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Black Carbon Facilitated Dechlorination of DDT and its Metabolites by Sulfide KAI DING, and WENQING XU Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03154 • Publication Date (Web): 04 Nov 2016 Downloaded from http://pubs.acs.org on November 7, 2016

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Environmental Science & Technology

Black Carbon Facilitated Dechlorination of DDT and its Metabolites by Sulfide

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Kai Ding1, Wenqing Xu1*

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Department of Civil and Environmental Engineering, Villanova University, Villanova, PA, 19085

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*Corresponding author phone: (610) 519-8549; fax: (610) 519-6754; e-mail: [email protected]

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1 ACS Paragon Plus Environment


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Abstract

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1,1,1-trichloro-2,2-di(4-chlorophenyl) ethane (DDT) and its metabolites 1,1-dichloro-2,2-

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bis(4-chlorophenyl) ethane (DDD) and 1,1-dichloro-2,2-bis(4-chlorophenyl) ethylene (DDE), are

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often detected in soils and sediments containing high concentrations of black carbon. Sulfide (~5

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mM) from biological sulfate reduction often co-exists with black carbon and serves as both a

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strong reductant and a nucleophile for the abiotic transformation of contaminants. In this study,

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we found that the abiotic transformation of DDT, DDD, and DDE (collectively referred to as

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DDX) require both sulfides and black carbon. 89.3±1.8% of DDT, 63.2±1.9% of DDD, and

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50.9±1.6% of DDE were degraded by sulfide (5 mM) in the presence of graphite powder (21 g/L)

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after 28 days at pH 7. Chloride was a product of DDX degradation. To better understand the

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reaction pathways, electrochemical cells and batch reactor experiments with sulfide-pretreated

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graphite powder were used to differentiate the involvement of black carbon materials in DDX

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transformation by sulfide. Our results suggest that DDT and DDD are transformed by surface

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intermediates formed from the reaction between sulfide and black carbon, while DDE

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degradation involves reductive dechlorination. This research lays the groundwork for developing

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an alternative in-situ remediation technique for rapidly decontaminating soils and sediments to

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lower toxic products under environmentally relevant conditions.


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Introduction 1,1,1-trichloro-2,2-di(4-chlorophenyl)ethane (DDT ) is a halogenated organic insecticide

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used worldwide since its discovery in 1874.1-3 Due to associated health risks and adverse

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environmental impact, DDT was banned in the United States in 1972 and became one of 21

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persistent organic pollutants (POPs) for immediate phasing out.3 DDT and its metabolites, 1,1-

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dichloro-2,2-bis(4-chlorophenyl) ethane (DDD) and 1,1-dichloro-2,2-bis(4-chlorophenyl)

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ethylene (DDE), are highly hydrophobic with the octanol and water partinion coefficients (log

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Poct/wat) of 6.91, 6.51, and 6.02, respectively.4 As a result, DDT, DDD and DDE (collectively

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refered to as DDX, Figure S1) bind strongly to soils and sediments, bioaccumulate to high

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concentrations in organisms at the top of the food chain, and exhibit biotoxicity to animals,

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including humans. Previous study suggested that DDX adversely affected the nervous system

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and reproductive capability of animals.4 According to the U.S. EPA, DDX are probable human

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carcinogens (Group B2), along with polychlorinated biphenyls (PCBs) and chloroform.3 Due to

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their environmental persistence, DDX are often detected in soils and sediments with estimated

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half-lives of 2-15 years.4 For instance, concentrations of DDX up to 45 ng/g in 32 soil samples

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from South Carolina and Georgia and 252 µg/g in San Francisco Bay have been reported.5,6

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According to the U.S. EPA, approximately 10% of the sediments beneath surface waters

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are heavily contaminated in the Unitated States alone and pose concerns for aquatic organisms

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and human health.7 Estimated costs are $250 billion in the United States using conventional

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remediation technology over the next 30 years.7 Traditional remediation approaches involve

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either dredging with subsequent landfill disposal, or capping, both of which are highly

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expensive. For instance, the dredging of the Lauritzen Canal in San Francisco Bay involves the

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removal of 82,000 m3 of contaminated sediment at an estimated cost of at least $12.1 million.8 In



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addition to the high cost, the National Research Council found that dredging and capping pose

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risks by disrupting benthic ecosystems, remobilizing contaminates back into the water column

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and, therefore, increasing the risk of toxic contaminants by bioaccumulation in the food chain.9

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An alternative approach is biodegradation.10 However, DDT and its metabolites are extremely

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insoluble in water. This limits their bioavailability to bioremedial microogranisms such as

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Pseudomonas and white rot fungus species.11-12 Moreover, the presence of organic compounds

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(e.g., ethanol, glucose) are often required for microoganism growth as DDT cannot serve as the

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sole carbon source.13 Recent studies have suggested that dechlorination of DDX can occur via a

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magnesium/palladium (Mg0/Pd 4+) bimetallic catalyst.14 However, the high cost of catalysts pose

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hurdles on this technique.14-17 Moreover, sulfide often naturally present in contaminated soils and

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sediments under anaerobic conditions, increasing the likelihood of catalyst poisoning.

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Previous research showed that black carbon in the presence of sulfide can foster the

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degradation of certain nitrogenous organic pollutants including nitroglycerin, 2,4-dinitrotoluene,

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3-bromonitrobenzene, and hexahydro-1,3,5-trinitro-1,3,5-triazine.18-21 Fu et al. has reported that

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one halogenated contaminant, hexachloroethane, can be degraded by sulfide in the presence of

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carbon nanomaterials, including both carbon nanotubes and graphene oxide.22 The redox

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properties of black carbon (oxygenated functional groups and conductivity) are proposed to

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facilitate the transformation of pollutant via either reduction or nucleophilic substitution,

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depending on the chemical structure of the contaminant.23 However, little is known regarding the

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effect of black carbon and sulfide on POPs (e.g., DDX). Previous studies of black carbon and

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DDX has focused on adsorptive properties of black carbon, which serves as a passive sorbent in

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sequestering DDX from the aqueous phase, reducing toxcitiy of DDX to benthic organisms.6, 24

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Intriguingly, Hale et al. observed that the amount of DDT adsorbed to sediments from field


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measurements was 32-fold lower than the estimated amount using the activated carbon-water

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partitioning coefficient obtained from a clean water system.25 Moreover, Erdem et al. reported

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that the desorption of DDT from fresh soils was much higher than from aged soils.26 This

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suggests that adsorbed DDX may undergo degradation by environmental reagents in soils and

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sediments.

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The main objective of the present study was to investigate the feasibility of DDX

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degradation by sulfide in the presence of black carbon materials (graphite powder, graphite

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sheet, and biochar) under environmentally relevant conditions. We evaluated the timescale of

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degradation, the reaction kinetics, concentration and type dependence of black carbon, and

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product formation. A second objective was to evaluate the reaction mechanism in detail. We

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used previously reported electrochemical cells capable of isolating solid phase-catalyzed

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reductive reactions involving electron transfer from sulfide to DDX using graphite sheet.19

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Moreover, we employed batch reactors containing sulfide-pretreated graphite in order to restrict

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observable solid phase-catalyzed reactions to those involving preformed sulfur species from

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reaction of graphite and sulfide. This study demonstrate that the use of black carbon and sulfide

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in remediating pollutants can be extended to POPs. Moreover, a novel reaction pathway

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involving the formation of surface species was reported for DDT and DDD decay.

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Materials and Methods

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Chemicals. Material sources and purity are provided in the Supporting Information (Text S1).

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All chemicals are used without further purification.

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Batch reactor experiments. All experiments were carried out in a glove box (Coy Laboratory

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Products Inc., Grass Lake, MI) to assure strict anaerobic conditions. 100 mg/L sodium azide was

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added to 20 mM pH 7 phosphate buffer as an aerobic metabolic inhibitor. Sulfide (H2S/HS-)



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stock solution was made daily by dissolving Na2S⋅9H2O in phosphate buffer. 5 mM sulfide

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concentration was achieved by introducing sulfide stock solution into 14 mL borosilicate glass

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reactors containing pre-weighed graphite powder (21 g/L) and filling up the reactors to eliminate

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headspace with phosphate buffer. Graphite powder was selected as a model black carbon with

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the minimum surface functional groups and surface area (Table S2). 10 µL of DDT, DDD or

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DDE stock solution (1000 mg/L DDX in acetone or methanol) were immediately spiked into all

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samples to initiate the reaction and to obtain an initial concentration of 0.714 mg/L (compared to

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43 mg/L used in Sayles et al.27) Vials were capped with Teflon-lined septa and placed on an end-

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to-end Rugged Rotator mixer (Glas-Col, Tere Haute, IN) in the dark at 30 rpm, 25 °C in a Model

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VRI6P incubator (VWR International, Radnor, PA). Controls containing graphite powder in the

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absence of sulfide, sulfide in the absence of graphite powder, and in the absence of both graphite

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powder and sulfide were set up. All experiments were carried out in triplicate.

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Samples were periodically analyzed. Reaction vials were centrifuged at 3000 rpm for 15

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minutes to separate aqueous and solid phases. The aqueous phase was extracted by shaking with

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10 mL hexane for 3 minutes. The solid phase was extracted by shaking with 10 mL

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hexane/acetone (1:1 by volume) for 3 minutes. All extracts were analyzed by gas

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chromatography and mass spectroscopy (GC: Agilent 6890N, MS: Agilent 5973 MS, Santa Clara,

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CA) to quantify the concentrations of DDX and their degradation products with o-p’-DDE as the

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internal standard. Details for the GC/MS analyses and oven program are provided in the

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Supporting Information (Table S1). Aqueous phase extraction efficiencies for DDT, DDD, and

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DDE were determined to be 93.2±0.5%, 94.8±0.9%, and 94.2±0.7%, respectively. Solid phase

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extraction efficiencies were 89.2±1.3%, 85.8±1.9%, and 86.3±2.7% for DDT, DDD, and DDE in

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the presence of graphite powder, respectively. Chloride was analyzed using a Shimazu ion


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chromatography (IC) with a conductivity detector at 45 °C using 3.6 mM Na2CO3 as the eluent at

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a flow rate of 0.8 mL/min. The aqueous phases of all samples containing chloride were analyzed

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directly, while the solid phase was extracted with 10 mL of deionized water. The solid phase

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extraction efficiency for chloride was determined to be 94.1±3.4% for in the presence of graphite

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powder. Method quantification limit for chloride was 0.14 µmol/L. Aqueous sulfide was also

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quantified using IC. To ensure that all sulfide were in the form of bisulfide (HS-), pH of all

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samples were adjusted to above 9 adding a small amount of sodium hydroxide prior to the IC

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measurements.

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Electrochemical cell experiments. Details of the electrochemical cell method was provided in

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our previous work.19 Briefly, the electrochemical cells were constructed by connecting two 14

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mL borosilicate glass reactors using insulated copper wire through Teflon-lined septa. Graphite

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sheets (0.13 mm thick; Alfa Aesar, Karlsruhe, Germany) served as electrodes, which were

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attached to the insulated copper wires by conductive NEM tape (Nisshin EMCO Ltd, Seoul,

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Korea). The electrical circuit was completed using a salt bridge made by filling Teflon tubing

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with agarose gel containing 1 M potassium chloride. DDX was spiked into the cathodic cell to

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achieve an initial concentration of 0.2 mg/L and gently mixed for 12 h on a rotating bed to reach

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sorption equilibrium. Sulfide was then spiked into the anodic cell at 5 mM to initiate the reaction.

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The electrochemical cells were returned back to the rotating bed at 30 rpm at 25 °C in the dark.

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At various time intervals, samples were collected and both aqueous and solid phases were

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extracted for chemical analysis.

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Black carbon preparation and characterization. Oak wood was used as the feedstock for

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biochar production via slow pyrolysis in a CM tube furnace (model 1600 serial) at various

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temperatures (550°C, 700 °C, 900 °C) under N2 flow of 1.5 L/min for 2 hours. Elemental



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analysis of all carbon materials was carried out by Galbraith Laboratories (Knoxville, TN) using

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a Flash 2000 Elemental Analyzer. Surface area of biochar, graphite powder was characterized by

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N2 sorption (Autosorb-3B, Quantachrome Instruments).

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Results and Discussion

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DDX degradation. Chemical transformation of DDX (Figure S1) was monitored over 28 days at

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pH 7. Because both aqueous and solid phases were analyzed for DDX and their decay products,

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the results were presented as the total DDX mass retrieved from both phases using previously

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determined extraction efficiencies. Over 99% DDX was adsorbed to the surface of graphite

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powder and the amount of DDX in the aqueous phase was negligible. As shown in Figure 1, no

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DDX decay was observed in controls lacking both sulfide and graphite, indicating that DDX

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hydrolysis was insignificant at pH 7. For controls containing only 21 g/L graphite powder, no

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DDX destruction occurred during the 28-day experimental time frame. Similarly, no significant

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DDX decay was observed for controls with 5 mM sulfide alone. In contrast, over 89.3±1.8% of

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DDT, 63.2±1.9% of DDD, and 50.9±1.6% of DDE were degraded by 5 mM sulfide in the

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presence of 21 g/L graphite powder after 28 days. All DDX decay followed first-order kinetics,

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with observed pseudo first-order rate constants (kobs) of 0.0875±0.0023 d-1, 0.0340±0.005 d-1,

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0.0239±0.004 d-1 and half-life (t1/2) of 7.9±0.2 days, 20.4± 0.5 days and 29.0±0.7 days for DDT,

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DDD, and DDE, respectively.

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To rule out the possible presence of catalytically active trace metals on the graphite

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powder we used, we prewashed the graphite in 1 N hydrochloric acid for 24 hours. The acid-

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treated graphite powder was dried and then used to react with DDT in the presence of 5 mM

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sulfide in batch reactors and untreated graphite powder was used as control. No significant

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difference was observed for DDT degradation kinetics for the experiments using acid-treated and


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untreated graphite power (Figure S2), suggesting trace metals were not responsible for the

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observed reactivity of graphite powder. Previous research suggests that DDX was extremely

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recalcitrant in the environment, with reported half-life (t½) on the time frame of years at neutral

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pH.28 In particular, DDE was considered as a terminal product of microbial transformation of

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DDT in soils and sediment.29 Therefore, the fast degradation kinetics demonstrated here shows

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great implications towards soils and sediments remediation.

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Product formation. As shown in Figure 2A, DDD and chloride were the main products for

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DDT breakdown by sulfide in the presence of graphite powder. In particular, 26.0±0.5 nmol

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DDT were degraded, producing 8.3±0.1 nmol DDD and 46.8±0.7 nmol chloride over 28 days,

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with a molar ratio yield of 1.8:1 for chloride. The mass balance on chlorine at the end of 28 days

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based on the mass of DDT transformed was 61.4±0.3 %. It is worth noting that both DDD and

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DDE were impurities in received DDT standards (< 2% by weight) as previously confirmed by

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other researchers.27 However, the amount of DDE remained at the background level during the

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28-day experimental time frame, suggesting DDE was unlikely to be a degradation product from

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DDT decay (Figure S4). As shown in Figure 2B, 2-chloro-1,1-bis(4-chlorophenyl) ethene

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(DDMU) and chloride were identified to be the main products from DDD degradation.

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Specifically, 19.7±0.6 nmol DDD was transformed into 2.82±0.1 nmol DDMU and 24.7±0.7

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nmol chloride over 28 days, with a molar ratio yield of 1.3:1 for chloride. The mass balance on

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chlorine at the end of 28 days based on the mass of DDD transformed was 42.1±0.5 %. Efforts

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were made to assess the possible presence of 1-chloro-2,2-bis(4-chlorophenyl) ethane (DDMS,

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Figure S1) as a transformation product. In particular, DDD samples containing graphite powder

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and sulfide after 28-days reaction were extracted and concentrated 10 times. One new peak

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eluted at 10.12 min after concentrating the samples. The MS results suggest that the observed



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product was DDMS (Figure S4). However, due to the lack of authentic standard for DDMS, our

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ability to further identify the product was limited. Nonetheless, our results indicate that DDD

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was transformed into DDMU and chloride, with possible DDMS at low concentrations.

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For DDE decay, DDMU and chloride were also found to be the main products (Figure

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2C). Over 28 days, 16.0±0.5 nmol DDE were degraded, while 2.2±0.1 nmol DDMU and

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16.6±0.5 nmol chloride were formed, suggesting a molar ratio yield of 1:1 for chloride. The mass

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balance on chlorine at the end of 28 days based on the mass of DDE transformed was 36.7±0.4%.

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The formation of chloride suggests that dechlorination of DDX took place in the presence of

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graphite powder and sulfide. All identified products, DDD, DDMU, and chloride, are less toxic

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than the parent compounds, indicating a detoxification pathway. In particular, the reported LD50

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values were 300 mg/kg for DDT, 4000-5000 mg/kg for DDD, and 880 mg/kg for DDE, where

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higher LD50 values indicate lower toxicity for DDD and DDE.4 The other degradation product,

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DDMU, was reported to be a detoxification product of DDD in both liver and kidney.30

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Reaction mechanisms. In order to understand the role of black carbon in facilitating DDX decay

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by sulfide, we propose two possible reaction mechanisms: (mechanism 1) the redox properties of

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black carbon (e.g., quinone functional groups and graphitic regions) promote electrons transfer

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from sulfide to adsorbed DDX, resulting in DDX degradation via reductive dechlorination;

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and/or (mechanism 2) surface intermediates formed via reaction between sulfide and black

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carbon, resulting in DDX degradation. Note that the proposed reaction mechanisms might also

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function synergistically. As previously described in our introduction, electrochemical cells were

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employed to isolate solid phase-catalyzed reductive reactions involving electron transfer from

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sulfide to DDX using sheet graphite. Accordingly, graphite sheets (one type of black carbon, 5.4

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g/L) served as both cathode and anode in the electrochemical cells. Physical separation between


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DDX and sulfide (5 mM) was achieved by having DDX in the cathodic cell and sulfide in the

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anodic cell. Both cells were filled with 20 mM phosphate buffer at pH 7. Thus, any DDX

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degradation in the cathodic cell can only be attributed to electron transfer from sulfide in the

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anodic cell via the graphite sheet (reaction mechanism 1). In order to further deduce if reaction

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mechanism 2 was responsible for the observed DDX decay, graphite sheet was pre-treated for 24

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hours with 5 mM sulfide prior to the introduction into the reaction system. Subsequently, the

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aqueous phase was removed and the solid phase was rinsed twice with 10 mL phosphate buffer

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to remove any residual sulfide adsorbed to the graphite surfaces. The vials containing the pre-

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treated graphite sheets were refilled with phosphate buffer and 0.2 mg/L DDX was added to

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initiate the reaction. Thus, any DDX degradation observed in these experiments could only be

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attributed to pre-formed surface intermediates on graphite upon exposure to sulfide (reaction

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mechanism 2). Batch reactors containing DDX (0.2 mg/L) and sulfide in the presence of graphite

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sheet (5.4 g/L) were set up as controls, which do not discern between the possible reaction

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mechanisms 1 and/or 2. In addition, batch reactors containing DDX (0.2 mg/L) and sulfide in the

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absence of graphite sheet were set up as controls.

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We found that 55.7±1.9% of DDT was degraded in batch reactors containing both sulfide

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and graphite sheet after 28 days (Figure 3A), which could be attributed to reaction mechanisms 1

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and/or 2. However, no degradation was observed in the cathodic cell, suggesting the contribution

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of mechanism 1 was negligible. Interestingly, 54.3±2.4% of DDT was transformed in reactors

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containing sulfide pre-treated graphite sheet after 28 days, suggesting that surface intermediates

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formed upon prior exposure of graphite sheet to sulfide were responsible for the observed DDT

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decay. Taken together, our results suggest that most of the observed DDT degradation in batch

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reactors containing both sulfide and graphite sheet can be attributed to mechanism 2. Similar



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results were obtained for DDD degradation (Figure 3B). In particular, 32.6±0.8 % of DDD

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decay occurred over 28 days in batch reactors under conditions that do not exclude any of the

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proposed reaction pathways. However, no DDD degradation was observed in the electrochemical

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cells, while 34.3±1.1% DDD degradation of was observed in the vials containing sulfide pre-

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treated graphite sheet. Overall, these results indicate that reaction mechanism 2 was responsible

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for both DDT and DDD decay, suggesting that the degradation of both compounds require the

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preformed surface species from the reaction between sulfide and black carbon.

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Interestingly, DDE degradation appeared to undergo a different reaction pathway. As

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shown in Figure 3C, 31.9±1.3% of DDE was degraded in batch reactors that do not exclude any

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of the proposed reaction pathways after 28 days. 29.7±1.1% of DDE was transformed in

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electrochemical cells, while no DDE was degraded in vials containing sulfide pre-treated

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graphite sheet. Taken together, these results suggest that DDE underwent surface-mediated

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reductive dechlorination by accepting electrons transferred from sulfide via the graphite sheet,

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yielding DDMU and chloride as the products (Figure S6). The reaction pathway was illustrated

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in Figure S3.

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Based on the results above and the identified transformation products of DDX, we

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propose that DDT and DDD could undergo two reaction pathways:31-32 1) nucleophilic

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substitution reaction (X-philic substitution), where the surface intermediate acts as a nucleophile

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attacking the halogen atom of DDX; and/or 2) elimination reaction (E1cB), where the surface

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intermediate acts as a base subtracting hydrogen atom on DDX. As illustrated in Figure S4, DDD

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was the main transformation product for DDT decay, indicating that X-philic substitution was

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important. For DDD decay, the elimination reaction pathway (E1cB) appeared to be important,

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as the formation of DDMU was identified to be predominant. Our results suggest that the formed


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surface intermediates can act as both nucleophile and base, facilitating the degradation of DDT

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and DDD. Further research is required to understand the nature of such surface intermediates.

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Previous research suggest that DDX could undergo reductive dechlorination in the

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presence of Fe0. The observed first order reaction rate constants for DDT, DDD, and DDE were

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1.7±0.4 d-1, 1.6±0.6 d-1 , and 0.95±0.66 d-1, respectively, with 15 g/L Fe0 loading.27 In this study,

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the rate constant for reductive dechlorination of DDE was 0.0239±0.004 d-1, which was much

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slower than the Fe0 system possibly due to the fact that sulfide is a weaker reductant.33 It is

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puzzling why DDT and DDD did not undergo reductive dechlorination in our electrochemical

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cells, although the one electron reduction potentials of DDT and DDD are expected to be lower

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than DDE. We speculate that the flat geometry of DDE molecule and the interaction between its

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C=C bond and the graphitic region of the carbon surface help stablize the reaction intermediate,

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which lower the activation energy of the reductive dechlorination. Previous research also

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reported an inverse relationship between the one electron reduction potential of some chlorinated

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ethenes and their degradation rates with Fe0, which was explained by the possible surface

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association via π–complexes.34-35

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Dependence of DDT and DDE degradation on graphite concentration. To understand the

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effect of graphite concentration on DDX decay by sulfide, both DDT and DDE were evaluated as

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our previous results suggested that DDT and DDE underwent different reaction mechanisms. In

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particular, DDT/DDE (0.714 mg/L) was spiked into batch reactors containing 5 mM sulfide in

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the presence of 7-35 g/L graphite powder at pH 7. The decay for DDT and DDE was monitored

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over 14 days and 28 days, respectively. The observed pseudo-first order rate constants (kobs) for

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both DDT and DDE decreased as the concentration of graphite powder increased (Figure 4) with



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a strong inverse linear correlation (DDT: kobs = -0.00375 (±0.0003) Cgraphite + 0.189 (±0.007), R2

282

=0.94; DDE: kobs = -0.00025 (±0.00003) Cgraphite + 0.03176 (±0.00007); R2 = 0.83).

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To further understand the observed inverse relationship, the amount of aqueous sulfide

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was measured in the presence of 7-35 g/L graphite powder using the analysis protocol previously

285

described. The amount of surface-associated S (sorbed sulfide and surface sulfur intermediates)

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was calculated using a mass balance approach. As shown in Figure S7, the concentration of both

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aqueous sulfide and surface-associated S decreased as graphite powder concentration increased

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from 7 g/L to 35 g/L, which was consistent with the observed inverse relationship between kobs

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for DDT/DDE and the concentration of graphite powder. Taken together, these results suggest

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that both aqueous and adsorbed sulfide contribute to DDT and DDE degradation in the presence

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of graphite powder. It is possible that more DDX molecules would be adsorbed within smaller

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micropores with higher adsorption energies at lower graphite concentration. However, the

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sorption sites have not been characterized.

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Environmental Relevance. Black carbon constitutes 5-30% of total organic carbon in soils and

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sediments (approximately 0.51 g/L ~67.4 g/L of black carbon in marine sediments, assuming

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black carbon constitutes 0.11~ 6.6 mg per grams of dry sediment, with a porosity of

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0.31~0.89).36,37 To evaluate whether or not other types of black carbon can foster the degradation

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DDX, we produced three different biochars using different pyrolysis temperatures. The reactivity

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of biochars in promoting DDX decay was then compared with graphite powder at the same

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concentration (14 g/L) in batch reactors over 14 days. The observed rate constants (kobs) for all

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carbon materials were shown in Figure 5. Our results suggest that degradation of DDX by sulfide

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in the presence of black carbon is not limited to graphite powder, but can be applied to other

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types of black carbon, such as biochars. Interestingly, graphite powder was found to be most


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reactive among all carbon materials investigated, although the relative surface areas of biochars

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were much higher than graphite powder (Table S2).38 Further investigation is required to better

306

understand the properties of black carbon materials in facilitating the degradation of DDX

307

contaminants.

308

As natural organic matter (NOM) is ubiquitous in the environment (0.2 to 15 mg C/L in

309

aquatic environments),39 it is of great interest to evaluate the impact of NOM on the degradation

310

kinetics of DDX by sulfide in the presence of black carbon materials. Specifically, 2 mg/L NOM

311

was introduced into batch reactors containing 21 g/L graphite powder and 5 mM sulfide.

312

Controls containing 1) 21 g/L graphite powder and 5 mM sulfide, 2) 21 g/L graphite powder and

313

2 mg/L NOM, and 3) 2 mg/L NOM and sulfide were also set up. DDX was spiked into all

314

samples to initiate the reaction and the decay of DDX was monitored over 7 days using the

315

protocol described previously. As shown in Figure 6, no degradation of DDX was observed in

316

samples containing only black carbon and NOM, or containing NOM and sulfide. In other words,

317

the presence of NOM did not promote the degradation of DDX by sulfide. DDX degradation was

318

observed in vials containing black carbon and sulfide as we previously discussed. Interestingly,

319

in the presence of NOM, degradation kinetics was slower by approximately 61.0±1.5% for DDT,

320

33.2±0.9% for DDD and 18.3±0.5% for DDE by sulfide in the presence of black carbon. In light

321

of these results, we speculate that NOM could compete for sorption sites pyrogenic carbon

322

materials and slow down the DDX degradation. Similar results were found where NOM could

323

compete with hydrophobic organic compounds via direct sites competition or pore blockage on

324

black carbon.40

325 326

Overall, these results suggest that various forms of pyrogenic carbon can foster the degradation of DDX by sulfide, likely pertinent to a wide range of naturally occurring carbon



Page 16 of 26

327

materials that exist in the environment. The presence of NOM appears to slow down DDX

328

degradation kinetics, but does not eliminate the reactions under environmentally relevant

329

conditions that we investigated. Taken together, the results of this study suggest a promising

330

alternative in situ remediation method for rapidly decontaminating soils and sediments to lower

331

toxic products.

332

Acknowledgment

333

The authors would like to acknowledge Prof. Gang Feng for assistance with the EDS

334

measurements, Professor Charles Coe, and Professor Yin Wang for their help with the BET

335

surface measurements.

336

Supporting Information Available: Materials, analytical method details, and additional figures

337

containing chemicals structures and product analysis. This information is available free of charge

338

via the Internet at http://pubs.acs.org.


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Figure 1. Pseudo-first order decay of DDX by 5 mM sulfide in the presence of 21 g/L graphite powder over 28 days in 20 mM phosphate buffer at pH 7.0 and 25 °C. ○ = 5 mM sulfide and 21 g/L graphite powder; □ = 21 g/L graphite powder only; △ = 5 mM sulfide only; ◇ = no graphite powder or sulfide. Error bars represent the standard deviation of triplicate. The solid line is obtained from the linear regression and the dash line represents the 95% confidence interval. A: DDT degradation (kobs = 0.0875±0.0023 d-1); B: DDD degradation (kobs = 0.0340±0.001 d-1); C: DDE degradation (kobs = 0.0239±0.006 d-1).


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Figure 2. DDX decay and product formation after reaction with 5 mM sulfide in the presence of 21 g/L graphite powder over 28 days in 20 mM phosphate buffer at pH 7 and 25 °C. Error bars represent the standard deviation of triplicate. ○ = mass of DDX; □ = mass of DDD (in A) or DDMU (in B & C); △ = mass of chloride. A: DDT degradation; B: DDD degradation; C: DDE degradation.



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Figure 3. Transformation of DDX in electrochemical cells and batch reactors after 28 days in 20 mM phosphate buffer at pH 7 and 25 °C. Error bars represent the standard deviation of triplicate. EC: experiments were carried out in electrochemical cells; Batch: experiments were carried out in batch reactors. (A) DDT; (B) DDD; (C) DDE.


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Figure 4. Pseudo-first order rate constants (kobs) for DDT and DDE degradation in the presence of 7-35 g/L graphite powder with 5 mM sulfide at pH 7. The solid line is obtained from the linear regression and dash lines represent the 95% confidence interval for the degradation kinetics. Error bars represent the standard deviation of triplicate. (A). DDT (kobs = -0.00375 (±0.0003) Cgraphite + 0.1888 (±0.007); R2 = 0.94); (B). DDE (kobs = -0.00025 (±0.00003) Cgraphite + 0.03176 (±0.00007); R2 = 0.83).



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Figure 5. The observed pseudo-first order rate constants (kobs) for DDX decay by 5 mM sulfide in the presence of 14 g/L different biochars and graphite powder at pH 7 and 25 °C. Error bars represent the standard deviation of experimental triplicate.


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Figure 6. The effect of natural organic matter (NOM) on DDX decay in the presence of 21 g/L graphite powder and 5 mM sulfide at pH 7 and 25 °C over 7 days. NOM concentration was 2 mg/L as total organic carbon. Error bars represent the standard deviation of triplicate.



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