LAUSR

Abstract The effect of a natural water matrix on the photodegradation of four phenolic compounds: phenol, 4-chlorophenol, 4-nitrophenol and methyl-p-hydroxybenzoate, has been studied with two commercial TiO2 catalysts with very different physico-chemical properties (Hombikat UV-100 and Aeroxide® P25). Small changes in the photodegradation rates of these four aromatic compounds were observed to compare both TiO2 photocatalysts. The photocatalytic process starts with addition of HO to the aromatic ring in a two-step process (addition-elimination). Hammett linear free energy relationships show the second step is rate-determining, electron-withdrawing substituents leading to a reduction in reaction rate. A photo-mechanism mediated by HO radicals formed from the photogenerated holes was the most significant when TiO2 Hombikat was used in tap water. On the contrary, the conduction band electrons and ROS generated from CB electrons played a major role in the photocatalytic degradation over TiO2 P25. The loss of activity found when P25 was employed in the presence of Na2CO3 or NaHCO3 was mainly due to the change in pH, because no effect on HO radical formation was detected. In contrast, the presence of ions enhanced photoactivity and faster HO radical formation was detected in the case of TiO2 Hombikat. These significant differences can be well related to the fact that Hombikat has presented a larger surface area and a more negatively charged surface than P25, which prevents anions from being strongly adsorbed.