Methyl Isocyanate

CAS RN: 624-83-9

Environmental Fate

TERRESTRIAL FATE: If methyl isocyanate is released to soil, it is expected to hydrolyze rapidly based on aqueous hydrolysis half-lives of 20 min at 15 deg C and 9 min at 25 deg C, calculated from measured rate constants of 5.9X10-4 sec-1 and 1.34X10-3 sec-1, respectively(1). Soil adsorption effects, volatilization, and biodegradation are not expected to be competing processes since hydrolysis proceeds so quickly. Methyl isocyanate is expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 348 mm Hg(2).
AQUATIC FATE: If methyl isocyanate is released to water, it is expected to hydrolyze rapidly based on half-lives of 20 min at 15 deg C and 9 min at 25 deg C, calculated from measured hydrolysis rate constants of 5.9X10-4 sec-1 and 1.34X10-3 sec-1, respectively(1). The products of hydrolysis may include N-carboxymethylamine, methylamine, carbon dioxide, and N,N'-dimethylurea(1,2). Bioconcentration, volatilization, adsorption to sediment and suspended solids, and biodegradation are not expected to be important fate processes(SRC) since hydrolysis proceeds so quickly.
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), methyl isocyanate, which has a vapor pressure of 348 mm Hg at 20 deg C(2), is expected to exist solely as a vapor in the ambient atmosphere. Vapor-phase methyl isocyanate 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 118 days(SRC), calculated from its rate constant of 1.4X10-13 cu cm/molecule-sec at 25 deg C(SRC), that was derived using a structure estimation method(3). Methyl isocyanate does not contain chromophores that absorb at wavelengths >290 nm and therefore is not expected to be susceptible to direct photolysis by sunlight(4).
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