VX

CAS RN:50782-69-9

Environmental Fate

TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 187(SRC), determined from a structure estimation method(2), indicates that VX is expected to have moderate mobility in soil(SRC). The pKa of VX is 9.12(3), indicating that this compound will partially exist in the cation form in the environment and cations generally adsorb more strongly to soils containing organic carbon and clay than their neutral counterparts(4). Tight adsorption to dry soil(5) and rapid adsorption to sand(6) have been observed. Simulated rainfall events were shown to desorb the VX from dry soil resulting in volatilization to the atmosphere(6). Volatilization of VX from moist soil surfaces where VX is dissolved in water may not be an important fate process(SRC) given an estimated Henry's Law constant of 1.0X10-8 atm-cu m/mole(SRC), derived from its vapor pressure, 8.78X10-4 mm Hg(7), and water solubility, 3.0X10+4 mg/L(8). VX does volatilize from dry surfaces, however, evaporation of VX from solid surfaces is a relatively slow process compared to other nerve agents such as GD or HD(6); the time for complete evaporation of VX was 6 days for a 0.5 micro-liter drop at 30 deg C on a glass surface(6). Laboratory and field studies indicate the disappearance of nerve agents from soil results from a combination of processes including evaporation, hydrolysis and microbial degradation(9). VX is susceptible to aqueous hydrolysis with rates varying with pH, temperature, presence of natural species (such as fulvic acid), and other factors(9,10); therefore, hydrolysis in moist soil is expected to be an important fate process(SRC). Field and closed-container studies indicate that approximately 90% of VX is lost from soil in 15 days(9); however, hydrolysis rates can vary markedly depending on conditions and requires further study(10).
TERRESTRIAL FATE: VX may be deposited on land as a result of a spill or as a result of its use in chemical warfare. When disseminated over bare or vegetated terrain, water may be quite limited and may be largely derived from the atmosphere; hydrolysis would therefore depend on the humidity and the diffusion of water vapor into an agent droplet. Deposition in cold regions presents a special problem since degradative processes are much slower and evaporative losses less. When disseminated over snow and ice, VX should spread on the surface. If freezing takes place after the agent is disseminated, the agent will be distributed in ice, segregated into concentrated pockets, or concentrated along grain boundaries depending upon the rate of cooling. The distribution and concentration of the agent may affect the hydrolysis rate(1).
TERRESTRIAL FATE: VX is moderately persistent on bare ground, remaining at high concentrations for about 2-6 days. The persistence of VX in soil is a function of temperature, organic carbon content of the soil, and possibly moisture content of the soil. In oven-dried soil or in soil where the temperature is below freezing, degradation is very slow(1). Little agent is lost through evaporation probably due to its low vapor pressure and Henry's Law constant. On vegetation, it evaporates in 24 to 48 hr(2). When VX was applied to humic sand, humic loam, and clayey peat in laboratory studies, it was rapidly hydrolyzed into ethyl hydrogen methylphosphonate that was slowly converted to methylphosphonic acid(3). After 1 day, about 3% of the applied VX remained in the humic loam and clayey peat and 20% remained in the humic sand. The remainder disappeared at a much slower rate and after 3 weeks only 0.1% was detected. In addition to degradation, it is thought that chemisorption may have contributed to the rapid initial disappearance of VX in the humic loam and clayey peat(SRC). VX was observed to have a high affinity for activated charcoal, moderate affinity for montmorillonite /(OH)4Si8Al4O20.nH2O/, and little affinity for goethite /hydrated iron xoide: FeO(OH)/(4). The affinity for goethite increased with the addition of fulvic acid, but did not increase its affinity to montmorillonite(4). Although dry goethite degraded VX in methylene chloride, wet goethite was observed to block electron interactions and reduce degradation in soil(4).
TERRESTRIAL FATE: Investigators studying the loss of VX (through radioisotope studies using 32S) in humic sand found that about 10% remained after 3 days; afterwards the rate loss was much less(1). The rate of VX application was 200 ppm. Eight days after treatment, the percentage of 32S remaining in humic sand, humic loam, and clayey peat soil as the degradation products, 2-(diisopropylamino)ethanethiol and/or N,N'-dithiodiethylene-bis(diisopropylamine), was 30%, 65%, and 77%, respectively(1). A laboratory study was made of the behavior of VX (1% concentration) in a sandy loam soil having a low clay, but high CEC content and a pH of 6.5 with moisture content of 0.045%, 20% and 50%(2). The results show a relatively rapid initial decomposition of VX to a roughly 5% of the initial concentration in about 10 days, followed by a much slower decline. The latter decay was similar for the moist soils, but much slower for the dry soil. The half-life, which was little affected by soil moisture, was approximately 2 days. The decomposition products were the disulfide and other organophosphorous compounds(SRC).
TERRESTRIAL FATE: Studies were performed in which the anticholinesterase activity of 0.01% VX in 9 soils of widely different properties was monitored. After the VX was introduced, the soil samples stood at room temperature in closed containers for varying intervals, after which they were extracted and the activity measured. In all cases, the activity declined rapidly; more than half the activity was gone in a day and in most cases only 10% of the activity remained after 1 wk of contact. No correlation was evident between soil characteristics and loss of activity. Since hydrolytic products had anticholinesterase activity, additional experiments were performed in an attempt to sort out the reaction sequences. The initial activity (1st week) was thought to be due to VX and diethyl dimethylpyrophosphonate, the latter being a product of the reaction of ethyl methylphosphonic acid, the hydrolytic product, with VX. In dry soil, the pyrophosphonate may be formed from the phosphonic acid through dehydration. The thiol or disulfide may also contribute to the anticholinesterase activity for about a month while the contribution by VX declines to negligible proportions(1). The fact that degradation is more rapid in soil than in water suggests that soil components catalyze the hydrolysis(SRC).
TERRESTRIAL FATE: In a field experiment designed to simulate a chemical attack under arctic conditions, a small sample of VX was placed on top of the snow surface; another sample was immediately covered with 5 cm of snow in order to simulate a snowfall after an attack. Between 0.1 and 10% of the sample was found 14 and 28 days after application. None of the agents studied, including VX, tended to migrate into the snow(1).
AQUATIC FATE: Based on a classification scheme(1), an estimated Koc value of 187(SRC), determined from a structure estimation method(2), indicates that VX is expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is not expected(3) based upon an estimated Henry's Law constant of 1.0X10-8 atm-cu m/mole(SRC), derived from its vapor pressure, 8.78X10-4 mm Hg(4), and water solubility, 3.0X10+4 mg/L(5). According to a classification scheme(6), an estimated BCF of 11(SRC), from its log Kow of 2.09(7) and a regression-derived equation(2), suggests the potential for bioconcentration in aquatic organisms is low(SRC). Hydrolysis half-lives of 100 days at pH 6.5 and 46 hrs at pH 10 have been reported(5). The major product, S-(2-diisopropylamino-ethyl)methylphosphonothioic acid, is almost as toxic as VX itself and is more stable toward hydrolysis(5). Hydrolysis half-lives in water at 25 deg C and pH 7 range from 17 to 42 days(7). Hydrolysis rates vary with pH(7,8); at pH 5, the half-life is on the order of 100 days, whereas at pH 8 the half-life is roughly 9 days(8). Batch hydrolysis experiments demonstrated an increasing hydrolysis rate as pH increased, but also indicated that dissolved aqueous constituents (e.g. fulvic acids, carbonates, clays) can cause major differences in absolute hydrolysis rates(8); for example, addition of natural dissolved material at pH 7 increased the hydrolysis rate by a factor of 2(8). Hydrolysis rates in environmental systems where natural dissolved species are present can depend on pH, species and concentration, and other factors(8). In seawater where the pH is slightly alkaline, the half-life of VX was approximately 5-14 days at 25 deg C and may be several years at 4 deg C(9). Sensitized photooxidation may occur in natural water exposed to sunlight and has been suggested as a means of decontaminating VX(9).
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), VX, which has a vapor pressure of 8.78X10-4 mm Hg at 25 deg(2), is expected to exist solely as a vapor in the ambient atmosphere at this temperature. Below 7 deg C (45 deg F), VX's vapor pressure is less than 1X10-4 mm Hg, where it can exist in both the vapor and particulate phases(SRC). Vapor-phase VX is degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals(SRC); the half-life for this reaction in air is estimated to be 2.5 hours(SRC), calculated from an estimated rate constant of 1.6X10-10 cu cm/molecule-sec at 25 deg C(SRC) that was derived using a structure estimation method(3). Particulate-phase VX may be removed from the air by wet and dry deposition(SRC). VX does not absorb UV radiation above 290 nm(4) and, therefore, is not expected to be susceptible to direct photolysis by sunlight(SRC).
ATMOSPHERIC FATE: When used as a chemical agent, VX will be disseminated using aerial sprays or munitions and will initially be in the form of droplets. These droplets would be subject to gravitational settling; VX from resulting deposits may reenter the atmosphere by evaporation; however, the vapor pressure of VX is very low and the time for 90% of a 1 mm drop to volatilize is 45 days at 10 deg C, and somewhat over a day at 37 deg C. Computer modeling indicates that under neutral atmospheric stability conditions, an effective release height of 30 m and a 30 m/sec wind speed, maximum deposition will occur at approximately 320, 500, and 700 m downwind for 100, 60 and 40 micron diameter particles(1). Significant deposits will occur at downwind distance of 2000 m and more(1).
Find more information on this substance at: Hazardous Substances Data Bank , TOXNET , PubMed