**1. Introduction**

338 Solar Radiation

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Taking into consideration the impacts of increasing concern as a result of human activity on the environment in recent decades, different approaches have been developed and proposed to minimize the persistence of organic and inorganic pollutants, not only the dangerous or biorecalcitrant. Industrial waste discharges, those from domestic sewage, and so-called emerging contaminants (pesticides, hormones and drugs), among others, have caused numerous problems for the sustainability of ecosystems (Amat et al.,2011).

In general, environmental problems are largely associated with the disposal of waste into sewers, rivers and eventually into the ocean. The damage caused to biota by these discharges is incalculable (Hermann & Guillard, 2002; Corcoran et al., 2010).

New and effective forms of wastewater treatment are essential to enable a responsible economic development of the planet ensuring its sustainability for future generations (Amat et al.,2011). These processes need to be environmentally safe, providing the elimination of contaminants and not just promoting a phase transfer, ensuring the reuse of water (Hermann & Guillard, 2002;Machado et al., 2003a; Sattler et al.,, 2004; Wojnárovits et al., 2007).

Besides the application in order to minimize the environmental impacts of human action, via photocatalytic processes, semiconductor oxides have also been employed in producing chemical raw materials through specific chemical reactions (Kanai et al., 2001; Murata et al., 2003; Amano et al., 2006; Denmark & Venkatraman, 2006; Hakki et al.,2009; Swaminathan & Selvam, 2011; Swaminathan & Krishnakumar, 2011), in the conversion of solar energy into electricity (Prashant, 2007; Patrocínio et al., 2010; Huang et al., 2011; Zhou et al., 2011) and

<sup>\*</sup> During his leave as Visiting Professor in the Departamento de Química at the Campus Catalão of Universidade Federal de Goiás.

production of hydrogen for subsequent generation of energy (Jing et al., 2010; Kim & Choi, 2010; Melo & Silva, 2011).
