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TECHNICAL REPORTS

Organic Compounds in the Environment

Remediating RDX-Contaminated Ground Water with Permanganate

Laboratory Investigations for the Pantex Perched Aquifer

^a Department of Civil Engineering, University of Nebraska, Lincoln, NE 68588-0531
^b School of Natural Resources, University of Nebraska, Lincoln, NE 68583-0915
^c Water Science Laboratory, University of Nebraska, Lincoln, NE 68583-0844

^* Corresponding author (scomfort{at}unl.edu)

Received for publication March 16, 2004.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Ground water beneath the U.S. Department of Energy Pantex Plant is contaminated with the high explosive RDX (hexahydro-1,3,5-trinitro-1,3,5 triazine). The USDOE Innovative Treatment and Remediation Demonstration (ITRD) program identified in situ oxidation by permanganate as a technology fit for further investigation. We evaluated the efficacy of KMnO₄ to transform and mineralize RDX by determining degradation kinetics and carbon mass balances using ¹⁴C-RDX. Aqueous RDX solutions (2–5 mg L^–1) and RDX-contaminated slurries (50% solids, w/v) were treated with KMnO₄ at 1000, 2000, 4000, and 20000 mg L^–1. Treating an aqueous RDX solution of 2.8 mg L^–1 with 20000 mg KMnO₄ L^–1 decreased RDX to 0.1 mg L^–1 within 11 d while cumulative mineralization proceeded for 14 d until 87% of the labeled carbon was trapped as ¹⁴CO₂. Similar cumulative mineralization was obtained when Pantex aquifer material was included in the solution matrix. Other experiments using 4000 mg KMnO₄ L^–1 showed that initial RDX concentrations (1.3–10.4 mg L^–1) or initial pH (4–11) had little effect on reaction rates. Attempts to identify RDX degradates and reaction products showed that N₂O was a product of permanganate oxidation and constituted 20 to 30% of the N balance. Time-course measurements of a ¹⁴C-RDX solution treated with KMnO₄ revealed few ¹⁴C-labeled degradates but through liquid chromatography–mass spectrometry (LC–MS) analysis, we present evidence that 4-nitro-2,4-diaza-butanol is formed. Aquifer microcosm studies confirmed that the transformation products not mineralized by KMnO₄ were much more biodegradable than parent RDX. These results indicate permanganate can effectively transform and mineralize RDX in the presence of aquifer material and support its use as an in situ chemical oxidation treatment for the Pantex perched aquifer.

Abbreviations: HPLC, high performance liquid chromatography • LC–MS, liquid chromatography–mass spectrometry • RDX, hexahydro-1,3,5-trinitro-1,3,5 triazine • USDOE, United States Department of Energy

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
THE U.S. DEPARTMENT OF ENERGY's Pantex Plant near Amarillo, Texas, has produced or dismantled conventional and nuclear weapons for the last 50 yr. Pre-1980 industrial operations included on-site disposal of high explosives (HE) and wastewater into unlined ditches. Surface runoff from these ditches into an aquifer-recharging playa has contaminated the perched aquifer beneath the Pantex Plant. The perched aquifer is contaminated with RDX, HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), TNT (2,4,6-trinitrotoluene), 2,4-DNT (2,4-dinitrotoluene), 1,2-DCA (1,2-dichloroethane), TCE (trichloroethene), PCE (tetrachloroethene), and chromium. Of these, considerable attention has focused on the high explosive RDX because it is the most widespread. Hydrogeological characteristics of the site make implementing remediation technologies formidable. Foremost is that the perched aquifer is approximately 90 m (300 ft) below the surface and 30 m (100 ft) above the Ogallala (High Plains) aquifer, a major source of pristine drinking water. Second, the saturated thickness of the aquifer is less than 4.5 m (15 ft) in many locations, making pump and treat systems ineffective. Migration of the contaminated plume beyond the bounds of the Pantex site and into privately owned lands has further exacerbated the problem.

The USDOE Innovative Treatment and Remediation Demonstration (ITRD) program was initiated to evaluate emerging technologies that may potentially replace inefficient or ineffective technologies. The ITRD process for the Pantex Plant recommended three candidate technologies for further testing: (i) oxidation by KMnO₄, (ii) anaerobic biodegradation, and (iii) chemical reduction by dithionite-treated (reduced) aquifer material. This research discusses in situ chemical oxidation of RDX by permanganate.

In situ chemical oxidation involves the addition of a chemical oxidant to destroy contaminants in-place. Potassium permanganate (KMnO₄) is an oxidizing agent with a strong affinity for organic compounds containing carbon–carbon double bonds, aldehyde groups, or hydroxyl groups. Considerable research with chlorinated solvents has shown that permanganate is attracted to the negative charge associated with the electrons of chlorinated alkenes such as tetrachloroethene, trichloroethene, dichloroethene, and vinyl chloride (Oberle and Schroder, 2000). Although the chemical structure of RDX does not readily lend itself to reaction with permanganate, IT Corporation and SM Stoller Corporation (2000) initially demonstrated effective RDX loss by KMnO₄ treatment. By using ¹⁴C-labeled RDX and samples of the Pantex perched aquifer sand, our objectives were to measure RDX transformation and mineralization using varying KMnO₄ concentrations and determine the effects of initial pH and RDX concentration on reaction rates. Because mineralization was incomplete (<90%) when lower KMnO₄ concentrations were used, we also attempted to identify transformation products and determined the ability of native biota to mineralize degradates produced by permanganate in aquifer microcosms.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Carbon-14 RDX Mass Balance Experiment
A continuous flow-through gas trapping system was used to measure loss of parent RDX, ¹⁴C, and production of ¹⁴CO₂. Experimental units consisted of 250-mL glass round-bottom flasks connected in series to a flow-through vacuum system with two midget gas bubbling traps (Bier et al., 1999). Inlet air to the reaction flasks was passed through 5-cm glass tubes packed with NaOH-covered pellets (Ascarite II) and glass wool. This continuous flow-through system produced gentle bubbling inside the reaction flasks during treatment and provided a slow but constant agitation. Trapping efficiency of the flow-through system using acid-treated NaH¹⁴CO₃ averaged 93.4% (STD_n–1 = 5.8%).

Initial tests treated replicated experimental units (n = 4) containing 150 mL of aqueous ¹⁴C-RDX (2.5 mg L^–1, 30000 dpm mL^–1, uniformly ring-labeled) with 20000 mg KMnO₄ L^–1. Because no ¹⁴C-labeled volatile organics were detected in initial trapping tests, subsequent experiments concentrated on measuring loss of parent RDX and ¹⁴C activity from solution and production of ¹⁴CO₂. Solution samples from the reaction flask and CO₂ traps were taken daily for the first 5 d and at 48-h intervals in the following weeks. Changes in solution RDX concentrations were determined by removing 1.2-mL aliquots, quenching the reaction with 120 µL of 0.5 mg MnSO₄·H₂O mL^–1, centrifuging at 12000 x g for 10 min, and transferring 1.0 mL of supernatant to a high performance liquid chromatography (HPLC) vial for analysis. RDX was quantified at 220 nm by HPLC using a Keystone NA column (Keystone Scientific, Bellefonte, PA) with an isocratic mixture of methanol and H₂O (30:70) at a flow rate of 1.5 mL min^–1 (Bier et al., 1999). Standards were prepared with analytical RDX obtained from AccuStandard (New Haven, CT). Changes in solution ¹⁴C activity were determined by removing 0.5 mL of sample, mixing with 18 mL of Ultima Gold scintillation cocktail (Packard, Meriden, CT), storing for 24 h, and then determining activity by liquid scintillation (LS) analysis. At the end of the experiment, we filtered the solution from the reaction flask through Whatman (Maidstone, UK) #1 filters, and allowed the precipitate to dry. We then mixed 0.5 g of precipitate with 400 mL of Combustaid (Packard), and determined precipitated ¹⁴C by combusting the precipitate in a Packard biological oxidizer. The oxidizer gas stream was trapped in a 3:2 (v/v) mixture of Carbosorb/Permafluor (Packard) and counted by LS analysis. RDX destruction kinetics were fit to a pseudo first-order rate equation by nonlinear regression analysis using the computer software SigmaPlot 2000 (SPSS, 2000).

Batch Experiments
Additional batch experiments were performed in 250-mL Erlenmeyer flasks and agitated with an orbital shaker. Initially, we duplicated the 20000 mg KMnO₄ L^–1 treatment used in the flow-through system (with and without aquifer solids), but subsequent treatments used lower KMnO₄ concentrations to treat RDX-contaminated slurries. For slurry experiments, we used fresh aquifer sediment obtained during drilling of the perched aquifer (approximately 90 m). Aquifer material was sent to our laboratory in ice-packed coolers and immediately transferred to a cold room (4.5°C) for storage. Soil analyses (Midwest Labs, Omaha, NE) determined that the aquifer material was approximately 91% (w/w) sand, 3% silt, and 6% clay with 0.1% organic material and had an alkaline pH (2:1 solution to soil ratio, pH = 9). Although mostly sand, our observations during handling of the Pantex material revealed that small pebble-sized occlusions of clay were also present.

Aquifer slurries consisted of 75 g aquifer material (dry weight) mixed with 150 mL of ¹⁴C-RDX (2.5 mg L^–1). Slurries were treated with 0, 1000, 2000, and 4000 mg KMnO₄ L^–1. In a second experiment, these same KMnO₄ treatments were repeated but after 21 and 50 d, additional KMnO₄ (dry solid) was added to the reaction flasks at the original concentrations to simulate repeated injections of KMnO₄. Sampling for temporal changes in RDX and ¹⁴C was conducted as described above.

To determine permanganate consumption by the Pantex perched aquifer, five concentrations of KMnO₄ (5–500 mg L^–1, 2:1 solution to soil ratio) were mixed with the Pantex sand and shaken on an orbital shaker for 48 h. Permanganate concentrations were measured with a UV/vis spectrophotometer (UV-2101PC; Shimadzu, Kyoto, Japan) at 525 nm.

Effects of Initial RDX Concentration and pH
To quantify the effects of initial RDX concentration and pH on destruction kinetics, we repeated the slurry experiments with 4000 mg KMnO₄ L^–1 and varied the initial RDX concentrations (1.3, 2.6, 4.3, 7.2, and 10.4 mg L^–1) and pH. The pH effects on RDX transformation and mineralization rates were determined by treating 5.0 mg RDX L^–1 with permanganate in the following buffers: 10 mM NaH₂PO₄ (initial pH = 4.1), 10 mM Na₂HPO₄ (pH = 8.3), 10 mM Na₃PO₄ (pH = 11.3), 5 mM NaH₂PO₄ + 5 mM Na₂HPO₄ (pH = 7.5), and an unbuffered control (pH = 9.6). Temporal changes in RDX, ¹⁴C, and pH were monitored.

RDX Transformation Products
To identify RDX transformation products, we used permanganate concentrations between 500 and 4000 mg L^–1 to treat ¹⁴C-labeled RDX solutions (150 mL) without aquifer material under batch conditions. Solutions were analyzed with an HPLC equipped with a photodiode array (Shimadzu) and radio-isotope detector (Packard). Several combinations of HPLC columns and mobile phases were employed in an attempt to separate and identify degradate peaks and included Keystone NA and Aquasil reverse phase (Keystone Scientific) and Supelcogel C-610H ion exclusion columns (Supelco). Typical mobile phases were varied and included 20:80 acetonitrile to water or 50:50 methanol to water. Nitrate was analyzed using cadmium reduction, following Lachat Method 12-107-01-1C (Lachat Instruments, Loveland, CO). Ammonium was analyzed colorimetrically with a Reflectoquant test kit (EM Science, Gibbstown, NJ).

Headspace above an aqueous RDX solution (150 mL, 12.5 mg L^{–1, 14}C-labeled) and slurry (150 mL, 75 g) was monitored for N₂O and ¹⁴CO₂ evolution following treatment with 4000 KMnO₄ L^–1. Each reactor contained a 7-mL scintillation vial filled with 6 mL of 0.5 M NaOH to trap evolved ¹⁴CO₂. Headspace, solution (RDX and ¹⁴C), and ¹⁴CO₂ traps were sampled weekly for four weeks. We obtained N₂O samples by removing 1000 µL from the headspace of the batch reactors with a gas tight syringe and transferring to a 12-mL N₂–flushed sample vial. Headspaces were subsequently exposed to the atmosphere for approximately 5 min while we sampled for solution RDX and exchanged ¹⁴CO₂ traps. Sample vials were transported to the University of Nebraska's Water Sciences Laboratory where 500 µL of the diluted gas was injected into a Hewlett-Packard (Palo Alto, CA) 5890 GC equipped with a Restek (Bellefonte, PA) 80/100 Porapak Q Column (2-m length x 2-mm diameter) and electron capture detector (ECD). Calibration standards were prepared by diluting N₂O standards (Scott Specialty Gases, Plumsteadville, PA) and used to quantify N₂O concentrations.

Two liquid chromatography–mass spectrometer (LC–MS) systems were used to identify RDX degradation products: an LCQ HPLC–MS system (Finnegan, Austin, TX), which employs an ion-trap mass analyzer, and a Quattro micro triple quadrupole LC–MS–MS (Waters, Milford, MA). Samples injected into the LCQ ion trap were separated with a BetaBasic C-18 column at 50°C using a mobile phase of 0.01 M NH₄OOCH (in H₂O) and isopropanol (80:20, pH = 8 with NH₄OH) at a flow rate of 0.2 mL min^–1 (Cassada et al., 1999). Data were collected in full scan mode (negative ion) from 50 to 600 amu. Samples injected into the triple quadrupole used the same column and flow rate, but the mobile phase consisted of 2% (v/v) formic acid–acetonitrile–methanol (60:24:16).

Biodegradation of Permanganate-Treated RDX
To determine the biodegradation potential of the RDX transformation products generated during KMnO₄ treatment, 12 flasks containing 75 g aquifer material and 150 mL of aqueous RDX (5 mg L^–1) were prepared. Eight of the flasks received 4000 mg KMnO₄ L^–1, while control flasks received none. We allowed KMnO₄ to react with RDX until RDX was undetected by HPLC (<50 µg L^–1). We then transferred 25 mL of solution from each of the flasks into 50-mL Teflon tubes, and quenched half (n = 4) with 400 µL MnSO₄ (0.5 mg MnSO₄·H₂O per mL of H₂O); all of the tubes were centrifuged at 5500 x g for 10 min. The supernatant containing ¹⁴C-RDX transformation products was then used as the stock solution (hereinafter referred to as ¹⁴C stock) for the aquifer microcosm experiments.

Aquifer microcosms were prepared in 250-mL glass jars with septa-containing screw-top lids. Each experimental unit contained 75 g (dry weight) of fresh aquifer sand, 15 mL of a ¹⁴C stock, and a ¹⁴CO₂ trap consisting of a 7-mL glass vial with 6 mL of 0.5 M NaOH. The 15 mL of added ¹⁴C stock solution saturated the Pantex sand and provide a thin film of overlying solution. Radioactivity (dpm) of the unquenched and control treatments was diluted with H₂O to provide the same initial radioactivity concentration as the quenched treatment. Carbon dioxide traps were sampled once per week to determine cumulative ¹⁴CO₂ produced.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Carbon-14 RDX Mass Balance Experiment
Treating aqueous RDX (pH 8–9) with 20000 mg KMnO₄ L^–1 reduced the RDX concentration from 2.8 to 0.1 mg L^–1 in 11 d (i.e., 96% loss; Fig. 1) . Loss of ¹⁴C activity paralleled transformation of RDX and mirrored the cumulative ¹⁴CO₂ recovered. Approximately 87% of the ¹⁴C activity was recovered as ¹⁴CO₂ after 18 d (Fig. 1). Combusting the precipitate (i.e., MnO₂) formed in the reaction flask during the experiment revealed insignificant activity (<0.1%) in the solid phase. The simultaneous decreases in solution RDX and ¹⁴C, combined with the cumulative production of ¹⁴CO₂, provided proof that KMnO₄ could effectively mineralize RDX.

View larger version (17K): [in this window] [in a new window]   Fig. 1. Loss of RDX and 14C and production of 14CO2 following treatment of RDX solution (without aquifer solids) with 20000 mg KMnO4 L–1 using a continuous flow-through trapping system. The term Co represents initial RDX or 14C in reaction flasks at t = 0 h. Bars on symbols represent standard deviations of means (n = 4); where absent, bars fall within symbols.

View larger version (28K): [in this window] [in a new window]   Fig. 2. Loss of RDX from an aquifer slurry treated with varying KMnO4 concentrations. Bars on symbols represent standard deviations of means (n = 4); where absent, bars fall within symbols.

View larger version (34K): [in this window] [in a new window]   Fig. 3. Loss of RDX and 14C from an aquifer slurry treated with repeated applications of varying KMnO4 concentrations. Bars on symbols represent standard deviations of means (n = 4); where absent, bars fall within symbols.

One potential problem with permanganate is the production of insoluble manganese dioxide. Li (2000) warned that MnO₂ can potentially cause plugging and flow diversion, and Lee et al. (2003) showed that low permeability zones can form along chlorinated solvent plumes, which decrease efficiency and lead to permanganate migrating away from the contaminant zone. Siegrist et al. (2002) showed that MnO₂ can commingle with clay and silt particles in water and increase the amount of filterable solids generated in situ. Our batch experiments with the Pantex aquifer solids support this, as we noticed that the slurry became progressively more viscous the longer the reaction took place, and sampling with a pipette became more tedious. This problem occurred even with the 1000 mg KMnO₄ L^–1 treatment but may have been exacerbated by the fact that the batch reactors were agitated during treatment, which suspended the clay in solution, rather than keeping the clay in lenses as in the undisturbed material. Although our laboratory batch experiments differ from the miscible displacement that would occur in the field, the potential for plugging within the perched aquifer still needs to be taken into consideration.

Effects of Initial RDX Concentration and pH
Initial RDX concentrations between 1.3 and 10.4 mg L^–1 did not greatly affect reaction rates when treated with 4000 mg KMnO₄ L^–1 (Fig. 4) . RDX transformation rates (i.e., k = 0.08–0.108 d^–1) and the extent of mineralization that occurred were similar among all RDX concentrations (Fig. 4). This indicates that slight differences in initial concentrations had negligible effects on destruction kinetics. The RDX concentrations observed in the Pantex perched aquifer (IT Corporation and SM Stoller Corporation, 2000) are similar to those used in our experiments. Monitoring solution ¹⁴C activity in the various RDX treatments revealed that roughly 40 to 50% of the activity was still present after 35 d (Fig. 4B). Because this was more than what we had observed in the previous experiment (10–30% ¹⁴C remaining after 70 d; Fig. 3), we acidified the slurry (pH < 3) at the end of the experiment and observed that an additional release of 35 to 40% of ¹⁴C activity (Fig. 4B). The retention of ¹⁴C activity by the permanganate-treated RDX solution (as dissolved CO₂ or HCO^–₃) is problematic and related to the alkaline conditions that occur during treatment (see initial pH results below). Consequently, acidification is needed for an accurate assessment of mineralization kinetics.

View larger version (38K): [in this window] [in a new window]   Fig. 4. (A) Effects of initial RDX concentration on RDX destruction and (B) loss of 14C following treatment of aquifer slurry with 4000 mg KMnO4 L–1. Effects of initial pH on RDX destruction kinetics in an aquifer slurry following treatment with 4000 mg KMnO4 L–1. Changes in (C) RDX concentration, (D) 14C activity, and (E) pH. Bars on symbols represent standard deviations of means (n = 4); where absent, bars fall within symbols.

The only pH treatment effect noted was the greater loss of solution ¹⁴C activity in the most acidic treatment (NaH₂PO₄) during the first few days (Fig. 4D). Because the NaH₂PO₄ treatment was significantly more acidic than the other treatments (pH 4–6), we again attributed this observation to the release of dissolved CO₂ at the lower pH.

RDX Transformation Products
Past research has shown that permanganate is effective in attacking compounds with carbon–carbon double bonds, aldehyde groups, or hydroxyl groups and is especially attracted to the negative charge associated with the electrons of chlorinated alkenes (e.g., tetrachloroethene, trichloroethene, dichloroethene, and vinyl chloride) (Oberle and Schroder, 2000). RDX has none of the above characteristics, and its complete oxidation (mineralization) can take several days to weeks (e.g., Fig. 2 and 3). By comparison, Yan and Schwartz (1999) observed half-lives ranged from less than a minute to 4 h for chlorinated ethylenes treated with permanganate (1 mM MnO^–₄). By comparison, when we treated 1,3,5-trinitrobenzene (TNB; 2 mg L^–1) with permanganate (4000 mg L^–1), it was undetectable within 30 min. The large difference in destruction kinetics between a nitroaromatic (TNB) and a nitramine (RDX) indicates that different destructive mechanisms by permanganate are likely operative.

Considering that most of the reaction between KMnO₄ and RDX occurred under alkaline conditions, one hypothesis is that permanganate is reacting with hydroxyl ions to form hydroxyl radicals (·OH) (Ladbury and Cullis, 1958; Wronska and Baranowska, 1964) and that the attack on RDX is indirect. Such an attack can occur at pH > 9 (Gates-Anderson et al., 2001). The limiting step would be the production of ·OH, which is responsible for the initial attack on the ring, and once that occurs, the permanganate concentration is high enough to quickly oxidize the RDX ring fragments. Although previous research supports ·OH formation under alkaline conditions, our pH experiments showed that pH had little influence on destruction kinetics (Fig. 4C). Moreover, earlier work by Bier et al. (1999) demonstrated that when ·OH attack on ¹⁴C-labeled RDX was the dominant mechanism (via the Fenton reaction), at least six radioisotope peaks were detected within 2 h during temporal sampling with HPLC-radioisotope detection. Using a similar analytical system, no major ¹⁴C-labeled degradates were detected during permanganate treatment of RDX (data not shown). The absence of detectable ¹⁴C-labeled peaks indicates that the initial attack of RDX by permanganate is likely the rate-limiting step and further degradation of the intermediates is relatively fast.

An alternative explanation is that permanganate is facilitating RDX hydrolysis, either directly or in concert with MnO₂ production. The slow reaction kinetics observed (e.g., several weeks; Fig. 2) are more indicative of a hydrolysis reaction and as previously reported (Hoffsommer et al., 1977), once proton abstraction from the ß carbon occurs and a nitro group is released (E2 reaction), the double bond formed on the triazine ring would favor attack by MnO^–₄. Although ·OH attack could also cause a double bond to form, Balakrishnan et al. (2003) studied alkaline hydrolysis of RDX and identified 4-nitro-2,4-diaza-butanal (4-NDAB) and N₂O as reaction products. We believe 4-NDAB is also a product of permanganate treatment of RDX. Liquid chromatographic analysis using both ion trap (Fig. 5) and triple quadrupole detection (data not shown) revealed the KMnO₄–treated RDX contained ion peaks at m/z 118 (M-H). Ion trap also revealed a m/z 164 peak (Fig. 5). 4-NDAB has a mass of 119 and based on the mobile phase used, we believe the m/z 164 detection peak is a negative formate adduct of 4-NDAB ([M + HCOO]^–). Adduct ion formation and detection is enhanced in electrospray using a buffer such as ammonium formate or formic acid (Cassada et al., 1999). Because 4-NDAB is not commercially available, we could not confirm it in our sample. However, Fournier et al. (2002) similarly observed a degradate peak of identical mass (m/z 118) during RDX biodegradation by Rhodococcus sp. Strain DN22 and matched their chromatograph to one produced by alkaline hydrolysis of RDX. Methylene dinitramine and other nitroso-RDX degradation products were not detected in the permanganate-treated RDX solutions analyzed by LC–MS.

View larger version (29K): [in this window] [in a new window]   Fig. 5. Ion chromatographs for permanganate-treated RDX and quenched with H2O2. (A–D) Total ion chromatograph and selected ions (m/z) using ion trap detection.

Measurement of nitrous oxide release showed that N₂O was a product of permanganate oxidation of RDX (solution and contaminated slurry) and constituted 20 to 30% of the N balance (Fig. 6) . Comparisons between the treated RDX solution and slurry showed that loss of parent RDX was similar in both medias but N₂O production was greater from the pure solution (Fig. 6). This indicates that other reactions (i.e., biotic) may be occurring when the aquifer solids are present and altering the distribution of reaction products.

View larger version (21K): [in this window] [in a new window]   Fig. 6. Loss of RDX and 14C and production of 14CO2 and N2O from permanganate-treated RDX. For N2O production, Co = maximum amount of N2O that could be obtained based on moles of RDX nitrogen added. Bars on symbols represent standard deviations of means (n = 4); where absent, bars fall within symbols.

View larger version (23K): [in this window] [in a new window]   Fig. 7. Cumulative 14CO2 produced (% of initial 14C) from aquifer microcosms incubated with KMnO4–treated RDX (quenched and unquenched) and parent RDX. All microcosms received the same initial radioisotope activity (i.e., dpm). Bars on symbols represent standard deviations of means (n = 4); where absent, bars fall within symbols.

	ACKNOWLEDGMENTS

 
This research was funded by a grant from Sandia National Laboratories. We appreciate the contributions of Dr. Patrick Shea and the technical assistance of Tom Machacek. Journal Series 14465, Agricultural Research Division, University of Nebraska–Lincoln.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 

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