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Environmental Pollution 159 (2011) 700e705

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Environmental Pollution

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Sequestration of organochlorine pesticides in soils of distinctorganic carbon content

Na Zhang a,b, Yu Yang a, Shu Tao a,*, Yan Liu a, Ke-Lu Shi a

a Laboratory for Earth Surface Processing, College of Urban and Environmental Sciences, Peking University, Beijing 100871, Chinab State Key Laboratory for GeoMechanics and Deep Underground Engineering, China University of Mining and Technology, Beijing 100083, China

The effect of soil organic matter on the sequestration of organochlorinin an innovative microcosm chamber.

e pesticides (HCHs and DDTs) in soils was investigated

a r t i c l e i n f o

Article history:Received 15 September 2010Received in revised form2 December 2010Accepted 3 December 2010

Keywords:Organochlorine pesticides (OCPs)SequestrationSoilsSource identification

* Corresponding author.E-mail address: taos@urban.pku.edu.cn (S. Tao).

0269-7491/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.envpol.2010.12.011

a b s t r a c t

In the present study, five soil samples with organic carbon contents ranging from 0.23% to 7.1% and agedwith technical dichlorodiphenyltrichloroethane (DDT) and hexachlorocyclohexane (HCH) for 15 monthswere incubated in a sealed chamber to investigate the dynamic changes of the OCP residues. The residuesin the soils decreased over the incubation period and finally reached a plateau. Regression analysis showedthat degradable fractions of OCPs were negatively correlated with soil organic carbon (SOC) except for a-HCH, while no correlation was found between degradation rate and SOC, which demonstrated that SOCcontent determines the OCP sequestration fraction in soil. Analysis of the ratio of DDT and its primarymetabolites showed that, since it depends on differential sequestration among them, magnitude of (p,p0-DDE þ p,p0-DDD)/p,p0-DDT is not a reliable criterion for the identification of new DDT sources.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Organochlorine pesticides (OCPs), such as dichlorodiphenyltri-chloroethane (DDT) and hexachlorocyclohexane (HCH), are a classof toxic, hydrophobic, chlorinated, organic compounds that havewidely been used in agriculture and public health across the world(Li and Macdonald, 2005). Their agricultural uses were generallyprohibited beginning in the early 1970s due to their accumulationin food chain and their toxic effects (Harner et al., 1999; Kurt-Karakus et al., 2006). To date, great attention has been paid totheir ubiquitous occurrence, unusual persistence, and adverseeffects in the environment (Turusov et al., 2002).

Soils are an important sink for persistent organic pollutants,especially for OCPs, because of their low solubility and volatility, aswell as their strong sorption by soil organic matter (SOM)(Koblizkova et al., 2009; Kumar et al., 2009; Tao et al., 2008). And ithas generally been accepted that most hydrophobic organiccompounds (HOCs) retained in soils were bound to SOM throughcovalent bonds, hydrophobic interactions and diffusion-controlledpartitioning (Marschner, 1999; Pignatello, 1998). Furthermore, SOMcan sequestrate a large fraction of HOCs due to their strong sorptionto glassy carbon domains anddiffusion into nano-scaledmicropores

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(Barriuso et al., 2008; Cornelissen et al., 2005; Luthy et al., 1997;Semple et al., 2003; Zimmerman et al., 2004). Consequently,sequestration can significantly decrease the bioavailability ofcontaminants and thereby reserve them in the soils for a long time.Therefore, a better understanding of how SOM controls OCPsequestration in soils is critical for predicting their fate and evalu-ating their health risks in the environment.

So far, intensive research on sequestration have mainly focusedon macroscopic observations to infer interaction/sorption mecha-nisms between HOCs and SOM and identify important factors(e.g. aqueous solubility, polarity, hydrophobicity, and molecularstructure of contaminants and the nature of SOM) that may affectsequestration rate and extent of HOCs (Abu and Smith, 2005; Luthyet al., 1997;White et al., 1997;Williamson et al., 2002). For example,researchers found the nature of SOM, especially with respect topolarity and aromatic carbon content, appears to be a critical factorin controlling reactivity with HOCs through determination the Kocof the model HOCs between different kinds of SOM and water(Kile et al., 1995). Sequestration dynamics of HOCs in aged soil wasalso examined in previous studies (Chung and Alexander, 1998;Lichtenstein et al., 1977). Chung and Alexander (1998) studied thesequestration dynamics of phenanthrene and atrazine in sterilizedsamples of 16 soils that differed greatly in physical and chemicalproperties, and found that both compounds became sequestered inindividual soils, but the rate and extent of sequestration variedmarkedly among the soils (Chung and Alexander, 1998).

Table 1Average concentrations (ng/g) of HCH and DDT isomers in soils of distinct organiccarbon content at each sampling time point over the incubation period.

Time (day) a-HCH b-HCH g-HCH d-HCH p,p0-DDT o,p0-DDT

0 165.3 67.9 134.4 437.2 32699.1 5554.43 109.1 44.1 98.0 379.3 31010.7 5077.07 83.9 28.6 77.8 239.6 24907.8 4807.915 73.3 21.6 58.7 190.7 16165.7 2891.930 52.4 19.7 42.5 150.3 15295.3 2565.672 15.5 15.6 25.8 131.8 12476.9 2399.4

N. Zhang et al. / Environmental Pollution 159 (2011) 700e705 701

However, direct evidence of an overall effect of soil organiccarbon (SOC) content on the sequestration of OCPs is still limited.The main objective of this study, therefore, was to investigate 1)how SOC content determines OCP sequestration fraction in soil; 2)sequestration differences among OCPs and their metabolites; 3) theeffect of sequestration on OCP residues in soil.

2. Materials and methods

2.1. Soil aging

The soils were collected from Shi San Ling, Changping County, Beijing, whereOCP residue levels are relatively low. The SOC content of the five samples was 0.23,1.06, 2.09, 4.53 and 7.06%. The samples were air-dried and finely ground beforesterilization in an autoclave. The sterilized soils were then spiked with technicalHCH and DDT in acetone. After acetone evaporation, the soils were sealed in amberglass bottles and kept in the dark at room temperature for 15 months to allowthorough interaction between pesticides and SOC.

2.2. Incubation

The aged soil samples were inoculated with a soil extract from a raw sample of7.06% SOC (1:100) and adjusted to 70% of the water holding capacity by addingdeionized water. The inoculated soils were incubated in wide-mouth glass bottles(2 L) linked by airtight connections. In each bottle, 15 soil samples (triplicate for eachSOC level) were installed for each sampling time point. Each sample was 3 g freshweight and kept in a plastic cap 29 mm in diameter. The depth of the soil sampleswas approximately 0.5 cm. The air in the chamber was circulated by an air-pumpattached to one of the bottles. The soils were incubated for 3, 7, 15, 30, and 72 days,and sampled by removing one bottle at a time. The air concentration of OCPs wasmonitored using polythene membranes of 1 cm � 1 cmwhich were deployed in thechamber (two in each bottle) as passive samplers for 6 h and sampled on the sameday as soil sampling.

2.3. Sample extraction and analysis

Extraction of total OCP residue from soils: the soil samples were freeze-driedand Soxhlet extracted using a mixture of acetone and dichloromethane (1:1, v/v) for15 h. The cleanup of the extracts was performed using silica gel columns eluted with25 ml of n-hexane and 35 ml of dichloromethane at a flow rate of 2 ml/min.

Passive air-sample extraction: the membranes were Soxhlet extracted byacetone for 8 h. The solvent volume was then reduced to 1 ml and changed ton-hexane for analysis. Concentrations of OCPs in the extracts were analyzed usinga GC with ECD detector (Agilent GC 6890/ECD 5973). TCMX (J&K Chemical, USA,1.0 mg/ml) was used as internal standard for sample quantification.

Butanol extraction of soil samples: Extraction followed a procedure from theliteratures (Nam et al., 1998; Northcott and Jones, 2001, 2003). In brief, an aliquot(w0.1 g; freeze-dried) of each soil sample was extracted with 1 ml n-butanol ina 1 ml plastic vial for 24 h on a horizontal shaker (150 rpm) at room temperature.The sample was then centrifuged at 3000 rpm for 15 min. The supernatant wasconcentrated to 100 mL on a rotary evaporator at 80 �C and the solvent was changedto n-hexane. The cleanup and analysis of the supernatant and the extraction of theresidue that could not be extracted by butanol followed the same procedure as fortotal residue extraction.

2.4. Quality control

One procedure blank was analyzed for every fifteen soil samples by extractingand cleaning up w1 g of anhydrous sodium sulfate. All procedures were carriedout in triplicate. Since there were no significant differences among blanks, theiraverage concentrations were used. Method recoveries for p,p0-DDT, o,p0-DDT, p,p0-DDE, o,p0-DDE, p,p0-DDD, o,p0-DDD, a-HCH, b-HCH, g-HCH and d-HCH in spiked soilsranged from 70.0 to 96.5%. The method detection limits were 0.1 (a-HCH)e4.5 (o,p0-DDT) ng/g.

3. Results and discussion

3.1. Dynamic changes of DDT and HCH residues in soils

Organic pollutants aged in field soil may undergo soil-airexchange, degradation, and otherdissipative processes like leaching(Semple et al., 2003). In the current indoor simulation experiment,the concentrations of DDTs (p,p0-DDT and o,p0-DDT) and HCHs(a-HCH, b-HCH, g-HCH and d-HCH) in all five soils greatly decreasedover the incubation period mostly due to soileair exchange and

biodegradation (Table 1; Fig. S1). The concentrations of HCHs andDDTs in soils with different SOC contents at time 0 (Table S1) wereassumed to be at a same level because all five soils were spikedwithsame amount of OCPs before aging treatment. However, smallvariations among soils could not be avoided because some minorerrors were inevitably introduced in the whole experimental andanalytical processes. On the other hand, the concentrations of DDTsand HCHs in air volatilized from soil were very low and onlychanged slightly over the incubation period (Fig. S2.). The averageconcentrations of p,p0-DDT, o,p0-DDT, a-HCH, b-HCH, g-HCH and d-HCH in air were 0.22, 0.18, 0.15, 0.32, 2.47 and 4.12 ng/m3, respec-tively. The mass ratios of the chemicals in air to that in soil were3.7 � 10�10 for p,p0-DDT, 1.8 � 10�9 for o,p0-DDT, 5.1 � 10�8 for a-HCH, 2.7 � 10�7 for b-HCH, 1.0 � 10�6 for g-HCH, and 5.2 � 10�7

for d-HCH. Lowmass ratios of the target compounds suggested thattheir soileair exchanges were extremely low, which can almost beignored in this incubation system. This was reasonable because theOCPs were not liable to volatilization due to their low vapor pres-sures (Zhang et al., 2009) and strong interaction with SOM afteraging for more than one year. Therefore, the reductions in OCPresidues in soils over the incubation period were primarily due totheir biological degradation.

Biodegradation dynamics of DDTs and HCHs in five soils ofdistinct SOC contents were fitted into a dynamic model:

y ¼ a��1� expð�b�xÞ

�(1)

(where parameter a represents the maximum degradation fraction;and parameter b represents the degradation rate), by regressing thedegradation loss fraction of the OCP residues against the incubationtime (Fig.1). The regressions had high R2 values formost of the OCPsin all soils (Table S2). For all the chemicals, the regression linesincreased steeply in the first 15 days and then gradually approacheda plateau level. The loss fractions in the soils rose swiftly in the first15 days from zero to 0.42e0.66 for p,p0-DDT, 0.26e0.80 for o,p0-DDT,0.55e0.75 for a-HCH, 0.51e0.83 for b-HCH, 0.50e0.78 for g-HCHand 0.54e0.66 for d-HCH, which suggested that marked degrada-tion of DDTs and HCHs occurred in the incubated soils in the earlyphase of the incubation. The loss fractions of OCPs in all soilsreached a plateau after 72 days of incubation indicating that theresidues finally retained in the soils were mainly unbiodegradablefractions sequestered or strongly bound by the SOM (Semple et al.,2003). Because the whole incubation was conducted in completedarkness and almost thermostatic environment, no photo-degradation or thermal degradation occurred, so the primarydegradationmechanismwas confined to biodegradation. Therefore,it is interesting to note that remarkable degradation of DDTs tookplace in the incubated soils, which is inconsistent with the tradi-tional perception that DDT is recalcitrant to degradation (Aislabieet al., 1997; Alexander, 1973; Barker et al., 1965; Corona-Cruzet al., 1999). Rather, DDT appears to be retained in soils because ofstrong sequestration. However, the bacteria capable of degradingDDT existing in the incubated soils need to be further identified for

Frac

tion

of L

oss

Frac

tion

of L

oss

Frac

tion

of L

oss

1.0 p,p’ -DDT

0.8

0.2

0.0 0 20 40 60 80

0.4

0.6

1.0 o,p’ -DDT

0.8

0.2

0.0 0 20 40 60 80

0.4

0.6

1.0 α

γ δ

β-HCH

0.8

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0.0 0 20 40 60 80

0.4

0.6

1.0 -HCH

0.8

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0.0 0 20 40 60 80

0.4

0.6

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0. 0 0 20 40 60 80

0.4

0.6

Time ( da y)

1.0

0.8

0.2

0.0 0 20 40 60 80

0.4

0.6

Time ( da y)

-HCH -HCH

SOC3 (2.1%) SOC1 (0.2%)

SOC2 (1.1%) SOC4 (4.5%)

SOC5 (7.1%)

Fig. 1. Dynamic loss of DDT and HCH isomers in soils of distinct organic carbon content, with regression lines.

N. Zhang et al. / Environmental Pollution 159 (2011) 700e705702

potential use in the remediation of soil/sediment contaminationby DDT.

tneiciffeocnoisserge

Ra

1.0

0.0

0.5

SOC (%)0 4 8

R2=0.99

2 6

γ -HCH

δ -HCH

p'p-DDT

o'p-DDT

β -HCH

R2=0.89

R2=0.80R2=0.95

R2=0.82

Fig. 2. Correlation relationships between the regression parameter a (Eq. (1)) and soilorganic carbon content for DDT and HCH isomers except a-HCH.

3.2. Effect of SOC content on sequestration fraction

It was observed that the parameter a (Eq. (1)) for all chemicalsexcept a-HCH was negatively correlated with SOC (p < 0.05)(Fig. 2). However, there were no correlations between parameterb (Eq. (1)) and SOC. This suggested that a lower fraction of OCPresidues in soil was degraded as the SOC content increased. It has tobe noted that the variation of parameters (a and b) from equation(1) may affect the correlation analysis. However, our data showedthat the variation was not too high to change the general trend ofthe analysis result (Table S3). For soils with low SOC, a portion ofOCPs could be deposited to soil particles or bound to minerals.Interaction between the deposited OCPs and minerals or soilparticles is weaker than that chemically bound to SOC, thus they aremore readily to be degraded by microorganisms. Moreover, thedeposited OCPs have high bio-accessibility which may facilitatetheir degradation.

Another possibility is that for soils with low SOC, the availabilityof organic carbon to microorganisms is limited. Microorganismsmay use OCPs as C source. In comparison, for soils samples withhigh SOC content, microorganisms would use their organic carbon

as C source. Therefore, it can be concluded that an increase in SOCcontent would enhance the sequestration of OCPs in soils. Theregression equation (Table S4) between parameter a and SOC can beused to estimate the degradable fraction of OCPs and the seques-tered fraction in soils of different SOC content.

N. Zhang et al. / Environmental Pollution 159 (2011) 700e705 703

Butanol extraction was done for soil samples incubated fordays 0 and 72 to further confirm the changes of the bioavailable andnon-bioavailable (sequestrated) fractions, because it is wellaccepted that the butanol-extractable portion represents thebioavailable fraction, while the non-extractable portion representsthe sequestrated fraction (Kelsey et al., 1997; Nam et al., 1998; TerLaak et al., 2006). It was shown that the butanol-extractablecontent decreased dramatically while the non-extractable contentremained at a stable level (Fig. S3), indicating that the bioavailablefraction was depleted by soil microorganisms after 72 days ofincubation, while the non-bioavailable residues were retained dueto sequestration of SOC.

In addition, the butanol-extractable fraction at day 0 (FBE0) andparameter a (Eq. (1)) were the same, and the positive correlationsbetween them for most of the OCPs (excluding a-HCH and b-HCH)verified that the butanol-extractable fraction was a good indicatorof the bioavailable fraction (Fig. S4). However, comparison ofparameter awith the experimental data indicated that the butanolextraction method overestimated the bioavailable fraction (Fig. S5).

3.3. Effect of octanol-air partition coefficienton sequestration fraction

The octanoleair partition coefficient (Koa) is a good predictor ofpartitioning between air and the organic phase for HOCs. The morehydrophobic chemicals usually have larger Koa. Relationshipsbetween degradation regression parameter a and the Koa of OCPswere examined to disclose the effect of physicochemical propertiesof pollutants on the sequestration mechanism. We found negativecorrelations (p < 0.05) between parameter a and logKoa for soilswith SOC higher than 2% (Fig. 3) indicating that more hydrophobiccompounds had higher fractions of sequestration in soils with SOChigher than 2%.

3.4. Dynamic change of association of DDT and HCH residueswith SOC

The correlation relationships between the DDT and HCH residueconcentrations in soils and SOC gradually strengthened over theincubation period (Figs. S6 and S7). The correlation coefficientsincreased from 0 at the beginning (0 d) to nearly 1.0 at the end ofthe incubation (72 d), giving a positive correlation (p < 0.05)between OCP concentrations and SOC after 72 days, which wascaused by the overall effect of degradation by soil microorganismsand sequestration by SOC of OCPs over the incubation period asdiscussed above.

tneiciffeoc noissergeR

a

1.0

0.0

0.5

log Koa

7.5 8.5 10.0

R2=0.82

8.0 9.5

SOC4 (4.5%)

SOC5 (7.1%)

SOC3 (2.1%)

R2=0.78

R2=0.80

9.0

Fig. 3. Correlation relationships between the regression parameter a (Eq. (1)) andlogKoa of DDT and HCH isomers in soils with organic carbon contents >2%.

3.5. Dynamic change of DDT metabolite accumulation in soils

DDE and DDD are the major primary degradation metabolites ofDDT in soil (Aislabie et al., 1997). Specifically, DDE is the mainproduct in an aerobic environment while DDD is produced underanaerobic conditions (Aislabie et al., 1997). We found that both DDEand DDD accumulated rapidly after the incubation began andreached peaks on day 15 because of significant early biodegradationof parent DDT in soils of all SOCs (Fig. 4). The concentrations of DDEand DDD then started to decrease, presumably because thedegradation rates of DDE and DDD exceeded that of DDT after day15. The technical DDT used in this study contained small fractions ofimpurities, including DDD (p,p0-DDD: 3.7%; o,p0-DDD: 2.1%) andDDE (p,p0-DDE: 0%; o,p0-DDE: 0.1%). So it was inferred that the DDDin soils was partially introduced from the technical DDT mixtureduring the aging process; the values were 795.70 ng/g for p,p0-DDDand 148.93 ng/g for o,p0-DDD at the starting point of incubation.However, although the total DDE accumulation in soils wasassumed to have been produced through DDT degradation, someDDEwas found at the beginning of incubation (118.14 ng/g p,p0-DDEand 128.05 ng/g o,p0-DDE). Generally, the accumulation dynamicsof DDE and DDD were governed by the degradation rates of DDT,and that of DDE and DDD themselves, which were limited by thesequestration rate to some extent.

DDE and DDD accumulation (accumulation ¼ concentrationt �concentrationt0) in soils attained a maximum around 15 days ofincubation (Fig. 4). Comparison of the increases of DDE and DDD insoils on day 15 showed that o,p0-DDE accumulation was muchhigher than o,p0-DDD in all soils and p,p0-DDEwas greater than p,p0-DDD in the soil with the lowest SOC (Fig. S8). Based on this, weconcluded that DDE was the dominant metabolite of DDT in ourincubated soils, which was reasonable considering the aerobicconditions in the incubation chamber. Furthermore, in soils oflower SOC the accumulation of DDE was higher than in other soilsbecause of the higher fraction of biodegradable DDT.

It was also very interesting to find that DDE accumulation insoils was considerably lower than DDD at the end of the incubation,and in lower SOC soils the accumulation was even negative, espe-cially for o,p0-DDE (Fig. S9). In particular, the ratios of p,p0-DDD top,p0-DDE accumulation and o,p0-DDD to o,p0-DDE accumulationwere 8.98 and 27.76 in soil of 7.06% SOC incubated for 72 days. All ofthe evidence indicated that DDD had stronger sorption to SOC thanDDE, so that more DDD than DDE was retained in soils because ofstronger sequestration, even though DDE was the dominantmetabolite.

The total accumulation of DDE and DDD in soils over the wholeincubation period was positively correlated with SOC (p < 0.1)(Fig. S10), illustrating that there was more accumulation ofmetabolites in soils of higher SOC probably due to strongersequestration. So it was concluded that SOC also played animportant role in controlling the sequestration fraction of DDTmetabolites even though the sequestration of DDE and DDDdiffered.

3.6. Dynamic change of (DDE þ DDD)/DDT

The ratio of DDT metabolites (DDE and DDD) to parent DDT(p,p0-DDE þ p,p0-DDD)/p,p0-DDT, has been used as a criterion tojudge if there are new sources of DDT input into the environment(Harner et al., 1999). It is generally accepted that, if the ration is >1,the DDT sources are regarded as old input, otherwise, there isa suspicion of potential new sources (Harner et al., 1999). This isbased on the simple concept that the longer the histories of DDTpollutants in soil, the more metabolites are generated by DDTdegradation. Also, it is traditionally believed that DDE and DDD are

)g/gn( .cnoc lioS

6000

0

3000

0 20 806040

)g/gn( .cnoc lioS

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Time (day) 0 20 806040

o,p’-DDE

4000

0

2000

0 20 80 6040

450

0

Time (day)0 20 80 6040

o,p’-DDD

p,p’-DDE p,p’-DDD

SOC1 (0.2%)

SOC2 (1.1%)

SOC3 (2.1%)

SOC4 (4.5%)

SOC5 (7.1%)

Fig. 4. Dynamic changes of DDE and DDD isomers in soils of distinct organic carbon content over the incubation period.

N. Zhang et al. / Environmental Pollution 159 (2011) 700e705704

more recalcitrant to degradation than DDT. Therefore, based onthese assumptions, some researchers suggested that the (p,p0-DDE þ p,p0-DDD)/p,p0-DDT ratio can indicate how long DDT hasbeen present in the environment and thus be used to differentiatenew sources from old.

However, the results of the current experiments showed thatthe ratio of (p,p0-DDE þ p,p0-DDD)/p,p0-DDT increased from nearlyzero at the start to a maximum on day 15, and then decreased toslightly above zero, instead of continuously increasing with time(Fig. 5). The ratios for soils with SOC in ascending order at day 15were 0.63, 0.28, 0.31, 0.23 and 0.20, while at the end of the incu-bation; the ratios were 0.23, 0.17, 0.15, 0.13 and 0.13.

The relationships between (p,p0-DDE þ p,p0-DDD)/p,p0-DDT andthe degradation fraction of DDT in the five soils showed no linearpositive correlation (Fig. S11). This further indicated that with thedegradation of a greater fraction of DDT, the ratio of (p,p0-DDE þ p,p0-DDD)/p,p0-DDT did not necessarily increase.

From the above, we suggest that (p,p0-DDE þ p,p0-DDD)/p,p0-DDT is not a reliable criterion for the identification of new DDTsources, because it is based on a controversial theoretical founda-tion. The key factors that dominated the ratio for aged DDT insoil were degradation and sequestration of both DDT and the

TD

D/)D

DD+E

DD(

0.70

0.35

0.000 8020

Time (day)40 60

SOC1 (0.2%)

SOC2 (1.1%)

SOC3 (2.1%)

SOC4 (4.5%)

SOC5 (7.1%)

Fig. 5. Dynamic changes of (p,p0-DDE þ p,p0-DDD)/p,p0-DDT in soils of distinct organiccarbon content over the incubation period.

metabolites DDE and DDD. We found that DDT, DDE, and DDD allhad significant degradation. So the ratio is finally controlled bytheir differences in sequestration.

Acknowledgement

Funding for this study was provided by the National BasicResearch Program (2007CB407301), the Ministry of EnvironmentalProtection (200809101), NIEHS (P42 ES016465), and the NationalNatural Science Foundation of China (40730737). We thank LisaCarlson and Iain Bruce for proof reading of the manuscript.

Appendix. Supplementary information

Supplementary information associated with this article can befound in the online version, at doi:10.1016/j.envpol.2010.12.011.

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