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May 31, 2013 - Role of Black Carbon Electrical Conductivity in Mediating Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) Transformation on Carbon Surfac...
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Role of Black Carbon Electrical Conductivity in Mediating Hexahydro1,3,5-trinitro-1,3,5-triazine (RDX) Transformation on Carbon Surfaces by Sulfides Wenqing Xu,† Joseph J. Pignatello,†,‡ and William A. Mitch†,* †

Department of Chemical and Environmental Engineering, Yale University, Mason Lab 313b, 9 Hillhouse Ave., New Haven, Connecticut 06520, United States ‡ Department of Environmental Sciences, Connecticut Agricultural Experiment Station, 123 Huntington St., New Haven, Connecticut 06504, United States S Supporting Information *

ABSTRACT: Recent research has demonstrated that black carbons catalyze the transformation of a range of nitrated explosives sorbed to the carbon surfaces in the presence of sulfides. Although surface oxygenated functional groups, particularly quinones, and electrical conductivity have both been hypothesized to promote these reactions, the importance of these properties has not been tested. In this work, the importance of electrical conductivity was addressed by producing chars of increasing electrical conductivity via pyrolysis of wood shavings at increasing temperature. The reactivity of chars with respect to transformation of the explosive RDX in the presence of sulfides correlated with electrical conductivity. Oxygenated functional groups were apparently not involved, as demonstrated by the elimination of reactivity of an activated carbon after ozone treatment or sorption of model quinones to the activated carbon surface. Although RDX transformation correlated with char electrical conductivity, no RDX transformation was observed when RDX was physically separated from sulfides but electrically connected through an electrochemical cell. RDX transformation occurred in the presence of a surface-associated sulfur species. The correlation with char electrical conductivity suggests that sulfides are oxidized on carbon surfaces to products that serve as potent nucleophiles promoting RDX transformation.

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toluene,7 and various methyl- or chloro-substituted nitrobenzenes13 by sulfides were enhanced in the presence of black carbons. Moreover, the black carbon-mediated transformation of organic compounds by sulfides has been extended from nitrated compounds to 2,4-dibromophenol,8 a halogenated contaminant. The mechanisms responsible for this black carbon-mediated transformation, however, are unclear. Previously, Dunnivant et al.14 observed enhanced transformation of a series of nitrobenzene structural analogues by sulfides in the presence of natural organic matter (NOM). They suggested that quinone moieties in NOM accelerated the electron transfer from sulfides to target contaminants, because faster contaminant reduction was observed in the presence of various model quinones.15 Aeschbacher et al.16 demonstrated that quinone moieties were key participants in the transfer of electrons to and from humic substances. Since quinone/carbonyl groups likely occur in black

INTRODUCTION As most rivers eventually discharge to estuaries, estuaries receive the cumulative loadings of a wide range of contaminants emitted to freshwaters. Hydrophobic contaminants tend to sorb strongly to marine sediments,1,2 where black carbon (i.e., soots and charcoals) and hydrogen sulfides coexist. Research over the past two decades has highlighted the important role of black carbons as inert geosorbents in sequestering organic contaminants.3,4 Indeed, activated carbon, an anthropogenic black carbon, has been applied to marine sediments to reduce the bioavailability of organic contaminants to benthic organisms.5 There has been increasing interest in the reactivity of black carbon in mediating the transformation of certain sorbed contaminants in the presence of hydrogen sulfides (i.e., H2S, HS−, S2−).6−9 Sulfides naturally exist at millimolar concentrations in estuarine pore waters due to biological sulfate reduction10 and can act as both nucleophiles and reductants.11 Most research has focused on the transformation of nitrated contaminants, which have been widely used as military explosives, precursors for pesticides, or as starting materials for antibiotics.12 The transformation of hexahydro-1,3,5trinitro-1,3,5-triazine (RDX),6 nitroglycerin,9 2,4-dinitro© XXXX American Chemical Society

Received: March 20, 2013 Revised: May 17, 2013 Accepted: May 31, 2013

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carbons,17,18 one hypothesis is that surface quinone functional groups promote the electron transfer between sulfides and contaminants sorbed to carbon surfaces.6,13 After applying a range of techniques (e.g., nitric acid treatment) to modify a parent char material, Yu et al.13 indicated that the reduction rate of nitrobenzene by sulfides correlates with the oxygenated functional group prevalence on the char surfaces. However, the wet chemical treatments typically used to modify carbon surfaces (i.e., nitric acid, ozonation) not only introduce an array of oxygenated functional groups, but also alter other carbon properties, including surface area, pore distribution, surface charge,19−21 and possibly electrical conductivity. Therefore, it is difficult to isolate the contribution of oxygenated functional groups. Another hypothesis is that conductive graphitic regions in black carbons promote electron transfer from hydrogen sulfides to sorbed organic compounds.7−9 Oh et al. first proposed this hypothesis based on previous results demonstrating that elemental iron initiated the reduction of heterocyclic nitramines, such as RDX, sorbed to graphite.22 However, the reactivity of black carbons in mediating contaminant transformation by sulfides has not yet been correlated with black carbon electrical conductivity, and so the importance of black carbon electrical conductivity remains unclear. The first objective of this work was to compare the contribution of surface oxygenated functional groups and electrical conductivity to the reactivity of black carbon with respect to mediating RDX transformation in the presence of sulfides. To isolate their importance, techniques were employed to vary either surface oxygenated functional groups or electrical conductivity with minimal alterations to other carbon properties. To evaluate the importance of electrical conductivity, we produced chars by pyrolysis of the same wood starting material at various temperatures, since previous literature suggested a strong correlation between the pyrolysis temperature and the electrical conductivity of the char.23,24 To isolate the importance of quinone groups, we introduced quinones by sorption of hydrophobic model compounds containing quinone functional groups. A second objective was to further characterize the pathway by which sulfides destroy RDX in the presence of black carbons. Previous research suggested that RDX transformation in abiotic systems involves reduction.7,25 However, the pathway responsible for black carbon-mediated RDX transformation in the presence of sulfides is unclear, as sulfides are both powerful reductants and nucleophiles.11 The products previously identified from the activated carbon-mediated transformation of RDX in the presence of sulfides under comparable conditions to those evaluated in this work included 1 mole of nitrite (33% of RDX N) and 2 moles of formaldehyde (67% of RDX C) per mole of RDX.6 These are the same products observed during alkaline hydrolysis.26 The nitrosated intermediate products observed during RDX reduction by Fe(II) species25 were not observed. The reaction pathway was further evaluated using an electrochemical cell containing sheet graphite electrodes.9 The anodic chamber containing sulfides was connected via a copper wire and salt bridge to the cathodic chamber containing RDX. Although the electrical connection would permit the sheet graphite to mediate electron transfer between sulfides and RDX, the physical separation of the sulfides and RDX would prevent nucleophilic substitution reactions. In this system, no RDX transformation was observed, suggesting that RDX transformation did not result from

reduction of RDX. This study further evaluated whether transformation of RDX by sulfides in the presence of black carbons involves reduction or nucleophilic substitution, or even a combination of the two processes.

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MATERIALS AND METHODS Chemicals. AccuStandard (New Haven, CT) RDX (1000 μg/mL in acetonitrile and methanol), Fisher (Pittsburgh, PA) granular activated carbon (GAC, 6−14 mesh), Acros (Fair Lawn, NJ) sodium sulfide nonahydrate (98+%) and benz[a]anthracene-7,12-dione (99%), Alfa Aesar (Ward Hill, MA) sheet graphite (0.13 mm thickness, catalog number 43078) and 1-thionaphthol (99%), Pfaltz & Bauer (Waterbury, CT) sodium tetrasulfide (90%), MP Biomedicals (Solon, OH) duroquinone (99.9%), Sigma-Aldrich (St. Louis, MO) p-benzoquinone (99.5%), JT Baker (Phillipsburg, NJ) dichloromethane, acetonitrile and methanol (HPLC grade), and Macron Chemicals (Center Valley, PA) ethyl acetate (ACS grade) were used without further purification. Deionized water (18 MΩ) was produced with a Millipore Elix 10/Gradient A10 water purification system. Black Carbon Preparation and Characterization. Red Oak wood chars were made in a three-zone tube furnace (Lindberg Blue) at different temperatures (600−900 °C) under N2 flow of 1.5 L/min for 2 h. Elemental analysis of all carbon materials was carried out by Galbraith Laboratories (Knoxville, TN) using a Flash 2000 Elemental Analyzer. Surface area was characterized by N2 sorption (Autosorb-3B, Quantachrome Instruments). Energy dispersive spectroscopy (EDS, Princeton Gamma Tech Inc.) using a liquid nitrogen-cooled germanium detector was used for surface analysis. The pH of the carbon materials was determined after equilibrating 1 g carbon in 20 mL deionized water for 48 h at 25 °C.19 To introduce model quinones to the carbon surface, GAC (10 g) was equilibrated with 1.3 mmole p-benzoquinone, 1.2 mmole of duroquinone, or 0.39 mmole benz[a]anthracene7,12-dione in methanol for 48 h. The concentrations of pbenzoquinone, duroquinone, and benz[a]anthracene-7,12dione in methanol were monitored by UV spectrophotometry at 243, 280, and 284 nm, respectively. No significant losses of quinones were observed in controls in the absence of GAC, indicating that sorption to the glass container wall was insignificant. The reductions in quinone concentrations observed in the presence of GAC were attributed to sorption to the carbon surface, and amounted to 128 μmole/g GAC for p-benzoquinone, 122 μmole/g GAC for duroquinone, and 39 μmole/g GAC for benz[a]anthracene-7,12-dione. GACs were then separated from methanol by filtration and air-dried for 24 h. To estimate the percentage of the GAC surface covered by various quinones, the surface areas of quinones were calculated using Chem Draw 3D 11.0. Treating compounds as approximately spherical, and assuming the entire GAC surface area is accessible and close packing of these spheres on the carbon surface, we calculated that 2446, 1580, 1231 μmole/g would be needed to achieve a theoretical 100% monolayer coverage for p-benzoquinone, duroquinone and benz[a]anthracene-7,12-dione, respectively. Accordingly, the sorption in our experiments achieved ∼5%, 8%, and 3% monolayer coverage for p-benzoquinone, duroquinone and benz[a]anthracene-7,12-dione, respectively. Electrical Conductivity Measurement. The electrical conductivity of all carbon samples was measured using a twoprobe bed technique adapted from Mochidzuki et al.27 In order B

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Table 1. Physical Properties of Various Black Carbons elemental analysis C samples 600 °C char 700 °C char 800 °C char 850 °C char 900 °C char activated carbon graphite a

H percentage by weight

84.1 87.2 84.9 84.7 84.0 71.4 99.7

2.6 1.7 1.7 1.4 1.4 2.7 1.6

O a

9.3 8.4 10.1 9.9 9.6 15.9 0.65 S·mm−1 suggests that the sorption was rate-limiting when the char concentration was 18.75 g/L. The results indicate that char electrical conductivity is important in influencing char reactivity, provided that the reaction is not limited by mass transfer from the aqueous to the solid phase. Previous research involving the pyrolysis of highly purified cellulose at a range of temperatures indicated a percolation temperature of 610 °C.24 Below this temperature, electrical conductivity, measured as AC conductivity, was dominated by polarizable carbonyl functional groups. Above this temperature, polarizable carbonyl functional groups were lost in favor of graphene regions capable of transferring electrons over longer distances, as measured by DC conductivity. No significant changes in the O/C ratio were observed for temperatures between 600 and 1000 °C. These results align with our results indicating that bulk electrical conductivity (similar to DC conductivity), was significant only above 600 °C. Our finding that krxn was observable in our experimental time frame only where there was measurable bulk electrical conductivity suggests that the percolation threshold must be exceeded to permit transport of electrons through the carbon. The broader applicability of the correlation between kobs and electrical conductivity for the chars was tested using GAC and sheet graphite. For the GAC, 62.5 g/L GAC was employed to achieve >95% sorption of RDX after 24 h equilibration. The aqueous phase was then decanted and replaced with fresh deaerated phosphate buffer to eliminate aqueous RDX. RDX transformation by sulfides in the presence of the GAC followed pseudo-first order decay as shown in Figure SI-5B. For sheet graphite, krxn was modeled as previously described, because it was not possible to isolate krxn at feasible sheet graphite concentrations. The proximity of the experimentally determined krxn for both GAC and graphite to those predicted based upon the electrical conductivity correlation for chars (Figure 2) indicates that electrical conductivity is an important factor determining black carbon reactivity with regard to RDX transformation. Importance of Oxygenated Functional Groups. Previous research has employed techniques such as treatment with nitric acid or ozone to increase the prevalence of surface oxygenated functional groups on black carbons.19,21 Deionized water (1 L) containing 10 g GAC was purged for 2 h with ozone generated by a Triogen LAB2B ozone generator using a pure oxygen source. After ozonation, 62.5 g/L of the GAC was equilibrated with 0.1 μmole RDX in deaerated deionized water buffered at pH 7. After 24 h, 94.4% (±5.6%) of the RDX was sorbed to untreated GAC, while 64.3% (±2.1%) of the RDX was sorbed to the ozonated GAC. For both untreated and ozone-treated GAC, the aqueous phases were then decanted and replaced with fresh deaerated phosphate buffer to eliminate aqueous RDX. In the presence of 3 mM sulfides, sorbed RDX followed a pseudofirst order decay over 24 h on untreated GAC (kobs = 1.1 (±0.2) × 10−5 s−1; n = 3). In contrast, RDX transformation was insignificant over 24 h in the presence of the ozone-treated GAC. The electrical conductivities of GAC before and after ozone treatment were 0.92 (±0.03) S·mm−1

Figure 2. Pseudo-first order surface reaction rate constants (krxn) for RDX transformation (0.1 μmoles) on black carbon surfaces by sulfides (3 mM) at pH 7.0 vs the measured conductivities of various black carbons. The dashed line presents the predicted correlation based on the experimental results for five red oak wood chars. Error bars represent the standard deviation from the linear regression of the total mass decay of RDX vs time.

and 0.85 (±0.02) S·mm−1, respectively. Neither the decrease in sorption of RDX to the GAC, nor the decrease in electrical conductivity could entirely explain the elimination of RDX transformation. Previous research has demonstrated that ozonation introduces a mixture of oxygenated functional groups, but simultaneously alters other carbon properties, including acidity, surface area, and pore distribution.19,21 Although the simultaneous alteration of carbon properties hinders attempts to understand the decline in reactivity upon ozonation, the use of ozonation to promote oxygenated functional group content did not increase GAC reactivity. Alternative explanations for the reduction in reactivity upon ozonation could include chemical alterations to the sites associated with sulfide or RDX reactions. Unfortunately, the simultaneous alteration of several carbon properties by this traditional treatment hinders attempts to isolate the black carbon alterations responsible for this loss in reactivity. However, the application of ozonation to promote oxygenated functional group content did not increase GAC reactivity. Because previous investigators have hypothesized a particular importance for quinone functional groups in the black carbonmediated transformation of contaminants,6,13 a more controlled technique for introducing quinones to the GAC surface that minimizes alteration of other carbon properties was sought. The model quinones, p-benzoquinone, duroquinone, and benz[a]anthracene-7,12-dione, were introduced to the GAC surface by sorption, achieving ∼5, 8 and 3% of the theoretical monolayer coverage, respectively. Model quinones were selected based on their two-electron reduction potentials at pH 7 (+0.28 V33 for p-benzoquinone, +0.005 V33 for duroquinone and approximately −0.2 V for benz[a]anthracene-7,12-dione based on analogy to the structurally similar quinones 9,10-anthraquinone-2,6-disulfonate (−0.18 V33) and 9,10-anthraquinone-2-sulfonate (−0.23 V33). The reduction potentials of these quinones lie above that of the twoelectron reduction potential of elemental sulfur to sulfides (−0.27 V11). The two-electron reduction potential of RDX was not available, but a subset of this range of quinones might mediate electron transfer between sulfides and RDX. In control experiments with RDX (0.1 μmole), quinones (0.2 μmole), and 3 mM sulfides, no RDX decay was observed in the absence of E

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formation may involve a nucleophilic substitution reaction. Reduced sulfur species (e.g., mercaptoquinone, polysulfides, etc.) are potent nucleophiles, as well as reductants.14,15,35 Indeed, the previously identified products of the black carbonmediated transformation of RDX in the presence of sulfides, 1 mole of nitrite and 2 moles of formaldehyde per mole of RDX,6 are the same products observed during alkaline hydrolysis.26 The nitrosated intermediate products observed during RDX reduction by Fe(II) species25 were not observed during RDX transformation by sulfides in the presence of black carbon.6 However, RDX transformation does not involve a direct nucleophilic substitution reaction between sulfides and RDX, since no RDX transformation was observed in controls containing RDX and sulfides in the absence of black carbons. To account for the correlation between black carbon electrical conductivity and reactivity with regard to RDX decay, we suggest a two-step reaction pathway, where sulfides are oxidized on black carbon surfaces to intermediates that are potent nucleophiles. These sulfur-based intermediates would react with RDX through a nucleophilic substitution reaction. Previous research has demonstrated that black carbons can oxidize sulfides to an array of sulfur products, including elemental sulfur and polysulfides (e.g., S42−).36 For example, thiophenol and tetrasulfide ion are close to 102 times more nucleophilic than HS−.11 If production of an active sulfur product were dependent on black carbon electrical conductivity, then such a pathway could account for the correlation between black carbon electrical conductivity and reactivity. Additional experiments indicated that the hypothesized reactive sulfur intermediates remain on the black carbon surface during RDX transformation. Sheet graphite (12 g/L) was exposed to 3 mM sulfides for 12 h in the absence of RDX. The sheet graphite was then transferred to another vial containing fresh deaerated phosphate buffer at pH 7 without addition of sulfides. When RDX (0.1 μmole) was injected into the solution remaining in the initial vial, no RDX transformation was observed after 24 h. However, RDX (0.1 μmole) injected into the fresh solution containing the pretreated sheet graphite was degraded (Figure SI-7). The loss rate appeared to decline over time, presumably due to the limited supply of the hypothesized sulfur intermediate, but only 41% ((±0.1%), n = 2) of the RDX remained after 25 h. As the association between the graphite and HS− was unlikely to be strong enough for HS− to be retained during transfer of the graphite between vials, these results suggest that another surface-associated species was responsible for RDX transformation. Soaking 12 g/L sheet graphite with 1 N hydrochloric acid to reduce the possible presence of trace metals resulted in no significant change in the kinetics of RDX (0.1 μmole) loss in the presence of 3 mM sulfides, suggesting that trace metals were not responsible for the observed reactivity. In order to detect the surface-sulfur species hypothesized to be responsible for the observed RDX decay, sheet graphite (12 g/L) was exposed to 3 mM sulfides for 12 h, and then rinsed with deionized water three times and air-dried overnight before surface analysis by EDS. EDS analysis of the sheet graphite surface indicated a peak at 2300 eV, characteristic of sulfur (Figure 3), at a prevalence of 0.22%. Although this technique did not further characterize the sulfur species, organic sulfides and polysulfides are candidates. The model organic sulfide, 1-thionaphthol, was sorbed to 2 g of sheet graphite in a fashion similar to that described for quinone sorption to GAC, achieving a final surface loading of

black carbons over 24 h. When 62.5 g/L of the quinone-treated GAC was equilibrated with 0.1 μmole RDX, 18% (±7%), 59% (±2%), and 24% (±6%) of the RDX sorbed after 24 h for pbenzoquinone, duroquinone, and benz[a]anthracene-7,12dione, respectively, compared to >95% sorption of the RDX for the unmodified GAC. The aqueous phase was decanted and replaced with fresh deaerated phosphate buffer to eliminate aqueous RDX. In controls in the absence of sulfides, no significant aqueous concentrations of RDX or quinones were measured after 24 h, indicating that desorption was insignificant. In the presence of 3 mM sulfides, the mass recovery of RDX after 24 h was 93% (±3%), 85% (±14%), and 76% (±12%) in the presence of GAC modified by pbenzoquinone, duroquinone and benz[a]anthracene-7,12dione, respectively, compared to 43.2% (±11.1%) in the presence of unmodified GAC. The reduction in GAC reactivity cannot be attributed to decreased conductivity; the conductivities of GAC modified by p-benzoquinone, duroquinone and benz[a]anthracene-7,12dione were 1.06 (±0.02) S·mm−1, 0.89 (±0.02) S·mm−1, and 1.18 (±0.01) S·mm−1, respectively, compared to 0.92 (±0.03) S·mm−1 for the unmodified GAC (Table 1). However, introduction of quinones to the GAC surface may have blocked specific reaction sites for sulfides or RDX. Regardless, the decrease in reactivity when quinones occupied such sites while maintaining electrical conductivity suggests that quinones were not the key functional groups at these reaction sites. Overall, results from the ozonation and quinone experiments suggest that the prevalence of oxygenated functional groups is much less important than electrical conductivity in promoting black carbon-mediated RDX transformation. Reaction Pathway. The correlation between black carbon electrical conductivity and the rate constant of surface reaction (krxn) (Figure 2) suggests that the reaction involves facilitated electron transfer. To test this hypothesis, we employed a previously developed electrochemical cell,9 in which the anodic chamber contained sulfides and the cathodic chamber contained the target contaminant and sheet graphites were used as electrodes (Figure SI-6). The graphite sheet electrodes were connected by a copper wire between the two chambers, while a salt bridge completed the circuit. Although the electrical connection would permit the sheet graphite to mediate electron transfer between sulfides and the target contaminant, the physical separation of the sulfides and target contaminant would prevent nucleophilic substitution reactions. The observation of nitroglycerin transformation in this system demonstrated that black carbon mediated nitroglycerin transformation via a reduction reaction,9 similar to iron oxides serving as a semiconductor.34 In contrast, RDX transformation was not observed in the electrochemical cell,9 a result replicated in this study using 0.2 μmoles RDX, 12 g/L sheet graphite, and 3 mM sulfides over 48 h. Thus, RDX and sulfides must be in direct contact for RDX transformation to occur. We can also eliminate the possibility that sulfides react with RDX in the presence of black carbon to reversibly produce an intermediate that then transfers electrons through the black carbon to reduce other sorbed RDX molecules: when RDX (0.2 μmole) and sulfides (3 mM) were spiked into the anodic chamber of the electrochemical cell and additional RDX (0.2 μmole) was spiked into the cathodic chamber, no RDX transformation was observed after 24 h in the right chamber. The requirement that RDX and sulfides have to be in contact for RDX transformation to occur suggests that RDX transF

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presence of sulfide-treated sheet graphite (Figure SI-7). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (203) 432-4386; fax: (203) 432-4387; e-mail: william. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a grant from the National Science Foundation (CBET-0747735).

Figure 3. EDS analysis of the sheet graphite surface before and after treatment with 3 mM sulfides for 12 h.

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12.2 μmole/g. No RDX (0.1 μmole) transformation was observed over 24 h upon exposure to this modified sheet graphite in the absence of additional sulfides. The results suggest that surface organic thiol functional groups are not responsible for RDX transformation. In contrast, RDX (0.1 μmole) exposed to 66 μM sodium tetrasulfide at pH 7 followed first-order decay in the presence of 12 g/L sheet graphite (kobs = 5.5 (±0.4) × 10−5 s−1; n = 2; Figure 4), but exhibited no

Figure 4. Transformation of the total mass (M) of RDX (0.1 μmoles) by 66 μM polysulfides in the presence of 12 g/L sheet graphite.

decay in the absence of black carbon after 36 h. Further characterization of the hypothesized surface-associated reduced sulfur nucleophiles is needed. Although a surface-mediated nucleophilic substitution reaction has been reported for the hydrolysis of carboxylate esters by mineral oxide surfaces,37 surface-mediated nucleophilic substitutions have not been reported previously for black carbons under environmentally relevant conditions.

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REFERENCES

(1) Luthy, R. G.; Aiken, G. R.; Brusseau, M. L.; Cunningham, S. D.; Gschwend, P. M.; Pignatello, J. J.; Reinhard, M.; Traina, S. J.; Weber, W. J.; Westall, J. C. Sequestration of hydrophobic organic contaminants by geosorbents. Environ. Sci. Technol. 1997, 31 (12), 3341−3347. (2) Cornelissen, G.; Gustafsson, O.; Bucheli, T. D.; Jonker, M. T. O.; Koelmans, A. A.; Van Noort, P. C. M. Extensive sorption of organic compounds to black carbon, coal, and kerogen in sediments and soils: Mechanisms and consequences for distribution, bioaccumulation, and biodegradation. Environ. Sci. Technol. 2005, 39 (18), 6881−6895. (3) Middelburg, J. J.; Nieuwenhuize, J.; van Breugel, P. Black carbon in marine sediments. Mar. Chem. 1999, 65 (3−4), 245−252. (4) Schmidt, M. W. I.; Noack, A. G. Black carbon in soils and sediments: Analysis, distribution, implications, and current challenges. Glob. Biogeochem. Cycle 2000, 14 (3), 777−793. (5) Oen, A. M. P.; Beckingham, B.; Ghosh, U.; Krusa, M. E.; Luthy, R. G.; Hartnik, T.; Henriksen, T.; Cornelissen, G. Sorption of organic compounds to fresh and field-aged activated carbons in soils and sediments. Environ. Sci. Technol. 2012, 46 (2), 810−817. (6) Kemper, J. M.; Ammar, E.; Mitch, W. A. Abiotic degradation of hexahydro-1,3,5-trinitro-1,3,5-triazine in the presence of hydrogen sulfide and black carbon. Environ. Sci. Technol. 2008, 42 (6), 2118− 2123. (7) Oh, S. Y.; Chiu, P. C. Graphite- and soot-mediated reduction of 2,4-dinitrotoluene and hexahydro-1,3,5-trinitro-1,3,5-triazine. Environ. Sci. Technol. 2009, 43 (18), 6983−6988. (8) Oh, S. Y.; Son, J. G.; Lim, O. T.; Chiu, P. C. The role of black carbon as a catalyst for environmental redox transformation. Environ. Geochem. Health 2012, 34, 105−113. (9) Xu, W.; Dana, K. E.; Mitch, W. A. Black carbon-mediated destruction of nitroglycerin and RDX by hydrogen sulfide. Environ. Sci. Technol. 2010, 44 (16), 6409−6415. (10) Zeng, T.; Chin, Y. P.; Arnold, W. A. Potential for abiotic reduction of pesticides in prairie pothole porewaters. Environ. Sci. Technol. 2012, 46 (6), 3177−3187. (11) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic Chemistry, 2nd ed.; Wiley & Sons: Hoboken, NJ, 2003. (12) Ju, K. S.; Parales, R. E. Nitroaromatic compounds, from synthesis to biodegradation. Microbiol. Mol. Biol. R 2010, 74 (2), 250− 272. (13) Yu, X. D.; Gong, W. W.; Liu, X. H.; Shi, L.; Han, X.; Bao, H. Y. The use of carbon black to catalyze the reduction of nitrobenzenes by sulfides. J. Hazard Mater. 2011, 198, 340−346. (14) Dunnivant, F. M.; Schwarzenbach, R. P.; Macalady, D. L. Reduction of substituted nitrobenzenes in aqueous solutions containing natural organic matter. Environ. Sci. Technol. 1992, 26 (11), 2133−2141. (15) Schwarzenbach, R. P.; Stierli, R.; Lanz, K.; Zeyer, J. Quinone and iron porphyrin mediated reduction of nitroaromatic compounds in

ASSOCIATED CONTENT

S Supporting Information *

Effect of compressive pressure on the empty bed resistance and piston length change (Figure SI-1); measured resistance vs packed carbon bed length for GAC (Figure SI-2); resistivity measurements under compressive pressure (Figure SI-3); experimental and modeling results for sheet graphite (text and Figure SI-4); importance of adsorption vs reaction for sheet graphite (Table SI-1); example first-order degradation plots for RDX on chars or GAC (Figure SI-5); electrochemical cell design (Figure SI-6); and degradation of RDX in the G

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dx.doi.org/10.1021/es4012367 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX