Removal of metronidazole by TiO2 and ZnO photocatalysis: a comprehensive comparison of process optimization and transformation products.

The photodegradation of antibiotic metronidazole (MNZ) was systematically studied and compared by using aqueous suspensions of TiO2 and ZnO catalysts under 100-W UV irradiation. The degradation conditions were optimized using the central composite design and response surface methodology. The optimal photodegradation conditions obtained were at pH 6.0 with 1.5 g L-1 of TiO2 (86.10% removal for 50 mg L-1 MNZ) and at pH 9.5 with 0.5 g L-1 of ZnO (60.32% removal for 30 mg L-1 MNZ) after 60-min irradiation at 20 °C. The degradation efficiency in the presence of TiO2 was higher than that of ZnO. The participation of active species such as hydroxyl radicals (OH·), holes (h+), and superoxide radicals (O2-·) during MNZ photodegradation over TiO2 and ZnO catalysts was also examined. Experimental results showed that MNZ oxidation was mainly driven by the presence of holes and superoxide radicals. Totally, 10 major intermediates were detected in UV/TiO2 and UV/ZnO photocatalysis of MNZ using LC-QTof/MS system, in which 5 same intermediates were found. The remaining different intermediates led to the variations of degradation pathways of both processes. Moreover, some bigger transformation products than the parent MNZ were detected.