 × 10–05 | 5.51 × 10–05 | 1.93 × 10–09 | 8.63 × 10–05 |
3.5.2 Risk assessment of PAHs in plants
Daily exposure to PAHs varies widely among different populations due to different plant daily intakes. Corn is a food product with a larger daily intake for humans. Therefore, the largest daily exposure of PAHs through ingestion was due to the consumption of corn and the risk of cancer caused by the consumption of corn was the highest, ranging from 1.21 × 10−7 to 2.43 × 10−5. The cancer risk of eating red dates and walnuts for different populations is at a safe level (ILCRs < 1.0 × 10−6), but the consumption of corn by different people is still at a potential risk level. Women face a higher risk of developing cancer from the ingestion of plants exposed to PAHs than men, and adults face a higher risk than children, which is consistent with the results of health risks caused by soil exposure in different populations through various routes (Table 6).
Carcinogenic risk of PAHs in plants for different populations
| ILCRcrop | Children | Adult | ||
|---|---|---|---|---|
| Female | Male | Female | Male | |
| Walnuts | 1.26 × 10–07 | 1.21 × 10–07 | 4.55 × 10–07 | 4.12 × 10–07 |
| Red dates | 1.42 × 10–06 | 1.38 × 10–06 | 5.16 × 10–06 | 4.68 × 10–06 |
| Corn | 6.73 × 10–06 | 6.49 × 10–06 | 2.43 × 10–05 | 2.21 × 10–05 |
4 Conclusion and recommendations
Total PAHs varied from 6.47 to 56.08 ng/g in agricultural soil samples, varied from 56.63 to 597.28 ng/g in plant samples, and ranged from 4183.85 to 24803.45 ng/L in water samples. Dominant compounds were 3-ring PAHs in soil, plants, and water. The PAHs in soil decreased gradually from surface to deep layer, the highest concentration of PAHs in plants were found in reed, followed by Tamarix ramosissima Lcdcb and walnuts. The contamination of PAHs in cotton showed the trend of leaf > root > flower, and the concentration in walnut leaves was higher than that in walnut fruits. Dominant origin of PAHs were biomass and coal combustion in agricultural soil, plants, and water. Health risks estimation indicated that PAHs had potential cancer risk for children through oral intake, and for adults by dermal contact exposure. The evaluation value of ILCR in corn samples was the highest, indicating the necessity to reduce the planting of corn in polluted areas. Female adults face highest risk of PAHs pollution in contrast to the other groups. Corresponding policies and regulations should be implemented and advocated against agricultural activities like farming and gardening, reducing human risk exposure time nearby the industrial estate.
However, winter is the heating season in Xinjiang, and pollution of PAHs will be more severe due to the large increase in coal combustion. Therefore, effective measures should be taken to control the pollution of PAHs. First, national and local environmental protection departments should formulate specific emission standards and use policies and regulations to limit the emission of PAHs. Second, some environmental protection measures should be taken, such as strictly control the emission of automobile exhaust gas and install a device to deal with automobile exhaust gas; use central heating instead of small coal furnace heating; briquette should be selected for industrial coal to full combustion; develop clean energy to replace coal and petroleum combustion with natural gas. Finally, it is recommended to use physical, chemical, and biological treatment technologies to remove or reduce the generated PAHs.
Acknowledgements
This work was supported by the National Natural Science Foundation of China [Grant no. 51968067], the Natural Science Foundation of Xinjiang Uygur Autonomous Region of China [Grant no. 2018D01C044], State Key Laboratory of Pollution Control and Resource Reuse Foundation, [Grant no. PCRRF19013], and State Key Joint Laboratory of Environment Simulation and Pollution Control, [Grant no. 22K01ESPCT].
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Author contributions: Mukadasi Abudureheman: investigation, methodology, conducting the experiment, data collation and analysis, and writing - original draft; Nuerla Ailijiang: investigation, supervision, conceptualization, writing - review & editing, formal analysis, and funding acquisition; Balati Maihemuti: investigation, helped with the sample collection; Anwar Mamat: investigation, assisted in sampling, and experiment guidance; Yusuyunjiang Mamitimin: writing – review & editing; Naifu Zhong: assisted in the experiment; and Nanxin Li: assisted in the experiment.
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Conflict of interest: The authors declare no conflict of interest.
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