Photodegradation rate

Chen ZM, Zabik MJ, Eeavitt RA. 1984. Comparative study of thin film photodegradation rates for 36 pesticides. Industrial and Engineering Chemistry Product Research and Development 23 5-11. 

Increase the photodegradation rate by absorbing light and initiating chemical changes. 

Figure 6. Photodegradation rate data on Tinuvin-type UV stabilizers (A) UV-visible spectrophotometric data (B) FTIR data on PMMA films...

Stage I 0 to 120% strain the structure of the elongated polymer is still very close to that of the original material, and, therefore, no large difference is seen in the photodegradation rate. 

The photostability of two polymorphs of nicardipine hydrochloride have been studied using a number of techniques [95]. After irradiation, the drug substance decomposes to a pyridine derivative, and the photodegradation of the /1-form exceeded that of the x-form. It was also found that the color of the two different forms differed with the polymorphic state, but that grinding the two forms lessened the difference in their photochemistries. A correlation between the heat of fusion (measured by differential scanning calorimetry) and the photodegradation rate constant was observed. 

Photolysis half-lives on different atmospheric particulate substrates (approx. 25 (xg/g on substrate) t,2 = 7.0 h on silica gel, t,/2 = 22 h on alumina and t,/2 = 29 h on fly ash (Behymer Hites 1985) first order daytime photodegradation rate constants for adsorption on wood soot particles in an outdoor Teflon chamber k = 0.0077 min-1 with 1000-2000 ng/mg loading and k = 0.0116 min-1 with 30-350 ng/mg loading (Kamens et al. 1988) ... 

Photolytic. The photodegradation rate of sulfotepp in water increased in the presence of humic acid. Under simulated sunlight, the half-lives of sulfotepp in water containing 10 and 100 mg/L of humic acid-potassium salt were 38.4 and 12.4 h, respectively (Jensen-Korte et al., 1987). 

Liberti et al. (1978) reported that TCDD will photodegrade provided a hydrogen donor is added in the presence of sunlight or high-pressure mercury lamps (X = 280-320 nm). The photodegradation rate was found to be related to the intensity of the irradition, temperature, and the medium in which TCDD is dispersed. The primary photodecompositions product of 2,3,7,8-TCDD were 2,3,7-trichlorodibenzo[Ae][l,4]dioxin and hydrogen chloride. 

The effects of surface nature and loading on the photodegradation rate of BeP adsorbed on silica and alumina are shown in Fig. 16.18. The type of adsorbent affects the rate of BeP photodegradation. On silica gel, the rate is approximately... 

Fig. 16.18 Effect of surface nature and loading on the photodegradation rate of adsorbed BeP. The photolysis experiments were carried out in a rotary cell, and the BeP remaining in the sample was determined by HPLC. Uncertainties in measurements are in the range of 4-10% of the absolute values. Reprinted with permission from Fioressi S, Arce R (2005) Photochemical transformation of benzo(e)pyrene in solution and adsorbed on silica gel and alumina surfaces. Environ Sci Technol 39 3646-3655. Copyright 2005 American Chemical Society...

Irradiation of BeP in hexane solution did not lead to the formation of the major derivates produced on adsorbed phases. This result suggests that BeP in nonpolar solvents photodegrades through a different reaction pathway than when adsorbed on surfaces. The type of solvent may affect, on the other hand, BeP photodegradation. In acetonitrile, for example, the photodegradation rate is faster than in hexane, and dione has been detected as one of the products. 

Photodegradation rates of ortho derivates present good correlation with the thermodynamic stability of sigma-complexes formed between the aromatic ring and the surface OH-radicals. Rates decrease in the order -OCH3 (guiacol) > -Cl (2-chlorophenol) -H (phenol) > -OH (catechol). ... 

This is consistent with a study by Feilberg and Nielsen (1999b), who investigated the influence of other aerosol components on the photodegradation rates of representative particle-associated nitro-PAHs in a model system consisting of the nitro-PAH dissolved in cyclohexane along with various known constituents of diesel exhaust and wood smoke particles. These cosolutes included PAHs, substituted phenols, hydroxy-PAHs, oxy-PAHs, and substituted benzaldehydes. 

In the absence of cosolutes, the photodegradation rates depended on the orientation of the nitro group. Thus 1-nitropyrene decayed relatively fast by the nitro-nitrite primary intramolecular photorearrangement process, followed by secondary radical reactions. However, 2-nitropyrene and 2-nitrofluoranthene were stable toward photolysis, consistent with the N02 group being in the same plane as the aromatic rings. 

However, when H-atom-donating cosolutes, e.g., certain phenols, were added, the photodegradation rates of both 1-nitropyrene and 3-nitrofluoranthene increased. In this case, the reaction occurred via H-atom abstraction from the phenol by the electronically excited nitro-PAHs. Feilberg and Nielsen concluded that the photodegradation of nitro-PAHs on both diesel particles and wood smoke proceeds primarily by radical formation. However, H-atom abstraction by the excited triplet states of 1-nitropyrene and 2-nitrofluoranthene may also contribute. 

The marked increase in die photodegradation rate, which shows a bellshaped dependence on pH, reflecting the distribution of =Ti—F. Because the degradation cannot be driven either by =TiO , or by SET, the processes should be mediated by OH ee formed through SET from the solvent (water). 

The photodegradation rate dependence on phenol concentration in the TiOi/F system shows a plateau in the 3 X IO " M to 3 X 10 M range, whereas for naked TiO2, a maximum is reached around 2 X 10 M of phenol followed by a decrease. This behavior is rationalized by the possibility of reductive back reactions of intermediates formed after the first oxidation step. The presence of fluoride could limit the occurrence of this detrimental effect, reducing the interaction of the formed intermediates with the surface. Moreover, also the change of the oxidation pathway changes the amount of the products. 

Titanium dioxide photodegradation rates can be significantly enhanced with H202. With the addition of H202, degradation times of trichloroethylene (TCE) dropped from 75 to 20 min in a study by Tanaka et al. (1989). This enhancement was most likely due to an increase of hydroxyl radicals. The half-lives of pesticides DDVP and DEP were demonstrated to be shortened with the addition of H2Oz (Harada et al., 1990). Similar enhancements were shown for the photodegradation of chloral hydrate, phenol, and chlorophe-nols (Venkatadri and Peters, 1993). 

Pan et al. (1999) reported detailed observations of PAH decay in aerosols under UV light of 253.7 nm. The study used a high volume sampler to collect the total suspended particles in air with quartz filters as the supporter. The results showed that the photo degradation of PAHs under UV radiation took place immediately, and was a first-order reaction. The photodegradation rate constant (k) is related to the oxidation half-wave potential (Ei/2) and the structure of the PAHs. 

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