Dimethoate

CAS RN: 60-51-5

Environmental Fate

Formothion & dimethoate should be considered together since their significant residues appear to be identical @ harvest; that is, effect of applying dimethoate & formothion will be additive.
TERRESTRIAL FATE: Based on a classification scheme(1), experimental Koc values which range from 5.2 to 50(2-6), indicate that dimethoate is expected to have very high mobility in soil(SRC). Volatilization of dimethoate from moist soil surfaces is not expected to be an important fate process(SRC) given an estimated Henry's Law constant of 2.4X10-10 atm-cu m/mole(SRC) derived from its vapor pressure, 1.87X10-6 mm Hg(15), and water solubility, 23,300 mg/L(15). Dimethoate soil half-life values of 2.2 days(16), 7 days(7-9), 11 days(10) and as much as 122 days(11) have been reported. After 6 days, average dimethoate losses due to evaporation from uncovered soil columns were 40.4, 32.3, 23.0, and 24.9% in sand, sandy clay loam, loam, and clay loam, respectively; dimethoate losses were directly proportional to water losses(12); this study indicates that dimethoate may be removed from terrestrial surfaces via evaporation(SRC). Dimethoate will be susceptible to hydrolysis in moist alkaline soils based on a reported base catalyzed rate constant of 756 1/m-hr at pH 9 which corresponds to a half-life of 3.7 days in water(13). Biodegradation may be an important fate process in soil(SRC) with 77% degradation reported in clay loam soil in 2 wks compared to 18 and 20% degradation in the same soil that had been autoclaved or irradiated(14).
AQUATIC FATE: Based on a classification scheme(1), experimental Koc values which range range from 5.2 to 50(2-6), indicate that dimethoate is not expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is not expected(7) based upon an estimated Henry's Law constant of 2.4X10-11 atm-cu m/mole(SRC) derived from its vapor pressure, 1.87X10-6 mm Hg(8), and water solubility, 23,300 mg/L(8). According to a classification scheme(9), BCFs of 1.1-2.4 and 2.7-6(10), suggests bioconcentration in aquatic organisms is low(SRC). Dimethoate hydrolyzes very slowly in sterile buffered solutions at pHs 5 and 7 with half-lives of 156 and 68 days respectively, but hydrolyzes rapidly to desmethyl dimethoate and dimethylthiophosphoric acid with a half-life of 4.4 days at pH 9(11). Photolysis and volatilization from water do not appear to be important removal process of dimethoate from water(SRC). Half-lives of 171, 173 and 219 days were given for river water, filtered river water and sea water, respectively, at 6 deg C, and 43, 29 and 36 days, respectively, at 22 deg C(12), suggesting that biodegradation may be a slow environmental fate process in water(SRC).
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), dimethoate, which has a vapor pressure of 1.87X10-6 mm Hg at 25 deg C(2), will exist in both the vapor and particulate phases in the ambient atmosphere. Vapor-phase dimethoate 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 5 hrs(SRC), calculated from its rate constant of 7.9X10-11 cu cm/molecule-sec at 25 deg C(SRC) that was derived using a structure estimation method(3). Particulate-phase dimethoate may be removed from air via dry deposition(SRC), and one monitoring study(4) indicates that dimethoate is removed from the atmosphere via wet deposition. Products of oxidation and hydrolysis accounted for 1.5 and 11%, respectively, of the dimethoate deposited on an untreated glass surface left in a greenhouse for 7 days(5). Therefore, some oxidation and hydrolysis may account for the removal of dimethoate from air(SRC). Based on a lack of absorbance of wavelengths of light above 290 nm(6), dimethoate is not expected to be susceptible to direct photolysis by sunlight(SRC).
When applied to plants, dimethoate was ... decomposed ... on surface ... by hydrolysis & oxidation. On the plant surface, dimethoate underwent non-enzymatic oxidation to the oxygen analog & hydrolysis to water sol deriv identified as dimethyl phosphoric & O,O-dimethyl phosphorothioic acids, desmethyl dimethoate ...
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