Tabun

CAS RN:77-81-6

Environmental Fate

TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 39(SRC), determined from a log Kow of 0.38(2) and a regression-derived equation(3), indicates that tabun is expected to have very high mobility in soil(SRC). Volatilization of tabun from moist soil surfaces is not expected to be an important fate process(SRC) given an estimated Henry's Law constant of 1.5X10-7 atm-cu m/mole(SRC), derived from its vapor pressure, 0.07 mm Hg(4), and water solubility, 9.8X10+4 mg/L(5). Hydrolysis in moist soil will attenuate the importance of volatilization under wet conditions(SRC). Persistence depends on moisture and weather conditions. Liquid tabun may persist for 1 to 2 days under average weather conditions and it evaporates about 20 times more slowly than water(2). In a Norwegian study, tabun was found to volatilize from snow slowly; tabun was present 2 weeks after being sprayed on snow, but was not measurable after 4 weeks(2). In another field study, the disappearance of tabun applied to snow found that <0.1% remained after 14 days(6). Tabun hydrolyzes in water at neutral or slightly acid pH and more rapidly under strong acid or alkaline conditions(7). Field studies measuring the evaporation rate of tabun at ambient temperatures from soil found that 50% of applied tabun evaporated in 1.71 hr and that 90% evaporated in 4.66 hr(8). Several species of bacteria, such as Alteromonas sp. and Flavobacterium sp. are capable of degrading G-type agents(9), but data are not available to assess the environmental importance of biodegradation(SRC).
AQUATIC FATE: Based on a classification scheme(1), an estimated Koc value of 39(SRC), determined from a log Kow of 0.38(2) and a regression-derived equation(3), indicates that tabun is not expected to adsorb to suspended solids and sediment(SRC). Tabun hydrolyzes in water at neutral or slightly acid pH and more rapidly under strong acid or alkaline conditions(2). The rate of hydrolysis also increases with increasing temperature(2). At neutral pH and 25 deg C, tabun persists in water for 14 to 28 hours; the half-life at 20 deg C and pH 7.4 is approximately 8 hours. The half-life in seawater at 20 deg C is shorter (4.5 hours) than in freshwater(2). Volatilization from water surfaces is not expected(4) based upon an estimated Henry's Law constant of 1.5X10-7 atm-cu m/mole(SRC), derived from its vapor pressure, 0.07 mm Hg(2), and water solubility, 9.8X10+4 mg/L(5) and tabun's susceptibility to hydrolysis. According to a classification scheme(6), an estimated BCF of 3(SRC), from its log Kow(2) and a regression-derived equation(3), suggests the potential for bioconcentration in aquatic organisms is low(SRC). Several species of bacteria, such as Alteromonas sp. and Flavobacterium sp. are capable of degrading G-type agents(7), but data are not available to assess the environmental importance of biodegradation(SRC).
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), tabun, which has a vapor pressure of 0.07 mm Hg at 25 deg C(2), is expected to exist solely as a vapor in the ambient atmosphere. Vapor-phase tabun 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 8 hours(SRC), calculated from its rate constant of 4.9X10-11 cu cm/molecule-sec at 25 deg C(SRC) that was derived using a structure estimation method(3). The UV absorption spectra of tabun in cyclohexane solution does not exhibit any absorption above 295 nm(4); therefore, tabun is not expected to be susceptible to direct photolysis by sunlight(SRC).
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