Photocatalytic oxidation of the antibiotic tetracycline on TiO2 and ZnO suspensions

The photocatalytic oxidation of the antibiotic tetracycline (TC) has been performed in aqueous suspension containing TiO2 or ZnO under simulated solar light. The experimental conditions to attain the optimal degradation rate for each catalyst were determined using multivariate analysis and response surface methodology (RSM). The optimal oxidation conditions found were 1.5 g L−1 and pH 8.7 for TiO2 and 1.0 g L−1 and pH 11 for ZnO. The catalytic activity of both catalysts indicates that ZnO presents a slightly higher oxidation rates than TiO2 when both catalysts were compared under optimised conditions. Using radical scavengers, it was determined that titania TC oxidation occurs principally by direct hole oxidation with participation of hydroxyl radicals and that the oxidation on ZnO is driven mainly by hydroxyl radical. The direct photolysis at pH 8.7 degrade TC to TC-derivatives that maintain their antibiotic activity. However the irradiation in the presence of catalyst completely removes the solution's antibacterial activity (AA). The microbiologic analyses were performed on agar plates loaded with Staphylococcus aureus.