Photochemical half-life

There is a clear dependence of the cross sections with temperature. Although the photochemical half-life increases with latitude, the decomposition is mainly thermal within the boundary layer. If the temperature dependence in the cross sections were not taken into account, calculations would predict a half-life governed solely by photolysis. 

Multicomponent liposomes reveal optimal UV protection in combination with me y-cyclodextrin complex of me vitamin present in me aqueous phases and lipid-soluble protectors (UV absorbers and antioxidants) present in me lipid phases (Loukas et al., 1995). The experimental photochemical half-life of riboflavin can be increased up to 266-fold by these complex liposome formulations. The liposomal composition is also important for me stabilizing effect obtained as incorporation of riboflavin into neutral and negatively charged liposomes increases photochemical stability, while association with positively charged liposomes leads to a decreased photochemical stability (Habib and Asker, 1991). Incorporation of retinol into multilamellar liposomes is demonstrated to extend the shelf-life of retinol under various conditions of pH, temperature, and light exposure (Lee et al., 2002). 

The rates for sunlight-caused photodecomposition of picloram in aqueous solutions were determined by five experiments. The photolysis follows pseudo first-order kinetics for concentrations up to 4.14 X 10 M and in circulating solutions as deep as 3.65 m. Hazy sunlight and water impurities had only a small effect on rate in the systems in which they were studied. A linear relationship relates the photochemical half-life of picloram to solution depth for solutions from 0.292-. 65 m deep. 

Aquatic fate volatilizes slowly (half lives river 23 days lakes 73 days). Atmospheric fate degrades photochemically (half life 9h). 

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 dominant transformation process for trichloroethylene in the atmosphere is reaction with photochemically produced hydroxyl radicals (Singh et al. 1982). Using the recommended rate constant for this reaction at 25 °C (2.36x10 cm /molecule-second) and a typical atmospheric hydroxyl radical concentration (5x10 molecules/cm ) (Atkinson 1985), the half-life can be estimated to be 6.8 days. Class and Ballschmiter (1986) state it as between 3 and 7 days. It should be noted that the half-lives determined by assuming first-order kinetics represent the calculated time for loss of the first 50% of trichloroethylene the time required for the loss of the remaining 50% may be substantially longer. 

Air t1/2 = 6 h with a steady-state concn of tropospheric ozone of 2 x 10-9 M in clean air (Butkovic et al. 1983) t/2 = 2.01-20.1 h, based on photooxidation half-life in air (Howard et al. 1991) calculated atmospheric lifetime of 11 h based on gas-phase OH reactions (Brubaker Hites 1998). Surface water computed near-surface of a water body, tl/2 = 8.4 h for direct photochemical transformation at latitude 40°N, midday, midsummer with tl/2 = 59 d (no sediment-water partitioning), t,/2 = 69 d (with sediment-water partitioning) on direct photolysis in a 5-m deep inland water body (Zepp Schlotzhauer 1979) t,/2 = 0.44 s in presence of 10 M ozone at pH 7 (Butkovic et al. 1983) calculated t,/2 = 59 d under sunlight for summer at 40°N latitude (Mill Mabey 1985) t,/2 = 3-25 h, based on aqueous photolysis half-life (Howard et al. 1991) ... 

Surface water computed near-surface t,/2 = 0.58 h for direct photochemical transformation at latitude 40°N, midday, midsummer, t,/2 = 4.2 h for direct photolysis in a 5-m deep inland water body with no sediment-water partitioning and t,/2 = 5.9 d with sediment-water partitioning (Zepp Schlotzhauer 1979) t,/2 = 0.68-2.04 h, based on estimated sunlight photolysis half-life in water (Lyman et al. 1982 quoted, Howard et al. 1991) ... 

Their mobility may be increased by the simultaneous presence of organic solvents such as mineral oil. The half-life of TCDD in soil has been reported as 10-12 years, whereas photochemical degradation seems to be considerably faster but with a large variation that might be explained by experimental differences (solvents used, etc.). Highly chlorinated PCDD/PCDFs seem to be more resistant to degradation than those with just a few chlorine atoms. 

Since the reverse reaction is thermally disallowed, the products may be unexpectedly stable. Thus cyclopentadiene can be partially converted photochemically into bicyclo [2,1,0] pentene, which in spite of having a strained structure has a half life of about two hours at room temperature. 

Endrin ketone may react with photochemically generated hydroxyl radicals in the atmosphere, with an estimated half-life of 1.5 days (SRC 1995a). Available estimated physical/chemical properties of endrin ketone indicate that this compound will not volatilize from water however, significant bioconcentration in aquatic organisms may occur. In soils and sediments, endrin ketone is predicted to be virtually immobile however, detection of endrin ketone in groundwater and leachate samples at some hazardous waste sites suggests limited mobility of endrin ketone in certain soils (HazDat 1996). No other information could be found in the available literature on the environmental fate of endrin ketone in water, sediment, or soil. 

Field studies on the transformation of endrin in the atmosphere were not located in the available literature. Photochemical isomerization of endrin, primarily to the pentacyclic ketone commonly called delta ketoendrin or endrin ketone, was observed after exposure of thin layers of solid endrin on glass to sunlight (Burton and Pollard 1974). Minor amounts of endrin aldehyde were also formed in this reaction. Results of seasonal studies indicated that this isomerization would proceed with a half-life (first-order kinetics) of 5-9 days in intense summer sunlight, with complete conversion to the pentacyclic ketone in 15-19 days. Knoevenagel and Himmelreich (1976) reported that photodegradation of solid endrin in the laboratory... 

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). 

Phenol is released into the air and discharged into water from both manufacturing and use. Based on its high water solubility (see Table 3-2) and the fact that it has been detected in rainwater, some phenol may wash out of the atmosphere however, it is probable that only limited amounts wash out because of the short atmospheric half-life of phenol. During the day, when photochemically produced hydroxyl radical concentrations are highest in the atmosphere, very little atmospheric transport of phenol is likely to occur. 

Decomposition rates Negligible rate of hydrolysis Half-life of 80 days in air with photochemically produced hydroxyl radicals Mabey and Mill 1978 Hampson 1980... 

Photolytic. Based on data for structurally similar compounds, acenaphthylene may undergo photolysis to yield quinones (U.S. EPA, 1985). In a toluene solution, irradiation of acenaphthylene at various temperatures and concentrations all resulted in the formation of dimers. In water, ozonation products included 1,8-naphthalene dialdehyde, 1,8-naphthalene anhydride, 1,2-epoxyacenaphthylene, and 1-naphthoic acid. In methanol, ozonation products included 1,8-naphthalene dialdehyde, 1,8-naphthalene anhydride, methyl 8-formyl-1-naphthoate, and dimethoxyacetal 1,8-naphthalene dialdehyde (Chen et al., 1979). Acenaphthylene reacts with photochemically produced OH radicals and ozone in the atmosphere. The rate constants and corresponding half-life for the vapor-phase reaction of acenaphthylene with OH radicals (500,000/cm ) at 25 °C are 8.44 x lO " cmVmolecule-sec and 5 h, respectively. The rate constants and corresponding half-life for the vapor-phase reaction of acenaphthylene with ozone at 25 °C are... 

Surface Water. In a 5-m deep surface water body, the calculated half-lives for direct photochemical transformation at 40 °N latitude, in the midsummer during midday were 3.2 and 13 d with and without sediment-water partitioning, respectively (Zepp and Schlotzhauer, 1979). The volatilization half-life of benzo [a] pyrene from surface water (1 m deep, water velocity 0.5 m/sec, wind velocity 1 m/sec) using experimentally determined Henry s law constants is estimated to be 1,500 h (Southworth, 1979). 

Photolytic. Fluorene reacts with photochemically produced OH radicals in the atmosphere. The atmospheric half-life was estimated to range from 6.81 to 68.1 h (Atkinson, 1987). Behymer and Hites (1985) determined the effect of different substrates on the rate of photooxidation of fluorene (25 tig/g substrate) using a rotary photoreactor. The photolytic half-lives of fluorene using silica gel, alumina, and fly ash were 110, 62, and 37 h, respectively. Gas-phase reaction rate constants for OH radicals, NO3 radicals, and ozone at 24 °C were 1.6 x lO , 3.5 x 10 and <2 x 10in cmVmolecule-sec, respectively (Kwok et al., 1997). 

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