## 5 Reactor Systems and Process Parameters
### Operational Parameters
For example, [PERSON] and [PERSON] (2017) found that the adsorption of dyes on TiO\({}_{2}\) is minimal at the isoelectric point, but increases under acidic conditions, due to favorable electrostatic interactions [154]. Adjusting the pH to suit the specific photocatalyst and pollutant is essential for maximizing the photocatalytic efficiency. Typically, TiO\({}_{2}\)-based systems perform better under slightly acidic to neutral conditions, enhancing  
Figure 11: Critical operational parameters influencing the efficiency of photocatalytic degradation processes, including catalyst loading, pH, dissolved oxygen, initial contaminant concentration, and ionic composition. These factors interact to affect light absorption, ROS generation, and pollutant-cataly interactions and must be optimized to enhance overall system performance. electrostatic attraction between catalyst and pollutants. The research of [PERSON] et al. [155] demonstrated that the photocatalytic degradation of tetracycline using TiO\({}_{2}\) was significantly influenced by pH. The photocatalytic degradation of tetracycline by TiO\({}_{2}\) was shown to be most efficient at pH 5-6, while performance dropped significantly at alkaline pH due to decreased electrostatic interactions between the pollutant and the catalyst surface [155]. At higher pH levels, the tetracycline changes to its neutral or anionic forms, reduces its interaction with the positively charged TiO\({}_{2}\) surface, thereby decreasing the degradation efficiency. Moreover, the photocatalytic behavior of other widely used semiconductors such as g-C\({}_{3}\)N\({}_{4}\) and ZnO is also significantly influenced by pH. For g-C\({}_{3}\)N\({}_{4}\), acidic conditions typically promote enhanced degradation due to increased electrostatic attraction between its negatively charged surface and positively charged contaminants. However, in alkaline environments, the surface deprotonation can reduce this interaction and lower the process efficiency [92]. For example, the degradation of rhodamine B by g-C\({}_{3}\)N\({}_{4}\) was reported to be most effective at pH 4-6, where increased adsorption and suppressed recombination of photogenerated charge carriers were observed [92]. In contrast, at higher pH values, the repulsion between deprotonated catalyst surfaces and anionic pollutants, as well as the decreased oxidative potential of photogenerated holes, led to reduced photocatalytic efficiency. ZnO, on the other hand, exhibits optimal photocatalytic activity under slightly acidic to neutral pH, as it tends to dissolve in strongly acidic or basic media, compromising the structural stability and active surface area [86; 99]. Studies have shown that the degradation of methylene blue and acid orange 7 using ZnO is most efficient in the pH range of 6-8, where both sufficient hydroxyl radical generation and stable surface interactions are maintained [86; 99]. Beyond this range, particularly above pH 9, the photocatalyst becomes prone to photocorrosion, leading to Zn\({}^{2+}\) leaching and a decline in reusability. Additionally, surface charge repulsion at high pH values can hinder the adsorption of anionic pollutants such as azo dyes, further reducing performance. These findings underscore the need to tailor the pH of the reaction system according to the specific photocatalyst and target pollutants to maximize performance and ensure material stability. Oxygen acts as a crucial electron acceptor in photocatalytic reactions, facilitating the formation of ROS necessary for pollutant degradation. The availability of dissolved oxygen directly influences the reaction rate. Research indicates that increasing the dissolved oxygen content can significantly improve the degradation rate constant. For instance, [PERSON] et al. (2024) assessed the photocatalytic activity of graphene oxide and hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) by photodegrading methylene blue (MB) in an aqueous solution [101].