Photocatalytic degradation kinetics and mechanism of environmental pharmaceuticals in aqueous suspension of TiO2: A case of sulfa drugs

The photocatalytic degradation kinetics of three sulfa pharmaceuticals has been investigated in TiO2 aqueous suspension. The disappearance of these three compounds follows a pseudo-first-order kinetics according to the Langmuir–Hinshelwood (L–H) model. The effects of catalyst amount, initial pH value, and initial concentration of each substrate on the photocatalytic degradation rates were measured in detail. It was observed that the surface reaction on TiO2 played an important role in the degradation of sulfa pharmaceuticals, and the further study of reactive oxygen species (ROSs) indicated that both photohole (h+) and especial hydroxyl radical (radical dotOH), were responsible for the major degradation of sulfa pharmaceuticals. The fates of the sulfur and nitrogen elements in various sulfa pharmaceuticals as well as total organic carbon (TOC) were examined following their photocatalytic transformation. The data showed that all three pharmaceuticals could be completely mineralized into CO2, H2O and inorganic ions within 240 min. These results indicated that many intermediates were produced during the photocatalytic transformation of sulfa pharmaceuticals process. Based on the identified intermediates, two tentative degradation pathways for the photocatalytic degradation of sulfa pharmaceuticals were proposed, for example hydroxylation addition to parent pharmaceuticals and the cleavage of S–N bond from the sulfaniline attacked by photohole.