In heterogeneous catalytic ozonation, surface properties, i.e., pHpzc and the density of the hydroxyl groups on catalyst’s surface have a fundamental utility in the ozonation process.
The surface of metal oxides would absorb water molecules due to their unsaturated electrons which cause the formation of hydroxyl groups on the surface. The solution pH is considered as the factor that has an influential effect on the surface properties of metal oxides including -Al2O3. The surface hydroxyl groups showed different charge properties at different solution pH 21, 22. Therefore, pH of solution is one of main effective factors in metal oxides such as -Al2O3 and ozone decomposition71 . Fei et al.
23 studied the influence of -Al2O3 surface properties on catalyzed ozonation of 2,4,6-trichloroanisole (TCA). They found that the solution pH was more effective on the rate of catalytic ozonation of TCA in the presence of -Al2O3. As when solution pH is closed to pHpzc of the -Al2O3, the oxides revealed the maximum effect on ozone.
Therefore, zero charge surface of -Al2O3 was more active than the electropositive or electronegative surface in catalytic ozonation of TCA. Hence, based on the results of our study, according to the pHpzc of alumina (8.6) and due to the economic problems caused by the increasing pH, the more experiments were carried out at pH of 8. Also, the effects of different pH (3, 8, and 10) resulted in drastic changes in the color of PCP solution during ozonation that is consistent with the result of Hong et al.
19. In their study and within the first 7 min of ozonation, the color of PCP solution was shifted from yellow to red due to the production of ketones and kinone compounds, and then within 45 min, the color intensity of the solution was gradually reduced. They also assessed the change in the color of the solution by changes in the optical absorption spectrum and indicated that optical spectra and changes in its absorption is quite different at various pH points during ozonation caused by generation of byproducts and balance of organic acids at different pHs.