Hexachlorocyclopentadiene

CAS RN:77-47-4

Environmental Fate

TERRESTRIAL FATE: Based on a classification scheme(1), the average Koc value of 4,265 from 15 different soils(2) indicates that hexachlorocyclopentadiene is expected to have low mobility in soil(SRC). Volatilization of hexachlorocyclopentadiene from moist soil surfaces is expected to be an important fate process(SRC) given a Henry's Law constant of 0.027 atm-cu m/mole(3). Hexachlorocyclopentadiene is not expected to volatilize from dry soil surfaces(SRC) based on a vapor pressure of 0.060 mm Hg(4). Volatilization losses of 0.84-1.60% from moist sand, 0.35-0.67% from moist loam, and 0.15-0.285% from moist humus were observed over a two hour period in a laboratory study, carried out at 25 deg C, where the initial hexachlorocyclopentadiene concentration was 50 ug/kg soil on a dry weight basis(5). Radiolabeled hexachlorocyclopentadiene at an initial concentration of 1 mg/kg was added to soil contained in a glass flask covered with perforated aluminum foil and kept on a laboratory shelf which was exposed to sunlight. After 7 days incubation, recovery of nonpolar compounds (hexachlorocyclopentadiene and nonpolar metabolites) was 6.1% in unaltered, nonsterile soil (approx 72% was polar and nonextractable), approx 6% in nonsterile soil at pH 4, approx 9% in nonsterile soil at pH 8, 36.1% in autoclaved soil (33.5% was polar and nonextractable), approx 15% in sodium azide treated soil, and approx 3% in flooded (anaerobic) soil. These data indicate that loss of hexachlorocyclopentadiene from soil is the result of abiotic and biotic degradation(6).
AQUATIC FATE: Based on a classification scheme(1), the average Koc value of 4,265 from 15 different soils(2) indicates that hexachlorocyclopentadiene is expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is expected(3) based upon a Henry's Law constant of 0.027 atm-cu m/mole(4); however adsorption may attenuate volatilization(3). Using this Henry's Law constant and an estimation method(3), volatilization half-lives for a model river and model lake are 2 hours and 7 days, respectively if adsorption is ignored(SRC). The volatilization half-life from a model pond is estimated as 37 days if adsorption is considered(5). According to a classification scheme(6), BCF values in the range of 100-1354 measured in fish(7-10), suggest bioconcentration in aquatic organisms is very high(SRC).If released to water, hexachlorocyclopentadiene will degrade primarily by photolysis and chemical hydrolysis. During daylight hours, hexachlorocyclopentadiene found in clear, shallow water is expected to photodegrade with a half-life on the order of several minutes(4). 2,3,4,4,5-Pentachloro-2-cyclopentenone, hexachloro-2-cyclopentenone, and hexachloro-3-cyclopentenone have been identified as primary photodegradation products of hexachlorocyclopentadiene. In unlit or deep, turbid water, chemical hydrolysis is expected to be an important fate process. Hydrolytic half-lives ranging from several hours to 2-3 weeks are predicted for waters with temperatures in the range of 20-30 deg C(4,11). An aqueous screening test showed that hexachlorocyclopentadiene was totally degraded in 7 days using domestic wastewater as inoculum(12). Both hydrolysis and biodegradation were potential degradation pathways.
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), hexachlorocyclopentadiene, which has a vapor pressure of 0.06 mm Hg at 25 deg C(2), is expected to exist solely as a vapor in the ambient atmosphere. Vapor-phase hexachlorocyclopentadiene 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 29 days(SRC), calculated from its rate constant of 3.9X10-13 cu cm/molecule-sec at 25 deg C(SRC) that was derived using a structure estimation method(3). Hexachlorocyclopentadiene absorbs light greater than 290 nm(4) and has been shown to undergo direct photolysis rapidly(5). Hexachlorocyclopentadiene adsorbed onto silica gel, underwent 46.0% photomineralization when irradiated with UV light (>290 nm) for 17 hours(5).
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