Soman

CAS RN: 96-64-0

Volatilization

The Henry's Law constant for soman is estimated as 4.7X10-6 atm-cu m/mole(SRC) derived from its vapor pressure, 0.41 mm Hg at 25 deg C(1), and water solubility, 2.1X10+4 mg/L(2). This Henry's Law constant indicates that soman is expected to volatilize from water surfaces(3). Based on this Henry's Law constant, the volatilization half-life from a model river (1 m deep, flowing 1 m/sec, wind velocity of 3 m/sec)(3) is estimated as 11 days(SRC). The volatilization half-life from a model lake (1 m deep, flowing 0.05 m/sec, wind velocity of 0.5 m/sec)(3) is estimated as 82 days(SRC). Soman's estimated Henry's Law constant indicates that volatilization from moist soil surfaces may occur(SRC). Laboratory and field tests have demonstrated that soman will evaporate from soil surfaces(4); soman evaporates at about one-fourth the rate of water(4). In a study where volatilization of alkyl methylphosphonates were estimated using models based upon surface deposition and contamination of bulk soil, it was determined that volatilization of soman may be substanital relative to its rate of hydrolysis(5). Using a surface deposition model, soman is expected to volatilize with a half-life of about 7.7 hours(6). Volatilization rates calculated using a bulk soil model were three orders of magnitude slower than those obtained using the surface deposition model(6). In cold environments, vaporization may be a controlling process. In a Norwegian study, soman was found to be removed from snow slowly, giving the appearance that volatilization was the cause of the loss of agent(5). After 48 days, soman was present in uncovered snow in concentrations less than 0.1 percent of 1 mg, and in covered snow in concentrations ranging from 0.1 to 10 percent of 1 mg(7). Field and laboratory tests have shown that soman absorbed to soil can be volatilized to the atmosphere (off-gassed) via rain events(4,8); the degree of volatilization varies with soil type, moisture content and length of time adsorbed, but this volatilization is an important fate process(4,8).
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