Malathion

CAS RN: 121-75-5

Environmental Fate

TERRESTRIAL FATE: Based on a classification scheme(1), Koc values of 927-17,620(2), indicate that malathion is expected to have low to no mobility in soil(SRC). Volatilization of malathion from moist soil surfaces is not expected to be an important fate process(SRC) given a Henry's Law constant of 4.89X10-9 atm-cu m/mole(3). Malathion is not expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 3.97X10-5 mm Hg at 30 deg C(4). The degradation half-life of malathion in five different soils was 1.33-4.14 days(5). Half-lives for malathion for radish and carrot soil for the seasons of winter, summer, and post-monsoon were determined to be 6.4, 2.1, and 5.3 days, and 6.6, 2.6, and 5.6 days, respectively(6). Malathion levels found in Southern California after a single spraying event at soil depths of 1 and 0.1 cm were 1.4 ug/g and 14.1 ug/g, respectively(7). Malathion is rapidly degraded in soils with reported degradation in 10 days in various non sterile loam soils of 81-94%, degradation in the same loam soils that were sterile were 5-19%(8).
TERRESTRIAL FATE: The degradation half-life of malathion was reported for five different soils in China(1).
pH % Organic Carbon % Sand/% Silt/% Clay Half-life (days)
8.1 1.9 74.4/21.9/3.7 1.79
7.2 1.3 83.2/15.0/1.7 4.14
6.9 1.1 41.9/52.6/5.5 2.02
5.0 0.7 30.2/28.1/41.7 1.38
4.8 1.7 36.8/61.5/1.7 1.33
AQUATIC FATE: Based on a classification scheme(1), Koc values of 927-17,620(2), indicate that malathion is expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is not expected(3) based upon a Henry's Law constant of 4.89X10-9 atm-cu m/mole(4). Malathion did not bioconcentrate in the freshwater fish topmouth gudgeon (Pseudorasbora parva)(5) or in pinfish (Lagodon sp)(6). Hydrolysis half-life for malathion in seawater/sediment systems at pH 7.3-7.7 is 2.0 days and in freshwater at pH 7.4 and 20 deg C is 11 days(7). Products of hydrolysis include malaoxon, malathion alpha and beta monoacid, O,O-dimethylphosphorodithioic acid, diethyl fumarate, diethyl thiomalate, O,O-dimethylphosphorothionic acid(8). Hydrolysis is the main route of malathion degradation in water with pH >7.0; at pH <7.0, the rate of hydrolysis is slow relative to the rate of biodegradation(9). Photodegradation may also compete with hydrolysis based on a study finding the first order photolysis rate of malathion in river water as 0.0013/min, this calculates to a half-life of 533 minutes(10). Malathion sorbed by algae was photodegraded more than 25 times faster than malathion in distilled water(11). The half-life for malathion in Limon River water was reported as 10.87, 10.41, 9.73 and 10.50 days under respective experimental conditions of filtered-dark, filtered-sunlight, non-filtered open to the air and non-filtered closed(12). The degradation half-life of malathion in different types of waters was 1.03-3.93 days(13). Malathion proposed degradation pathway in river water proceeds via hydrolysis to O,O-dimethyldithiophosphoric acid followed by oxidation to O,O-dimethylthiophosporic acid(14).
AQUATIC FATE: The degradation half-life (T1/2) and first-order degradation rate constant (k/day) of malathion were reported for five different waters in China(1).
Water source Hafl-life (days) k/day
underground 1.91 0.3622
Shangzhuang reservoir 1.65 0.4184
rain 3.93 0.1762
Qinhe River 1.03 0.6714
Jingmi diversion canal 1.51 0.4413
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), malathion, which has a vapor pressure of 3.97X10-5 mm Hg at 30 deg C(2), will exist in both the vapor and particulate phases in the ambient atmosphere. Vapor-phase malathion 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 hours(SRC), calculated from its rate constant of 7.7X10-11 cu cm/molecule-sec at 25 deg C(SRC) that was derived using a structure estimation method(3). Particulate-phase malathion may be removed from the air by wet or dry deposition(SRC). Malathion absorbs light at wavelengths >290 nm(4) and, therefore, may be susceptible to direct photolysis by sunlight(SRC).
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