Remediation of Dicofol Type Ddts-Contaminated Sediments by Ferrous Activated Sodium Persulfate Oxidation

In recent years, contamination by dicofol-type DDTs has attracted immense concern as a new source of DDT pollution. In this study, sediment samples from a dicofol manufacturing factory in Tianjing, China exhibited serious DDT contamination [p,p'-DDE (115.27 mg kg) and p,p'-DDT (11.84 mg kg)]. Results of the batch experiments showed that total DDT degradation rates increase as S2O8/Fe molar ratios increase. The S2O8/Fe molar ratios used in this study were as follows: 60/10 < 10/30 < 20/30 < 60/50 < 60/20 < 40/30 < 60/40 < 60/30 < 80/30. Their corresponding degradation rates were 31, 43, 52, 69, 70, 71, 72, 89, and 91 μg g, respectively. The optimal S2O8/Fe molar ratio was 60/30, which resulted in 64% and 96% degradation of p,p'-DDE and p,p'-DDT, respectively. However, when an excessive amount of ferrous ion was used (<S2O8/Fe molar ratio of 60/30), then competition for SO4 between ferrous ion and DDTs resulted in decreased DDT degradation efficiency and increased persulfate decomposition (represented by the generated amount of sulfate). Our results implied that a slow and steady production of sulfate free radicals is favorable for DDT degradation, and that Fe availability plays an important role in controlling persulfate reactions activated by ferrous ion. Fe-activated persulfate oxidation may be significant in developing environment friendly and fast-remediation options for DDT-contaminated sediments and soil. Therefore, this study contributes to current knowledge on remediating DDT contamination.


Introduction
DDT [1,1,1-trichoro-2,2-bis(p-chlorophenyl)-ethane] is one of the persistent organic pollutants (POPs) identified by the Stockholm Convention on POPs which has been extensively used for controlling agricultural pests and disease-carrying insects such as malaria vectors (Zitko, 2003;Kamanavalli & Ninnekar, 2004).DDT is more stable than other organochlorine pollutants because of its chlorinated aliphatic and aromatic structures.Exposure to DDTs (DDT and its homologues) may damage the human nervous and reproductive systems (Guo et al., 2009).Although the manufacture and application of DDTs have been restricted since the 1970s because of their negative effects, traces of DDTs can still be detected in air, water, soil, sediments, and organisms (Bettinetti et al., 2008;Yao et al., 2006).Moreover, the 2001 Stockholm Convention on POPs still allows the use of DDTs in several countries, such as South Africa, to control the transmission of malaria.DDE [1-Chloro-2-[2,2-dichloro-1-(4-chlorophenyl)ethenyl]-benzene] is a common metabolite of DDT (Ssebugere et al., 2010;Yang et al., 2010).Environmental DDE originates from the metabolites of DDT resulting from aerobic biotic, abiotic, and photochemical degradation, as well as from technical-grade DDT contaminants (Thomas et al., 2008).DDE has been reported to be more persistent than DDT and can be detected in soil decades after the application of DDT (Thomas et al., 2008).According to the United States Geological Survey (USGS), p,p'-DDE content in America was 60% in urban areas and 48% in rural areas in 1999 (Thomas et al., 2008).As a potent androgen antagonist (Kelce et al., 1995), DDE has also been found to be the most abundant DDT component in sediments (Eganhouse & Pontolillo, 2008), fish, and humans (Kamanavalli & Ninnekar, 2004).controlling mites.Dicofol is usually synthesized from technical p,p'-DDT.During synthesis reaction (Scheme 1), p,p'-DDT is first chlorinated into Cl-DDT, and then hydrolyzed into dicofol (Qiu et al., 2005).

Scheme 1. Synthesis reaction of dicofol by p,p'-DDT
To date, China still produces 5000 tons to 6000 tons of DDT per year as raw and processed materials for dicofol production (Huang et al., 2007).Moreover, approximately 8770 tons of DDTs were released into the environment in China by dicofol-type DDT contamination from 1988 to 2002 (Qiu et al., 2005;Turgut et al., 2009).
Although biodegradation and anaerobic reductive dechlorination for remediating DDTs have been well studied (Li et al., 2010;You et al., 1996), investigations on aerobic oxidative degradation for remediating DDTs is limited.Chemical oxidation via persulfate oxidation activated by ferrous ion has been evaluated as an option for treating chlorinated organic contaminants, such as trichloroethylene (TCE; Liang et al., 2004aLiang et al., , 2004b;;Liang et al., 2008); tetrachloroethylene, dichloroethylene, and dichloroethane (Abranovic et al., 2006); polychlorinated biphenyls and polycyclic aromatic hydrocarbons (Block et al., 2004); and lindane (γ-HCH; Cao et al., 2008).Sulfate free radicals (SO 4 -) can be formed rapidly through persulfate-ferrous ion reaction at ambient temperature (20 °C) (Liang et al., 2004a).These free radicals can potentially degrade organic contaminants within soil mass by in situ chemical oxidation.The stoichiometric reaction between persulfate and ferrous ion is shown in the following equations (Kolthoff et al., 1951): The ratio of reaction between S 2 O 8 2-and Fe 2+ is dependent on the concentration of each reactant.When the reaction is near to stall, increasing the concentration of Fe 2+ will accelerate the reaction shown in Equation (1).However, the target chlorinated organic contaminant and the excess Fe 2+ will compete for SO 4 -, as shown in Equation (2).Gradual addition of small quantities of Fe 2+ is necessary to optimize S 2 O 8 2-oxidative degradation of the target chlorinated organic contaminant and to control the reaction.To our knowledge, no Fe 2+ -activated persulfate oxidation technique for remediating DDT contamination in sediments has yet been reported.
The primary purpose of this study is to investigate the contamination caused by dicofol-type DDTs in a dicofol manufacturing factory in Tianjin, China and to evaluate the effectiveness of Fe 2+ -activated S 2 O 8 2-oxidative degradation for DDTs (p,p'-DDE and p,p'-DDT).In addition, the effects of various initial Fe 2+ and S 2 O 8 2concentrations on DDT degradation in sediments at ambient temperature (20 °C) are also investigated.

Sediment Collection and Preparation
Sediment samples were collected from Tianjin Renong Pharmaceutical Factory in China (Scheme 2).Sampling sites were highly contaminated by DDTs because of dicofol manufacturing before 2002.The three sampling sites were the factory floor, sewage, and drainage ditch.Sediments were collected from 20 cm to 40 cm sections mainly composed of fine sand with silt.The vessels were filled with sediments and completely sealed.The sediments were air-dried and ground with mortar until they could pass through a 2.0 mm sieve.The DDTs in the samples were detected via gas chromatography with mass selective detection (Table 1).Sample no. 3 was selected as the object for the batch study because it is highly contaminated with p,p'-DDE (115.27mg kg -1 ) and p,p'-DDT (11.84 mg kg -1 ).The soluble sulfate of sample no. 3 was 56.28 mg kg -1 and its pH was 7.7.In addition, small amounts of p,p'-DDD and o,p'-DDE were also detected in all samples.However, no attempt was made to quantify p,p'-DDD and o,p'-DDE because their concentrations were very low.
Scheme 2. The study area (Tianjin, China) and the sampling site (Tianjin Renong Pharmaceutical Factory) For each set of experiment, 1 mL reagent solution was added in the following order: (1) sodium persulfate and (2) ferrous ion (one-fifth every 5 min) (Liang et al., 2004a).Afterwards, the sediments and solutions were vortex-mixed and incubated at ambient temperature (20 °C).Sampling and detection of the remaining amounts of DDTs were conducted at 0, 1, 2, 3, and 4 h.The concentration of sulfate in the supernatant was detected by ion chromatography at 4 h.

Extraction and Analysis of DDTs
The DDTs in the sediments were extracted using the ultrasonic extraction method.Hexane (10 mL) was added to the serum bottles containing the sediments.The bottles were then sonicated for 30 min in 6 L water in an ultrasonic bath (Soniclean, Australia) to ensure particle and solvent mixing (Thangavadivel et al., 2011).Furthermore, the contents were vortex-mixed and the solvent with DDTs was separated and passed through a funnel filled with 2.0 g anhydrous sodium sulfate (Na 2 SO 4 ) to eliminate the remaining water in the samples (Hussen et al., 2006).Then, the extraction solvent with DDTs was evaporated to 2.0 mL using nitrogen gas.
The DDTs were analyzed by an Agilent 7890A gas chromatograph (Agilent Technologies, CA, USA) equipped with an Agilent 5975C mass selective detector (Agilent Technologies, CA, USA).The column used was HP-5MS, 30.00 m × 0.25 mm id with a film thickness of 0.25 μm.The injector temperature was 250 °C and the helium gas flow rate was 1.0 mL min -1 .The column temperature was initially set to 40 °C for 1 min, and then later increased at a rate of 30 °C min -1 to 130 °C min -1 .Afterwards, column temperature was switched to a rate of 5 °C min -1 to 160 °C min -1 for 6 min, and then, to a rate of 10 °C min -1 to 190 °C for 3 min.Finally, the temperature was switched to a rate of 20 °C min -1 to 300 °C and maintained isothermally for 2 min (Manirakiza et al., 2000).The injection volume was 1.0 μL in a splitless mode.

Analysis of the Sulfate in the Supernatant
According to Equations ( 1) and ( 2), persulfate will decompose into sulfate during reactions activated by ferrous ion.To evaluate the amount of decomposed persulfate, the concentration of supernatant sulfate was determined by ion chromatography.After 4 h reaction, 0.5 mL supernatant was passed through a 0.45 μm filter to eliminate impurities which may interfere with ion chromatography.The ion chromatography system, Dionex Ionpac AS14 column (4.6 mm × 3100.0 mm; Thermo Scientific, CA, USA), comprises a GP50 gradient pump, a column oven LC25, and an electrochemical detector ED50.Elution buffer was made of 3.5 mM sodium bicarbonate and 1.0 mM sodium carbonate.The flow ratio was 1.2 mL min -1 .

Assessment of DDT Contamination in Sampling Sites
Although the sampling site was a dicofol manufacturing factory which closed in 2002, no dicofol was detected in the samples because this pesticide is highly degradable in natural environments.However, DDT contamination remains serious even after nine years of production cessation.Dicofol impurity was proposed to possibly contribute to DDTs in the environment.This hypothesis was supported by the investigation of air samples collected over Taihu Lake, China during the summer of 2002, where very high concentrations of DDTs were found to be related to dicofol applications (Qiu et al., 2004), thus suggesting that dicofol is a possible source of DDTs which may constantly evaporate from soil to air.
According to the guidelines of the Chinese Environmental Quality Standard for Soil (GB15618-1995), the quality of soil can be classified as: with background pollution (grade I), low pollution (grade II), and high pollution (grade III).All three sampling sites were highly contaminated by DDTs, as shown in Table 1.p,p'-DDE and p,p'-DDT were the main DDT components and their concentration (1 mg kg -1 ) was much higher than that of grade III soil.Moreover, p,p'-DDE concentration was higher than p,p'-DDT concentration in all samples.The results confirmed previous findings that DDE is the most abundant DDT component in soil and sediments (Guo et al., 2009), and is hardly degraded compared with other DDT components (Thomas et al., 2008;de la Cal et al., 2008).Further investigations and an effective remediation option for dicofol-type DDT contamination are recommended.The samples were collected in 2010.Mean ± SE values (mg kg -1 ) are shown (n = 3).Means are significantly different (one-way ANOVA: p < 0.05).

Figure 1 .
Figure 1.Dynamics of DDT degradation with different persulfate concentrations . The possible consumption of sulfate free radicals may result from reactions with H 2 O, S 2 O 8 2− , and excess Fe 2+(Kolthoff et al.,   & Waygood, 1990), as shown in Equations (

Figure 3 .
Figure 3.Time course of DDT degradation with different concentrations of ferrous ion

Table 1 .
Concentrations of DDTs in samples obtained from the three sampling sites

Table 3 .
Influence of different ferrous ion concentrations on the degradation of (A) DDE and (B) DDTThe assessment of the site for a dicofol manufacturing factory showed serious DDT (p, p'-DDE and p, p'-DDT) contamination in sediments.DDT degradation and persulfate decomposition were observed by calculating the amount of ferrous ion and persulfate at ambient temperature (20 °C).This study demonstrated that sulfate free radicals formed by ferrous ion activation are capable of degrading DDTs in sediments.Total DDT degradation ratios at S 2 O 8 2− /Fe 2+ molar ratios of 31,