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Use of Organic Solvents to Extract Organochlorine Pesticides (OCPs) from Aged Contaminated Soils
Pedosphere 23(1): 10–19, 2013 ISSN 1002-0160/CN 32-1315/P c 2013 Soil Science Society of China Published by Elsevier B.V. and Science Press
Use of Organic Solvents to Extract Organochlorine Pesticides (OCPs) from Aged Contaminated Soils∗1 YE Mao1 , YANG Xing-Lun1 , SUN Ming-Ming1 , BIAN Yong-Rong1 , WANG Fang1 , GU Cheng-Gang1 , WEI Hai-Jiang1 , SONG Yang1 , WANG Lei1 , JIN Xin1 and JIANG Xin1,∗2 State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008 (China) (Received June 24, 2012; revised November 5, 2012)
ABSTRACT Problems associated with organochlorine pesticide (OCP)-contaminated sites in China have received wide attention. To solve such problems, innovative ex-situ methods of site remediation are urgently needed. We investigated the feasibility of the extraction method with different organic solvents, ethanol, 1-propanol, and three fractions of petroleum ether, using a soil collected from Wujiang (WJ), China, a region with long-term contamination of dichlorodiphenyltrichloroethanes (DDTs). We evaluated different influential factors, including organic solvent concentration, washing time, mixing speed, solution-to-soil ratio, and washing temperature, on the removal of DDTs from the WJ soil. A set of relatively better parameters were selected for extraction with 100 mL L −1 petroleum ether (60–90 ◦ C): washing time of 180 min, mixing speed of 100 r min−1 , solution-to-soil ratio of 10:1, and washing temperature of 50 ◦ C. These selected parameters were also applied on three other seriously OCP-polluted soils. Results demonstrated their broad-spectrum effectiveness and excellent OCP extraction performance on the contaminated soils with different characteristics. Key Words:
dichlorodiphenyltrichloroethane, ex-situ soil washing, extraction performance, petroleum ether, site remediation
Citation: Ye, M., Yang, X. L., Sun, M. M., Bian, Y. R., Wang, F., Gu, C. G., Wei, H. J., Song, Y., Wang, L., Jin, X. and Jiang, X. 2013. Use of organic solvents to extract organochlorine pesticides (OCPs) from aged contaminated soils. Pedosphere. 23(1): 10–19.
INTRODUCTION Organochlorine pesticides (OCPs) are a set of persistent organic pollutants (POPs) that are toxic, persist in the environment for long periods of time, and biomagnify through the food chain (Wang et al., 2011). The amount of dichlorodiphenyltrichloroethane (DDT) produced and applied in China before it was banned in 1983 is 0.4 million tons, accounting for 20% of the total global production (Zhang et al., 2002; Nakata et al., 2005). Technical chlordane is still being extensively used as a termiticide at 500–800 t year−1 in China in agriculture, gardens, and buildings (POPs Research Center, 2005). Mirex is widely produced and used as a termiticide at up to 30 t year−1 while banned in agriculture (Wong et al., 2005). With the signing of the Stockholm Convention and the development of global monitoring programs, many OCP-contaminated sites are left by hundreds of abandoned OCP production factories in large cities in China (Sun et al., 2012). Currently, most of these contaminated sites face ∗1 Supported
urgent land use conversion and redevelopment for commerce (Li et al., 2008; Fu et al., 2009; Yang, L. Y. et al., 2010; Yang, X. L. et al., 2010), and likely pose a threat on residents and the environment. Chinese soil engineers need to do research on and develop new soil remediation techniques suitable for specific national conditions in order to cope with these emergent problems (Zhu et al., 2005; Karstensen et al., 2006; Yang, L. Y. et al., 2010; Yang, X. L. et al., 2010; Sun et al., 2011) A potential technique for rapid removal of OCPs from soils is ex-situ soil washing with organic solvents since it has relatively high removal capacity as well as cost-effectiveness (Meguro et al., 2008; Wan et al., 2009). As desorption of organic contaminant is a ratelimiting step, water-miscible organic solvents are generally selected for their ability to transport pollutants from solid phase to aqueous bulk phase, and to increase the solubility of the organic contaminant in the liquid phase (Silva et al., 2005; Frankki et al., 2007). The use of organic solvents has been investigated at many POP-polluted sites; for instance, Smith et al. (2004)
by the National High Technology Research and Development Program of China (No. 2009AA063103) and the National Natural Science Foundation of China (No. 41030531). ∗2 Corresponding author. E-mail: [email protected].
SOLVENT EXTRACTION OF PESTICIDES FROM SOILS
investigated the effectiveness of different concentrations of ethanol, 1-propanol, and 2-propanol in removing p, p -DDT from a soil contaminated with DDTs for nearly 40 years and Jonssonly et al. (2010) reported extraction of approximately 80% polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) from a contaminated soil in the presence of 75% ethanol at 60 ◦ C after 10 washing cycles. The extraction of polynuclear aromatic hydrocarbons (PAHs) from aged field soils in the presence of different organic solvents (i.e., ethyl acetate, acetone, ethanol, 2-propanol, and 1-pentanol) has also shown promising results in several studies (Khodadoust et al., 2000; Lee and Hosomi, 2001; Nam et al., 2001; Silva et al., 2005). Most previous studies differ in their focus on influencing factors (Chang et al., 2000; Juhasz et al., 2003; Zhao et al., 2005; Haapea et al., 2006; Peng et al., 2011) but there have been relatively few systematic studies conducted on these factors, especially those that attempt to select a set of broad-spectrum washing parameters for extracting contaminants from different OCP-contaminated soils. Therefore, the aim of this study was to assess the potential applicability of organic solvent washing as a remediation technique for OCP-contaminated soils. The effects of certain factors, organic solvent concentration, washing time, mixing speed, solution-tosoil ratio, and washing temperature, on OCP-removal efficiency were examined with five organic solvents, i.e., ethanol, 1-propanol, and three different petroleum ether fractions (30–60, 60–90, and 90–120 ◦ C). A set of selected parameters were tested for the robustness and potential of the technique on other three contaminated soils. MATERIALS AND METHODS Chemicals, reagents and soil samples Standard samples of o, p -DDT, p, p -DDT, p, p dichlorodiphenyldichloroethane (DDD), o, p -DDT, p, p -DDT, cis-chlordane, trans-chlordane, and mirex (purity > 99.5%) were purchased from Dr. Ehrenstorfer (Augsburg, Germany). Hexane (HPLC-grade) was purchased from the Tedia Company, USA. Ethanol, 1-propanol, 30–60 ◦ C petroleum ether (PE 1), 60– 90 ◦ C petroleum ether (PE 2), and 90–120 ◦ C petroleum ether (PE 3) (Nanjing Chemical Reagent Co., China) are all analytical grade. Acetone and dichloromethane (analytical grade, Nanjing Chemical Reagent Co., China) were distilled prior to use. Anhydrous sodium sulfate (Na2 SO4 , Nanjing Chemical Reagent Co., China) was oven-dried at 400 ◦ C for 4
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h before use. Silica gel (100 mesh) was activated at 130 ◦ C for 18 h and deactivated using 3.3% deionized water based on the EPA standard method (U.S. EPA, 1996). Other chemical reagents were of analytical grade, as required. All soil samples were taken from four abandoned pesticide factories, which produced OCPs between the 1960s and the 1980s, located in the old city district of Wujiang (WJ), Liyang (LY), Lishui (LS), and Yancheng (YC), Jiangsu Province, China. Soil samples were collected from the surface to a depth of 200 mm, where a relatively higher concentration of OCPs was detected. All soil samples were air dried for 7 d, then homogenized, ground to pass a 2-mm sieve, and stored in glass bottles at 4 ◦ C until analysis. The physicochemical properties of the soils and the concentrations of OCPs are given in Table I. Soil washing Organic solvent concentration. The WJ soil (25 g) was placed into a 1-L flask with Teflon-lined caps, to which 250 mL each of ethanol, 1-propanol, PE 1, PE 2, and PE 3 were added. The operating parameters, namely, the volume fractions of organic solvents divided by the total volume fraction of distilled water plus organic solvents, were adjusted in terms of organic solvent concentrations as 10, 50, 100, 20, 300, 400, and 500 mL L−1 . The flask was shaken in a temperatureregulated shaker (20 ± 2 ◦ C) at a fixed mixing speed of 100 r min−1 for 60 min. Then, the soil suspensions were centrifuged at 2 500 r min−1 for 30 min. The soil and supernatant were separated for further OCP extraction and analysis. Each treatment was performed in triplicate. Washing time. Various time intervals were investigated at a solution-to-soil ratio of 10:1 (v/v). Soil suspensions containing the WJ soil (25 g) with selected concentrations of the five organic solvents were prepared in triplicate and shaken at a fixed mixing speed of 100 r min−1 for 1, 5, 10, 20, 30, 60, 180, 360, and 720 min. At each interval, the soil suspensions were centrifuged and separated for analysis. Mixing speed. To study the effect of mixing speed, the WJ soil samples (25 g) at a solution-tosoil ratio of 10:1 (v/v) with selected concentrations of the five organic solvents were prepared in triplicate, shaken at a speed of 0, 50, 100, 150, 200, and 250 r min−1 for 60 min, and then centrifuged and separated for analysis. Solution-to-soil ratio. To understand the effect of solution-to-soil ratio (v/v), the WJ soil (25 g) samples at four solution-to-soil ratios of 2:1, 5:1, 10:1, and
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TABLE I Physico-chemical properties and the concentrations of organochlorine pesticides (OCPs) in the soils collected from different sites selected Item
Soil sampling site Wujiang
Liyang
Lishui
Yancheng
Soil property Organic matter (g kg−1 ) Sand (%) Silt (%) Clay (%) pH CEC (cmol kg−1 ) Texture
6.6 7.7 66.8 25.5 6.91 18 Sandy silt
10.2 9.7 53.2 37.1 6.79 8.91 Sandy silt
43.5 5.9 77.5 16.6 6.85 9.31 Sandy silt
66.2 5.1 37.2 57.7 6.23 9.50 Clay silt
OCP (mg kg−1 dry soil)a) p, p -DDE o, p -DDE p, p -DDD o, p -DDT p, p -DDT cis-chlordane trans-chlordane Mirex Total
1.69±0.09 NDb) 15.55±0.89 6.92±0.35 58.81±3.80 ND ND ND 82.99±4.23
48.15±3.67 76.08±1.96 1.62±0.04 122.84±9.14 47.15±1.72 25.37±1.23 5.32±0.34 19.63±0.92 346.16±14.23
240.71±19.68 44.37±1.29 12.66±0.26 122.11±6.45 31.53±0.79 460.90±12.08 270.17±4.30 212.92±8.37 1 395.37±26.05
1 109.73±15.90 1 060.21±38.90 ND 3 295.53±136.49 502.33±7.47 2 427.90±104.22 289.43±11.30 ND 8 685.13±268.71
Aging time (years)
25
20
20
30
a) DDE b) Not
= dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; DDT = dichlorodiphenyltrichloroethane. detected.
20:1 with selected concentrations of the five organic solvents were prepared in triplicate. The soil solution was shaken for 60 min, and then centrifuged and separated for analysis. Washing temperature. Various washing temperatures (30, 40, 50, 60, and 70 ◦ C) were investigated at a solution-to-soil ratio of 10:1 (v/v). The WJ soil samples (25 g) with selected concentrations of the five organic solvents were prepared in triplicate. Soil suspensions were shaken for 60 min, and then centrifuged and separated for analysis. Performance assessment. After studying the factors influencing the removal of OCPs from the WJ soil, a set of washing parameters were selected for one promising solvent based on OCP removal efficiency and cost-effectiveness. To testify the applicability and robustness, five sequential washing cycles were performed on other three OCP-polluted soils (LY, LS, and YC soils). At the end of each washing cycle, the soil suspensions were treated as mentioned above. OCP extraction and analysis Sample extraction and cleanup. With the approach of accelerated solvent extractions (ASE), the total soil OCP extraction was performed with an ASE200 accelerated solvent extraction system (Dionex, USA) at a temperature of 100 ◦ C, pressure of 1 500
psi (10.345 MPa), and static time of 5 min. Hexane/acetone (1:1, v/v) was used as extraction solvent. To eliminate water, about 2 g of Na2 SO4 was added into each vial of the soil extracts. Then the extract was concentrated first to 2 mL by a rotary evaporator, and cleaned up with a Florisil solid phase extraction (SPE) cartridge (1 g, 6 mL, Supelco, USA). After the sample was transferred onto the cartridge, it was then eluted with 50 mL acetone/hexane (2:98, v/v). The elution was concentrated to 1 mL for gas chromatograph (GC) analysis. To assure data quality, spike recovery samples containing known quantities of the OCP standard were used to evaluate the extraction efficiency. OCP analysis. The OCP concentrations were measured by gas chromatograph (Agilent 6890, Agilent Technologies, Inc., USA) equipped with a DB5 capillary column (30 m length × 0.32 mm inside diameter × 0.25 μm film thickness), a 63 Ni electron capture detector, and an HP 7683 auto-sampler (HP Company, USA). The carrier gas was nitrogen with a flow of 0.7 mL min−1 . The injector and detector temperatures were 225 and 300 ◦ C, respectively. The oven temperature program consisted of an initial temperature of 60 ◦ C for 1 min, which was increased to 100 ◦ C at a rate of 20 ◦ C min−1 , held for 2 min, to 160 ◦ C at a rate of 10 ◦ C min−1 , to 230 ◦ C at 4 ◦ C min−1 , held for 5 min, and finally to 280 ◦ C at 10 ◦ C min−1 . The
SOLVENT EXTRACTION OF PESTICIDES FROM SOILS
injection volume was 1 μL in a splitless mode. Quality control. To confirm the OCP analysis results, every set of ten samples a procedural/laboratory blank and standard sample was run. Method performance was assessed by quality parameters such as recovery, repeatability, and limits of detections (LODs) and quantification (LOQs). LODs and LOQs were calculated on the basis of signal-to-noise ratio (S/N) of 3 and 10, respectively. Recovery was assessed by analyzing uncontaminated soil samples (n = 6) spiked with each of the studied OCPs, including p, p -DDT, p, p -dichlorodiphenyldichloroethylene (DDE), p, p -DDD, o, p -DDT, cis-chlordane, transchlordane, and mirex, at 50, 100, 200, 500, and 1 000 ng g−1 . Mean recovery of OCPs in the spiked soil samples was 94% ± 8% for DDTs, 101% ± 12% for cischlordane, 95% ± 4% for trans-chlordane, and 92% ± 14% for mirex. In addition, the analytical precision of the results for the soil replicates (n = 3) with relative standard deviations (RSD) ranging from 0.5% to less than 5% was acceptable. Calculation and statistical analysis The OCP removal efficiency (RE) could be obtained from the residual OCP concentrations in soil
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samples (ROCP) and the initial OCPs concentrations in the soil (IOCP): RE =
IOCP − ROCP × 100% IOCP
(1)
Statistical analysis was carried out using the SPSS 14.0 software package for Windows. Mean values were compared using the least significant difference (LSD) test at the probability level ≤ 5%. RESULTS AND DISCUSSION Organic solvent concentration Fig. 1 illustrated that higher concentrations of the five organic solvents resulted in higher DDT removal efficiency from the WJ soil, but their performances were not the same. For example, the addition of 100 mL L−1 PE 1 resulted in the desorption of 75% p, p -DDE, 74% p, p -DDD, 69% o, p -DDT, and 76% p, p -DDT, compared with 9% and 31% p, p -DDE, 9% and 9% p, p -DDD, 13% and 20% o, p -DDT, and 0% and 15% p, p -DDT when using 100 mL L−1 ethanol and 100 mL L−1 1-propanol, respectively. A similar phenomenon was observed for 100 mL L−1 PE 2 and 100 mL L−1 PE 3. Many researches have demonstrated that petro-
Fig. 1 Effects of different organic solvents of different concentrations on organochlorine pesticide (OCP) desorption from the soil collected at Wujiang. DDT = dichlorodiphenyltrichloroethane; DDE = dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; PE 1 = 30–60 ◦ C petroleum ether; PE 2 = 60–90 ◦ C petroleum ether; PE 3 = 90–120 ◦ C petroleum ether. Error bars represent the standard deviations of the means (n = 3).
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leum ether showed excellent extraction performance for OCP-contaminated soils (Aydin et al., 2006; Dumitran et al., 2009; Ozcan et al., 2009), but an excessive volume was consumed to achieve a high extraction efficiency during lab analysis. In our study, we used petroleum ether mixed with distilled water as the washing solution, which manifested three mainly distinct advantages. Firstly, less organic extraction solvent was used in such a washing system, soil particles dispersed evenly in the ex-situ soil washing system, and also petroleum ether could contact with the contaminated soil intimately and constantly. Secondly, the adsorption of petroleum ether by soil particles could be reduced because water has a barrier effect on the sorption of petroleum ether to soil particles as well as on soil structure and soil fauna (Lee and Hosomi, 2001; Wu et al., 2011). Thirdly, the extraction performances of 100 mL L−1 PE 1, 100 mL L−1 PE 2, and 100 mL L−1 PE 3 were comparable to those of 500 mL L−1 ethanol and 300 mL L−1 1-propanol. Therefore, considering the potential extraction efficiency and cost of the five organic solvents, 100 mL L−1 PE 1, 100 mL L−1 PE 2, 100 mL L−1 PE 3, 500 mL L−1 ethanol, and 300 mL L−1 1-propanol were chosen as the washing solutions for the tests on the effects of other factors on extraction efficiency.
M. YE et al.
Washing time Kinetic studies were conducted using the five selected washing solutions to determine if the extraction time would affect DDT removal from the WJ soil (Fig. 2). The changes in removal efficiency of DDTs with time were similar for the five organic solvents. About 60% DDTs desorbed quickly during the first 30 min of extraction. Increasing extraction time from 30 to 180 min resulted in a slow increase of the DDT removal percentage. However, after 180 min, only a small percentage increase of DDTs desorbed was observed. Desorption is of particular importance in exsitu soil washing because desorption is a rate-limited factor for extraction in most cases (Kraaij et al., 2002; Lepp¨ anen et al., 2003; Semple et al., 2004). Desorption of contaminants from soil particles generally proceeds an initial rapid desorbing phase followed by the slow and then very slow desorbing phase (Pignatello and Xing, 1995), which in fact is a balanced redistribution of contaminants between soil particles and washing solution. Once the contaminants achieve a steady state during the washing process, removal efficiency reaches a maximum value. Thus, 180 min was chosen as the relative better extraction time for the five organic solvents.
Fig. 2 Desorption kinetics of organochlorine pesticides (OCP) from the soil collected at Wujiang with selected organic solvents of a given concentration. DDT = dichlorodiphenyltrichloroethane; DDE = dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; PE 1 = 30–60 ◦ C petroleum ether; PE 2 = 60–90 ◦ C petroleum ether; PE 3 = 90–120 ◦ C petroleum ether. Error bars represent the standard deviations of the means (n = 3).
SOLVENT EXTRACTION OF PESTICIDES FROM SOILS
Mixing speed Residual DDTs in the WJ soil decreased with the increase in mixing speed from 0 to 100 r min−1 for the five selected washing solutions (Fig. 3), which indicated that DDTs extracted increased with increasing mixing speed. However, the speed higher than 100 r min−1 did not enhance the DDT extraction efficiency remarkably. Therefore, from the perspective of energy saving, the mixing speed of 100 r min−1 was selected for further potential application. Solution-to-soil ratio Solution-to-soil ratios are an important parameter in ex-situ soil washing. Generally, higher solution-tosoil ratios lead to higher desorption of contaminants. It can be seen from Fig. 4 that extraction of 30%–46% p, p -DDE, 26%–41% p, p -DDD, 18%–33% o, p -DDT, and 11%–34% p, p -DDT was achieved at the solutionto-soil ratio of 2:1 for the five selected solvents, while the removal efficiency up to 80%–98% p, p -DDE, 84%– 95% p, p -DDD, 75%–94% o, p -DDT, and 73%–94% p, p -DDT was achieved at 20:1. Although the highest DDT removal efficiency occurred at a solution-to-soil ratio of 20:1, a higher solution-to-soil ratio would pose a higher requirement for the equipment and energy and
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generate more wastewater for post-treatment. Therefore, the solution-to-soil ratio of 10:1 could be taken as the relatively better parameter in this case. Temperature Fig. 5 revealed that the DDT removal performance from the WJ soil was affected by temperature within the tested range. An increase in temperature from 20 to 70 ◦ C for 500 mL L−1 ethanol promoted the amounts of p, p -DDE, p, p -DDD, o, p -DDT, and p, p DDT desorbed from 78% to 99%, 78% to 95%, 77% to 95%, and 67% to 98%, respectively. In addition, In the case of 300 mL L−1 1-propanol at a temperature of 70 ◦ C, approximately 99% p, p -DDE, 97% p, p DDD, 93% o, p -DDT, and 96% p, p -DDT was desorbed, compared with approximately 77% p, p -DDE, 80% p, p -DDD, 77% o, p -DDT, and 64% p, p -DDT at 20 ◦ C. Rising temperatures in this range played a positive role in desorption of DDTs for the two selected solvents. This can be attributed to the fact that an increased temperature decreased the partition coefficient of the contaminants between soil particles and aqueous phase, thereby increasing the desorption rate of the DDTs from the soil to the aqueous phase. Raising temperature, however, had different influences on DDTs removal efficiency when using the three
Fig. 3 Effect of mixing speed on organochlorine pesticide (OCP) desorption from the soil collected at Wujiang with selected organic solvents of a given concentration. DDT = dichlorodiphenyltrichloroethane; DDE = dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; PE 1 = 30–60 ◦ C petroleum ether; PE 2 = 60–90 ◦ C petroleum ether; PE 3 = 90–120 ◦ C petroleum ether. Error bars represent the standard deviations of the means (n = 3).
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Fig. 4 Effect of solution-to-soil ratios from 2:1 to 20:1 on organochlorine pesticide (OCP) desorption from the soil collected at Wujiang with selected organic solvents of a given concentration. DDT = dichlorodiphenyltrichloroethane; DDE = dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; PE 1 = 30–60 ◦ C petroleum ether; PE 2 = 60–90 ◦ C petroleum ether; PE 3 = 90–120 ◦ C petroleum ether. Error bars represent the standard deviations of the means (n = 3).
different petroleum ether fractions. The temperature increase from 20 to 70 ◦ C with 100 mL L−1 PE 1 resulted in a decrease in the removal of DDTs. This result may be caused by the intrinsic nature of PE 1 (30–60 ◦ C) which is very volatile and will limit its use. With a rise in temperature from 20 to 50 ◦ C, 100 mL L−1 PE 2 resulted in increasing desorption of p, p DDE, p, p -DDD, o, p -DDT, and p, p -DDT from 73% to 90%, 72% to 81%, 68% to 84%, and 62% to 80%, respectively. It may be due to the fact that the temperature rise was conducive to desorption of DDTs and accelerated the transfer of the contaminants from soil particles to PE 2. However, from 50 to 70 ◦ C, the DDT removal efficiency encountered a slight decrease, which may be attributed to the evaporation of PE 2 (60–90 ◦ C) at a higher range of temperature. With 100 mL L−1 PE 3, the DDT removal efficiency increased only slightly within the tested temperature range, which may be caused by the much more matchable response of the boiling range of PE 2 (60–90 ◦ C) with the operational temperature (30–70 ◦ C) than PE 1 and PE 3. In
short, the results indicated that 100 mL L−1 PE 2 (60– 90 ◦ C) showed an excellent extraction performance at a temperature of 50 ◦ C and demonstrated more potential than other four organic solvents. Therefore, 50 ◦ C was selected as a suitable temperature for extraction with 100 mL L−1 PE 2. Performance assessment By comparing the effects of different factors on the removal of DDTs from the WJ soil, a set of washing parameters, washing time of 180 min, mixing speed of 100 r min−1 , solution-to-soil ratio of 10:1, and tempera ture of 50 ◦ C, are selected for soil washing with 100 mL L−1 PE 2, which had a good removal efficiency and low cost. To verify the feasibility, five sequential washing cycles were applied to three other OCP-polluted soils (LY, LS, and YC soils) with varying characteristics and contamination levels. The characteristics of the three soils are given in Table I. Table II demonstrated that a high OCP removal
SOLVENT EXTRACTION OF PESTICIDES FROM SOILS
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Fig. 5 Effect of temperature on organochlorine pesticide (OCP) desorption from the soil collected at Wujiang with selected 5 organic solvents of a given concentration. DDT = dichlorodiphenyltrichloroethane; DDE = dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; PE 1 = 30–60 ◦ C petroleum ether; PE 2 = 60–90 ◦ C petroleum ether; PE 3 = 90–120 ◦ C petroleum ether. Error bars represent the standard deviations of the means (n = 3).
efficiency could be achieved under the selected procedure. The OCPs were most efficiently removed from the LY soil (Table II), a sandy silt soil with an organic matter content of 10.2 g kg−1 and an OCP concentration of 346.16 mg kg−1 dry soil. Approximately 94% of the total OCPs were extracted after five washing cycles. Up to 85% of the total OCPs were removed from the LS soil which was rich in organic matter (43.5 g kg−1 ) and contained total OCPs of 1 395.37 mg kg−1 dry soil. The soil with the lowest total OCP removal was the YC soil (75% removal efficiency), a clay soil with a high organic matter content (66.2 g kg−1 ) and a high OCP concentration of 8 685.13 mg kg−1 dry soil. The sandy characteristics could be the reason for the high removal efficiency of OCPs from the LY soil (Tables I and II). Extraction techniques such as solvent washing are often most efficient when applied to coarse soils (Freeman and Harris, 1995) because the washing solvent can penetrate the particles and effectively desorb the contaminants. In addition, the larger the soil particles, the smaller the surface area and the lower the abundance of contaminant sorption sites and the adsorptive capacity of the soil (Khodadoust et al., 1999; Frankki et al., 2007). In literature, it was reported that less of the contaminant is desorbed in soils with higher organic matter contents (Wang et al., 2006,
2007; Xu et al., 2006). These observations are consistent with the high extraction efficiency from the LY soil, but not with the high extraction efficiency from the LS soil. The lowest removal efficiency was from the YC soil, a clay silt soil with the highest organic matter content, probably due to entrapment of contaminants in clay and silt conglomerates and the strong affinity of OCPs to the organic matter, particularly to organic carbon (Cornelissen et al., 2005; Palomo and Bhandari, 2006). Results in Table II also demonstrated that multiple washing cycles could remove more, but not all, of OCPs from the contaminated soils, especially the heavily polluted soils such as the LS and YC soils. Several factors such as the chemical nature of the contaminants, the different initial concentrations in the soil, and the duration of soil contamination can explain the different washing results (Spark and Swift, 2002; Gevao et al., 2003). Considering that the OCP contamination of the three soils occurred more than 20 years prior to the present study, it was possible that a long period of contact increased adsorption of the contaminants. CONCLUSIONS A set of soil washing parameters (washing time of
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TABLE II Removal efficiency of organochlorine pesticides (OCPs) from three soils during sequential washing cycles at a mixing speed of 100 r min−1 , washing time of 180 min, solution-to-soil ratio of 10:1, and 50 ◦ C with 100 mL L−1 60–90 ◦ C petroleum ether Soil sampling site
OCPa)
Washing cycle 1st
Liyang
Lishui
Yancheng
p, p -DDE o, p -DDE p, p -DDD o, p -DDT p, p -DDT cis-chlordane trans-chlordane Mirex p, p -DDE o, p -DDE p, p -DDD o, p -DDT p, p -DDT cis-chlordane trans-chlordane Mirex p, p -DDE o, p -DDE p, p -DDD o, p -DDT p, p -DDT cis-chlordane trans-chlordane Mirex
65.68±0.97 52.80±6.92 85.01±12.05 57.67±2.52 65.37±8.39 52.68±2.80 83.42±2.40 63.96±2.74 49.43±2.37 67.23±3.32 65.03±2.55 59.19±2.46 78.52±3.56 39.08±1.63 49.76±2.77 51.43±1.94 25.63±2.07 21.99±3.02 NDb) 28.88±1.83 37.37±2.56 17.57±0..89 41.32±2.30 ND
2nd
3rd
4th
5th
81.16±4.23 69.35±3.56 94.43±2.08 67.27±2.43 77.11±0.89 67.26±2.46 92.37±4.02 79.96±4.23 68.11±2.54 81.44±5.09 83.65±2.12 72.44±1.45 88.11±0.89 54.26±2.46 62.37±4.02 69.96±4.23 43.55±5.15 48.61±2.34 ND 51.27±3.22 57.66±3.34 31.43±3.79 67.45±3.30 ND
% 93.81±4.74 78.81±3.34 98.23±3.34 74.66±2.05 84.68±3.42 76.52±1.01 95.51±1.63 83.34±1.95 75.71±3.47 89.87±4.01 87.32±1.49 84.33±5.43 96.29±1.53 65.52±1.01 74.51±1.63 80.34±1.95 57.81±2.21 61.48±3.33 ND 62.34±5.37 66.84±3.44 42.61±2.88 79.50±3.39 ND
94.47±3.64 85.34±2.86 98.70±2.36 86.33±1.46 92.54±0.98 81.13±0.97 95.26±2.33 88.74±2.36 83.52±3.09 94.11±4.52 88.89±3.63 90.06±1.57 97.56±5.75 73.13±0.97 81.26±2.33 85.74±2.36 64.40±3.11 67.52±5.37 ND 68.77±4.73 74.21±2.12 50.83±3.33 87.26±2.56 ND
95.34±2.43 86.34±3.65 98.89±6.85 88.99±2.03 94.50±5.09 86.62±3.38 96.76±5.63 89.11±4.25 85.81±2.56 94.34±4.54 88.18±2.82 92.43±6.74 96.73±4.55 80.62±1.38 83.51±6.46 86.54±6.53 71.44±3.52 69.24±2.51 ND 73.82±4.42 79.22±2.14 58.13±2.31 92.61±4.42 ND
a) DDE b) Not
= dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; DDT = dichlorodiphenyltrichloroethane. detected
180 min, mixing speed of 100 r min−1 , solution-to-soil ratio of 10:1, and temperature of 50 ◦ C) were selected for extraction of POPs from the contaminated soils with 100 mL L−1 PE 2, which had a high removal efficiency and cost-effectiveness. The selected parameters demonstrated broad-spectrum effectiveness and excellent OCP extraction efficiency from the contaminated soils with different characteristics. Further studies should consider the feasibility on a larger, more technical scale, as well as the secondary effects of PE 2 on soil structure and soil fauna. REFERENCES Aydin, M. E., Tor, A. and Ozcan, S. 2006. Determination of selected polychlorinated biphenyls in soil by miniaturised ultrasonic solvent extraction and gas chromatography-massselective detection. Anal. Chim. Acta. 577: 232–237. Chang, M. C., Huang, C. R. and Shu, H. Y. 2000. Effects of surfactants on extraction of phenanthrene in spiked sand. Chemosphere. 41: 1295–1300. ¨ 2005. Cornelissen, G., Haftka, J., Parsons, J. and Gustafsson, O. Sorption to black carbon of organic compounds with varying polarity and planarity. Environ. Sci. Technol. 39: 3688– 3694. Dumitran, C., Onut¸u, I. and Dinu, F. 2009. Extraction of hy-
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surfactant washing in batch experiments. Water Air Soil Pollut. 151: 71–86. Spark, K. M. and Swift, R. S. 2002. Effect of soil composition and dissolved organic matter on pesticide sorption. Sci. Total Environ. 298: 147–161. Sun, M. M., Fu, D. Q., Teng, Y., Shen, Y. Y., Luo, Y. M., Li, Z. G. and Christie, P. 2011. In situ phytoremediation of PAHcontaminated soil by intercropping alfalfa (Medicago sativa L.) with tall fescue (Festuca arundinacea Schreb.) and associated soil microbial activity. J. Soil. Sediment. 11: 980– 989. Sun, M. M., Luo, Y. M., Christie, P., Jia, Z. J., Li, Z. G. and Teng, Y. 2012. Methyl-β-cyclodextrin enhanced biodegradation of polycyclic aromatic hydrocarbons and associated microbial activity in contaminated soil. J. Environ. Sci. 24: 926–933 U.S. EPA. 1996. U.S. EPA SW-846. Test Methods for Evaluating Solid Waste. Method 3630. Silica Gel Cleanup. Revision 3. U.S. Government Printing Office, Washington D.C. Wan, J. Z., Yuan, S. H., Mark, K. T., Chen, J., Li, T. P., Lin, L. and Lu, X. H. 2009. Enhanced washing of HCB contaminated soils by methyl-β-cyclodextrin combined with ethanol. Chemosphere. 75: 759–764. Wang, C., Lu, G. H., Wang, P. F., Wu, H., Qi, P. D. and Liang, Y. 2011. Assessment of environmental pollution of Taihu lake by combining active biomonitoring and integrated biomarker response. Environ. Sci. Technol. 45: 3746–3752. Wang, W. T., Meng, B. J., Lu, X. X., Liu, Y. and Tao, S. 2007. Extraction of polycyclic aromatic hydrocarbons and organochlorine pesticides from soils: A comparison between Soxhlet extraction, microwave-assisted extraction and accelerated solvent extraction techniques. Anal. Chim. Acta. 602: 211–222. Wang, X. J., Piao, X. Y., Chen, J., Hu, J. D., Xu, F. L. and Tao, S. 2006. Organochlorine pesticides in soil profiles from Tianjin, China. Chemosphere. 64: 1514–1520. Wong, M. H., Leung, A. O. W., Chan, J. K. Y. and Choi, M. P. K. 2005. A review on the usage of POP pesticides in China, with emphasis on DDT loadings in human milk. Chemosphere. 60: 740–752. Wu, G. Z., Li, X. G., Coulon, F., Li, H., Lian, J. Y. and Sui, H. 2011. Recycling of solvent used in a solvent extraction of petroleum hydrocarbons contaminated soil. J. Hazard. Mater. 186: 533–539. Xu, S. S., Liu, W. X. and Tao, S. 2006. Emission of polycyclic aromatic hydrocarbons in China. Environ. Sci. Technol. 40: 702–708. Yang, L. Y., Xia, X. H., Liu, S. D. and Bu, Q. W. 2010. Distribution and sources of DDTs in urban soils with six types of land use in Beijing, China. J. Hazard. Mater. 174: 100– 107. Yang, X. L., Wang, F., Gu, C. G. and Jiang, X. 2010. Tenax TA extraction to assess the bioavailability of DDTs in cotton field soils. J. Hazard. Mater. 179: 676–683. Zhang, G., Parker, A., House, A., Mai, B., Li, X., Kang, Y. and Wang, Z. 2002. Sedimentary records of DDT and HCH in the Pearl River delta, South China. Environ. Sci. Technol. 36: 3671–3677. Zhao, B. W., Zhu, L. Z. and Gao, Y. Z. 2005. A novel solubilization of phenanthrene using Winsor I microemulsion-based sodium caster oil sulfate. J. Hazard. Mater. B119: 205– 211. Zhu, Y. F., Liu, H., Xi, Z. Q., Cheng, H. X. and Xu, X. B. 2005. Organochlorine pesticides (DDTs and HCHs) in soils from the outskirts of Beijing, China. Chemosphere. 60: 770–778.
Use of Organic Solvents to Extract Organochlorine Pesticides (OCPs) from Aged Contaminated Soils∗1 YE Mao1 , YANG Xing-Lun1 , SUN Ming-Ming1 , BIAN Yong-Rong1 , WANG Fang1 , GU Cheng-Gang1 , WEI Hai-Jiang1 , SONG Yang1 , WANG Lei1 , JIN Xin1 and JIANG Xin1,∗2 State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008 (China) (Received June 24, 2012; revised November 5, 2012)
ABSTRACT Problems associated with organochlorine pesticide (OCP)-contaminated sites in China have received wide attention. To solve such problems, innovative ex-situ methods of site remediation are urgently needed. We investigated the feasibility of the extraction method with different organic solvents, ethanol, 1-propanol, and three fractions of petroleum ether, using a soil collected from Wujiang (WJ), China, a region with long-term contamination of dichlorodiphenyltrichloroethanes (DDTs). We evaluated different influential factors, including organic solvent concentration, washing time, mixing speed, solution-to-soil ratio, and washing temperature, on the removal of DDTs from the WJ soil. A set of relatively better parameters were selected for extraction with 100 mL L −1 petroleum ether (60–90 ◦ C): washing time of 180 min, mixing speed of 100 r min−1 , solution-to-soil ratio of 10:1, and washing temperature of 50 ◦ C. These selected parameters were also applied on three other seriously OCP-polluted soils. Results demonstrated their broad-spectrum effectiveness and excellent OCP extraction performance on the contaminated soils with different characteristics. Key Words:
dichlorodiphenyltrichloroethane, ex-situ soil washing, extraction performance, petroleum ether, site remediation
Citation: Ye, M., Yang, X. L., Sun, M. M., Bian, Y. R., Wang, F., Gu, C. G., Wei, H. J., Song, Y., Wang, L., Jin, X. and Jiang, X. 2013. Use of organic solvents to extract organochlorine pesticides (OCPs) from aged contaminated soils. Pedosphere. 23(1): 10–19.
INTRODUCTION Organochlorine pesticides (OCPs) are a set of persistent organic pollutants (POPs) that are toxic, persist in the environment for long periods of time, and biomagnify through the food chain (Wang et al., 2011). The amount of dichlorodiphenyltrichloroethane (DDT) produced and applied in China before it was banned in 1983 is 0.4 million tons, accounting for 20% of the total global production (Zhang et al., 2002; Nakata et al., 2005). Technical chlordane is still being extensively used as a termiticide at 500–800 t year−1 in China in agriculture, gardens, and buildings (POPs Research Center, 2005). Mirex is widely produced and used as a termiticide at up to 30 t year−1 while banned in agriculture (Wong et al., 2005). With the signing of the Stockholm Convention and the development of global monitoring programs, many OCP-contaminated sites are left by hundreds of abandoned OCP production factories in large cities in China (Sun et al., 2012). Currently, most of these contaminated sites face ∗1 Supported
urgent land use conversion and redevelopment for commerce (Li et al., 2008; Fu et al., 2009; Yang, L. Y. et al., 2010; Yang, X. L. et al., 2010), and likely pose a threat on residents and the environment. Chinese soil engineers need to do research on and develop new soil remediation techniques suitable for specific national conditions in order to cope with these emergent problems (Zhu et al., 2005; Karstensen et al., 2006; Yang, L. Y. et al., 2010; Yang, X. L. et al., 2010; Sun et al., 2011) A potential technique for rapid removal of OCPs from soils is ex-situ soil washing with organic solvents since it has relatively high removal capacity as well as cost-effectiveness (Meguro et al., 2008; Wan et al., 2009). As desorption of organic contaminant is a ratelimiting step, water-miscible organic solvents are generally selected for their ability to transport pollutants from solid phase to aqueous bulk phase, and to increase the solubility of the organic contaminant in the liquid phase (Silva et al., 2005; Frankki et al., 2007). The use of organic solvents has been investigated at many POP-polluted sites; for instance, Smith et al. (2004)
by the National High Technology Research and Development Program of China (No. 2009AA063103) and the National Natural Science Foundation of China (No. 41030531). ∗2 Corresponding author. E-mail: [email protected].
SOLVENT EXTRACTION OF PESTICIDES FROM SOILS
investigated the effectiveness of different concentrations of ethanol, 1-propanol, and 2-propanol in removing p, p -DDT from a soil contaminated with DDTs for nearly 40 years and Jonssonly et al. (2010) reported extraction of approximately 80% polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) from a contaminated soil in the presence of 75% ethanol at 60 ◦ C after 10 washing cycles. The extraction of polynuclear aromatic hydrocarbons (PAHs) from aged field soils in the presence of different organic solvents (i.e., ethyl acetate, acetone, ethanol, 2-propanol, and 1-pentanol) has also shown promising results in several studies (Khodadoust et al., 2000; Lee and Hosomi, 2001; Nam et al., 2001; Silva et al., 2005). Most previous studies differ in their focus on influencing factors (Chang et al., 2000; Juhasz et al., 2003; Zhao et al., 2005; Haapea et al., 2006; Peng et al., 2011) but there have been relatively few systematic studies conducted on these factors, especially those that attempt to select a set of broad-spectrum washing parameters for extracting contaminants from different OCP-contaminated soils. Therefore, the aim of this study was to assess the potential applicability of organic solvent washing as a remediation technique for OCP-contaminated soils. The effects of certain factors, organic solvent concentration, washing time, mixing speed, solution-tosoil ratio, and washing temperature, on OCP-removal efficiency were examined with five organic solvents, i.e., ethanol, 1-propanol, and three different petroleum ether fractions (30–60, 60–90, and 90–120 ◦ C). A set of selected parameters were tested for the robustness and potential of the technique on other three contaminated soils. MATERIALS AND METHODS Chemicals, reagents and soil samples Standard samples of o, p -DDT, p, p -DDT, p, p dichlorodiphenyldichloroethane (DDD), o, p -DDT, p, p -DDT, cis-chlordane, trans-chlordane, and mirex (purity > 99.5%) were purchased from Dr. Ehrenstorfer (Augsburg, Germany). Hexane (HPLC-grade) was purchased from the Tedia Company, USA. Ethanol, 1-propanol, 30–60 ◦ C petroleum ether (PE 1), 60– 90 ◦ C petroleum ether (PE 2), and 90–120 ◦ C petroleum ether (PE 3) (Nanjing Chemical Reagent Co., China) are all analytical grade. Acetone and dichloromethane (analytical grade, Nanjing Chemical Reagent Co., China) were distilled prior to use. Anhydrous sodium sulfate (Na2 SO4 , Nanjing Chemical Reagent Co., China) was oven-dried at 400 ◦ C for 4
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h before use. Silica gel (100 mesh) was activated at 130 ◦ C for 18 h and deactivated using 3.3% deionized water based on the EPA standard method (U.S. EPA, 1996). Other chemical reagents were of analytical grade, as required. All soil samples were taken from four abandoned pesticide factories, which produced OCPs between the 1960s and the 1980s, located in the old city district of Wujiang (WJ), Liyang (LY), Lishui (LS), and Yancheng (YC), Jiangsu Province, China. Soil samples were collected from the surface to a depth of 200 mm, where a relatively higher concentration of OCPs was detected. All soil samples were air dried for 7 d, then homogenized, ground to pass a 2-mm sieve, and stored in glass bottles at 4 ◦ C until analysis. The physicochemical properties of the soils and the concentrations of OCPs are given in Table I. Soil washing Organic solvent concentration. The WJ soil (25 g) was placed into a 1-L flask with Teflon-lined caps, to which 250 mL each of ethanol, 1-propanol, PE 1, PE 2, and PE 3 were added. The operating parameters, namely, the volume fractions of organic solvents divided by the total volume fraction of distilled water plus organic solvents, were adjusted in terms of organic solvent concentrations as 10, 50, 100, 20, 300, 400, and 500 mL L−1 . The flask was shaken in a temperatureregulated shaker (20 ± 2 ◦ C) at a fixed mixing speed of 100 r min−1 for 60 min. Then, the soil suspensions were centrifuged at 2 500 r min−1 for 30 min. The soil and supernatant were separated for further OCP extraction and analysis. Each treatment was performed in triplicate. Washing time. Various time intervals were investigated at a solution-to-soil ratio of 10:1 (v/v). Soil suspensions containing the WJ soil (25 g) with selected concentrations of the five organic solvents were prepared in triplicate and shaken at a fixed mixing speed of 100 r min−1 for 1, 5, 10, 20, 30, 60, 180, 360, and 720 min. At each interval, the soil suspensions were centrifuged and separated for analysis. Mixing speed. To study the effect of mixing speed, the WJ soil samples (25 g) at a solution-tosoil ratio of 10:1 (v/v) with selected concentrations of the five organic solvents were prepared in triplicate, shaken at a speed of 0, 50, 100, 150, 200, and 250 r min−1 for 60 min, and then centrifuged and separated for analysis. Solution-to-soil ratio. To understand the effect of solution-to-soil ratio (v/v), the WJ soil (25 g) samples at four solution-to-soil ratios of 2:1, 5:1, 10:1, and
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M. YE et al.
TABLE I Physico-chemical properties and the concentrations of organochlorine pesticides (OCPs) in the soils collected from different sites selected Item
Soil sampling site Wujiang
Liyang
Lishui
Yancheng
Soil property Organic matter (g kg−1 ) Sand (%) Silt (%) Clay (%) pH CEC (cmol kg−1 ) Texture
6.6 7.7 66.8 25.5 6.91 18 Sandy silt
10.2 9.7 53.2 37.1 6.79 8.91 Sandy silt
43.5 5.9 77.5 16.6 6.85 9.31 Sandy silt
66.2 5.1 37.2 57.7 6.23 9.50 Clay silt
OCP (mg kg−1 dry soil)a) p, p -DDE o, p -DDE p, p -DDD o, p -DDT p, p -DDT cis-chlordane trans-chlordane Mirex Total
1.69±0.09 NDb) 15.55±0.89 6.92±0.35 58.81±3.80 ND ND ND 82.99±4.23
48.15±3.67 76.08±1.96 1.62±0.04 122.84±9.14 47.15±1.72 25.37±1.23 5.32±0.34 19.63±0.92 346.16±14.23
240.71±19.68 44.37±1.29 12.66±0.26 122.11±6.45 31.53±0.79 460.90±12.08 270.17±4.30 212.92±8.37 1 395.37±26.05
1 109.73±15.90 1 060.21±38.90 ND 3 295.53±136.49 502.33±7.47 2 427.90±104.22 289.43±11.30 ND 8 685.13±268.71
Aging time (years)
25
20
20
30
a) DDE b) Not
= dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; DDT = dichlorodiphenyltrichloroethane. detected.
20:1 with selected concentrations of the five organic solvents were prepared in triplicate. The soil solution was shaken for 60 min, and then centrifuged and separated for analysis. Washing temperature. Various washing temperatures (30, 40, 50, 60, and 70 ◦ C) were investigated at a solution-to-soil ratio of 10:1 (v/v). The WJ soil samples (25 g) with selected concentrations of the five organic solvents were prepared in triplicate. Soil suspensions were shaken for 60 min, and then centrifuged and separated for analysis. Performance assessment. After studying the factors influencing the removal of OCPs from the WJ soil, a set of washing parameters were selected for one promising solvent based on OCP removal efficiency and cost-effectiveness. To testify the applicability and robustness, five sequential washing cycles were performed on other three OCP-polluted soils (LY, LS, and YC soils). At the end of each washing cycle, the soil suspensions were treated as mentioned above. OCP extraction and analysis Sample extraction and cleanup. With the approach of accelerated solvent extractions (ASE), the total soil OCP extraction was performed with an ASE200 accelerated solvent extraction system (Dionex, USA) at a temperature of 100 ◦ C, pressure of 1 500
psi (10.345 MPa), and static time of 5 min. Hexane/acetone (1:1, v/v) was used as extraction solvent. To eliminate water, about 2 g of Na2 SO4 was added into each vial of the soil extracts. Then the extract was concentrated first to 2 mL by a rotary evaporator, and cleaned up with a Florisil solid phase extraction (SPE) cartridge (1 g, 6 mL, Supelco, USA). After the sample was transferred onto the cartridge, it was then eluted with 50 mL acetone/hexane (2:98, v/v). The elution was concentrated to 1 mL for gas chromatograph (GC) analysis. To assure data quality, spike recovery samples containing known quantities of the OCP standard were used to evaluate the extraction efficiency. OCP analysis. The OCP concentrations were measured by gas chromatograph (Agilent 6890, Agilent Technologies, Inc., USA) equipped with a DB5 capillary column (30 m length × 0.32 mm inside diameter × 0.25 μm film thickness), a 63 Ni electron capture detector, and an HP 7683 auto-sampler (HP Company, USA). The carrier gas was nitrogen with a flow of 0.7 mL min−1 . The injector and detector temperatures were 225 and 300 ◦ C, respectively. The oven temperature program consisted of an initial temperature of 60 ◦ C for 1 min, which was increased to 100 ◦ C at a rate of 20 ◦ C min−1 , held for 2 min, to 160 ◦ C at a rate of 10 ◦ C min−1 , to 230 ◦ C at 4 ◦ C min−1 , held for 5 min, and finally to 280 ◦ C at 10 ◦ C min−1 . The
SOLVENT EXTRACTION OF PESTICIDES FROM SOILS
injection volume was 1 μL in a splitless mode. Quality control. To confirm the OCP analysis results, every set of ten samples a procedural/laboratory blank and standard sample was run. Method performance was assessed by quality parameters such as recovery, repeatability, and limits of detections (LODs) and quantification (LOQs). LODs and LOQs were calculated on the basis of signal-to-noise ratio (S/N) of 3 and 10, respectively. Recovery was assessed by analyzing uncontaminated soil samples (n = 6) spiked with each of the studied OCPs, including p, p -DDT, p, p -dichlorodiphenyldichloroethylene (DDE), p, p -DDD, o, p -DDT, cis-chlordane, transchlordane, and mirex, at 50, 100, 200, 500, and 1 000 ng g−1 . Mean recovery of OCPs in the spiked soil samples was 94% ± 8% for DDTs, 101% ± 12% for cischlordane, 95% ± 4% for trans-chlordane, and 92% ± 14% for mirex. In addition, the analytical precision of the results for the soil replicates (n = 3) with relative standard deviations (RSD) ranging from 0.5% to less than 5% was acceptable. Calculation and statistical analysis The OCP removal efficiency (RE) could be obtained from the residual OCP concentrations in soil
13
samples (ROCP) and the initial OCPs concentrations in the soil (IOCP): RE =
IOCP − ROCP × 100% IOCP
(1)
Statistical analysis was carried out using the SPSS 14.0 software package for Windows. Mean values were compared using the least significant difference (LSD) test at the probability level ≤ 5%. RESULTS AND DISCUSSION Organic solvent concentration Fig. 1 illustrated that higher concentrations of the five organic solvents resulted in higher DDT removal efficiency from the WJ soil, but their performances were not the same. For example, the addition of 100 mL L−1 PE 1 resulted in the desorption of 75% p, p -DDE, 74% p, p -DDD, 69% o, p -DDT, and 76% p, p -DDT, compared with 9% and 31% p, p -DDE, 9% and 9% p, p -DDD, 13% and 20% o, p -DDT, and 0% and 15% p, p -DDT when using 100 mL L−1 ethanol and 100 mL L−1 1-propanol, respectively. A similar phenomenon was observed for 100 mL L−1 PE 2 and 100 mL L−1 PE 3. Many researches have demonstrated that petro-
Fig. 1 Effects of different organic solvents of different concentrations on organochlorine pesticide (OCP) desorption from the soil collected at Wujiang. DDT = dichlorodiphenyltrichloroethane; DDE = dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; PE 1 = 30–60 ◦ C petroleum ether; PE 2 = 60–90 ◦ C petroleum ether; PE 3 = 90–120 ◦ C petroleum ether. Error bars represent the standard deviations of the means (n = 3).
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leum ether showed excellent extraction performance for OCP-contaminated soils (Aydin et al., 2006; Dumitran et al., 2009; Ozcan et al., 2009), but an excessive volume was consumed to achieve a high extraction efficiency during lab analysis. In our study, we used petroleum ether mixed with distilled water as the washing solution, which manifested three mainly distinct advantages. Firstly, less organic extraction solvent was used in such a washing system, soil particles dispersed evenly in the ex-situ soil washing system, and also petroleum ether could contact with the contaminated soil intimately and constantly. Secondly, the adsorption of petroleum ether by soil particles could be reduced because water has a barrier effect on the sorption of petroleum ether to soil particles as well as on soil structure and soil fauna (Lee and Hosomi, 2001; Wu et al., 2011). Thirdly, the extraction performances of 100 mL L−1 PE 1, 100 mL L−1 PE 2, and 100 mL L−1 PE 3 were comparable to those of 500 mL L−1 ethanol and 300 mL L−1 1-propanol. Therefore, considering the potential extraction efficiency and cost of the five organic solvents, 100 mL L−1 PE 1, 100 mL L−1 PE 2, 100 mL L−1 PE 3, 500 mL L−1 ethanol, and 300 mL L−1 1-propanol were chosen as the washing solutions for the tests on the effects of other factors on extraction efficiency.
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Washing time Kinetic studies were conducted using the five selected washing solutions to determine if the extraction time would affect DDT removal from the WJ soil (Fig. 2). The changes in removal efficiency of DDTs with time were similar for the five organic solvents. About 60% DDTs desorbed quickly during the first 30 min of extraction. Increasing extraction time from 30 to 180 min resulted in a slow increase of the DDT removal percentage. However, after 180 min, only a small percentage increase of DDTs desorbed was observed. Desorption is of particular importance in exsitu soil washing because desorption is a rate-limited factor for extraction in most cases (Kraaij et al., 2002; Lepp¨ anen et al., 2003; Semple et al., 2004). Desorption of contaminants from soil particles generally proceeds an initial rapid desorbing phase followed by the slow and then very slow desorbing phase (Pignatello and Xing, 1995), which in fact is a balanced redistribution of contaminants between soil particles and washing solution. Once the contaminants achieve a steady state during the washing process, removal efficiency reaches a maximum value. Thus, 180 min was chosen as the relative better extraction time for the five organic solvents.
Fig. 2 Desorption kinetics of organochlorine pesticides (OCP) from the soil collected at Wujiang with selected organic solvents of a given concentration. DDT = dichlorodiphenyltrichloroethane; DDE = dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; PE 1 = 30–60 ◦ C petroleum ether; PE 2 = 60–90 ◦ C petroleum ether; PE 3 = 90–120 ◦ C petroleum ether. Error bars represent the standard deviations of the means (n = 3).
SOLVENT EXTRACTION OF PESTICIDES FROM SOILS
Mixing speed Residual DDTs in the WJ soil decreased with the increase in mixing speed from 0 to 100 r min−1 for the five selected washing solutions (Fig. 3), which indicated that DDTs extracted increased with increasing mixing speed. However, the speed higher than 100 r min−1 did not enhance the DDT extraction efficiency remarkably. Therefore, from the perspective of energy saving, the mixing speed of 100 r min−1 was selected for further potential application. Solution-to-soil ratio Solution-to-soil ratios are an important parameter in ex-situ soil washing. Generally, higher solution-tosoil ratios lead to higher desorption of contaminants. It can be seen from Fig. 4 that extraction of 30%–46% p, p -DDE, 26%–41% p, p -DDD, 18%–33% o, p -DDT, and 11%–34% p, p -DDT was achieved at the solutionto-soil ratio of 2:1 for the five selected solvents, while the removal efficiency up to 80%–98% p, p -DDE, 84%– 95% p, p -DDD, 75%–94% o, p -DDT, and 73%–94% p, p -DDT was achieved at 20:1. Although the highest DDT removal efficiency occurred at a solution-to-soil ratio of 20:1, a higher solution-to-soil ratio would pose a higher requirement for the equipment and energy and
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generate more wastewater for post-treatment. Therefore, the solution-to-soil ratio of 10:1 could be taken as the relatively better parameter in this case. Temperature Fig. 5 revealed that the DDT removal performance from the WJ soil was affected by temperature within the tested range. An increase in temperature from 20 to 70 ◦ C for 500 mL L−1 ethanol promoted the amounts of p, p -DDE, p, p -DDD, o, p -DDT, and p, p DDT desorbed from 78% to 99%, 78% to 95%, 77% to 95%, and 67% to 98%, respectively. In addition, In the case of 300 mL L−1 1-propanol at a temperature of 70 ◦ C, approximately 99% p, p -DDE, 97% p, p DDD, 93% o, p -DDT, and 96% p, p -DDT was desorbed, compared with approximately 77% p, p -DDE, 80% p, p -DDD, 77% o, p -DDT, and 64% p, p -DDT at 20 ◦ C. Rising temperatures in this range played a positive role in desorption of DDTs for the two selected solvents. This can be attributed to the fact that an increased temperature decreased the partition coefficient of the contaminants between soil particles and aqueous phase, thereby increasing the desorption rate of the DDTs from the soil to the aqueous phase. Raising temperature, however, had different influences on DDTs removal efficiency when using the three
Fig. 3 Effect of mixing speed on organochlorine pesticide (OCP) desorption from the soil collected at Wujiang with selected organic solvents of a given concentration. DDT = dichlorodiphenyltrichloroethane; DDE = dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; PE 1 = 30–60 ◦ C petroleum ether; PE 2 = 60–90 ◦ C petroleum ether; PE 3 = 90–120 ◦ C petroleum ether. Error bars represent the standard deviations of the means (n = 3).
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Fig. 4 Effect of solution-to-soil ratios from 2:1 to 20:1 on organochlorine pesticide (OCP) desorption from the soil collected at Wujiang with selected organic solvents of a given concentration. DDT = dichlorodiphenyltrichloroethane; DDE = dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; PE 1 = 30–60 ◦ C petroleum ether; PE 2 = 60–90 ◦ C petroleum ether; PE 3 = 90–120 ◦ C petroleum ether. Error bars represent the standard deviations of the means (n = 3).
different petroleum ether fractions. The temperature increase from 20 to 70 ◦ C with 100 mL L−1 PE 1 resulted in a decrease in the removal of DDTs. This result may be caused by the intrinsic nature of PE 1 (30–60 ◦ C) which is very volatile and will limit its use. With a rise in temperature from 20 to 50 ◦ C, 100 mL L−1 PE 2 resulted in increasing desorption of p, p DDE, p, p -DDD, o, p -DDT, and p, p -DDT from 73% to 90%, 72% to 81%, 68% to 84%, and 62% to 80%, respectively. It may be due to the fact that the temperature rise was conducive to desorption of DDTs and accelerated the transfer of the contaminants from soil particles to PE 2. However, from 50 to 70 ◦ C, the DDT removal efficiency encountered a slight decrease, which may be attributed to the evaporation of PE 2 (60–90 ◦ C) at a higher range of temperature. With 100 mL L−1 PE 3, the DDT removal efficiency increased only slightly within the tested temperature range, which may be caused by the much more matchable response of the boiling range of PE 2 (60–90 ◦ C) with the operational temperature (30–70 ◦ C) than PE 1 and PE 3. In
short, the results indicated that 100 mL L−1 PE 2 (60– 90 ◦ C) showed an excellent extraction performance at a temperature of 50 ◦ C and demonstrated more potential than other four organic solvents. Therefore, 50 ◦ C was selected as a suitable temperature for extraction with 100 mL L−1 PE 2. Performance assessment By comparing the effects of different factors on the removal of DDTs from the WJ soil, a set of washing parameters, washing time of 180 min, mixing speed of 100 r min−1 , solution-to-soil ratio of 10:1, and tempera ture of 50 ◦ C, are selected for soil washing with 100 mL L−1 PE 2, which had a good removal efficiency and low cost. To verify the feasibility, five sequential washing cycles were applied to three other OCP-polluted soils (LY, LS, and YC soils) with varying characteristics and contamination levels. The characteristics of the three soils are given in Table I. Table II demonstrated that a high OCP removal
SOLVENT EXTRACTION OF PESTICIDES FROM SOILS
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Fig. 5 Effect of temperature on organochlorine pesticide (OCP) desorption from the soil collected at Wujiang with selected 5 organic solvents of a given concentration. DDT = dichlorodiphenyltrichloroethane; DDE = dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; PE 1 = 30–60 ◦ C petroleum ether; PE 2 = 60–90 ◦ C petroleum ether; PE 3 = 90–120 ◦ C petroleum ether. Error bars represent the standard deviations of the means (n = 3).
efficiency could be achieved under the selected procedure. The OCPs were most efficiently removed from the LY soil (Table II), a sandy silt soil with an organic matter content of 10.2 g kg−1 and an OCP concentration of 346.16 mg kg−1 dry soil. Approximately 94% of the total OCPs were extracted after five washing cycles. Up to 85% of the total OCPs were removed from the LS soil which was rich in organic matter (43.5 g kg−1 ) and contained total OCPs of 1 395.37 mg kg−1 dry soil. The soil with the lowest total OCP removal was the YC soil (75% removal efficiency), a clay soil with a high organic matter content (66.2 g kg−1 ) and a high OCP concentration of 8 685.13 mg kg−1 dry soil. The sandy characteristics could be the reason for the high removal efficiency of OCPs from the LY soil (Tables I and II). Extraction techniques such as solvent washing are often most efficient when applied to coarse soils (Freeman and Harris, 1995) because the washing solvent can penetrate the particles and effectively desorb the contaminants. In addition, the larger the soil particles, the smaller the surface area and the lower the abundance of contaminant sorption sites and the adsorptive capacity of the soil (Khodadoust et al., 1999; Frankki et al., 2007). In literature, it was reported that less of the contaminant is desorbed in soils with higher organic matter contents (Wang et al., 2006,
2007; Xu et al., 2006). These observations are consistent with the high extraction efficiency from the LY soil, but not with the high extraction efficiency from the LS soil. The lowest removal efficiency was from the YC soil, a clay silt soil with the highest organic matter content, probably due to entrapment of contaminants in clay and silt conglomerates and the strong affinity of OCPs to the organic matter, particularly to organic carbon (Cornelissen et al., 2005; Palomo and Bhandari, 2006). Results in Table II also demonstrated that multiple washing cycles could remove more, but not all, of OCPs from the contaminated soils, especially the heavily polluted soils such as the LS and YC soils. Several factors such as the chemical nature of the contaminants, the different initial concentrations in the soil, and the duration of soil contamination can explain the different washing results (Spark and Swift, 2002; Gevao et al., 2003). Considering that the OCP contamination of the three soils occurred more than 20 years prior to the present study, it was possible that a long period of contact increased adsorption of the contaminants. CONCLUSIONS A set of soil washing parameters (washing time of
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TABLE II Removal efficiency of organochlorine pesticides (OCPs) from three soils during sequential washing cycles at a mixing speed of 100 r min−1 , washing time of 180 min, solution-to-soil ratio of 10:1, and 50 ◦ C with 100 mL L−1 60–90 ◦ C petroleum ether Soil sampling site
OCPa)
Washing cycle 1st
Liyang
Lishui
Yancheng
p, p -DDE o, p -DDE p, p -DDD o, p -DDT p, p -DDT cis-chlordane trans-chlordane Mirex p, p -DDE o, p -DDE p, p -DDD o, p -DDT p, p -DDT cis-chlordane trans-chlordane Mirex p, p -DDE o, p -DDE p, p -DDD o, p -DDT p, p -DDT cis-chlordane trans-chlordane Mirex
65.68±0.97 52.80±6.92 85.01±12.05 57.67±2.52 65.37±8.39 52.68±2.80 83.42±2.40 63.96±2.74 49.43±2.37 67.23±3.32 65.03±2.55 59.19±2.46 78.52±3.56 39.08±1.63 49.76±2.77 51.43±1.94 25.63±2.07 21.99±3.02 NDb) 28.88±1.83 37.37±2.56 17.57±0..89 41.32±2.30 ND
2nd
3rd
4th
5th
81.16±4.23 69.35±3.56 94.43±2.08 67.27±2.43 77.11±0.89 67.26±2.46 92.37±4.02 79.96±4.23 68.11±2.54 81.44±5.09 83.65±2.12 72.44±1.45 88.11±0.89 54.26±2.46 62.37±4.02 69.96±4.23 43.55±5.15 48.61±2.34 ND 51.27±3.22 57.66±3.34 31.43±3.79 67.45±3.30 ND
% 93.81±4.74 78.81±3.34 98.23±3.34 74.66±2.05 84.68±3.42 76.52±1.01 95.51±1.63 83.34±1.95 75.71±3.47 89.87±4.01 87.32±1.49 84.33±5.43 96.29±1.53 65.52±1.01 74.51±1.63 80.34±1.95 57.81±2.21 61.48±3.33 ND 62.34±5.37 66.84±3.44 42.61±2.88 79.50±3.39 ND
94.47±3.64 85.34±2.86 98.70±2.36 86.33±1.46 92.54±0.98 81.13±0.97 95.26±2.33 88.74±2.36 83.52±3.09 94.11±4.52 88.89±3.63 90.06±1.57 97.56±5.75 73.13±0.97 81.26±2.33 85.74±2.36 64.40±3.11 67.52±5.37 ND 68.77±4.73 74.21±2.12 50.83±3.33 87.26±2.56 ND
95.34±2.43 86.34±3.65 98.89±6.85 88.99±2.03 94.50±5.09 86.62±3.38 96.76±5.63 89.11±4.25 85.81±2.56 94.34±4.54 88.18±2.82 92.43±6.74 96.73±4.55 80.62±1.38 83.51±6.46 86.54±6.53 71.44±3.52 69.24±2.51 ND 73.82±4.42 79.22±2.14 58.13±2.31 92.61±4.42 ND
a) DDE b) Not
= dichlorodiphenyldichloroethylene; DDD = dichlorodiphenyldichloroethane; DDT = dichlorodiphenyltrichloroethane. detected
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