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TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 35(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). Volatilization of sarin from moist soil surfaces is not expected to be an important fate process(SRC) given an estimated Henry's Law constant of 5.7X10-7 atm-cu m/mole(SRC), derived from its vapor pressure, 2.9 mm Hg(4), and assigned value for water solubility of 1.0X10+6 mg/L (miscible)(5). Sarin is expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 2.9 mm Hg(4). Biodegradation data for sarin were not available(SRC, 2007).
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(1); one study found that the disappearance rate increased with increases in soil moisture(1); 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(3); 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 35(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. The hydrolysis is pH and temperature dependent(8,9); with the fastest rates of hydrolysis occurring below pH 4 and above 6.5(9), the hydrolysis half-life is about 237 hr at 25 deg C(9). Hydrolysis takes place faster in seawater than in distilled water due to the presence of metal cations in seawater(8,9); the hydrolysis half-lives in seawater at 25 deg C and pHs 7.6 and 7.9 are 58.1 and 25 min, respectively(9); in seawater at 25 deg C, the time required to decompose 99.9% of initial sarin is about 8 hr(9); 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(9). Volatilization from water surfaces is not expected(3) based upon an estimated Henry's Law constant of 5.7X10-7 atm-cu m/mole(SRC), derived from its vapor pressure, 2.9 mm Hg(4), and assigned value for water solubility of 1.0X10+6 mg/L (miscible)(5). According to a classification scheme(6), an estimated BCF of 3.2(SRC), from an estimated log Kow(2) and a regression-derived equation(7), suggests the potential for bioconcentration in aquatic organisms is low(SRC). Biodegradation data for sarin were not available(SRC, 2007).
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 2.9 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(4). The UV absorption spectrum of sarin in cyclohexane solution do not exhibit any absorption above 290 nm(5); therefore, sarin is not expected to degrade through direct photolysis in the environment(SRC).