Degradation of
(RDX) Using Zerovalent Iron
G H I N W A N A J A , ‡
hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX) (56%),
less than 5% of RDX degraded under aerobic conditions, but
more than 75% of the nitramine degraded under anaerobic
a pseudofirst order constant k1 was obtained. The reaction
constants (Supporting Information Table S-1) were deter-
mined (...
fication process in an anoxic environment or in the presence
of specific kind of anaerobic bacteria (38).
Hydrazine (NH2NH2)...
indicating the involvement of the two aza nitrogens in the
formation of CH4N4O3. We tentatively identified the inter-
(7) Bhushan, B.; Halasz, A.; Spain, J. C.; Thiboutot, S.; Ampleman,
G.; Hawari, J. Diaphorase catalyzed biotransformation ...
of 7

Naja 2008 ZVI RDX EST

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Transcripts - Naja 2008 ZVI RDX EST

  • 1. Degradation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) Using Zerovalent Iron Nanoparticles G H I N W A N A J A , ‡ A N N A M A R I A H A L A S Z , ‡ S O N I A T H I B O U T O T , † G U Y A M P L E M A N , † A N D J A L A L H A W A R I * , ‡ Defence Research Establishment, Valcartier (Quebec), 2459 Blvd Pie IX, Canada G0A 1R0, and Biotechnology Research Institute, National Research Council of Canada, Montreal, Quebec, Canada H4P 2R2 Received November 8, 2007. Revised manuscript received March 18, 2008. Accepted March 27, 2008. Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a common contaminant of soil and water at military facilities. The present study describes degradation of RDX with zerovalent iron nanoparticles (ZVINs) in water in the presence or absence of a stabilizer additive such as carboxymethyl cellulose (CMC) or poly(acrylic acid) (PAA). The rates of RDX degradation in solution followed this order CMC-ZVINs > PAA-ZVINs > ZVINs with k1 values of 0.816 ( 0.067, 0.082 ( 0.002, and 0.019 ( 0.002 min-1, respectively. The disappearance of RDX was accompanied by the formation of formaldehyde, nitrogen, nitrite, ammonium, nitrous oxide, and hydrazine by the intermediary formation of methylenedinitramine (MEDINA), MNX (hexahydro- 1-nitroso-,3,5-dinitro-1,3,5-triazine),DNX(hexahydro-1,3-dinitroso- 5-nitro-1,3,5-triazine), TNX (hexahydro-1,3,5-trinitroso-1,3,5- triazine). When either of the reduced RDX products (MNX or TNX) was treated with ZVINs we observed nitrite (from MNX only), NO (from TNX only), N2O, NH4 +, NH2NH2 and HCHO. In the case of TNX we observed a new key product that we tentatively identified as 1,3-dinitroso-5-hydro-1,3,5-triazacyclo- hexane. However, we were unable to detect the equivalent denitrohydrogenated product of RDX and MNX degradation. Finally,duringMNXdegradationwedetectedanewintermediate identified as N-nitroso-methylenenitramine (ONNHCH2NHNO2), theequivalentofmethylenedinitramineformedupondenitration of RDX. Experimental evidence gathered thus far suggested that ZVINs degraded RDX and MNX via initial denitration and sequential reduction to the corresponding nitroso derivatives prior to completed decomposition but degraded TNX exclusively via initial cleavage of the NsNO bond(s). Introduction The extensive use of explosives such as hexahydro-1,3,5- trinitro-1,3,5-triazine (RDX) has led to widespread contami- nation of soil and water (1, 2). RDX is known to be toxic to various aquatic and terrestrial organisms (3), thus neces- sitating its removal from polluted environments. Recently our group has conducted several studies to elucidate the degradationpathwaysofRDXundervariousabioticandbiotic conditions in an effort to help in the design of in situ remediation strategies. For example we found that initial denitration can lead to decomposition and the formation of the two key intermediates 4-nitro-2,4-diazabutanal (NDAB) and methylenedinitramine (MEDINA) whose formation depends on the stoichiometry of the released nitrite ion. The loss of 2 NO2 - from RDX normally leads to NDAB as has been found during alkaline hydrolysis (4) and aerobic biodegrada- tion with Rhodococcus sp. strain DN22 (5) and XplA (6). Whereas the loss of 1 NO2 - leads to the predominant formation of methylenedinitramine (MEDINA) as has been observed during RDX treatment with diaphorase enzyme (7) and XplA (6). RDX transformation to the corresponding nitroso derivatives (8) has also been documented to occur under various abiotic reducing and biotic anaerobic condi- tions (9, 10). Although more recently Kemper et al. (11) did not observe any of RDX nitroso products using hydrogen sulfide and black carbon. RDX removal using zerovalent iron has been extensively reported (8, 12–14) where several products including MNX, DNX, and TNX have been identified, but little is known on how these nitroso products cleave. In the present study we chose to examine highly reactive zerovalent iron nanopar- ticles (ZVINs) capable of generating intermediates in suf- ficient concentrations to allow investigation of their decom- position routes. Recently, zerovalent iron nanoparticles (ZVINs) have been developed for several environmental remediation technologies (15), especially for the treatment of chlorinated organic compounds (16), metal ions (17), pesticides (18), organic dyes (19) and inorganic anions (20). However, the integration of ZVI nanoparticles in environ- mental processes has been held back by the key technical barrier represented by the tendency of iron nanoparticles to agglomerate and, thereby, rapidly lose their chemical reac- tivity and mobility. Extensive studies have been devoted to the stabilization of the ZVINs. While Schrick et al. (21) used hydrophilic carbon as delivery vehicles to support ZVI nanoparticles, He et al. (22) reported a new strategy for stabilizing palladized iron nanoparticles with sodium car- boxymethyl cellulose. To our knowledge, no reports are available for the degradation of RDX with ZVI nanoparticles whose unique properties present a novel technological potential. The current work examines the reaction between RDX and ZVINs focusing on the identification of the RDX transformation pathway pinpointing the products, intermediates of the reaction as well as their yields. For this purpose, ZVI nanoparticles were used to degrade RDX in water in the presence and absence of the two stabilizers, carboxymethyl cellulose (CMC) and poly(acrylic acid) (PAA). Addition of the polymeric stabilizers into the system is intended to keep the nanoparticles well dispersed, to facilitate their even distribu- tion and smooth penetration through soil when applied in accelerated in situ RDX degradation at contaminated sites. Finally, due to the formation of MNX and TNX as potential RDX intermediates during RDX reduction with ZVI nano- particles, we thus investigated their reaction under the same conditions to gain further insight into the degradation pathway(s) of RDX. Experimental Section Chemicals. Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) (>99%), hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX) (99%) and ring-labeled [15 N]-RDX (98%) were obtained from DefenceResearchandDevelopmentCanada(Valcartier,QC). * Corresponding author phone: +1-514-496 6267; fax +1-514-496 6265; e-mail address: ‡ Biotechnology Research Institute. † Defence Research Establishment. Environ. Sci. Technol. 2008, 42, 4364–4370 4364 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008 10.1021/es7028153 CCC: $40.75  2008 American Chemical Society Published on Web 05/14/2008
  • 2. Hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine(MNX)(98%), hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX) (56%), ring-labeled [15 N]-MNX, ring-labeled [15 N]-TNX, methylene- dinitramine(MEDINA)and4-nitro-2,4-diazabutanal(NDAB) were provided by Dr. R. J. Spanggord from SRI International (Menlo Park, CA). Hydrazine sulfate (99%) was obtained from Aldrich, Canada. Sodium borohydride (g98.5%) and poly- (acrylic acid) (M.W. 1800) were purchased from Aldrich, and sodium carboxymethyl cellulose, polymer of cellulose- (OsCH2sCOO- )n, CMC, (M.W. 90 000) and nitrous oxide standard (1000 ppm) were procured from Sigma-Aldrich. Ferrous sulfate (99%) and sodium hydroxide (99%) were obtained from Anachemia and EMD, respectively. Nitrite and ammonium standards (1000 ppm) were purchased from Alltech. All other chemicals were reagent grade, and all solutions were prepared using Milli-Q-UV Plus Ultrapure water system (>18 MΩ, Millipore, MA). Zerovalent Iron Nanoparticles Preparation. Zerovalent iron nanoparticles (ZVINs), poly(acrylic acid) modified zerovalent iron nanoparticles (PAA-ZVINs), and carboxym- ethyl cellulose modified zerovalent iron nanoparticles (CMC- ZVINs) were prepared according to the methods published by Liu et al. (15), Schrick et al. (21) and He et al. (22), respectively, with modifications detailed in the Supporting Information. Physical Characterization. Transmission electron mi- croscope (TEM) micrographs were recorded using a Philips CM20 200 kV electron microscope equipped with an Oxford Instruments energy dispersive X-ray spectrometer (Link exl II) and an UltraScan 1000 CCD camera. To obtain the TEM images, the nanoparticle suspensions were diluted with methanolandsonicatedvigorously.TwentyµLofeachsample was then dropped on a holey carbon film 300 mesh copper grid and allowed to air-dry. The N2sBET specific surface area of the nanoparticles was measured using a TriStar 3000 gas adsorption analyzer and the multipoint method (Micromeritics Analytical Ser- vices, GA) with a resolution of 0.05 mmHg. Reaction of RDX with ZVINs. Deionized water (10 mL) was mixed with 3 mg of ZVINs (methanol solution) in 15 mL serum bottles and crimp-sealed with Teflon-coated septa. The solution was either made anaerobic by purging the headspace for 10 min with argon or kept under a blanket of air for the aerobic experiments. Following 15 min of gentle shaking (rotary shaker, 150 rpm) to equilibrate the pH value (5.9-6.1), 0.82 µmol of RDX in methanol (0.2 mL) was added to each bottle and the reaction was allowed to take place at room temperature. In some experiments ring-labeled [15 N]- RDX was used under the same conditions to help identify RDX products. All experiments were made in triplicate. Controls containing ZVINs in water with no RDX were also performed to ensure the ZVINs stability in water. Controls containing RDX in water with no ZVINs were not necessary since RDX hydrolysis could be neglected within the range of pH values examined (4). The reactions were stopped after 3, 6, 10, 20, 40, 60, 120, 240, and 480 min. For each time measurement three bottles were sacrificed for analysis. Parallel RDX anoxic batch experiments were performed using CMC-ZVINs and PAA-ZVINs (0.3 g L-1 of nanometal) to compare their reactivity with the nonstabilized ZVINs based on the same initial iron nanoparticle concentration. Other batch experiments were conducted using ZVINs (0.3 g L-1 ) and either MNX (80 µM) or TNX (80 µM) to determine products formed and thus know their role in the degradation of RDX. In some experiments ring-labeled [15 N]- MNX and [15 N]-TNX were used under the same conditions to help identify the degradation products. To determine the eventual fate of nitrogen-containing RDX degradation prod- ucts such as NO2 -, NH4 +, N2O, MEDINA, and NH2NH2 we allowed ZVINs (0.3 g L-1) to react with 20 mg L-1 of each chemical separately. Chemical Analysis. The gas phase in the headspace of the chemical assays was sampled using a gastight syringe (250 or 100 µL) and then analyzed for nitrogen, nitrous oxide, andhydrogenbyagaschromatograph(Hewlett-Packard6890 GC, Mississauga, ON) connected to either a TCD or an ECD detector (23). Aliquots of the aqueous phase of the reaction mixtureswerefilteredthrough0.22µmfilters(Millipore)prior to analyses of RDX, intermediates and final products. The nitrosoderivativesMNX,DNX,andTNXandtheringcleavage products methylenedinitramine (MEDINA) and 4-nitro-2,4- diazabutanal (NDAB) were analyzed as described by Hawari et al. (24) and by Bushan et al. (25). Formaldehyde, formic acid, ammonium, nitrate, nitrite were analyzed as described by Monteil-Rivera et al. (26). Denitrosation of TNX was followed by monitoring nitric oxide (NO) using Apollo 4000 free radical analyzer (WPI, U.S.) specific for NO analysis. Whereas hydrazine formation was monitored by analyzing aliquots of the reaction mixture after derivatization with salicylaldehyde (98%, Aldrich) followed by LC/MS analysis as described by Monteil-Rivera et al. (26). The concentration of iron in CMC-ZVINs and PAA-ZVINs, ferric, and ferrous ionsweredeterminedbyspectroscopicmethodsasdescribed by Schrick et al. (21). Results and Discussion ZVI Nanoparticle Characterizations. The TEM images (Figure 1) indicated that the three types of nanoparticles (ZVINs, CMC-ZVINs, and PAA-ZVINs) were mostly spherical inshapeandformedaggregates.TheZVINsfeaturedadistinct core and amorphous shell structure as clearly presented in Figure 1b. The TEM images (Figure 1e) also showed that the CMC-ZVINs had the smallest average particle diameter of 15 ( 4 nm compared to the ZVINs with 32 ( 7 nm and to the PAA-ZVINs with 173 ( 40 nm as an average particle diameter. These measurements were in agreement with those found in the literature. For instance, Sun et al. (27) used the same procedureformingironnanoparticleswithamediandiameter of approximately 60.2 nm, whereas He et al. (22) reported CMCstabilizedFe-Pdnanoparticleswiththeaverageparticle diameter of 4.3 nm. When using poly(acrylic acid) to prepare ironnanoparticles,Schricketal.(21)observedironaggregates measuring approximately 100 nm. ThespecificsurfaceareasofZVINs,CMC-ZVINs,andPAA- ZVINs were 42.6, 11.3, and 5.9 m2 g-1 , respectively. These experimental surface areas were compared to the theoretical values calculated using eq 1 (28) (24.0, 51.3, and 4.4 m2 g-1 for ZVINs, CMC-ZVINs, and PAA-ZVINs, respectively). specific surface area ) 6 Fd (1) where F and d are the particle density (7.8 × 106 g m-3) and diameter (m), respectively. The observed difference between the calculated and measured areas may be caused by the density difference (27). In fact, the surfaces of those different types of nano- particles differed whereby iron was largely present as iron hydroxides with the ZVINs, and it was surrounded by polymers in the two other cases coating the nanoparticles with a thick film and thus decreasing the value of the specific surface area. Kinetics of RDX Degradation with ZVINs. Since the physicochemical properties of RDX indicate that it has an importantpotentialforleaching,remediationtreatmentsthat rapidly transform RDX and promote its degradation are of great importance. In the present study, ZVINs (3 g L-1 ) completely degraded 82 µmol L-1 of RDX in five minutes under both aerobic (98.3%) and anaerobic (100%) conditions. However, when reducing the amount of ZVINs to 0.3 g L-1 VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 4365
  • 3. less than 5% of RDX degraded under aerobic conditions, but more than 75% of the nitramine degraded under anaerobic conditions in one hour. Hundal et al. (29) reported the complete transformation of RDX (144 µmol L-1 ) with micro ZVI (10 g L-1 ) in 96 h, but Wanaratna et al. (13) reported that 50 µmol L-1 of RDX can be degraded using micro ZVI in less than 10 min at pH 3.5 by applying an excess amount of ZVI (32 g L-1 ). In the present study, the rapid removal of RDX with ZVINs was attributed to the high reactivity of the nano metal (30–32) due to the small particle size (32 nm) offering a large surface area (42.6 m2 g-1 ) to facilitate the reaction. The slow-down of RDX degradation in the presence of air was attributed to the possible corrosion of the surface of Fe due to its reaction withy oxygen (33). The kinetics of RDX (82 µmol L-1 ) degradation was then followed in anoxic batch experiments using 0.3 g L-1 of the three types of ZVINs (nonstabilized ZVINs, and stabilized CMC-ZVINs and PAA-ZVINs). The comparison of the reac- tivity was based on the same amount of nano metal. Within one hour, more than 75% of RDX was degraded using the three types of nanoparticles (Figure 2), with CMC-ZVINs showing the highest degradation percentage. The latter was very reactive and 6 min were sufficient to degrade 100% of RDX. Assuming that RDX degradation followed a second order reaction rate with respect to RDX and iron concentrations (13), and by maintaining the change in the iron concentration insignificant compared to the change in RDX concentration, FIGURE 1. TEM micrographs of the (a) nonstabilized ZVINs and (b) its enlargement; (c) CMC-ZVINs; and (d) PAA-ZVINs.; (e) probability density function of the particle size distribution of the iron nanoparticles (2 corresponds to the CMC-ZVINs (n ) 100), 0 to the ZVINs (n ) 100), and b to PAA-ZVINs (n ) 45)). 4366 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008
  • 4. a pseudofirst order constant k1 was obtained. The reaction constants (Supporting Information Table S-1) were deter- mined (min-1 ) as well as the corresponding BET surface area- normalized rate constants (h-1 m-2 L). The results indicated thattheCMC-ZVINsapplicationproducedthehighestkinetic constant(0.816min-1 ),almost40timeshigherthantheZVINs (0.019 min-1 ). The obtained values fall within the range of those found in the literature. The k1 constant values obtained by Wanaratna et al. (13) varied between 0.095 min-1 and 0.976 min-1 when ZVI powder was used to remediate RDX- contaminated water. Oh et al. (8) estimated the k1 constant at 0.016 min-1 (value close to the one obtained in the present study) when following the degradation of RDX using iron having 25.8 m2 of surface area per liter of solution. The high reactivity of the CMC-ZVINs has been observed by He et al. (22) who reported that the stabilized nano reagent can degrade trichloroethylene 17 times faster than the nonstabilized ZVINs. This high reactivity of the CMC-ZVINs could be explained by the higher dispersion of the nano- particles caused by the negatively charged carboxylic groups that inhibit aggregation and thus reduce the adhesion coefficient between the nanoparticles (21). The reactivity differencecouldalsobepartlyattributedtotheboroncontent of these particles since different ratios of iron to boron were used during the syntheses. Indeed, it has been speculated in several studies that boron may be responsible for the unique reactivity of ZVINs (prepared using borohydride reducing agent) when compared to nano iron synthesized by the gas- phase reduction of iron oxides (34, 35). However, since in the present case the reactivity was compared for the same amount of nano metal, the effect of boron on the RDX degradation could not be definitively determined. Products and Degradation Pathways. Figure 3 represents RDX degradation with the simultaneous appearance of the ring cleavage products (MEDINA, HCHO, NO2 - N2O, NH4 +, NH2NH2, and N2). The degradation of RDX was also ac- companied by the formation of the nitroso derivatives (MNX, DNX, and TNX) (Supporting Information Table S-2A). Most of the detected products have been observed during the RDX degradation with micro ZVI (8, 14). However, in the present study several new products were detected giving new insights into the initial steps involved in the degradation pathways of RDX (discussed below). At the end of the reaction, which lasted 4 h, most of RDX detectedintermediatestransformedfurthertoeventuallygive N2O, NH4 + , and N2 as the main N-containing products and HCHO as the main C-containing product. In all 2.86 HCHO molecules were produced per one RDX molecule cleaved, accounting for more than 95.6% of the total carbon in RDX after cleavage (Supporting Information Table S-2B). In the case of nitrogen-containing degradation products, 1.44 moleculesofN2Oand1.25moleculesofNO2 -,NH2NH2,NH4 +, and N2 were produced, accounting for more than 79% of the total nitrogen of RDX after cleavage (Supporting Information Table S-2B). The presence of the stabilizer did not seem to drastically affect product distribution. As shown in Tables S-2 and S-3 RDX degradation using the three types of iron nanoparticlesledtothesameintermediateandfinalproducts. But the nitroso intermediates seemed to disappear faster in the presence of CMC and PAA. The identity of nitrogen as an RDX degradation product was confirmed using the ring-labeled [15 N]-RDX and GC/MS analysis. We detected N2 with a molecular mass ion at both 28 Da (14 N14 N) and 29 Da (15 N14 N), confirming the formation of the gas (29 Da) from the original NsNO2 group in RDX and from further reduction of nitrite following initial deni- tration. When either NO2 - or N2O was allowed to come in contact with ZVINs we detected N2 and NH4 +. Nitrous oxide is a decomposing product of MEDINA in water (36) but it could also arise from the reduction of NO2 - by ZVINs (data not shown). Comparatively, NH4 + was weakly degraded into N2 and was probably adsorbed on the surface of the nanoparticles. The iron-aided NO2 - reduction into N2 has already been reported by Huang et al. (37). The present experimental findings mimic the generally known denitri- FIGURE 2. Time course of RDX (0.82 µmol L-1 ) degradation expressed in percentage using the three types of iron nanoparticles. (O corresponds to the CMC-ZVINs, 9 to the ZVINs, and 4 to PAA-ZVINs). The concentration of iron nanoparticles was 0.3 g L-1. The standard deviations were within 5% of the corresponding values. FIGURE 3. Time course of RDX degradation (82 µmol L-1 ) using ZVINs (0.3 g L-1 ) showing the formation of (a) formaldehyde, nitrous oxide, and MEDINA; (b) ammonium, nitrogen, and nitrite. The standard deviations were within 7% of the corresponding values, except for nitrogen where the standard deviation was within 15% of the value. VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 4367
  • 5. fication process in an anoxic environment or in the presence of specific kind of anaerobic bacteria (38). Hydrazine (NH2NH2) was also detected as has been the case when HMX was treated with ZVI (26). As it is toxic, we conducted experiments to determine its origin in the degradation process and its eventual fate. We found that NH2NH2 was a transient species which was transformed further to give NH4 +. After nine days, almost 90% of the initial amount of hydrazine disappeared (Figure S-1). When TNX was treated with ZVINs under the same conditions used for RDX, hydrazine was also detected, suggesting that NH2NH2 might have originated from the NsNO functional group of TNX formed during RDX reductive transformation (Sup- porting Information Table S-4). The hydrazine derivative was detected at a retention time of 15 min with a m/z [M+H]+ of 241 Da, but when the 15 N-labeled RDX (or [15 N]-TNX) was used the product showed a m/z [M+H]+ of 242 Da, representing an increase of one Da due to the introduction of one 15 N atom (the aza nitrogen) (Supporting Information Figure S-1). Furthermorewedetectedtracesofnitricoxide(NO)during treatment of RDX with ZVINs (Figure 4a). To confirm the origin of its formation we treated TNX with the nano metal under the same conditions and found appreciable amounts of NO being formed that did not persist indefinitely (Figure 4a). The formation of NO from TNX was consistent with the observation of the novel intermediate II with [M - H]- at 144 Da, representing an empirical formula of C3H7N5O2 (Figure 4b). When ring labeled [15 N-TNX] was used, the [M - H]- was detected at 147 Da, representing an increase of 3 Da corresponding to the three 15 N aza labeled N in the original nitramine (Figure 4c). We tentatively identified the initial TNX degradation intermediate as 1,3-dinitroso-5- hydro-1,3,5-triazacyclohexane (Figures 4b and c). The ex- pected denitrohydrogenated product of RDX (I) (Figure 5) was not detected. Presumably the latter was less stable than II under these reducing conditions. For example, Bonner et al. (39) reported that intermediate I was unstable and decomposed into MEDINA. Likewise when MNX was treated with the nano metal we observed nitrite, indicating initial denitration, and traces of NO possibly from the denitrosation of its reduced TNX product (Figure 4a). Denitration of MNX was supported by the detection of another new intermediate with [M - H]- at m/z119DamatchinganempiricalformulaofCH4N4O3. When the ring-labeled [15 N]-MNX was used the [M - H]- was detected at m/z 121 Da representing an increase of 2 Da, FIGURE 4. (a) Nitric oxide (NO) production during TNX, MNX, and RDX (80 µmol L-1 ) degradation with ZVINs (0.3 g L-1 ); (b) ES(-) ion mass spectrum of denitrosed hydrogenated compound (II) from TNX; (c) ES(-) ion mass spectrum of denitrosed hydrogenated compound (II) from [15 N]-TNX. 4368 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008
  • 6. indicating the involvement of the two aza nitrogens in the formation of CH4N4O3. We tentatively identified the inter- mediate as N-nitroso-methylenenitramine (ONNHCH2- NHNO2) (III). Compound III is the equivalent of MEDINA formed following denitration of RDX (Figure 5). We did not observe MEDINA when MNX was treated with ZVINs, suggesting that NO originated from its reduced product TNX. Supporting Information Table S-4 summarizes the product distribution observed from MNX and TNX treatment with ZVINs (Figure 5). Experimental evidence gathered thus far on products distribution, stoichiometry, and time courses indicate that RDX degraded via two initial routes. The first route involved initial denitration of RDX giving the suspected unstable denitrohydrogenatedintermediateIwhichwoulddecompose in water to produce MEDINA (Figure 5, path a). The second route involved the stepwise reduction of the NsNO2 func- tional groups to give MNX, DNX, and TNX. Likewise MNX would undergo either denitration prior to ring cleavage or reduction to eventually give TNX which underwent denit- rosation (cleavage of NsNO bond) followed by ring cleavage (Figure 5, path b). Environmental Significance. The use of CMC as stabiliz- ers for ZVINs, which kept the metal well dispersed in water, degradedRDXfortytimesfasterthanthenonstabilizedZVINs. The use of surfactants in many industrial remediation technologies often enhances the remediation process by increasing mobility and solubility in water of insoluble or sparingly soluble contaminants that, in turn, improves their mass removal and the overall process performance. Also the presentreactionsystemdegradedRDXtoobnoxiousproducts such as formaldehyde (biodegradable), nitrous oxide, am- monia, and nitrogen. Although the three nitroso products MNX, DNX, and TNX were also detected as RDX products none of these hazardous chemicals persisted indefinitely rather they all degraded further to produce HCHO and hydrazine, the latter degraded to ammonia. These experi- mental findings can constitute the basis for the development of in situ remediation technologies for contaminated sites. Understanding the dynamics and pathways of RDX degra- dation would help optimizing the in situ remediation of water contaminated with explosives including groundwater and marine sediment. Acknowledgments We thank Mr. Dashan Wang from the ICPET for the transmission electron microscope images. We also thank Dr. Fanny Monteil-Rivera for helpful discussions and Louise Paquet and Stephane Deschamps for conducting the analy- ses. Financial support was provided by DRDC, Valcartier, Canada,andOfficeofNavalResearch(ONR)U.S.Navy(Award N000140610251). Supporting Information Available Experimental section (ZVI nanoparticles preparation and modification), Figure S-1 and Tables S-1, S-2, S-3 and S-4 as mentioned in the text. This material is available free of charge via the Internet at Literature Cited (1) Brannon, J. M.; Price, C. B.; Yost, S. L.; Hayes, C.; Porter, B. Comparison of environmental fate and transport process descriptors of explosives in saline and freshwater systems. Mar. Pollut. Bull. 2005, 50, 247–251. (2) Eisentraeger, A.; Reifferscheid, G.; Dardenne, F.; Blust, R.; Schofer, A. Hazard characterization and identification of a former ammunition site using microarrays, bioassays, and chemical analysis. Environ. Toxicol. Chem. 2007, 26, 634–646. (3) Robidoux,P.Y.;Dubois,C.;Hawari,J.;Sunahara,G.I.Assessment of soil toxicity from an antitank firing range using Lumbricus terrestris and Eisenia andrei in mesocosms and laboratory studies. Ecotoxicology 2004, 13, 603–614. (4) Balakrishnan, V. K.; Halasz, A.; Hawari, J. Alkaline hydrolysis of cyclic nitramine explosives RDX, HMX, and CL-20: New insights into degradation pathways obtained by the observation of novel intermediates. Environ. Sci. Tecnol. 2003, 37, 1838–1843. (5) Fournier, D.; Halasz, A.; Spain, J.; Fiurasek, P.; Hawari, J. Determination of key metabolites during biodegradation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) with Rhodococcus sp. strain DN22. Appl. Environ. Microbiol. 2002, 68, 166–172. (6) Jackson, R. G.; Rylott, E. L.; Fournier, D.; Hawari, J.; Bruce, N. C. Exploring the biochemical properties and remediation applica- tion of the unusual explosive-degrading P450 system XplA/B. Proc. Natl. Acad. Sci. USA 2007, 104, 16822–16827. FIGURE 5. Proposed pathway for degradation of RDX in the presence of ZVINs: (a) Denitration route; (b) Nitro reduction to nitroso followed by denitrosation of TNX formed. VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 4369
  • 7. (7) Bhushan, B.; Halasz, A.; Spain, J. C.; Thiboutot, S.; Ampleman, G.; Hawari, J. Diaphorase catalyzed biotransformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) via N-denitration mechanism. Biochem. Biophys. Res. Commun. 2002, 296, 779– 784. (8) Oh, S. Y.; Cha, D. K.; Kim, B. J.; Chiu, P. C. Reductive transformation of hexahydro-1,3,5-trinitro-1,3,5-triazine, oc- tahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, and methylene- dinitramine with elemental iron. Environ. Toxicol. Chem. 2005, 24, 2812–2819. (9) McCormick, N. G.; Cornell, J. H.; Kaplan, A. M. Biodegradation of hexahydro-1,3,5-trinitro-1,3,5-triazine. Appl. Environ. Mi- crobiol. 1981, 42, 817–823. (10) Zhao, J.-Z.; Paquet, L.; Halasz, A.; Hawari, J. Metabolism of hexahydro-1,3,5-trinitro-1,3,5-triazine through initial reduction to hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine followed by denitrationinClostridiumbifermentansHAW-1.Appl.Microbiol. Biotechnol. 2003, 63, 187–193. (11) Kemper, J. M.; Ammar, E.; Mitch, W. A. Abiotic degradation of hexahydro-1,3,5-trinitro-1,3,5-triazine in the presence of hy- drogen sulfide and black carbon. Environ. Sci. Technol. 2008, ASAP Article. (12) Comfort, S. D.; Shea, P. J.; Machacek, T. A.; Satapanajaru, T. Pilot-scale treatment of RDX-contaminated soil with zerovalent iron. J. Environ. Qual. 2003, 32, 1717–1725. (13) Wanaratna, P.; Christodoulatos, C.; Sidhoum, M. Kinetics of RDX degradation by zero-valent iron (ZVI). J. Hazard. Mater. 2006, 136, 68–74. (14) Oh, B. T.; Just, C. L.; Alvarez, P. J. J. Hexahydro-1,3,5-trinitro- 1,3,5-triazine mineralization by zerovalent iron and mixed anaerobic cultures. Environ. Sci. Technol. 2001, 35, 4341–4346. (15) Liu, Y. Q.; Majetich, S. A.; Tilton, R. D.; Sholl, D. S.; Lowry, G. V. TCE dechlorination rates, pathways, and efficiency of nanoscale iron particles with different properties. Environ. Sci. Technol. 2005, 39, 1338–1345. (16) Doong, R. A.; Chen, K. T.; Tsai, H. C. Reductive dechlorination of carbon tetrachloride and tetrachloroethylene by zerovalent silicon-iron reductants. Environ. Sci. Technol. 2003, 37, 2575– 2581. (17) Leupin, O. X.; Hug, S. J. Oxidation and removal of arsenic (III) from aerated groundwater by filtration through sand and zero- valent iron. Water Res. 2005, 39, 1729–1740. (18) Ghauch, A.; Rima, J.; Amine, C.; Martin-Bouyer, M. Rapid treatment of water contaminated with atrazine and parathion with zero-valent iron. Chemosphere 1999, 39, 1309–1315. (19) Pereira, W. S.; Freire, R. S. Azo dye degradation by recycled waste zero-valent iron powder. J. Braz. Chem. Soc. 2006, 17, 832–838. (20) Li, Z. H.; Jones, H. R.; Bowman, R. S.; Helferich, R. Enhanced reduction of chromate and PCE by palletized surfactant- modified zeolite/zerovalent iron. Environ. Sci. Technol. 1999, 33, 4326–4330. (21) Schrick, B.; Hydutsky, B. W.; Blough, J. L.; Mallouk, T. E. Delivery vehicles for zerovalent metal nanoparticles in soil and ground- water. Chem. Mater. 2004, 16, 2187–2193. (22) He, F.; Zhao, D. Y.; Liu, J. C.; Roberts, C. B. Stabilization of Fe-Pd nanoparticles with sodium carboxymethyl cellulose for en- hanced transport and dechlorination of trichloroethylene in soil and groundwater. Ind. Eng. Chem. Res. 2007, 46, 29–34. (23) Sheremata, T. W.; Hawari, J. Mineralization of RDX by the white rot fungus Phanerochaete chrysosporium to carbon dioxide and nitrous oxide. Environ. Sci. Technol. 2000, 34, 3384–3388. (24) Hawari, J.; Halasz, A.; Groom, C.; Deschamps, S.; Paquet, L.; Beaulieu, C.; Corriveau, A. Photodegradation of RDX in aqueous solution: A mechanistic probe for biodegradation with Rhodo- coccus sp. Environ. Sci. Technol. 2002, 36, 5117–5123. (25) Bhushan, B.; Paquet, L.; Halasz, A.; Spain, J. C.; Hawari, J. Mechanism of xanthine oxidase catalyzed biotransformation of HMX under anaerobic conditions. Biochem. Biophys. Res. Commun. 2003, 306, 509–515. (26) Monteil-Rivera, F.; Paquet, L.; Halasz, A.; Montgomery, M. T.; Hawari, J. Reduction of octahydro-1,3,5,7-tetranitro-1,3,5,7- tetrazocine by zerovalent iron: Product distribution. Environ. Sci. Technol. 2005, 39, 9725–9731. (27) Sun, Y. P.; Li, X. Q.; Cao, J. S.; Zhang, W. X.; Wang, H. P. Characterization of zero-valent iron nanoparticles. Adv. Colloid Interface Sci. 2006, 120, 47–56. (28) Cao, J. S.; Clasen, P.; Zhang, W. X. Nanoporous zero-valent iron. J. Mater. Res. 2005, 20, 3238–3243. (29) Hundal, L. S.; Singh, J.; Bier, E. L.; Shea, P. J.; Comfort, S. D.; Powers, W. L. Removal of TNT and RDX from water and soil using iron metal. Environ. Pollut. 1997, 97, 55–64. (30) Li, X. Q.; Elliott, D. W.; Zhang, W. X. Zero-valent iron nano- particles for abatement of environmental pollutants: Materials and engineering aspects. Crit. Rev. Solid State Mat. Sci. 2006, 31, 111–122. (31) Zhang, W. X. In situ remediation with nanoscale iron particles. Abstr. Pap. Am. Chem. Soc. 2003, 225, U961–U962. (32) Zhang, W. X. Nanoscale iron particles for environmental remediation: An overview. J. Nanopart. Res. 2003, 5, 323–332. (33) Keenan, C. R.; Sedlak, D. L. Factors affecting the yield of oxidants from the reaction of nanoparticulate zero-valent iron and oxygen. Environ. Sci. Technol. 2008, 42, 1262–1267. (34) Liu, Y. Q.; Choi, H.; Dionysiou, D.; Lowry, G. V. Trichloroethene hydrodechlorination in water by highly disordered monome- tallic nanoiron. Chem. Mater. 2005, 17, 5315–5322. (35) Ponder, S. M.; Darab, J. G.; Bucher, J.; Caulder, D.; Craig, I.; Davis, L.; Edelstein, N.; Lukens, W.; Nitsche, H.; Rao, L. F.; Shuh, D. K.; Mallouk, T. E. Surface chemistry and electrochemistry of supported zerovalent iron nanoparticles in the remediation of aqueous metal contaminants. Chem. Mater. 2001, 13, 479–486. (36) Halasz, A.; Spain, J.; Paquet, L.; Beaulieu, C.; Hawari, J. Insights into the formation and degradation mechanisms of methyl- enedinitramine during the incubation of RDX with anaerobic sludge. Environ. Sci. Technol. 2002, 36, 633–638. (37) Huang, Y. H.; Zhang, T. C. Nitrite reduction and formation of corrosion coatings in zerovalent iron systems. Chemosphere 2006, 64, 937–943. (38) Garber, E. A. E.; Hollocher, T. C. Positional isotopic equivalence of nitrogen in N2O produced by the denitrifying bacterium Pseudomonas-Stutzeri. Indirect evidence for a nitroxyl pathway. J. Biol. Chem. 1982, 257, 4705–4708. (39) Bonner, T. G.; Hancock, R. A.; Roberts, J. C. The N-nitroxymethyl derivativesof1,3-dinitroperhydro-1,3,5-triazine,piperidine,and succinimide. J. Chem. Soc., Perkin Trans. 1. 1972, 1902–1907. ES7028153 4370 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008

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