The Henry's Law constant for methyl mercaptan is estimated as 0.0031 atm-cu m/mole(SRC) derived from its vapor pressure, 1,510 mm Hg(1), and water solubility, 15,400 mg/L(2). This Henry's Law constant indicates that methyl mercaptan is expected to volatilize rapidly 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 0.8 hours(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 2.8 days(SRC). Methyl mercaptan's Henry's Law constant indicates that volatilization from moist soil surfaces is expected to occur(SRC). Methyl mercaptan is expected to volatilize rapidly from dry soil surfaces based upon its vapor pressure and because it is a gas a temperatures above 6 deg C(SRC). However, gaseous methyl mercaptan gas has been found to strongly adsorb to moist and dry soil surfaces suggesting that adsorption might be an environmental sink for methyl mercaptan(4). Therefore, the importance of volatilization from soil surfaces may be attenuated by adsorption(SRC).
Literature: (1) Daubert TE, Danner RP; Physical and Thermodynamic Properties of Pure Chemicals Data Compilation. Washington, DC: Taylor and Francis (1989) (2) Hine J, Mookerjee PK; J Org Chem 40: 292-8 (1975) (3) Lyman WJ et al; Handbook of Chemical Property Estimation Methods. Washington, DC: Amer Chem Soc pp. 15-1 to 15-29 (1990) (4) Smith KA et al; Soil Sci 116: 313-9 (1973)
Using a structure estimation method based on molecular connectivity indices(1), the Koc of methyl mercaptan can be estimated to be 13(SRC). According to a classification scheme(2), this estimated Koc value suggests that methyl mercaptan is expected to have very high mobility in soil. Gaseous methyl mercaptan has been observed to partition to soils(3). For example, when gaseous methyl mercaptan was passed over six air-dried and moist (50% field capacity) soils, 2.4-32.1 mg/g and 2.2-21.4 mg/g of methyl mercaptan rapidly adsorbed to the dry and moist soils, respectively(3). Neither the capacity or rate of sorption was correlated to soil pH, organic matter content, or clay content; sterile controls ruled out the involvement of microorganisms(3); it was suggested that adsorption to soil surfaces might be an environmental sink for gaseous methyl mercaptan(3).
Literature: (1) US EPA; Estimation Program Interface (EPI) Suite. Ver. 4.1. Jan, 2011. Available from, as of July 19, 2012: http://www.epa.gov/oppt/exposure/pubs/episuitedl.htm (2) Swann RL et al; Res Rev 85: 17-28 (1983) (3) Smith KA et al; Soil Sci 116: 313-9 (1973)