**3. Prospectives and Photocatalytic Approaches in Depollution Technologies**

The novelty of this study comes from the fact that this work makes a clear discrimination between oxidative degradation of pollutants, typically leading to the formation of various intermediates, and mineralization, consisting of oxidation to unharmful CO2.

The first condition to apply photocatalysis in depollution technologies is that the organic pollutant should be mineralized to unharmful CO2. The advantage of using solar light in depollution is obvious. Other restrictive conditions hindering of large– scale application of photocatalysis in pollutant removal are the low reaction rates and photocatalytic material-related. Despite the considerable advances in the abatement of numerous recalcitrant compounds, there are remaining challenges to overcome for largescale practical application. In the meantime, the by-products should be identified and quantified, and their environmental toxicity should be assessed.

The stringent goals in photocatalytic depollution technologies are: (i) high selectivity to CO2; (ii) performant materials characterized by non-toxicity, low band gap, stability, low production costs, high recyclability and (iii) high reaction rates. The efficiency and selectivity of a photocatalyst to carbon dioxide or other unharmful compounds depend on the type of the photocatalytic material and the operating reaction parameters [193]. Another challenge consists in the fact that the majority of photocatalytic systems are based on TiO2 or ZnO, which have large band gap energies (3.1–3.3 eV), thus are mostly active in UV radiation which accounts for only 5% of sunlight [194,195].
