Photolysis half-life time

The compound-specific data required for exposure assessments comprise the 1-octanol/water partition coefficient (log water solubility (S ), vapour pressure (p ), Henry s law constant (H, H ), soil sorption coefficient hydrolysis half-life time, photolysis half-life time and information on biodegradability (OECD, 1993c). These parameters generally relate to steady-state conditions - conditions that are rarely met in the real environment. The experimental data underlying the QSAR models are preferably determined by standardized protocols, but, even then, the absolute values are of variable reliability and precision, which clearly affects the accuracy of the predictions based on the acquired QSARs. The endpoints discussed in the following sections were selected because of their consideration in regulatory evaluation schemes in, for example, the EU (EEC, 1990). 

CD measurements allowed the conversion of 3, the less stable product of photolysis of 2, into the more stable isomer 4 (half-life time 190 min at 300 K) to be followed ... 

TeBDE appeared at the longest exposure time (244 hours). In sand, the half-life of DeBDE exposed to UV radiation was between 12 and 37 hours. Lower PBDEs and other compounds (not specified) were found after UV radiation (Sellstrom et al. 1998). In a laboratory study of soil treated with DeBDE and exposed to UV light, a photolysis half-life of 185 hours for DeBDE was measured. The debromination process was the same as that seen for other studies. 

Hence, when considering the whole epilimnion, the direct photolysis half-life of 4-NP is about 10 times longer as compared to the half-life at the surface. Note that in contrast to the near-surface situation, because of the very different screening factors, the reaction of the dissociated species is about four times more important in determining the overall direct photolysis rate of 4NP in the well-mixed epilimnion. 

Measurement of the vapor phase photolysis of 2,3,7,8-TCDD Is experimentally a major challenge due to the low volatility. Podoil and coworkers (3 ), however, have estimated the half life of 2,3,7,8-TCDD In the vapor phase to be 55 minutes. This estimate assumed that the quantum yield Is the same as In hexane and Is Invariant with wavelength, that the spectral properties are the same as In solution, and that the mechanisms for transformation are the same. This photolysis half-life Is approximately 200 times smaller than that expected for reaction with hydroxyl radical. However, both vapor phase photolysis and reactions with OH radicals would be unimportant If TCDD Is predominantly sorbed on particles In the atmosphere. Experimental verification of these estimates Is needed, because atmospheric transformation may be an Important fate of 2,3,... 

Photolysis Half-Life (P-ti ) The photolysis half-life of a chemical is the time required for the parent chemical to reach one-half or 50% of its original concentration. Chemicals will undergo photolysis if they can absorb sunlight. Photolysis can occur in air, soil, water and plants. The rate of photolysis is dependent upon the pH, temperature, presence of sensitizers, sorption to soil, and depth of the compound in soil and water. Lyman et al. (1982) present an excellent overview of the photolysis proeess. 

Studies have appeared where photolysis in natural bodies of water under normal sunlight conditions has been examined. For example, metolachlor was slowly photodegraded by sunlight in lake water, with a half-life of 22 days in summer and 205 days in winter (28). Addition of a 5% solution of dissolved organic matter to the water extended the half-Hves two to three times longer, depending on the season (see PHOTOCHEMICAL TECHNOLOGY, photocatalysis). ... 

The Half-life in the Environment data reflect observations of the rate of disappearance of the chemical from a medium, without necessarily identifying the cause of mechanism of loss. For example, loss from water may be a combination of evaporation, biodegradation and photolysis. Clearly these times are highly variable and depend on factors such as temperature, meteorology and the nature of the media. Again, the reader is urged to consult the original references. 

The most important transformation process for di-w-octylphthalate present in the atmosphere as an aerosol is reaction with photochemically produced hydroxyl radicals. The half-life for this reaction has been estimated to be 4.5 14.8 hours (Howard et al. 1991). Actual atmospheric half-lives may be longer since phthalate esters sorbed to wind-entrained particulates may have long atmospheric residence times (Vista Chemical 1992). Direct photolysis in the atmosphere is not expected to be an important process (EPA 1993a HSDB 1995). 

CASRN 51707-55-2 molecular formula C9H8N4OS FW 220.20 Soil. The reported half-life in soil is approximately 26-144 d (Hartley and Kidd, 1987). Photolytic. Klehr et al. (1983) studied the photolysis of thiadiazuron on adsorbed soil surfaces. Irradiation was conducted using artificial sunlight having a wavelength <290 nm. The amount of thiadiazuron remaining after irradiation times of 0.25, 0.5, 1, 2, 3.75, and 18.0 h were 56.4, 42.8, 35.7, 23.8, 25.0, and 67.2%, respectively. The primary photoproduct identified was l-phenyl-3-(l,2,5-thiadiazol-3-yl)urea and five unknown polar compounds. The unknown com pounds could not be identified because the quantities were too small to be detected. 

The results of Herzberg and Shoosmith94 indicate that the half-life of CH2 in the presence of CH2N2 is of the order of the collision time, in contrast to the earlier report of Pearson et al.103 of a half-life of 5 X 10-3 sec. for methylene in the pyrolysis and photolysis of CH2N2. 

In Table 15.7 the reaction quantum yields are given for some selected organic pollutants. As can be seen, reaction quantum yields vary over many orders of magnitude, with some compounds exhibiting very small Oir values. However, since the reaction rate is dependent on both ka and Oir (Eq. 15-34), a low reaction quantum yield does not necessarily mean that direct photolysis is not important for that compound. For example, the near-surface direct photolytic half-life of 4-nitrophenolate (Oir = 8.1 x 10 6) at 40°N latitude is estimated to be in the order of only a few hours, similar to the half-life of the neutral 4-nitrophenol, which exhibits a Oir more than 10 times larger (Lemaire et al., 1985). The reason for the similar half-lives is the much higher rate of light absorption of 4-nitrophenolate as compared to the neutral species, 4-nitrophenol (compare uv/vis spectra in Fig. 15.5 and Illustrative Example 15.3). As a second example, comparison of the near-surface photolytic half-lives (summer, 40°N... 

Phenyltrimethyldisilene (15) and (E)- and (Z)-l,2-dimethyl-l,2-diphenyldisilene (16) were also observed as transient absorption spectra by laser flash photolysis of the precursors in methylcyclohexanes28. The absorption band at 380 nm, assigned to the disilene 15, reached maximum intensity at ca 10 ns after the excitation and then started to decrease. The half-life assigned to 15 was 700 ns. The logarithm of the decay profile of the transient absorption at 380 nm versus time shows a very good linear relationship, indicating that the decay of the transient absorption fits first-order kinetics. This result shows that intramolecular isomerization or proton abstraction from the solvent is the origin for the decay of the disilene 15, which survives in solution only for several nanoseconds. 

Trithiepine 47 was dissolved in DCCI3 and the photolysis was monitored by 111 NMR spectroscopy to determine the kinetic parameter of the reaction. In the spectra, the decrease of the intensity of signals of 47 and the appearance and increase of the intensity of new signals of 48 were observed. The plot of ln(trithiepine) versus the reaction time reveals that the desulfurization of 47 to 48 follows a first-order kinetics with respect to the substrate concentration. The rate constant and the half-life period of this photoreaction were calculated to be 7 = (2.82 1.11) x 10-4 and /1/2 = 41.0min <2000TL1801>. 

Since L- values are averaged over 24 hours and over each 3-month season, the accuracy of rate estimates depend on the rate of the photolysis process compared with the averaging interval. For example, a compound which photolyzes with a half life of 1 hour will photo-lyze three times faster at solar noon than at 800 or 1600 hrs. However, if the half life is one week near mid-season, diurnal variations make little difference. 

Volatilization half-life of about few minutes (Mills et al. 1982) loss half-lives in marine mesocosm were estimated to be 20 d in spring at 8-16°C and t,/2 = 13 d in winter at 3-7°C (Wakeham et al. 1983). Photolysis photolytic dissociation of atmospheric CFC13 and CF2C12 gives chlorine atoms which destroys the ozone layer, these halomethanes may remain at altitudes of 20-40 km for 40-150 yr and will reach saturation values of 10-30 times the present levels (Molina Rowland 1974). 

The loss of 2,3,7,8-TCDD in contaminated soil has been studied under natural conditions in experimental plots at the Dioxin Research Facility, Times Beach, Missouri (Yanders et al. 1989). The 2,3,7,8-TCDD concentration profiles of sample cores taken at Times Beach in 1988 were virtually the same as those in cores taken in 1984. The authors concluded that the loss of 2,3,7,8-TCDD due to photolysis at Times Beach was minimal in the 4 years covered by the study (Yanders et al. 1989). Estimates of the half-life of TCDD on the soil surface range from 9 to 15 years, whereas the half-life in subsurface soil may range from 25 to 100 years (Paustenbach et al. 1992). 

In a CH4 matrix at 12 K, (24) yields, upon UV photolysis, the CO-bridged dimer (25). When (24) is subjected to flash photolysis in cyclohexane solution at room temperature, time-resolved IR studies show that both (25) and the mononuclear radical [CpFe(CO)2] are formed within 5 ps. The radical has a half-life of <25 ps while (25) has a half-life of 1.5 ms under the experimental conditions. If the cyclohexane solution is doped with the two-electron donor molecule MeCN it is possible to monitor the formation of the substitution product [Cp2Fe2(CO)3L] (L = MeCN) (equation 19). 

Table III gives the half-life of radicals, for the same range of variables as are presented in Tables I and II. In an experiment with flash photolysis, the radical concentration may be about 1 mm. and the half-life is about 1 iMsec. In an experiment with a intense steady source close to the reaction vessel, the radicals may reach a concentration of lO Ycm. and the half-life is about 0.01 sec. In an experiment with weak sources or light sent through a typical monochromator, the radical concentration is typically 10 /cm. and the lifetime is 1 to 10 sec. In the latter case radicals will have time to diffuse to the walls, and thus two investigators may unwittingly be studying two totally different reactions even when they wanted to study the same photolysis, if one had a weak light source and wall reactions and the other had a strong light source with short-lived radicals.

The formation and decay of OH was observed following flash photolysis of mixtures containing 0.1-0.4 Torr NO2, 2.5 Torr H2 and 20-300 Torr He. A typical oscilloscope trace is shown in fig. 2. In this system OH radicals are produced as a result of processes (1) to (3). The rate coefficients for reactions (2) and (3) are /t2 = 2x lO" cm molecule" s and 3 = 5x 10 cm molecule s With the NO2 and H2 concentrations used in these experiments, the half-life of H atoms was 6 s, that of 0( D) atoms much shorter, and OH radicals were essentially formed completely within the life-time of the photolytic flash. The chlorine filter almost completely prevented long wavelength photolysis of NO2 and even if 0( P) atoms were produced, either by photolysis or by quenching of 0( /)) atoms, they would react quite rapidly with NO2... 

Most of the releases of carbonyl sulfide to the environment are to air, where it is believed to have a long residence time. The half-life of carbonyl sulfide in the atmosphere is estimated to be 2 years. It may be degraded in the atmosphere via a reaction with photochemically produced hydroxyl radicals or oxygen, direct photolysis, and other unknown processes related to the sulfur cycle. Sulfur dioxide, a greenhouse gas, is ultimately produced from these reactions. Carbonyl sulfide is relatively unreactive in the troposphere, but direct photolysis may occur in the stratosphere. Also, plants and soil microorganisms have been reported to remove carbonyl sulfide directly from the atmosphere. Plants are not expected to store carbonyl sulfide. 

As with most organochlorine insecticides, heptachlor and its epoxide are highly persistent in soils, with a reported representative field half-life of 250 days. Heptachlor and its epoxide are moderately bound to soils. This should significantly limit their mobility. Due to their persistence, even low mobility may result in appreciable movement. Therefore, heptachlor and heptachlor epoxide may pose a risk of groundwater contamination over time. Heptachlor epoxide exhibits a low susceptibility to biodegradation, photolysis, oxidation, or hydrolysis in the environment. 

Perutz reported the time resolved infrared (TRIR) study of CpRh(CO)2 in cyclohexane solution. A species was observed with a lifetime of 15 ms, assigned as CpRh(CO)(c-hexyl)H. If CO is present (1.5 atm), the intermediate decays with a half-life of 1.7 ms. Similar observations were made if CpRh(CO)(C2H4) was used to prepare the reactive intermediate. Laser flash photolysis experiments show the formation of the hydrocarbon activation adduct within 400 ns of the flash, but did not provide evidence for an intermediate prior to its formation [34]. 

PROBABLE FATE photolysis C-C bond photolysis can occur, not important in aquatic systems, photooxidation by U.V. light in aqueous medium 90-95°C, time for the formation of CO2 (% theoretical) 24% 3 hr, 50% 17.4 hr, 75% 45.8 hr, photooxidation in air 9.24 hrs-3.85 days oxidation probably not an important process hydrolysis very slow, not important, first-order hydrolytic half-life 207 days volatilization not an important process, calculated half-life in water 4590 hr 25°C and 1 m depth, based on an evaporation rate of 1.5x10 m/hr sorption important for transport to anaerobic sludges, 30-40% adsorbed on aquifer sand 5°C after 3-100 hr equilibrium time, 75-100% disappearance from soils 3-10 yrs biological processes biotransformation is the most important process other reac-tions/interactions electrochemical reduction with products of benzene and gamma-TCCH has been studied... 

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