Photochemical and photocatalytic degradation of salicylic acid with hydrogen peroxide over TiO2/SiO2 fibres

The effect of hydrogen peroxide on the photo-oxidation of salicylic acid (SA) in aqueous solutions under UV-irradiation was investigated in both purely homogeneous and photocatalytic conditions. Results obtained show that in absence of photocatalyst and under UV-irradiation, the SA degradation rate increases with increasing the amount of hydrogen peroxide added, although for high concentrations of H2O2 the enhancement of the activity is progressively attenuated. In the case of the photocatalytic degradation, nanocrystallites of anatase immobilised over SiO2 fibres were used as catalyst. In the present of TiO2/SiO2, the addition of hydrogen peroxide results in almost a three-fold increment of the photoactivity at low concentration (29.1 mM). In contrast, when the amount of H2O2 added exceeds certain limit, the rate of SA removal becomes slightly lower than those measured in absence of photocatalyst. A simple kinetic model which considers the summation of both photochemical and photocatalytic contributions is used to account for the dependence of the rate on the H2O2 concentration. The efficiency of the SA mineralization was also evaluated by monitoring the evolution of total organic carbon (TOC) with irradiation time. In this respect, remarkable differences are found when comparing pure photochemical degradation, which only reduce the TOC concentration in about a 3% after 300 min of irradiation, with the photocatalytic treatment, which achieve a 38% of SA mineralization in the same period. (c) 2006 Elsevier B.V. All rights reserved.