## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & \multicolumn{4}{c}{**Photo-Fenton with Hydrogen Peroxide**} \\ \hline
**Catalyst Type** & **Model** & **Process** & **Rate** & **Iron Amount in** & **Oxidant** & **pH** & **Reference** \\  & **Dye(s)** & **Efficiency** & **Constants k** & **Solution** & **Amount** & **pH** & **Reference** \\ \hline Fe-Based & & & & & & & \\ Metal-Organic & MB & Discoloration eff.: & & & & & \\  & & max. 99\% & n.a. & 100 mg/L & 735 mg/L & 6.5 & [46] \\ \hline FeCu bimetallic & & & & & & & \\ system & & & & & & & \\  & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._Furthermore, there is a decrease up to a maximum of 4.5% in the iron content of the material. An increase in the oxygen content on the steel surface and iron dissolution has also been observed in the case of electro-Fenton processes [22; 50]. The dissolved iron provides the ions necessary to initiate and sustain the photo-Fenton photodegradation process. As mentioned in Tables 1 and 2, the iron concentration in solution ranges from 0.13 to 3.95 mg/L. In water, iron concentrations below 0.3 mg/L can be characterized as unnoticeable, whereas levels of 0.3-3 mg/L are generally acceptable [26]. As previously stated, using stainless steel at near-neutral pH generates much less iron in solution than \"traditional\" processes using ZVI iron (Table 3). Regarding Ni and Cr, the other major constituents of stainless steel responsible for its characteristic corrosion resistance in aqueous environments, Table 4 reveals that there is a 0.05% reduction in the weight percentage of Ni on the surface of the stainless steel catalyst submitted to the photo-Fenton process (H\({}_{2}\)O\({}_{2}\) oxidant), while for Cr, a reduction of 0.01% is registered for both oxidants. In solution, at the experimental pH values used in this study, nickel should exist as NiO and/or Ni(OH)\({}_{2}\), as these species are stable at pH 5-15. Consequently, these species tend to separate from aqueous solution and precipitate [51], posing a low cause for environmental concern. Cr oxidation products such as Cr\({}^{3+}\) and/or Cr(VI) ions are rarely present at pH values over 6 because hydrated chromium oxide (Cr(OH)\({}_{3}\)) is hardly water-soluble, and Cr(VI) can be reduced to Cr\({}^{3+}\) (thus precipitating it out of the solution) by the Fe\({}^{2+}\) ions [52; 53]. More probably, since Ni and Cr are known to form a thin oxide layer on the surface of stainless steel in the presence of atmospheric oxygen (oxygen is present in the surface chemistry of the steel material, as in Table 4), the reduction in the surface Ni and Cr in steel could be more likely due to the microabrasion of this oxide layer than metallic Ni and Cr oxidation to water-soluble species [54]. \begin{table} \begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{4}{c}{**Relative Element Concentration (wt.\%)**} & \multirow{2}{*}{**Fe Chemical Species (2p)**} \\ \cline{2-2} \cline{4-7}  & **Fe2p** & **Cr2p** & **Ni2p** & **O1s** & **C1s** & **(rel. \%)** \\ \hline \multirow{2}{*}{St-0} & \multirow{2}{*}{70.48} & \multirow{2}{*}{17.9} & \multirow{2}{*}{9.54} & \multirow{2}{*}{2.02} & \multirow{2}{*}{0.06} & Fe\({}^{0}\) & 98.4 \\  & & & & & & Fe\({}^{3+}\) & 1.6 \\ \hline \multirow{2}{*}{St-H\({}_{2}\)O\({}_{2}\)} & \multirow{2}{*}{68.47} & \multirow{2}{*}{17.8} & \multirow{2}{*}{9.50} & \multirow{2}{*}{4.19} & \multirow{2}{*}{0.04} & Fe\({}^{0}\) & 90.3 \\  & & & & & Fe\({}^{2+}\) & 7.3 \\ \multirow{2}{*}{St-S\({}_{2}\)O\({}_{8}\)\({}^{2-}\)} & \multirow{2}{*}{67.37} & \multirow{2}{*}{17.8} & \multirow{2}{*}{9.55} & \multirow{2}{*}{5.24} & Fe\({}^{3+}\) & 2.4 \\ \hline \hline \end{tabular} 
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{St-S\({}_{2}\)O\({}_{8}\)\({}^{2-}\)} & \multirow{2}{*}{67.37} & \multirow{2}{*}{17.8} & \multirow{2}{*}{9.55} & \multirow{2}{*}{5.24} & Fe\({}^{0}\) & 89.1 \\  & & & & & Fe\({}^{2+}\) & 7.8 \\ \multirow{2}{*}{St-S\({}_{2}\)O\({}_{8}\)\({}^{2-}\)} & \multirow{2}{*}{67.