Article pubs.acs.org/est
VX Fate on Common Matrices: Evaporation versus Degradation Ishay Columbus,* Daniel Waysbort, Itzhak Marcovitch, Lea Yehezkel, and Dana M. Mizrahi* Department of Organic Chemistry, Israel Institute for Biological Research, Ness-Ziona 74100, Israel S Supporting Information *
ABSTRACT: A study of the volatilization rate of the nerve agent VX (O-ethyl S-2-(N,N-diisopropylamino)ethyl methylphosphonothiolate) from various urban matrices in a specially designed climatic chamber (model system) is described. The performance of the model system combined with the analytical procedure produced profiles of vapor concentration obtained from samples of VX dispersed as small droplets on the surfaces of the matrices. The results indicated that the bitumencontaining surfaces such as asphalt blocks and bitumen sheets conserve VX and slow-release part of it over a long period of time. No complete mass balance could be obtained for these surfaces. Influence of environmental and experimental parameters as well as the efficacy of decontamination procedure were also measured. From smooth surface tiles a fast release of VX was measured and almost a complete mass balance was obtained, which characterizes the behavior of inert surfaces. Experiments carried out on concrete blocks showed fast decay of the concentration profile along with a very poor reconstruction of the initial quantity of VX, implying that this matrix degraded VX actively due to its multiple basic catalytic sites. To complement this study, solid-state NMR measurements were compared to add data concerning agent-fate within the matrices.
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INTRODUCTION During the last two decades, the fate of chemical warfare agents (CWAs) in environmental matrices has been the focus of extensive research. In view of rising world terrorism, remediation and cleanup became a civilian issue as important as the military one.1,2 Moreover, when dealing with the remediation of a civilian area, one has to consider a population much more diverse and sensitive than the military. As a result, the accepted level for hazardous materials is lower by orders of magnitude for civilian population, as compared to the military, and often requires special detection apparatus. Investigating the fate of CWAs on and in various matrices is crucial for determining the risk arising from such contaminated surfaces. Defense measures require a database of matrice-, timeand exposure-dependent risk. It will enable to assess the need for active decontamination instead of passive weathering, and to decide on the various decontamination procedures. Extensive research has been dedicated to the fate of the nerve agent VX (O-ethyl S-2-(N,N-diisopropylamino)ethyl methylphosphonothiolate) on a range of matrices.3 These studies were focused on concrete (including an in-depth investigation on droplet size and concrete age influence),4−7 several types of soils8−10 and asphalt.11,12 In recent work we described the fate of VX on common matrices and divided them into three categories, depending on their ability to conserve or degrade VX.13 During that study we performed periodical solid-state (SS) 31P MAS NMR analyses according to a widely used method on VX contaminated matrices.14−21 Among these reports, few studies were focused on evaluating the long-term evaporation of VX from various matrices.22,23 A previous publication by our group described a specially © 2012 American Chemical Society
constructed climatic chamber (model system) featuring controlled temperature, humidity, and air velocity. This article contained detailed information concerning the reaction chamber and the experimental methods utilized.11 The performance of the model system and the analytical procedures produced meaningful concentration profiles of vapors obtained from VX dispersed as small droplets over common urban matrices. Initially, the model system was used to measure VX volatilization from stainless steel and asphalt surfaces (1 g/m2) at 40 °C. The time dependence of VX volatilization from an inert stainless steel plate and the rate of its desorption from the system walls after removal of the plate were measured. The results showed that more than 95% of the applied VX on the stainless steel plate was recovered. This indicated that the model system would enable an almost complete mass balance of the vapor phase in future experiments. Preliminary results of the volatilization of VX from asphalt block showed that the released vapors amounted to ∼30% of the applied mass. The time course was best fitted to a triexponential curve with rate constants changing over time from 2.2 to 0.03 h−1. As reported recently, wind tunnel experiments on sand and glass slides were carried out to investigate the role that these surfaces are playing in VX evaporation.8 It was determined that simple exponential profiles are not applicable in these cases. Concentration profiles in sand and on glass slides showed a similar behavior, but the maximum values were lower in the Received: Revised: Accepted: Published: 3921
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In a typical experiment, 15 mg of VX (75 drops of 0.2 μL) were dispersed as small droplets over a 10 × 16 cm piece of the matrix, and its volatilization profile was measured. Quantitative data of selected volatilization experiments are presented in the Supporting Information (Table S1). In the decontamination experiment, a solution of STB26 (1:3 STB:water, 16 mL, 1 lit/m 2 ) was dispersed on the contaminated asphalt surface 1 h after VX contamination. Measurement of the decontaminated asphalt surface in the inner chamber began only when the chlorine air concentration was reduced to 0.1 ppm (ca. 4 h) to prevent analysis inaccuracies due to chlorine-induced enzyme inhibition. The accumulated amount of VX volatilized from the surface after the decontamination was compared to the amount without decontamination, and enabled the evaluation of the decontamination procedure efficacy. Degradation Experiments. As previously reported,13 samples of matrices (ca. 100 mg) were used to fill a 4-mm ZrO2 rotor. Neat VX (5 μL, 5% w/w) was applied via syringe to the center of the sample. The rotor was sealed with a fitted KelF cap. 31P MAS NMR experiments were carried out on a 500 MHz Avance (Bruker) spectrometer equipped with a 4-mm standard CP-MAS probe using direct excitation (no CP). The observation frequency for 31P was 202 MHz. Typical spinning rate was 3−5 kHz. Chemical shifts were referenced to TMP (trimethylphosphate) as 0 ppm. The spectra were measured periodically to determine remaining VX quantity and qualitatively identify degradation products. The amount of the agent was measured by integration of its peaks, in comparison to the integration of the degradation products peaks. Caution: VX is a very toxic nerve agent, and care must be taken to prevent exposure to liquid and vapor. These experiments should only be performed by trained personnel using applicable safety procedures.
sand experiments, due to the penetration of VX. Approximately 9% of the VX on sand and 60% of the VX on glass were recovered as VX vapor. Herein, we describe new evaporation results from our model system on various urban matrices, including asphalt blocks, bitumen sheets, concrete surfaces, and smooth surface tiles. For the first time, we compare them to our periodical 31P MAS NMR analyses of the matrices.13 In addition, an example of concentration profile of a decontaminated surface is given.
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EXPERIMENTAL SECTION Materials. Asphalt. Blocks of asphalt were collected from a local road and kept at ambient temperature. For the evaporation experiments, surfaces with the dimensions of 16 × 10 cm2 were cut to fit the sample carrier. SS NMR experiments were done on samples ground by a Fritsch ball mill to a fine powder (mean particle size 27.6 μm, SD 15.4%).13 Bitumen Sheets. Two types of commercial polymermodified bitumen sheets (for roofs and insulation) were used, either with aggregate or sand upper layer. Pieces of the sheets were cut to fit the sample carrier (in the case of the evaporation measurements) or the MAS rotor (in the case of SS NMR experiments).13 Concrete. Blocks of fresh standard concrete were obtained from The Standards Institution of Israel (Department of building, 80−94% Portland cement, 6−20% coarse limestone aggregates) and kept at ambient temperature. Prior to solidstate NMR experiments, the chips were ground to a fine powder by a Fritsch micro steel ball mill (mean particle size 11.3 μm, SD 11.1%).13 Commercial Red Sidewalk Bricks. The red sidewalk bricks were purchased from the Acker-stone Ind. company. Particulate percentage of components: conc. sand 40−50%, crushed rock 40−50%, Portland cement 10−20%, Admix 0−1%, iron oxide 0−1%. Smooth Surface Tiles. Three types of smooth tiles were tested in the evaporation experiments: matt and shiny granite porcelain and Hevron marble. Evaporation Experiments. The evaporation experiments were performed in our model system, situated in a climate chamber, where temperature, relative humidity and air velocity are controlled and monitored. A detailed description of this laboratory setup was reported recently.11 The volatilized VX was collected continuously in a Hepes buffer aqueous solution (pH 7.8). Its analysis was based on Ellman’s enzymatic method, utilizing various choline esterases.24,25 The typical detection limit of VX in the buffer solutions was 0.15−0.35 ng/mL. The determination of VX concentration in the buffer solutions enabled the calculation of VX concentration in the air during a certain sampling period. Altogether, along the duration of one experiment, 6 orders of magnitude of the VX concentration can be detected. The following ranges of environmental parameters were studied: temperature (20−40 °C), relative humidity (2− 70%) and wind speed (0.7 and 1.8 m/sec). The contamination parameters were droplet size (1 and 0.2 μL) and surface concentration (1 and 10 g/m2). In our experimental setup, a variable flow of preconditioned air is passed through the evaporation chamber. With 40 and 100 L of air flowing each minute through the chamber (equivalent to wind velocity of 0.7 and 1.8 m/sec, respectively), the system is not in an equilibrium. Yet, the flow has a minor influence on the evaporation rates from the surface.
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RESULTS AND DISCUSSION Evaporation Experiments. Asphalt Surface. Figure 1 shows examples of VX volatilization profiles from asphalt blocks at two different temperatures: 20° and 40 °C. The data points were best fitted to biexponential curves. Appreciable amounts of VX were evaporated over several weeks in the two experiments. As it can be seen, VX concentration in the air above the contaminated surface did not decrease to the
Figure 1. Volatilization of 15 mg of VX from asphalt road samples (1 g/m2) at 20° and 40 °C. The data points were fitted to biexponential curve. The American threshold is shown for comparison. 3922
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previous American general population threshold limit (GPL, 0.003 ng/L air) in the time course of the measurements (displayed as a gray horizontal line). The differences between the two experiments were expressed by the initial air concentrations (higher at 40 °C) and by the rate of evaporation from the asphalt surface (faster at 40 °C). Removal of the asphalt block after 5 days (in a separate experiment; Figure 2)
Figure 2. Volatilization of 15 mg of VX from asphalt road samples (1 g/m2) at 40 °C. After five days the sample was removed from the chamber.
resulted in an 8-fold decrease in VX air concentration immediately, and ca. 100-fold over 1.5 days. This proved that long-term measurements did not result from absorbance of the walls of the system, but expressed actual VX evaporation from the contaminated examined surface. The influence of the other parameters (droplet size, wind speed, relative humidity) was minor, but the general trend was of higher rate of VX volatilization from bigger droplets (1 μL, Figure 3, top), higher wind speed (1.8 m/s, Figure 3, bottom) and lower relative humidity (18%) conditions. No complete mass balance of the VX deposited on asphalt could be obtained after extractions of the asphalt block by MeOH (extraction of the surface and few layers deep). In the 40 °C experiment, ca. 40% of VX were missing, while in the 20 °C experiment ca. 55% of VX were missing (Table S2 in the Supporting Information). Even toluene treatment, characterized by destruction of the asphalt block and extraction of the bituminous inner layers, did not provide complete mass balance. This phenomenon may be explained by the highly efficient dissolution of VX in the organic layer of the asphalt (bitumen) along with some degradation processes (see below). Bitumen Sheets. Figure 4 shows examples of VX volatilization profiles from bitumen sheets at 20° and 40 °C. It is seen that in comparison to asphalt, VX kept evaporating from bitumen sheets for a longer time (months), with minor influence of the temperature. On the other hand, the upper layer of the bitumen sheets (aggregate or sand) had an influence on the VX evaporation (Figure 5), and it may be suspected that in the long run the upper layer acts as a barrier for secondary evaporation. The relatively long evaporation times from bitumen sheets longer than measured for asphalt blocksmay result from the efficient dissolution of VX into bitumen (the main ingredient of these sheets), causing a very long-term slow release of VX. Smooth Surface Tiles (Floor). Figure 6 shows examples of VX volatilization profiles from three types of smooth surface tiles: matt and shiny granite porcelain and Hevron marble. All
Figure 3. Volatilization of 15 mg of VX from asphalt road samples (1 g/m2) at 40 °C in two droplet sizes (0.2 and 1 μL) (top) and in two air speeds (0.7 and 1.8 m/s) (bottom).
Figure 4. Volatilization of 15 mg of VX from aggregate bitumen sheet samples (1 g/m2) at 20° and 40 °C. The data points were fitted to biexponential curves.
experiments were conducted at 40 °C. In general, VX evaporation from the tiles lasted for 3−4 days, and most of the VX was recovered. For example, in the marble tile experiments, 13.1−13.9 mg of VX (87−93%) were collected in the air. The evaporation from the marble tile is somewhat slower compared to the other tiles, maybe due to enhanced porosity of this tile. Practically, these tiles are inert and do not seem to undergo secondary evaporation for long periods of time. Concrete. Figure 7 (blue curve) shows VX volatilization profile from a relatively “new” (less than 1 year old) concrete surface. The initial air concentration of VX measured from concrete was lower than from asphalt or tiles (150 vs 400−700 3923
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Figure 7 (red curve) shows the decay profile of VX air concentration above this surface. It is noticeable that the decay of VX evaporation is slower relative to the ‘naked’ concrete surface. In addition, 33% of the VX could be collected in the air. This difference in behavior is probably due to the fact that the cement content (the “active” material) in the upper layer of the sidewalk brick is only 10−20%, while the content in the “naked” concrete surface is nearly 90%. The influence of VX surface concentration and of temperature were determined by conducting experiments on the commercial bricks with VX surface concentration of 1 and 10 g/m2 and temperature of 20 and 40 °C (Figure 8). Figure 5. Volatilization of 15 mg of VX from aggregate and sand bitumen sheets samples (1 g/m2) at 40°. The data points were fitted to biexponential curves.
Figure 8. Volatilization of 15 and 150 mg of VX from commercial red bricks (1 and 10 g/m2) at 20 and 40 °C.
Figure 6. Volatilization of 15 mg of VX from three types of smooth surface tile samples (1 g/m2) at 40 °C.
Interestingly, a gap of ca. 1 order of magnitude in the VX air concentration between the 1 g/m 2 and the 10 g/m 2 experiments was observed in both temperatures. This gap reflects almost accurately the difference in the initial concentrations dispersed on the bricks. Lower temperature experiments showed lower initial VX air concentrations (factor of 5 in 20 °C versus 40 °C) and slower decay of evaporation from the surface. During most of the time course of the measurements, the 20 °C curves showed higher VX air concentrations at any given time point, compared to the 40 °C curves. The reason of that is probably the relatively fast penetration of VX into the brick and its high rate of decomposition inside it at 40 °C experiments. Both processes left less VX available for volatilization. Evaporation Experiment Following Decontamination. Following a chemical warfare attack, the contaminated urban matrices must be treated with the appropriate decontamination means. In order to assess the decontamination efficacy, it is necessary to measure the volatilization profile of the residual chemical agent from the decontaminated matrix. Thus, we performed several experiments measuring the volatilization of VX from an asphalt surface following treatment with STB slurry (super tropical bleach, composed of Ca(OCl)2 and CaO).26 Figure 9 shows VX volatilization profiles from asphalt surfaces, with and without STB decontamination. The initial air concentration of VX above the decontaminated surface (0.05 ng/L air) is lower by 4 orders of magnitude relative to the nondecontaminated surface. One-day long evaporation was sufficient to reduce VX concentration below the GPL American threshold of 0.003 ng/L air. After eight days, the traces of the STB cover were scraped and removed from the surface, and the
Figure 7. Volatilization of 15 mg of VX from concrete surface and commercial red brick (1 g/m2) at 40 °C.
ng/L air, respectively). In addition, the concentration rapidly decreased, and already after 4 days it was lower than the American GPL threshold. Only 9% (1.3 mg) of VX were collected in the air over this period. There are two optional explanations for this rapid reduction and poor mass balance. The first takes into consideration the strong adsorption of VX on the surface, which slows down evaporation and interferes with extraction. The second is based on the chemical degradation of VX in the basic sites of the concrete. An additional nondestructive technique was required to characterize the interaction of VX and concrete (see below). For comparison purpose, we conducted another experiment of VX volatilization from commercial red sidewalk bricka concrete brick containing added iron oxide as a red pigment. 3924
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On one of the samples, VX degradation began shortly after deposit of the VX on the surface. On another sample degradation started about ten days after contamination. In both samples VX was evident for as long as 42 days after contamination. Looking into the characteristics of bitumen, it is obvious that this organic mixture of asphaltenes and petrolenes contributes little if any to the hydrolysis of VX. It is much more likely that sand, rock and whitewash are the active components that degrade VX. However, bitumen serves as a “solvent” for VX, actually shading it from those active inorganic particles, and by that causing the delay in VX degradation. This is probably the explanation for the very long slow-release of VX from the bitumen sheets in our model system. The degradation rate of VX in concrete, measured in the NMR experiments, was rapid in comparison with the other matrices. VX began degrading as soon as it was deposited on concrete and degradation lasted for less than a week (Figure 11). The active degradation process in concrete can be attributed to the basic nature of concrete active sites. These results are in full agreement with the experiments carried out in the model system, showing a fast decay of the vapor concentration profile along with a very poor reconstruction of VX quantity. Implication for Decontamination and Remediation Planners. Combining the data arising from the evaporating model system with the MAS NMR results may give a good assessment of the CWA fate for a given matrix. Climatic chamber volatilization experiments deal mostly with the physical processes between the contaminant and surface, such as desorption and adsorption. The complementary MAS NMR experiments offer an insight on the chemical agent-surface interactions involved. For example, incomplete volatilization of a contaminant may be regarded as a long-term hazard, since it is deduced that the rest of the contaminant remains intact in the matrix. However, MAS NMR studies may show that this contaminant fraction was actually degraded by the matrix and poses no residual hazard. Thus, a more complete picture of CWAs fate in environmental matrices is obtained. It is noteworthy that in the cases of asphalt and concrete, the NMR samples were ground, which may enhance VX degradation due to increased surface area.13 However, it is impractical to attempt predicting accurately the fate of CWAs in unexamined matrices, due to high
Figure 9. Volatilization of 15 mg of VX from asphalt surfaces (1 g/m2) at 40 °C with (blue) and without (red) treatment of STB. The STB layer was removed after eight days.
asphalt matrix was inserted back into the chamber. The VX concentration immediately increased above the American threshold to a level of 0.065 ng/L air, and then gradually decreased to the threshold level over additional six days, indicating the threat of secondary volatilization from the asphalt matrix. Comparison to Solid State NMR Experiments. As part of our ongoing research on the interaction of CWAs with matrices, it was interesting to compare the results of the evaporation model system to our recently published results on VX natural weathering in various urban matrices, measured by 31 P solid-state MAS NMR.13 Figure 10 describes MAS NMR results of VX decay on two different asphalt samples, showing a delay in the degradation process. The first sample began degrading VX after ca. 20 days and VX was present for as long as 60 days after contamination. VX degradation process in the second sample started after ca. 10 days and was completed after 24 days. These differences may be ascribed to asphalts’ wellknown heterogeneous nature. The splitting of the VX peak after two weeks is attributed to a certain portion of the VX being adsorbed into the asphalt. The incomplete mass balance that we have observed for asphalt in the evaporation studies could be explained by the slow VX degradation showed in the NMR measurement. The VX decay on two bitumen sheet samples showed similar behavior as asphalt (Figure S1 in the Supporting Information).
Figure 10. 31P MAS NMR results of the degradation profile of VX (2 samples) on ground asphalt (left) and the spectra of sample 2 (right) (a) 0.1 days, (b) 6 days, (c) 10 days, (d) 14 days, (e) 17 days, (f) 20 days, and (g) 24 days after the contamination. 3925
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Figure 11. 31P MAS NMR results of the degradation profile of VX on ground concrete (left) and the spectra of the sample (right) (a) 1 h, (b) 5 h, (c) 24 h, (d) 48 h, (e) 72 h (f) 96 h, and (g) 168 h after the contamination. concrete: Rate of degradation by direct surface interrogation using an ion trap secondary ion mass spectrometer. Environ. Sci. Technol. 2002, 36, 4790−4. (6) Wagner, G. W.; O’ Connor, R. J.; Procell, L. R. Preliminary study on the fate of VX in concrete. Langmuir 2001, 17, 4336−41. (7) 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, 7146−50. (8) Brevett, C.; Sumpter, K. B.; Pence, J. J.; Nickol, R. G.; King, B. E.; Giannaras, C. V.; Durst, H. D. Evaporation and degradation of VX on silica sand. J. Phys. Chem. C 2009, 113, 6622−33. (9) Love, A. H.; Vance, A. L.; Reynolds, J. G.; Davisson, M. L. Investigating the affinity and persistence of VX nerve agent in environmental matrices. Chemosphere 2004, 57, 1257−64. (10) Mount, C.; Begos, A; Bellier, B. Extraction of nerve agent VX from soils. Anal. Chem. 2004, 76, 2791−97. (11) 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 S2-(N,N-diisopropylamino)ethyl methylphosphonothiolate on asphalt. Environ. Sci. Technol. 2004, 38, 2217−23. (12) 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. (13) Mizrahi, D. M.; Columbus, I. 31P MAS NMR: A useful tool for the evaluation of VX natural weathering on various urban matrixes. Environ. Sci. Technol. 2005, 39, 8931−5. (14) Wagner, G. W.; Bartram, P. W.; Koper, O.; Klabunde, K. J. Reaction of VX, GD and HD with nanosize MgO. J. Phys. Chem. B 1999, 103, 3225−8. (15) Wagner, G. W.; Koper, O.; Lucas, E.; Decker, S.; Klabunde, K. J. Reactions of VX, GD and HD with nanosize CaO. Autocatalytic dehydrogenation of HD. J. Phys. Chem. B 2000, 104, 5118−23. (16) Wagner, G. W.; Procell, L. R.; Munavalli, S. 27Al, 47,49Ti, 31P and 13 C MAS NMR study of VX, GD and HD reactions with nanosize Al2O3, conventional Al2O3 and TiO2 and aluminum and titanium metal. J. Phys. Chem. C 2007, 111, 17564−9. (17) Wagner, G. W.; Chen, Q.; Wu, Y. Reactions of VX, GD, and HD with nanotubular titania. J. Phys. Chem. C 2008, 112, 11901−6. (18) Gershonov, E.; Columbus, I.; Zafrani, Y. Facile hydrolysis-based chemical destruction of the warfare agents VX, GB, and HD by alumina-supported fluoride reagents. J. Org. Chem. 2009, 74, 329−38. (19) Zafrani, Y.; Goldvaser, M.; Dagan, S.; Feldberg, L.; Mizrahi, D.; Waysbort, D.; Gershonov, E.; Columbus, I. Degradation of sulfur mustard on KF/Al2O3 supports: Insights into the products and the reactions mechanisms. J. Org. Chem. 2009, 74, 8464−7.
heterogeneity of the surfaces. Yet, we have constructed a database concerning various groups of matrices. Therefore, with knowledge of the structure of the new matrix (or the components composing the matrix) we may characterize the expected behavior as being limited between two known similar matrices that have been examined, and act as recommended for the more problematic matrix.
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ASSOCIATED CONTENT
S Supporting Information *
Table S1, Quantitative data of selected volatilization experiments; Table S2, Mass balance of VX deposited on asphalt surfaces; Figure S1, 31P MAS NMR results of the degradation of VX on polymer-modified bitumen sheets. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +97289381453; fax: +97289381548; e-mail: ishayc@ iibr.gov.il (I.C.),
[email protected] (D.M.M.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was internally funded by the Israeli Prime Minister’s office. We thank Mr. Michael Goldvaser for his help in the experiments done in the model system.
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REFERENCES
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