**2. Chemical and biological mechanisms involved in homogeneous photocatalysis**

#### **2.1 Chemistry**

The chemical mechanisms involved in the Fenton reaction are well known since early of the past century. The reactions of iron (II) salts with hydrogen peroxide have been widely studied for decades and the main reactions involved are summarized in Table 1 (Orozco et al., 2008; Gallard and De Laat, 2000; Gallard et al., 1999).


Table 1. Chemical reactions involved in the Fenton reaction (Fe (II) and H2O2).

When the process is carried out under radiation, called the photo-Fenton process, the influence of radiation increases the oxidation rate of the pollutant when compared to the conventional Fenton reaction (Bandala et al., 2007). Several possible reaction schemes for the photo-induced system have been proposed as well as the formation of different complexes studied by spectrophotometry. From these studies, the influence of iron oxidation state and the specific counter ion of the iron salt employed have been demonstrated to affect both the decomposition of H2O2 as well as the overall efficiency of the photo-Fenton process for the oxidation of some model pollutants.


Table 2. Suggested side reactions related with the iron salt counter ion during Fenton reaction.
