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Potential of different species for use in removal of DDT from the contaminated soils
Chemosphere 73 (2008) 120–125
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Potential of different species for use in removal of DDT from the contaminated soils Ce-Hui Mo a,*, Quan-Ying Cai b, Hai-Qin Li b,1, Qiao-Yun Zeng b, Shi-Rong Tang c, Yue-Chun Zhao b a b c
Department of Environmental Engineering, Jinan University, Guangzhou 510632, China College of Resources and Environment, South China Agricultural University, Guangzhou 510642, China Centre for Research in Ecotoxicology and Environmental Remediation, Institute of Agricultural Environmental Protection, The Ministry of Agriculture, Tianjin 300191, China
a r t i c l e
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Article history: Received 19 July 2007 Received in revised form 19 April 2008 Accepted 28 April 2008 Available online 16 June 2008 Keywords: Phytoextraction Organochlorine pesticide Removal rate Bioconcentration factor Translocation factor
a b s t r a c t Dichlorodiphenyltrichloroethane (DDT) and its main metabolites, p,p0 -DDD and p,p0 -DDE (DDTs in this study included DDT, DDD and DDE), are frequently detected in agricultural soils even though its usage in agriculture was banned in 1980s or earlier. In this study, eleven plants including eight maize (Zea mays) cultivars and three forage species (alfalfa, ryegrass and teosinte) widely cultivated in China were grown in the soils spiked with DDTs to investigate their potential for removal of DDT from the contaminated soils. The plants varied largely in their ability to accumulate and translocate DDTs, with the bioconcentration factor (BCF; DDT concentration ratio of the plant tissues to the soils) ranging from 0.014 to 0.25 and the translocation factor (TF; DDT concentration ratio of the shoots to the roots) varying from 0.35 (Zea mays cv Chaotian-23) to 0.76 (Zea mays spp. mexicana). The amount of DDT phytoextraction ranged from 3.89 lg (ryegrass) to 27.0 lg (teosinte) and accounted for <0.1% of the total initial DDTs spiked in the soils. After 70 d, the removal rates reached 47.1–70.3% of the total initial DDTs spiked in the soils with plants while that was only 15.4% in the soils without plant. Moreover, the higher removal rates of DDTs occurred at the first 20 d of experiment, and then the removal rate decreased with time. The highest amount of DDTs phytoextracted was observed in teosinte, followed by Zea mays spp. mexicana, but the highest removal rate of DDTs was found in maize (Zea mays cv Jinhai-6). Even though phytoextraction is not the main removal process for DDTs, the plant species especially Zea mays cv Jinhai-6 showed high potential for removing DDTs from the contaminated soils. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Dichlorodiphenyltrichloroethane (DDT) was one of the most widely used pesticides in agriculture in many countries until the 1970s. Estimates showed that China produced at least 435,200 tonnes of DDT and its main metabolites [1,1-dichloro-2,2-bis(4-chlorophenyl)ethylene, p,p0 -DDE; 1,1-dichloro-2,2-bis(4-chlorophenyl)ethane, p,p0 -DDD] from the 1950s to May 1983 when their production was banned, accounting for 20% of the global production (Hua and Shan, 1996). Part of DDT was released into the environment during the past decades and this trend continues, because of its usage as an anti-malaria agent or of an impurity in other pesticides such as dicofol (Wong et al., 2005). DDT and its two metabolites are classified as persistent organic pollutants (POPs) and their pollution is of worldwide concern due to their persistence in the environment, bioaccumulation, and negative effects on soil microbial, plant, animal life and humans
* Corresponding author. Tel./fax: +86 20 85226615. E-mail addresses: [email protected] (C.-H. Mo), [email protected] (Q.-Y. Cai), [email protected] (S.-R. Tang). 1 Present address: Jize Environmental Science and Technology Ltd., Kunshan 215316, China. 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.04.082
(Katsoyiannis and Samara, 2004; Turusov et al., 2002). Recently, high levels of DDT and its main metabolites have been frequently detected in various media such as soils, water, sediments and plants in China, as well as in other countries (Waliszewski et al., 2004; Chau, 2005; Nakata et al., 2005; Kurt-Karakus et al., 2006; Li et al., 2006; Katsoyiannis and Samara, 2007; Cai et al., 2008). Finding of higher DDT concentrations in the cultivated soils than in the non-cultivated ones reveals either that DDT was used in the past or that is currently illegally used for agricultural reasons (Chau, 2005). DDT residues in agricultural soils are of great concern due to the uptake of DDT by plants, accumulation in food chain, and re-emission from soils to the atmosphere. Thus, the removal of DDT and its metabolites from the agricultural environment has intrigued many scientists in recent years. Phytoremediation is an in situ and cost-effective technology in which the plants are applied to extract, directly or indirectly degrade or remove organic and inorganic contaminants from the contaminated natural media, including POPs such as DDT and its metabolites (Cunningham and Ow, 1996; Tang, 2005). Literature review, however, showed that few publications were available on phytoremediation of DDT in spite of its widespread occurrence (Garrison et al., 2000; Lunney et al., 2004). Garrison et al. (2000) reported biotransformation of DDT by plant cultures. White et al.
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(2005, 2006) investigated phytoextraction of p,p0 -DDE, and found that accumulation and translocation of DDT or DDE by plants were somewhat species-specific or cultivar-specific (Lunney et al., 2004). But until now, no literature reported which plant has the greatest potential for phytoremediation of DDT in soils. Further research should be done in this direction. The objectives of the present study were to investigate the uptake and translocation of DDT in soil-plant system, and to compare the potential of various plant species and cultivars endemic to China for removal of DDT from the contaminated soils. 2. Materials and methods 2.1. Chemicals and materials In order to compare the phytoextraction potential of different species and cultivars for DDT, two plant species were selected, including maize (Zea mays) and forage. The forage species included alfalfa (Medicago sativa), ryegrass (Lolium multiflorum), and teosinte (Zea mays ssp. parviglumis) which may have potential for phytoremediation of PAHs (Fan et al., 2007) or DDT (Lunney et al., 2004). Eight representative cultivars of maize were selected from the cultivars grown widely in China, namely, Jinhai-6 (MV1), Wanqin (MV2), Zea mays spp. mexicana (MV3), Huanong-1 (MV4). Huahai (MV5), Huidan (MV6), Baiyunuo (MV7) and Chaotian-23 (MV8). The latter was sweet maize, while the others were feed maize. All the plant seeds for experiment were obtained from Guangdong Academy of Agricultural Sciences, China. The DDTs (DDTs in this study stands for p,p0 -DDT, o,p0 -DDT, p,p0 DDE and p,p0 -DDD) of analytical grade for pot trial were obtained from Taigu Chemical Factory of Tianjin, China. A composite stock standard solution of DDTs (100 mg/l) for analysis was used, containing p,p0 -DDE, p,p0 -DDD and p,p0 -DDT. The working standard solutions were prepared by diluting appropriate volumes of the stock standard solution. 2,4,5,6-Tetrachloro-m-xylene (TCmX) and pentachloronitrobenzene were used as surrogate and internal standard solution, respectively. These standards were purchased from J&K Chemical Limited Co. (Beilinwei, Beijing, China). Another standard solution, o,p0 -DDT, was purchased from China’s Research Center of Standard Material. Analytical grade organic solvents including n-hexane, dichloromethane (DCM), methanol and acetone were redistilled prior to use. Silica gel (100–180 mesh, Guangzhou Chemical Reagent Co.) was Soxhlet-extracted with DCM and methanol, respectively, for 12 h, dried at 130–140 °C for 4 h before use. Neutral alumina (80–100 mesh, Guangzhou Chemical Reagent Co.) was activated at 250 °C, for 12 h. Anhydrous sodium sulfate was dried at 250 °C for 4 h and stored in a sealed desiccator. These materials were purchased from Guangzhou Chemical Reagent Co., China. All the glassware were soaked in K2CrO4–H2SO4 solution for 30 min, washed with tap water and redistilled water and then were dried at 250 °C for 2 h. 2.2. Experimental design The experiment was carried out in the glasshouse of South China Agricultural University, Guangzhou, China. The soil was paddy soil with 24.9 g/kg dry weight (d.w.) of organic matter, 1.02 g/kg (d.w.) of total N, 0.92 g/kg (d.w.) of total P, 19.35 g/kg (d.w.) of total K, 78.8 mg/kg (d.w.) of available N, 29.0 (d.w.) mg/kg of available P, 6.28 of pH and 310 g/kg (d.w.) of water-holding capacity. The soil was air-dried, crushed, mixed thoroughly and passed through a 5-mm sieve. There were 5.0 kg of soils in each pot. Prior to cultivation, the soil was fertilized 0.20 g/kg N, 0.15 g/kg P and 0.15 g/kg K with urea, superphosphate and potassium chloride, respectively.
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The stock solution of DDTs was prepared by diluting DDTs with acetone. The stock solution was mixed thoroughly with paddy soil (500 g, 2 mm) and allowed to evaporate for 48 h. The treated soil was mixed thoroughly with ‘‘clean” bulk soils (4500 g, 5 mm) to obtain the initial DDT concentration which were determined by Soxhlet extraction-gas chromatography coupled with electron capture detector (GC-ECD) as other samples. Considering the toxicity of DDTs to the plants and the concentrations used in the previous study (32.0–88.9 mg/kg d.w.) by Kiflom et al. (1999), the total initial concentration of DDTs in soils of the present study was 29.9 mg/kg (d.w.). Soils (5.0 kg) contaminated with DDTs were placed in a ceramic pot (27 17 cm, I.D. height). The pot trial included 11 treatments with different species or cultivars given above and a treatment without plant (defined as Control). Immediately after being prepared, they were arranged in a completely randomized block design with five replications in a glasshouse. The soils were kept at the water-holding capacity with redistilled water for seven days and then mixed thoroughly. Twenty seeds of maize were sown in each pot and, after germination, seedlings were manually thinned twice and three uniform seedlings were established per pot. Fifty seedlings of alfalfa or ryegrass were established to each pot. The redistilled water (200 ml) was supplied for each pot including the control treatment every day during the growth. There was no effluence from the bottom of the pots. No chemical pesticide was used. At 10, 20, 30, 50 and 70 d of growth, soil samples were carefully collected from the pot (at least 10 sites, randomly) using a 1.0 cm stainless steel sampler. The plants were harvested after 70 d of growth. The shoots (including stem and leaves) and roots of plants were carefully removed from the soils and their fresh weights were measured. The plant samples were rinsed with redistilled water, oven-dried at 55 °C and then ground (1-mm sieve). The soil samples (approximately 100 g) were air-dried, ground to pass through a 1-mm sieve, and refrigerated until analysis. 2.3. Analytical procedure, QA/QC measures and performances Sample extraction and cleanup were performed according to USEPA methods 3550B (Ultrasonic extraction) and 3630C (Silica gel column) with modification, respectively. Dried plant (10 g) and soil samples (20 g) spiked with surrogate were extracted in triplicate with 30 ml ethyl acetate and with 30 ml acetone/DCM mixture (1:1, v/v) in sonicator (SK5200H, China) for 20 min, respectively. After each extraction, separation was accomplished by centrifuging at 3000 rpm for 5 min. The supernatant was combined and concentrated carefully to 2 ml in a rotary vacuum evaporator (Yarong, China). The concentrated extracts were loaded on a combined column of silica gel and alumina. The glass chromatography column (25 1 cm I.D., Guangzhou, China), fitted with a Teflon stopcock, was packed, bottom-up, with cotton-wool (Soxhlet-extracted with DCM for 72 h before use), 3 cm alumina, 10 cm extracted silica, followed by 2 cm anhydrous sodium sulfate. DCM/n-hexane mixture (1:1, v/v, 50 ml) was used for elution. The collected extract was blown-down under a gentle stream of nitrogen (N2) and diluted with n-hexane to an appropriate volume. Measurements of DDTs in extract were performed following USEPA method 8081A with slight modification. The analysis was carried out by gas chromatography (GC, Hewlett–Packard 5890 Series II, Agilent Technology) coupled with electron capture detector (ECD). A HP-5 30 m 0.32 mm I.D., 0.17 lm membrane thickness (Agilent Technology, US) was used. The GC oven temperature was raised from 150 to 280 °C at 4.0 °C/min. Nitrogen (N2) was the carrier gas, at a flow of 2.5 ml/min. The injection was set on a splitless mode at 280 °C. The injection volume was 1.0 ll. A 63Ni electron capture detector was used and its temperature was at 300 °C.
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The instruments were calibrated daily and the relative percent differences between the six-point calibration and the daily calibrations were <15% for all target analytes. Identification of individual compounds was based on the comparison of retention time data between samples and the standard solution. The internal calibration method for quantification was applied based on seven-point calibration curve (0–400 lg/ml). Method blanks (solvent), duplicate samples and spiked blanks (standards spiked into solvent) were analyzed. In addition, surrogate standards were added to the samples to monitor procedural performance and matrix effects. The average recoveries were 75.0–89.5% for 2,4,5,6-tetrachloro-mxylene and 75.2–92.0% for DDTs in plant samples, while those were 75.7–86.5% for 2,4,5,6-tetrachloro-m-xylene and 83.2–102% for DDTs in soil samples, conforming to the reported ranges of USEPA method 8081A. 2.4. Statistical analysis Experimental results were expressed on a dry weight basis except the plant biomass. The sum concentrations of p,p0 -DDT, o,p0 DDT, p,p0 -DDE and p,p0 -DDD were expressed as RDDTs. Data was subjected to analysis including the analysis of variance (ANOVA) and mean values were compared by Duncan’s multiple range (P < 0.05) test performed using the statistics analysis system (SAS, version 8.0) for Windows software package. 3. Results and discussion 3.1. DDT concentrations in the soils Fig. 1a presents the concentrations of DDT residues in the soils after plant growth. The RDDTs values varied from 8.88 (MV1) to 25.3 mg/kg (Control), being considerably lower than the initial one (29.9 mg/kg) in the soils, and far lower than those observed in the soils after cowpeas growth (32.0–36.8 mg/kg) (Kiflom et al., 1999). This might be related to the experiment conditions and the initial soil concentration of DDT. In the study of Kiflom
Fig. 1. The concentrations of RDDTs residues in the soils (a) and plants (b).
et al. (1999), the initial soil concentrations of DDT were up to 60.4 and 88.9 mg/kg. The DDT concentrations of treatments after plant growth were significantly lower than that in control (without plant), especially the one of MV1. The same phenomenon was recorded by Kiflom et al. (1999). 3.2. Vegetation concentrations of DDTs The concentrations of DDTs in plant shoots and roots are showed in Fig. 1b. The data exhibited that the DDTs were taken up and translocated into the plants grown in the DDTs spiked soils. For all species or cultivars, the concentrations of RDDTs in roots were considerably higher than those in shoots, being similar to other plant species (e.g., tall fescue, mustard, crimson clover) in other studies (Kiflom et al., 1999; White et al., 2005, 2006), but different from the distribution in pigeonpea, zucchini, pumpkin (Lunney et al., 2004; White et al., 2005). This might be attributed to the variations in the degree of exposure of either part to DDTs (Kiflom et al., 1999). Moreover, the results of White et al. (2005) indicated that, under same exposure conditions (field plot), distributions of DDTs in shoots and roots were somewhat species-specific. The RDDTs accumulated by the shoots varied from 0.206 (MV1) to 1.23 mg/kg (alfalfa), while the RDDTs in the roots ranged between 0.385 (MV1) and 3.23 mg/kg (alfalfa), being comparable with the RDDTs in cowpeas (3.24–7.77 mg/kg) (Kiflom et al., 1999) or in the plants grown in high DDT-contaminated soils (Lunney et al., 2004). Those results indicated that maize and forage (e.g., alfalfa) have similar accumulation for DDTs to cowpeas etc which were planted by Kiflom et al. (1999) or Lunney et al. (2004). It should be pointed out that, in the present study, the initial concentration of DDTs in soils was 29.9 mg/kg, being seven times higher than that in the high DDT-contaminated soils (3.70 mg/kg) reported by Lunney et al. (2004) but far lower than those spiked by Kiflom et al. (1999). But the RDDTs in the roots of alfalfa and ryegrass (3.23 and 1.23 mg/kg) were remarkably lower than those in the abovementioned study (4.69 and 2.73 mg/kg), whereas the RDDTs in shoots of alfalfa and ryegrass (1.23 and 0.658 mg/kg) were substantially higher than those in the aforementioned study (0.026 and 0.060 mg/kg). This might be partly attributed to the different physico-chemical properties of the soils and subsequently the different bioavailability of DDTs and transport processes (Kiflom et al., 1999). In the present study DDTs were freshly spiked, while they are weathered DDTs in the soils of the study reported by Lunney et al. (2004), which might result in the different bioaccumulation in the same plant. As shown in Fig. 1b, the variation of RDDTs in the shoots and roots of different plants under investigation by a factor of >6, respectively, suggested that the ability of the plant species and cultivars to accumulate and translocate DDTs varied to a great extent. Bioconcentration factor (BCF), calculated as the dry weight DDT concentration ratio of plant tissue to that in the soils ([RDDTs]root or shoot/[RDDTs]soil), was applied to compare the relative abilities of different species and cultivars to take up and translocate DDTs in the shoots and roots. The average BCFs were lower than 1.0, even lower than 0.10, varying from 0.013 (MV4) to 0.094 (alfalfa) for the shoots and from 0.024 (MV4) to 0.25 (alfalfa) for the roots (Table 1). The BCFs for DDTs in this study were similar to those for cowpeas (Kiflom et al., 1999) or for rice (Yao et al., 2007), but considerably different from those reported for alfalfa and ryegrass in the study of Lunney et al. (2004), where most of BCFs for DDTs in the roots were greater than 1.0. These results confirmed again that there was variation in the accumulation of DDTs in plants grown in soils spiked with fresh DDTs and weathered DDTs. Translocation factor (TF), expressed as the DDT concentration ratio of the shoots to the roots, is another value of interest reflecting DDTs transfer to the shoots from the roots. The present study
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C.-H. Mo et al. / Chemosphere 73 (2008) 120–125 Table 1 Bioconcentration factor (BCF) and translocation factor (TF) for DDTs in different species and cultivars Plant species
Bioconcentration factor (BCF)a Shoot
MV1 MV2 MV3 MV4 MV5 MV6 MV7 MV8 Alfalfa Ryegrass Teosinte
0.023 ± 0.002 0.014 ± 0.002 0.037 ± 0.005 0.013 ± 0.001 0.016 ± 0.001 0.015 ± 0.001 0.017 ± 0.002 0.024 ± 0.002 0.094 a 0.045 c 0.072 ± 0.008
Table 2 Uptake, residue and removal of DDTs
Translocation factor (TF)b
Biomass (g/pot)a
Amount of uptake (lg d.w.)
Phytoextracted (% of initial RDDTs)
Residual in soil (mg d.w.)
Removal rate (%)b
145 bcc 143 bcd 150 bc 158 b 208 a 157 b 134 cd 147 bc 2.24 e 4.59 e 126 d –d
6.65 5.58 18.6 5.27 7.66 5.52 5.67 10.5 4.48 3.89 27.0 –
0.45 0.37 1.24 0.37 0.51 0.37 0.38 0.70 0.30 0.26 1.81 –
44.4 d 70.3 bc 72.2 bc 71.9 bc 58.7 bcd 67.9 bc 55.1 cd 79.0 b 65.0 bcd 72.4 bc 68.1 bc 126 a
70.3 52.9 51.6 51.8 60.7 54.6 63.1 47.1 56.5 51.5 54.4 15.4
Root dc d c d d d d d
b
0.044 ± 0.009 de 0.030 ± 0.005 de 0.054 ± 0.020 d 0.024 ± 0.0003 e 0.030 ± 0.004 de 0.034 ± 0.010 de 0.047 ± 0.004 de 0.036 ± 0.006 de 0.25 a 0.085 c 0.12 ± 0.006 b
0.55 ± 0.06 0.47 ± 0.05 0.76 ± 0.18 0.54 ± 0.05 0.53 ± 0.07 0.74 ± 0.22 0.44 ± 0.02 0.35 ± 0.02 0.38 ± 0.16 0.54 ± 0.22 0.62 ± 0.09
MV1 MV2 MV3 MV4 MV5 MV6 MV7 MV8 Alfalfa Ryegrass Teosinte Control
de def b def d def def c ef f a
a b b b ab b ab c b b b d
a
BCF or bioconcentration factor; dry weight ratio of DDT concentration in plant to the soil. b TF or translocation factor; ratio of shoot BCF to root BCF. No significant differences were observed among TFs of various treatments (P > 0.05). c Mean ± S.D. (n = 5) followed by the same letters within a column were not significantly different (P > 0.05).
showed that the calculated TFs for the plant species were lower than 1.0 with great variation among the different species and cultivars, but no significant differences were observed (Table 1). This implied that most of the DDTs absorbed were retained in the roots while small amount of DDTs were translocated into the shoots, resulting in accumulation in the roots. Kiflom et al. (1999) suggested that, apart from the biological process of DDTs for entering the roots, it is likely that some DDTs could remain adsorbed on the roots even though the roots were rinsed thoroughly and consequently the DDT concentration in the roots was overestimated. It was possible that this happened to the present study. Although the roots usually retained some amount of DDTs absorbed, this amount of DDTs still comprises small amount, since the roots biomass was normally much less than that of the shoots (White, 2002; Kay et al., 2006). As a consequence, the amounts of DDTs accumulated in the shoots were markedly higher than that in the roots in terms of the biomass of shoots and roots. It should be noted that the ability to transfer DDTs from roots to shoots were speciesspecific or cultivar-specific (Table 1), but no significant difference was recorded for TFs between different plants. To evaluate accumulation and translocation of DDTs, the absolute amount of DDTs accumulated in each plant was calculated by multiplying the DDT concentration of plant tissues by its relevant biomass on a dry weight basis, and the results were presented in Table 2. The total fresh weight of the plant species varied from 2.24 (alfalfa) to 208 g (MV5). The amounts of DDT accumulated in the plants ranged from 3.89 (ryegrass) to 27.0 lg (teosinte). The percentage of DDTs phytoextracted in the various treatments varied by a factor of 7, being comparable with the values in other studies (Lunney et al., 2004; White et al., 2005). It is clear that a similar variation for DDT phytoextraction also exists at the cultivar level within seven maize cultivars where MV3 showed the highest amount of uptake. As stated above, the concentrations of DDTs in the tissues of alfalfa and ryegrass were significantly higher than those in the maize, but the biomass of the latter were higher than those of alfalfa and ryegrass by 26-fold or greater. As a result, the amounts of DDT accumulated in maize were more than in alfalfa and ryegrass. This finding was in accordance with the previous studies (Lunney et al., 2004; White et al., 2005). The results indicated that the amount of DDTs accumulated in plants depends not only on the concentration in plant tissues but also on their biomass. The bigger the biomass is, the more DDTs the plants accumulate (Kiflom et al., 1999).
a
Wet weight. Removal rate (%) = (C0 Ct)/C0 100. In which C0 is the initial concentration of DDTs in the soil (29.9 mg/kg); Ct is the residual concentration of DDTs after plant growth (Fig. 1). c Mean (n = 5) followed by the same letters within a column were not significantly different (P > 0.05). d Without plant. b
3.3. Removal rate of DDTs Compared to the control, seventy-day’s plant growth on the spiked soils decreased the concentrations of DDT residue in soils to a great extent. The DDT levels in the soils with plants and without plant (Control) accounted for 29.7–52.9% and 84.6% of the total DDTs spiked, respectively, while the removal rates of DDTs varied from 47.1% to 63.1% for the treatments with plants and 15.4% for control (Table 2), indicating that the plants could enhance the DDT removal from the soils. It is obvious that the pattern of DDTs removal from the spiked soils fitted the assuming pseudo-first order kinetics (Fig. 2). In the control (without plant) the removal rate was only 3.9% of the total DDTs during the first 10 d of experiment, while those of the treatments with plants (except from MV3) exhibited removal rates up to 12.6–25.6%, for the same period. Moreover, the DDT removal rates of the treatments with plants in the other periods (except from MV6 during the 10–20-day growth) were also higher than those without plant, suggesting that plants enhanced the removal of DDTs from the soils. Nevertheless, even though the plants had the ability to enhance the removal of DDTs, phytoextraction is not the main removal process of DDTs. Calculation showed that the percentage of DDTs phytoextracted in this study accounted for <0.1% of the total amount of DDTs spiked (Table 2), being far lower than the removal amount from the soils if an approximate mass balance in the soil-plant system was taken into consideration. This is very similar to the result reported by Lunney et al. (2004) and White et al. (2005). It is important to note that there was variation in uptake and translocation of DDTs, depending upon plant species or cultivars and being similar to plant uptake and translocation of p,p0 -DDE (White et al., 2005). Overall, levels of DDTs removed from the soils were low, as the predicted bioavailability of DDTs to plants was quite low. In general, there was the metabolic conversion and dehydrochlorination of DDTs during plant growth. The plant-enhancing removal of DDTs may be attributed to the promotion of DDT metabolism by plants (White et al., 2005) or plant-assisted volatilization, because of the nature of DDTs freshly added. Furthermore, plant root exudates significantly increased DDT desorption from the soils (Luo et al., 2006), and thereby increased the bioavailability of DDTs. Nevertheless, further research is necessary to better understand the DDT plant availability under different soil conditions and different removal processes.
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35 Control MV1 MV2 MV3 MV4 MV5 MV6 MV7 MV8 Alfalfa Ryegrass Teosinte
Concentration (mg/kg)
30 25 20 15 10 5 0
10
20
30 Time (day)
50
70
Fig. 2. The concentrations of RDDTs in the soils at different time of plant growth.
As shown in Fig. 2, the concentrations of DDTs in soils decreased with the growth time of plants. For seven out of eleven plants (e.g., MV1, MV2, MV5, MV6, MV8, Alfalfa and Ryegrass), the highest removal rates were obtained in the first 10 d of growth, and then the removal rate decreased gradually; whereas the highest removal rates in the treatments with MV3, MV4 and MV7 were found during the period of 10–20 d experiment, implying that the effects of different plants to enhance the removal of DDTs were not constant. A total of 29.3–55.1% of the initial DDTs was removed during 20 d; less than 15% during 20–30 d (except MV1) and during 30–50 d, and less than 5% during 50–70 d. These results indicated that removal rates of DDTs decreased with the increasing growth time, and that the removal rates against the increasing growth time fit assuming pseudo-1st order kinetics. As previously stated, the DDT metabolism or biodegradation might be the main removal process of DDTs from the soils. The fresh-spiked DDTs in the soils have increased bioavailability compared to the aged ones (Yao et al., 2007), and thus the rate of in situ degradation was higher during the first 10 d than the following period and more degradation of DDT was found during the initial stages (Suresh et al., 2005). It is interesting to note that the effects of different species and cultivars varied largely. The removal amount or removal rate in the treatment of MV1 was significantly higher than in the other treatments (Table 2), and also higher than the values reported by Lunney et al. (2004). As described by Luo et al. (2006), the ability of different species root to increase in DDT desorption from the soils varied considerably. The relatively low biomass of ryegrass roots had the lowest effect on DDT desorption than maize and thereby the availability of DDT in the treatment with ryegrass might be less than that with maize. As a result, the removal rate of DDTs in the treatment with ryegrass was lower than with maize. On the other hand, the percentage of contaminant phytoextraction from soils was less than 1.0%, which did not approach those observed in the phytoremediation of heavy metals by ‘‘hyperaccumulating” species. But, when comparing the total removal rate of DDTs from the soils between the treatments with the plant growth and the control (without plant) (e.g., by subtracting the removal rate of the control from the lowest or the highest removal rate of plant treatment), the former was higher by 31.7–54.9%, than the control, indicating the potential for a plant-assisted removal approach of DDTs from the contaminated soils. 4. Conclusion The plant species and cultivars investigated in this study exhibited various abilities to accumulate and translocate DDTs in soils.
The percentage of phytoextraction accounted for less than 1% of the total amount of DDTs spiked in the soils. Compared to the control, plants could enhance the removal of DDTs from the soils, and a net removal of 31.7–54.9% of the initial DDTs was obtained after plant growth of 70 d. The removal rate was higher in the first 20 d than in the following days. The variation in ability to remove DDTs from contaminated soils was observed not only within different species but also within different cultivars of same species, but the removal efficiencies were generally low. Despite this, further research is worth conducting, in order to screen plant species with high phytoextraction ability, biomass, and especially removal rate. In addition, an optimization of the conditions is considered necessary for the phytoremediation potential to be enhanced. Acknowledgements This work was supported by the Natural Science Foundation of China (Nos. 30471007, 30671208), Key Scientific Research Project of Ministry of Education of China (No. 02112), the Natural Science Foundation of Guangdong Province (Nos. 021011, 036716, 043005970) and Project of Department of Science and Technology of Guangdong (Nos. 01C21202, 03A20504, 03C34505, 05B20801002), and the Research Foundation of State Key Laboratory of Organic Geochemistry, Chinese Academy of Sciences, and the Project of Bureau of Science and Technology of Guangzhou (2007Z3-E0471). References Cai, Q.Y., Mo, C.H., Wu, Q.T., Katsoyiannis, A., Zeng, Q.Y., 2008. The status of soil contamination by semivolatile organic chemicals (SVOCs) in China: a review. Sci. Total Environ. 389, 209–224. Chau, K.W., 2005. Characterization of transboundary POP contamination in aquatic ecosystems of Pearl River delta. Mar. Pollut. Bull. 51, 960–965. Cunningham, S.D., Ow, D.W., 1996. Promises and prospects of phytoremediation. Plant Physiol. 110, 715–719. Fan, S., Li, P., Gong, Z., He, N., Zhang, L., Ren, W., Verkhozina, V.A., 2007. Study on phytoremediation of phenanthrene-contaminated soil with alfalfa (Medicago sativa L.). Environ. Sci. 28, 2080–2084 (in Chinese). Garrison, A.W., Valentine, A.Z., Jimmy, K.A., Jackson, J.E., Jones, W.J., Rennels, D., Wolfe, N.L., 2000. Phytodegradation of p,p-DDT and the enantiomers of o,p0 DDT. Environ. Sci. Technol. 34, 1663–1670. Hua, X.M., Shan, Z.J., 1996. The production and application of pesticides and factor analysis of their pollution in environment in China. Adv. Environ. Sci. 4, 33– 45. Katsoyiannis, A., Samara, C., 2004. Persistent organic pollutants (POPs) in the sewage treatment plant of Thessaloniki, northern Greece: occurrence and removal. Water Res. 38, 2685–2698. Katsoyiannis, A., Samara, C., 2007. Comparison of active and passive sampling for the determination of persistent organic pollutants (POPs) in sewage treatment plants. Chemosphere 67, 1375–1382. Kay, B.D., Hajabbasi, M.A., Ying, J., Tollenaar, M., 2006. Optimum versus non-limiting water contents for root growth, biomass accumulation, gas exchange and the rate of development of maize (Zea mays L.). Soil Till. Res. 88, 42–54. Kiflom, W.G., Wandiga, S.O., Ng’ang’a, P.K., Kamau, G.N., 1999. Variation of plant p,p0 -DDT uptake with age and soil type and dependence of dissipation on temperature. Environ. Int. 25, 479–487. Kurt-Karakus, P.B., Bidleman, T.F., Staebler, R.M., Jones, K.C., 2006. Measurement of DDT fluxes from a historically treated agricultural soil in Canada. Environ. Sci. Technol. 40, 4578–4585. Li, J., Zhang, G., Qi, S., Li, X., Peng, X., 2006. Concentrations, enantiomeric compositions, and sources of HCH, DDT and chlordane in soils from the Pearl River Delta, South China. Sci. Total Environ. 372, 215–224. Lunney, A.I., Zeeb, B.A., Reimer, K.J., 2004. Uptake of weathered DDT in vascular plants: potential for phytoremediation. Environ. Sci. Technol. 38, 6147–6154. Luo, L., Zhang, S., Shan, X.Q., Zhu, Y.G., 2006. Oxalate and root exudates enhance the desorption of p,p0 -DDT from soils. Chemosphere 63, 1273–1279. Nakata, H., Hirakawa, Y., Kawazoe, M., Nakabo, T., Arizono, K., Abe, S.I., Kitano, T., Shimada, H., Watanabe, I., Li, W., Ding, X., 2005. Concentrations and compositions of organochlorine contaminants in sediments, soils, crustaceans, fishes and birds collected from Lake Tai, Hangzhou Bay and Shanghai city region, China. Environ. Pollut. 133, 415–429. Suresh, B., Sherkhane, P.D., Kale, S., Eapen, S., Ravishankar, G.A., 2005. Uptake and degradation of DDT by hairy root cultures of Cichorium intybus and Brassica juncea. Chemosphere 61, 1288–1292. Tang, S.R., 2005. The Principles and Methods of Phytoremediation of Contaminated Environment. Chinese Scientific Press, Beijing. pp. 1–298.
C.-H. Mo et al. / Chemosphere 73 (2008) 120–125 Turusov, V., Rakitsky, V., Tomatis, L., 2002. Dichlorodiphenyltrichloroethane (DDT): ubiquity, persistence and risks. Environ. Health Persp. 110, 125–128. Waliszewski, S.M., Carvajal, O., Infanzon, R.M., Trujillo, P., Aguirre, A.A., Maxwell, M., 2004. Levels of organochlorine pesticides in soils and rye plant tissues in a field study. J. Agric. Food Chem. 52, 7045–7050. White, J.C., 2002. Differential bioavailability of field-weathered p,p0 -DDE to plants of the Cucurbita and Cucumis genera. Chemosphere 49, 143–152. White, J.C., Parrish, Z.D., Isleyen, M., Gent, M.P.N., Iannucci-Berger, W., Eitzer, B.D., Mattina, M.J.I., 2005. Uptake of weathered p,p0 -DDE by plant species effective at accumulating soil elements. Microchem. J. 81, 148–155.
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White, J.C., Ross, D.W., Gent, M.P.N., Eitzer, B.D., Mattina, M.I., 2006. Effect of mycorrhizal fungi on the phytoextraction of weathered p,p0 -DDE by Cucurbita pepo. J. Hazard. Mater. 137, 1750–1757. Wong, M.H., Leung, A.O.W., Chan, J.K.Y., Choi, M.P.K., 2005. A review on the usage of POP pesticides in China. Chemosphere 60, 740–752. Yao, F., Yu, G., Bian, Y., Yang, X., Wang, F., Jiang, X., 2007. Bioavailability to grains of rice of aged and fresh DDD and DDE in soils. Chemosphere 68, 78–84.
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Potential of different species for use in removal of DDT from the contaminated soils Ce-Hui Mo a,*, Quan-Ying Cai b, Hai-Qin Li b,1, Qiao-Yun Zeng b, Shi-Rong Tang c, Yue-Chun Zhao b a b c
Department of Environmental Engineering, Jinan University, Guangzhou 510632, China College of Resources and Environment, South China Agricultural University, Guangzhou 510642, China Centre for Research in Ecotoxicology and Environmental Remediation, Institute of Agricultural Environmental Protection, The Ministry of Agriculture, Tianjin 300191, China
a r t i c l e
i n f o
Article history: Received 19 July 2007 Received in revised form 19 April 2008 Accepted 28 April 2008 Available online 16 June 2008 Keywords: Phytoextraction Organochlorine pesticide Removal rate Bioconcentration factor Translocation factor
a b s t r a c t Dichlorodiphenyltrichloroethane (DDT) and its main metabolites, p,p0 -DDD and p,p0 -DDE (DDTs in this study included DDT, DDD and DDE), are frequently detected in agricultural soils even though its usage in agriculture was banned in 1980s or earlier. In this study, eleven plants including eight maize (Zea mays) cultivars and three forage species (alfalfa, ryegrass and teosinte) widely cultivated in China were grown in the soils spiked with DDTs to investigate their potential for removal of DDT from the contaminated soils. The plants varied largely in their ability to accumulate and translocate DDTs, with the bioconcentration factor (BCF; DDT concentration ratio of the plant tissues to the soils) ranging from 0.014 to 0.25 and the translocation factor (TF; DDT concentration ratio of the shoots to the roots) varying from 0.35 (Zea mays cv Chaotian-23) to 0.76 (Zea mays spp. mexicana). The amount of DDT phytoextraction ranged from 3.89 lg (ryegrass) to 27.0 lg (teosinte) and accounted for <0.1% of the total initial DDTs spiked in the soils. After 70 d, the removal rates reached 47.1–70.3% of the total initial DDTs spiked in the soils with plants while that was only 15.4% in the soils without plant. Moreover, the higher removal rates of DDTs occurred at the first 20 d of experiment, and then the removal rate decreased with time. The highest amount of DDTs phytoextracted was observed in teosinte, followed by Zea mays spp. mexicana, but the highest removal rate of DDTs was found in maize (Zea mays cv Jinhai-6). Even though phytoextraction is not the main removal process for DDTs, the plant species especially Zea mays cv Jinhai-6 showed high potential for removing DDTs from the contaminated soils. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Dichlorodiphenyltrichloroethane (DDT) was one of the most widely used pesticides in agriculture in many countries until the 1970s. Estimates showed that China produced at least 435,200 tonnes of DDT and its main metabolites [1,1-dichloro-2,2-bis(4-chlorophenyl)ethylene, p,p0 -DDE; 1,1-dichloro-2,2-bis(4-chlorophenyl)ethane, p,p0 -DDD] from the 1950s to May 1983 when their production was banned, accounting for 20% of the global production (Hua and Shan, 1996). Part of DDT was released into the environment during the past decades and this trend continues, because of its usage as an anti-malaria agent or of an impurity in other pesticides such as dicofol (Wong et al., 2005). DDT and its two metabolites are classified as persistent organic pollutants (POPs) and their pollution is of worldwide concern due to their persistence in the environment, bioaccumulation, and negative effects on soil microbial, plant, animal life and humans
* Corresponding author. Tel./fax: +86 20 85226615. E-mail addresses: [email protected] (C.-H. Mo), [email protected] (Q.-Y. Cai), [email protected] (S.-R. Tang). 1 Present address: Jize Environmental Science and Technology Ltd., Kunshan 215316, China. 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.04.082
(Katsoyiannis and Samara, 2004; Turusov et al., 2002). Recently, high levels of DDT and its main metabolites have been frequently detected in various media such as soils, water, sediments and plants in China, as well as in other countries (Waliszewski et al., 2004; Chau, 2005; Nakata et al., 2005; Kurt-Karakus et al., 2006; Li et al., 2006; Katsoyiannis and Samara, 2007; Cai et al., 2008). Finding of higher DDT concentrations in the cultivated soils than in the non-cultivated ones reveals either that DDT was used in the past or that is currently illegally used for agricultural reasons (Chau, 2005). DDT residues in agricultural soils are of great concern due to the uptake of DDT by plants, accumulation in food chain, and re-emission from soils to the atmosphere. Thus, the removal of DDT and its metabolites from the agricultural environment has intrigued many scientists in recent years. Phytoremediation is an in situ and cost-effective technology in which the plants are applied to extract, directly or indirectly degrade or remove organic and inorganic contaminants from the contaminated natural media, including POPs such as DDT and its metabolites (Cunningham and Ow, 1996; Tang, 2005). Literature review, however, showed that few publications were available on phytoremediation of DDT in spite of its widespread occurrence (Garrison et al., 2000; Lunney et al., 2004). Garrison et al. (2000) reported biotransformation of DDT by plant cultures. White et al.
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(2005, 2006) investigated phytoextraction of p,p0 -DDE, and found that accumulation and translocation of DDT or DDE by plants were somewhat species-specific or cultivar-specific (Lunney et al., 2004). But until now, no literature reported which plant has the greatest potential for phytoremediation of DDT in soils. Further research should be done in this direction. The objectives of the present study were to investigate the uptake and translocation of DDT in soil-plant system, and to compare the potential of various plant species and cultivars endemic to China for removal of DDT from the contaminated soils. 2. Materials and methods 2.1. Chemicals and materials In order to compare the phytoextraction potential of different species and cultivars for DDT, two plant species were selected, including maize (Zea mays) and forage. The forage species included alfalfa (Medicago sativa), ryegrass (Lolium multiflorum), and teosinte (Zea mays ssp. parviglumis) which may have potential for phytoremediation of PAHs (Fan et al., 2007) or DDT (Lunney et al., 2004). Eight representative cultivars of maize were selected from the cultivars grown widely in China, namely, Jinhai-6 (MV1), Wanqin (MV2), Zea mays spp. mexicana (MV3), Huanong-1 (MV4). Huahai (MV5), Huidan (MV6), Baiyunuo (MV7) and Chaotian-23 (MV8). The latter was sweet maize, while the others were feed maize. All the plant seeds for experiment were obtained from Guangdong Academy of Agricultural Sciences, China. The DDTs (DDTs in this study stands for p,p0 -DDT, o,p0 -DDT, p,p0 DDE and p,p0 -DDD) of analytical grade for pot trial were obtained from Taigu Chemical Factory of Tianjin, China. A composite stock standard solution of DDTs (100 mg/l) for analysis was used, containing p,p0 -DDE, p,p0 -DDD and p,p0 -DDT. The working standard solutions were prepared by diluting appropriate volumes of the stock standard solution. 2,4,5,6-Tetrachloro-m-xylene (TCmX) and pentachloronitrobenzene were used as surrogate and internal standard solution, respectively. These standards were purchased from J&K Chemical Limited Co. (Beilinwei, Beijing, China). Another standard solution, o,p0 -DDT, was purchased from China’s Research Center of Standard Material. Analytical grade organic solvents including n-hexane, dichloromethane (DCM), methanol and acetone were redistilled prior to use. Silica gel (100–180 mesh, Guangzhou Chemical Reagent Co.) was Soxhlet-extracted with DCM and methanol, respectively, for 12 h, dried at 130–140 °C for 4 h before use. Neutral alumina (80–100 mesh, Guangzhou Chemical Reagent Co.) was activated at 250 °C, for 12 h. Anhydrous sodium sulfate was dried at 250 °C for 4 h and stored in a sealed desiccator. These materials were purchased from Guangzhou Chemical Reagent Co., China. All the glassware were soaked in K2CrO4–H2SO4 solution for 30 min, washed with tap water and redistilled water and then were dried at 250 °C for 2 h. 2.2. Experimental design The experiment was carried out in the glasshouse of South China Agricultural University, Guangzhou, China. The soil was paddy soil with 24.9 g/kg dry weight (d.w.) of organic matter, 1.02 g/kg (d.w.) of total N, 0.92 g/kg (d.w.) of total P, 19.35 g/kg (d.w.) of total K, 78.8 mg/kg (d.w.) of available N, 29.0 (d.w.) mg/kg of available P, 6.28 of pH and 310 g/kg (d.w.) of water-holding capacity. The soil was air-dried, crushed, mixed thoroughly and passed through a 5-mm sieve. There were 5.0 kg of soils in each pot. Prior to cultivation, the soil was fertilized 0.20 g/kg N, 0.15 g/kg P and 0.15 g/kg K with urea, superphosphate and potassium chloride, respectively.
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The stock solution of DDTs was prepared by diluting DDTs with acetone. The stock solution was mixed thoroughly with paddy soil (500 g, 2 mm) and allowed to evaporate for 48 h. The treated soil was mixed thoroughly with ‘‘clean” bulk soils (4500 g, 5 mm) to obtain the initial DDT concentration which were determined by Soxhlet extraction-gas chromatography coupled with electron capture detector (GC-ECD) as other samples. Considering the toxicity of DDTs to the plants and the concentrations used in the previous study (32.0–88.9 mg/kg d.w.) by Kiflom et al. (1999), the total initial concentration of DDTs in soils of the present study was 29.9 mg/kg (d.w.). Soils (5.0 kg) contaminated with DDTs were placed in a ceramic pot (27 17 cm, I.D. height). The pot trial included 11 treatments with different species or cultivars given above and a treatment without plant (defined as Control). Immediately after being prepared, they were arranged in a completely randomized block design with five replications in a glasshouse. The soils were kept at the water-holding capacity with redistilled water for seven days and then mixed thoroughly. Twenty seeds of maize were sown in each pot and, after germination, seedlings were manually thinned twice and three uniform seedlings were established per pot. Fifty seedlings of alfalfa or ryegrass were established to each pot. The redistilled water (200 ml) was supplied for each pot including the control treatment every day during the growth. There was no effluence from the bottom of the pots. No chemical pesticide was used. At 10, 20, 30, 50 and 70 d of growth, soil samples were carefully collected from the pot (at least 10 sites, randomly) using a 1.0 cm stainless steel sampler. The plants were harvested after 70 d of growth. The shoots (including stem and leaves) and roots of plants were carefully removed from the soils and their fresh weights were measured. The plant samples were rinsed with redistilled water, oven-dried at 55 °C and then ground (1-mm sieve). The soil samples (approximately 100 g) were air-dried, ground to pass through a 1-mm sieve, and refrigerated until analysis. 2.3. Analytical procedure, QA/QC measures and performances Sample extraction and cleanup were performed according to USEPA methods 3550B (Ultrasonic extraction) and 3630C (Silica gel column) with modification, respectively. Dried plant (10 g) and soil samples (20 g) spiked with surrogate were extracted in triplicate with 30 ml ethyl acetate and with 30 ml acetone/DCM mixture (1:1, v/v) in sonicator (SK5200H, China) for 20 min, respectively. After each extraction, separation was accomplished by centrifuging at 3000 rpm for 5 min. The supernatant was combined and concentrated carefully to 2 ml in a rotary vacuum evaporator (Yarong, China). The concentrated extracts were loaded on a combined column of silica gel and alumina. The glass chromatography column (25 1 cm I.D., Guangzhou, China), fitted with a Teflon stopcock, was packed, bottom-up, with cotton-wool (Soxhlet-extracted with DCM for 72 h before use), 3 cm alumina, 10 cm extracted silica, followed by 2 cm anhydrous sodium sulfate. DCM/n-hexane mixture (1:1, v/v, 50 ml) was used for elution. The collected extract was blown-down under a gentle stream of nitrogen (N2) and diluted with n-hexane to an appropriate volume. Measurements of DDTs in extract were performed following USEPA method 8081A with slight modification. The analysis was carried out by gas chromatography (GC, Hewlett–Packard 5890 Series II, Agilent Technology) coupled with electron capture detector (ECD). A HP-5 30 m 0.32 mm I.D., 0.17 lm membrane thickness (Agilent Technology, US) was used. The GC oven temperature was raised from 150 to 280 °C at 4.0 °C/min. Nitrogen (N2) was the carrier gas, at a flow of 2.5 ml/min. The injection was set on a splitless mode at 280 °C. The injection volume was 1.0 ll. A 63Ni electron capture detector was used and its temperature was at 300 °C.
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The instruments were calibrated daily and the relative percent differences between the six-point calibration and the daily calibrations were <15% for all target analytes. Identification of individual compounds was based on the comparison of retention time data between samples and the standard solution. The internal calibration method for quantification was applied based on seven-point calibration curve (0–400 lg/ml). Method blanks (solvent), duplicate samples and spiked blanks (standards spiked into solvent) were analyzed. In addition, surrogate standards were added to the samples to monitor procedural performance and matrix effects. The average recoveries were 75.0–89.5% for 2,4,5,6-tetrachloro-mxylene and 75.2–92.0% for DDTs in plant samples, while those were 75.7–86.5% for 2,4,5,6-tetrachloro-m-xylene and 83.2–102% for DDTs in soil samples, conforming to the reported ranges of USEPA method 8081A. 2.4. Statistical analysis Experimental results were expressed on a dry weight basis except the plant biomass. The sum concentrations of p,p0 -DDT, o,p0 DDT, p,p0 -DDE and p,p0 -DDD were expressed as RDDTs. Data was subjected to analysis including the analysis of variance (ANOVA) and mean values were compared by Duncan’s multiple range (P < 0.05) test performed using the statistics analysis system (SAS, version 8.0) for Windows software package. 3. Results and discussion 3.1. DDT concentrations in the soils Fig. 1a presents the concentrations of DDT residues in the soils after plant growth. The RDDTs values varied from 8.88 (MV1) to 25.3 mg/kg (Control), being considerably lower than the initial one (29.9 mg/kg) in the soils, and far lower than those observed in the soils after cowpeas growth (32.0–36.8 mg/kg) (Kiflom et al., 1999). This might be related to the experiment conditions and the initial soil concentration of DDT. In the study of Kiflom
Fig. 1. The concentrations of RDDTs residues in the soils (a) and plants (b).
et al. (1999), the initial soil concentrations of DDT were up to 60.4 and 88.9 mg/kg. The DDT concentrations of treatments after plant growth were significantly lower than that in control (without plant), especially the one of MV1. The same phenomenon was recorded by Kiflom et al. (1999). 3.2. Vegetation concentrations of DDTs The concentrations of DDTs in plant shoots and roots are showed in Fig. 1b. The data exhibited that the DDTs were taken up and translocated into the plants grown in the DDTs spiked soils. For all species or cultivars, the concentrations of RDDTs in roots were considerably higher than those in shoots, being similar to other plant species (e.g., tall fescue, mustard, crimson clover) in other studies (Kiflom et al., 1999; White et al., 2005, 2006), but different from the distribution in pigeonpea, zucchini, pumpkin (Lunney et al., 2004; White et al., 2005). This might be attributed to the variations in the degree of exposure of either part to DDTs (Kiflom et al., 1999). Moreover, the results of White et al. (2005) indicated that, under same exposure conditions (field plot), distributions of DDTs in shoots and roots were somewhat species-specific. The RDDTs accumulated by the shoots varied from 0.206 (MV1) to 1.23 mg/kg (alfalfa), while the RDDTs in the roots ranged between 0.385 (MV1) and 3.23 mg/kg (alfalfa), being comparable with the RDDTs in cowpeas (3.24–7.77 mg/kg) (Kiflom et al., 1999) or in the plants grown in high DDT-contaminated soils (Lunney et al., 2004). Those results indicated that maize and forage (e.g., alfalfa) have similar accumulation for DDTs to cowpeas etc which were planted by Kiflom et al. (1999) or Lunney et al. (2004). It should be pointed out that, in the present study, the initial concentration of DDTs in soils was 29.9 mg/kg, being seven times higher than that in the high DDT-contaminated soils (3.70 mg/kg) reported by Lunney et al. (2004) but far lower than those spiked by Kiflom et al. (1999). But the RDDTs in the roots of alfalfa and ryegrass (3.23 and 1.23 mg/kg) were remarkably lower than those in the abovementioned study (4.69 and 2.73 mg/kg), whereas the RDDTs in shoots of alfalfa and ryegrass (1.23 and 0.658 mg/kg) were substantially higher than those in the aforementioned study (0.026 and 0.060 mg/kg). This might be partly attributed to the different physico-chemical properties of the soils and subsequently the different bioavailability of DDTs and transport processes (Kiflom et al., 1999). In the present study DDTs were freshly spiked, while they are weathered DDTs in the soils of the study reported by Lunney et al. (2004), which might result in the different bioaccumulation in the same plant. As shown in Fig. 1b, the variation of RDDTs in the shoots and roots of different plants under investigation by a factor of >6, respectively, suggested that the ability of the plant species and cultivars to accumulate and translocate DDTs varied to a great extent. Bioconcentration factor (BCF), calculated as the dry weight DDT concentration ratio of plant tissue to that in the soils ([RDDTs]root or shoot/[RDDTs]soil), was applied to compare the relative abilities of different species and cultivars to take up and translocate DDTs in the shoots and roots. The average BCFs were lower than 1.0, even lower than 0.10, varying from 0.013 (MV4) to 0.094 (alfalfa) for the shoots and from 0.024 (MV4) to 0.25 (alfalfa) for the roots (Table 1). The BCFs for DDTs in this study were similar to those for cowpeas (Kiflom et al., 1999) or for rice (Yao et al., 2007), but considerably different from those reported for alfalfa and ryegrass in the study of Lunney et al. (2004), where most of BCFs for DDTs in the roots were greater than 1.0. These results confirmed again that there was variation in the accumulation of DDTs in plants grown in soils spiked with fresh DDTs and weathered DDTs. Translocation factor (TF), expressed as the DDT concentration ratio of the shoots to the roots, is another value of interest reflecting DDTs transfer to the shoots from the roots. The present study
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C.-H. Mo et al. / Chemosphere 73 (2008) 120–125 Table 1 Bioconcentration factor (BCF) and translocation factor (TF) for DDTs in different species and cultivars Plant species
Bioconcentration factor (BCF)a Shoot
MV1 MV2 MV3 MV4 MV5 MV6 MV7 MV8 Alfalfa Ryegrass Teosinte
0.023 ± 0.002 0.014 ± 0.002 0.037 ± 0.005 0.013 ± 0.001 0.016 ± 0.001 0.015 ± 0.001 0.017 ± 0.002 0.024 ± 0.002 0.094 a 0.045 c 0.072 ± 0.008
Table 2 Uptake, residue and removal of DDTs
Translocation factor (TF)b
Biomass (g/pot)a
Amount of uptake (lg d.w.)
Phytoextracted (% of initial RDDTs)
Residual in soil (mg d.w.)
Removal rate (%)b
145 bcc 143 bcd 150 bc 158 b 208 a 157 b 134 cd 147 bc 2.24 e 4.59 e 126 d –d
6.65 5.58 18.6 5.27 7.66 5.52 5.67 10.5 4.48 3.89 27.0 –
0.45 0.37 1.24 0.37 0.51 0.37 0.38 0.70 0.30 0.26 1.81 –
44.4 d 70.3 bc 72.2 bc 71.9 bc 58.7 bcd 67.9 bc 55.1 cd 79.0 b 65.0 bcd 72.4 bc 68.1 bc 126 a
70.3 52.9 51.6 51.8 60.7 54.6 63.1 47.1 56.5 51.5 54.4 15.4
Root dc d c d d d d d
b
0.044 ± 0.009 de 0.030 ± 0.005 de 0.054 ± 0.020 d 0.024 ± 0.0003 e 0.030 ± 0.004 de 0.034 ± 0.010 de 0.047 ± 0.004 de 0.036 ± 0.006 de 0.25 a 0.085 c 0.12 ± 0.006 b
0.55 ± 0.06 0.47 ± 0.05 0.76 ± 0.18 0.54 ± 0.05 0.53 ± 0.07 0.74 ± 0.22 0.44 ± 0.02 0.35 ± 0.02 0.38 ± 0.16 0.54 ± 0.22 0.62 ± 0.09
MV1 MV2 MV3 MV4 MV5 MV6 MV7 MV8 Alfalfa Ryegrass Teosinte Control
de def b def d def def c ef f a
a b b b ab b ab c b b b d
a
BCF or bioconcentration factor; dry weight ratio of DDT concentration in plant to the soil. b TF or translocation factor; ratio of shoot BCF to root BCF. No significant differences were observed among TFs of various treatments (P > 0.05). c Mean ± S.D. (n = 5) followed by the same letters within a column were not significantly different (P > 0.05).
showed that the calculated TFs for the plant species were lower than 1.0 with great variation among the different species and cultivars, but no significant differences were observed (Table 1). This implied that most of the DDTs absorbed were retained in the roots while small amount of DDTs were translocated into the shoots, resulting in accumulation in the roots. Kiflom et al. (1999) suggested that, apart from the biological process of DDTs for entering the roots, it is likely that some DDTs could remain adsorbed on the roots even though the roots were rinsed thoroughly and consequently the DDT concentration in the roots was overestimated. It was possible that this happened to the present study. Although the roots usually retained some amount of DDTs absorbed, this amount of DDTs still comprises small amount, since the roots biomass was normally much less than that of the shoots (White, 2002; Kay et al., 2006). As a consequence, the amounts of DDTs accumulated in the shoots were markedly higher than that in the roots in terms of the biomass of shoots and roots. It should be noted that the ability to transfer DDTs from roots to shoots were speciesspecific or cultivar-specific (Table 1), but no significant difference was recorded for TFs between different plants. To evaluate accumulation and translocation of DDTs, the absolute amount of DDTs accumulated in each plant was calculated by multiplying the DDT concentration of plant tissues by its relevant biomass on a dry weight basis, and the results were presented in Table 2. The total fresh weight of the plant species varied from 2.24 (alfalfa) to 208 g (MV5). The amounts of DDT accumulated in the plants ranged from 3.89 (ryegrass) to 27.0 lg (teosinte). The percentage of DDTs phytoextracted in the various treatments varied by a factor of 7, being comparable with the values in other studies (Lunney et al., 2004; White et al., 2005). It is clear that a similar variation for DDT phytoextraction also exists at the cultivar level within seven maize cultivars where MV3 showed the highest amount of uptake. As stated above, the concentrations of DDTs in the tissues of alfalfa and ryegrass were significantly higher than those in the maize, but the biomass of the latter were higher than those of alfalfa and ryegrass by 26-fold or greater. As a result, the amounts of DDT accumulated in maize were more than in alfalfa and ryegrass. This finding was in accordance with the previous studies (Lunney et al., 2004; White et al., 2005). The results indicated that the amount of DDTs accumulated in plants depends not only on the concentration in plant tissues but also on their biomass. The bigger the biomass is, the more DDTs the plants accumulate (Kiflom et al., 1999).
a
Wet weight. Removal rate (%) = (C0 Ct)/C0 100. In which C0 is the initial concentration of DDTs in the soil (29.9 mg/kg); Ct is the residual concentration of DDTs after plant growth (Fig. 1). c Mean (n = 5) followed by the same letters within a column were not significantly different (P > 0.05). d Without plant. b
3.3. Removal rate of DDTs Compared to the control, seventy-day’s plant growth on the spiked soils decreased the concentrations of DDT residue in soils to a great extent. The DDT levels in the soils with plants and without plant (Control) accounted for 29.7–52.9% and 84.6% of the total DDTs spiked, respectively, while the removal rates of DDTs varied from 47.1% to 63.1% for the treatments with plants and 15.4% for control (Table 2), indicating that the plants could enhance the DDT removal from the soils. It is obvious that the pattern of DDTs removal from the spiked soils fitted the assuming pseudo-first order kinetics (Fig. 2). In the control (without plant) the removal rate was only 3.9% of the total DDTs during the first 10 d of experiment, while those of the treatments with plants (except from MV3) exhibited removal rates up to 12.6–25.6%, for the same period. Moreover, the DDT removal rates of the treatments with plants in the other periods (except from MV6 during the 10–20-day growth) were also higher than those without plant, suggesting that plants enhanced the removal of DDTs from the soils. Nevertheless, even though the plants had the ability to enhance the removal of DDTs, phytoextraction is not the main removal process of DDTs. Calculation showed that the percentage of DDTs phytoextracted in this study accounted for <0.1% of the total amount of DDTs spiked (Table 2), being far lower than the removal amount from the soils if an approximate mass balance in the soil-plant system was taken into consideration. This is very similar to the result reported by Lunney et al. (2004) and White et al. (2005). It is important to note that there was variation in uptake and translocation of DDTs, depending upon plant species or cultivars and being similar to plant uptake and translocation of p,p0 -DDE (White et al., 2005). Overall, levels of DDTs removed from the soils were low, as the predicted bioavailability of DDTs to plants was quite low. In general, there was the metabolic conversion and dehydrochlorination of DDTs during plant growth. The plant-enhancing removal of DDTs may be attributed to the promotion of DDT metabolism by plants (White et al., 2005) or plant-assisted volatilization, because of the nature of DDTs freshly added. Furthermore, plant root exudates significantly increased DDT desorption from the soils (Luo et al., 2006), and thereby increased the bioavailability of DDTs. Nevertheless, further research is necessary to better understand the DDT plant availability under different soil conditions and different removal processes.
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35 Control MV1 MV2 MV3 MV4 MV5 MV6 MV7 MV8 Alfalfa Ryegrass Teosinte
Concentration (mg/kg)
30 25 20 15 10 5 0
10
20
30 Time (day)
50
70
Fig. 2. The concentrations of RDDTs in the soils at different time of plant growth.
As shown in Fig. 2, the concentrations of DDTs in soils decreased with the growth time of plants. For seven out of eleven plants (e.g., MV1, MV2, MV5, MV6, MV8, Alfalfa and Ryegrass), the highest removal rates were obtained in the first 10 d of growth, and then the removal rate decreased gradually; whereas the highest removal rates in the treatments with MV3, MV4 and MV7 were found during the period of 10–20 d experiment, implying that the effects of different plants to enhance the removal of DDTs were not constant. A total of 29.3–55.1% of the initial DDTs was removed during 20 d; less than 15% during 20–30 d (except MV1) and during 30–50 d, and less than 5% during 50–70 d. These results indicated that removal rates of DDTs decreased with the increasing growth time, and that the removal rates against the increasing growth time fit assuming pseudo-1st order kinetics. As previously stated, the DDT metabolism or biodegradation might be the main removal process of DDTs from the soils. The fresh-spiked DDTs in the soils have increased bioavailability compared to the aged ones (Yao et al., 2007), and thus the rate of in situ degradation was higher during the first 10 d than the following period and more degradation of DDT was found during the initial stages (Suresh et al., 2005). It is interesting to note that the effects of different species and cultivars varied largely. The removal amount or removal rate in the treatment of MV1 was significantly higher than in the other treatments (Table 2), and also higher than the values reported by Lunney et al. (2004). As described by Luo et al. (2006), the ability of different species root to increase in DDT desorption from the soils varied considerably. The relatively low biomass of ryegrass roots had the lowest effect on DDT desorption than maize and thereby the availability of DDT in the treatment with ryegrass might be less than that with maize. As a result, the removal rate of DDTs in the treatment with ryegrass was lower than with maize. On the other hand, the percentage of contaminant phytoextraction from soils was less than 1.0%, which did not approach those observed in the phytoremediation of heavy metals by ‘‘hyperaccumulating” species. But, when comparing the total removal rate of DDTs from the soils between the treatments with the plant growth and the control (without plant) (e.g., by subtracting the removal rate of the control from the lowest or the highest removal rate of plant treatment), the former was higher by 31.7–54.9%, than the control, indicating the potential for a plant-assisted removal approach of DDTs from the contaminated soils. 4. Conclusion The plant species and cultivars investigated in this study exhibited various abilities to accumulate and translocate DDTs in soils.
The percentage of phytoextraction accounted for less than 1% of the total amount of DDTs spiked in the soils. Compared to the control, plants could enhance the removal of DDTs from the soils, and a net removal of 31.7–54.9% of the initial DDTs was obtained after plant growth of 70 d. The removal rate was higher in the first 20 d than in the following days. The variation in ability to remove DDTs from contaminated soils was observed not only within different species but also within different cultivars of same species, but the removal efficiencies were generally low. Despite this, further research is worth conducting, in order to screen plant species with high phytoextraction ability, biomass, and especially removal rate. In addition, an optimization of the conditions is considered necessary for the phytoremediation potential to be enhanced. Acknowledgements This work was supported by the Natural Science Foundation of China (Nos. 30471007, 30671208), Key Scientific Research Project of Ministry of Education of China (No. 02112), the Natural Science Foundation of Guangdong Province (Nos. 021011, 036716, 043005970) and Project of Department of Science and Technology of Guangdong (Nos. 01C21202, 03A20504, 03C34505, 05B20801002), and the Research Foundation of State Key Laboratory of Organic Geochemistry, Chinese Academy of Sciences, and the Project of Bureau of Science and Technology of Guangzhou (2007Z3-E0471). References Cai, Q.Y., Mo, C.H., Wu, Q.T., Katsoyiannis, A., Zeng, Q.Y., 2008. The status of soil contamination by semivolatile organic chemicals (SVOCs) in China: a review. Sci. Total Environ. 389, 209–224. Chau, K.W., 2005. Characterization of transboundary POP contamination in aquatic ecosystems of Pearl River delta. Mar. Pollut. Bull. 51, 960–965. Cunningham, S.D., Ow, D.W., 1996. Promises and prospects of phytoremediation. Plant Physiol. 110, 715–719. Fan, S., Li, P., Gong, Z., He, N., Zhang, L., Ren, W., Verkhozina, V.A., 2007. Study on phytoremediation of phenanthrene-contaminated soil with alfalfa (Medicago sativa L.). Environ. Sci. 28, 2080–2084 (in Chinese). Garrison, A.W., Valentine, A.Z., Jimmy, K.A., Jackson, J.E., Jones, W.J., Rennels, D., Wolfe, N.L., 2000. Phytodegradation of p,p-DDT and the enantiomers of o,p0 DDT. Environ. Sci. Technol. 34, 1663–1670. Hua, X.M., Shan, Z.J., 1996. The production and application of pesticides and factor analysis of their pollution in environment in China. Adv. Environ. Sci. 4, 33– 45. Katsoyiannis, A., Samara, C., 2004. Persistent organic pollutants (POPs) in the sewage treatment plant of Thessaloniki, northern Greece: occurrence and removal. Water Res. 38, 2685–2698. Katsoyiannis, A., Samara, C., 2007. Comparison of active and passive sampling for the determination of persistent organic pollutants (POPs) in sewage treatment plants. Chemosphere 67, 1375–1382. Kay, B.D., Hajabbasi, M.A., Ying, J., Tollenaar, M., 2006. Optimum versus non-limiting water contents for root growth, biomass accumulation, gas exchange and the rate of development of maize (Zea mays L.). Soil Till. Res. 88, 42–54. Kiflom, W.G., Wandiga, S.O., Ng’ang’a, P.K., Kamau, G.N., 1999. Variation of plant p,p0 -DDT uptake with age and soil type and dependence of dissipation on temperature. Environ. Int. 25, 479–487. Kurt-Karakus, P.B., Bidleman, T.F., Staebler, R.M., Jones, K.C., 2006. Measurement of DDT fluxes from a historically treated agricultural soil in Canada. Environ. Sci. Technol. 40, 4578–4585. Li, J., Zhang, G., Qi, S., Li, X., Peng, X., 2006. Concentrations, enantiomeric compositions, and sources of HCH, DDT and chlordane in soils from the Pearl River Delta, South China. Sci. Total Environ. 372, 215–224. Lunney, A.I., Zeeb, B.A., Reimer, K.J., 2004. Uptake of weathered DDT in vascular plants: potential for phytoremediation. Environ. Sci. Technol. 38, 6147–6154. Luo, L., Zhang, S., Shan, X.Q., Zhu, Y.G., 2006. Oxalate and root exudates enhance the desorption of p,p0 -DDT from soils. Chemosphere 63, 1273–1279. Nakata, H., Hirakawa, Y., Kawazoe, M., Nakabo, T., Arizono, K., Abe, S.I., Kitano, T., Shimada, H., Watanabe, I., Li, W., Ding, X., 2005. Concentrations and compositions of organochlorine contaminants in sediments, soils, crustaceans, fishes and birds collected from Lake Tai, Hangzhou Bay and Shanghai city region, China. Environ. Pollut. 133, 415–429. Suresh, B., Sherkhane, P.D., Kale, S., Eapen, S., Ravishankar, G.A., 2005. Uptake and degradation of DDT by hairy root cultures of Cichorium intybus and Brassica juncea. Chemosphere 61, 1288–1292. Tang, S.R., 2005. The Principles and Methods of Phytoremediation of Contaminated Environment. Chinese Scientific Press, Beijing. pp. 1–298.
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