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Distribution and transport of PAHs in soil profiles of different water irrigation areas in Beijing, China Aifang Jin,ab Jiangtao He,*a Sunuan Chenc and Guoxin Huangd Vertical distribution characteristics and transport mechanisms of polycyclic aromatic hydrocarbons (PAHs) in soil profiles (0–5.5 m) of different water irrigation areas in the southeast suburb of Beijing were analyzed and compared. 16 priority PAHs on the United States Environmental Protection Agency (US EPA) list were analyzed using gas chromatography and mass spectrometry (GC-MS). The relationship between the properties of soil and PAHs was also studied by statistical analyses. The results showed that total PAH concentrations in the topsoils of the wastewater irrigation (WWI) area, reclaimed water irrigation (RWI) area, groundwater irrigation (GWI) area were much higher than those in the deep soils, with the concentrations of 726.0, 206.8 and 42.8 mg kg1 (dry wt), respectively. The low molecular weight (LMW) PAHs (2–3 ring) including naphthalene (Nap), phenanthrene (Phe), fluorene (Fl) dominated the layers (0.5–5.5 m) underneath the surfaces. The migration of LMW PAHs was faster than that of high molecular weight (HMW) PAHs and LMW PAHs were transported in dissolved matter. The different soil textures of three sites caused the differences in the variation ranges of PAHs in the profiles. The statistical analyses
Received 21st November 2013 Accepted 25th March 2014
showed a significant linear positive correlation between PAHs and total organic carbon (TOC). The 2–4 ring PAHs were detected in the wastewater and reclaimed waters, which was consistent with those in
DOI: 10.1039/c3em00623a
the soil profiles. The presence of PAHs in the soil profiles was mainly due to the irrigation of wastewater.
rsc.li/process-impacts
Wastewater reuse guidelines and standards for irrigation should be established urgently.
Environmental impact Wastewater irrigation plays a positive role in alleviating water shortages and decreasing fertilizer use, however little attention was paid to soil and groundwater environmental problems caused by persistent organic compounds such as PAHs during a long-term irrigation process. On the other hand, little is known about PAHs pollution, and their distribution and transport in deep soil. Therefore, the typical wastewater, reclaimed water and groundwater areas were chosen in Beijing, China, and the distribution and transport of PAHs in soil proles (0–5.5 m) were identied. The ndings would provide an important basis for establishing wastewater reuse guidelines and standards for irrigation.
Introduction In recent years, reclaimed water has been progressively adopted to replace wastewater for irrigation purposes in various areas, particularly in arid and semi-arid areas, as it can signicantly reduce environmental problems and health risks. Water quality standards of reclaimed water irrigation (RWI) in developed countries have been established to ensure safe irrigation.1,2 These guidelines have prescribed acceptable concentrations of bulk parameters such as COD, BOD and heavy metals. However,
a
Beijing Key Laboratory of Water Resources & Environmental Engineering, China University of Geosciences (Beijing), Beijing 100083, China. E-mail: jinaifang0813@ 163.com; [email protected]; Tel: +86-10-82322080
b
China Institute of Geo-Environment Monitoring, Beijing 100081, China
c
Hainan Institute of Geo-Environment Monitoring, Hainan 570216, China. E-mail: [email protected] d
China Meat Research Centre, Beijing Academy of Food Sciences, Beijing 100068, China. E-mail: [email protected]; Tel: +86-10-67264755
1526 | Environ. Sci.: Processes Impacts, 2014, 16, 1526–1534
the organic pollutants are not specically regulated. Polycyclic aromatic hydrocarbons (PAHs) as a group of stable and ubiquitous persistent organic pollutants (POPs) are receiving increased attention due to their toxicity, carcinogenicity and mutagenicity. PAHs are introduced into the environment primarily by the incomplete combustion of fossil fuels and other organic materials. Comparatively stable molecular structures, slow down photochemical decomposition and biodegradation rates contribute to their persistence in the environment.3 The United States Environmental Protection Agency (US EPA) has identied 16 PAHs as priority pollutants, 7 out of which have been classied as probable human carcinogens. The fate of PAHs in the environment is primarily controlled by properties of PAHs and environmental media. Soil is an important media for PAHs accumulation and PAHs are initially concentrated in topsoils. In soil, PAHs are subject to various partitionings, such as adsorption, degradation and transport processes, which will ultimately control their vertical distribution. PAHs generally accumulate in organic matter-rich layers
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because of their persistence and affinity for soil organic matter.4 Many surveys of PAHs contamination in water bodies have been conducted in China. In Tianjin, the total concentration of 16 PAHs ranged from 227 to 601 ng L1 in reclaimed waters with a mean value of 352 ng L1 and from 1765 to 35 210 ng L1 in surface waters (main rivers, tributaries, ditches, etc.) with a mean value of 14 066 ng L1.5 The total 16 PAH concentrations in inuent and effluent of a sewage treatment plant in a city reached as high as 1777.9 and 1380.1 ng L1, respectively.6 If topsoils are frequently irrigated or saturated with PAHs contaminated water for a long time, there is a signicant increased likelihood of PAHs leaching from the contaminated zone to the deeper layers to endanger the groundwater safety.7 Therefore, a survey for soil PAHs contamination under irrigation is needed to avoid the food and groundwater pollution risk. So far, many studies have investigated the surface soil PAHs pollution levels.8–10 Nevertheless, few studies have explored the PAHs pollution in deep soils, especially in WWI (wastewater irrigation) and RWI (reclaimed water irrigation) areas. In China, studies associated with soil PAHs pollution in WWI areas were mainly conducted in Beijing, Tianjin and Shenyang.11–14 However, there were still no reports on the soil PAHs pollution in a RWI area. The objectives of this study were to attain and compare the vertical distribution and transport of PAHs in soil proles (0–5.5 m) across the WWI area, the RWI area and the GWI area of Beijing. Also, different irrigation waters and soil properties were analyzed to further explain the distribution of PAHs in different soil proles.
Materials and methods Study areas and sample collection Study area description. The WWI area is located at downstream of an industrial district in the southeast suburb of Beijing, where rainfall and surface water cannot meet the need for crop growth. In order to ensure agricultural production, Liangshui River, the second largest sewage-received river, has been used for irrigation since the 1960s. The depth of groundwater in the WWI area is about 60 m. The RWI area is located at the Duozi Village in the southeast suburb of Beijing. A branch of Tonghui Canal which passes through this district from north to south provides water irrigation for agriculture. Tonghui Canal was built in 1958 and received raw municipal sewage at the early stage. Since the 1990s, the canal has received the secondary effluent from the Gaobeidian sewage treatment plant, which has been used for irrigation. The depth of groundwater in this area is also approximately 60 m. The GWI area is located at Yongledian of Tongzhou District of Beijing. The irrigation water was collected from motorpumped wells at the depth of 30 m. For the three areas, the irrigation time from April to November was considered, and the irrigation frequency was similar, about 2–4 times per year. The average amount of irrigation water was about 1214 m3 per acre per year. Sampling. 36 soil samples in November 2007, April 2008 and June 2008 were separately collected from 3 boreholes forming
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Environmental Science: Processes & Impacts
an equilateral triangle of 1 m length in each irrigation area where wheat was grown. The diameter of the boreholes was 38 mm and the maximum depth of the boreholes drilled using an Eijkelkamp sampler was 5.5 m from the surface. Soil samples were collected every 0.5 m along the prole from each borehole and analyzed separately. The locations of sampling sites in the three areas are shown in Fig. 1. Surface water samples including irrigation water were collected along the canals at different irrigation seasons. Wastewater samples were numbered WGB-1, WGB-2, WGB-3, and WGB4, while recycled water samples were numbered ZSB-1 and ZSB-2. All the samples were collected into pre-cleaned brown glass bottles and stored in a cool box (4 C). Aer returning to the laboratory, the soil samples were dried at room temperature, passed through a 20-mesh sieve and then stored in brown glass bottles at 4 C, while the water samples were stored at 4 C prior to analysis for PAHs. Sample extraction and cleanup According to EPA method 3550,15 soil samples (15 g dry wt) and anhydrous sodium sulfate (3 g) were weighed precisely and placed into brown glass jars of 40 mL, with 20 mL acetone– hexane solvent (1 : 1 v/v) and sonicated for 20 min at 50 C and 400 W power in an ultrasonic shaking apparatus. The mixture was then centrifuged for 3 min at 3000 rpm and the extract was collected. The same extraction was repeated twice again by adding 20 mL acetone–hexane solvent (1 : 1 v/v) to the lter residue. The three extract solutions were combined and anhydrous sodium sulfate was added for drying, and then concentrated to around 1 mL by rotary evaporation at 35–40 C. According to EPA method 3510,16 the water samples were ltered by an APFF glass ber membrane. 1 L water sample was measured precisely and placed into a separatory funnel, with 20 mL cyclohexane. The separatory funnel was sealed and shaken vigorously for 1–2 min with periodic venting to release excess pressure, and stilled for 10 min to separate the organic
Fig. 1
Map of the soil sampling sites in Beijing.
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layer from the water phase. The organic layer was collected. The extraction was repeated twice using fresh portions of solvent. The three solvent extracts were combined and anhydrous sodium sulfate was added for drying, and then concentrated to around 1 mL by rotary evaporation at 35–40 C. According to EPA method 3630,17 the concentrated extracts were cleaned up using a column (10 mm i.d.) consisting of 10 g activated silicagel and capped with anhydrous sodium sulfate. The chromatographic column was pre-eluted with 25 mL hexane at 2 mL min1 and the eluate was discarded. Just prior to exposure of the sodium sulfate layer to the air, the concentrated extracts were transferred into the column using an additional 2 mL hexane, then cleaned using 25 mL hexane and discarded the above eluate and nally eluted with 25 mL mixture of dichloromethane and hexane (2 : 3 v/v). Aerward, this eluate was concentrated exactly to 1 mL under a gentle steam of pure nitrogen, and subsequently transferred into vials for PAHs analysis. Unless otherwise indicated, all chemicals and solvents were of high purity analytical reagent grade and used as received. PAHs analysis The concentrations of 16 PAHs including naphthalene (Nap), acenaphthylene (Acy), acenaphthene (Ace), uorene (Fl), phenathrene (Phe), anthracene (Ant), uoranthene (Flu), pyrene (Pyr), benzo[a]anthracene (BaA), chrysene (Chr), benzo[b]uoranthene (BbF), benzo[k]uoranthene (BkF), benzo[a]pyrene (BaP), indeno[1,2,3-cd]pyrene (InP), dibenzo[a,h]anthracene (DBA), and benzo[g,h,i]perylene (BghiP) in the extracts were determined by an Agilent 6890 gas chromatography coupled with an Agilent 5973 mass spectrometer and a 7683 autosampler. The HP-5MS capillary column (50 m 0.25 mm i.d. 0.25 mm lm thickness) was used with helium as the carrier gas at a constant ow rate of 1.0 mL min1. GC temperature was programmed from initial 80 (1 min) to 215 C (1 min) at 30 C min1, then to 255 C (1 min) at 5 C min1, to 263 C (1 min) at 1 C min1, and nally to 300 C (3 min) at 25 C min1 1 mL of the extract was injected in the splitless mode and the solvent delay was 3 min. The mass spectrometer was operated in scan mode with an electron impact ionization of 70 eV, an electron multiplier voltage of 1906 V, and an ion source of 230 C. The identication of individual PAHs was based on the retention time and the mass spectrum with appropriate individual standards at full scan mode. The molecular ions were selected as the target ions for quantication and another two or three characteristic ions were selected for conrmation.
Paper
standard deviation (RSD), ranged between 1.87% and 5.64%, which met the requirements of EPA. Physical and chemical analysis of soil samples TOC and classication of soil samples were characterized by conventional standard procedures. TOC was measured using the potassium dichromate oxidation process.18 Particle size distribution was carried out using the hydrometer method.19
Results and discussion Concentrations and compositions of PAHs in the topsoils The concentrations of PAHs in the surfaces are given in Fig. 2. The contents and compositions of PAHs were different in the topsoils of the three sites. The total concentration of PAHs in the topsoils of three areas was in the order: WWI area (726.0 mg kg1) > RWI area (206.8 mg kg1) > GWI area (42.8 mg kg1) (dry wt). Clearly, the total concentration in the WWI area was much higher than that in the other two areas, indicating that the use of wastewater irrigation may lead to the accumulation of PAHs in the soils. The results were consistent with other studies.20–23 A literature survey revealed that the total concentration of PAHs in the WWI area was obviously lower than that present in wastewater irrigation areas in suburbs of Beijing and Tianjin, China,12 but higher than Hunpu wastewater-irrigation area of Liaoning, China.23 The concentration of PAHs in the RWI area was close to that in farmland soils irrigated by effluents from biological treatment plants in the southeast suburb of Beijing.24 According to the theory of Maliszewska Kordybach,25 the total content of PAHs in three sites could be regarded as medium contaminated, light contaminated and none-contaminated. Most of 16 PAHs were detected in the surface layers of three sites. 14 of PAHs except Ant and BaA were found in the WWI area. 2-ring and 4–6 ring PAHs respectively contributed to 59% and 31% of the total concentration, while 3-ring PAHs had the lowest proportion of 10%. The most abundant compound was Nap with an average value of 399.0 mg kg1, which accounted for
QA/QC measures All data were subject to strict quality control procedures. Quantication was done using an external calibration method. The method detection limits for 16 PAHs ranged from 0.3 to 0.5 mg kg1 for soil samples, and from 2.0 to 4.0 ng L1 for water samples. Method blanks (solvent) and sample duplicates were analyzed routinely. The recovery efficiencies with spiked PAHs in soil and water were in the range of 74.2–117.1% and 85.0– 109.5% respectively. The precision, expressed as the relative
1528 | Environ. Sci.: Processes Impacts, 2014, 16, 1526–1534
Fig. 2 The concentrations of PAHs in the surfaces of different irrigation areas. (Here the concentration of Nap in the WWI area was logtransformed (base 10).)
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55% of the total content of PAHs. The concentrations of the other PAHs were all less than 50 mg kg1. The concentration of Ace was relatively low, with a value of 4.1 mg kg1. In the RWI and GWI areas, the high molecular weight (HMW) PAHs (4–6 ring) were the mainly dominant PAHs with a total contribution of 70% and 60% respectively, followed by 3-ring PAHs. Flu dominated the PAHs in the RWI area, with a value of 40.1 mg kg1, accounting for 19% of the detected PAHs. The rest of PAHs were all below 30 mg kg1. Nevertheless, the highest concentration of PAHs in the GWI area was of Phe, with a value of 5.1 mg kg1. The values of the detected PAHs in the GWI area were signicantly lower than the ones in the other two sites. According to Canadian soil quality guidelines and interim remediation criteria for agricultural soil,26,27 only Nap in the WWI area was fourfold higher than the target value (100 mg kg1), the other individual PAHs in the different irrigation areas were all below their limits. However, the sum of the 7 PAH compounds as human carcinogens in the three areas reached 166.4, 103.4, and 18.6 mg kg1, respectively. This represented a potential threat to human health. Compared with the common PAHs detected in the surfaces, the individual PAHs (except Flu) in the three sites were in the following order: WWI area > RWI area > GWI area. The values of HMW PAHs were maintained at the same order of magnitude in the WWI and RWI areas, and were 1 order of magnitude higher than those in the GWI area. The low molecular weight (LMW) PAH (2–3 ring) concentrations in the RWI area were 1–2 orders of magnitude lower than those in the WWI area, but higher than those in the GWI area. This was due to the fact that LMW PAHs were more easily removed than HMW PAHs in sewage treatment processes. The persistence of PAHs was strongly affected by the number of rings, with PAHs with more rings more difficult to degrade.28 This was the reason why HMW PAHs predominated over other PAHs in the RWI area. In addition, the linear distance of sampling sites between the WWI and RWI areas was merely 3.9 km, and they belonged to the same land use type, but the percentages of HMW and LMW PAHs were markedly different. Therefore, it can be inferred that the distribution and concentrations of PAHs in the soils of different irrigation areas were closely related to the quality of irrigation water.
Migration of individual PAHs in different soil proles Vertical distribution of individual PAHs. Although the PAHs were mainly enriched in the topsoils at high levels, and could be easily absorbed by soil organic matter preventing them from migrating downward, they were also detected in the soil proles of different irrigation areas. This fact suggested that a long history of different water irrigation had a remarkable effect on PAHs mobility in soils. The LMW PAHs were the main PAHs detected in the soils and the distribution characteristics of individual PAHs were nearly the same. The highest contents of PAHs appeared in the surface layers, and decreased with depth. Nap, Fl, Phe and Flu contents of the soil proles were compared in different areas (Fig. 3). The order of concentrations of 4 PAHs in the proles was WWI area > RWI area > GWI area and the remarkable variation
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Environmental Science: Processes & Impacts
of Nap in the vertical sections of different irrigation areas was found. While the variation of Flu was minor, the contents of Flu were similar to each other in the three sites. The variation of PAHs in the WWI area was more signicant than those in the other areas, which was mainly due to the differences in properties of soil proles. Additionally, it should be noted that the variation of Nap in the RWI area was obviously different from that in the WWI and GWI areas. Nap was found at a low level in the surface of the RWI area, and increased with increasing depth, which may be ascribed to high permeability of the soil proles. Simultaneously, Nap in the reclaimed water was low, meaning volatilization and photolysis might be another factors affecting the distribution of Nap. Migration of PAHs. It is well-known that TOC was an important factor in inuencing the vertical distribution of PAHs.29 Hydrophobic organic contaminants such as PAHs were readily absorbed by organic-matter-rich layers due to their high affinity,4,30,31 which prevents PAHs from leaching into deep layers. Changes of the vertical distribution of TOC and total PAHs in each irrigation area are presented in Fig. 4. The change of total PAHs along the soil proles was basically consistent with that of the TOC content. The relationship between total PAHs and TOC in the three sites was analyzed by soware SPSS. Normality testing showed that the concentrations of PAHs and TOC followed normal distribution. The statistical analyses showed a signicant linear positive correlation between PAHs and TOC, with the Pearson correlation coefficients of 0.529, 0.547 and 0.704 for the WWI, RWI, and GWI areas respectively, which were statistically signicant at a level of 0.01. It was therefore concluded that PAHs in the current study areas were signicantly affected by soil TOC, which was consistent with other investigations.9,32,33 However, Zhang et al.34 noted that PAHs were not signicantly inuenced by TOC content until it arrived at a certain level in the weakly polluted soils. Simpson et al.35 reported that the relationship between total PAHs and organic carbon was signicant only for highly contaminated sites. It was supposed that there was only adsorption in the migration of PAHs, with the exception of degradation, and the contents of PAHs in the solid phase were the values that had achieved the adsorption equilibrium with the liquid phase.36,37 Therefore, the percentage of PAHs content, which was dened by the PAHs content (CD, mg kg1) at depth D (m)/the level (C0, mg kg1) in the surface (D ¼ 0 m), was used to compare the migration of LMW PAHs among different irrigation areas. The percentages of individual PAHs in different irrigation areas are presented in Fig. 5. The more quickly the percentages decreased with depth, the more easily PAHs were adsorbed to the soils, and the lower the migration of PAHs was. As shown in Fig. 5, the order of migration ability of different individual PAHs was: Ace > Fl > Nap > Phe > Acy > Flu for the WWI area, Nap > Fl > Phe > Pyr > Flu for the RWI area, and Nap > Fl > Phe > Flu for the GWI area. The mobilization of Nap and Fl in the RWI area was higher than that in the WWI area due to the higher permeability of the soil proles in the former area. The migration ability of PAHs in the GWI area was similar to that in the WWI area, but was lower in these two areas than that in the
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Fig. 3 The vertical distribution of individual PAHs in different irrigation areas. (Here the concentration of Nap in the WWI area was log-transformed (base 10).)
Fig. 4 The vertical distribution of TOC and PAHs in the soil profiles.
RWI area. Apparently, the migration order of PAHs in the three sites was roughly consistent with their properties including octanol–water partitioning coefficients (Kows) and soil organic carbon–water partitioning coefficients (Kocs) (Table 1): higher Kocs or Kows are correlated with less mobile organic chemicals,
1530 | Environ. Sci.: Processes Impacts, 2014, 16, 1526–1534
whereas lower Kocs or Kows are correlated with more mobile organic chemicals. Hence, 2–3 ring PAHs were the main pollutants in the proles. Comparing the compositions of PAHs between the topsoils and the deep soils, it was evident that the PAHs in the deep soils
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Fig. 5
Table 1
Environmental Science: Processes & Impacts
The percentages of individual PAHs in different irrigation areas. Kows and Kocs for 16 PAHs38
PAHs
Kocs (mL g1)
log Kows (L kg1)
Acenaphthene (Ace) Acenaphthylene (Acy) Fluorene (Fl) Naphthalene (Nap) Anthracene (Ant) Fluoranthene (Flu) Phenanthrene (Phe) Benzo[a]anthracene (BaA) Benzo[b]uoranthene (BbF) Benzo[k]uoranthene (BkF) Chrysene (Chr) Pyrene (Pyr) Benzo[a]pyrene (BaP) Dibenz[a,h]anthracene (DBA) Indeno[1,2,3-cd]pyrene (InP) Benzo[g,h,i]perylene (BghiP)
7.1 4.8 1.4 2.0 3.0 1.1 2.3 4.0 1.2 1.2 4.0 1.1 1.0 3.8 3.5 7.8
3.9 10 3.9 10 4.2 10 3.4 10 4.6 10 5.1 10 4.6 10 5.7 10 6.2 10 6.2 10 5.7 10 5.1 10 6.1 10 6.7 10 6.7 10 7.1 10
103 103 104 103 104 105 104 105 106 106 105 105 106 106 106 106
particles were classied into three sizes:
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Cite this: Environ. Sci.: Processes Impacts, 2014, 16, 1526
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Distribution and transport of PAHs in soil profiles of different water irrigation areas in Beijing, China Aifang Jin,ab Jiangtao He,*a Sunuan Chenc and Guoxin Huangd Vertical distribution characteristics and transport mechanisms of polycyclic aromatic hydrocarbons (PAHs) in soil profiles (0–5.5 m) of different water irrigation areas in the southeast suburb of Beijing were analyzed and compared. 16 priority PAHs on the United States Environmental Protection Agency (US EPA) list were analyzed using gas chromatography and mass spectrometry (GC-MS). The relationship between the properties of soil and PAHs was also studied by statistical analyses. The results showed that total PAH concentrations in the topsoils of the wastewater irrigation (WWI) area, reclaimed water irrigation (RWI) area, groundwater irrigation (GWI) area were much higher than those in the deep soils, with the concentrations of 726.0, 206.8 and 42.8 mg kg1 (dry wt), respectively. The low molecular weight (LMW) PAHs (2–3 ring) including naphthalene (Nap), phenanthrene (Phe), fluorene (Fl) dominated the layers (0.5–5.5 m) underneath the surfaces. The migration of LMW PAHs was faster than that of high molecular weight (HMW) PAHs and LMW PAHs were transported in dissolved matter. The different soil textures of three sites caused the differences in the variation ranges of PAHs in the profiles. The statistical analyses
Received 21st November 2013 Accepted 25th March 2014
showed a significant linear positive correlation between PAHs and total organic carbon (TOC). The 2–4 ring PAHs were detected in the wastewater and reclaimed waters, which was consistent with those in
DOI: 10.1039/c3em00623a
the soil profiles. The presence of PAHs in the soil profiles was mainly due to the irrigation of wastewater.
rsc.li/process-impacts
Wastewater reuse guidelines and standards for irrigation should be established urgently.
Environmental impact Wastewater irrigation plays a positive role in alleviating water shortages and decreasing fertilizer use, however little attention was paid to soil and groundwater environmental problems caused by persistent organic compounds such as PAHs during a long-term irrigation process. On the other hand, little is known about PAHs pollution, and their distribution and transport in deep soil. Therefore, the typical wastewater, reclaimed water and groundwater areas were chosen in Beijing, China, and the distribution and transport of PAHs in soil proles (0–5.5 m) were identied. The ndings would provide an important basis for establishing wastewater reuse guidelines and standards for irrigation.
Introduction In recent years, reclaimed water has been progressively adopted to replace wastewater for irrigation purposes in various areas, particularly in arid and semi-arid areas, as it can signicantly reduce environmental problems and health risks. Water quality standards of reclaimed water irrigation (RWI) in developed countries have been established to ensure safe irrigation.1,2 These guidelines have prescribed acceptable concentrations of bulk parameters such as COD, BOD and heavy metals. However,
a
Beijing Key Laboratory of Water Resources & Environmental Engineering, China University of Geosciences (Beijing), Beijing 100083, China. E-mail: jinaifang0813@ 163.com; [email protected]; Tel: +86-10-82322080
b
China Institute of Geo-Environment Monitoring, Beijing 100081, China
c
Hainan Institute of Geo-Environment Monitoring, Hainan 570216, China. E-mail: [email protected] d
China Meat Research Centre, Beijing Academy of Food Sciences, Beijing 100068, China. E-mail: [email protected]; Tel: +86-10-67264755
1526 | Environ. Sci.: Processes Impacts, 2014, 16, 1526–1534
the organic pollutants are not specically regulated. Polycyclic aromatic hydrocarbons (PAHs) as a group of stable and ubiquitous persistent organic pollutants (POPs) are receiving increased attention due to their toxicity, carcinogenicity and mutagenicity. PAHs are introduced into the environment primarily by the incomplete combustion of fossil fuels and other organic materials. Comparatively stable molecular structures, slow down photochemical decomposition and biodegradation rates contribute to their persistence in the environment.3 The United States Environmental Protection Agency (US EPA) has identied 16 PAHs as priority pollutants, 7 out of which have been classied as probable human carcinogens. The fate of PAHs in the environment is primarily controlled by properties of PAHs and environmental media. Soil is an important media for PAHs accumulation and PAHs are initially concentrated in topsoils. In soil, PAHs are subject to various partitionings, such as adsorption, degradation and transport processes, which will ultimately control their vertical distribution. PAHs generally accumulate in organic matter-rich layers
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Published on 25 March 2014. Downloaded by California Institute of Technology on 08/10/2014 01:20:27.
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because of their persistence and affinity for soil organic matter.4 Many surveys of PAHs contamination in water bodies have been conducted in China. In Tianjin, the total concentration of 16 PAHs ranged from 227 to 601 ng L1 in reclaimed waters with a mean value of 352 ng L1 and from 1765 to 35 210 ng L1 in surface waters (main rivers, tributaries, ditches, etc.) with a mean value of 14 066 ng L1.5 The total 16 PAH concentrations in inuent and effluent of a sewage treatment plant in a city reached as high as 1777.9 and 1380.1 ng L1, respectively.6 If topsoils are frequently irrigated or saturated with PAHs contaminated water for a long time, there is a signicant increased likelihood of PAHs leaching from the contaminated zone to the deeper layers to endanger the groundwater safety.7 Therefore, a survey for soil PAHs contamination under irrigation is needed to avoid the food and groundwater pollution risk. So far, many studies have investigated the surface soil PAHs pollution levels.8–10 Nevertheless, few studies have explored the PAHs pollution in deep soils, especially in WWI (wastewater irrigation) and RWI (reclaimed water irrigation) areas. In China, studies associated with soil PAHs pollution in WWI areas were mainly conducted in Beijing, Tianjin and Shenyang.11–14 However, there were still no reports on the soil PAHs pollution in a RWI area. The objectives of this study were to attain and compare the vertical distribution and transport of PAHs in soil proles (0–5.5 m) across the WWI area, the RWI area and the GWI area of Beijing. Also, different irrigation waters and soil properties were analyzed to further explain the distribution of PAHs in different soil proles.
Materials and methods Study areas and sample collection Study area description. The WWI area is located at downstream of an industrial district in the southeast suburb of Beijing, where rainfall and surface water cannot meet the need for crop growth. In order to ensure agricultural production, Liangshui River, the second largest sewage-received river, has been used for irrigation since the 1960s. The depth of groundwater in the WWI area is about 60 m. The RWI area is located at the Duozi Village in the southeast suburb of Beijing. A branch of Tonghui Canal which passes through this district from north to south provides water irrigation for agriculture. Tonghui Canal was built in 1958 and received raw municipal sewage at the early stage. Since the 1990s, the canal has received the secondary effluent from the Gaobeidian sewage treatment plant, which has been used for irrigation. The depth of groundwater in this area is also approximately 60 m. The GWI area is located at Yongledian of Tongzhou District of Beijing. The irrigation water was collected from motorpumped wells at the depth of 30 m. For the three areas, the irrigation time from April to November was considered, and the irrigation frequency was similar, about 2–4 times per year. The average amount of irrigation water was about 1214 m3 per acre per year. Sampling. 36 soil samples in November 2007, April 2008 and June 2008 were separately collected from 3 boreholes forming
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Environmental Science: Processes & Impacts
an equilateral triangle of 1 m length in each irrigation area where wheat was grown. The diameter of the boreholes was 38 mm and the maximum depth of the boreholes drilled using an Eijkelkamp sampler was 5.5 m from the surface. Soil samples were collected every 0.5 m along the prole from each borehole and analyzed separately. The locations of sampling sites in the three areas are shown in Fig. 1. Surface water samples including irrigation water were collected along the canals at different irrigation seasons. Wastewater samples were numbered WGB-1, WGB-2, WGB-3, and WGB4, while recycled water samples were numbered ZSB-1 and ZSB-2. All the samples were collected into pre-cleaned brown glass bottles and stored in a cool box (4 C). Aer returning to the laboratory, the soil samples were dried at room temperature, passed through a 20-mesh sieve and then stored in brown glass bottles at 4 C, while the water samples were stored at 4 C prior to analysis for PAHs. Sample extraction and cleanup According to EPA method 3550,15 soil samples (15 g dry wt) and anhydrous sodium sulfate (3 g) were weighed precisely and placed into brown glass jars of 40 mL, with 20 mL acetone– hexane solvent (1 : 1 v/v) and sonicated for 20 min at 50 C and 400 W power in an ultrasonic shaking apparatus. The mixture was then centrifuged for 3 min at 3000 rpm and the extract was collected. The same extraction was repeated twice again by adding 20 mL acetone–hexane solvent (1 : 1 v/v) to the lter residue. The three extract solutions were combined and anhydrous sodium sulfate was added for drying, and then concentrated to around 1 mL by rotary evaporation at 35–40 C. According to EPA method 3510,16 the water samples were ltered by an APFF glass ber membrane. 1 L water sample was measured precisely and placed into a separatory funnel, with 20 mL cyclohexane. The separatory funnel was sealed and shaken vigorously for 1–2 min with periodic venting to release excess pressure, and stilled for 10 min to separate the organic
Fig. 1
Map of the soil sampling sites in Beijing.
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layer from the water phase. The organic layer was collected. The extraction was repeated twice using fresh portions of solvent. The three solvent extracts were combined and anhydrous sodium sulfate was added for drying, and then concentrated to around 1 mL by rotary evaporation at 35–40 C. According to EPA method 3630,17 the concentrated extracts were cleaned up using a column (10 mm i.d.) consisting of 10 g activated silicagel and capped with anhydrous sodium sulfate. The chromatographic column was pre-eluted with 25 mL hexane at 2 mL min1 and the eluate was discarded. Just prior to exposure of the sodium sulfate layer to the air, the concentrated extracts were transferred into the column using an additional 2 mL hexane, then cleaned using 25 mL hexane and discarded the above eluate and nally eluted with 25 mL mixture of dichloromethane and hexane (2 : 3 v/v). Aerward, this eluate was concentrated exactly to 1 mL under a gentle steam of pure nitrogen, and subsequently transferred into vials for PAHs analysis. Unless otherwise indicated, all chemicals and solvents were of high purity analytical reagent grade and used as received. PAHs analysis The concentrations of 16 PAHs including naphthalene (Nap), acenaphthylene (Acy), acenaphthene (Ace), uorene (Fl), phenathrene (Phe), anthracene (Ant), uoranthene (Flu), pyrene (Pyr), benzo[a]anthracene (BaA), chrysene (Chr), benzo[b]uoranthene (BbF), benzo[k]uoranthene (BkF), benzo[a]pyrene (BaP), indeno[1,2,3-cd]pyrene (InP), dibenzo[a,h]anthracene (DBA), and benzo[g,h,i]perylene (BghiP) in the extracts were determined by an Agilent 6890 gas chromatography coupled with an Agilent 5973 mass spectrometer and a 7683 autosampler. The HP-5MS capillary column (50 m 0.25 mm i.d. 0.25 mm lm thickness) was used with helium as the carrier gas at a constant ow rate of 1.0 mL min1. GC temperature was programmed from initial 80 (1 min) to 215 C (1 min) at 30 C min1, then to 255 C (1 min) at 5 C min1, to 263 C (1 min) at 1 C min1, and nally to 300 C (3 min) at 25 C min1 1 mL of the extract was injected in the splitless mode and the solvent delay was 3 min. The mass spectrometer was operated in scan mode with an electron impact ionization of 70 eV, an electron multiplier voltage of 1906 V, and an ion source of 230 C. The identication of individual PAHs was based on the retention time and the mass spectrum with appropriate individual standards at full scan mode. The molecular ions were selected as the target ions for quantication and another two or three characteristic ions were selected for conrmation.
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standard deviation (RSD), ranged between 1.87% and 5.64%, which met the requirements of EPA. Physical and chemical analysis of soil samples TOC and classication of soil samples were characterized by conventional standard procedures. TOC was measured using the potassium dichromate oxidation process.18 Particle size distribution was carried out using the hydrometer method.19
Results and discussion Concentrations and compositions of PAHs in the topsoils The concentrations of PAHs in the surfaces are given in Fig. 2. The contents and compositions of PAHs were different in the topsoils of the three sites. The total concentration of PAHs in the topsoils of three areas was in the order: WWI area (726.0 mg kg1) > RWI area (206.8 mg kg1) > GWI area (42.8 mg kg1) (dry wt). Clearly, the total concentration in the WWI area was much higher than that in the other two areas, indicating that the use of wastewater irrigation may lead to the accumulation of PAHs in the soils. The results were consistent with other studies.20–23 A literature survey revealed that the total concentration of PAHs in the WWI area was obviously lower than that present in wastewater irrigation areas in suburbs of Beijing and Tianjin, China,12 but higher than Hunpu wastewater-irrigation area of Liaoning, China.23 The concentration of PAHs in the RWI area was close to that in farmland soils irrigated by effluents from biological treatment plants in the southeast suburb of Beijing.24 According to the theory of Maliszewska Kordybach,25 the total content of PAHs in three sites could be regarded as medium contaminated, light contaminated and none-contaminated. Most of 16 PAHs were detected in the surface layers of three sites. 14 of PAHs except Ant and BaA were found in the WWI area. 2-ring and 4–6 ring PAHs respectively contributed to 59% and 31% of the total concentration, while 3-ring PAHs had the lowest proportion of 10%. The most abundant compound was Nap with an average value of 399.0 mg kg1, which accounted for
QA/QC measures All data were subject to strict quality control procedures. Quantication was done using an external calibration method. The method detection limits for 16 PAHs ranged from 0.3 to 0.5 mg kg1 for soil samples, and from 2.0 to 4.0 ng L1 for water samples. Method blanks (solvent) and sample duplicates were analyzed routinely. The recovery efficiencies with spiked PAHs in soil and water were in the range of 74.2–117.1% and 85.0– 109.5% respectively. The precision, expressed as the relative
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Fig. 2 The concentrations of PAHs in the surfaces of different irrigation areas. (Here the concentration of Nap in the WWI area was logtransformed (base 10).)
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55% of the total content of PAHs. The concentrations of the other PAHs were all less than 50 mg kg1. The concentration of Ace was relatively low, with a value of 4.1 mg kg1. In the RWI and GWI areas, the high molecular weight (HMW) PAHs (4–6 ring) were the mainly dominant PAHs with a total contribution of 70% and 60% respectively, followed by 3-ring PAHs. Flu dominated the PAHs in the RWI area, with a value of 40.1 mg kg1, accounting for 19% of the detected PAHs. The rest of PAHs were all below 30 mg kg1. Nevertheless, the highest concentration of PAHs in the GWI area was of Phe, with a value of 5.1 mg kg1. The values of the detected PAHs in the GWI area were signicantly lower than the ones in the other two sites. According to Canadian soil quality guidelines and interim remediation criteria for agricultural soil,26,27 only Nap in the WWI area was fourfold higher than the target value (100 mg kg1), the other individual PAHs in the different irrigation areas were all below their limits. However, the sum of the 7 PAH compounds as human carcinogens in the three areas reached 166.4, 103.4, and 18.6 mg kg1, respectively. This represented a potential threat to human health. Compared with the common PAHs detected in the surfaces, the individual PAHs (except Flu) in the three sites were in the following order: WWI area > RWI area > GWI area. The values of HMW PAHs were maintained at the same order of magnitude in the WWI and RWI areas, and were 1 order of magnitude higher than those in the GWI area. The low molecular weight (LMW) PAH (2–3 ring) concentrations in the RWI area were 1–2 orders of magnitude lower than those in the WWI area, but higher than those in the GWI area. This was due to the fact that LMW PAHs were more easily removed than HMW PAHs in sewage treatment processes. The persistence of PAHs was strongly affected by the number of rings, with PAHs with more rings more difficult to degrade.28 This was the reason why HMW PAHs predominated over other PAHs in the RWI area. In addition, the linear distance of sampling sites between the WWI and RWI areas was merely 3.9 km, and they belonged to the same land use type, but the percentages of HMW and LMW PAHs were markedly different. Therefore, it can be inferred that the distribution and concentrations of PAHs in the soils of different irrigation areas were closely related to the quality of irrigation water.
Migration of individual PAHs in different soil proles Vertical distribution of individual PAHs. Although the PAHs were mainly enriched in the topsoils at high levels, and could be easily absorbed by soil organic matter preventing them from migrating downward, they were also detected in the soil proles of different irrigation areas. This fact suggested that a long history of different water irrigation had a remarkable effect on PAHs mobility in soils. The LMW PAHs were the main PAHs detected in the soils and the distribution characteristics of individual PAHs were nearly the same. The highest contents of PAHs appeared in the surface layers, and decreased with depth. Nap, Fl, Phe and Flu contents of the soil proles were compared in different areas (Fig. 3). The order of concentrations of 4 PAHs in the proles was WWI area > RWI area > GWI area and the remarkable variation
This journal is © The Royal Society of Chemistry 2014
Environmental Science: Processes & Impacts
of Nap in the vertical sections of different irrigation areas was found. While the variation of Flu was minor, the contents of Flu were similar to each other in the three sites. The variation of PAHs in the WWI area was more signicant than those in the other areas, which was mainly due to the differences in properties of soil proles. Additionally, it should be noted that the variation of Nap in the RWI area was obviously different from that in the WWI and GWI areas. Nap was found at a low level in the surface of the RWI area, and increased with increasing depth, which may be ascribed to high permeability of the soil proles. Simultaneously, Nap in the reclaimed water was low, meaning volatilization and photolysis might be another factors affecting the distribution of Nap. Migration of PAHs. It is well-known that TOC was an important factor in inuencing the vertical distribution of PAHs.29 Hydrophobic organic contaminants such as PAHs were readily absorbed by organic-matter-rich layers due to their high affinity,4,30,31 which prevents PAHs from leaching into deep layers. Changes of the vertical distribution of TOC and total PAHs in each irrigation area are presented in Fig. 4. The change of total PAHs along the soil proles was basically consistent with that of the TOC content. The relationship between total PAHs and TOC in the three sites was analyzed by soware SPSS. Normality testing showed that the concentrations of PAHs and TOC followed normal distribution. The statistical analyses showed a signicant linear positive correlation between PAHs and TOC, with the Pearson correlation coefficients of 0.529, 0.547 and 0.704 for the WWI, RWI, and GWI areas respectively, which were statistically signicant at a level of 0.01. It was therefore concluded that PAHs in the current study areas were signicantly affected by soil TOC, which was consistent with other investigations.9,32,33 However, Zhang et al.34 noted that PAHs were not signicantly inuenced by TOC content until it arrived at a certain level in the weakly polluted soils. Simpson et al.35 reported that the relationship between total PAHs and organic carbon was signicant only for highly contaminated sites. It was supposed that there was only adsorption in the migration of PAHs, with the exception of degradation, and the contents of PAHs in the solid phase were the values that had achieved the adsorption equilibrium with the liquid phase.36,37 Therefore, the percentage of PAHs content, which was dened by the PAHs content (CD, mg kg1) at depth D (m)/the level (C0, mg kg1) in the surface (D ¼ 0 m), was used to compare the migration of LMW PAHs among different irrigation areas. The percentages of individual PAHs in different irrigation areas are presented in Fig. 5. The more quickly the percentages decreased with depth, the more easily PAHs were adsorbed to the soils, and the lower the migration of PAHs was. As shown in Fig. 5, the order of migration ability of different individual PAHs was: Ace > Fl > Nap > Phe > Acy > Flu for the WWI area, Nap > Fl > Phe > Pyr > Flu for the RWI area, and Nap > Fl > Phe > Flu for the GWI area. The mobilization of Nap and Fl in the RWI area was higher than that in the WWI area due to the higher permeability of the soil proles in the former area. The migration ability of PAHs in the GWI area was similar to that in the WWI area, but was lower in these two areas than that in the
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Fig. 3 The vertical distribution of individual PAHs in different irrigation areas. (Here the concentration of Nap in the WWI area was log-transformed (base 10).)
Fig. 4 The vertical distribution of TOC and PAHs in the soil profiles.
RWI area. Apparently, the migration order of PAHs in the three sites was roughly consistent with their properties including octanol–water partitioning coefficients (Kows) and soil organic carbon–water partitioning coefficients (Kocs) (Table 1): higher Kocs or Kows are correlated with less mobile organic chemicals,
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whereas lower Kocs or Kows are correlated with more mobile organic chemicals. Hence, 2–3 ring PAHs were the main pollutants in the proles. Comparing the compositions of PAHs between the topsoils and the deep soils, it was evident that the PAHs in the deep soils
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Fig. 5
Table 1
Environmental Science: Processes & Impacts
The percentages of individual PAHs in different irrigation areas. Kows and Kocs for 16 PAHs38
PAHs
Kocs (mL g1)
log Kows (L kg1)
Acenaphthene (Ace) Acenaphthylene (Acy) Fluorene (Fl) Naphthalene (Nap) Anthracene (Ant) Fluoranthene (Flu) Phenanthrene (Phe) Benzo[a]anthracene (BaA) Benzo[b]uoranthene (BbF) Benzo[k]uoranthene (BkF) Chrysene (Chr) Pyrene (Pyr) Benzo[a]pyrene (BaP) Dibenz[a,h]anthracene (DBA) Indeno[1,2,3-cd]pyrene (InP) Benzo[g,h,i]perylene (BghiP)
7.1 4.8 1.4 2.0 3.0 1.1 2.3 4.0 1.2 1.2 4.0 1.1 1.0 3.8 3.5 7.8
3.9 10 3.9 10 4.2 10 3.4 10 4.6 10 5.1 10 4.6 10 5.7 10 6.2 10 6.2 10 5.7 10 5.1 10 6.1 10 6.7 10 6.7 10 7.1 10
103 103 104 103 104 105 104 105 106 106 105 105 106 106 106 106
particles were classied into three sizes:
