Heterogeneous photocatalytic degradation of gallic acid under different experimental conditions

Abstract

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.