n-Octane

CAS RN: 111-65-9

Environmental Fate

TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 3.1X10+4(SRC), determined from a log Kow of 5.18(2) and a regression-derived equation(3), indicates that n-octane is expected to be immobile in soil(SRC). Volatilization of n-octane from moist soil surfaces is expected to be an important fate process(SRC) given an estimated Henry's Law constant of 3.21 atm-cu m/mole(SRC), derived from its vapor pressure, 14.1 mm Hg(4), and water solubility, 0.66 mg/L(5). However, adsorption to soil is expected to attenuate volatilization(SRC). n-Octane is expected to volatilize from dry soil surfaces(SRC) based upon its vapor pressure(4). n-Octane, present at one mg/mL silt loam soil suspension, exhibited average Theoretical biological oxygen demands of 13, 58, 70 and 69% after 2, 5, 10 and 20 days, respectively(6), suggesting that biodegradation may be an important environmental fate process in soil(SRC).
AQUATIC FATE: Based on a classification scheme(1), an estimated Koc value of 3.1X10+4(SRC), determined from a log Kow of 5.18(2) and a regression-derived equation(3), indicates that n-octane is expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is expected(4) based upon an estimated Henry's Law constant of 3.2 atm-cu m/mole(SRC), derived from its vapor pressure, 41.1 mm Hg(5), and water solubility, 0.66 mg/L(6). Using this Henry's Law constant and an estimation method(4), volatilization half-lives for a model river and model lake are 3 hrs and 4 days, respectively(SRC). However, volatilization from water surfaces is expected to be attenuated by adsorption to suspended solids and sediment in the water column(SRC). The estimated volatilization half-life from a model pond is 11 months if adsorption is considered(7). According to a classification scheme(8), an estimated BCF of 1200(SRC), from its log Kow(2) and a regression-derived equation(3), suggests the potential for bioconcentration in aquatic organisms is very high(SRC). When evaporation rates are low, biodegradation of n-octane under aerobic conditions may be important in water(SRC). For example, a 49% loss of n-octane occurred within 5 days and completely disappeared within 15 days when 1 mL of crude oil was added to a 100 mL simulated seawater solution inoculated with sediment samples from Fukae of Kobe harbor, Japan and incubated at 20 deg C(9). Although complete recovery was reported for the control samples, no account was made of volatilization losses(SRC). In a similar study using a jet fuel mixture and freshwater at 25 deg C, a 99% loss of n-octane in sample controls was attributed to evaporation(10).
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), n-octane, which has a vapor pressure of 14.1 mm Hg at 25 deg C(2), is expected to exist solely as a vapor in the ambient atmosphere. Vapor-phase n-octane 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 44 hrs(SRC), calculated from its rate constant of 8.68X10-12 cu cm/molecule-sec at 25 deg C(3). Experimental data also showed that 33.2% of the n-octane fraction in a dark chamber reacted with nitrate radicals to form the corresponding alkyl nitrate(4,5), suggesting nighttime reactions with nitrate radicals may contribute to the atmospheric transformation of n-octane, especially in urban environments(SRC). n-Octane does not contain chromophores that absorb at wavelengths >290 nm(6) and, therefore, is not expected to be susceptible to direct photolysis by sunlight(SRC).
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