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TECHNICAL REPORTS

Organic Compounds in the Environment

Remediating Munitions-Contaminated Soil with Zerovalent Iron and Cationic Surfactants

School of Natural Resources, University of Nebraska–Lincoln, 256 Keim Hall, Lincoln, NE 68583-0915

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

Received for publication May 8, 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soils contaminated from military operations often contain mixtures of HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), and TNT (2,4,6-trinitrotoluene) rather than a single explosive. Differences among explosives in solubility and reactivity make developing a single remediation treatment difficult. When Fe⁰ was used to treat a munitions-contaminated soil, we observed high rates of destruction for RDX and TNT (98%) but not HMX. Our objective was to determine if HMX destruction by Fe⁰ could be enhanced by increasing HMX solubility by physical (temperature) or chemical (surfactants) means. To determine electron acceptor preference, we treated RDX and HMX with Fe⁰ in homogeneous solutions and binary mixtures. Increasing aqueous temperature (20 to 55°C) increased HMX solubility (2 to 22 mg L^–1) but did not increase destruction by Fe⁰ in a contaminated soil slurry that also contained RDX and TNT. Batch experiments using equal molar concentrations of RDX and HMX demonstrated that RDX was preferentially reduced over HMX by Fe⁰. By testing various surfactants, we found that the cationic surfactants (HDTMA [hexadecyltrimethylammonium bromide], didecyl, and didodecyl) were most effective in increasing HMX concentration in solution. Didecyl and HDTMA were also found to be highly effective in facilitating the transformation of HMX by Fe⁰. Using HDTMA or didecyl solutions (3%, w/v) containing solid-phase HMX, we observed that 100% of the added HMX was transformed by Fe⁰ in the didecyl matrix and 60% in the HDTMA matrix. These results indicate that cationic surfactants can increase HMX solubility and facilitate Fe⁰–mediated transformation kinetics but HMX destruction rates will be slowed when RDX is present.

Abbreviations: CMC, critical micelle concentration • HDTMA, hexadecyltrimethylammonium bromide • HE, high explosives • HMX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine • HPLC, high performance liquid chromatography • LANL, Los Alamos National Laboratory • LUMO, lowest unoccupied molecular orbital • RDX, hexahydro-1,3,5-trinitro-1,3,5-triazine • TNT, 2,4,6-trinitrotoluene

	INTRODUCTION

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PAST SITE ASSESSMENTS of munition production facilities have determined that soils contaminated from military operations often contain mixtures of RDX, HMX, and TNT rather than a single explosive. Two reasons for this heterogeneity include the manufacturing practice of blending two or more explosives for specific compositions (e.g., octol contains approximately 75% TNT and 25% RDX; Akhavan, 1998) and synthesis impurities. The two grades of HMX used for military purposes contain between 2 and 7% (w/w) RDX. Likewise, the Bachmann synthesis of RDX (Bachmann and Sheehan, 1949) commonly results in HMX impurities of 8 to 12% (Fedoroff and Sheffield, 1966). Consequently, a remediation treatment designed and optimized for a single high explosive may have little practical value unless it is robust enough to remediate multiple energetics.

Although HMX and RDX are similar in that both consist of multiples of the CH₂ = N–NO₂ monomeric unit, these polynitramines differ with HMX being less water soluble (approximately 5 mg L^–1) than RDX (40 mg L^–1) and chemically more stable and resistant to attack by strong base (Akhavan, 1998). Recent biodegradation studies have also confirmed that HMX is more resistant to microbial attack than RDX (Shen et al., 2000).

Past research has demonstrated that zerovalent iron (Fe⁰) can abiotically degrade RDX and TNT in soil and water under laboratory conditions (Hundal et al., 1997; Singh et al., 1998, 1999). In an attempt to scale-up this treatment technology, we conducted pilot-scale experiments (70 kg soil) with Fe⁰ and treated contaminated soil from an outwash pond that had previously been used for munitions wastewater disposal (Los Alamos National Laboratory, NM). Zerovalent iron effectively removed 98% of the RDX and TNT within 120 d (Comfort et al., 2003). Because HMX is considered less toxic than RDX (McLellan et al., 1988a, 1988b), it was not initially considered a contaminant of concern. Further soil analysis, however, revealed that HMX was present at very high concentrations (>40000 mg kg^–1) and that this energetic compound was not effectively destroyed by the Fe⁰ treatment.

Our objective was to determine if HMX destruction by Fe⁰ could be enhanced by increasing HMX solubility by physical (temperature) or chemical (surfactants) means. We accomplished this by increasing temperature or adding surfactants to increase HMX solubility. To determine how mixtures of explosives would affect destruction rates, we compared Fe⁰–mediated destruction kinetics of RDX and HMX in homogeneous solutions versus binary mixtures.

	MATERIALS AND METHODS

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Chemical Reagents and Soils
Technical-grade RDX and TNT were obtained from the U.S. Biomedical Research and Development Laboratory (Frederick, MD). HMX was obtained from Sandia National Laboratories (Albuquerque, NM). Analytical standards of all explosives were obtained from the Indian Head Division, Naval Warfare Center (Indian Head, MD); additional analytical RDX was obtained from AccuStandard (New Haven, CT). Surfactants (Table 1) were obtained from Aldrich (Milwaukee, WI). Two forms of Fe⁰ (Peerless Metal Powders, Detroit, MI) were used in batch experiments: annealed (heat treated under N₂ and H₂ atmosphere) and unannealed iron. Specific surface areas were 0.134 m² g^–1 (annealed Fe⁰) and 2.55 m² g^–1 (unannealed Fe⁰) (Micromeritics, Norcross, GA).

Temperature Effects on High Explosive Solubility and Destruction by Zerovalent Iron
TNT, RDX, and HMX aqueous solubilities were determined at varying temperatures. This was accomplished by shaking replicates (n = 3 or 4) of a saturated high explosives (HE)–H₂O solution (distilled, deionized water containing solid-phase HE) at temperatures ranging from 20 to 55°C. To describe temperature effects on HE solubility, polynomial or power function equations were fit to temperature versus TNT and RDX concentration data.

The effect of temperature on Fe⁰–mediated destruction of HMX, RDX, and TNT was determined by conducting batch experiments with LANL soil in 50-mL Teflon tubes. Experimental units consisted of 3 g of LANL soil, 0.15 g unannealed iron, and 10 mL H₂O. The soil slurries were placed on a reciprocating shaker and agitated at 25 or 55°C. Controls (no Fe⁰) were run at each temperature and all treatments were replicated (n = 4). Temperature treatments of 25 and 55°C were imposed by placing experimental units inside an insulated case containing coils of circulating water connected to a water bath. This insulated case was placed on a reciprocating shaker during Fe⁰ treatment. After 24 h of agitation, 15 mL of acetonitrile was added to the soil slurry to extract the HE. Experimental units were sonicated at 30°C for 18 h and centrifuged (5000 x g), and 1.5 mL of the supernatant was removed. The supernatant was then centrifuged again (13000 x g) and analyzed for HMX, RDX, and TNT by high performance liquid chromatography (HPLC).

RDX and HMX Destruction by Zerovalent Iron: Homogenous versus Binary Mixtures
Batch experiments determined rates of HMX and RDX degradation in the presence of annealed and unannealed Fe⁰. Near equal molar concentrations (0.005–0.0075 mM; 1.13–2.07 mg L^–1) of HMX and RDX were prepared in distilled deionized water and treated together or alone in 100-mL aliquots with 5 g of Fe⁰. Experimental units consisted of 250-mL Erlenmeyer flasks covered with Parafilm and agitated on a reciprocating shaker. Solutions were sampled (1.5 mL) at 0, 1, 2, 3, 4, 6, 8, and 24 h, centrifuged at 13000 x g for 10 min, and analyzed by HPLC.

Temporal changes in HMX and RDX concentrations followed zero- or first-order kinetics and the appropriate equations were fit to the data by linear (zero-order) and nonlinear (first-order) regression using SigmaPlot 2000 computer software (SPSS, 2000).

In an attempt to explain differences in chemical reactivities toward Fe⁰, lowest unoccupied molecular orbital (LUMO) energies of RDX and HMX were calculated with a Hartree Fock approximation using the 6-31G* basis set. The geometry of each compound was optimized and calculations were performed with the Windows-based SPARTAN (Version 02) computer program (Wavefunction, 2002). It should be noted that attempts to obtain LUMO energies with semi-empirical models (i.e., AM1) did not yield similar relative differences between HMX and RDX.

Surfactant Effects on High Explosive Solubility and Destruction by Zerovalent Iron
Increasing HMX solubility was determined by adding solid-phase HMX in excess of its aqueous solubility (i.e., 0.1 g HMX added to 100 mL deionized, distilled H₂O) and spiking in the surfactants (HDTMA, Tween 80, and Triton X-100; Table 1) and the solvent DMSO (dimethyl sulfoxide) as individual treatments. Concentrations of surfactants and DMSO ranged between 1 and 5% (v/v or w/v). HDTMA was later compared with didecyl and didodecyl in a separate experiment. The effect of individual surfactant concentrations was determined by first adding 1% of surfactant to the reaction flasks (n = 3). Flasks were then agitated at 25°C for 24 h and a 1.5-mL sample removed, centrifuged (12000 x g), and analyzed for HE by HPLC. Higher surfactant concentrations were obtained by spiking in an additional 1% surfactant each day for the next 4 d and repeating the analysis steps.

To determine the effects of the surfactants on TNT, RDX, and HMX concentrations in a contaminated soil, soil slurries were prepared by mixing 3 g of LANL soil with 25 mL H₂O and spiking in 5% HDTMA, Tween 80, or Triton X-100. Replicates (n = 3) from each treatment were agitated at 25°C for 24 h before determining HE concentrations.

Because HDTMA and didecyl were the most successful at increasing HMX concentration, we repeated the equal molar concentration experiment using 3% didecyl or HDTMA and treated HMX and RDX as homogenous and binary mixtures with 5% unannealed Fe⁰. To determine if Fe⁰ could continuously transform HMX as the solid phase dissolved in a cationic surfactant matrix, we added 75 mg HMX to 25 mL of H₂O and spiked in HDTMA (5%). This experiment was later repeated using 3% (v/v) didecyl. Reaction flasks were agitated at 25°C for 48 h to allow partial dissolution of the solid-phase HMX and equilibrium HMX concentrations were determined. Five grams of Fe⁰ was then added to half of the experimental units to produce the following paired treatments: control (H₂O only), H₂O + Fe⁰; HDTMA (5%), HDTMA + Fe⁰; and didecyl (3%), didecyl + Fe⁰. After 6 d of additional shaking at 25°C (following Fe⁰ additions), all flasks were extracted with 100 mL CH₃CN to determine the total mass of HMX remaining.

Long-term changes in the surface morphology of unannealed iron following treatment of HMX, with and without didecyl, were determined by mixing 1 g unannealed Fe⁰ with 25 mL H₂O (with and without 2% didecyl) and solid-phase HMX (1.125 g). Batch flasks were agitated on a reciprocating shaker for 7 mo and then filtered, and iron samples were dried in a desiccator. Photographs were taken by mounting samples of the iron treatments with carbon tabs, sputter-coating with gold-palladium, and observing with a Hitachi (Tokyo, Japan) S-3000N scanning electron microscope (SEM) operated at 15 kV.

Chemical Analyses
HMX, RDX, and TNT were extracted from soil (3 g) with 15 mL CH₃CN by sonicating for 18 h at 30°C, centrifuging at 5000 x g, removing the supernatant, and microcentrifuging (12000 x g) before analyzing by HPLC.

Aqueous samples and acetonitrile extracts (10–25 µL) were injected into a Keystone NA column (Keystone Scientific, Bellefonte, PA) with an isocratic (30:70) mixture of methanol and H₂O at a flow rate of 1.0 mL min^–1 and quantified spectrophotometrically at 220 nm. Linear response ranges were established to bracket concentrations observed during experiments (0.25–10 mg L^–1 without surfactants; 1–500 mg L^–1 with surfactants). Limits of quantitative detection for RDX and HMX were 0.1 mg L^–1.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Effect of Temperature on High Explosive Solubility
We conducted a temperature versus HMX aqueous solubility experiment and compared results to our previous findings with RDX and TNT (Bier et al., 1999; Li et al., 1997). HMX solubility increased linearly from approximately 2 mg L^–1 at 20°C to 8 mg L^–1 at 45°C and then rose abruptly to 22 mg L^–1 at 55°C (Fig. 1). These results differed significantly from those observed with RDX and TNT and clearly indicate that HMX is the least soluble of the three major HE across a range of temperatures. Lynch et al. (2001) reported similar differences in HE solubilities in response to increasing temperature.

View larger version (21K): [in this window] [in a new window]   Fig. 1. Effect of temperature on the aqueous concentration of HMX, RDX, and TNT.

Because modest increases in HMX concentration were observed with increased temperature, we conducted an experiment in which the LANL soil was treated with Fe⁰ at two temperatures (25 and 55°C). HMX destruction by Fe⁰ did not occur at either temperature after agitating for 24 h (Table 2). By contrast, Fe⁰ effectively reduced RDX and TNT and was more effective at the higher temperature (Table 2). While the relative increase in destruction at the higher temperature was the same for RDX and TNT (both increased by 7%), this increase was only significant ( = 0.05) for TNT. Because TNT was the most soluble at the higher temperatures (Fig. 1), total destruction by Fe⁰ was greatest for TNT (98%).

View larger version (28K): [in this window] [in a new window]   Fig. 2. Destruction of HMX and RDX by zerovalent iron (annealed and unannealed) when present alone or in combination. Error bars on symbols represent sample standard deviations of means (n = 4); where absent, bars fall within symbols.

Differences in chemical reactivity toward Fe⁰ are linked, at least in principle, to differences in the susceptibility of a molecule to accept electrons. The ease with which oxidized compounds can be reduced varies considerably and attempts have been made to correlate this process with a number of chemical descriptors such as bond dissociation energies, electronegativities of the leaving groups, stabilities of the carbon radicals, one-electron reduction potentials (Jafvert and Wolfe, 1987; Schwarzenbach and Gschwend, 1990; Larson and Weber, 1994), and LUMO energies. While many researchers have developed successful linear free energy relationships (LFER) using one-electron reduction potentials, Scherer et al. (1998) developed a LFER between the LUMO energies of various chlorinated aliphatics and destruction rate constants observed with Fe⁰ treatment. Of the quantum-chemical descriptors, LUMO energies are perhaps the most justified because they represent the frontier molecular orbital into which electron transfer takes place. Compounds with lower LUMO energies would be favored over compounds with higher LUMO energies in electron transfer reactions. Scherer et al. (1998) showed that for a series of chlorinated aliphatics, the lower the LUMO energy the greater the destruction kinetics observed with zerovalent iron. Nam and Tratnyek (2000) similarly reported a correlation between LUMO energies of a series of azo dyes and reaction rates for reduction by Fe⁰. Our calculated LUMO energies for RDX (2.49 eV) and HMX (2.76 eV) indicate that RDX would be favored over HMX in reduction by Fe⁰ but without a larger data set (series of nitramines and rate constants) to develop a correlation LFER, it is unclear if this relative difference is enough to explain our observed results. Moreover, differences in HMX destruction kinetics (both rate and order) between iron sources complicate LFER development for predictive purposes.

Although our abiotic approach to treating HMX and RDX demonstrates that HMX is more difficult to destroy, several researchers have come to this same conclusion using microbial treatments (McCormick et al., 1981; Kitts et al., 1994; Shen et al., 2000). One reason given for HMX recalcitrance is that HMX has more steric constraints caused by the crowding of atoms in the –CH₂–N–NO₂ reacting group (Croce and Okamoto, 1979; Hawari, 2000). Consequently, differences between RDX and HMX reactivity toward Fe⁰ may perhaps also be a function of accessibility or differences in orientation of the molecules on the iron oxide surface.

Based on results from the equal molar and temperature experiments, it became apparent that for Fe⁰ to effectively destroy HMX, higher concentrations than what was achieved by increased temperatures may be needed. To accomplish this, we surveyed a variety of surfactants to determine their effects on HMX solubility.

Effect of Surfactants on High Explosive Solubility
A factorial test of three surfactants (HDTMA, Tween 80, and Trition X-100) and DMSO at five concentrations demonstrated that HDTMA was far superior in increasing HMX solubility (Fig. 3A). Using concentrations well above the critical micelle concentration (CMC) for these surfactants (Table 1), HMX concentrations were not significantly increased until surfactants were added in percentage concentrations. At the 5% concentration, HDTMA increased HMX solution concentration to >250 mg L^–1 in a pure HMX-H₂O suspension. When we tested the surfactants (5% HDTMA, Tween 80, or Triton X-100) on the LANL soil, we found that HDTMA increased HMX concentration to 230 mg L^–1 in a contaminated soil slurry. In addition to increasing HMX concentration, RDX concentration also increased dramatically in the HDTMA matrix (Fig. 3B). The nonionic surfactants (Tween 80 and Triton X-100) were not as effective in increasing HMX concentration but did increase RDX concentrations in the soil slurry. TNT was also slightly increased by the nonionic surfactants (Fig. 3B) but considering that TNT never reached its solubility limit in the control (H₂O only), it is likely that solid-phase TNT was not present in the LANL soil and therefore our observations do not reflect the potential of the nonionic surfactants to increase TNT concentrations.

View larger version (37K): [in this window] [in a new window]   Fig. 3. (A) Surfactant concentration effects on HMX solubility in an aqueous matrix. (B) Surfactant effect on HMX, RDX, and TNT concentration in a soil slurry. Error bars represent sample standard deviations of means.

View larger version (28K): [in this window] [in a new window]   Fig. 4. Destruction of HMX and RDX alone or in combination by unannealed Fe0 in a 3% (w/v) didecyl or HDTMA matrix. Error bars represent sample standard deviations of means.

What makes remediating the LANL soil such a formidable task is the sheer magnitude of HMX present. As the Fe⁰ transforms the HMX dissolved in the soil solution, solid-phase HMX will continue to replenish the soil solution with HMX. Given that both HDTMA and didecyl effectively increased HMX concentration and Fe⁰ effectively transformed HMX in the presence of the surfactants, we combined Fe⁰ with HDTMA or didecyl to treat an aqueous solution containing solid-phase HMX. Results from this experiment showed that only about 3% of the added HMX was destroyed by Fe⁰ when used alone whereas approximately 60% of the HMX was transformed within 6 d when HDTMA was added with the Fe⁰ (Table 4). The Fe⁰ + didecyl treatment, however, removed all of the solid-phase HMX present in the batch reactor (Table 4).

From an Fe⁰–treatment perspective, there are basically two ways in which the surfactant could increase destruction kinetics. The surfactant could increase solubilization of the contaminant or interact with the iron-oxide surface and act as a surface film for partitioning of the contaminant. While increased solubilization is often the desired effect, the surfactant will have little value if it inhibits contact with the iron surface. Bizzigotti et al. (1997) found that Fe⁰–mediated destruction of PCE decreased as the concentration of cyclodextrin (hydroxypropyl-ß-cyclodextrin) increased. By contrast, Li (1998) observed that precoating Fe⁰ with HDTMA increased PCE destruction kinetics threefold over the untreated iron. While we demonstrated that the cationic surfactants were effective in increasing HMX and RDX solubilities, we recognize that the cationic surfactants will be adsorbed to polar-oxide surfaces largely through electrostatic cation exchange reactions. Although iron oxides do not possess high cation exchange capacity (CEC) (McBride, 1994), the oxides formed during Fe⁰ corrosion have been shown to exhibit an adsorption edge for cations as the pH approaches or exceeds the zero point of charge (ZPC). Zero points of charge can range between 6.5 to 8.5 depending on oxides formed [e.g., -Fe₂O₃, ZPC = 6.7; -FeOOH, 7.8; and amorphous Fe(OH)₃, 8.5; Stumm and Morgan, 1981]. Considering the pH of our Fe⁰–surfactant systems varied with surfactant (pH 7.2–7.8 for didecyl; pH 9.1–9.3 for HDTMA; Fig. 4), differences in adsorption of cationic surfactants to the iron-oxide surface may have occurred. The relative degree of surface coverage by the surfactant (monomers, hemimicelles, admicelles) will be influenced by CEC because low charge density minerals have been speculated to form only monolayer complexes with HDTMA (Deng and Dixon, 2002). This was confirmed by Li (1998), who observed that maximum adsorption of HDTMA on Fe⁰ was only 4 mmol kg^–1. When this concentration was normalized to iron surface area and the molecular size of the HDTMA hydrophilic region (head), surface coverage was estimated at only 25%, indicating that HDTMA was probably positioned parallel to the surface, forming an incomplete monolayer or patches of hemimicelles.

There has been some question as to whether electron transfer from the iron core to the organic contaminant could occur through a hydrophobic surface film. This process and its impact on contaminant reduction can be expected to vary with film composition and thickness in addition to contaminant structure, properties, and the mechanism(s) of reduction. While Tratnyek et al. (2001) observed that natural organic matter inhibited CCl₄ and TCE reduction by Fe⁰, organic matter had little effect on nitrobenzene reduction. Li and Farrell (2000) reported an increase in direct TCE reduction after coating iron electrodes with a hydrophobic polymer, which they attributed to inhibition of water reduction. The observations of Li (1998) and others (Alessi and Li, 2001; Tratnyek et al., 2001) as well as our own measurements with HMX indicate that reductive transformations can occur when cationic surfactants are combined with Fe⁰.

One of the most important factors influencing electron transfer from Fe⁰ is the nature and extent of oxides formed during corrosion. It is perhaps noteworthy that the long-chain (C₁₂–C₁₈) amine surfactants have been routinely used in industry as corrosion inhibitors to protect metal surfaces from water, salts, and acids (Rosen, 1989). Saturated, long-chain amines give close-packed hydrophobic surface films and have been added to fuels and lubricating oils to prevent corrosion of metal containers (Rosen, 1989). Consequently, even if electron transfer is impeded by the surfactant film (as opposed to the bare Fe⁰), this loss in electron transfer could be offset by less iron passivation and a cleaner iron surface, which could then result in greater overall contaminant destruction. While we do not know the chemical makeup of the type of iron oxides formed in the presence of the surfactants, scanning electron microscope (SEM) photos of the iron surface taken before and after treating HMX with and without 2% didecyl revealed striking differences in the physical nature of the surface. The didecyl treatment resulted in a much smoother iron surface (Fig. 5), which may represent less oxide growth or a surface coated with didecyl. In either case, the didecyl-treated iron was drastically different from the iron without didecyl, which was much more irregular and rough with some noticeable cracks in the oxide observed (Fig. 5). These photos illustrate that cationic surfactants influence the physical characteristics of the oxides formed during contaminant treatment and may help explain the increased effectiveness of the Fe⁰ + cation surfactant treatment in removing solid-phase HMX from an aqueous solution.

View larger version (74K): [in this window] [in a new window]   Fig. 5. (A) Scanning electron microscope (SEM) photo of unannealed iron. (B) Surface of unannealed iron after treating an aqueous solution containing solid-phase HMX. (C) Surface of unannealed iron following treatment of an aqueous–solid-phase HMX mixture with 2% (v/v) didecyl.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

	ACKNOWLEDGMENTS

 
We thank Sandia National Laboratory, the Nebraska Research Initiative, University of Nebraska Water Center, Nebraska Center for Environmental Toxicology, and the School of Natural Resources for financial support. Appreciation is also expressed to Dr. Gordon Gallup and Dr. Mohammad Qasim for assistance with LUMO energy calculations. A contribution of Agric. Res. Div. Projects NEB-40-002 and NEB-40-019, Journal Ser. no. 14206.

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