Estimate, of half-life

Half-life estimates of approximately 28 days for thiophanate-methyl indicate a very slow decay compared to methiocarb with an estimate of half-life of about 11 days. The application of a model based on a first-order decay process resulted in fairly high R2 and significant fit. The results suggest that both pesticides are relatively stable compared to other compounds under similar environmental conditions (Brouwer et al., 1994). With respect to the objectives of the study and the proposed model, it can be stated that the results confirm the assumption of a linear relationship between application rate (for both application techniques) and the increase of dislodgeable foliar residue. This relationship holds for modeling purposes. The contribution of the crop density or total crop surface area to the process of interception cannot be quantified with the results of the present study. Because the interception factor ranges from about 0.35 to 0.9 (Willis and McDowell, 1987), the... 

No modern studies of the human pharmacokinetics of LSD have been done, largely because human experimentation has virtually stopped. An older study that used a spectrofluorometric technique for measuring plasma concentrations of LSD was done in humans given doses of 2 Mg/kg i.v. After equilibration had occurred in about 30 min, the plasma level was between 6 and 7 ng/ml. Subsequently, plasma levels gradually fell until only a small amount of LSD was present after 8 hr. The half-life of the drug in humans was calculated to be 175 min (2). Subsequent pharmacokinetic analysis of these data indicated that plasma concentrations of LSD were explained by a two-compartment open model. Performance scores were highly correlated with concentration in the tissue (outer) compartment, which was calculated at 11.5% of body weight. The new estimation of half-life for loss of LSD from plasma, based on this model, was 103 min (47). 

Estimates of half-life range from 2.8 X 10 to 1.13 X 10 years... 

The a- and [3-isomers of endosulfan undergo photolysis in laboratory tests after irradiation in polar solvents and upon exposure to sunlight on plant leaves. The a-isomer also undergoes isomerization to the P-isomer, which is relatively more stable (Dureja and Mukerjee 1982). A photolytic half-life of about 7 days was reported for endosulfan by EPA (1982c). The primary photolysis product is endosulfan diol, which is subsequently photodegraded to endosulfan a-hydroxyether. Endosulfan sulfate is stable to direct photolysis at light wavelengths of >300 nm however, the compound reacts with hydroxy radicals, with an estimated atmospheric half-life of 1.23 hours (HSDB 1999). 

Mathematical modeling of trichloroethylene volatilization from a rapidly moving, shallow river (1 meter deep, flowing 1 meter per second, with a wind velocity of 3 meters per second) has estimated its half-life at 3.4 hours (Thomas 1982). Measured volatilization half-lives in a mesocosm, which simulated the Narragansett... 

EPIsuite estimate among other Xow, K00, KOM Henry s Law constant, [27] melting and boiling points, aerobic and anaerobic biodegradability of organic chemicals, biodegradation of half-life of hydrocarbons, and bioconcentration factors... 

Pharmacokinetic studies in patients yielded an estimated product half-life of approximately 20 days (11-50 days range) and the product clearance was found to be variable according to body weight, gender and tumour burden. Safety and efficacy were established by three randomized, controlled trials. The first study was a randomized double-blind trial involving 813 patients. The primary end-point measured was overall survival, which was extended from a median of 15.6 months to 20.3 months. 

Di- -octylphthalate may also undergo photolysis in surface waters as a result of its absorption of electromagnetic radiation at wavelengths less than 290 nm. The estimated photolytic half-life of the compound in surface water is 144 days (EPA 1992a). Photolysis was predicted to be the most important removal mechanism after volatilization for di- -octylphthalate losses from oligotrophic lakes (Wolfe et al. 1980). 

The transport of disulfoton from water to air can occur due to volatilization. Compounds with a Henry s law constant (H) of <10 atm-m /mol volatilize slowly from water (Thomas 1990). Therefore, disulfoton, with an H value of 2.17x10" atm-m /mol (Domine et al. 1992), will volatilize slowly from water. The rate of volatilization increases as the water temperature and ambient air flow rate increases and decreases as the rate of adsorption on sediment and suspended solids increases (Dragan and Carpov 1987). The estimated gas- exchange half-life for disulfoton volatilization from the Rhine River at an average depth of 5 meters at 11 °C was 900 days (Wanner et al. ] 989). The estimated volatilization half-life of an aqueous suspension of microcapsules containing disulfoton at 20 °C with still air was >90 days (Dragan and Carpov 1987). 

Following an accidental discharge of stored chemicals including disulfoton, the estimated biodegradation half-life of disulfoton in Rhine River water was between 7 and 41 days at 10 °C (Wanner et al. 1989). Therefore, biodegradation of disulfoton is expected to be important in water, and the rate will depend on the initial concentration. A theoretical model predicted that over 12 days biodegradation and photolysis would account for an 80% mass loss of disulfoton in the Rhine River after an accident spill incident (Mossman et al. 1988) however, the removal of disulfoton by chemical processes was much slower than by biodegradation (Capel et al. 1988). 

Photolytic. Fukuda et al. (1988) studied the photodegradation of acenaphthene and alkylated naphthalenes in distilled water and artificial seawater using a high-pressure mercury lamp. Based upon a rate constant of 0.23/h, the photolytic half-life of acenaphthene in water is 3 h. Behymer and Hites (1985) determined the effect of different substrates on the rate of photooxidation of acenaphthene using a rotary photoreactor equipped with a 450-W medium pressure mercury lamp (X = 300-410 nm). The photolytic half-lives of acenaphthene absorbed onto silica gel, alumina, and fly ash were 2.0, 2.2, and 44 h, respectively. The estimated photooxidation half-life of acenaphthene in the atmosphere via OH radicals is 0.879 to 8.79 h (Atkinson, 1987). 

Chemical/Physical. The estimated hydrolysis half-life of acetonitrile at 25 °C and pH 7 is >150,000 yr (Ellington et al., 1988). No measurable hydrolysis was observed at 85 °C at pH values 3.26 and 6.99. At 66.0 °C (pH 10.42) and 85.5 °C (pH 10.13), the hydrolysis half-lives based on first-order rate constants were 32.2 and 5.5 d, respectively (Ellington et al., 1987). The presence of hydroxide or hydronium ions facilitates hydrolysis transforming acetonitrile to the intermediate acetamide which undergoes hydrolysis forming acetic acid and ammonia (Kollig, 1993). Acetic acid and ammonia formed react quickly forming ammonium acetate. 

In distilled water, acrolein is hydrolyzed to p hydroxypropionaldehyde (Burczyk et ah, 1968 Reinert and Rodgers, 1987 Kollig, 1993). The estimated hydrolysis half-life in water is 22 d (Burczyk et al, 1968). Bowmer and Higgins (1976) reported a disappearance half-life of 69 and 34 d in buffered water at pH values of 5 and 8.5, respectively. 

Slowly hydrolyzes in water forming HCl and benzyl alcohol. The estimated hydrolysis half-life in water at 25 °C and pH 7 is 15 h (Mabey and Mill, 1978). The hydrolysis rate constant for benzyl chloride at pH 7 and 59.2 °C was determined to be 0.0204/min, resulting in a half-life of 34 min (Ellington et al, 1986). 

Photolytic. The estimated photolytic half-life of bis(2-ethylhexyl) phthalate in water is 143 d (Wolfe et al., 1980). 

Surface Water. The estimated volatilization half-life of 1,3-butadiene in a model river 1 m deep, flowing 1 m/sec and a wind speed of 3 m/sec is 3.8 h (Lyman et al., 1982). 

Chemical/Physical. Under laboratory conditions, carbon tetrachloride partially hydrolyzed in aqueous solutions forming chloroform and carbon dioxide (Smith and Dragun, 1984). Complete hydrolysis yields carbon dioxide and HCl (Ellington et al., 1993 Kollig, 1993). The estimated hydrolysis half-life in water at 25 °C and pH 7 is 7,000 yr (Mabey and Mill, 1978) and 40.5 yr (Jeffers et al., 1989 Ellington et al, 1993). The estimated hydrolysis half-life reported by Mabey and Mill (1978) was based on second-order neutral kinetics. Jeffers et al. (1996) reported that hydrolysis of carbon tetrachloride is first-order, contrary to findings of Mabey and Mill (1978). Jeffers et al. (1996) report that the extrapolated environmental half-life at 25 °C is 40 years. 

Atkinson and Carter (1984) estimated a half-life of 320 d for the reaction of dichlorvos with ozone in the atmosphere. 

Photolytic. Atkinson (1985) reported an estimated photooxidation half-life of 10.5 h for the reaction of furfural with OH radicals in the atmosphere. 

Soil. Methylene chloride undergoes biodegradation in soil under aerobic and anaerobic conditions. Under aerobic conditions, the following half-lives were reported 54.8 d in sand (500 ppb) 1.3, 9.4, and 191.4 d at concentrations of 160, 500, and 5,000 ppb, respectively, in sandy loam soil 12.7 d (500 ppb) in sandy clay loam soil 7.2 d (500 ppb) following a 50-d lag time. Under anaerobic conditions, the half-life of methylene chloride in clay following a 70-d lag time is 21.5 d (Davis and Madsen, 1991). The estimated volatilization half-life of methylene chloride in soil is 100 d (Jury et al., 1990). 

Tuazon et al. (1984a) investigated the atmospheric reactions of TV-nitrosodimethylamine and dimethylnitramine in an environmental chamber utilizing in situ long-path Fourier transform infared spectroscopy. They irradiated an ozone-rich atmosphere containing A-nitrosodimethyl-amine. Photolysis products identified include dimethylnitramine, nitromethane, formaldehyde, carbon monoxide, nitrogen dioxide, nitrogen pentoxide, and nitric acid. The rate constants for the reaction of fV-nitrosodimethylamine with OH radicals and ozone relative to methyl ether were 3.0 X 10 and <1 x 10 ° cmVmolecule-sec, respectively. The estimated atmospheric half-life of A-nitrosodimethylamine in the troposphere is approximately 5 min. 

When parathion was released in the atmosphere on a sunny day, it was rapidly converted to the photochemical paraoxon. The estimated photolytic half-life is 2 min (Woodrow et al., 1978). The reaction involving the oxidation of parathion to paraoxon is catalyzed in the presence of UV light, ozone, soil dust, or clay minerals (Spencer et al., 1980, 1980a). 

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