Oxidation of p-hydroxybenzoic acid by UV radiation and by TiO2/UV radiation: comparison and modelling of reaction kinetic

The phenolic compound p-hydroxybenzoic acid is very common in a great varie ty of agroindustrial wastewaters (olive oil and table olive industries, dis tilleries). The objective of this work was to study the photocatalytic acti vity of TiO2 towards the decomposition of p-hydroxybenzoic acid. In order t o demonstrate the greater oxidizing power of the photocatalytic system and to quantify the additional levels of degradation attained, we performed exp eriments on the oxidation of p-hydroxybenzoic acid by W radiation alone and by the TiO2/UV radiation combination. A kinetic model is applied for the p hotooxidation by UV radiation and by the TiO2/UV system. Experimental resul ts indicated that the kinetics for both oxidation processes can be fitted w ell by a pseudo-first-order kinetic model. The second oxidation process can be explained in terms of the Langmuir-Hinshelwood kinetic model. The value s of the adsorption equilibrium constant, K-pHB, and the second order kinet ic rate constant, k(C), were 0.37 ppm(-1) and 6.99 ppm min(-1), respectivel y. Finally, a comparison between the kinetic rate constants for two oxidati on systems reveals that the constants for the TiO2/UV system are clearly gr eater (between 220-435%) than those obtained in the direct W photooxidation . (C) 2001 Elsevier Science B.V. All rights reserved.