## 2 Materials and Methods
### Photocatalytic Test  
In order to determine the photocatalytic degradation of each pollutant of the experiments shown in Table 2, photocatalytic tests in cylindrical round-bottomed photocatalytic cells were performed. These cells have a diameter of 4.5 \(\times\) 10\({}^{-2}\) m and a volume of 39.7 \(\times\) 10\({}^{-6}\) m\({}^{3}\) and are made of quartz glass. For all the different pollutants used, their aqueous solutions were prepared. The initial absorbance of the pollutant is computed by the spectrophotometer UV-Vis (U-2001, Hitachi, Tokyo, Japan) and set to 7 \(\times\) 10\({}^{-6}\) M (A\({}_{\text{initial}}\)), in pH = 6.  
The experimental procedure starts with the oxygenation of the pollutant by purging it with ultrapure O\({}_{2}\) gas in a dark bubbler container for 1 h. A sufficient amount of N-doped TiO\({}_{2}\) powder and pollutant are then placed in the cells so that the photocatalyst per pollutant load (g/L) of every test is met. Prior to the irradiation test, the powders dispersed in the pollutant solution are kept in the dark, so that there are no absorption phenomena altering the photocatalytic results. The cells are then placed in a photoreceptor (0.5 m \(\times\) 0.4 m \(\times\) 0.3 m dimensions) with reflective walls (I\({}_{\text{cutoff}}\) = 320 nm), equipped with white light LED tape of 6 W/m, 870 lm/m, and 12 V on top. All photocatalytic tests are conducted under visible light irradiation and continuous stirring (300 rpm). To measure the absorption of every pollutant at a specific time, the shift and intensity of the maximum of the absorption curve is monitored using the spectrophotometer.  
In the photocatalytic tests of 100 min total duration, a quantity of the pollutant was selected every 10 minutes to measure its concentration with the aid of the spectrophotometer. In the 150 min total duration tests, a similar process was followed every 30 min, while in  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Levels**} & \multicolumn{4}{c}{**Parameters (Factors)**} \\ \cline{2-5}  & **A-Photocatalyst/Pollutant** & **B-Pollutant** & **C-Distance from Light** & **D-Irradiation Time** \\  & **(g/L)** & **Source (cm)** & **(Total min/Measuring** \\  & & & & **Gap Time)** \\ \hline
1 & 0.1 & Methylene Blue & 5 & 100/10 \\
2 & 1 & Rhodamine B & 10 & 150/30 \\
3 & 10 & Brilliant Green & 15 & 200/20 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Taguchi orthogonal array L9 factors and their levels.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Experiment No.**} & \multicolumn{4}{c}{**Control Factors**} \\ \cline{2-5}  & **A** & **B** & **C** & **D** \\ \hline
1 & 0.1 g/L & Methylene Blue & 5 cm & 100 min/10 min \\
2 & 0.1 g/L & Rhodamine B & 10 cm & 150 min/30 min \\
3 & 0.1 g/L & Brilliant Green & 15 cm & 200 min/20 min \\
4 & 1 g/L & Methylene Blue & 10 cm & 200 min/20 min \\
5 & 1 g/L & Rhodamine B & 15 cm & 100 min/10 min \\
6 & 1 g/L & Brilliant Green & 5 cm & 150 min/30 min \\
7 & 10 g/L & Methylene Blue & 15 cm & 150 min/30 min \\
8 & 10 g/L & Rhodamine B & 5 cm & 200 min/20 min \\
9 & 10 g/L & Brilliant Green & 10 cm & 100 min/10 min \\ \hline \hline \end{tabular}
\end{table}
Table 2: Modified L9 orthogonal array.  
the case of the 200 min total duration tests, the process was followed every 20 min. The ratio of the measured absorption each time (A) to the initial (Ainitial) one corresponds to the ratio of \(C/C_{0}\) pollutant concentrations. The cuvettes used for measuring the absorption were made of quartz.