Oxidative Degradation of 2,4-Dihydroxybenzoic Acid by the Fenton and Photo-Fenton Processes: Kinetics, Mechanisms, and Evidence for the Substitution of H2O2 by O-2

The kinetics and mechanisms of the oxidative degradation of 2,4-dihydroxybenzoic acid (2,4-DHBA) by the Fenton and photo-Fenton processes were investigated in detail by a combination of H PLC, IC, and TOC analyses. The formation of 2,3,4-trihydroxybenzoic acid (2,3,4-THBA) at an early oxidation stage shows that hydroxylation of the aromatic ring is the first step of the process. This intermediate was able to reduce Fe-III and to contribute to the recycling of Fe-II. Complete mineralization could only be achieved under irradiation (photo-Fenton). A detailed study of the dependence of the rate of mineralization on the concentration of H2O2 and dissolved O-2 was carried out. It was found that, even at a low initial concentration of H2O2, mineralization by the photo-Fenton process was complete in a relatively short time, provided that the O-2 concentration was high enough, indicating that O-2 may, at least in part, substitute H2O2. Channeling reaction pathways toward O-2 rather than H2O2 consumption is of particular interest for the technical development of the photo-Fenton process.