In
heterogeneous catalytic ozonation, surface properties including pHpzc
and the density of the hydroxyl groups on catalyst’s surface have an important
function 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 one of the most important
factors affecting 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
decomposition. 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.