## 2 Materials and Methods  
The methylene blue (MB) cationic dye (C\({}_{16}\)H\({}_{18}\)ClN\({}_{3}\)S, M\({}_{w}\) = 319.85) and methylorange (MO) anionic dye (C\({}_{14}\)H\({}_{14}\)N\({}_{3}\)NaO\({}_{3}\)S, M\({}_{w}\) = 327.33) were purchased from Sigma-Aldrich. The austenitic stainless steel (1.4301; X5 CrNi18-10, AISI 304) had a nominal weight percent chemical composition of 0.07% C, 1% Si, 1.5% Mn, 18% Cr, 9% Ni, Fe balance, determined by atomic emission spectroscopy. The microstructure of the steel, as seen in reflexion microscopy mode (200\(\times\), aqua regia etchant), consists of equiaxed austenite grains with a small amount of d-ferrite at the austenite grain boundaries (Figure 1a). The reagents used in the spectrophotometric determination of the total Fe content--namely, 1,10-phenanthroline (Phen), hydroxylamine, and trisodium citrate--were purchased from Sigma-Aldrich and were of analytical grade. The hydrogen peroxide was used as a 30% wt. aqueous solution (Chemical Company SA, Iasi, Romania). The potassium persulfate (K\({}_{2}\)S\({}_{2}\)O\({}_{8}\)) oxidant of 99.5% purity was purchased from Sigma-Aldrich and used as a 4% vol. aqueous solution. The photodegradation experiments were performed in a closed cylindrical reactor (height 460 mm, diameter 310 mm) under irradiation with four visible domain-emitting light bulbs (5700 K, wavelength distribution of ~400-750 nm) placed annular to the photoreceptor (Figure 1b). The steel samples (20 mm \(\times\) 20 mm \(\times\) 8 mm) were placed in glass beakers, in contact with 20 mL of aqueous MB and MO solutions of 10\({}^{-5}\) M initial concentration (c\({}_{0}\)) at pH values of ~6.5 (the typical pH values of the aqueous dye solutions). The value of the irradiance at the rim of the beaker containing the samples was 1 mW/cm\({}^{2}\). The photodegradation effect was studied with different amounts of added H\({}_{2}\)O\({}_{2}\) solution to make the concentration of peroxide 333 \(\mu\)g/L, 666 \(\mu\)g/L, 999 \(\mu\)g/L, and 1665 \(\mu\)g/L. In the other batch of experiments, a potassium persulfate starting solution was added to the dye  
Figure 1: (**a**): Optical microscopy of the austenitic stainless-steel surface; (**b**): principle scheme of the photoreactor. solutions so as the initial concentration of S2O82- to reach the same values as in the case of peroxide. Each experiment was performed at ambient temperature (22 \({}^{\circ}\)C) in triplicate; the average values are presented in this paper. A Spekol 11 mono-channel spectrophotometer (Carl Zeiss Jena, Germany) was used for monitoring the MB and MO dye solutions decolorization at various time intervals (c), based on their absorbance values at wavelength maxima of \(\lambda\) = 665 nm (MB) and \(\lambda\) = 465 nm (MO). The photodegradation efficiencies (\(\eta\)) were calculated with Equation (1):  
\[\eta=\frac{\varsigma_{0}-\varsigma_{\mathrm{t}}}{\varsigma_{0}}\cdot 100\ (\%) \tag{1}\]  
In addition, to elucidate the type of radicals responsible for the photodegradation, the photo-Fenton experiments were performed with each of the dye solutions spiked with 50 \(\mu\)L tert-butanol (99.8%, Scharlau), which is an efficient quencher of hydroxyl (HO\(\bullet\)) or sulphate (SO42-\(\bullet\)) radicals [25]. To provide a complete picture of the advanced decomposition of MB and MO, not only based on the degradation of chromophore groups, which determines the VIS absorbance decrease, the chemical oxygen demand (COD) values of the irradiated solutions containing the two dyes were determined using the spectrophotometric method [26]. The COD, expressed in mg/L, is proportional to the amount of oxidizable organic substances in water [27].