CAS RN: 333-41-5

Environmental Fate

TERRESTRIAL FATE: Based on a classification scheme(1), a range of experimental Koc values of 191-1,842(2,3), indicates that diazinon is expected to have moderate-to-low mobility in soil(SRC). Volatilization of diazinon from moist soil surfaces is not expected to be an important fate process(SRC) given a Henry's Law constant of 1.17X10-7 atm-cu m/mole(4). Diazinon is not expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 9.0X10-5 mm Hg(5). Biodegradation is expected to be a major fate process in soils(SRC). Half-lives of <1, 2, and 5 weeks have been reported in non-sterile soils as compared to half-lives of 6, 6.5, and 12.5 weeks in sterile soils(6,7); at 7.5 and 20 deg C, dissipation was approximately 65-95% in non-sterile soil and only 20-25% in sterile soil over 120 days(8). Overall persistence in soils has been reported to be 3 to 14 weeks(9). Field dissipation half-lives are reported to range from 2.8 to 48 days with an average of 7 days(3). Laboratory studies in top soil and subsoil at 20 and 7.5 deg C have reported soil half-lives of 7 to 25 days(10); disappearance of diazinon was a combined effect of biotic and abiotic hydrolysis with hydrolysis rates depending on pH (especially important in acidic organic soil)(11). Hydrolysis of diazinon was reported to be more rapid in soils than in water; 11% of diazinon applied to Poygan sand was degraded per day(11). Diazinon absorbs light at wavelengths >290 nm(12), and therefore is susceptible to direct photolysis(SRC). The percent photolysis (difference between samples exposed to light closely approximating that of sunlight and covered samples) in 24 hrs was reported to be 51% on moist soil surfaces and 44% on dry soil surfaces at 45 deg C, and 36% in pH 7.0 buffer solutions at 25 deg C(13). The photolysis rate constant of diazinon on soil at 25 deg C is reported to be 0.151 per day(3) which corresponds to a half-life of 4.6 days(SRC). Soil degradation of diazinon involves oxidation to the phosphate (diazoxon)(5) and hydrolysis to 2-isopropyl-6-methyl-pyrimidinol(14).
TERRESTRIAL FATE: ... Diazinon is relatively nonpersistent in soil. Most diazinon applied is lost from soil through chemical and biologic degradation within about 2 months of application. ... about 46% of diazinon added to neutral aqueous solution remained after 2 wks.
TERRESTRIAL FATE: Degraded in soil to 10% in 3 weeks
AQUATIC FATE: Based on a classification scheme(1), a range of Koc values of 191-1,842(2,3), indicates that diazinon is expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is not expected(4) based upon a Henry's Law constant of 1.17X10-7 atm-cu m/mole(5). According to a classification scheme(6), a range of BCFs from 3-200(7-11), suggests the potential for bioconcentration in aquatic organisms is low-to-moderate(SRC). Hydrolysis may be a significant fate process(SRC) with reported half-lives of 31 days (pH 5), 185 days (pH 7.4), and 136 days (pH 9) at 20 deg C(12), 2-3 weeks in distilled water at pH 6 at room temperature(13), and 12 days (pH 5), 139 days (pH 7) and 77 days (pH 9) at 25 deg C(3). Major products of hydrolysis were 2-isopropyl-4-methyl-6-hydroxypyrimidine and diethyl thiophosphoric acid or diethyl phosphoric acid(12). Photolysis is an important degradation process(SRC); 36% of diazinon photolyzed in a pH 7 buffer solution exposed to light with wavelengths >290 nm in a period of 24 hrs(14). The photolysis rate constant in water at 25 deg C is reported to be 0.136 per day(3) which corresponds to a half-life of 5.1 days(SRC). Diazinon disappeared more quickly from unsterilized natural water (12 weeks) than from unsterilized distilled or sterilized natural water (>16 weeks), suggesting that degradation is both biological and chemical in nature in natural waters(15). Using water samples from two nursery recycling ponds as inoculum at 22 deg C, half-lives ranged from 34.4 to 76.6 days in non-sterile water and 69.5 to 134.4 days sterile water(16).
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), diazinon, which has a vapor pressure of 9.0X10-5 mm Hg at 25 deg C(2), will exist in both the vapor and particulate phases in the ambient atmosphere(SRC). Vapor-phase diazinon 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 1.8 hrs(SRC), calculated from its experimental rate constant of 2X10-10 cu cm/molecule-sec(3). Particulate-phase diazinon may be removed from the air by wet and dry deposition(SRC). Diazinon absorbs light in the environmental spectrum(4) and has the potential for direct photolysis in the atmosphere(SRC). The detection of diazinon in lake waters in Canada (arctic, sub-arctic and remote locations) at long distances (hundreds of kilometers) from known source areas indicates diazinon can undergo long range atmospheric transport(5).
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