Sarin

CAS RN: 107-44-8

Environmental Fate

TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 16(SRC), determined from a log Kow of 0.30(2) and a regression-derived equation(3), indicates that sarin is expected to have very high mobility in soil(SRC). The importance of leaching in moist soil will be attenuated(SRC) due to sarin's ability to hydrolyze in water(4). Volatilization of sarin from both dry and moist soil surfaces is expected to be an important fate process(SRC) based on results of field and laboratory studies(4). Biodegradation data for sarin were not available(SRC, 2013). Sarin is considered nonpersistent in the environment since it is volatile, soluble in water, and subject to hydrolysis(4).
TERRESTRIAL FATE: Two soil persistence studies conducted by the US Army found that 90% or more of sarin added to soil will be lost in the first five days; one study found that the disappearance rate increased with increases in soil moisture; in humus and loam soils (12.8-36.8% moisture content, pH 6.5), 100% disappearance occurred in 24 hr(1). The persistence times of sarin droplets applied to soil (dry to wet conditions) at 25 deg C ranged from 7 to 14 hrs(2). Field studies measuring the disappearance of sarin applied to snow surfaces found that <0.1% remained after 14 days on uncovered snow; persistence times increased when the surface applications were covered by snowfall thereby decreasing the evaporation rates(3).
AQUATIC FATE: Based on a classification scheme(1), an estimated Koc value of 16(SRC), determined from a log Kow of 0.30(2) and a regression-derived equation(3), indicates that sarin is not expected to adsorb to suspended solids and sediment(SRC). Aqueous hydrolysis is expected to be the major degradation process for sarin in water forming the products HF and isopropyl methylphosphonic acid(4). The hydrolysis is pH and temperature dependent(4-6); with the fastest rates of hydrolysis occurring below pH 4 and above 6.5(6), the hydrolysis half-life is about 237 hr at 25 deg C(6). Hydrolysis takes place faster in seawater than in distilled water due to the presence of metal cations in seawater(5,6); the hydrolysis half-lives in seawater at 25 deg C and pHs 7.6 and 7.9 are 58.1 and 25 min, respectively(6); in seawater at 25 deg C, the time required to decompose 99.9% of initial sarin is about 8 hr(6); in distilled water, the time required to decompose 99.9% varies from about 750 to 7.5 hr at pHs of 7.0 to 9.0(6). Sarin has been shown to volatilize slowly from water(5) and its estimated Henry Law constant of 5.7X10-7 atm-cu m/mole(SRC) derived from its vapor pressure, 2.86 mm Hg at 25 deg C(1), and water solubility, and assigned value for water solubility of 1.0X10+6 mg/L (miscible)(7), also indicates that sarin will volatilize slowly from water surfaces(8). Using this Henry's Law constant and an estimation method(8), volatilization half-lives for a model river and model lake are 76 and 555 days, respectively(SRC). Hydrolysis will attenuate the environmental importance of volatilization(SRC). According to a classification scheme(9), an estimated BCF of 3(SRC), from its log Kow of 0.30(10) and a regression-derived equation(3), suggests the potential for bioconcentration in aquatic organisms is low(SRC). Biodegradation data for sarin were not available(SRC, 2013). Sarin is considered nonpersistent in the environment since it is volatile, soluble in water, and subject to hydrolysis(4).
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), sarin, which has a vapor pressure of 2.86 mm Hg at 25 deg C(2), is expected to exist solely as a vapor in the ambient atmosphere. Vapor-phase sarin 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 9.6 hours(SRC), calculated from its rate constant of 4.0X10-11 cu cm/molecule-sec at 25 deg C(SRC) that was derived using a structure estimation method(3). The UV absorption spectrum of sarin in cyclohexane solution does not exhibit any absorption above 290 nm(4); therefore, sarin is not expected to degrade through direct photolysis in the atmosphere(SRC).
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