UV/TiO2-heterogeneous photocatalysis was tested as a process to degrade gallic acid (Gal) in oxygenated solutions at pH 3. In the absence of oxidants other than oxygen, decay followed a zero order rate at different concentrations and was slow at concentrations higher than 0.5 mM. Addition of Fe3+, H2O2 and the combination Fe3+/H2O2 improved Gal degradation. In the absence of H2O2, an optimal Fe : Gal molar ratio of 0.33 : 1 was found for the photocatalytic decay, beyond which addition of Fe3+ was detrimental and even worse in comparison with the system in the absence of Fe3+. TiO2 addition was beneficial compared with the same system in the absence of the photocatalyst if Fe3+ was added at low concentration (0.33 : 1 Fe : Gal molar ratio), while at high concentration (1 : 1 Fe : Gal molar ratio) TiO2 did not exert any significant effect. H2O2 addition (1 : 0.33 Gal : H2O2 molar ratio, absence of Fe(III)) also enhanced the heterogeneous photocatalytic reaction. Simultaneous addition of Fe3+ and H2O2 was more effective than the addition of the separate oxidants. This system was compared with Fenton and photo-Fenton systems. At low H2O2 concentration (0.33 : 1 : 0.2 Fe : Gal : H2O2 molar ratio), the presence of TiO2 also enhanced the reaction. The influence of the thermal charge transfer reaction between Gal and Fe(III), which leads to an important Gal depletion in the dark with formation of quinones, was analysed. The mechanisms taking place in these complex systems are proposed, paying particular attention to the important charge transfer reaction of the Fe(III)–Gal complex operative in dark conditions.
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