Redox photodegradation of 2,4-dichlorophenoxyacetic acid over TiO<inf>2</inf>

The photodegradation of 2,4-dichlorophenoxyacetic acid using titanium dioxide as photocatalyst was investigated. The yield of 2,4-D photodegradation depends on the mass of the semiconductor, temperature and light intensity. The highest conversion rates were obtained at pH = 3.5, which is close to the pKa of the acid and lower than the point of zero charge of TiO2. The experimental data fit a Langmuir-Hinshelwood kinetic model. The rate and equilibrium adsorption constants were obtained from those data at different temperatures. Some aromatic intermediate products of 2,4-D photodegradation were detected (2,4-dichlorophenol, hydrohydroquinone, chlorohidroquinone). An analysis of total organic carbon (TOC) showed that a complete mineralization of 2,4-D can be easily achieved. On the other hand, the results obtained in experiments using platinized TiO2 lead us to assume that both oxidation and reduction steps should be taken into account if an explanation of the experimental data is to be made. © 1995 Elsevier Science B.V. All rights reserved.