Photocatalytic degradation of citric acid under different conditions: TiO2 heterogeneous photocatalysis against homogeneous photolytic processes promoted by Fe(III) and H2O2

Degradation of citric acid (Cit) at relatively high concentrations by photocatalysis over TiO2 under air bubbling and near UV-light was tested under different conditions. The reaction was improved by addition of H2O2, Fe(III) or both. Minor amounts of H2O2 were effective to enhance the oxidation and the mineralization. H2O2 addition impacts more on the initial rate, while the effect of Fe(III) is more important after prolonged irradiation. The kinetic profiles were analyzed and compared, and the best ratio of reagents was found (1:1:1 H2O2/Cit/Fe molar ratio). However, a lower amount of Fe (1:1:0.2 H2O2/Cit/Fe ratio) was also effective. Similar experiments under the optimal conditions in the absence of TiO2 were performed for comparison. When both H2O2 and Fe(III) were present, the behavior with and without TiO2 was very similar. So far, 3-oxoglutaric acid was detected as a reaction intermediate. A mechanistic analysis is initiated to explain some features of these complex systems.