In radical mechanism pathways like that involved in Fenton processes

As previously stated for ozonation in the presence of ion-exchanged montmorillonite [59] and [60], intermediate pH values ranging from 2.5 to 4.5 appear to involve a synergistic effect between adsorption and oxidation, and between ozonation on the catalyst surface and in bulk solution.
The effect of pH variations should be strongly correlated to the presence and structures of the catalysts employed. These pH changes may be enhanced by the presence of catalyst, which, in turn, may undergo modifications in both chemical composition and surface properties.