## 3 Results and Discussion
### Photocatalysis Results and Modeling  
Figure 4 reports the results of the catalyst concentration optimization step on both OMW and synthetic phenol solution. It is possible to observe that the optimal photocatalyst concentration was equal to 2 g L\({}^{-1}\) (a similar value was obtained in previous work using the same catalyst but without N-doping [48]) and 1.5 g L\({}^{-1}\) for OMW and synthetic phenol solution, respectively. The same experiments were conducted at T = 45 \({}^{\circ}\)C, the maximum value chosen for the experimental campaign, and it was observed that the optimal concentration values were the same as those used at lower temperature values, whereas the  
Figure 4: COD and phenols sorption removal obtained at a different photocatalyst concentration (T = 25 \({}^{\circ}\)C, initial OMW COD = 9.4 g L\({}^{-1}\), synthetic solution initial concentration = 25 mg L\({}^{-1}\), the contact time in dark = 30 min, q = mg of COD or phenols sorbed/g of photocatalyst and q\({}_{\max}\) is the maximum value of q, impeller rotational velocity = 200 rpm). Figure 3: Diffusion coefficient (D) calculated trend (T = 25 \({}^{\circ}\)C, impeller rotational velocity = 20 rpm, time = 120 min, \(\sigma\) < 4%). q values underwent a slight reduction (about 7% for OMW and 10% for phenols). Another important result was that the pH of the pretreated OMW, 5.6, was lower in comparison to the pH of zero-charge of the photocatalyst, which resulted as positively charged and able to adsorb deprotonated organic acids and other negatively charged organic species present in the OMW. Figure 5 reports the kinetic experimental and modeled data of OMW and synthetic phenol solution photodegradation. In both cases, a T increase led to faster kinetics of the photodegradation process. In detail, the OMW COD removal efficiency at 180 min was equal to 45.8, 46.1, 47, and 47.3% for T = 25, 35, 40, and 45 \({}^{\circ}\)C, respectively, showing that the temperature increase did not modify substantially the asymptotic value, whereas it influenced the apparent kinetic constant k\({}_{\rm C}\), which increased from \(4.94\times 10^{-4}\) L\({}^{1+n}\)/min mg\({}^{n-1}\) at 25 \({}^{\circ}\)C to 6.91 \(\times\) 10\({}^{-4}\) L\({}^{1+n}\)/min mg\({}^{n-1}\) at 45 \({}^{\circ}\)C, with a reaction order always in the range 1.24-1.25. The COD removal efficiency was higher than that reported in a previous study where the same catalyst without N-doping was used in similar operative conditions [48]. In a  
Figure 5: OMW (**a**) and synthetic phenol solution (**b**) kinetic photodegradation results and modeling at different T values (T = 25, 35, 40, and 45 \({}^{\circ}\)C; initial OMW COD = 9.4 g L\({}^{-1}\); synthetic solution initial concentration = 25 mg L\({}^{-1}\); impeller rotational velocity = 200 rpm). recent study of OMW, the photo-Fenton treatment at neutral pH was active only for a few minutes due to stability issues, and 80% microcontaminants degradation was reported. However, the photo-Fenton efficiency was tested in diluted OMW, and it would be not possible to reuse the catalyst [21]. In this study, the catalyst could be reused and there were no stability issues observed. Regarding the results of synthetic phenol solution photodegradation, the phenols removal efficiency at 180 min was equal to 89.4, 89.9, 90.1, and 90.5% for T = 25, 35, 40, and 45 \({}^{\circ}\)C, respectively, whereas the apparent kinetic constant kP values were 5.8 \(\times\) 10\({}^{-3}\), 6.3 \(\times\) 10\({}^{-3}\), 6.5 \(\times\) 10\({}^{-3}\), and 6.8 \(\times\) 10\({}^{-3}\) at 25, 35, 40, and 45 \({}^{\circ}\)C, respectively, with a reaction order always in the range 1.4-1.42. Therefore, the synthetic phenol solution photodegradation was characterized by faster kinetics in comparison with that observed for OMW photothermal. Even if a proper comparison cannot be made, OMW is very complex wastewater, containing various recalcitrant pollutants in addition to phenol compounds.