Kinetic study of salicylic acid photocatalytic degradation using sol–gel anatase thin film with enhanced long-term activity

Abstract

The scope of this work was to evaluate the photocatalytic degradation of salicylic acid (SA) using the anatase TiO2 thin film (hereafter: ATF) under UV-A irradiation and to explore the tentative long-term activity of ATF. The ATF was immobilized by the sol–gel procedure on the inner side of outer wall of annular glass reactor for maximized exposure area to applied irradiation. The ATF was used for 41 consecutive photocatalytic runs (trun = 60 min) and then irradiated under natural sunlight over a period of 15 days. Photocatalytic runs were repeated with irradiated catalyst, showing the unchanged photocatalytic activity with observed reaction rate constant k SA,obs = 4.0 × 10−7 min−1 W−0.5 m1.5. The degradation and mineralization kinetics of SA and the degradation by-products were studied. A tentative reaction scheme was presented. 2,5-Dihydroxybenzoic acid (2,5-DHBA) and 2,3-dihydroxybenzoic acid (2,3-DHBA) were identified as the main degradation by-products. The incident photon flux was determined along the inner reactor wall, i.e. on the thin film surface (I tf, W m−2) using two radiation emission models. To obtain kinetic parameters independently of photocatalyst optical properties and irradiation conditions, its lump value was introduced to kinetic model. A detailed kinetic study revealed first-order kinetics for SA, 2,5-DHBA, 2,3-DHBA and residual organics degradation and confirmed the preferred degradation pathway via ·OH radical attack.