2,3,7,8-Tetrachlorodibenzo-p-dioxin

CAS RN: 1746-01-6

Environmental Fate

TERRESTRIAL FATE: Based on a classification scheme(1), a mean Koc value of 2.45X10+7(2), indicates that TCDD is expected to be immobile in soil(SRC). Volatilization of TCDD from moist soil surfaces is expected to be an important fate process(SRC) given an estimated Henry's Law constant of 3.2X10-6 atm-cu m/mole(SRC), based upon its vapor pressure, 1.5X10-9 mm Hg(3), and water solubility, 2.0X10-4 mg/L(4). However, adsorption to soil is expected to attenuate volatilization(SRC). TCDD is not expected to volatilize from dry soil surfaces(SRC) based upon its vapor pressure(3). TCDD was photo-degraded 99.9% during and immediately after soil application in test site studies(5). Once below the surface, TCDD remained in the top 15 cm 14 years after application(5). After 350 days, 56% of the initially applied TCDD was recovered from the sandy soil, while 63% was recovered from the silty clay loam for all concentrations; incubation was at 28 deg C in foil-sealed beakers(6). Estimates of the half-life of TCDD in subsurface soil may range from 25 to 100 years(7). These results indicate that biodegradation is not an important environmental fate process in soil(SRC).
AQUATIC FATE: Based on a classification scheme(1), a mean Koc value of 2.45X10+7(2), indicates that TCDD is expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is expected(3) based upon an estimated Henry's Law constant of 3.2X10-6 atm-cu m/mole(SRC), derived from its vapor pressure, 1.5X10-9 mm Hg(4), and water solubility, 2.0X10-4 mg/L(5). Using this Henry's Law constant and an estimation method(3), volatilization half-lives for a model river and model lake are 21 and 160 days, respectively(SRC). However, volatilization from water surfaces is expected to be attenuated by adsorption to suspended solids and sediment(SRC). The estimated volatilization half-life from a model pond is 5.1X10+4 years if adsorption is considered(6). TCDD does not hydrolyze in environmental waters and is not expected to react with photochemically produced singlet oxygen or peroxy radicals in water(7). According to a classification scheme(8), BCF values of 1585 to 5.1X10+6 for fish(9-11) suggest bioconcentration in aquatic organisms is very high(SRC). The photolysis half-life for TCDD in natural waters from marshes, ponds and lakes in New York was 3.5-5.9 hours(12). In laboratory studies using aquatic sediments and lake water, TCDD was found to be recalcitrant to microbial attack(13). Screening analyses using 100 microbial strains (which had previously shown the ability to degrade persistent pesticides) determined that only 5 strains showed some ability to degrade TCDD(14). These results indicate that biodegradation is not an important environmental fate process in water(SRC). Based on studies of Agent Orange containing TCDD as a contaminant, photodegradation is expected to be the most relevant fate(15).
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), TCDD, which has a vapor pressure of 1.5X10-9 mm Hg at 25 deg C(2), is expected to exist solely in the particulate phase in the ambient atmosphere. Particulate-phase TCDD may be removed from the air by wet and dry deposition(SRC). After 40 hours of illumination in simulated sunlight, TCDD sorbed onto small-diameter fly ash particulates suspended in air exhibited no loss to as much as 8% photo-degradation(3).
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