Parathion

CAS RN:56-38-2

Environmental Fate

Aquatic fate: ... Parathion ... Shown to be 2-3 times more persistent than methyl parathion in natural water systems.
Terrestrial fate: persistence of parathion was partially dependent on soil type. In some soils degradation was rapid & probably through combination of hydrolysis & strong microbial activity. In other soils ... Loss was slow & attributable to hydrolysis.
TERRESTRIAL FATE: Based on a classification scheme(1), Koc values ranging from 314 to 15,860(2) indicate that parathion is expected to have moderate to no mobility in soil(SRC). Volatilization of parathion from moist soil surfaces is not expected to be an important fate process(SRC) given a Henry's Law constant of 2.98X10-7 atm-cu m/mole(4). Parathion is not expected to volatilize from dry soil surfaces based upon a vapor pressure of 6.68X10-6 mm Hg at 20 deg C(4). The half-life for photodecomposition of parathion on 3 soils with various water contents ranged from 31 to 70 hr(5). After 8 weeks of incubation in an organic and a mineral soil, <2 and 6%, respectively, of the 1 ppm parathion applied remained; in sterilized controls 80 and 95% remained(6). Prior exposure of soils to p-nitrophenol resulted in increased mineralization of parathion to carbon dioxide(7). Metabolic pathways involve both oxidative and reductive reactions(8). The primary oxidative pathway involves an initial hydrolysis to p-nitrophenol and diethylthiophosphoric acid; a second oxidative pathway involves oxidation to paraoxon(8). Under low oxygen conditions reduction to aminoparathion occurs(8).
AQUATIC FATE: Based on a classification scheme(1), sediment Koc values ranging from 3,086 to 38,000(2) indicate that some adsorption of parathion to suspended solids and sediment in the water column is expected(SRC). Parathion is not expected to volatilize from water surfaces(3) based upon a Henry's Law constant of 2.98X10-7 atm-cu m/mole(4). According to a classification scheme(5), BCFs ranging from 63 to 462(6), suggest bioconcentration in aquatic organisms is moderate to high(SRC). Irradiation of parathion for 10 hr in aerated distilled water resulted in 88% degradation(7). 20% of parathion was lost by photolysis in 2 hr in Okeefenokee Swamp water(8). Reported hydrolysis half-lives at 20 deg C at environmentally relevant pHs range from 3 weeks at pH 9(9) to 43 weeks at pH 5(10); the half-life for hydrolysis in sterile sea water has been reported to be approximately 1 yr at 4 deg C(11). It is thought that divalent cations catalyze hydrolysis(11). Parathion biodegrades in acclimated natural waters within several weeks(12,13). Parathion, 5 ppm, completely degraded within 2 weeks in acclimated water from Holland Marsh, Ontario, being almost quantitatively converted to aminoparathion; only 10% degradation occurred in 16 weeks when the water was sterilized(12). After 30 days incubation in non-sterile (sterile) coastal river water, only 21, 14, and 6% (64, 57, and 49%) of parathion remained at pH values of 6,7, and 8.16, respectively(14).
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), parathion, which has a vapor pressure of 6.68X10-6 mm Hg at 20 deg C(2), will exist in both the vapor and particulate phases in the ambient atmosphere. Vapor-phase parathion 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 4.2 hours(SRC) from the estimated rate constant(3). Particulate-phase parathion may be removed from the air by wet and dry deposition(SRC). Parathion absorbs radiation with wavelengths shorter than 320 nm(4), suggesting a potential for direct photolysis(SRC).
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