**3.2.2 Photodegradation of pollutants**

During the same period, Frank and Bard applied TiO2 photocatalysis in the destruction of pollutants (Frank & Bard, 1997). Their preliminary studies on the oxidation of cyanide to cyanate, by using four different forms of TiO2 under xenon lamp, opened the way for the photodegradation of environmental pollutants.

In the last three decades, TiO2 powders have known wide applications into the recovery of water of industrial, agricultural or civil origin, as well as the decontamination of atmosphere and soil, through the mineralization of the pollutants, or at least their transformation into non-toxic compounds (Fox & Dulay, 1993; Gaya & Abdullaha, 2008; Hoffmann et al., 1995; Ravelli et al., 2011). Of course, the great advantage of these photocatalytic systems depends on the fact that they do not need the use of stoichiometric quantities of chemicals, potentially themselves polluting, but they act under light (often sunlight represents the best choice) in the presence of oxygen (Malato et al., 2007).

Moreover, the high band-gap value of TiO2 allow to oxidize various organic substrates, such as hydrocarbons and their derivatives to lower molecular weight ozidized species and CO2, volatile organic compounds (VOCs) and nitrogen oxides (NOx) present in urban atmosphere (Carp et al., 2004).
