| Volatilization | The Henry's Law constant for methyl bromide is 7.34X10-3 atm-cu m/mole(1). This Henry's Law constant indicates that methyl bromide is expected to volatilize rapidly from water surfaces(2). 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)(2) is estimated as 3.0 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)(2) is estimated as 3.9 days(SRC). A volatilization half-life of 72 min for methyl bromide was obtained by an experiment in which 100 ppm of methyl bromide was placed in a 39.4 cm aeration cylinder and air bubbled through for 4 hr(3). The mass transfer coefficient of methyl bromide is 22.56 cm/hour at 25 deg C(3), and this value was used to calculate a volatilization half-life of 3.1 hr for methyl bromide from 1 m of water(2). Methyl bromide's Henry's Law constant indicates that volatilization from moist soil surfaces may occur(SRC). The volatility of methyl bromide from treated soils (initial concentration 2,733 ug/g soil; soil moisture = -33 kPa) incubated at 15 deg C, 25 deg C, and 35 deg C were 32%, 35%, and 54%, respectively(4); more than 86% of the total methyl bromide flux occurred within 3 hrs at all three temperatures(4). The volatility increased with increasing soil moisture(4). A measured 4%, 35%, and 65% of the applied methyl bromide volatilized from soil samples with -300 kPa, -33 kPa, and -3 kPa soil moisture content, respectively(4). Volatilization of methyl bromide from dry soil surfaces may occur(SRC) based upon a vapor pressure of 1620 mm Hg(5). Using a soil screening model, the half-lives for the volatilization of methyl bromide from 1 and 10 cm were estimated to be 0.2 and 0.5 days, respectively(6).
Literature: (1) Yates SR, Gan J; J Agric Food Chem 46: 755-61 (1998) (2) Lyman WJ et al; Handbook of Chemical Property Estimation Methods. Washington, DC: Amer Chem Soc pp. 15-1 to 15-29 (1990) (3) Nelly WB; Predicting the Flux of Organics Across the Air/Water Interface. Control Hazard Materials Spills Proc Nat Conf 3rd, pp. 197-200 (1976) (4) Rice PJ et al; Environ Toxicol Chem 15: 1723-9 (1996) (5) Daubert TE, Danner RP; Physical and Thermodynamic Properties of Pure Chemicals: Data Compilation. Design Inst Phys Prop Data, Amer Inst Chem Eng New York, NY: Hemisphere Pub Corp 5 Vol (1989) (6) Jury WA et al; J Environ Qual 13: 573-9 (1984) |
| Soil Adsorption | Reported Koc values for methyl bromide range from 9 to 22(1). According to a classification scheme(2), these Koc values suggest that methyl bromide is expected to have very high mobility in soil(SRC). The adsorption coefficient, Kd, for methyl bromide was below measurable for Greenfield sandy loam, Linne clay loam, and Carsetas loamy sand soils; Kd was equal to 0.2 for potting mix soil(3).
Literature: (1) USDA; ARS Pesticide Properties Database on Methyl Bromide (74-83-9). Available from, as of Sept 26, 2012: http://www.ars.usda.gov/Services/docs.htm?docid=14147 (2) Swann RL et al; Res Rev 85: 17-28 (1983) (3) Gan J, Yates SR; J Agric Food Chem 44: 4001-8 (1996) |
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