Degradation, estimating

The persistence of fenthion in the environment is dependent on several factors, including photolysis, metabolism in plants and insects, and microbial degradation. Estimates of the half-life of fenthion in soil vary from < 1 day in studies cited by the US EPA for aerobic soil metabolism to 3-6 weeks, cited by Extoxnet. Half-lives for aquatic degradation range from 2.9 to 21.1 days for various ocean, river, swamp, or lake aquatic conditions. Sunlight accelerates degradation of fenthion 20-fold in river water and fivefold in seawater. 

Occupational exposure may occur through oral, inhalation, or dermal contact at facilities where this chemical is used in research. In air, it exists solely as vapor where it is degraded (estimated half-life of 10 days) by reaction with photochemically produced hydroxyl radicals. It hydrolyzes in water (half-life of 1.2 h at pH 7 at 20°C). A limited number of research laboratory workers may also be possibly exposed several accidents have been reported in which laboratory personnel were exposed when the compound exploded at room temperature. 

Chemical stability Measure stability of compounds in solution through non-enzyme degradation Estimate compound stability... 

The half-lives for these four compounds taken from the literature allowed the estimation of the Four reaction rates necessai to model their degradation [18], As a first approximation, the rate of hydrolysis of the C-Cl bond of all Four, -triazine compounds was assumed to be the same and to be 5.0 x 10 s on the basis of literature precedence. This approximation seems reasonable as the four structures differ only in the alkyl groups at a site quite remote from the C-CI bond. Furthermore, among the Four reaction steps hydrolysis is the slowest anyway. 

Urea and uracil herbicides tend to be persistent in soils and may carry over from one season to the next (299). However, there is significant variation between compounds. Bromacil is debrominated under anaerobic conditions but does not undergo further transformation (423), linuron is degraded in a field soil and does not accumulate or cause carryover problems (424), and terbacd [5902-51-2] is slowly degraded in a Russian soil by microbial means (425). The half-hves for this breakdown range from 76 to 2,475 days and are affected by several factors including moisture and temperature. Finally, tebuthiuron apphed to rangeland has been shown to be phytotoxic after 615 days, and the estimated time for total dissipation of the herbicide is from 2.9 to 7.2 years (426). 

Fig. 10. Nomograph for estimating the rate of hydrolytic degradation of pyrophosphate and tripolyphosphate (tetramethyl ammonium salts) (27). For...

Emissions During Exterior End Use. When flexible PVC is used in exterior appHcations plasticizer loss may occur due to a number of processes which include evaporation, microbial attack, hydrolysis, degradation, exudation, and extraction. It is not possible, due to this wide variety of contribution processes, to assess theoretically the rate of plasticizer loss by exposure outdoors. It is necessary, therefore, to carry out actual measurements over extended periods in real life situations. Litde suitable data have been pubHshed with the exception of some studies on roofing sheet (47). The data from roofing sheet has been used to estimate the plasticizer losses from all outdoor appHcations. This estimate may weU be too high because of the extrapolation involved. Much of this extracted plasticizer does not end up in the environment because considerable degradation takes place during the extraction process. 

Environment. Detection of environmental degradation products of nerve agents directly from the surface of plant leaves using static secondary ion mass spectrometry (sims) has been demonstrated (97). Pinacolylmethylphosphonic acid (PMPA), isopropylmethylphosphonic acid (IMPA), and ethylmethylphosphonic acid (EMPA) were spiked from aqueous samples onto philodendron leaves prior to analysis by static sims. The minimum detection limits on philodendron leaves were estimated to be between 40 and 0.4 ng/mm for PMPA and IMPA and between 40 and 4 ng/mm for EMPA. Sims analyses of IMPA adsorbed on 10 different crop leaves were also performed in order to investigate general apphcabiflty of static sims for... 

The presence of errors within the underlying database fudher degrades the accuracy and precision of the parameter e.stimate. If the database contains bias, this will translate into bias in the parameter estimates. In the flash example referenced above, including reasonable database uncertainty in the phase equilibria increases me 95 percent confidence interval to 14. As the database uncertainty increases, the uncertainty in the resultant parameter estimate increases as shown by the trend line represented in Fig. 30-24. Failure to account for the database uncertainty results in poor extrapolations to other operating conditions. 

Polypropylene differs from polyethylene in its chemical reactivity because of the presence of tertiary carbon atoms occurring alternately on the chain backbone. Of particular significance is the susceptibility of the polymer to oxidation at elevated temperatures. Some estimate of the difference between the two polymers can be obtained from Figure 1J.7, which compares- the rates of oxygen uptake of eaeh polymer at 93°C. Substantial improvements can be made by the inclusion of antioxidants and such additives are used in all commercial compounds. Whereas polyethylene cross-links on oxidation, polypropylene degrades to form lower molecular weight products. Similar effects are noted... 

Estimate the concentration of a discharged chemical at the intake point by 1) simple dilution, or by a simplified degradation equation with dilution. 

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