| 環境中動態 | TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 13(SRC), determined from a structure estimation method(2), indicates that methyl mercaptan is expected to have very high mobility in soil(SRC). 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). It was suggested that adsorption to soil surfaces might be an environmental sink for gaseous methyl mercaptan(3). Volatilization of methyl mercaptan from moist soil surfaces is expected to be an important fate process(SRC) given an estimated Henry's Law constant of 0.0031 atm-cu m/mole(SRC), based upon its vapor pressure, 1,510 mm Hg at 25 °C(4), and water solubility, 15,400 mg/L(5). 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 °C(SRC). However, the importance of volatilization from soil surfaces may be attenuated by gas-phase adsorption to soil surfaces(SRC). Methyl mercaptan is produced under anaerobic conditions (e.g., in salt marshes) by a variety of organisms and is often liberated during the decay of sulfur-containing organic matter(6). It may be formed by sulfate-reducing and methane-producing bacteria from methionine, or reversibly formed from dimethyl sulfide and dimethyl disulfide(6). Ultimately, methyl mercaptan may be mineralized to methane, carbon dioxide, and hydrogen sulfide(6). The oxidation and consequent dimerization of methyl mercaptan to dimethyl disulfide occurs for soils supplemented with methionine and its sulfoxide, and sulfone or S-methyl cysteine(7). This results in part from the formation of methyl mercaptan and subsequent dimerization to dimethyl disulfide(7).AQUATIC FATE: Based on a classification scheme(1), an estimated Koc value of 13(SRC), determined from a structure estimation method(2) indicates that methyl mercaptan is not expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is expected(3) based upon an estimated Henry's Law constant of 0.0031 atm-cu m/mole(SRC) derived from its vapor pressure, 1,510 mm Hg(4), and water solubility, 15,400 mg/L(5). Using this Henry's Law constant and an estimation method(3), volatilization half-lives for a model river and model lake are 0.8 hours and 2.8 days, respectively(SRC). According to a classification scheme(6), an estimated BCF of 3(SRC), from its estimated log Kow of 0.78(2) and a regression-derived equation(2), suggests the potential for bioconcentration in aquatic organisms is low(SRC). Methyl mercaptan is not expected to undergo hydrolysis in the environment due to the lack of functional groups that hydrolyze under environmental conditions(3). Methyl mercaptan is produced under anaerobic conditions (e.g. in salt marshes) by a variety of organisms and is often liberated during the decay of sulfur-containing organic matter(7). It may be formed by sulfate-reducing and methane-producing bacteria from methionine, or reversibly formed from dimethyl sulfide and dimethyldisulfide(7). Ultimately, methyl mercaptan may be mineralized to methane, carbon dioxide, and hydrogen sulfide(7). When incubated in anaerobic lake sediment from Lake Mendota, WI for 4 hr, methyl mercaptan was rapidly mineralized(8).ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), methyl mercaptan, which has a vapor pressure of 1510 mm Hg at 25 °C(2) is expected to exist solely as a vapor/gas in the ambient atmosphere. Methyl mercaptan has a normal boiling point of 5.95 °C(2), so at temperatures above 5.95 °C, methyl mercaptan will exist as a gas in the atmosphere(SRC). Gas-phase methyl mercaptan 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 11.7 hours(SRC), calculated from its rate constant of 3.29X10-11 cu cm/molecule-sec at 25 °C(3). The half-life for the reaction of methyl mercaptan with ozone in the atmosphere has been reported to be 140 hr(4). The rate constant for the gas-phase reaction of methyl mercaptan with nitrate radicals that exist in the night-time atmosphere ranged from 8.1X10-13 to 12.2X10-13 cu cm/molecule-sec(5)this corresponds to an average atmospheric half-life of about 1 hour(SRC) at an atmospheric concentration of 2.4X10+8 nitrate radicals per cu cm(5). The UV spectrum of methyl mercaptan shows a log epsilon of 0.7 at 290 nm(6) suggesting that methyl mercaptan may be susceptible to direct photolysis by sunlight(SRC). |
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