Discussion of Hydrolysis Reactions of GB, VX, and H
The risk posed by agents depends upon their tendency to partition to phases where exposure could occur and on their stability and the toxicity of their degradation by-products. Thus, consideration of the physical and chemical properties of the agents provides a basis for evaluation of the potential risks of residual contamination. The risk associated with agents can be prolonged if they are sequestered in occluded spaces, and this tendency is also related to agent physical properties. Therefore, a brief review of the volatilization and hydrolysis reactivity of GB (sarin), VX, and mustard (H) are provided in the following paragraphs.
PROPERTIES OF GB (SARIN)
Although all three agents are considered semivolatile liquids, GB has a markedly higher vapor pressure (2.9 mm Hg at 25ºC) and will volatilize, leading to the conclusion that any residual GB would have been depleted by volatilization by the time facility destruction occurs (Reutter, 1999). Under normal environmental conditions, it also undergoes rapid hydrolysis, forming non-toxic products isopropyl methylphosphonic acid and fluoride (Kingery and Allen, 1995). GB can permeate into polymeric or porous materials, and there has been a report of unhydrolyzed GB in paint in an Iraqi shell fragment several years after exposure to the atmosphere (Black et al., 1994). The small residual levels of GB detected in this example suggest, however, that the potential exposure to residual GB after permeation into a polymeric or porous surface is likely minimal. In soil samples collected during the same Iraqi sampling campaign, intact GB was not detected (Black et al., 1994). Because GB is volatile and diffuses fairly rapidly, materials containing occluded spaces would be expected to release GB during the years between exposure and demolition. On the basis of these considerations, GB is considered to be a relatively nonpersistent agent.
PROPERTIES OF VX
VX has a much lower vapor pressure compared to GB (only 7 × 10–4 mm Hg at 25ºC) (Reutter, 1999), and exhaustive depletion due to volatilization from occluded spaces in porous or permeable surfaces may not occur. In situations where VX fills cracks or diffuses into permeable materials, volatilization will be inhibited, but subsequent disturbances of the system could expose intact VX, resulting in a potential exposure scenario resulting from volatilization or more likely from direct dermal contact.
For the most part, hydrolysis of VX results in detoxification. VX can be detoxified rapidly (rate constant on the order of 0.1 day–1) via hydrolysis reactions; however, not all hydrolysis reactions detoxify VX (Davisson et al., 2005; Love et al., 2004).1 The compound undergoes hydrolytic degradation via three pathways, involving cleavage of the P-S, S-C, and P-O bonds (Epstein et al., 1973; Munro et al., 1999). The principal pathway is cleavage of the P-S bond,
which forms ethyl methylphosphonic acid (EMPA) and 2-(diisopropylamino) ethane thiol (DESH), which are both relatively nontoxic (Kingery and Allen, 1995; Munro et al., 1999). Cleavage of the S-C bond is a less prevalent process, and it also produces relatively nontoxic products. Basic sites such as those found on concrete have been shown to greatly increase the rates of hydrolysis via P-S and S-C cleavage (Groenewold et al., 2002; Williams et al., 2005).
VX hydrolysis via P-O cleavage is a matter of concern because this furnishes S-[2-(diisopropylamino)ethyl] methylphosphonothioic acid and ethanol (Yang et al., 1990). The former product, known as EA-2192, retains much of the neurotoxicity of the intact agent, and hence presence of this compound is an ongoing source of concern. In fact, the state of Utah requires measurement of EA-2192 to ensure detoxification to closure standards (see Chapter 5). Concerns related to EA-2192 are reasonably mitigated, however, by the following considerations:
EA-2192 has no volatility and poses no inhalation hazard.
EA-2192 does not diffuse through the skin barrier.
Hydrolysis of the EA-2192 proceeds fairly rapidly, with a rate constant on the order of that of the parent compound (0.1 day–1) (Kaaijk and Frijlink, 1977; Verweij and Boter, 1976).
With regard to occluded spaces and permeable polymers, it should be noted that there may be potential for survival of intact VX sequestered in these environments. This may occur because VX thus sequestered may be protected from hydrolysis.
PROPERTIES OF H (MUSTARD AGENT)
Bis-(2-chloroethyl)sulfide, or sulfur mustard, can refer to H, HD (distilled mustard), or HT (distilled mustard mixed with bis-(2-(2-chloroethylthio)ethyl) ether) in the context of this report. H is relatively involatile, with a vapor pressure of 9 × 10–2 mm Hg at 25ºC (Reutter, 1999). Thus H, in any form, would be expected to display some persistence.
Mustard is detoxified by hydrolysis, but in general, rates of mustard hydrolysis are slower than those of the nerve agents. Nevertheless, hydrolysis would be expected to result in depletion of mustard under most situations if enough time passes between the end of operations and demolition. Chemical decontamination will accelerate rates of hydrolysis.
The generally slower rates of hydrolysis and low volatility serve to make the compound susceptible to surviving for extended periods of time in occluded spaces. This phenomenon is exacerbated by H polymerization reactions that can form a “skin” (Yang et al., 1988) over the surface of intact mustard. The skin can protect the underlying agent from exposure to water and other naturally occurring hydrolysis reagents. Rupture of the skin during scabbling2 or other demolition activities could release mustard and result in a toxic exposure risk. In addition to occupying pores, mustard will also permeate many polymeric materials, and it can be released later either as a result of demolition activities or by heating the polymer.
Black, R., R. Clarke, R. Read, and M. Reid. 1994. Application of Gas Chromatography-Mass Spectrometry and Gas Chromatography-Tandem Mass Spectrometry to the Analysis of Chemical Warfare Samples, Found to Contain Residues of the Nerve Agent Sarin, Sulphur Mustard and Their Degradation Products. Journal of Chromatography A 662(2): 301-321.
Davisson, M., A. Love, A. Vance, and J. Reynolds. 2005. UCRL-TR-209748 Environmental Fate of Organophosphorus Compounds Related to Chemical Weapons. Livermore, CA: Lawrence Livermore National Laboratory.
Epstein J., J. Callahan, and V. Bauer. 1973. The Kinetics and Mechanisms of Hydrolysis of Phosphonothiolates in Dilute Aqueous Solutions. Phosphorus 4: 157-163.
Groenewold, G., J. Williams, A. Appelhans, G. Gresham, J. Olson, M. Jeffery, and B. Rowland. 2002. Hydrolysis of VX on Concrete: Rate of Degradation by Direct Surface Interrogation Using an Ion Trap Secondary Ion Mass Spectrometer. Environmental Science & Technology 36(22): 4790-4794.
Kaaijk, J., and C. Frijlink. 1977. Degradation of S-2-di-isopropylaminoethyl O-ethyl methylphosphonothioate in Soil. Sulphur-Containing Products. Pesticide Science 8(5): 510-514.
Kingery, A., and H. Allen. 1995. The Environmental Fate of Organophosphorus Nerve Agents: A Review. Toxicological & Environmental Chemistry 47(3): 155-184.
Love, A., A. Vance, J. Reynolds, and M. Davisson. 2004. Investigating the Affinities and Persistence of VX Nerve Agent in Environmental Matrices. Chemosphere 57(10): 1257-1264.
Munro, N., S. Talmage, G. Griffin, L. Waters, A. Watson, J. King, and V. Hauschild. 1999. The Sources, Fate, and Toxicity of Chemical Warfare Agent Degradation Products. Environmental Health Perspectives 107(12): 933-974.
Reutter, S. 1999. Hazards of Chemical Weapons Release during War: New Perspectives. Environmental Health Perspectives 107(12): 985-990.
Verweij, A., and H. Boter. 1976. Degradation of S-2-Di-isopropylaminoethyl O-ethyl Methylphosphonothioate in Soil: Phosphorus-Containing Products. Pesticide Science 7(4): 355-362.
Williams, J., B. Rowland, M. Jeffery, G. Groenewold, A. Appelhans, G. Gresham, and J. Olson. 2005. Degradation Kinetics of VX on Concrete by Secondary Ion Mass Spectrometry. Langmuir 21(6): 2386-2390.
Yang, Y.-C., L. Szafraniec, W. Beaudry, and D. Rohrbaugh. 1990. Oxidative Detoxification of Phosphonothiolates. Journal of the American Chemical Society 112(18): 6621-6627.
Yang, Y.-C., L. Szafraniec, W. Beaudry, and J. Ward. 1988. Kinetics and Mechanism of the Hydrolysis of 2-chloroethyl Sulfides. Journal of Organic Chemistry 53(14): 3293-3297.