Elsevier

Journal of Environmental Sciences

Volume 38, December 2015, Pages 87-94
Journal of Environmental Sciences

Accelerated anaerobic dechlorination of DDT in slurry with Hydragric Acrisols using citric acid and anthraquinone-2,6-disulfonate (AQDS)

https://doi.org/10.1016/j.jes.2015.05.005Get rights and content

Abstract

The application of electron donor and electron shuttle substances has a vital influence on electron transfer, thus may affect the reductive dechlorination of 1,1,1-trichoro-2,2-bis(p-chlorophenyl)ethane (DDT) in anaerobic reaction systems. To evaluate the roles of citric acid and anthraquinone-2,6-disulfonate (AQDS) in accelerating the reductive dechlorination of DDT in Hydragric Acrisols that contain abundant iron oxide, a batch anaerobic incubation experiment was conducted in a slurry system with four treatments of (1) control, (2) citric acid, (3) AQDS, and (4) citric acid + AQDS. Results showed that DDT residues decreased by 78.93%–92.11% of the initial quantities after 20 days of incubation, and 1,1-dichloro-2,2-bis(4-chlorophenyl)-ethane (DDD) was the dominant metabolite. The application of citric acid accelerated DDT dechlorination slightly in the first 8 days, while the methanogenesis rate increased quickly, and then the acceleration effect improved after the 8th day while the methanogenesis rate decreased. The amendment by AQDS decreased the Eh value of the reaction system and accelerated microbial reduction of Fe(III) oxides to generate Fe(II), which was an efficient electron donor, thus enhancing the reductive dechlorination rate of DDT. The addition of citric acid + AQDS was most efficient in stimulating DDT dechlorination, but no significant interaction between citric acid and AQDS on DDT dechlorination was observed. The results will be of great significance for developing an efficient in situ remediation strategy for DDT-contaminated sites.

Introduction

1,1,1-Trichoro-2,2-bis(p-chlorophenyl)ethane (DDT) is one of the most extensively used organochlorine pesticides worldwide (Bidlan and Manonmani, 2002). Although the agricultural use of DDT has been banned in most countries since 1983, it was still discharged by the application of dicofol that contained DDT as an impurity in the following 20 years in China (Yang et al., 2008). Contamination by DDT is of great concern due to its recalcitrance to degradation, bioaccumulation in the food chain, and potential toxicity to humans and wildlife (Yao et al., 2006). In recent years, DDT has been detected in various environmental compartments, and its concentration in soil is much higher than in air and water (Yang et al., 2008). Therefore, it is of great significance to develop an efficient remediation technology for DDT-contaminated soils.

Reductive dechlorination is a crucial pathway for DDT degradation under anaerobic conditions, because the five electrophilic chlorine atoms on the DDT molecule make aerobic oxidative degradation difficult. Reductive dechlorination is a form of anaerobic respiration in which the chlorinated compound is used as the terminal electron acceptor by dechlorinating microorganisms (Holliger and Schumacher, 1994; EI Fantroussi et al., 1998). Reductive dechlorination requires the addition of two electrons for each chlorine removed. Hydrogen (H2) is one of the substrates (and in some cases, the only one) that can serve as a direct electron donor (Fennell and Gossett, 1997). In natural systems, including contaminated soils, most H2 becomes available to hydrogenotrophic microorganisms through the fermentation of organic substrates by the methanogens and other anaerobic microbial consortium (Fennell and Gossett, 1997, Löffler et al., 1999). In recent years, evidence has been accumulated that organic acids, alcohols, glucose, and complex organic materials acting as electron donor substances were commonly used in contaminated sites for the reductive dechlorination of chlorinated organic pollutants (Fennell and Gossett, 1997, Wu et al., 1998, Fan et al., 2006, Aulenta et al., 2007, Chen et al., 2013).

The process of electron transfer is vital for reductive dechlorination. It has been widely reported that humus, as the major organic matter in soil, can play an important role acting as a redox mediator in microbial redox reactions (Cervantes et al., 2004, Doong and Chiang, 2005, Van der Zee and Cervantes, 2009, Aulenta et al., 2010). In the last several years, studies found that humus as an electron shuttle could influence microbial redox transformation of polychlorinated organic compounds (Luijten et al., 2004, Van der Zee and Cervantes, 2009, Aulenta et al., 2010). Quite a lot of documents suggested that quinone was the dominant redox functional moiety in humus (Lovley et al., 1996, Scott et al., 1998, Cervantes et al., 2002, Cervantes et al., 2004, Roden et al., 2010), and further studies also suggested that non-quinone functional groups in humic samples significantly contributed to their electron-transfer capacities (Ratasuk and Nanny, 2007, Hernández-Montoya et al., 2012, Martinez et al., 2013). Anthraquinone-2,6-disulfonate (AQDS), which contains the quinone moiety functional group, is used as a humus analogue in many assays due to the complicated composition of humus. Some studies showed that AQDS actually served as an electron shuttle between the electrode surface and anaerobic bacteria in microbial redox reactions (Aulenta et al., 2010, Cao et al., 2012, Chen et al., 2013). Nevertheless, the effect of AQDS on DDT dechlorination in soils has not been well elucidated.

Hydragric Acrisols are widely distributed in tropical and subtropical regions, which are the main producing areas of paddy rice in China. There are abundant iron oxides such as Fe(III) and Fe(II) in Hydragric Acrisols. Quite a lot of reports have shown that iron oxide under anaerobic conditions can accelerate reductive dechlorination of chlorinated organic compounds (Li et al., 2009, Li et al., 2010, Wei and Finneran, 2011). Thus the redox reactions of natural iron oxides in Hydragric Acrisols with prevalent anaerobic conditions may influence the reductive dechlorination of DDT. Furthermore, the iron oxides, electron donor substances, and AQDS may demonstrate an interactive effect on reductive dechlorination, which needs investigation.

In the present study, we explored the application of citric acid, acting as an electron donor substance, and AQDS, acting as a humus analogue, for DDT dechlorination in a Hydragric Acrisols slurry, using a batch anaerobic incubation experiment. The specific objectives of this research were to (1) examine the reductive dechlorination rate of DDT in Hydragric Acrisols, (2) investigate the individual and interactive effects of citric acid and AQDS on the transformation of DDT by soil microorganisms, and (3) elucidate the relationship between DDT dechlorination capability and methanogenesis rate. The results will be of great significance for developing efficient agricultural remediation strategies for chlorinated compound-contaminated sites.

Section snippets

Chemicals and reagents

DDT, 1,1-dichloro-2,2-bis(4-chlorophenyl)-ethane (DDD), 1,1-dichloro-2,2-bis(4-chlorophenyl)ethylene (DDE) and 1-chloro-2,2-bis(p-chlorophenyl)ethylene (DDMU), purity > 99.5%, were purchased from Dr. Ehrenstorfer (Augsburg, Germany). AQDS (purity > 97%) and 1,4-piperazinediethanesulfonic acid (PIPES, purity > 98%), were obtained from Sigma-Aldrich (St. Louis, MO, USA). Sodium sulfate (Na2SO4, Nanjing Chemical Reagent Co., Nanjing, Jiangsu, China) was oven-dried at 400°C for 4 hr before use. Silica gel

Eh values of the reaction systems

The Eh values of various reaction systems at each sampling time are presented in Table 1. From the beginning to the 16th day of incubation, Eh values of the reaction systems were significantly different for every two treatments (P < 0.05), and the order for slurry Eh values was: citric acid + AQDS < citric acid < AQDS < control. At the end of incubation, the highest slurry Eh was in the control treatment, and followed by that in the AQDS treatment (P < 0.05). No significant difference was observed between

DDT transformation in the reaction systems

The results revealed that the reductions in the amounts of extractable DDT residues were 63.18–73.15 μmol/L, but the sum of all DDT metabolites was only 16.54–32.89 μmol/L in slurries for different treatments. Moreover, the amounts of DDT and its metabolites lost from slurries due to volatility should be quite slight due to their large values of octanol/water partition coefficient and low vapor pressures. Therefore, non-extractable (bound) DDT and its metabolites, which accounted for considerable

Conclusions

In anaerobic slurries, DDT was dechlorinated obviously with the dominant metabolite of DDD. Non-extractable DDT and its metabolites were formed because of the adsorption and binding by soil organic matter and clay particles. The application of citric acid accelerated DDT dechlorination weakly in the early days due to the competition for electrons by methanogens, and then the effect became better in the later period of incubation while the methanogenesis rate decreased. AQDS as an electron

Acknowledgments

This study was supported by the National Natural Science Foundation of China (No. 41201314) and the Open Fund Project of State Key Laboratory of Soil and Sustainable Agriculture (No. 0812201227).

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