Photochemical degradation processes

Schmitt-Kopplin, P, Hertkorn, N., Schulten, H. R., and Kettrup, A. (1998). Structural changes in a dissolved soil humic acid during photochemical degradation processes under 02 and N2 atmosphere. Environ. Sci. Technol. 32, 2531-2541. 

Reactions 1 and 2 describe reversible photochromism, and 3 and 5 irreversible photochemical degradation processes having rate constants of Pad and pDB - The thermal degradation processes are reactions 4 and 6, with the latter having a negligibly small rate constant as SPs do not degrade appreciably in the absence of UV light. 

Drug substances that are liquids should be exposed in chemically inert and transparent containers. At the end of the exposure period, the samples should be examined for any changes in physical properties (e.g., appearance, clarity, or color of solution) and for assay. The degradants must be examined by a method suitably validated for products likely to arise from photochemical degradation processes. 

Keywords sunlight, heat, hydroperoxides, autoxidation, weathering, photochemical degradation, processing, phosphites, sulphur compounds, metal thiolates, nickel complexes, UV absorbers, HALS, metal deactivators, hindered phenols. 

The characteristics of plastics can change depending on the exposure to thermal, mechanical (shear), oxidative and photochemical degradation processes. The characteristics of the recyclate may be quite different from those of the original virgin plastic. 

Complexed arenediazonium salts are stabilized against photochemical degradation (Bartsch et al., 1977). This effect was studied in the former German Democratic Republic in the context of research and development work on diazo copying processes (Israel, 1982 Becker et al., 1984) as well as in China (Liu et al., 1989). The comparison of diazonium ion complexation by 18-crown-6 and dibenzo-18-crown-6 is most interesting. Becker at al. (1984) found mainly the products of heterolytic dediazoniation when 18-crown-6 was present in photolyses with a medium pressure mercury lamp, but products of homolysis appeared in the presence of dibenzo-18-crown-6. The dibenzo host complex exhibited a charge-transfer absorption on the bathochromic slope of the diazonio band. Results on the photo-CIDNP effect in the 15N NMR spectra of isotopically labeled diazonium salts complexed by dibenzo-18-crown-6 indicate that the primary step is a single electron transfer. 

These results emphasise the important role played by HC1 not only as a catalyst for the dehydrochlorination process but in influencing the distribution of polyene sequences which result from the primary part of the degradation process and the photochemical cross-linking reactions of the polyenylic cations. 

Following biological degradation, the extract is exposed to photochemical degradation, which removes uranium from solution as polyuranate. The metals and uranium are captured in separate treatment steps, allowing for the separation of wastes into radioactive and nonradioactive waste streams. This treatment process does not create additional hazardous wastes and allows for the reuse of the contaminated soil. The technology has been the subject of bench-scale tests and is not currently commercially available. 

The mechanism of the photochemical degradation of catenated silicon derivatives has received considerable attention (25). Substituted cyclic derivatives photochemically extrude a silylene fragment which can be intercepted by appropriate trapping reagents (e.g., trialkylsilanes or 2,3-dimethyl butadiene). This extrusion results in the formation of the corresponding ring contracted cyclopolysilane. The process continues upon additional irradiation until a cyclotetrasilane results which then undergoes... 

The photoionization of amines, which are strong electron donors, is a process of significance in the photochemical degradation of living matter (see section 5.5). 

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