## 3 Results and Discussion  
The decolorization of both methylene blue and methylorange dyes was successfully achieved through the use of the stainless steel photocatalyst in the presence of both types of oxidant species--namely, peroxide and persulfate ions. High photodecoloration efficiencies (>99.1%) were registered in the case of MB for all hydrogen peroxide concentrations, indicating that the disruption of the chromophore groups for this dye occurs in a more facile manner than for methylorange, for which efficiencies of 91-93.5% were registered (Figure 2a,b). As far as photodecoloration is concerned, for the hydrogen peroxide concentration range reported in this paper and under close to neutral pH conditions, there seems to be a less marked influence of peroxide ion concentration on the photo-Fenton process efficiency for both dyes. Photodecoloration in the presence of persulfate ions also occurs with a high efficiency in the case of MB (88.4% at a 999 mg/L persulfate concentration, Figure 2a), although in this case, after the aforementioned concentration, there is a sharp efficiency decrease. This could be due to the fact that the higher amount of iron ions generated from stainless steel through persulfate-aided oxidation hinders the efficient harvesting of visible light [29,30]. In the experiments where the photodecoloration occurs in the presence of the sulfate and hydroxyl radicals scavenger tert-butanol (TBu), a decrease of up to 55% (for MB, Figure 2a) and up to 62% (for MO, Figure 2b) in the photo-Fenton efficiencies was registered for both oxidants, indicating the critical role that these species play in the degradation of organic matter. A more significant drop in photodecoloration efficiency was registered for persulfate due to tert-butanol inhibiting the generation of both sulfate and hydroxyl radicals. When mineralization comes into discussion, hydrogen peroxide presents the highest photodegradation efficiency for MB (74.5% vs. 69.9% for persulfate, at 1666 \(\mathrm{\SIUnitSymbolMicro g}\)/L, Figure 2c), while for MO, persulfate is more efficient at 999 \(\mathrm{\SIUnitSymbolMicro g}\)/L (57%, Figure 2c). The decolorization kinetic was fitted with the pseudo-first-order kinetic model (Langmuir-Hinshelwood) from Equation (3) [31] and expressed in Figure 3a,b (MB), and Figure 3c,d (MO). \[\mathrm{ln}\bigg{(}\frac{c_{0}}{c_{t}}\bigg{)}=\mathrm{kt} \tag{3}\]  
Figure 2: (a): Photodecoloration efficiency for MB (without and with added tert-butanol scavenger); (b): decolorization efficiency for MO (without and with added tert-butanol scavenger); (c): COD-based mineralization efficiency for MB and MO. In Equation (3), k represents the pseudo-first-order rate constant. The fitting parameters for this model (including the correlation coefficients, R\({}^{2}\)) are depicted in Tables 1 and 2. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **c\(\mathbf{c_{2}O_{2}}\)** \\ **(\(\mu\)g/L)** \\ \end{tabular} } & \multicolumn{2}{c}{**Kinetic Data for MB**} & \multicolumn{2}{c}{**Kinetic Data for MO**} \\ \cline{2-7}  & **Total Fe** & \multicolumn{2}{c}{**Rate Constants**} & \multicolumn{2}{c}{**Total Fe**} & \multicolumn{2}{c}{**Rate Constants**} \\ \cline{2-7}  & **(mg/L)** & **\(k_{1}\) (h\({}^{-1}\))** & **\(k_{2}\) (h\({}^{-1}\))** & **(mg/L)** & **\(k_{1}\) (h\({}^{-1}\))** & **\(k_{2}\) (h\({}^{-1}\))** \\ \hline
333 & 0.13 & 0.77 (0.992) & 2.35 (0.995) & 0.15 & 0.33 (0.962) & 0.35 (0.983) \\
666 & 0.16 & 1.00 (0.987) & 2.39 (0.998) & 0.18 & 0.34 (0.996) & 0.87 (0.980) \\
999 & 0.17 & 1.23 (0.985) & 3.21 (0.996) & 0.21 & 0.39 (0.993) & 1.03 (0.984) \\
1665 & 0.28 & 1.26 (0.992) & 4.05 (0.978) & 0.32 & 0.47 (0.983) & 1.33 (0.989) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Rate constants of the photo-Fenton process as a function of H\({}_{2}\)O\({}_{2}\) concentration.