Distribution of polycyclic aromatic hydrocarbons in soils and corns around Zhongyuan Oil Field, China

中国环境学会  2011年 06月21日

  ① Qingdao University of Science and Technology, Qingdao 266042;② Key Lab of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan;③ State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China
   
  Abstract: The concentrations of sixteen individual polycyclic aromatic hydrocarbons (PAHs) in the samples of soils, corn roots and corn leaves around three different oil sludges in Zhongyuan Oil Field were investigated. The contents of PAHs in the soil samples from high to low were the Third Wenming oil sludges (3W) > the Third Mazhai oil sludges (3M)> the Fourth Wen’er oil sludges (4W), and the PAHs with 2--4 rings were the most predominant components of PAHs in the soil samples. The contents of PAHs in the corn root samples from high to low were 3W > 3M > 4W, and the PAHs with 2--4 rings were the most predominant components of PAHs in the corn root samples, which were consistent with those in the soil samples. The contents of PAHs in the corn leaf samples from high to low were 3W> 4W > 3M, which were inconsistent with those of the soils and corn root samples. And the PAHs with 5--6 rings were the most predominant components of PAHs in the corn leaf samples.
  Key Words: soil of oil field; corn; polycyclic aromatic hydrocarbons (PAHs)
   
  Polycyclic aromatic hydrocarbons(PAHs)which mainly come into being in the incomplete combustion of organic compounds are teratogenic, carcinogenic, and mutagenic, so they have been listed by many countries into the black or gray list about the first-controlled pollutants, where there have been sixteen matters regarded as first-controlled organic pollutants by American Environmental Protection Agency (AEPA) and seven by China (United Nations Environment Programme, http://www.chem.unep.ch/gmn/default.htm, 2008; Blasco et al, 2006; Baran et al, 2004; Oleszczuk and Baran, 2005). PAHs have low water-solubility and hydrophobic property, and they can intensively diffuse in the non water phase, being adsorbed on the granules, so the soils become one of their primary environmental habitations. The residual PAHs in the soils not only influence the normal functions of the soils and decrease their environmental qualities, but also gather gradually in both animals and plants after entering the cropland ecosystem. And they even enter human bodies through food chains, which cause serious harm to human health and lives (Huang and Wei, 2003).
  Presently, a great deal of researches about the PAHs in soils and plants have been carried out at home and abroad, and the main focuses are as follows: the contents and sources of the PAHs in the soils, distribution and migration, the relation between their physicochemical properties and behaviors in the soils, the relation between the environment factors and their behaviors, risk assessment and management; the mechanism of PAHs being adsorbed by plants, the influence factors, remediation of soils polluted by PAHs and so on. However, the researches about PAHs in the soils and plants around oily sludges are relatively less(Ling et al, 2005, 2006; Ge et al, 2005; Zhang et al, 2005; Ding et al, 2004; Zhan et al, 2003; Tao et al, 2003; Liste and Alexander, 2000; Menzies et al, 1992). In our research, the agricultural soils and corns planted in them around Zhongyuan Oil Field were chosen to be studied. In order to carry out the pollution prevention and sound distribution of regional agriculture production, the distribution of PAHs excellently controlled by EPA in the agricultural soils, the corn roots and the corn leaves were analyzed, and the main factors which influenced their distribution were discussed.
   
  SAMPLES COLLECTION AND ANALYSIS


  Samples Collection
  According to the local monsoon feature, the soils and corns samples were collected from the agricultural soils and the corns planted in them which were respectively 10, 20, 50, 100, 200 and 500 m away from the oil sludges on July 24th and 28th, 2007, and the oil sludges included the Third Wenming oil sludges (3W), the Third Mazhai oil sludges (3M) and the Fourth Wen’er oil sludges (4W), which were all located in Zhongyuan Oil Field. The basic conditions and numbers of the samples were displayed in Table 1.

  

     Samples Treatment
  The soil samples were immediately air-dried in the shade after being collected, grinded and sieved into a particle size of 60 meshes. After being sieved, they were placed into the polyvinyl chloride bags, sealed and stored at room temperature. In the process of the soil samples being grinded, they were grinded in the order of being from farnes to nearness according to their distances from the oil sludgea, thus preventing the cross-infection among them. The corn samples collected were cut up, frozen by Freeze-Dryer, and grinded by agate mortar. And then they were placed into the brown jar, frozen and stored at low-temperature, waiting to be analysed.
  Instruments
  The instruments which were used in the research included High Performance Liquid Chromatography (HPLC, Shimadzu, Japan), Fluorescence Detector, VP-ODS Separating Column (Serial No. 9122504, 150L×4.6), Ultrasonic Washer (KQ5200, Kun Shan in China), Rotary Evaporator (Shen Sheng SENCO-R, China), Thermostat Waterbath (Shen Sheng W201B, China), Low-speed Tabletop Centrifuge (TDL-40B, Shanghai), Multipurpose Water-cycling Vacuum Pump (SHB-Ⅲ, Zheng Zhou in China) and High-speed Mixing Instrument (Chang Zhou, China).
  Reagents
  The reagents which were used in the research included dchloromethane, cyclohexane, methanol, acetonitrile, silica, and anhydrous sodium sulfate. Among them, dchloromethane, cyclohexane and methanol were all analytically pure, and acetonitrile was chromatographically pure. Methanol needed to be distilled before being used. Silica was used in the stratographic analysis, which demanded that its diameter should be 0.15~0.074 mm and it should be activated for 16h at 130℃ before being used. Anhydrous sodium sulfate was heated for 2h at 500℃, and was placed into the glass container to be sealed and stored after being cooled to be the room temperature. The standard matters of PAHs were sixteen first-detectioned mixed samples of PAHs in accordance with USEPA Method 610, which were bought from Supelco Corporation of America.
   
  Extraction, Purification and Determination of PAHs


  (1) The extraction, concentration and purification of PAHs. 5.00 g of samples were measured by analytical balance and put into the centrifuge bottle of 100 mL, and then 20.00 mL of dichloromethane were added. After the samples were put into the dichloromethane, they were kept in the ultrasonic washer of 40 °C for 2 h, and then were transferred into the centrifugal vacuum pump of 2000 r/min, and were kept for 5 min. 10.00 mL of supernatant were moved into the egg yard type bottle, and all the liquids were dried by rotary evaporator at 40 °C. After that, the resulting residues were dissolved in 2.00 mL of cyclohexane, and 0.50 mL of the solution were put on the silicamini column packed with 1.0 g of silica gel. The column was eluted with methylene chloride/hexane (v/v=1:1), and the eluate was discarded with 1.00mL. The second fraction of 2.00mL containing PAHs was collected and blown to be dry by nitrogen. The residues were redissolved in 1mL of acetonitrile and stored at 4 °C.
  (2) Determination of PAHs. Chromatographic Conditions: HPLC, Fluorescence Detector, VP-ODS Separating Column; acetonitril-water (9:1) solution being as the mobile phase, with flow rate of 0.5 mL/min; the column temperature being 30 °C. Chromatographic Determination: the samples concentrated solution and standard mixed solution which had the same volume were extracted respectively, and they were determined by external standard method.
  Table 2 Concentrations of PAHs in soil and corn samples (ng/g)

sample

ΣPAHs in soil samples

ΣPAHs in corn root samples

ΣPAHs in corn leaf samples

3M-1

1994.8

605.76

1025.43

3M-2

1427.5

505.06

910.43

3M-3

759.4

397.84

667.43

3M-4

524.8

262.52

581.43

3M-5

316.9

147.45

140.43

3M-6

246.6

117.38

124.43

3W-1

2047.5

637.93

1121.43

3W-2

1404.6

537.92

925.43

3W-3

703.5

379.01

710.43

3W-4

522.9

262.6

601.43

3W-5

334.6

149.5

142.43

3W-6

238.5

112.32

128.43

4W-1

1543.2

465.13

1069.43

4W-2

1173.5

371.69

920.43

4W-3

536.3

269.67

689.43

4W-4

396.8

191.42

584.43

4W-5

267.8

127.07

147.43

4W-6

215.1

103.14

121.43

   
  The qualitative analysis was carried out by the compare of the retention time of PAHs standard compounds, while the quantitative analysis was done by the determination of PAHs with external standard method. The analytical work included blank analysis, the standard samples being added to the substrates, parallel analysis and iterative determination of the samples. The recovery of standard samples being added to the substrates was 47.5%-103.4%, and the relative standard deviation was 5.8%-9.7%. In addition, analysis of the principal components was conducted with SPSS1 1.0. Kinds and contents of PAHs in samples were displayed in Table 2.

 

  RESULTS AND DISCUSSION
 

  Distribution Characteristics of PAHs in Aagricultural Soil
  Total contents of sixteen PAHs are usually used to evaluate the pollution level in the researches (Wu et al, 2003; Zeng and Vista, 1997). The PAH contents in soil samples around three different oil sludges are listed in Table 2. The results indicate that all the sixteen PAHs can be detected in soil samples. The contents of PAHs in soil samples of 3M, 3W and 4W are 246.6-1994.8 ng/g, 238.5-2047.5ng/g and 215.1-1543.2ng/g, respectively, so the order of pollution degree of the soils around the three oil sludges is 3W>3M>4W. The contents of PAHs in soil samples are correlative with the distances from oily sludges, and the contents are drastically decreasing with the increase of the distance from the oil sludges (Fig 1). The toxicity and cancer-causing function are different, because of the difference among the structures of PAHs. The research shows that PAHs of Low ring (2-and 3- ring) exhibit strong and acute toxicity comparatively, while PAHs of High ring (4- and more ring) have “three causing” function to many living creatures. Concentrations of PAHs with various rings in soil samples are listed in Fig 2, which show that PAHs of 2-4 rings are the most predominant components of PAHs in the soil around the Oil Field. The analysis results of the soil samples around the Oil Field in winter show that the PAHs contents of the soil samples of 3M, 3W and 4W are 499.9~2241.3ng/g, 578.1~2408.8ng/g and 434.5~1822.0ng/g respectively, which are much higher than those of the soils in the summer (Kuang et al, 2008). The result is related with the corns growing in the soils in summer. Not only do the corns absorb the PAHs in the soils, but their roots offer the surface for the growing of microbes and change the soil environment (e.g. oxidation and reduction, humidity, ventilation, etc. ) to degrade PAHs. Xu et al (2006) in his research found that the corns could absorb and gather phenanthrene and pyrene of the polluted soils, and the quantities increaseed with the increase of their contents.Yoshitomi also found that the corn roots could bring on the growth of microbes and degradation of pyrene (Yoshitomi and Shann, 2001). In addition, the air temperature in summer is higher than that in winter, and it is easier to evaporate and degrade for PAHs of low molecular weight, therefore, the contents of PAHs detected in the soil samples in summer are much lower.
   
  Distribution Characteristics of PAHs in Corn Roots
  The results (Table 2) indicate that all the sixteen PAHs can be detected in corn root samples. The contents of PAHs in corn root samples of 3M, 3W and 4W are 117.38-605.76ng/g, 112.32-637.93ng/g and 103.14-465.13ng/g respectively, so the order of PAHs contents in corn root samples is 3W>3M>4W, which is consistent with that in the soils around the three oil sludges. The regression analysis of the contents of PAHs in the soils and the corn roots around oil sludges indicates that they have good linear relationship with a correlation coefficient of 0.9628, and they present remarkable positive correlation, which illustrates that the PAHs contents in soils around the three oil sludges influence the absorption of PAHs in the corn roots. Concentrations of PAHs with various rings in corn root samples are listed in Fig 3, which shows that PAHs of 2-4 rings are the most predominant components of PAHs in the corn roots around the Oil Field, which are consistent with the characteristics of PAHs contents in soils around the three oil sludges.
     
  Distribution Characteristics of PAHs in Corn Leave
  The contents of PAHs in corn leaf samples of 3M, 3W and 4W are 124.43-1025.43ng/g, 128.43-1121.43ng/g and 121.43-1069.43ng/g respectively (Table 2). The order of PAHs contents in corn leaf samples is 3W>4W>3M, which is inconsistent with that in soils around the three oil sludges. Concentrations of PAHs with various rings in corn leaf samples are listed in Fig 4, which shows that PAHs of 5-6 rings are the most predominant in all the PAHs in corn leaves around Oil Field, which are inconsistent with the characteristics of PAHs contents in soils and corn roots around the three oil sludges. The reason is that PAHs in the corn leaves mainly originate from atmosphere, and different PAHs have different volatilities, which lead to that the presence modality and mutual transformation characteristics of different PAHs are greatly different (Dong et al, 1999). Generally, gaseity is the main existing form of PAHs with 2-3 rings, the allocation of 4-ringed PAHs in the gaseity and granules is balanced, and PAHs with 5 and more rings mainly exist in the granules(Halsall and Coleman, 1994). The corn leaves are narrow and long, and full of pilosity, which make them have large contact with the atmosphere. During the long time of the atmosphere granules being absorbed by PAHs, the PAHs with little rings can return the atmosphere again, so the PAHs with 5-6 rings are the most predominant components of PAHs in the corn leaves.
  
  CONCLUSIONS
  (1) The contents of PAHs in soil samples of 3M, 3W and 4W are 246.6-1994.8ng/g, 238.5-2047.5ng/g and 215.1-1543.2ng/g respectively. It can be seen from the regional distribution that the contents of PAHs in soils around Zhongyuan Oil Field are correlative with the distances from the oil sludges, and the contents are drastically decreasing with the increase of the distance from the oil sludges.
  (2) The contents of PAHs in corn root samples of 3M, 3W and 4W are 117.38-605.76ng/g, 112.32-637.93ng/g and 103.14-465.13ng/g respectively. They are closely related with the contents of PAHs in soils around the oil sludges.
  (3) The contents of PAHs in corn leaf samples of 3M, 3W and 4W are 124.43-1025.43ng/g, 128.43-1121.43ng/g and 121.43-1069.43ng/g respectively. Influenced by the structures of the corn leaves, the PAHs with 5-6 rings are the most predominant components of PAHs in the corn leaves around oil sludges, and the characteristics of their contents are different from those in soils and the corn roots around the three oil sludges.
   
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  * The study is jointly supported by National Natural Science Foundation of China (20977055), Shandong Provincial Natural Science Foundation of China (Y2007E03), Key Lab Foundation of Biogeology and Environmental Geology of Ministry of Education (No. BGEGF200804) and State Key Laboratory Foundation of Geological Processes and Mineral Resources (GPMR200924).
  

 
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