Photocatalytic Ozonation Reactions

Heterogeneous photocatalysis and ozonation are advanced oxidation processes for eliminating organic contaminants in wastewater. The combination of photocatalysis 

Synergistic effects have been evidenced for perovskites of LaTio isCuo.ssOs type [99]. The best results in terms of pyruvic acid removal and mineralization degree were achieved for the combination of these perovskites with ozone under UV radiation. Under these conditions, a complete decomposition was determined in only 45 min, where the TOC abatement was roughly 80% [58,99]. 

Various parameters such as pH, temperature, light intensity, ozone dosage, and catalyst dosage were found to influence the efficiency of the complex gas-liquid processes. All of these closely relate to the type of substrates as well as to the predominant reaction mechanism [101]. The decomposition rate of formic add by the combination of photocatalysis on Cu(n)/Ti02 and ozonation was found to be 31% higher at maximum than the sirni of the decomposition rates when formic acid was individually decomposed by the two methods, which confirms once more the presence of a synergistic effed of the photocatalysis and ozonation [102], 

A similar behavior was observed for the photocatalytic ozonation of dyes on copper ferrite (CuFe204) nanoparticles. The mineralization occurred via aliphatic carboxylic acids that were deteded as dominant aliphatic intermediates where they were further oxidized slowly to CO2. Finally, inorganic anions were detected as the photocatalytic mineralization products of dyes [103]. 

Liquid-phase oxidation is a green route to producing both the selective synthesis of valuable products and complete mineralization of wastes. Most of these 

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A varying and much more complex mechanistic situation exists in heterogeneous photocatalysis (Fig. 5-13). With respect to the transient oxygen species, comparable overall oxidation reactions are usually observed, but the set of primary reactive oxygen species is slightly different. It is commonly assumed, that superoxide radical anions and hydroxyl radicals are the primary species formed after photogeneration of the electron-hole pair of a semiconductor catalyst in the presence of water and air (Serpone, 1996). In the presence of ozone, ozonide radical anions or are formed by fast electron transfer reaction of superoxide radical anions with O3 molecules. The combination Ti02-03-UV/VIS is called photocatalytic ozonation (Kopf et al., 2000). For example, it was applied for the decomposition of tri-chloroethene in the gas phase (Shen and Kub, 2002). 

Intensification can be achieved using this approach of combination of cavitation and advanced oxidation process such as use of hydrogen peroxide, ozone and photocatalytic oxidation, only for chemical synthesis applications where free radical attack is the governing mechanism. For reactions governed by pyrolysis type mechanism, use of process intensifying parameters which result in overall increase in the cavitational intensity such as solid particles, sparging of gases etc. is recommended. 

As reported by Augugliaro et al. [64] the photocatalysis can be combined with chemical or physical operations. In the first case, when the coupling is with ozonation [65, 66], ultrasonic irradiation, photo-Fenton reaction or electrochemical treatment, which influence the photocatalytic mechanism, an increase of the efficiency of the process is obtained. 

Furthermore, it should be mentioned that photocatalytic processes with the aid of Ti02 can be used for environmental purification [1]. This is due to the fact that the oxidation potential of Ti02 (3.0 V) is considerably higher than that of more conventional oxidizing agents such as chlorine (1.36 V) and ozone (2.07 V). Due to its chemical inertness and non-toxicity Ti02 is compatible with many types of practical catalytic systems. Many photodegradation reactions of noxious, malodorous chemicals, oil on water etc. have been reported. 

As shown in Chapter 5.3.5, from ozone via OH, H2O2 is also produced through the Fenton reaction. Moreover, it is seen that photocatalytic dioxygen reduction is less effective compared with O3 because many reactions scavenge superoxide (return it to O2) and the OH formation needs a three-electron transfer (O2 + 3 e + 2H2O 3 OH + OH) compared with the one-electron transfer step into O3. Therefore, the presence of O3 will enhance all interfacial oxidation processes. 

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