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Environmental fate & pathways

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Acetonitrile is a volatile liquid with a vapour pressure of 9,864 Pa at 25ºC and rapid volatilization to the atmosphere is expected when acetonitrile is released to the environment.

Henry's Law constants have been reported for acetonitrile ranging from 2.07 - 3.5 Pa-m3/mole (Snider and Dawson, 1985; Hine and Mookerjee, 1975; Gaffney, 1987 as cited in HSDB, 2009). These values indicate that volatilization of acetonitrile from surface waters and moist soils is likely to be significant (Howard, 1993).

US EPA HENRYWIN software (v3.10) calculated a Henry's Law Constant of 7.902E-5 atm-m3/mole (8.0 Pa-m3/mole) for acetonitrile based on vapor pressure of 200 mmHg and water solubility of 1.37E5 mg/L. Based on this value, volatilization half-lives for a model river and model lake were estimated to be 18 hours and 7 days, respectively.

The removal of acetonitrile from the atmosphere by precipitation has been quoted in several reports as one of the main fate processes for this compound, as important as the photo-oxidation reaction with hydroxyl radicals (Hamm et al., 1989; Hamm and Warneck, 1990). A residence time of about 3 years has been estimated for acetonitrile in the troposphere due to wet deposition (Hamm et al., 1984; Arijs and Brasseuur, 1986).

An overall residence time for acetonitrile near 1.5 years was calculated considering the two principal removal processes, precipitation and reaction with hydroxyl radicals (Hamm and Warneck, 1990; Arijs and Brasseur, 1986). However, a shorter residence time was calculated based on the tropospheric mass balance of this compound (Hamm and Warneck, 1990). They estimated a mass content (G) in the troposphere of 370-570 x 103 tonnes acetonitrile and a global source strength (Q) of 600-1,100 x 103 tonnes acetonitrile / year, which would yield an overall residence time, calculated as G/Q, of 0.23-0.95 years. The dissolution in the ocean was proposed as an effective process for acetonitrile removal, that would take up 65% of the total acetonitrile emission, and the combined residence time for the process (precipitation, hydroxyl radical reaction and ocean dissolution) was estimated to be of 0.45 years, which is in agreement with the mass balance estimate.

The adsorption coefficient, Koc, can be estimated for acetonitrile using the octanol-water partition coefficient, log Pow = -0.34 (Leo et al., 1971), and Koc values of 0.3-16 l/kg were obtained using regression equations reported in Lyman et al (1982).EPA PCKOC software (v1.66) generated an estimated Koc of 4.5 for acetonitrile. These Koc values indicate a low potential for adsorption to soils; given the high water solubility of acetonitrile leaching to groundwater may be important when spilled on soil (Howard, 1993).

US EPA EPIWIN software (v3.20) Level III Fugacity Model predicted the following percent distribution and half-lives for acetonitrile in environmental media:

Air - 13%, 1544 hours

Water - 44.1%, 360 hours

Soil - 42.8%, 720 hours

Sediment - 0.0811%, 3240 hours