Hydrogen cyanide

CAS RN: 74-90-8

Environmental Fate

TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 15(SRC), determined from a log Kow of -0.25(2) and a regression-derived equation(3), indicates that hydrogen cyanide is expected to have very high mobility in soil(SRC). The pKa of hydrogen cyanide is 9.2(4), indicating that this compound will exist partially in the anion form in the environment and anions generally do not adsorb more strongly to soils containing organic carbon and clay than their neutral counterparts(5). By analogy to the fate of cyanides in water, it is predicted that the fate in soil would be pH dependent. Cyanide may occur in the form of hydrogen cyanide, alkali metal salts, or immobile metallocyanide complexes. At soil surfaces with pH <9.2, it is expected that volatilization of hydrogen cyanide would be an important loss mechanism for cyanides. In subsurface soil, cyanide present at low concentrations would probably biodegrade(6). Volatilization of hydrogen cyanide from moist soil surfaces is expected to be an important fate process(SRC) given a Henry's Law constant of 1.33X10-4 atm-cu m/mole(7). Hydrogen cyanide is expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 742 mm Hg at 25 deg C(8). Hydrogen cyanide can be biodegraded by acclimated microbial cultures and sludges, but is usually toxic at high concentrations to unacclimated microbial systems(9), indicating that biodegradation may not be an important environmental fate process in soil(SRC).
AQUATIC FATE: Based on a classification scheme(1), an estimated Koc value of 15(SRC), determined from a log Kow of -0.25(2) and a regression-derived equation(3), indicates that hydrogen cyanide is not expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is expected(4) based upon a Henry's Law constant of 1.33X10-4 atm-cu m/mole(5). Using this Henry's Law constant and an estimation method(4), volatilization half-lives for a model river and model lake are 5 hrs and 3 days, respectively(SRC). A pKa of 9.2(6) indicates hydrogen cyanide will exist partially in the anion form at pH values of 5 to 9 and, therefore, volatilization from water surfaces is not expected to be an important fate process(SRC). According to a classification scheme(7), an estimated BCF of 3(SRC), from its log Kow(2) and a regression-derived equation(3), suggests the potential for bioconcentration in aquatic organisms is low(SRC). Hydrogen cyanide can be biodegraded by acclimated microbial cultures and sludges, but is usually toxic at high concentrations to unacclimated microbial systems(8), indicating that biodegradation may not be an important environmental fate process in water(SRC).
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), hydrogen cyanide, which has a vapor pressure of 742 mm Hg at 25 deg C(2), is expected to exist solely as a vapor in the ambient atmosphere. Vapor-phase hydrogen cyanide 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 530 days(SRC), calculated from its rate constant of 3.0X10-14 cu cm/molecule-sec at 25 deg C(3). Hydrogen cyanide does not contain chromophores that absorb at wavelengths >290 nm(3) and, therefore, is not expected to be susceptible to direct photolysis by sunlight(SRC).
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