Evaporation and Degradation of VX on Silica Sand - The Journal of

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J. Phys. Chem. C 2009, 113, 6622–6633

Evaporation and Degradation of VX on Silica Sand Carol A. S. Brevett,*,† Kenneth B. Sumpter,‡ John Pence,† Robert G. Nickol,† Bruce E. King,‡ Chris V. Giannaras,† and H. Dupont Durst‡ SAIC, P.O. Box 68, Gunpowder, Maryland 21010, and Research and Technology Directorate, U.S. Army Edgewood Chemical Biological Center (ECBC), Aberdeen ProVing Ground, Maryland 21010-5423 ReceiVed: December 16, 2008; ReVised Manuscript ReceiVed: February 3, 2009

The evaporation and degradation of VX (O-ethyl S-[2-N,N-(diisopropylamino)ethyl] methylphosphonothioate) on silica sand and borosilicate glass were measured using vapor analysis and 31P solid state magic angle spinning nuclear magnetic resonance (SSMAS NMR) spectroscopy. 31P SSMAS NMR studies of VX degradation on crushed glass, air-dried sand, and oven-dried sand at a variety of temperatures showed that the final product was nontoxic ethyl methylphosphonic acid (EMPA), produced via the toxic diethyl dimethylpyrophosphonate intermediate. The reaction exhibited a lag time that depended upon the quantity of water and EMPA that were present in the initial sample of VX and was autocatalytic; EMPA was both the catalyst and product for the degradation of VX. 31P SSMAS NMR studies of VX on moist sand at a variety of temperatures showed that the final product was nontoxic EMPA; observed intermediates were protonated VX, O-ethyl methylphosphonothioic acid, and toxic EA-2192 (S-[2-N,N-(diisopropylamino)ethyl methylphosphonothioic acid). The VX degradation mechanism differed on the moist and dry surfaces; the activation energies were 80 kJ/mol on moist sand and ∼46 kJ/mol on glass, air-dried, and oven-dried sand. The VX degradation rate was 5 to 9 times slower on moist sand than on air-dried sand. Analysis of the vapor emitted showed that the maximum vapor concentration coincided with the maximum surface area of the droplets, which was at ∼300 min. After four days, no further change in the concentration of vapor emitted was detected; approximately 9% of the incident VX drop on sand and ∼60% of the incident VX drop on glass had been recovered as VX vapor. 1. Introduction The possible fates of the highly toxic chemical warfare agent VX, O-ethyl S-[2-N,N-(diisopropylamino)ethyl] methylphosphonothioate (CH3P(O)(OEt)SCH2CH2N(isoPr)2, after deposition onto an exterior surface are evaporation, degradation, absorption followed by degradation, and absorption with no degradation. The longevity of the VX on various environmental surfaces will affect the decision to decontaminate the site or allow the VX to degrade to nontoxic byproduct over a period of time. Various studies have used NMR, a bulk technique to study the degradation of VX on sand, asphalt,1 and concrete.2,3 The NMR studies were performed in sealed rotors, with no ability for the VX to evaporate. By contrast, in an external environment, the VX does have the ability to evaporate, despite the fact that it is generally regarded as a nonvolatile agent. One common technique for gauging the amount of VX evaporation from a surface is by extracting the VX remaining; this method has the complications of VX solubility in the solvent, in the surface, and the possibility of not detecting the VX degradation products.4,5 Verweij and Boter6 used extractions followed by GC and anticholinesterase activity measurements to show that VX on clayey peat, humic loam, and humic sand degraded to EMPA and then MPA. The anticholinesterase activity, which was taken to represent the loss of VX, had a half-life of less than one day on the humic loam, ∼1 day on the clayey peat, and ∼3 days on the humic sand. Kaaijk and Frijlink7 used extractions followed by GC on the same systems to show that bis(diisopropylami† ‡

SAIC. U.S. Army Edgewood Chemical Biological Center.

noethane) disulfide was the only 32S-containing product and was tightly bound to the soil. Mizrahi et al.1 used 31P solid state magic angle spinning nuclear magnetic resonance spectroscopy (SSMAS NMR) to show that the half-life for VX degradation on desert sand was first-order, with a 4-day half-life, whereas the degradation on beach sand had a lag time of ten days, followed by a half-life of ∼2 days. EMPA was the sole product on the desert sand; EMPA and 3% O-ethyl methylphosphonothiolate salt were formed on the beach sand. Waysbort et al.8 studied the combined effects of VX evaporation from asphalt and extraction of the residual VX from the asphalt. At 40 °C approximately 30% of the VX evaporated, and an additional 38% of VX and the degradation products O-ethyl methylphosphonothioic acid (CH3P(O)(OC2H5)SH), toxic S-[2-N,N-(diisopropylamino)ethyl methylphosphonothioic acid (CH3P(O)(OH)SCH2CH2N(isoPr)2, EA-2192), and ethyl methylphosphonic acid, (EMPA, CH3P(O)(OC2H5)OH) were detected in the extracts of the asphalt. Degradation studies by Groenewold et al.9 used mass spectrometry to study the hydrolysis of VX on concrete and found that submonolayer amounts decontaminated on the concrete over a period of hours. Williams at al.10 found that the half-life for trace amounts of VX on concrete was 2-3 h and decreased with surface temperature and pH. Wagner et al. 3used 31 P NMR to show that the half-life for the degradation of VX to EMPA on a ∼20-year old monolith of concrete was ∼3 months; when the same concrete was crushed, 12% of the VX, an amount equivalent to a monolayer, reacted with a 2-h halflife; the remaining VX degraded with a 1-month half-life. On monoliths of concrete that were less than 2 months old, the

10.1021/jp8111099 CCC: $40.75  2009 American Chemical Society Published on Web 03/27/2009

Evaporation and Degradation of VX on Silica Sand

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6623

SCHEME 1

degradation of VX to EMPA and diisopropylaminoethanethiol (DESH) was complete within 4 days for 1-µL droplets of VX and within 11 days for a 4-µL droplet.2 Mizrahi et al. obtained a 2-day half-life for the degradation of VX to EMPA on crushed concrete.1 Studies of the degradation of VX on nanosize alumina,11 magnesia,12 and calcium oxide13 surfaces showed that nontoxic EMPA was formed; on alumina further degradation of the EMPA to MPA was observed. Epstein et al.14 studied the hydrolysis of VX in dilute aqueous solution at 25 °C and a pH range from 2 to 13.5. The reaction was first-order with observed half-lives of over a month for pH values of 7 and below, the addition of base increased the rate of the reaction, and at a pH value of 13.5, the half-life of VX was 3 min. The phosphorus-derived products were EMPA, EA2192, and O-ethyl methylphosphonothioic acid (CH3P(O)(OC2H5)SH, EMPT); the sulfur-derived products were diisopropylaminoethanethiol (DESH, HSCH2CH2N(isoPr)2), bis(diisopropylaminoethane) disulfide {(DES)2, [SCH2CH2N(isoPr)2]2}, and bis(diisopropylaminoethane) sulfide {(DE)2S, S[CH2CH2N(isoPr)2]2}. Yang et al.15 studied the hydrolysis of VX in systems that consisted of ton container VX plus 2%, 5%, and 8% added water. Yang et al. found that the degradation rate of VX was slower when more water was added, whereas the degradation of the diethyl dimethylpyrophosphonate intermediate species ([CH3(O)P(OCH2CH3)]2O, called “pyro”) to EMPA was faster with added water, exhibiting a first-order half-life of 6 h at 23 °C. Although the initial rate of VX degradation was faster with 2% added water, the reaction needed 8% water to proceed to completion. The reaction profile exhibited an initial lag time, a fast reaction, and then a slower reaction; ∼300 h (12 days) were required for complete reaction of the VX. In this work, the kinetics of VX degradation on moist and ambient silica sand and glass at various temperatures were measured using 31P SSMAS NMR, and the results were compared to the evaporation rate measured in the wind tunnel and the extraction results. 2. Experimental Procedures 2.1. Evaporation of VX from Sand in the Wind Tunnels. The 5-cm wind tunnels that were used in these experiments have been previously described.16,17 For the analysis of VX, a silver fluoride pad (CAMSCO, Houston, TX) was inserted onto the end of Tenax TA thermal desorption tubes (Markes International, Llantrisant, UK) to convert any VX in the stream to its G-analog, ethyl methylphosphonofluoridate (EMPF, EA-1207, ethyl GB), which is more volatile and therefore easier to analyze by thermal desorption gas chromatography. The chemical equation for the reaction is (Scheme 1): The ethyl methylphosphonofluoridate was desorbed from the thermal desorption tubes using a Markes UNITY/ULTRA Desorption system (Markes International, Llantrisant, UK) and analyzed on an Agilent 6890/5973 GC/MSD (Agilent, Santa Clara, CA) using a HP-5MS capillary column (30 m long, 0.25 mm id, 0.25 µm film thickness, (5%-phenyl)-methylpolysiloxane stationary phase).18 Oven parameters were 60 °C (1.5 min) to 250 at 50 °C/min. Thermal desorber parameters were a tube

Figure 1. Retention times of ethyl methylphosphonofluoridate, 1 min, internal standard bromofluorobenzene, 2.1 min, and hydroxymandelic acid, ethyl ether, 3.4 min.

Figure 2. Mass spectrum of the 1.1 min ethyl methylphosphonofluoridate peak.

desorption temperature of 250 °C for 2.5 min and a 10 mL/min split flow. The tube purge/sweep flow was 50 mL/min, and the trap desorption temperature was 300 °C for 2.0 min. The sample was carried into a split/splitless injection port at 250 °C. The split vent was turned on at 0.5 min with a purge flow of 50 mL/min of helium. Retention times were 1 min for ethyl methylphosphonofluoridate, 2.1 min for the internal standard bromofluorobenzene, and 3.4 min for 3-hydroxymandelic acid, ethyl ether (impurity, not from VX) (Figure 1). The mass spectrometer was operated in EI mode and scanned from 35 to 300 amu in 2.78 s. The molecular ion for ethyl methylphosphonofluoridate (m/z ) 126) was not typically observed, but the characteristic fragment ion at m/z ) 99 was observed (Figure 2). The ability of the Tenax TA tubes to collect the VX vapor was verified by injecting a known amount of VX vapor into a 20 ft. copper tube that had flowing air and showing that the VX was quantitatively recovered. 2.2. Cameras. The camera used to take still photographs of VX on sand was a digital Fuji Finepix S6000fd. The data were processed using Microsoft Paint software. The camera used to photograph the spreading of VX on sand in the wind tunnel was a Sony XC-ST50 CCD camera with a Fuji HF50HA-1 lens mounted above the sample. The data were accumulated with an Imperx Pro PCMCIA video capture card and processed using proprietary video software (Battenkill Technologies Inc., West Newton, MA). The camera was not used for the spread rate studies of VX on glass, as the light required (ELS-24DC 24W Solarc Hi-Lux Light Source) turned the VX yellow. 2.3. GC/MSD Liquid Analysis. The analyses were performed on an Agilent 6890 gas chromatograph (Agilent, Santa Clara, CA) using a HP-5MS (30 m long, 0.25 mm id, 0.25 µm film thickness, (5%-phenyl)-methylpolysiloxane stationary phase) capillary column and an Agilent 5973 mass selective detector. Oven parameters were 45 °C for 5 min to 265 at 10 °C/min. A liquid injection of 1 µL of sample was made into a split/splitless injection port at 250 °C. The split vent was turned on at 0.5 min with a purge flow of 50 mL/min of helium. Retention times

6624 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Figure 3. GC of the internal standard (16.4 min) VX (20.1 min) and degradation product (DES)2 (23.5 min).

Figure 4. Mass spectrum of the 20.1 min VX peak; the major characteristic ion seen is m/z ) 114.

were pentadecane (internal standard, 16.5 min), VX (20.0 min), and bis(diisopropylaminoethyl) disulfide ((DES)2, 23.6 min) (Figure 3). The mass spectrometer was operated in EI mode and scanned from 40 to 350 amu in 4.58 s. The molecular ion for VX, m/z ) 267 was not seen; the highest mass fragment ion was m/z ) 252 and the most prevalent was m/z ) 114 (Figure 4). 2.3.1. Analysis of VX on Aged Sand Samples. A 25 mm column of sand was placed in a 100 × 13 mm screw-top test tube, 3 µL VX was added, and the sample was capped. After the requisite period of time, 10 mL of 0.1 M Tris buffer, pH 8, was added, followed by vortexing for 1 min. Five milliliters (or as much as possible) of the Tris extract was removed and placed in screw-top test tube and extracted with 1 mL of methylene chloride by vortexing for 1 min. The organic layer was allowed to separate from the aqueous Tris and was then removed and set aside. The Tris was extracted a second time with 1 mL of methylene chloride extract, and the two 1-mL extracts were combined into one vial, of which 1 mL was placed into a GC vial. To this was added 10 µL of a pentadecane internal standard, yielding 2.3 mg/mL, and the sample was analyzed by GC/MSD. Compounds detected by this method were VX (20.00 min), pyro, 2-(diisopropylamino)ethanethiol (12.12 min), O-ethyl-S-diisopropylaminoethyl ethylphosphonothioate (21.41 min), bis(diisopropylaminoethyl) disulfide (23.62 min), and 1-[(2-diispropylamino)ethylthio]-2-[(2-diisopropylamino)ethyl dithio]ethane [iPr2-NCH2CH2SCH2CH2SSCH2CH2NiPr2] (26.95 min). 2.3.2. Analysis of Spent NMR Samples. Samples from the rotors were poured into a vial and extracted with 5 mL of acetonitrile (Fisher Scientific, Fairlawn, NJ) by sonicating for 30 min. A 1.8 mL aliquot of the acetonitrile extract was filtered through a 0.45 PFTE filter to removed sediment. Two samples were prepared from the filtered extract. The first sample was prepared by derivatizing 100 µL of the acetonitrile extract with 2 µL of BSTFA (bis(trimethylsilyl)trifluoroacetamide Supelco, Bellefonte, PA) and heating at 60 °C for 30 min. The second

Brevett et al. sample was prepared by diluting 100 µL of the acetonitrile extract with 300 µL of methylene chloride. Both were analyzed by GC/MSD. The remaining 3.2 mL of acetonitrile extract were blown down to dryness with an air stream and reconstituted in 2 mL of methylene chloride. One microliter of the methylene chloride sample was analyzed by GC/MSD. Compounds of interest detected using this method included EMPA, EMPT, and MPA, which were identified as their TMS derivatives. Retention times were: EMPA (11.48 min), EMPT (12.42 min), MPA (12.58 min), bis(diisopropylaminoethyl) disulfide (23.60 min), and 1-[(2-diispropylamino)ethylthio]-2-[(2-diisopropylamino)ethyl dithio]ethane (26.94 min). 2.3.3. Analysis of Neat Ton Container VX. Two samples were prepared for analyses by GC/MSD. The first sample was prepared by adding 2 µL of VX into 200 µL of acetonitrile. From this dilute mixture, a 10 µL aliquot was further diluted into 0.6 mL of methylene chloride, which was analyzed by GC/ MS, of which 1 µL was injected onto the GC/MS. Compounds of interest detected were as follows: 2-(diisopropylamino)ethanol, 12.1 min; diethyl dimethylpyrophosphonate, 16.2 min; VX, 20.0 min; bis(diisopropylaminoethyl) disulfide 23.5 min. The second sample was prepared by adding 2 µL of BSTFA and 2 µL of VX to 200 µL of acetonitrile. The mixture was then heated at 60 °C for 20 min. After the heating period, 10 µL of the derivatized mixture were added to 0.6 mL of methylene chloride, and the resulting solution (1 µL) was analyzed by GC/MS. Compounds of interest detected were as follows: TMS-derivatized-EMPA, 11.5 min; 2-(diisopropylamino)ethanol, 12.1 min; TMS-derivatized-EMPT, 12.4 min; S-trimethylsilyl-2-(diisopropylamino)ethanol, 16.0 min; diethyl dimethylpyrophosphonate,16.2min;VX,20.0min;bis(diisopropylaminoethyl) disulfide 23.5 min. 2.4. NMR Instrumentation. The 31P spectra of ton container VX on sand were collected at 9.4 T using a Varian Inova NMR spectrometer (Palo Alto, CA) equipped with a Doty Scientific (Columbia, SC) 7 mm standard VT-MAS probe using direct polarization, spinning rates of ∼1500 Hz, and a 90° pulse width of 7.5 ms. The silicon nitride rotors were packed with 200 mg sand, spiked with 4 µL of VX and optionally 8 µL of water using a microliter syringe, weighed, and sealed with Doty Scientific double O-ring Kel-F caps. The delay times between pulses were at least 5 times the measured T1 relaxation time, and spectra were referenced to external phosphoric acid at 0 ppm. The kinetic data were modeled using Berkeley Madonna Software (University of California, Berkeley, CA). 2.5. Reagents. The sand used was AFS-50 Fine Sand produced by Warmwell Quarry (Bardon Aggregates, Dorset, UK). The physical properties from the specifications sheet of the supplier indicate a predominant particle size of 0.25 to 0.5 mm, 98.6% SiO2, 0.39% aluminum oxide, 0.09% ferric oxide, and a skeletal density of 2.65 g/cm3. The sand had a surface area of 0.23 m2/g, of which 0.0382 m2/g were micropores19 and the pH of 0.1 g of sand in 2 mL of water, measured after 24 h using pH paper, was 6. The measured bulk and tapped densities of the sand were 1.34 and 1.48 g/cm3, respectively, yielding void volumes of 50% and 44%, respectively. The sand was used after drying in ambient air for 48 h (labeled air-dried sand, ADS); oven drying indicated that ∼2% water had adsorbed to the sand. Sand that was dried at 110 °C in an oven for 24 h and used immediately was abbreviated ODS. The glass slides used in the wind tunnels and crushed for the NMR experiments were Schott Borofloat glass 1.45 in. ( 0.050 in. diameter × 0.7 mm thick (Valley Design Corporation) and were washed in laboratory detergent, (Sparkleen Biodegradable



Figure 5. Ton container grade VX on sand: (a) initial drop (6 mm diameter), (b) after 30 min (9.5 mm diameter), (c) after 40 min and shaking sand with a line drawn for contrast (9 mm diameter), (d) ped removed (9 mm diameter). Gradations in ruler are 1 mm apart.

Figure 6. (a) Concentration of VX in mg/m3 from glass ([,2,9) and sand (4,],O,0) at 42 °C. (b) Surface areas of the same 6-µL drops of VX on sand with time.

Laboratory Detergent from Fisher), thrice rinsed with DI water, submerged in nitric acid for 24 h (EMD), rinsed with DI water, dried with a paper towel (Kimwipes EXL), rinsed with hexane (EMD OmniSOLV HR-GC Hexanes), and redried with a Kimwipe. The surface areas were 0.046 m2/g for the as-is crushed glass (abbreviated AIG) and 0.066 m2/g for the crushed washed glass (abbreviated WG). The Tris buffer was made from Trizma brand tris(hydroxymethyl)aminomethane, (Sigma, St. Louis, MO). The VX used was ton container grade. Caution: VX (Oethyl S-[2-N,N-(diisopropylamino)ethyl] methylphosphonothiolate) is a potent nerve agent, and care must be taken to prevent exposure to liquid or vapor. It should only be manipulated by trained personnel employing appropriate engineering controls and personal protective equipment. 3. Results 3.1. Evaporation of VX from Glass and Sand. Photography of a 6-µL drop of VX placed onto 14 g of sand showed that the drop initially occupied only a 6 mm diameter (Figure 5a) in a cup that had a 240 mm diameter and 15 mm depth. The VX

spread to a 9.5 mm diameter spot (Figure 5b), and by 30 min the drop had begun to visibly fade (Figure 5c), indicating a loss of agent at the surface. Gently shaking the cup resulted in a loosening of the clump of VX and sand that had formed, which was roughly hemispherical, with a 9 mm diameter and 5 mm depth (Figure 5d). A 6-µL drop of VX on glass was placed into a wind tunnel that was at 42 °C and had 181 standard liters per minute (SLPM) air flow (corresponding to 1.5 m/s) within 3 min of deposition. The evaporation of the VX was measured until no further decrease in the concentration of vapor was detected, at which time the experiment was terminated (Figure 6a). During the first 1500 min, the VX vapors emanating from the glass substrate were above the IDLH limit of 3 × 10-3 mg/m3 (IDLH is the OSHA Immediately Dangerous to Life and Health limit).20 A 6-µL drop of VX on sand evaporated similarly yielded VX vapors that were close to the IDLH, yet consistently above the STEL (short-term exposure limit) of 1 × 10-5 mg/m3. Summation of the concentrations over time yielded the cumulative amount of VX that had evaporated. The VX vapor recovered for 14 sand samples under these conditions was averaged 10.5%;


Figure 7. VX (2), S-products ([), and the total (9) detected by extraction from ADS with Tris buffer and VX (4), S-products (]), and the total (0) detected by extraction from ADS with acetonitrile followed by GC/MSD.

the range was 2.9% to 19.5%, and five of the samples had vapors above the IDLH during the first 1500 min of the evaporation. The VX vapor recovered for four glass samples under these conditions was averaged 59%; the range was 40% to 75%, and all of the samples had vapors above the IDLH during the first 1500 min of the evaporation. Photography was used to measure the size of the drop on sand during the evaporation experiment in the wind tunnel (Figure 6b). The wet spot caused by the droplet became difficult to detect using photography at 700 to 1000 min, whereas the vapor concentration showed no further decrease after 1500 min. The maximum surface area was observed at 300 min, in agreement with the time at which the maximum vapor concentration was measured. Photography was not used during the VX on glass experiments because the light flux required discolored the VX. 3.2. Extraction Experiments. Twenty-four vials of 5 g of air-dried sand (ADS) were spiked with 3 µL (12 µmol) of VX and sealed. Two vials were extracted with Tris buffer, and two with acetonitrile, at 1.5, 26, 47 and 144 h. The amount of VX was quantified based upon a standard calibration curve; the amounts of diisopropylaminoethanethiol and bis(diisopropylaminoethane) disulfide were estimated by using the diisopropylaminoethanethiol and bis(diisopropylaminoethane) disulfide present in the VX calibration curve. The Tris buffer extracted all of the initial VX aliquot and ∼40% of the projected thiol and disulfide products. The acetonitrile extracted 83% of the initial VX aliquot, which decreased with time, falling to zero after six days, and 67% of the projected diisopropylaminoethanethiol and bis(diisopropylaminoethane) disulfide products (Figure 7). The first-order half-lives for VX nonextractability were 22 h for acetonitrile and 26 h for Tris. Twenty 1.5 in. diameter glass disks were spiked with 1 µL (4 µmoles) of VX and maintained at 22 °C; half-were covered with a Petri dish lid, and the other half-were exposed to 150 ft/min air flow. The glass disks were rinsed with acetonitrile: methylene chloride at 1.5, 24, 48, 72, and 96 h for GC/MSD analysis; this extract was also derivatized with BSTFA prior to GC/MSD analysis. The amount of VX was quantified based upon a standard calibration curve; the amounts of EMPA, EMPT, and MPA were estimated based upon an internal standard. The amount of VX extracted decreased with time, falling to zero after four days for the uncovered sample and 14% after four days for the covered sample. After four days, the uncovered sample yielded 5% of the expected EMPA product; the balance of the incident VX and products had evaporated. MPA (a solid) was not detected since the EMPA, and VX precursors had evaporated. After four days, the covered

Brevett et al.

Figure 8. VX loss ([) and product formation of EMPA (0) and EMPT (]) on covered glass slides.

Figure 9. 31P SSMAS NMR spectra of the degradation of VX on airdried silica sand at (a) room temperature and (b) 50 °C; integrals are shown.

sample yielded 75% of the calculated EMPA product and 14% VX remained (Figure 8). 3.3. Kinetics and Products of VX Degradation. 3.3.1. AirDried Silica Sand. The degradation of VX on air-dried silica sand using 31P SSMAS NMR was studied at 22, 30, 40, and 50 °C. EMPA was observed as the major product (Figure 9), and the chemical shift ranged from 19 to 20 ppm, depending on the sample. Extraction with acetonitrile and derivatization with BSTFA followed by GC/MSD confirmed the presence of EMPA. An intermediate peak was observed slightly downfield of the EMPA in the 30, 40 and 50 °C samples, (Figure 9b) and was attributed to pyro, which was confirmed qualitatively by GC/MSD. The T1 relaxation times of the pyro were 0.6 s compared to 0.04 s for the EMPA (Figure 10). The phosphorus mass balance, as measured by the total integrated peak area in the NMR spectra, averaged 81%. For the 50 °C sample, the mass balance initially observed was 80%; the addition of acetonitrile, which mobilized the products, increased the observed product to 91%. Trace amounts of EMPT were observed in a few of the samples via NMR (75 ppm, Figure 9) and more consistently via GC/MSD (Table 1). No EA-2192 (43 ppm) was observed in these samples, and MPA was only observed in the GC/MSD. The sulfur-based products were



Figure 10. 31P spectra obtained by the inversion-recovery method to calculate the T1 relaxation times of pyro (22.0 ppm) and EMPA (19.5 ppm) on air-dried silica sand after 18 h at 50 °C.22

TABLE 1: 31P-Containing Product Analysis for VX Degradation on Air-Dried Silica Sand, Oven-Dried Silica Sand, and Glassa substrate

temp, °C

%EMPA NMR/GCb

%EMPT NMR/GC

MPA GC

pyrob NMR/GC

ADS ADS ADS WG WG AIG AIG ODS ODS ODS ODS ADS ADS ADS TC VX

22 22 22 22 22 22 22 35 22 22 35 30 40 50 22

100/nm 97/93 100/97 100/91 64/86b 100/94 71/93b 100/92 100/93 70/92b 100/94.5 100/nm 100/95 100/96 1.5e

0/nm 3.1/3 1.6/2.2 0/2 0/5.5 0/0.6 0/0.3 0/2.1 0/0.9 0/1.4 0/1.4 0/nm 0/1.5 0/0.6 0.4

nm 3.6 0.4 7 6 5.4 6.5 5.7 5.9 5.8 2.6 nm 3.3 3.5 0

0/nm 0/ 0/obsdb /tr 30/obsdb /0 10/obsdb 0/0 0/0 17/obsdb 0/0 0/nm 0/0 0/0 0.5

a The percentages given for EMPT and EA-2192 were observed shortly after the loss of VX and VXH+; these species also degraded to EMPA over the period of a month. nm ) not measured. b The pyro was observed in the GC/MSD when present; in most samples the pyro had hydrolyzed to EMPA before the extraction and GC/ MSD.

∼1.4% diisopropylaminoethanethiol, 90% bis(diisopropylaminoethane) disulfide, and 0.3-7% 1-[(2-diispropylamino)ethylthio]-2-[(2-diisopropylamino)ethyl dithio]ethane which seemed to increase in concentration as the sample aged; no bis(diisopropylaminoethane) sulfide was detected. First-order kinetic plots for VX loss are shown in Figure 11. Further examination of the 22 and 30 °C plots showed the presence of a lag time early in the reaction (Figure 12). The corresponding plots for the gain of EMPA are also shown and were not first-order. The half-lives ranged from 8 h at 50 °C to 140 h at room temperature (Table 2), and the activation energy Ea was 46 kJ/mol, calculated using the Arrhenius equation, ln kobs ) Ae(-Ea/RT) and the kobs from Tables 2 and 3 (Figure 13).21 3.3.2. Moist Silica Sand. The degradation of VX on air-dried silica sand with added water was studied at 22, 30, 35, 40, and 50 °C. When a 25- to 35-fold molar excess of water was added to the VX on sand, the VX (56 ppm) became protonated (VXH+, 62 ppm), and this protonated species reacted to form EMPA (25.9 to 26.6 ppm). Toxic EA-2192 was also observed (43 ppm, Figure 14). In the 22 °C sample (Figure 14), multiple peaks were observed at the VX 56 ppm chemical shift; these were all considered to be VX. The first-order kinetic plots for VX and VXH+ loss and EMPA gain at 22 °C are shown in Figure 15; the loss of the VX and VXH+ followed first-order kinetics until all of the species had been depleted. The gain of EMPA and

Figure 11. First-order kinetic plots for VX on air-dried silica sand at 22 (0), 30 (4), 40(O), and 50 (]) °C.

Figure 12. First-order kinetic plots for VX loss at 22 (0) and 30 (4) °C and EMPA formation at 22 (9) and 30 (2) °C on air-dried silica sand.

TABLE 2: Kinetic Results for VX Degradation on Ambient Substrates temp, °C

substrate

kobs, h-1

half-life, h

initial %VX

maximum %pyro

22 22 22 22 22 22 35 35 22 30 40 50

AIG AIG WG WG ODS ODS ODS ODS ADS ADS ADS ADS

0.0148 0.0057a 0.0114 0.0059 0.0109 0.0082a 0.0269 0.0214 0.0181a 0.0426 0.0853 0.0912

47 ∼180b 61 130 64 ∼120b 26 32 ∼70b 16 8.1 7.6

80 97 80 100 80 95 75 96 97 89 93 83

22 32 50 38 35 26 44 33 2c 13 34 42

a

A lag time was observed before the reaction began. b First half-life, measured directly from kinetic plots. c On two additional samples, for which only the latter half of the reaction was observed, 18 and 25% pyro were detected.

EA-2192 followed first-order kinetics until all of the VX had been depleted. At this point, there was no further change in the amount of EA-2192, but the EMPA continued to accrue. The rate constant for the loss of VX was 0.0076 h-1, giving a halflife of 91 h, compared to 225 h for the sum of the VX and VXH+ species. The EMPT was present at 2% abundance when most of the VX had been depleted and remained unchanged during the remainder of the study. The identity of the VXH+ was verified by the addition of 14 µL of 11 M KOH solution to a reaction of VX on moist sand;


Brevett et al.

TABLE 3: Kinetic Results for VX Degradation on Moist Silica Sand temp, °C 22 30 30 35 40 50

kobs, h-1 0.00308 0.00534 0.00473 0.00771 0.0169 0.0457

half-life, h a

225 130a 147a,b 90a 41a 15a

initial %VX 97 89 89 n/a n/a 83

a The half-life given is that for the total VX (sum of VX and protonated VX). b Calculated from four data points.

Figure 15. First-order kinetic plots for VX (]), the sum of VX and VXH+ (O) and products EA-2192(0), EMPA(4), and EMPT ([) on wet silica sand at 22 °C.

Figure 13. Arrhenius plot for VX degradation on air-dried (0), ovendried (9), and wet (]) silica sand, kobs from Tables 2 and 3.

Figure 16. 31P SSMAS spectra of VX and on wet silica sand. Bottom spectrum: After 150 h at 30 °C; top spectrum: 45 min after the addition of KOH.

Figure 14. 13C SSMAS spectra of the degradation of munitions VX on wet silica sand at 22 °C.

the VXH+ peak at 62 ppm was completely removed, yet the VX peak at 52 ppm remained (Figure 16). The protonation of VX and the concomitant change in chemical shift have been previously observed in aqueous solution and zeolites.3 The firstorder half-lives for total VX loss ranged from 15 h at 50 °C to 225 h at room temperature (Figure 17, Table 3); the energy of activation was 80 kJ/mol (Figure 13). At the end of the reaction, the phosphorus products (measured via NMR) were typically 80-95% EMPA, 3-5% EMPT, and 4-12% EA-2192 (Table 4). The mass balance of phosphorus species, as measured by the total integrated peak area, averaged 85%. For the 50 °C sample the phosphorus mass balance of 54% increased to 76% upon the addition of acetonitrile, which mobilized the products. The GC/MSD of the extract detected organic products: 0.1 to

Figure 17. First-order kinetic plots for total VX on moist silica sand at 22 (9), 30 (4), 35 (]), 40 (O), and 50 (0) °C.

3% diisopropylaminoethanethiol and 91 to 99% bis(diisopropylaminoethane) disulfide in all of the samples and 6 to 8% bis(diisopropylaminoethane) sulfide in samples that were analyzed within a week after the end of the degradation; the disulfide:sulfide ratio was ∼14:1. The compound 1-[(2-diispropylamino)ethylthio]-2-[(2-diisopropylamino)ethyl dithio]ethane, [iPr2-NCH2CH2SCH2CH2SSCH2CH2N-iPr2] was detected in samples that were extracted ∼2 months after the VX degradation was complete in 0.2 to 5% yield.



TABLE 4: 31P-Containing Product Analysis for VX Degradation on Moist Silica Sanda temp, °C

%EMPA NMR/GC

%EMPT NMR/GC

%EA-2192 NMR

MPA GC

22 30 30 35 40 50

81/91 90-95/94 88/tr 92/92 91/92 96/93

3/9 0 0/tr 3/4.9 5/4.6 4/5.2

9 5-10 12 5 4 0

0 0.7 0 3.4 3.4 2.2

a The percentages given for EMPT and EA-2192 were observed shortly after the loss of VX and VXH+; these species also degraded to EMPA over the period of a month. The ton container VX used contained 1.5% EMPA and 0.4% EMPT as measured via derivatization and GC/MS.

Figure 19. First-order kinetic plots for ∼80% pure VX degradation at 22 °C on ODS (b), washed glass (0) and as-is glass (]), and for 97% VX on washed glass (O) and as-is glass (4) derived from the 31P SSMAS experiments.

SCHEME 2: Formation of EMPA, EMPT, EA-2192, and DESH from VX

Figure 18. First-order kinetic plots for VX degradation at 22 °C on air-dried sand with 97% initial [VX] (4); oven-dried sand with 96% (0) and 80% (]) initial [VX]; at 35 °C on oven-dried sand with 96% (9) and 80% ([) initial [VX].

3.3.3. OWen-Dried Silica Sand. The degradation of VX on ODS sand using 31P SSMAS NMR was studied at 22 and 35 °C. Two batches of ton container VX were used at each condition; one batch had 9% EMPA and 2.5% pyro degradation products, whereas the other was purer, containing only 0.5% EMPA. In all four experiments the degradation of the VX was complete, indicating that even the oven-dried sand retained enough water for the VX degradation to go to completion. On the ODS at both temperatures, the purer VX degraded slightly slower than the impure VX (Figure 18). At 22 °C the 97% pure VX exhibited a lag time on the air-dried sand, although once the degradation began, it was complete in ∼240 h (Figure 18). When oven-dried sand was used, the 80% purity VX decayed according to first-order kinetics, whereas the 96% pure VX exhibited a lag time and overall took twice as long to degrade as did similar VX on the air-dried sand (Table 2). At 35 °C the VX degradation on oven-dried sand followed first-order kinetics and no lag times were observed (Figure 18). The activation energy for VX degradation on the oven-dried sand was 46 kJ/ mol, similar to the air-dried sand (Figure 13). 3.3.4. Washed and As-ReceiWed Glass. Comparison NMR experiments were also performed on as-is and washed glass, the same substrate that was used in the wind tunnel experiments. The vast majority of the VX evaporated in the wind tunnel in 30 h; little degradation of VX had occurred at this point in time. The NMR experiments on glass showed three phenomena: the impure VX degraded faster, the rates on the washed and unwashed glass were the same, and the washed glass gave strong spinning side bands, which may be indicative of VX interacting with the acid-washed glass surface. The degradation of the purer VX samples exhibited a lag time; the impure VX sample had a

shorter lag time and gave a similar rate to impure VX on ovendried sand which had no lag time (Figure 19). The reactions were extracted prior to the degradation of all of the pyro to EMPA, and pyro was detected in the GC/MSD. 3.4. Modeling. The kinetic data were modeled (Berkeley Madonna Software, Berkeley, CA) based upon Scheme 2, although not all reactions were operative for VX degradation on both the wet and air-dried sands. Scheme 2 was derived from a consideration of the intermediates, products, and reactions known from the literature. 3.4.1. Air-Dried Sand, OWen-Dried Sand, and Glass. On airdried sand, oven-dried sand, and glass, the pyro intermediate was observed and accumulated in all samples except the 22 °C


Brevett et al.

TABLE 5: Reactions for VX Degradation on Air-Dried Sand, Oven-Dried Sand, and Glass reaction

rate constant

reaction modeled

VX f EMPA + DESH VX + EMPA f Pyro + DESH Pyro f 2 EMPA

k1 k2 k3

k1[VX] k2[VX][EMPA] k3[Pyro]

sample, before decomposing to EMPA. On air-dried sand, the minor product EA-2192 was not observed in any of the samples, EMPT was observed only in the 22 °C sample, 2% VXH+ was observed only in the 50 °C sample and only in the first hour. Thus, the predominant relevant rate constants for VX degradation on air-dried sand from Scheme 2 were k1, k2, and k3. The rate constant k3 was calculated in two ways: as a second-order rate constant and as a pseudo-first-order rate constant, assuming a large excess of water. These two treatments were numerically equivalent once the excess concentration of water was taken into account; hence, only the data for the pseudo-first-order treatment are given. The rate constant k1 was calculated similarly. The reactions and differential equations are given in Table 5 and equations 1-4. The rate constants k1 and k2 increased with temperature, whereas k3 decreased with temperature, consistent with the greater accumulation of pyro as the temperature increased (Table 2). The rate constant k2 was second-order with units of mol-1 h-1. Sample simulations of the kinetic plots at 22 and 50 °C are shown in Figure 20, and the calculated rate constants are in Table 6. Reverse reactions were not included for any of the steps in the mechanism. On the dry sand, EMPT was only observed at 22 °C, thus indicating that the faster k2 and k1 reactions predominated over the slower k6. Once formed, EMPA did not revert to pyro.

d(VX)/dt ) -k1[VX]-k2[VX][EMPA]

(1)

d(EMPA)/dt ) k1[VX]-k2[VX][EMPA] + 2k3[pyro] (2) d(pyro)/dt ) k2[VX][EMPA]-k3[pyro]

(3)

d(DESH)/dt ) k1[VX] + k2[VX][EMPA]

(4)

3.4.2. Wet Sand. On the wet sand exposed to VX, protonated VX was detected, no pyro intermediate was observed, and the major decomposition product was EMPA. The minor products EMPT and EA-2192, formed early in the reaction, persisted and in a few cases, when the reaction was monitored for sufficient time, were observed to degrade to EMPA. The production of EMPT and EA-2192 ceased once all of the VX had been depleted (Figure 16); thus, it was surmised that EMPT and EA-2192 formed from VX, not VXH+. The loss of both VX and the sum of VX and VXH+ was plotted; once the VX was depleted, the rate observed was that for only VXH+ degradation, and EMPA was the only product that increased in concentration. For this reason, the column in Table 8 was labeled ktotal (k1H). The relevant rate constants from Scheme 2 for the VX degradation on wet sand were k1H, k1, k4, k5, and k6. Reaction k2H was eliminated since the loss of [VXH+] was observed to be first order, which would require [EMPA] ∼ 10[VXH+]. However, the measured EMPA concentration was less than the VX concentration during the beginning of the reaction and was only ∼10 times the VXH+ concentration at the end of the reaction. In the reaction scheme employed, all rate constants

were pseudo-first-order, since the water was present in a large molar excess over the VX. The kobsVX calculated in Table 8 was the sum of the calculated k4, k1, k5, and k6. The rate constants k5 and k6 were reflective of and consistent with the amount of products formed. The reactions modeled and the differential equations are shown in Table 7 and eqs 5-9. Typical graphical results for the modeling of the 22 and 50 °C reactions are shown in Figure 21.

d(VX)/dt ) -k4[VX]-k5[VX]-k6[VX]-k1[VX] ) kobsVX (5) d(VXH+)/dt ) k4[VX]-k1H[VXH+]

(6)

d(DESH)/dt ) d(EMPA)/dt ) k1H[VXH+] + k1[VX] (7) d(DIAZ+)/dt ) d(EMPT)/dt ) k6[VX]

(8)

d(EA-2192)/dt ) d(EtOH)/dt ) k5[VX]

(9)

4. Discussion 4.1. Evaporation of VX from Glass and Sand. The evaporation of VX from glass, a nonporous surface, from which all agents were expected to evaporate, was compared to the porous sand surface, from which the ability of the VX to evaporate was unknown. The VX soaked into the sand, whereas on glass the droplet remained on the surface. The maximum concentration of VX vapor was ∼3-5 times less over the sand than it was over the glass and was reached in ∼300 to 400 min for both sand and glass. After ∼1500 min (25 h), the concentration of VX vapors over both substrates had decreased to ∼10-5 mg/m3 and had reached a plateau. The differences in the profiles of the evaporation curves on the same substrate were attributed to differences in the weight of the droplet and the exact shape of the droplet when it was first deposited. Although there was a ∼3-min lag time between the deposition of the drop of VX and when the first measurements were taken, the amount of undetected initial evaporation was likely minimal, due to the low evaporation rate of VX at room temperature. The time at which the maximum VX vapor flux from the sand was detected coincided with the time at which the maximum surface area of the VX droplet on sand was reached, indicating that the evaporation occurred from the surface of the sand; a larger surface area yielded a higher evaporation rate. The sand in the cup had a void volume of 44% based on the tapped and skeletal densities of the sand. Thus, if the 6-µL (6 mm3, radius of 1.1 mm) droplet of VX were to go directly into the voids within the sand, the volume occupied would be 2.3 times the original 6 µL, corresponding to a volume of 14 mm3 and thus a radius of 1.5 mm. The observation was that the droplet initially formed a 6-mm diameter spot on the sand, which grew to 9.5 mm as it evaporated over a 30-min period of time and was ∼5 mm deep when removed. Thus, assuming that the ped of VX and sand maintained a hemispherical shape, the volume increased from 57 mm3 to 190 mm3 over a 30-min period of time. The volume of sand occupied by the VX was 190 mm3, although only 14 mm3 would have been required if the VX had completely filled the voids between the grains of sand. A monolayer of 6-µL (6 mg, 0.0225 mmol) VX would have required 10.7 m2 surface area (based upon a molecular



Figure 20. Observed and calculated moles VX, EMPA, and pyro as a function of time for VX degradation on air-dried sand at 22 and 50 °C.

TABLE 6: Comparison of Measured and Calculated Rate Constants for the Degradation of VX on Air-Dried Sand temp, °C

kobs, h-1

calcd k1, h-1

calcd k2, mol-1 h-1

calcd k3, h-1

calcd kobs, h-1 b

22 30 40 50

0.018a 0.043 0.085 0.091

0.00013 0.014 0.031 0.055

2618 4474 9627 12222

0.75 0.146 0.067 0.026

0.018 0.045 0.098 0.14

a This reaction exhibited a lag time. b Calculated from kobs ) k1 + k2[EMPA]; the concentration of EMPA at the midpoint of the reaction was used.

TABLE 7: Reactions for VX Degradation on Moist Sand reaction VX f VX f VXH+ VX f VX f

EMPA + DESH VXH+ f EMPA + DESH EMPT + DIAZ+ EA-2192 + EtOH

rate constant

reaction modeled

k1 k4 k1H k6 k5

k1[VX] k4[VX] k1H[VXH+] k6[VX] k5[VX]

surface area of 79 Å2 3). By comparison, the 190 mm3 of sand occupied by the VX had a surface area of 0.065 m2. Hence, the ped formed contained much more than a monolayer of VX, yet it was not completely dense and apparently had some entrained air. The weight/weight sand/VX ratio obtained was 281 mg/6 mg ) 47. 4.2. Extraction of VX from Sand. The extraction of VX from sand with time was consistent with the NMR experiments. As the degradation of the VX proceeded, products were detected in the extracts. The phosphorus-containing products, EMPA and diethyl dimethylpyrophosphonate, were not observed in the GC/ MSD (derivatization was not performed on these samples), although the sulfur-containing products, DESH and (DES)2, formed from the air oxidation of DESH were. Tris buffer was chosen since the pH of 9 would deprotonate any VXH+ that might have formed on the sand.5

4.3. Kinetics and Products of VX Degradation. The sand/ VX ratio (by mass) in the NMR rotor was 50, similar to that observed by photography when VX was dosed onto sand. Therefore, the NMR studies were performed under conditions that were realistic compared to environmental conditions. The addition of water, a ∼30-fold excess, would result in one phase of VX/water coating the sand, due to the miscibility of VX and water. The ultimate phosphorus-containing product from VX degradation on both wet and air-dried sand was nontoxic EMPA. There was apparently enough water present on the wet, ambient, and oven-dried substrates to degrade all of the VX. In the absence of added water the rate of VX loss on airdried silica sand was faster, and the activation energy was lower than on moist silica sand (Figure 13). Yang et al.15 observed that water added to neat ton container VX would both initiate and retard the rate of VX degradation; with 2% added water the reaction was faster but incomplete, whereas with 8% water the reaction was slower but went to completion. The change in rate with added water was ascribed to the enhanced nucleophilic reactivity of the EMPA anion (pKa ∼ 2) in the absence of water.15 The degradation of neat ton container VX with 2% to 8% added water at 22 °C took 150 to 300 h to reach termination and had an initial lag time. The beginning of the degradation involved an intramolecular intermediate (Scheme 3), catalyzed by the EMPA- anion which attacked the VX; once all of the water was depleted, the reaction ceased. The degradation of VX on air-dried sand and glass at 22 °C reached completion in ∼1 week, similar to the neat ton container VX with added water; a slight lag time was also detected in the beginning of the reaction when high-purity VX was used. When low-purity VX was used, the EMPA that would otherwise form during the lag time was already present; thus, first-order kinetics were observed. Scheme 3 elaborates on the attack of VX and VXH+ by the nucleophilic EMPA- anion. Consideration of the intramolecular intermediate of the substitution step showed that the

TABLE 8: Comparison of Measured and Calculated Rate Constants (h-1) for the Degradation of VX on Moist Sand temp, °C

measd kobsVX,

measd ktotal (k1H)

calcd k1H

calcd k4

calcd k5

calcd k6

calcd k1

calcda kobsVX

22 30 35 40 50

0.0076 0.0210 0.0273 too fast too fast

0.00308 0.00534 0.00771 0.0169 0.0457

0.0028 0.0047 0.0079 0.015 0.038

0.0038 0.011 0.022 0.059 0.084

0.0008 0.0011 0.0014 0.0033 0.0099

0.0002 0.00034 0.0011 0.0033 0.0047

0.0021 0.0044 0.0064 0.0060 0.030

0.0072 0.017 0.031 0.071 0.13

a

kobsVX ) k4 + k5 + k6 + k1


Brevett et al.

Figure 21. Observed and calculated %compounds as a function of time for VX degradation on wet sand at (a) 22 and (b) 50 °C.

SCHEME 3: Mechanism for the Formation of Diethyl Dimethylpyrophosphonate from EMPA, VX, and VXH+ (after Yang15)

and dilute aqueous solutions of VX. The degradation of VX on air-dried sand, oven-dried sand, and glass resides near the system of neat VX with small amounts of water added. The degradation of VX on the moist sand occurred at an intermediate rate between the two extremes; at 22 °C the half-life on moist sand was 225 h, compared to ∼35 h on air-dried sand and 99 days in aqueous solution at pH ) 6.14 The occurrence of similar degradation rates and products on air-dried sand, oven-dried sand, and glass, measured by both NMR and extraction, was consistent with the VX degradation being autocatalytic and having no dependence on the surface. By contrast, prior studies of VX extraction from soil5 using Tris buffer showed that although the %VX recovery decreased with time, 8-10% was recovered after 100 days. In these prior experiments the VX was added to 100 g of soil as a dichloromethane solution of 100 or 1000 µg/mL. The soil had been dried at 40 °C and then sieved; the moisture content when spiked was unknown. The dilution of the VX in solvent would allow for spreading of VX and EMPA over the soil, and thus the concentration EMPA needed for the autocatalysis would be lower than when neat VX was used, hence retarding the rate. The major product and overall degradation rate at room temperature obtained on this sand were similar to the prior 31P NMR studies of VX degradation on desert sand,1 although the pyro and VXH+ intermediates were not observed on the desert sand. The production of EMPT and EA-2192 on wet sand showed that multiple degradation pathways were available. The formation of the bis(diisopropylaminoethane) sulfide was mechanistically and quantitatively consistent with the formation of the EMPT (Scheme 2). 5. Conclusions

VXH+-EMPA transition state would form an internal ion-pair,15 and thus be stabilized relative to VX-EMPA. On the air-dried sand, the EMPA- anion is expected to be a more reactive nucleophile than on wet sand, where it would be solvated. The lower activation energy for the reaction on air-dried sand and glass (46 kJ/mol) compared to wet sand (80 kJ/mol) was reflective of the stronger nucleophilicity of the EMPA- anion under dry conditions compared to the wet sand. The reaction of VX with water may be thought of having two extremes: neat VX with small amounts of water added,

When VX was deposited on air-dried sand, ∼9% evaporated and the balance degraded to nontoxic EMPA over the course of a week, via the toxic intermediate diethyl dimethylpyrophosphonate. The addition of water to the air-dried sand resulted in a slower degradation rate for the VX. The activation energy for the reaction on air-dried sand was 46 kJ/mol, compared to 80 kJ/mol on the wet sand. The net reaction for VX degradation on both air-dried and moist sand was hydrolysis to EMPA. When the system was water-starved, the reaction intermediate was pyro, and the reaction was autocatalytic. On moist sand, with an excess of water, the reaction between water and VX was direct and additional products were observed. The rate and intermediates for the degradation of VX on silica sand were highly dependent upon the amount of water present. Thus, in

Evaporation and Degradation of VX on Silica Sand the event of contamination of silica sand or glass with VX, the possible production of toxic intermediates and products would require testing for the absence of not only VX, but also VXH+, EA-2192, and diethyl dimethylpyrophosphonate in order to declare the area acceptable for re-entry and reuse. Acknowledgment. The authors thank Drs. James Savage and Mark Brickhouse for programmatic support. The work described in this report was performed under SAIC Contract No DAAD1303-D-0017 with funding from the Defense Threat Reduction Agency (DTRA) under project BA07TAS041. The authors acknowledge Mr. Carroll Cook and Mr. Joe Myers for assistance with the agent operations, and Dr. George Wagner for many useful discussions. References and Notes (1) Mizrahi., D. M.; Columbus, I. 31P MAS NMR: a useful tool for the evaluation of VX natural weathering in various urban matrices. EnViron. Sci. Technol. 2005, 39, 8931–8935. (2) Wagner, G. W.; O’Connor, R. J.; Edwards, J. L.; Brevett, C. A. S. Effect of drop size on the degradation of VX in concrete. Langmuir 2004, 20, 7146–7150. (3) Wagner, G. W.; O’Connor, R. J.; Procell, L. R. Preliminary study on the fate of VX in concrete. Langmuir 2001, 17, 4336–4341. (4) Gura, S.; Tzanani, N.; Hershkovitz, M.; Barak, R.; Dagan, S. Fate of the chemical warfare agent VX in asphalt: a novel approach for the quantitation of VX in organic surfaces. Arch. EnViron. Contam. Toxicol. 2006, 51, 1–10. (5) Mountauban, C.; Begos, A.; Bellier, B. Extraction of nerve agent VX from soils. Anal. Chem. 2004, 76, 2791–2797. (6) Verweij, A.; Boter, H. L. Degradation of S-2-di-isopropylamionoethyl O-ethyl methylphosphonothiolate in soil: phosphorus-containing products. Pestic. Sci. 1976, 7, 355–362. (7) Kaaijk, J.; Frilink, C. Degradation of S-2-di-isopropylamionoethyl O-ethyl methylphosphonothiolate in soil, sulfur-containing products. Pestic. Sci. 1977, 8, 510–514. (8) Waysbort, D.; Manisterski, E.; Leader, H.; Manisterski, B.; Ashani, Y. Laboratory setup for long-term monitoring of the volatilization of hazardous materials: preliminary tests of O-ethyl S-2-(N,N-diisopropylamino)ethyl] methylphosphonothioate on asphalt. EnViron. Sci. Technol. 2004, 38, 2217–2223. (9) Groenewold, G. S.; Williams, J. M.; Appelhans, A. D.; Grisham, G. L.; Olson, J. E.; Jeffery, M. T.; Rowland, B. Hydrolysis of VX on

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6633 Concrete: Rate of degradation by direct surface interrogation using an ion trap secondary ion mass spectrometer. EnViron. Sci. Technol. 2002, 36, 4790–4794. (10) Williams, J. M.; Rowland, B.; Jeffery, M. T.; Groenewold, G. S.; Appelhans, A. D.; Grisham, G. L.; Olson, J. E. Degradation kinetics of VX on concrete by secondary ion mass spectrometry. Langmuir 2005, 21, 2386–2390. (11) Wagner, G. W.; Procell, L. R.; O’Connor, R. J.; Munavalli, S.; Carnes, C. L.; Kapoor, P. N.; Klabunde, K. J. Reactions of VX, GB, GD and HD with Nanosize Al2O3. Formation of Aluminophosphates. J. Am. Chem. Soc. 2001, 123, 1636–1644. (12) Wagner, G. W.; Bartram, P. W.; Koper, O.; Klabunde, K. J. Reactions of VX, GD, and HD with nanosize MgO. J. Phys. Chem. B 1999, 103, 3225–3228. (13) Wagner, G. W.; Koper, O. B.; Lucas, E.; Decker, S.; Klabunde, K. J. Reactions of VX, GD, and HD with nanosize CaO: Autocatalytic dehydrohalogenation of HD. J. Phys. Chem. B 2000, 104, 5118–5123. (14) Epstein, J.; Callahan, J. J.; Bauer, V. E. The kinetics and mechanisms of hydrolysis of phosphonothiolates in dilute aqueous solution. Phosphorus 1974, 4, 157–163. (15) Yang, Y. C.; Szafraniec, L. L.; Beaudry, W. T.; Rohrbaugh, D. K.; Procell, L. R.; Samuel, J. B. Autocatalytic hydrolysis of V-type nerve agents. J. Org. Chem. 1996, 61, 8407–8413. (16) Weber, D. J.; Scudder, M. K.; Moury, C. S.; Shuely, W. J.; Molnar, J. W.; Miller, M. C. Development of the 5-cm Agent Fate Wind Tunnel, Edgewood Chemical Biological Center, Technical Report ECBC-TR-327, AD-A462 884, December 2006. (17) Weber, D. J. Scudder, M. K. Moury, C. S. Donnelly, T. Park, K. D’Onofrio, T. G. Molnar, J. W. Shuely, W. J. Nickol, R. G. King, B. Danberg, J. Miller, M. C. Micro Wind Tunnel for Hazardous Chemical Fate Studies. Proceedings of the 45th AIAA Conference; AIAA-2007-0960, Reno, Nevada, January, 8–11, 2007. (18) Smith Jr, J. E. Boyd, W. D. Mason, D. W. Depot Area Air Monitoring System and VX Study, Southern Research Institute; Final Report Contract DAAK11-77-C-0087, November 1982. (19) Surface areas were from five-point BET measurements, and the micropore area was from an adsorption t-plot, using nitrogen gas. Collected by Micromeritics Inc., Norcross, GA. (20) Committee on ReView and Assessment of the Army Non-Stockpile Chemical Materiel Demilitarization Program: Workplace Monitoring, National Research Council, Impact of ReVised Airborne Exposure Limits on Non-Stockpile Chemical Materiel Program ActiVities; National Academy of Science: Washington, DC, 2005; p 37. (21) Pilling, M. J.; Seakins, P. W. Reaction Kinetics; Oxford University Press: New York, 2005; p 20. (22) Braun, S.; Kalinowski, H.-O.; Berger, S. 150 and More Basic NMR Experiments; Wiley-VCH: New York, 1998; pp 155-158.

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