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Waste Water Treatment and Reutilization 2011 Part 10

Tham khảo tài liệu 'waste water treatment and reutilization 2011 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 232 Waste Water - Treatment and Reutilization Fig. 9. Normalized concentration profiles of model substrates obtained in the photo-Fenton process. kPhot _ kDark PEF Arc-hkt 56 kApp where kDarkApp and kPhotApp are the rate constants linked to the dark and photo enhanced reactions respectively. The PEF is a useful index that allows evaluating the contribution of photo induced processes in photo-Fenton systems. In addition the apparent half-lives can be used to define an overall photo enhancement factor PEFO by the following relation t hot PEFO 1 57 t1 2 The analysis of PEFO values corresponding to the normalized profiles showed that higher photo enhancements are found for conditions where the dark reaction is slower. This behavior may be interpreted assuming that the rates of the photo induced reactions mostly depend on the photon flux and do not significantly depend on the nature of the substrate or the reaction conditions. Therefore for a relatively constant photochemical contribution the slower the dark reaction is the greater the effect of photoinduced pathways results. 4. Product yields and mechanism of nitrobenzene transformation Nitrobenzene thermal degradation was investigated using Fenton s reagent in several experimental conditions. This section deals with the analysis of the distributions of intermediate reaction products and the mechanisms of nitrobenzene decomposition. 4.1 Initial steps of NBE transformation From the analysis of reaction products distributions as a function of NBE conversion degree a mechanism was proposed for NBE degradation in AOP systems Carlos et al. 2008 . The first steps involve two main pathways hydroxylation pathways which yield phenolic derivatives and the nitration pathway which yields 1 3-dinitrobenzene scheme 1 . Hydroxyl radicals usually react with benzene derivatives by electrophilic addition to form hydroxycyclohexadienyl-like radicals Walling 1975 Oturan and Pinson 1995 that can undergo different processes according to the