Photodegradation of pharmaceutical drugs in aqueous TiO2 suspensions: mechanism and kinetics

Abstract

The degradation and mineralization of Tamoxifen (TAM) and Gemfibrozil (GEM) drugs, whose molecular structures exhibit ethereal bonds, have been carried out by irradiating aqueous suspensions of TiO2 with near-UV light at pH 10. Two commercial polycrystalline TiO2 powders (Degussa P25 and Merck) were used as the photocatalysts. A remarkable TAM degradation and the formation of stable intermediates which are not mineralized occur in homogeneous system under irradiation through the breakage of the ethereal O-C (sp3) bond. Heterogeneous photocatalysis plays a minor role on TAM oxidation; in fact the addition of the photocatalyst does not modify the pathway and the rate of primary steps of TAM degradation but it determines the complete mineralization of intermediate products. In irradiated homogeneous solutions GEM undergoes a small partial oxidation while the addition of the photocatalyst determines its complete and fast degradation and mineralization. The identification of some stable intermediate compounds allows one to hypothesise that the breakage of the ethereal bond is also occurring in the primary steps of GEM photocatalytic oxidation. The disappearance rate of TAM is higher than that of GEM both in homogeneous and heterogeneous systems, while the total organic carbon concentration decreases more quickly for GEM. For both drugs the photoreactivity results in homogeneous system indicate a first order kinetics with respect to the drug concentration. The GEM photocatalytic results have been modelled by the Langmuir-Hinshelwood relation which allows one to determine the values of the kinetic constants and the equilibrium adsorption constants. TiO2 Degussa P25 showed to be the most active photocatalyst for both the degradation and mineralization of GEM.