# Immobilized TiO\({}_{2}\)-Polyethylene Glycol: Effects of Aeration and pH of Methylene Blue Dye
## 3 Result and Discussion
### Effects of pH  
Researchers have paid much attention to studying the effect of pH on the photocatalytic degradation of dyes because the real wastewater influences from industries may not be have a neutral pH (pH 7). Moreover, the pH of the reaction mixture influences the surface-charged- properties of  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Aeration (mL/min)** & \(k\)**, Rate Constant (min\({}^{-1}\))** & **Correlation Coefficient (\(R^{2}\))** \\ \hline
25 & 0.035 & 0.9969 \\
50 & 0.058 & 0.9950 \\
75 & 0.068 & 0.9913 \\
100 & 0.053 & 0.9940 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The kinetic constants (\(k\)) of decolorizing MB at different aeration flow rates for optimum loading of 0.3 g of immobilized TiO\({}_{2}\)/PEG.  
Figure 4: Aeration flow rate (mL/min) of immobilized TiO\({}_{2}\)/PEG photocatalyst film.  
the photocatalysts. The effect of pH is important to study, as it impacts the surface state of titanium and the ionization state of the dye molecules [33]. In this study, the effect of pH on MB degradation was evaluated at various pH conditions (i.e., 3, 6, 8, 10, 11, and 12). The significant effect of pH on the calculated pseudo first-order rate constant for the photocatalytic decolourization of MB is depicted in Figure 5a. It was observed that the photocatalytic degradation rate increased by increasing the pH value. pH 11 showed the highest photocatalytic activity as compared to other pH. This is also due to the significant effect of high adsorption existing between positively-charged cationic MB dye with the negative charge on the surface of immobilized TiO\({}_{2}\)/PEG. As can be seen in Figure 5, there is a difference in the values of MB photodegradation rate between pH 4 and pH 8. Figure 5b shows the plot graph of the pH value corresponding to the point of zero charge (pH\({}_{\text{pzc}}\)) of immobilized TiO\({}_{2}\)/PEG surface where the value of pH\({}_{\text{pzc}}\) is 6.5. This means that the surface charge of immobilized TiO\({}_{2}\)/PEG is positive in acidic pH (pH < 6.5) and negatively charged in alkaline pH (pH > 6.5). The pH\({}_{\text{pzc}}\) is a pH value where the net electrical charge of the photocatalyst is zero and is used to qualitatively assess the TiO\({}_{2}\) surface charge. Due to the negatively-charged TiO\({}_{2}\)/PEG surface in alkaline conditions, adsorption dominates between the negative TiO\({}_{2}\) surface and the cationic MB dye (pKa < 2) [34]. Hence, a pH value lower that 6.5 will cause a poor degradation of MB dye, and this explains the gap in photocatalytic degradation rate obtained beyond pH 6.5 seen in Figure 5b. However, the photoactivity efficiency decreased at pH 12 to become 0.062 min\({}^{-1}\) due to the rapid adsorption of dye which covered the photocatalyst surface, thus resulting in a slower photocatalytic activity [35]. Nonetheless, the removal rate of MB by TiO\({}_{2}\)/PEG film was largely attributed to adsorption, since the visible blue-stained surface indicated that the film was not fully photocatalytically degraded. This is due to the agglomeration of MB, which reduced the amount of light needed for photocatalysis [36]. Thus, the optimal pH obtained in this study was discovered to be at pH 11.