| Volatilization | The Henry's Law constant for methyl iodide is 0.00526 atm-cu m/mole at 25 deg C(1). This Henry's Law constant indicates that methyl iodide should volatilize 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 1.3 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 4.8 days(SRC). The Henry's Law constant in seawater of salinity 30.4 g dissolved inorganic matter/kg seawater was 0.00354 atm-cu m/mole at 20 deg C(4) indicating a lower volatization rate for methyl iodide in seawater(SRC). Dissipation of methyl iodide from open surface water was found to be primarily a result of volatilization(4). Experiments conducted under indoor conditions resulted in a first-order half-life of 29 hours under static conditions and 6.5 hours when stirred at low speed with a magnetic stirrer(5). After 6 days, less that 1% of the methyl iodide was detected as iodide ion. Methyl iodide's Henry's Law constant(1) indicates that volatilization from moist soil surfaces may occur(SRC). Methyl iodide is expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 405 mm Hg at 25 deg C(3).
Literature: (1) Hunter-Smith RJ et al; Tellus Ser B B35: 170-6 (1983) (2) Lyman WJ et al; Handbook of Chemical Property Estimation Methods. Washington, DC: Amer Chem Soc pp. 15-1 to 15-29 (1990) (3) Boublik T et al; The Vapor Pressures of Pure Substances Amsterdam: Elsevier p. 51 (1984) (4) Moore RM et al; Chemosphere 30: 183- 91 (1995) (5) Gan J, Yates SR; J Agric Food Chem 44: 4001-8 (1996)
Literature: #Experiments were conducted to assess the volatilization loss of methyl iodide, applied at 30 cm, from 60-cm packed soil columns with different soils and under various soil surface conditions(1). In Greenfield sandy loam, the greatest cumulative loss occurred in nontarp applications, 94%, and the least in a high-barrier plastic tarp treatment, 75%. Volatilization losses with a polyethylene film was 90% and therefore this treatment was ineffective at preventing volatilization loss. Volatilization losses using a polyethylene film were significantly lower, 38% and 53%, from two soils high in organic matter and capable of rapidly degrading the chemical.
Literature: (1) Gan J et al; J Environ Qual 26: 1107-15 (1997) |
| Soil Adsorption | Using a structure estimation method based on molecular connectivity indices(1), the Koc for methyl iodide can be estimated to be 14(SRC). According to a classification scheme(2), this estimated Koc value suggests that methyl iodide should have very high mobility in soil. The soil/water distribution coefficient of methyl iodide in various soils were (soil, Kd): Greenfield sandy loam, 0.09; Linne clay loam, 0.15; Carsetas loamy sand, 0.16; and potting mix, 0.55(3).
Literature: (1) Meylan WM et al; Environ Sci Technol 26: 1560-67 (1992) (2) Swann RL et al; Res Rev 85: 17-28 (1983) (3) Gan J, Yates SR; J Agric Food Chem 44: 4001-8 (1996) (4) Gan J et al; J Environ Qual 26: 1107-15 (1997) |
| Vapor Pressure | | Pressure | Reference |
|---|
| 4.05X10+2 mm Hg @ 25 deg C | Boublik, T., Fried, V., and Hala, E., The Vapour Pressures of Pure Substances. Second Revised Edition. Amsterdam: Elsevier, 1984., p. 51 |
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