Research Note pubs.acs.org/IECR
Hydrothermal Degradation of Chemical Warfare Agents on Activated Carbon: Rapid Chemical-Free Decontamination Ruth Osovsky,*,† Doron Kaplan,† Hadar Rotter,† Ido Nir,† and Ishay Columbus*,‡ †
Department of Physical and ‡Organic Chemistry, Israel Institute for Biological Research, P.O. Box 19, Ness Ziona 74100, Israel S Supporting Information *
ABSTRACT: Hydrothermal treatment of activated carbon contaminated with adsorbed HD, VX, or sarin at temperatures of 90−120 °C decomposes >95% of the adsorbed chemical warfare agents within a period of 0.5−4 h, in an environmentally friendly route that is free of corrosive chemicals and ends in nontoxic products.
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INTRODUCTION
Scheme 1. Hydrolysis Products of CWAs
Investigations nowadays are focused on the development of efficacious and safe decontamination approaches for chemical warfare agents (CWAs), based on the use of standard and environmentally friendly techniques (green chemistry) and avoidance of corrosive and harmful chemicals.1−3 Of special importance is the decontamination of chemical protective equipment, for example, gas-mask filters and protective garments, universally used by military, industrial, and civilian protection teams. Filters and protective garments are based on activated carbon (AC) as prime adsorbent. Because adsorbed CWAs on AC are relatively stable even in the presence of humidity, contaminated protective equipment may pose a longterm environmental problem.4−8 For example, on a coconut shell carbon with 13% water content, ∼27% of HD (bis(2chloroethyl) sulfide, sulfur mustard) remained after 111 days at 90 °C.4 On dry BPL carbon at 50 °C decomposition of adsorbed HD was completed within a week.5 On highly moisturized BPL carbon (39% water) decomposition ended within 24 h. Reported half-lifetimes of VX (O-ethyl S-[2(diisopropylamino)ethyl] methylphosphonothioate) and sarin (isopropyl methylphosphonofluoridate, GB) on ACs at room temperature are days to weeks.6,7 Reaction mechanisms for the hydrolysis of adsorbed CWAs on AC are shown in Scheme 1. The main hydrolysis products of HD, sarin, and VX are, respectively, thiodiglycol (TDG),8 isopropyl methylphosphonic acid (IMPA),7 and ethyl methylphosphonic acid (EMPA).6 In view of the stability of adsorbed CWAs on AC, techniques for the accelerated degradation of the adsorbed CWAs should be sought. Recently, our group reported on accelerated degradation of adsorbed HD on AC in a hydrothermal treatment.8 Near-complete decontamination (>95% within 0.5 h) was achieved with adsorbed HD on a water-saturated AC from a protective garment at temperatures of 120−160 °C. The success of the hydrothermal treatment for decontamination of adsorbed HD motivated us to investigate its potential for adsorbed nerve agents as well. In the present note, we show that the hydrothermal treatment provides efficient decontamination of adsorbed HD, sarin, and VX on AC, even at lower temperatures than before. © 2013 American Chemical Society
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EXPERIMENTAL SECTION Chemicals and Activated Carbon. Caution. These experiments should only be performed by trained personnel using applicable safety procedures. HD, sarin, and VX were locally obtained. HD* (13C-enriched HD) was synthesized with 50% labeling of each ethylenic moiety to eliminate 13C−13C coupling effects on the spectra. Spherical (0.5 mm mean diameter) microporous AC (1140 m2/g surface area, 0.63 cm3/g micropore volume) from a SARATOGA permeable protective garment (Blücher, Germany) was used. Prior to contamination with CWAs, the AC was either dried at 120 °C for 2−3 h or humidified at room temperature and relative humidity of 75%.7 Thermal and Hydrothermal Treatments. Hydrothermal treatments were performed on samples of ∼40 mg AC packed into 4 mm diameter ZrO2 MAS NMR rotors and contaminated with 5 wt % CWA. The sample rotor was positioned in a brass Received: Revised: Accepted: Published: 9705
May 13, 2013 June 23, 2013 June 26, 2013 June 26, 2013 dx.doi.org/10.1021/ie401517a | Ind. Eng. Chem. Res. 2013, 52, 9705−9708
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Research Note
Table 1. Hydrothermal Reaction Rates of Adsorbed HD, Sarin, and VX on SARATOGA AC from MAS-NMR Spectra room temperature agent HD sarin VX a
dry carbon no decomp. after 2 monthsb no decomp. after 6 months ∼14 d half-life time
wet carbon
thermal treatment 100 °C
∼45 d half-life timeb ∼4 d half-life time no decomp. after 2h ∼14 d half-life NDa after 2 h time
120 °C
hydrothermal treatment 160 °C
40% remaining after 4 hb a
ND after 0.5 h NDa after 0.5 h
90 °C a
ND after 4h NDa after 3h NDa after 1h
100 °C a
ND after 2h NDa after 2h NDa after 1h
120 °C a
ND after 0.5 hb NDa after 0.5 h NDa after 0.5 h
Not detected. bPrevious work.8
Figure 1. Selected 13C MAS NMR spectra of HD* on activated carbon before and after hydrothermal treatment at 120 °C (a), 100 °C (b), and 90 °C (c).
Figure 2. Selected 31P MAS NMR spectra of sarin on activated carbon before and after hydrothermal treatments at 120 °C (a), 100 °C (b), and 90 °C (c).
Swagelok-type tube fittings reactor as previously described.8 For the hydrothermal experiment, 200 μL of water was added
to the reactor, outside the rotor, in order to saturate the contaminated AC. For the thermal experiment, the AC and the 9706
dx.doi.org/10.1021/ie401517a | Ind. Eng. Chem. Res. 2013, 52, 9705−9708
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Research Note
Figure 3. Selected 31P MAS NMR spectra of VX on activated carbon before and after hydrothermal treatments at 120 °C (a), 100 °C (b), and 90 °C (c).
reactor were kept dry. The reactor was heated at 90−120 °C for 0.5- 4 h and cooled to room temperature, and the rotor was capped and analyzed by MAS NMR. MAS NMR Measurements. MAS NMR spectroscopy was used to determine the amount of remaining CWAs on the AC and identify degradation products. 31P and 13C MAS NMR spectra were obtained at frequencies of 202 and 125 MHz, respectively, on a 11.7 T spectrometer (Bruker Avance 500), equipped with a 4 mm standard CP-MAS probe, using direct polarization (no cross-polarization). Typical spinning rate was 5 kHz. 31P and 13C chemical shifts were referenced to external trimethyl phosphate and CDCl3, respectively, as 0 ppm. The pulse delay was 2 s, which is considered sufficient for relaxation in organophosphorous esters on solid matrices. Typically 200 transients were accumulated per spectrum. For comparison, spectra were recorded under identical conditions.
same procedure could be effective for all three CWAs even at lower temperatures. Indeed, improved decontamination of HD, sarin, and VX on AC was achieved in the present work by a hydrothermal treatment at temperatures of 90−120 °C. Heating the watersaturated contaminated AC to 120 °C degraded the three CWAs within just 0.5 h. At 90 °C, the order of the rates of degradation of the CWAs was found to be VX > sarin > HD (Table 1 and Figures 1−3), and the longest degradation time (HD) was 4 h. The distribution of water in the closed reactor among the three phases: liquid, vapor, and adsorbed in the AC, depends on the temperature. As can be concluded from the linewidths and chemical shifts in the spectra in Figures 1−3 and our previous data,6−8 the CWAs are predominantly adsorbed within the micropores of the AC particles. We assume that most of the CWA in the reactor remains adsorbed, and that adsorbed CWA would chemically react with adsorbed water. We also assume that only water from the liquid phase is adsorbed within the micropores of the AC. The fraction of the liquid phase (x) is estimated from eq 1.8
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RESULTS AND DISCUSSION Practical decontamination of adsorbed CWAs should be accomplished rapidly, preferably at mild conditions. Our previous study8 and present results (Table 1 below) indicate that separately, humidity and heating are insufficient for the decontamination of adsorbed CWAs on AC. The results in Table 1 show that even on the highly moisturized AC (∼40% wt water content) HD, sarin, and VX degrade only slowly at room temperature. The shortest lifetime (t1/2 ≈ 4 d) was observed for sarin on the humidified AC. Adsorbed HD is expected to be more difficult to decontaminate than the organophosphorous agents. Indeed, Table 1 shows that HD on the wetted carbon is distinctly stable, with 50% remaining after ∼45 d of degradation. Even five months after contamination, ∼25% of the HD remains on the carbon. About half of the adsorbed VX decomposed within 14 d on dry or wet carbon. On the other hand, the thermal treatments degrade sarin and VX on dry AC at 120 °C within about 0.5 h (Table 1). Thermal treatment of HD on dry AC is less efficient, since only 60% of the initial adsorbed quantity decomposes at 160 °C after 4 h. Since the hydrothermal approach was found to decompose HD on water-saturated AC at 120−160 °C, it was hoped that the
v = vLx + vG(1 − x)
(1)
where v is the reactor internal volume divided by the total mass of added water, and vL and vG are the specific volumes of the liquid and gas phases (known from the phase diagram), respectively. The mass of liquid water (WL) can be directly calculated from x and the amount of added water (0.20 g). For example, for vG = 891.9 and vL = 1.06 cm3/g at 120 °C, WL is 0.18 g. The micropore volume of the hydrothermally treated, uncontaminated carbon was ∼23 μL/40 mg sample. Hence, at 120 °C and lower temperatures the micropores will be practically saturated with water. A lower temperature is also favorable since the evolved pressure inside the reactor is lower. Since the water content of our moisturized AC is comparable to that of other microporous ACs, for example BPL (Calgon, USA), we believe that the hydrothermal treatment reported here will be equally effective for other ACs as well. 9707
dx.doi.org/10.1021/ie401517a | Ind. Eng. Chem. Res. 2013, 52, 9705−9708
Industrial & Engineering Chemistry Research
Research Note
(7) Kaplan, D.; Shmueli, L.; Nir, I.; Waysbort, D.; Columbus, I. Degradation of Adsorbed Sarin on Activated Carbons: A 31P-MASNMR study. Clean 2007, 35 (2), 172−177. (8) Osovsky, R.; Kaplan, D.; Rotter, H.; Kendler, S.; Goldvaser, M.; Zafrani, Y.; Columbus, I. Hydrothermal Degradation of Adsorbed Sulfur Mustard on Activated Carbon. Carbon 2011, 49, 3899−3906.
From the corresponding MAS NMR spectra (Figures 1-3), the mechanism and product identities can be deduced. HD* decomposes to TDG via chlorohydrin (CH). The hydrolysis product, TDG, further reacts to give two products: TDG-dimer and, interestingly, the oxidation product TDG-Sulfoxide (TDGSO) (Figure S1 in the Supporting Information). This distribution of products differs from the distribution obtained after hydrothermal treatment at 160 °C.8 The degradation of the nerve agents, sarin and VX, yielded only the nontoxic hydrolysis products IMPA and EMPA, respectively. In the VX spectra there is no evidence for the presence of the toxic degradation product S-(2-diisopropylaminoethyl) methylphosphonothioic acid (desethyl-VX). Peak assignments were verified on the basis of MAS NMR spectra recorded after spiking the sample with authentic products, as well as by extraction of the carbon sample with methanol and GC−MS analysis of the extract.
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CONCLUSIONS A greater than 95% decomposition of adsorbed HD, sarin, and VX on an activated carbon that is used in protective garments can be achieved within 0.5−4 h by hydrothermal treatment of the carbon. The combination of high water content in the micropores and temperatures of 90−120 °C is the key to effective decomposition of the three CWAs despite the differences in degradation mechanisms. Decontamination is achieved without environmentally unfriendly chemicals and ends in nontoxic products.
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ASSOCIATED CONTENT
S Supporting Information *
Additional scheme of proposed decontamination mechanism. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Wagner, G. W.; Yang, Y. −C. Rapid Nucleophilic/Oxidative Decontamination of Chemical Warfare Agents. Ind. Eng. Chem. Res. 2002, 41, 1925−1928. (2) Wagner, G. W.; Procell, L. R.; Sorrick, D. C.; Lawson, G. E.; Wells, C. M.; Reynolds, F. M.; Ringelberg, D. B.; Foley, K. L.; Lumetta, G. J.; Blanchard, D. L., Jr. All-Weather Hydrogen PeroxideBased Decontamination of CBRN Contaminants. Ind. Eng. Chem. Res. 2010, 49, 3099−3105. (3) Wagner, G. W. Decontamination of Chemical Warfare Agents Using Household Chemicals. Ind. Eng. Chem. Res. 2011, 50, 12285− 12287. (4) Karwacki, C. J.; Buchanan, J. H; Mahle, J. J.; Buettner, L. C.; Wagner, G. W. Effect of Temperature on the Desorption and Degradation of Mustard from Activated Carbon. Langmuir 1999, 15, 8645−50. (5) McGarvey, D.; Mahle, J.; Wagner, G. Chemical Agent Hydrolysis on Dry and Humidified Adsorbents. In: ECBC-TR-334-Report, Aberdeen Proving Ground, 2003. (6) Columbus, I.; Waysbort, D.; Shmueli, L.; Nir, I.; Kaplan, D. Decomposition of Adsorbed VX on Activated Carbons Studied by 31P MAS NMR. Environ. Sci. Technol. 2006, 40, 3952−3958. 9708
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