Fate mechanism

If areas identified as likely to receive significant atmospheric contaminant concentrations include areas supporting edible biota, the biouptake of contaminants must be considered as a possible environmental fate pathway. Direct biouptake from the atmosphere is a potential fate mechanism for lipophilic contaminants. Biouptake from soil or water following transfer of contaminants to these media must also be considered as part of the screening assessments of these media. 

Fig. 3 The MCM domains and fate mechanisms incorporated into CHEMGL [38]...

A bioconcentration factor (BCF) relates the concentration of a chemical in plants or animals to the concentration of that chemical in the medium in which they live. A BCF of about 7 was calculated for 2-hexanone (Lande et al. 1976) using the empirical regression of Neely et al. (1974). This low BCF indicates that bioconcentration is probably not an important fate mechanism for 2-hexanone released into the environment. Biomagnification of 2-hexanone is also not expected to occur to any great extent (Lande et al. 1976). However, no experimental data on the biomagnification potential of 2-hexanone were located to corroborate these assumptions. 

The major fate mechanism of atmospheric 2-hexanone is photooxidation. This ketone is also degraded by direct photolysis (Calvert and Pitts 1966), but the reaction is estimated to be slow relative to reaction with hydroxyl radicals (Laity et al. 1973). The rate constant for the photochemically- induced transformation of 2-hexanone by hydroxyl radicals in the troposphere has been measured at 8.97x10 cm / molecule-sec (Atkinson et al. 1985). Using an average concentration of tropospheric hydroxyl radicals of 6x10 molecules/cm (Atkinson et al. 1985), the calculated atmospheric half-life of 2-hexanone is about 36 hours. However, the half-life may be shorter in polluted atmospheres with higher OH radical concentrations (MacLeod et al. 1984). Consequently, it appears that vapor-phase 2-hexanone is labile in the atmosphere. 

Assessment of Toxicology and Fate Mechanism of Action Toxicogenomics... 

Benzene toxicity has been studied extensively, and the current understanding of benzene toxicity includes its metabolic fate, mechanism of toxicity, pharmacokinetic models for disposition, and impact of exposure on human health. These aspects of benzene toxicity will be addressed in the following sections. 

The primary environmental fate mechanism followed by stored or buried HD is hydrolysis. Although HD is rapidly hydrolyzed (a half-life of 4 to 8 min at 25° C in distilled water has been reported [Bartlett and Swain, 1949]), the overall process of hydrolytic destruction is limited by the very low water solubility of HD. Intermediate hydrolysis products and/or water-insoluble thickeners that can coat or encapsulate droplets of mustard retard hydrolysis. Because of low water solubility and formation of intermediate products, bulk amounts of HD may persist undispersed under water for some time. However, HD dispersed as droplets or mist, as in the case of an aerial attack, is expected to hydrolyze rapidly in humid air. 

The fate mechanisms of GB in soil includes hydrolysis, evaporation and leaching the phos-phonic acid hydrolysis products are subject to biodegradation. Depending on temperature, > 90% of GB added to soil may be lost in 5 days (Small, 1984). As shown by field studies under snow in Norway, low temperatures would increase persistence. In this setting, approximately 55% was removed by evaporation within 5 h and 15% was removed by hydrolysis. Hydrolysis products and several impurities were present up to four weeks later (NMFA, 1982-1983 Johnsen and Blanch, 1984). Hydrolytic half-lives are highly dependent upon pH and temperature. Hydrolytic half-lives are shorter in acidic and basic solutions than at a neutral pH. At 20° C and the pH of natural waters where the half-life is a maximum, estimates of the half-life range from 461 h (pH 6.5) to 46 h (pH 7.5) (Clark, 1989). At 25°C, the half-life ranges from 237 h (pH 6.5) to 24 h (pH 7.5). A half-life of 8300 h at 0°C and a pH of 6.5 was estimated. Durst et al, (1988) have documented a half-life of 3 s at a pH of 12. 

PROBABLE FATE photolysis direct photochemical degradation in the atmosphere or in the upper layers of surface waters should not be an important fate process half-life for the atmospheric reaction with photochemically produced hydroxyl radicals 10 hrs oxidation could occur, but too slow to be important hydrolysis gradual hydrolysis of carbon-chlorine bond is a probable principle fate mechanism, can be expected in comparison to other chlorine containing compounds, half-life for this pH independent process 0.5-2 yrs volatilization not important, volatilization from water should be a slow process half-life from a model pond 1 lyrs, volatilization from the soil to the atmosphere might occur, but will be a slow process, volatilization from moist soil should not be an important fate process sorption possible importance as catalyst for hydrolysis biological processes biodegradation not expected to be an important fate process, but there is not enough data to draw a conclusion... 

PROBABLE FATE photolysis-, direct photolysis is probably not important, if released to atmosphere, will degrade by reaction with photochemically produced hydroxyl radicals (estimated half-life 1.15 days) oxidation photooxidation in atmosphere can occur, photooxidation half-life in air 4.61-46.1 hrs hydrolysis slow hydrolysis of carbon-chlorine bond, may be important fate mechanism volatilization if released to water, volatilization is expected to be the principle removal process, but may be slow, volatilization half-lives for a model river (1 m deep) and a model environmental pond 13.9 hr, and 6.6 days respectively sorption adsorption on organic matter is possible biological processes no data on bioaccumulation or biodegradation... 

PROBABLE FATE photolysis, no information available pertaining to the rate of photodissociation in aqueous environment, photodissociation to formyl chloride may occur in stratosphere, predominate fate process, if released to the atmosphere, is the reaction with photochemi-cally produced hydroxyl radicals with an estimated half-life of 40 days, less than 1% will eventually diffuse above the ozone layer where it will be destroyed by photolysis, direct photolysis is not important oxidation photooxidation in troposphere is the primary fate mechanism, photo-... 

PROBABLE FATE photolysis, photodissociation in stratosphere may be important, not important in aquatic environment, photooxidation half-life in air >7.3->73 yrs photooxidation by U.V. in aqueous medium 90-95 C time for the formation of C02 (% theoretical) 25% 25.2 hr, 50% 93.7 hr, 75% 172.0 hr oxidation not an important process hydrolysis probably unimportant volatilization some volatilization occurs, importance as a fate mechanism is unknown, measured half-life for evaporation from 1 ppm aqueous solution 25 C, still air, and an average depth of 6.5 cm 40.7 min, should volatilize slowly from dry soil surfaces, if released to water, volatilization appears to be the most dominant removal mechanism (half-life 15 hrs from a model river) sorption no data available, moderate to slight adsorption to suspended solids and sediments may occur biological processes high Log Kow indicates possibility of bioaccumulation... 

PROBABLE FATE photolysis-, photochemical reactions in aqueous media are probably unimportant, slow decomposition in the troposphere in the presence of nitrogen oxides is possible, appreciable photodissociation may occur in stratosphere, photooxidation half-life in air 19.1-191 days oxidation-, probably unimportant, in troposphere, oxidation by hydroxyl radicals to CO2, CO, and phosgene is important fate mechanism hydrolysis not an important fate process, first-order hydrolytic half-life 704 yrs volatilization due to high vapor pressure, volatilization to the atmosphere is rapid and is a major transport process sorption sorption to inorganic and organic materials is not expected to be an important fate mechanism biological processes bioaccumulation is not expected, biodegradation may be possible but very slow compared with evaporation... 

A Kjj value of less flian 10" atm-mol/m suggests that volatilization would probably not be a significant fate mechanism for flie dissolved solvent. The rate of volatiUzation is... 

Plasticizers have also been detected in river and lake sediment samples (Table 18.7). Because adsorption is a major fate mechanism for many plasticizers (see 18.2.5), it is expected that, depending on their organic carbon content, sediments can act as a sink for dissolved plasticizers. The largest reported concentration for di-n-butyl phthalate in river sediments (1,100 mg/kg) confirms this expectation. However, some of the sediment and soil samples may have been biased in that they were collected near chemical plants or in industrial areas, and hence, the results may reflect point sources rather than some type of background levels resulting from non-point mechanisms of dispersal. Kohli et al noted that it was difficnlt to determine background levels for phthalate esters because of their wide occurrence. 

Photodegradation is another family of chemical reactions where the plasticizer may react directly with solar radiation or with dissolved constituents made reactive by solar radiation. Photodegradation may be an important fate mechanism in the atmosphere (see 18.5), but it is generally thought that plasticizers do not photodegrade significantly by either photolysis or photooxidation. 

The reaction of some of the solvents with ozone may be much slower. For example, the half-life for the reaction of benzene with ozone may be longer than 100 years.Solvents such as carbon tetrachloride, 1,1,1-trichloroethane, and the chlorinated fluorocarbons may be relatively resistant to photo-oxidation. The major fate mechanism of atmospheric 1,1,1-trichloroethane, for example, may be wet deposition. 

AF, Atmospheric fate, mechanisms, and final products of atmospheric reactions CS, Cross sections, absorption (a) for the compound... 

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