Photochemical Advantages And Disadvantages

Photochemical Oxidation:

The photochemical technologies present the advantages to be simple, clean, relatively inexpensive, and generally more efficient than chemical AOPs. Also, they can disinfect waters, and destroy pollutants. Consequently, UV radiations have been coupled with powerful oxidants such as O3 and H2O2, including, in some cases, a catalysis with Fe3+ or TiO2, resulting in various kinds of important photochemical AOPs. These photochemical processes are able to degrade pollutants by means of three possible reactions, including photodecomposition, based on UV irradiation, excitation and degradation of pollutant molecules, oxidation by direct action of O3 and H2O2, and oxidation by photocatalysis (with Fe3+ or TiO2), inducing the

Generally, the reaction rate is larger in alkaline medium at pH > 10, which can be attributed to the fact that the HO2− anion, resulting from the ionization of H2O2, can strongly absorb UV radiations and produce free radicals (HO2• and •OH). However, a drawback of this AOP is that the molar absorption coefficient of H2O2 is relatively weak in the UV region, and, consequently, it is necessary to use a rather strong concentration of hydrogen peroxide for an efficient oxidation of organic pollutants like cyanides, benzene, trichloroethylene, tetrachloroethylene by UV irradiation in the presence of hydrogen

1; Simplified diagram of the heterogeneous photocatalytic processes occurring at an illuminated TiO2 particle.

Moreover, the photogenerated holes are strong oxidants, and the photogenerated electrons are reducing enough to yield superoxide from dioxygen. The energy band diagram for TiO2 is presented in Figure 2. As can be seen, the redox potential for photogenerated holes is 2.53 V versus the standard electrode hydrogen (SHE). In these potential conditions, the photogenerated holes are able to either directly oxidize the absorbed pollutants or oxidize the hydroxyl groups located at the TiO2 surface to form •OH radicals, whose redox potential is only slightly decreased (Fujishima et al., 2000).

In addition, it is possible to increase the number of •OH radicals by adding into the photoreactor H2O2 or O3 which can be photolyzed by UV irradiation. During the heterogeneous photocatalytic process, the TiO2 catalyst can be utilized either under dispersed form (powder, aqueous suspension) or in thin film form (fixed TiO2 catalytic layer on solid support). However, a drawback of the dispersed form is the progressive formation of dark catalytic sludge, which diminishes the efficiency of UV irradiation and reduces the photoreactor performances. In contrast, for TiO2 films, there is no need to separate the catalytic particles at the end of the process, but the catalytic layer must be very stable and