## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{1}{c}{\multirow{2}{*}{**Catalyst Type**}} & \multicolumn{5}{c}{**Photo-Fenton with Hydrogen Peroxide**} \\ \cline{2-9}  & **Model** & **Process** & **Rate** & **Iron Amount in** & **Oxidant** & \multirow{2}{*}{**pH**} & \multirow{2}{*}{**Reference**} \\  & **Dye(s)** & **Efficiency** & **Constants k** & **Solution** & **Amount** & & \\ \hline \multirow{8}{*}{Stainless steel} & \multirow{3}{*}{MB} & Discoloration eff.: & & & & & \\  & & max. 99.5\%, & & 0.77–4.05 h\({}^{-1}\) & 0.13–0.28 mg/L & & & \\  & & eff. 74.5\% & & & & & \\ \cline{1-1} \cline{2-9}  & & & Discoloration eff.: & & & & \\ \cline{1-1}  & & max. 93.5\%, & & 0.33–1.33 h\({}^{-1}\) & 0.15–0.32 mg/L & & & \\ \cline{1-1}  & & & eff. 49.9\% & & & & \\ \hline Nano ZVI & \multirow{3}{*}{MB} & Discoloration eff.: & & & & & \\ \cline{1-1}  & particles & & max. 98\% & & & & \\ \hline \multirow{4}{*}{Fe-Ni/SiO\({}_{2}\)} & \multirow{3}{*}{MB} & Discoloration eff.: & & & & & \\ \cline{1-1}  & & max. 99.80\% at & & & & & \\ \cline{1-1}  & & pH 3 & & & & & \\ \cline{1-1}  & & and 83\% at pH=9 & & & & & \\ \hline Au-Fe\({}_{3}\)O\({}_{4}\)/ & \multirow{3}{*}{MB} & Discoloration eff.: & & & & & \\ \cline{1-1}  & graphene & & max. 99\% & & & & \\ \hline Fe\({}_{3}\)O\({}_{4}\) and & \multirow{3}{*}{MB} & Discoloration eff.: & & & & & \\ \cline{1-1}  & & max. 95\% & & & & & \\ \hline nitrilotriacetic & \multirow{3}{*}{MB} & Discoloration eff.: & & & & & \\ acid-bonded Fe\({}^{2+}\) & & & max. 97.4\% & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Parameters of selected photo-Fenton processes from the reference literature compared to those of the present study. By analysing the data presented in Table 3, it can be seen that using austenitic stainless steel as a photo-Fenton catalyst determines good mineralization and discoloration efficiencies at near-neutral pH values. Compared with the literature data, attained using hydrogen peroxide or persulfate and Fe\({}^{2+}\) salts or ZVI nanoparticles as an iron source, at the same pH values as used in this study (6-7), the mineralization efficiency can be considered practically nil [33; 42; 48; 49]. Comparable degradation efficiencies of already-reported photo-Fenton systems are reported to be achieved only under acidic pH values (3-4), which generate total iron concentrations in solution ranging from 0.6 to 150 mg/L. Compared to this study, these values are up to 38 times higher, necessitating further separation steps in a real-life wastewater treatment pilot installation. Also, photo-Fenton on solid stainless steel supports, as reported herein, can operate at oxidant concentrations of 20 to 460 times lower than those reported in the reference literature [44; 46], reducing the costs and the oxidative degradation of the installation. The reaction rates obtained are similar to those reported in the reference literature for ZVI nanoparticles or iron salts (except when using UV irradiation) but under nearly-neutral pH conditions. A comparison of this study's reaction rates with those from the reference literature under neutral pH conditions was not possible due to the lack of such data for MB and MO. Regarding the surface chemistry of the catalyst, the relative wt.% of the elements of interest (Fe, Cr, Ni, C, O) before and after the photo-Fenton degradation process for both hydrogen peroxide and potassium persulfate ions at their highest concentration (1667 ug/L) used in this study was determined through XPS spectroscopy (Figure 4a, Table 4). It can be seen that the most notable modification in the surface chemistry of stainless steel is the increase in oxygen content with 107% in the case of hydrogen peroxide and with 159% in the case of potassium persulfate.