## 3 Results and Discussion
### Taguchi Methods Results
Taking into account the abovementioned, a photocatalytic protocol is proposed for testing the degradation of azo dyes in the presence of N-TiO\({}_{2}\) nanoparticles under visible light irradiation. For Factor A, the ratio of photocatalyst to pollutant load (g/L), it is revealed that as the amount of catalyst increases, so does the degradation of the pollutant. The 10 g/L choice results in the highest degradation of the pollutant, though this is not considered to be an economical choice. Therefore, the amount of 1 g/L is preferable and proposed for the protocol, as it exhibits a similar final degradation. It is obvious that the third level of 0.1 g/L leads to a low degradation of the pollutant, sometimes equivalent to photolysis results. Regarding Factor B, pollutant, the intrinsic effect of each pollutant should be investigated in order to choose. As the photocatalysis results showed, BG and MB exhibit a high percentage of photolysis. Consequently, their final results of C/C\({}_{0}\) degradation are fictitious. It is clear that Rhodamine B is more stable, has a lower photo-sensitivity, and exhibits a better fitting to the pseudo-first-order kinetics model among the three tested pollutants. The choice of Rhodamine B will ensure that the results of the photocatalysis are connected as much as possible with the effect of the catalyst itself. For Factor C, distance from the irradiation source, while a higher degradation would be expected at 5 cm distance, the exact opposite is observed. At 5 cm, the lowest degradation is recorded. This is probably due to the lack of light reflecting on the walls of the reactor and reaching back to the cell, resulting in enhanced radiation. At 10 cm, the degradation is slightly greater than that achieved at 15 cm distance from the light source, so the level of 10 cm is chosen for the proposed protocol. Finally, Factor D, irradiation time, affects the process as expected. That is, the more time the pollutant is irradiated, the more the degradation proceeds. The chosen level for the protocol is that of 150/30 min, as the choice of 200/20 min is not cost beneficial. The proposed protocol of photocatalytic tests regarding the degradation of azo dyes in the presence of N-TiO\({}_{2}\) nanoparticles under visible light irradiation is summarized in Table 4. The protocol applies to a batch, horizontal, rectangular, vis-LED equipped reactor with reflective walls. The main contribution of the Taguchi method lies in the economic benefit it offers, as proved in this study where 9 experiments provided the optimized parameters of photocatalysis, whereas all possible combinations of parameters and levels would require the realization of 3\({}^{4}\) = 81 experiments in total. Similar studies adopting the Taguchi method can be found in the literature for photocatalysis under UV irradiation, with a design of 4-5 factors with 3-4 different levels, as well. These studies focus mainly on ZnO-composite NPs, for degrading Lidocaine HCl [59] and reducing Cr(VI) [60]. In the case of Lidocaine HCl, photocatalysis of CuO/ZnO NPs was studied and the four factors were the irradiation time, pH, catalyst load, and concentration of drug and dosage of H\({}_{2}\)O\({}_{2}\) (for promoting photocatalytic degradation). The findings \begin{table}
\begin{tabular}{c c} \hline \hline
**Factor** & **Level for Protocol** \\ \hline A-Photocatalyst/pollutant (g/L) & 1 \\ B-Pollutant & Rhodamine B \\ C-Distance from light source (cm) & 10 \\ D-Irradiation time (total min/measuring gap time) & 150/30 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Proposed photocatalysis protocol. showed a pH of 6-8, 360 min of irradiation, 0.48 g/1 catalyst load, 20-30 mg/L Lidocaine HCl, and 7 mM H\({}_{2}\)O\({}_{2}\) are the optimum conditions. Regarding the reduction of Cr (VI), ZnO/Todoroke nanocatalyst was synthesized and the catalysis process was optimized, analyzing the pH, Cr concentration, catalyst load, and irradiation.