Degradation kinetics of VX

Abstract

O-ethyl S-(2-diisopropylaminoethyl)phosphonothiolate (VX) is among the most toxic of chemical warfare agents. VX is an oily liquid that is relatively involatile and is slow to hydrolyze, and thus may persist for weeks or longer in the environment, creating long term contamination of the territory. Concern over prolonged risk from VX exposure is exacerbated because it poses a dermal contact hazard -lethal dose on bare skin is about 10 mg/70 kg and it is readily absorbed through the skin. Therefore, a detailed understanding of its volatilization behavior and degradation pathways and rates is necessary. Volatilization has not been considered to be an important depletion mechanism, however, recent studies have shown a significant fractio n of VX may volatilize. VX degradation reactions and their rates have been difficult to measure in many environmental media. For this reason, VX persistence has generally been described in terms of half lives. In this review, rates of VX degradation are compared on the basis of pseudo-first order rate constants in order to p rovide a basis for assessing VX persistence in a given environment. An issue of concern is that one VX degradation pathway produces S-2-(diisopropylaminoethyl) methylphosphonothioic acid (known as EA2192), a degradation product that is almost as toxic as VX. Consequently degradation pathways and rates for EA2192 are also discussed. Among countries with abundant chemical warfare (CW) stockpiles, O-ethyl S-(2-diisopropylaminoe- thyl)phosphonothiolate (VX) is of great interest due to its toxicity (1), slow rates of oxidative or hydrolytic detoxification, and the ability of its degradation products to retain much of the original lethality. Many studies have revealed that volatilization, migration and m ultiple degradation reactions of VX can occur, and thus residual toxicity can be variable. The rates of the d egradation reactions vary significantly, and the period of time between initial release and detoxificatio n by natural means is also variable. The degradation chemistry of VX is widely scattered across open source literature and U. S. Gov- ernment reports. With this information, known degradation pathways are compared and overall trends regarding routes and rates to decontamination are identifie d. The degradation behavior of VX has been previously reviewed by Nancy Munro et al (2,3) and Yu-Chu Yang (4), who compiled extensive descriptions of VX, G agents, and mustard degradation. They catalogued many descriptions of VX degradation that were derived from experiments conducted in the environment and in the laboratory. In many of these studies, it was difficult to effectively identi fy degradation pathways or measure reaction rates in environmental matrices. These studies, many of which are described only in U. S. Army reports, provide good phenomenological descriptions of VX behavior that are based on actual field observations. Another salient review was published in 1995 by Kingery and Allen, which describes past studies of environmental VX degradation that emphasized the behavior of the alkyl methylphosphonic acids (5). Since these reviews, a significant body of work has appeared i n the peer-reviewed literature that has quantified VX degradation as it occurs in contact with a varie ty of mineral oxide and synthetic materials.