Complete Dechlorination of DDE/DDD Using Magnesium/Palladium System

By Gautam, Sumit Kumar; Suresh, Sumathi

ABSTRACT:

Kinetic studies on the dechlorination of 1,1-dichloro-2,2 bis (4,- chlorophenyl) ethane (DDD) and 1,1,dichloro-2,2 bis (4,- chlorophenyl) ethylene (DDE) in 0.05% biosurfactant revealed that the reaction follows second-order kinetics. The rate of reaction was dependent on the presence of acid, initial concentrations of the target compound, and zerovalent magnesium/tetravalent palladium. Gas chromatography-mass spectrometry analyses of DDE dechlorination revealed the formation of a completely dechlorinated hydrocarbon skeleton, with diphenylethane as the end product, thereby implying the removal of all four chlorine atoms of DDE. In the case of DDD, we identified two partially dechlorinated intermediates [namely, 1, 1-dichloro-2, 2 bis (phenyl) ethane and 1, chloro-2, 2 bis (phenyl) ethane] and diphenylethane as the end product. On the basis of products formed from DDD dehalogenation, we propose the removal of aryl chlorine atoms as a first step. Our investigation reveals that biosurfactant may be an attractive solubilizing agent for DDT and its residues. The magnesium/palladium system is a promising option because of its high reactivity and ability to achieve complete dechlorination of DDE and DDD. Water Environ. Res., 79, 430 (2007).

KEYWORDS: 1,1-dichloro-2,2 bis (4,-chlorophenyl) ethane; 1,1,dichloro-2,2 bis (4,-chlorophenyl) ethylene; dechlorination; diphenylethane; magnesium; palladium; zerovalent.

doi: 10.2175/106143006X115336

Introduction

The major objective of our investigations presented in this paper was to evaluate a bimetallic system, namely a magnesium/ palladium system, for its efficacy to dehalogenate 1,1-dichloro-2,2 bis (4,- chlorophenyl) ethane (DDD) and 1,1,dichloro-2,2 bis (4,- chlorophenyl) ethylene (DDE), which are the major dead-end, lipophilic products formed through restricted transformation of the pesticide 1,1-trichloro-2, 2-bis (4-chlorophenyl) ethane (DDT). Recently, Pandit et al. (2001) reported the presence of high concentrations of DDE and DDD in the coastal marine environment of Mumbai, India. Kumari et al. (1996) reported the accumulation of DDE and DDD in soils and ponds in India. Most often, the enhanced level of DDD and DDE in the natural environment is a direct consequence of incomplete microbial biotransformation of DDT, and these products are more recalcitrant compared with the parent compound (Jablonski and Ferry, 1992; Kale et al., 1999; Pfaender and Alexander, 1972; You et al., 1996). The main source of DDD in the contaminated environment is from the breakdown of DDT under reducing conditions, catalyzed by both biotic and abiotic systems. DDD was also marketed as an insecticide under trade names such as rothane and dilene (Bumpus and Aust, 1987). DDE is formed from DDT by removal of a single chlorine atom via a process called dehydrodechlorination.

DDE and DDD are known to disturb the metabolic functioning of living organisms. DDE acts as an antiandrogen, binds to the androgen receptor, and inhibits transcriptional activation, causing sexual abnormalities (World Health Organization, 2003). Ferreira et al. (1997) demonstrated that DDE and DDD cause uncoupling of oxidative phosphorylation, an important energy-yielding process in higher organisms. Yanez et al. (2004) reported DDE- and DDD-related damage of DNA in blood cells of women. Based on the above described toxic effects of DDD and DDE on the crucial metabolic functions in biological systems, it becomes imperative to develop technologies for remediation of these recalcitrant compounds in water and soil compartments of the environment.

Although there has been extensive research around the world to develop methods for remediating DDT, there is very little information related to degradation of DDE and DDD in the literature. Zerovalent-metal-mediated reductive dechlorination is a potent and emerging technology for remediating organochlorine pesticides. There are number of reports on the successful dechlorination of environmental pollutants, such as polychlorinated biphenyls (PCBs), DDT, chlorinated methanes, chlorinated ethylene, and chlorophenols by zerovalent metal systems. For example, Graham and Jovanovic (1999) successfully dechlorinated p-chlorophenol using zerovalent iron (Fe)/palladium (Pd) in a magnetically stabilized fluidized bed reactor. Engelmann et al. (2001) demonstrated dechlorination of 50 mg/L (50 ppm) DDT using zerovalent magnesium (Mg)/tetravalent palladium (Pd^sup +4^) using water-acetone as the reaction phase. Grittini et al. (1995) demonstrated dechlorination of PCBs completely, within 5 to 10 minutes, using the bimetallic system divalent palladium (Pd^sup 2+^)/Fe. Iron acted as the electron donor and palladium as the catalyst to dechlorinate PCBs via hydrogenation. Dechlorination reactions are typically initiated by the ionization of zerovalent metals, as proposed by Matheson and Tratnyek (1994). In the next step, electrons are captured by protons to generate nascent hydrogen (H*), which, in turn, are presumed to react rapidly with a catalytic metal (palladium, nickel, cobalt, or platinum), if present, to produce the corresponding metal hydride. The target compounds react rapidly with the metal hydride and are reductively dehalogenated. Major factors that influence the rates and extent of dechlorination by zerovalent metal systems are the following:

(1) Ionization potential and electrode potential (E) of the zerovalent metal,

(2) Solubility of the metal hydroxide formed following corrosion of metal,

(3) Availability of protons, and

(4) Solubility of the target compound.

Zerovalent magnesium (Mg/divalent magnesium [Mg^sup +2^]) with an ionization potential of -2.2V (compared with -0.44 V for Fe/ divalent iron [Fe^sup +2^]) and higher solubility of magnesium hydroxide favors the faster dechlorination reaction. Lower pH ensures the solubility of metal hydroxide (which, in turn, promotes faster corrosion of zerovalent metal) and availability of adequate protons for generation of nascent hydrogen. Finally, lipophilic compounds, such as DDD/DDE, require a suitable solvent to enhance their solubility and mass transfer to the hydrogenating catalyst. Solubilizing agents, such as organic solvents or surfactants, need to be included in the reaction phase. However, the use of a solvent, such as acetone, is expensive and also poses a health risk. Therefore, the use of alternate ecofriendly solubilizing agents, such as a biosurfactant, may be a better option. To our knowledge, no study appeared in the literature aimed at complete dechlorination of DDD and DDE using a bimetallic system. Here we present our research on the complete dechlorination of DDD and DDE using the Mg/ Pd^sup +4^ system in 0.05% v/v biosurfactant solution in distilled water.

Materials and Methodology

Source of Chemicals. Magnesium (Mg) granules (approximately 20 mesh); hexachloropalladate (FV) dipotassium (K^sub 2^PdCl^sub 6^); DDD; DDE; 1-chloro-2, 2-bis (p-chlorophenyl) ethylene (DDMU); 2, 2- bis (p-chlorophenyl) acetic acid (DDA); and 2, 2-bis (pchlorophenyl) ethanol (DDOH) were purchased from SigmaAldrich Chemical Company (St. Louis, Missouri). Acetone and cyclohexane were purchased from Merck India Ltd. (Mumbai). All chemicals were of high purity and analytical grade. No pretreatment was performed with any of the chemicals, and they were used as received. Biosurfactant JBR425 was received from Jeneil Biosurfactant Company Ltd. (Saukville, Wisconsin). The JBR425 was an aqueous solution of rhamnolipids at 25% concentration. Two major rhamnolipids present in solution were decanoic acid (3-[(6deoxy-L-mannopyranosyl) oxyl]-, l- (carboxymethyl) octyl ester) and decanoic acid (3-[[6-deoxy-2-O-(6- dexoy-L-mannopyranosyl)L-mannopyranosyl] oxyl-, l-(carboxymethyl) octyl ester). The pH of the biosurfactant was 6.5 to 7.0. Biosurfactant (0.05% v/v) solution was prepared in distilled water, and 2 mL of this solution was used for the experiments.

Experimental Protocol for Dechlorination of DDD and DDE. Dechlorination kinetic experiments were conducted in 2 mL 0.05%v/v biosurfactant solution in distilled water. Three different concentrations of target compounds (DDD = 93.75, 156.25, and 312.5 M and DDE = 93.34,157.23, and 313.47 M) were used to establish the order of reaction and rate constant (k) values. Desired concentrations of DDE or DDD were added to reaction tubes from 1000- mg/L (1000-ppm) stock solutions prepared in acetone. The glass reaction tubes were of 10 mL capacity. A small volume of acetone (100 L) that was introduced through addition of DDD/ DDE from stock solution was evaporated to total dryness. Subsequently, the dried ODD/DDE was taken up in 0.05% v/v biosurfactant solution in distilled water, to which 20 L/mL of glacial acetic acid was added. Appropriate volumes of K^sub 2^PdCl^sub 6^ stock solution (1 mg/mL) were added in each reaction tube to attain the required final concentration of 0.1 mg palladium/mL. Reactions were initiated by the addition of Mg granules (2.0 mg/mL). An instantaneous reduction of Pd^sup +4^ was confirmed by the disappearance of the reddish color of the salt followed by the formation of palladized magnesium. Typical duration of th\e reaction was 0.75 hours, that is, the entire 2-mL reaction mixture samples were used for extraction between O and 0.75 hours to monitor the kinetics of removal of DDD or DDE. Two sets of appropriate control experiments were conducted in parallel to determine the extent of DDD and DDE dechlorination, if any (1) using Pd^sup +4^ (concentration mentioned above) in the absence of Mg and (2) using Mg (concentration mentioned above) in the absence of Pd^sup +4^ under the same conditions as the test samples. Also, 24-hour experiments were conducted to determine the extent of dechlorination of 30-mg/ L (30-ppm) DDD and DDE in the absence of acid (experimental conditions were similar to the studies done in the presence of acid). All reactions were conducted in triplicate under atmospheric pressure, with continuous shaking in a water bath maintained at 30C. No precautions were taken to exclude oxygen or reduce oxidation-reduction (redox) potential of the reaction phase. Entire reaction mixtures were used at chosen time points, extracted thrice using cyclohexane (6 mL total), and 0.2 or 1.0 L volume of the pooled hexane extracts were injected for gas chromatographyelectron capture detection (GC-ECD) analyses. The identity of the dechlorinated intermediate and end products formed following the reaction were established by gas chromatography-mass spectrometry analyses.

Gas Chromatographic Analyses. Gas Chromatography-Electmn Capture Detection Analyses. Analyses of extracted samples were done using an Agilent model 6890 gas chromatography instrument equipped with a nickel(63) electron capture detector (Agilent Technologies, Wilmington, Delaware). The column used was an HP-5 capillary column of 0.32 mm internal diameter, 0.25 m film thickness, and 30 m length (Agilent Technologies). Injection was made in splitless mode using nitrogen as the carrier gas. The following temperature program was used: the initial oven temperature was 150C, with a hold time of 4 minutes, then ramped up by 6C/min to a final temperature of 250C, with a hold time of 4 minutes. The detector temperature was set at 300C. The residual concentrations of ODD/DDE, dechlorinated intermediates, and end products were quantified from peak areas obtained through automated integration and also by comparison with known concentrations of pure standard compounds.

Gas Chmmatography-Mass Spectrometry Analyses. The GCMS analyses were performed using a Hewlett Packard model G1800A gas chromatography instrument interfaced with an electron ionization detector (Hewlett Packard, PaIo Alto, California). An HP-1 capillary column was used with helium as the carrier gas (Agilent Technologies). The injection volume was 1 L. The column temperature was ramped up from 100 to 250C at 10C/min, to obtain separation and relative retention time data. The 70 eV electron impact mass spectra were obtained at the maximum of each peak eluted from the chromatogram. The mass spectral data coupled with systematic reduction in the retention times of dechlorinated products (resulting from successive loss of chlorine atoms) allowed identification of the intermediates and end products with reasonable certainty.

Results and Discussion

Stacked GC-ECD profiles following 30 minutes of reaction of DDE and DDD with Mg/Pd^sup +4^ in 0.05% v/v biosurfactant solution in distilled water in the presence of acid are depicted in Figures 1 and 2, respectively. Results indicate that the reaction is fast, with approximately 75% removal of 314.47 M DDE and approximately 60% removal of 312.5 M DDD within 30 minutes of reaction. We were unable to detect partially dechlorinated products during the course of the reaction or after its completion. The kinetic plots shown in Figures 3 and 4 (for DDE and DDD, respectively) indicate second-order kinetics of the dechlorination reaction for both compounds. The calculated second-order rate constants (k) listed in Table 1 clearly show that the rate of dechlorination was faster with DDE compared with DDD. This may be the result of a higher oxidized state of DDE, which is likely to be dechlorinated via the reductive route at a much faster rate. In the absence of acid, the extent of removal of 30 mg/L (30 ppm) of DDD and DDE by Mg/ Pd^sup +4^ were only 44 and 42%, respectively, even after 24 hours of reaction. Thus, these results clearly indicate the importance of acid for achieving complete removal of DDD and DDE. The control experimental results obtained following the reaction with magnesium alone (in the absence of palladium) showed only 36% removal of 30 mg/L (30 ppm) DDE after 24 hours of reaction, and there was insignificant removal of DDD. This removal of DDE may be explained on the basis of direct electron transfer from Mg to DDE, although the rate of reaction was extremely slow. Insignificant removal of DDD and DDE was observed following reaction with palladium alone (in the absence of magnesium). Sayles et al. (1997) studied dechlorination of DDT by Fe in the presence of a chemical surfactant, such as triton X-114. Additional metals, such as cobalt, nickel, and palladium, were not added, and rate constant values observed with and without surfactant were 1.7, and 3.0 days^sup -1^, respectively. The research group proposed dechlorination of DDT via direct electron transfer. The lower rate constants observed by Sayles et al. (1997) may be attributed to (1) lower rates of electron transfer and dechlorination by Fe and (2) precipitation of iron hydroxide under alkaline conditions and inhibition (passivation) of Fe corrosion.

The identity of intermediates and end products formed following the reaction of DDE/ODD with Mg/Pd^sup +4^ were determined through GC-MS and analyzed according to Weiss and LaPierre (1980). The GC- MS pattern following 1 hour of reaction of 100-mg/L (100-ppm) DDE with Mg/Pd^sup +^ (7.5 mg/0.15 mg/mL) in 0.05% v/v biosurfactant solution in distilled water in the presence of acid revealed the presence of a major product peak at 8.11 minutes (Figure 5). Based on the molecular ion fragmentation of this peak (Figure 6), the product formed after Mg/Pd^sup +4^ catalyzed degradation of DDE was identified as diphenylethane (DPE), characterized by molecular ion mass/charge (m/e) of 182. A peak at m/e of 167 is formed following elimination of methyl radical (CH^sub 3^), as seen in Figure 6.

The GC-MS analysis following 20 hours of reaction of 100-mg/L (100 ppm) DDD with Mg/Pd^sup +^ (7.5 mg/0.15 mg/mL) in 0.05% v/v biosurfactant solution in distilled water in the presence of acid revealed the presence of three major product peaks at 10.47, 9.27, and 6.77 minutes (Figure 7). Based on the molecular ion fragmentation pattern of the above peaks, the corresponding products were identified as l,l-dichloro-2,2 bis (phenyl) ethane (peak characterized by m/e of 250, Figure 8); 1, chloro-2, 2 bis (phenyl) ethane, (peak characterized by m/e of 216, Figure 9); and dipheny!ethane (peak characterized by m/e of 182, Figure 10). Based on the end products identified, we propose a pathway for DDD dechlorination as shown in Figure 11. We predict that aryl chlorine atoms are removed first, followed by sequential removal of alkyl chlorine atoms, leading to the formation of diphenylethane as the end product. The reason for the faster removal of aromatic chlorine atoms may be related to the triangular configuration of the biphenyl moiety of the DDD molecule, which is coupled with rotational ability (Figure 12). This configuration allows the aromatic rings to easily access and adsorb onto the Mg/Pd interfacial regions. This adsorption step is followed by the expulsion of the two aromatic chlorine atoms. We hypothesize that the heterogeneous phase reductive catalysis would follow the same pathway for DDD, DDE, and DDT, because the basic structure and configuration of all three compounds are similar.

Conclusions

To our knowledge, there is little literature available on the dechlorination of DDE and DDD. Our investigations clearly reveal that up to 100 mg/L (100 ppm) of these highly persistent and stable intermediates of DDT can be dechlorinated completely using Mg/ Pd^sup +4^ within 1 hour, provided the target compounds are solubilized through addition of biosurfactant in the aqueous phase. The GC-ECD and GC-MS studies suggest complete dechlorination of both DDE and DDD to DPE. The aryl chlorine atoms are removed first, followed by sequential elimination of alkyl chlorine atoms. It was observed that the presence of acid enhanced the rate of reaction, by providing adequate protons for nascent hydrogen production and by solubilizing metal hydroxide. We conclude that it may be worthwhile to evaluate the Mg/Pd^sup +4^ reactive system for designing indigenous permeable barriers or construction of reactors for DDE/ DDD and DDT-contaminated water. Currently, experiments are in progress to test the efficacy of the Mg/Pd^sup +4^ system for the treatment of DDT, DDD, and DDE aged in soil for a long time. This technique can also be useful in treating effluent originating from industries manufacturing chlorinated pesticides. The major advantages of the proposed system are the following:

* Higher rate of dechlorination as compared to microbial systems.

* Target pollutants can be solubilized using biosurfactants, and there is no requirement for addition of organic solvents. The addition of biosurfactant to contaminated sites and large-scale industrial effluent could have two effects; on one hand, the biosurfactant may solubilize other contaminants (i.e., nonaqueous- phase pollutants, such as oil hydrocarbons) and enhance their biodegradation rates under natural environmental conditions; on the other hand, certain unwanted organics may be solubilized by the presence of biosurfactant, and such mobilized compounds may sorb onto the Mg/Pd surface and reduce the reactivity of the bimetallic system towards the main target contaminant, suchas DDT.

* No accumulation of partially dechlorinated products, such as 1- chloro-2,2-bis (p-chlorophenyl) ethane (DDMS); DDMU; 2, 2-bis (p- chlorophenyl) ethane (DDNS); and 2,2-bis (p-chlorophenyl) ethane (DDNU).

* No precautions are required to lower the redox potential of the reaction phase or exclude oxygen.

Credits

The authors thank the Department of Biotechnology, Ministry of Science and Technology, Government of India, for providing financial support for this project. We also thank Regional Sophisticated Instrument Centre, IIT-Bombay, Mumbai, India, for allowing us to use their GC-MS facility.

Submitted for publication September 12,2005; revised manuscript submitted April 7, 2006; accepted for publication April 17, 2006.

The deadline to submit Discussions of this paper is July 15,2007.

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Sumit Kumar Gautam1, Sumathi Suresh2*

1 Ph.D. student. Centre for Environmental Sciences and Engineering, Indian Institute of Technology-Bombay, Mumbai, India.

2^sub *^ Associate Professor, Centre for Environmental Sciences and Engineering, Indian Institute of Technology-Bombay, Mumbai- 400076, India; e-mail: sumathis@iitb.ac.in.

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