Reaction photo-initiated AOPs

Photo-initiated AOPs are subdivided into VUV and UV oxidation that are operated in a homogeneous phase, and in photocatalysis (Fig. 5-15). The latter can be conducted in a homogeneous aqueous phase (photo-enhanced Fenton reaction) or in a heterogeneous aqueous or gaseous phase (titanium dioxide and certain other metal oxide catalysts). These techniques apply UV-A lamps or solar UV/VIS radiation and they are in pre-pilot or pilot status. According to Mukhetjee and Ray (1999) the development of a viable and practical reactor system for water treatment with heterogeneous photocatalysis on industrial scales has not yet been successfully achieved. This is mainly related to difficulties with the efficient distribution of electromagnetic radiation (UV/VIS) to the phase of the nominal catalyst. 

Tab. 5-2 presents several examples of recent research concerning photo-initiated AOPs that are related to the photooxidation or photominerahzation of specific compounds or to the diminution of global water parameters. It is a long way from being a comprehensive list (this would inevitably go beyond the scope of this book), and it was randomly selected from the current Hterature. Nevertheless, it demonstrates the enormous amount of research activity in the field of photo-initiated AOPs in aqueous systems that elucidate reaction mechanisms and that establish the broad application potential of these processes. Examples of O3-UV AOPs are not included. They were reviewed by GoUschalk et al. (2000). 

Well studied primary reactive species in radiation- or photo-initiated reactions of auxiliary oxidants in an aqueous phase are hydrated electrons (eaq), hydrogen atoms (H ) and hydroxyl radicals ( OH), the last being by far the most important ones in photo-initiated AOPs. The formation and reactivity of ejq and of H were described by Hart and Anbar (1970) and by Buxton et al. (1988). Hydrated electrons can be produced by VUV photolysis of water, by photolysis of aqueous solutions of [FelCNlq]" or of V with formation of [Fe(CN)5] and il2, respectively (cf. Buxton et al, 1988). 

From this table the following conclusions may be drawn. Firstly, the effectiveness of photo-initiated AOPs is strongly dependent on the nature of the substrates secondly, phenol and its halogenated derivatives are very sensitive to the photo-Fmton reaction and thirdly, this table demonstrates that the photo-Fenton, the H2O2-UV and the O3-UV AOP seem to be very efficient in oxidative degradation of various organic substrates. 

However, it must be stated that the reaction pathways are different during the various treatment routes. This fact manifests itself in the different number and types of intermediate oxidation products that have been identified with identical substrates during different AOPs (cf Rajeshwar, 1996). Further, the optimum conditions for a specific photo-initiated AOP treatment depend mainly on the nature of the waste or model water. For instance, H2O2-O3 treatment (without irradiation) can have advantages in the treatment of waters with high inherent UV ab-... 

Levenspid earlier presented, in 1972, a qualitative discussion about the product distribution related to photochemical reactions comparing batch and batch recirculation photochemical reactors. The essentials of this discussion can be transferred to photo-initiated AOPs (at least to the H2O2-UV process), which at low concentrations of a pollutant M ([M] <100 mg L ) usually follow an overall first order reaction kinetics (Bolton et al, 1996). 

Fig. 8.15 Summary of factors influencing the reaction kinetics and the process efficiency of photo-initiated AOPs.

As already menhoned above, AOPs mainly rely on photo-inihated oxidahons (Fig. 5-10, reactions 4). However, depending on the particular situahon of water or air contamination, photo-induced oxidation and photooxygenahon reachons may compete effechvely with photo-initiated oxidations generating a complex mechanishc scheme (cf. Zepp, 1988). 

The implications of the versatile reaction mechanisms depicted in Figs. 7-1 to 7-4 are profound with respect to the complete understanding and hence to the kinetic modeling of AOPs. Despite the complexity of these photo-initiated reactions, it is possible to model AOPs with sufficient precision if all the rate constants of OH radical reactions involved and those of all other elementary reactions are known (Crittenden et al., 1999). Most importantly, the structures and the concentrations of all intermediary reaction products must be known. In addition, photoreactor specific parameters have to be included, such as the incident photon flow d>p and the dimensions of the irradiated volume. This task can be achieved for example... 

Thus, two rate equations (Eqs. 8-9 and 8-10) were derived by steady-state analysis, which describes the zero and the first order overall reaction rates that are common for many photo-initiated (and other) AOPs in water and air. 

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