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Pollution characteristics and potential ecological risk assessment of polycyclic aromatic hydrocarbons in wastewater irrigated soil.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) are organic substances that contain two or more benzene rings. Due to their ring-shaped structure, PAHs have relatively high chemical stability and are rarely affected by photochemical and biological oxidation. They are persistent and bioaccumulative. Thousands of different kinds of PAHs exist, and most of them are strongly carcinogenic, teratogenic, and mutagenic (Chen & Liao, 2006; Ifegwu & Anyakora, 2015). The U.S. Environmental Protection Agency (U.S. EPA) lists 16 PAHs as priority pollutants (Keith & Telliard, 1979).

The soil is an important environmental medium and is rich in organic matter. PAHs are lipophilic in nature, and thus show high affinity to the soil. Soil bears more than 90% of the environmental load of PAHs, and is therefore a storage and transfer station for PAHs (Wang et al., 2007). Studies show that the amount of PAHs entering human bodies from the soil is significantly higher than that from air and water (Feng, Fu, Zhao, & Gao, 2011). Moreover, the PAHs in soil can affect human health indirectly via the soil-groundwater-atmosphere connection and endanger human health directly via the soil-plant-food chain (Aleem & Malik, 2003; Zohair, Salim, Soyibo, & Beck, 2006).

With industrial development and a rising living standard, the amount of industrial water and urban sewage increases year by year, resulting in the increasingly prominent problem of surface water pollution and the contamination of clean irrigation water. To solve the problem of irrigation water shortages, more areas are forced to use wastewater for irrigation, leading to deterioration of soil quality. The contaminants in wastewater include not only heavy metals but also many organic compounds such as PAHs (Hao, Wang, & Li, 2010; Zhang et al., 2012).

Currently in many countries including China, requirements on the limit of heavy metals have been included in the standards of agricultural land use, but the evaluation and monitoring of PAHs have not yet been incorporated in these standards (Tang, Tang, Zhu, Zheng, & Miao, 2005), leading to a lack of control for PAHs in soil. Therefore, it is critical to investigate the level of PAHs in different types of soil, and develop appropriate standards for level of PAHs in soil as soon as possible. We collected soil samples from two typical wastewater-irrigated farmlands, Farmlands A and B, in Tangshan, China, and a clean-water irrigated farmland, Farmland C, was used as the control area. PAHs in the soil were analyzed and evaluated to determine their characteristics and ecological risk, and to provide a theoretical basis for the prevention and treatment of PAHs, as well as derive a critical limit for PAHs for standards and guidelines.

Materials and Methods

Equipment and Reagents

The chromatographic analysis was performed with Aglient 1200 high-performance liquid chromatography (HPLC). An ultrasonic oscillation water bath was purchased from Kunshan Ultrasonic Instrument (KQ5200DB).

The primary reagents were acetone, petroleum ether, cyclohexane, and methanol. All solvents were of HPLC grade. Silica gel, purchased from Tianjin Chemical Reagent Plant, was activated at 130[degrees]C for 16 hours, and water was added before usage (silica gel/water = 95/5). Anhydrous ammonium sulfate, purchased from Tianjin Chemical Reagent Plant, was heated at 650[degrees]C for 4 hours before usage and stored in a desiccator. PAHs were purchased from Supelco Technical Service, and consisted of the 16 PAHs listed by U.S. EPA as priority pollutants, namely naphthalene (NAP), acenaphthylene (ACY), acenaphthene (ACE), fluorine (FLU), phenanthrene (PHE), anthracene (ANT), fluoranthene (FLA), pyrene (PYR), benzo[a] anthracene (BaA), chrysene (CHR), benzo[b] fluoranthene (BbF), benzo[k] fluoranthene (BkF), benzo [a] pyrene (BaP), dibenz[a,h] anthracene (DahA), benzo[g,h,i]perylene (BghiP), and indeno[1,2,3-cd]pyrene (IcdP).

Sample Collection and Treatment

The area of study is Tangshan, China. Tangshan (latitude 39[degrees]37'51.12"N and longitude 118[degrees]10'48.7"E) is a heavily industrialized city in northeastern Hebei Province. Two typical wastewater-irrigated farmlands from Tangshan, China, were selected for investigation. There are several chemical plants upstream of Farmland A, and a paper mill upstream of Farmland B. Both farmlands are irrigated with water from the river, whose water quality is categorized as below class V, exceeding the Standards for Irrigation Water Quality according to the Environmental Protection Agency of China (GB3838-2002), and is categorized as a wastewater irrigation area. The main parameter exceeding the standard limit is NH3-N and chemical oxygen demand. Farmland C is irrigated by groundwater and lies 15 km away from A and B; it was chosen as the control area. The irrigation water quality for Farmland C satisfies the Standard for Drinking Water (GB 5749-2006). The locations of Farmlands A, B, and C can be seen in Figure 1. Following the Technical Specification for Environmental Monitoring (State Environmental Protection Administration of China, 2004), surface soils up to a depth of 20 cm were collected and mixed for each farmland. Five sampling spots were selected for each of Farmlands A, B, and C. Samples were collected at the four corners and at the midpoint of each sampling spot. All samples were mixed, and each mixture of sample weighed 1-2 kg.

We removed any stones and leaves from the sample before letting the sample dry naturally. Samples were sieved through a 2-mm mesh nylon sieve and crushed until the soil passed through the 40-mesh sieving screen. Each sample was then blended and stored at -20[degrees]C.

The method for extraction and purification was performed as described by Gao and coauthors (2010), with slight modification. A sample of 10 g of the soil was mixed with 10 g of anhydrous sodium sulfate. A mixture of acetone, petroleum ether, and cyclohexane (1:1:1, v/v/v) was added, and the mixture was allowed to sit for at least 10 hours. The mixture was extracted by the ultrasonic oscillator for 30 min, and the supernatant was decanted after the mixture was centrifuged at 2,000 rpm for 10 min. The supernatant was then collected. The precipitate was extracted 3 times using the extracting reagent. All the supernatant was put into K-D concentrator, with 70[degrees]C water bath until volume decreased to 1 ml.

Glass fiber, 4 g of silica gel (water content 5%), and 4 g of anhydrous sodium sulfate were filled into the glass chromatography column (150 mm x 10 mm) sequentially. The chromatography column was eluted with 10 ml of cyclohexane to remove organic impurities. The concentrated sample was transferred into the chromatography column and eluted with 20 ml cyclohexane continuously. The resultant eluent was collected and put into a K-D concentrator and concentrated to dry. Lastly, 0.5 ml of benzene was added as the solvent and stored for HPLC analysis.

The extracts were analyzed with Agilent Eclipse XDB-C18 (250.0 mm x 4.6 mm, 5 pm) at column temperature 25[degrees]C. The injection volume was 20 pl. The ratio of methanol increased from 60-100% during the first 20 min, and stayed 100% during the second 20-min interval, with detected wavelength 254 nm, flow rate at 1 ml/min. The wavelength of the fluorescence detector was Ex 340 nm, Em 425 nm.

Quality control was carried out according to the method by Ma and coauthors (2011), with slight modification. Soil samples were extracted repeatedly. The extracts were first dried and used as the soil matrix. Then 10 soil matrices were extracted, purified using the aforementioned method, and analyzed by HPLC. Limit of detection for this group was set at 3 times the standard deviation. The results showed that for 16 PAHs, the minimum detection limit was 0.01-0.16 pg/kg. Then, seven soil matrices were taken, and a mixture of the standard samples of 16 PAHs was added. The new mixture was extracted, purified using the aforementioned method, and analyzed by HPLC. Finally, the recovery and relative standard deviation were calculated for this group. We found that the average recovery of the 16 PAHs group was 85-110.2% and relative standard deviation was 2.6-5.1%, showing that the result meets the requirements of PAH trace analysis.

Results and Discussion

PAH Contamination Levels in Soils

The concentration of 16 PAHs detected and their total PAH contents ([SIGMA]PAHs) are presented in Table 1. It can be seen that the PAH concentrations of wastewater-irrigated farmlands were significantly higher than that found in the control area. Sixteen kinds of PAHs were observed in Farmland A, and the [SIGMA]PAHs was 1,046.2 [micro]g/kg. Sixteen kinds of PAHs were observed in Farmland B, with [SIGMA]PAHs of 1,308.1 [micro]g/kg. Fifteen kinds of PAHs were present in Farmland C, and the [SIGMA]PAHs was 189.1 [micro]g/kg. Edwards (1983) found that the level of endogenous PAHs in the soil should be around 1-10 pg/kg, and they mainly come from the degradation of vegetation and natural fire. It can, therefore, be inferred that the PAH concentrations observed in this research were above normal natural values, and the soil under investigation was affected by human behavior.

Presently no uniform standard for the evaluation of PAHs in soils exists. To evaluate the contamination level of PAHs, this study used the criterion proposed by Maliszewska-Kordybach (1996), which is widely used and referenced. Maliszewska-Kordybach classified the contamination of PAHs in soils into four levels according to the PAH content in soil in Europe: no contamination (<200 pg/kg), slight contamination (200-600 [micro]g/kg), moderate contamination (600-1,000 [micro]g/kg), and heavy contamination (>1,000 [micro]g/kg). According to this classification, the PAH content in Farmlands A and B both reached a level of heavy contamination, and the control area can be classified as a level of no contamination.

Studies on residual concentrations of PAHs in soil in some cities show that different cities and different regions display very different residual concentrations. PAH content in Fuzhou, China, is 100.2-1,215.1 pg/kg (Han, Yang, Yang, & Ni, 2008); in Cixi, Zhejiang, it is 70.4-325.0 [micro]g/kg (Li et al., 2007); in Hong Kong it is 21.1-544.0 pg/kg (Chung, Hu, Cheung, & Wong, 2007); and in South Korea it is 23.3-2,834 pg/kg with a mean of 236 pg/kg (Nam, Song, Eom, Lee, & Smith, 2003). Compared with these regions, the soil investigated in this research showed a very high level of PAHs. The concentration of PAHs near typical farmlands irrigated by wastewater exceeds previous values found by other researchers elsewhere. Urgent attention and further investigation should be focused on PAHs in wastewater-irrigated farmlands.

Distribution Characteristics of PAH Rings

The environmental behavior of PAHs is related to their chemical and physical properties (Douben, 2003; Luo, Liu, & He, 2014). PAHs can be divided into two categories according to their physical and chemical properties, namely low molecular weight aromatic benzene with 2-3 rings and high molecular weight aromatic hydrocarbons with 4-6 phenyl rings. Low molecular weight PAHs have low boiling points. They are volatile and their distribution is affected by environmental factors (air movement, temperature, and lighting). Such low molecular weight PAHs show acute toxicity. On the contrary, high molecular weight PAHs have high boiling points and are less volatile. They tend to remain in the soil and many such PAHs are carcinogenic, muagenic, and teratogenic (Chen & Liao, 2006; Douben, 2003).

In this research, analysis of the results show that PAHs in the wastewater-irrigated areas mainly consist of 4-6 rings. We found fewer 2-3 rings. High-ring PAHs contents were 83.1% and 60.2% for Farmlands A and B, respectively.

Past studies show similar results on the concentration of PAHs in soil. Li and coauthors (2007) investigated the spatial distribution and sources of PAHs in soils from typical oil-sewage irrigation areas in Northeast China and found out that 47% of the PAHs present were 4-ring PAHs. Ge and coauthors (2005) analyzed the wastewater-irrigated soil near a steel mill; the detection rate of PAHs was 100% and mainly consisted of 4-ring and higher PAHs. Such high dominance of higher-ring PAHs in soil was also observed by Peng and coauthors (2011). They studied PAHs in the urban soil of Beijing and found that 4-6 ring PAHs accounted for 83% of the total PAH content of the soil.

Our results echo these findings and further show that wastewater-irrigated soils have mainly 4-ring and above PAHs. These higher-ring PAHs are highly carcinogenic, teratogenic, and mutagenic. Our research suggests that in developing national standards and guidelines, the relevant authority could place an initial control and limits over the allowable concentration of 4-ring and above PAHs in the soil. This suggested process can be the most effective way to monitor PAHs in soil, as the majority of the PAHs will be regulated and therefore most harm could be potentially reduced.

Evaluation of Ecological Risk of PAHs

This research used the single-factor index method and Nemerow index comprehensive method to evaluate the ecological risk of PAHs. Single-factor index method can effectively reflect the degree of individual contaminant and is often used to evaluate the level of contamination of one specific pollutant. The mathematical expression is as follows:

[P.sub.i] = [C.sub.i]/[S.sub.i]

where [P.sub.i] is the single-factor index of one specific pollutant i, [C.sub.i] is the value of pollutant i as measured, and [S.sub.i] is the evaluation standard value for pollutant i.

When evaluating the effect of combined contamination of several pollutants, single-factor index method is combined with comprehensive index method to determine the level of contamination. Nemerow comprehensive index method is widely used in the evaluation of PAH contamination. Its mathematical expression is:

[P.sub.n] = [square root of ([[P.sub.max.sup.2] + [P.sub.sav.sup.2]]/2)]

where [P.sub.n] is the Nemerow comprehensive pollution index, [P.sub.max] is the maximum value in the single-factor pollution index, and [P.sub.sav] is the average value in the single-factor pollution index. It can be seen from the formula that for the Nemerow compresensive index method, the pollutant with the highest single-factor index is considered favorably in the computation of [P.sub.n], therefore reflecting the degree of pollution with the severity of the most dominant pollutant. The grading standards for both single-factor index and Nemerow index are presented in Table 2.

As there are no uniform standard values for the evaluation of PAHs in soil, this study used the standard value for PAH management for agricultural soils in the Netherlands (Annokkee, 1990). This method is straightforward, relatively widely used, and generally accepted. The indicators include NAP, PHE, PYR, BaA, BbF, BaP, DahA, and BghiP. Their standard values are all 100 pg/kg. The single-factor index and Nemerow comprehensive index for PAHs in different sampling areas are shown in Table 3.

Table 3 indicates that in the single-factor index, mainly PYR, DahA, and BghiP had a high pollution index. PYR, DahA, and BghiP in Farmland A were 0.89, 4.18, and 1.81, respectively, reaching the warning limit of pollution grade, moderate pollution, and slight pollution, respectively; PYR, DahA, and BghiP in Farmland B were 0.77, 1.35, and 4.36, respectively, reaching the warning limit, slight pollution, and heavy pollution level, respectively. Other indicators of single-factor index were less than 0.7, belonging to the clean grade. Farmland C had a single-factor index less than 0.7, belonging to the clean grade.

PYR, DahA, and BghiP have 4-, 5-, and 6-rings, respectively, and they are all highly carcinogenic, teratogenic, and mutagenic. Our research shows that the wastewater-irrigated soil contains these toxic properties, which is confirmed by the results discussed earlier.

The Nemerow comprehensive index shows that [P.sub.n] in Farmlands A and B were 3.05 and 3.16, respectively, graded as heavy pollution. [P.sub.n] in Farmland C was 0.34, graded as clean limit. This result is consistent with the evaluation of the [SIGMA]PAHs according to Maliszewska-Kordybach (1996). These results indicate that soils in wastewater-irrigated soil are under ecological risk--mainly the risk of carcinogenic, teratogenic, and mutagenic effects.

Conclusion

The PAH concentrations detected in wastewater-irrigated areas were significantly higher than PAH concentrations found in the control area: [SIGMA]PAHs were 1,046.2 [micro]g/kg and 1,308.1 [micro]g/kg for Farmlands A and B, respectively, reaching a level of heavy pollution. Based on the distribution characteristics of numbers of rings of PAHs, wastewater-irrigated soil contains mainly higher-ring PAHs (4 and above). They are strongly carcinogenic, teratogenic, and mutagenic.

The ecological risk assessment showed that in wastewater-irrigated soil, the PAHs exceeding the standard value are mainly PYR, DahA, and BghiP, whose Nemerow comprehensive indices are greater than 3, reaching a level of heavy pollution. This finding indicated that there is an ecological risk of wastewater irrigation of agricultural soil. Given the ecological risks associated with PAHs in wastewater-irrigated soils, the management of agricultural irrigation water quality should be urgently strengthened and standard limits for PAHs for different types of soil should be developed as soon as possible in order to facilitate the monitoring, prevention, and remediation of contamination in soil. Our research methods, including the evaluation criteria of using single-factor index and Nemerow comprehensive index, could be a potential reference for developing guidelines and standards on PAH control in wastewater-irrigated soil. AM

Hongxia Gao

Yingli Liu

Weijun Guan

Nan Liu

Shoufang Jiang

Hebei Province Key Laboratory of Occupational Health and Safety School of Public Health North China University of Science and Technology

Acknowledgements: This work was financially supported by the Science Research Program of Ministry of Education of Hebei (2011230) and Medical Science Research Chief Project of Hebei Province (20100129).

Corresponding Author: Hongxia Gao, Hebei Province Key Laboratory of Occupational Health and Safety, School of Public Health, North China University of Science and Technology, Tangshan 063000, Hebei, China. E-mail: ghxgao@126.com.

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Caption: FIGURE 1 Map of Sampling Area Locations
TABLE 1

Concentrations of Polycyclic Aromatic Hydrocarbons (PAHs) in Soils
Among Different Areas ([micro]g/kg)

PAH Name   Rings   Farmland A       Farmland B       Farmland C

                   [bar.x]    s     [bar.x]    s     [bar.x]    s

NAP          2       3.3     1.4     12.5     5.3      5.4     1.9
ACY          3      67.4     21.1    119.4    30.8    44.1     11.6
FLU          3       3.1     1.2     152.5    45.9     3.9     1.6
PHE          3      65.2     35.3    49.5     20.2    14.2     5.6
ACE          3      19.8     4.7     172.5    50.3     6.5     3.1
ANT          3      18.1     6.2     13.8     4.7      ND       ND
BaA          4      37.4     12.1    12.2     3.2      2.6     1.1
CHR          4      16.8     5.3     11.9     3.5      3.9     1.3
FLA          4      37.5     12.9     7.7     2.8      4.2     1.5
PYR          4      88.7     22.6    77.2     24.1    13.8     3.2
BaP          5      19.5     4.8      8.3     3.7      5.7     2.1
DahA         5      417.8    92.9    134.6    30.8    46.6     12.9
BbF          5      53.6     18.9    19.6     6.2      4.9     2.1
BkF          5      12.9     5.1     23.2     6.8     12.7     3.6
BghiP        6      171.2    41.4    436.3    90.7    13.5     4.3
IcdP         6      13.9     3.2     56.9     9.5      7.1     2.7
[SIGMA]     --     1,046.2    --    1,308.1    --     189.1     --
   PAHs

NAP = naphthalene; ACY = acenaphthylene; FLU = fluorine; PHE =
phenanthrene; ACE = acenaphthene; ANT = anthracene; BaA =
benzo[a]anthracene; CHR = chrysene; FLA = fluoranthene; PYR =
pyrene; BaP = benzo[a]pyrene; DahA = dibenz[a,h]anthracene; BbF =
benzo[b]fluoranthene; BkF = benzo[k]fluoranthene; BghiP =
benzo[g,h,i]perylene; IcdP = indeno[1,2,3-cd]pyrene; ND = not
detected.

TABLE 2

Grading Standard for Soil
Contamination

Pollution Index                     Pollution Grade
([P.sub.i] or [P.sub.n])

P [less than or equal to] 0.7       Clean
0.7 [less than or equal to] P <1    Warning limit
1 [less than or equal to] P <2      Slight pollution
2 [less than or equal to]  P <3     Moderate pollution
P >3 Heavy pollution

TABLE 3

Single-Factor Index and Nemerow Comprehensive Index for
Polycyclic Aromatic Hydrocarbons in Different Sampling Areas

Sampling               Single-Factor Index (Pi)
Area

             NAP    PHE    PYR    BaA    BbF    BaP    DahA   BghiP

Farmland A   0.03   0.65   0.89   0.37   0.54   0.20   4.18   1.81
Farmland B   0.13   0.50   0.77   0.12   0.20   0.08   1.35   4.36
Farmland C   0.05   0.14   0.14   0.03   0.05   0.06   0.48   0.13

Sampling          Nemerow
Area           Comprehensive
             Index ([P.sub.n])

Farmland A         3.05
Farmland B         3.16
Farmland C         0.34

NAP = naphthalene; PHE = phenanthrene; PYR = pyrene; BaA =
benzo[a]anthracene; BbF = benzo[b]fluoranthene; BaP = benzo[a]pyrene;
DahA = dibenz[a,h]anthracene; BghiP = benzo[g,h,i]perylene.
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Title Annotation:INTERNATIONAL PERSPECTIVES
Author:Gao, Hongxia; Liu, Yingli; Guan, Weijun; Liu, Nan; Jiang, Shoufang
Publication:Journal of Environmental Health
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Date:May 1, 2017
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