Oxidation of p-hydroxybenzoic acid by UV radiation and by TiO 2/UV radiation: comparison and modelling of reaction kinetic

Abstract

The phenolic compound p-hydroxybenzoic acid is very common in a great variety of agroindustrial wastewaters (olive oil and table olive industries, distilleries). The objective of this work was to study the photocatalytic activity of TiO 2 towards the decomposition of p-hydroxybenzoic acid. In order to demonstrate the greater oxidizing power of the photocatalytic system and to quantify the additional levels of degradation attained, we performed experiments on the oxidation of p-hydroxybenzoic acid by UV radiation alone and by the TiO 2/UV radiation combination. A kinetic model is applied for the photooxidation by UV radiation and by the TiO 2/UV system. Experimental results indicated that the kinetics for both oxidation processes can be fitted well by a pseudo-first-order kinetic model. The second oxidation process can be explained in terms of the Langmuir–Hinshelwood kinetic model. The values of the adsorption equilibrium constant, K pHB, and the second order kinetic rate constant, k c, were 0.37 ppm −1 and 6.99 ppm min −1, respectively. Finally, a comparison between the kinetic rate constants for two oxidation systems reveals that the constants for the TiO 2/UV system are clearly greater (between 220–435%) than those obtained in the direct UV photooxidation.