Solar-light promoted photodegradation of metronidazole over ZnO-ZnAl2O4 heterojunction derived from 2D-layered double hydroxide structure

Abstract The photodegradation of metronidazole (MNZ) in water was successfully achieved under sunlight irradiation in the presence of ZnO-ZnAl2O4 heterojunction catalyst derived from layered-double hydroxide structure. The catalyst was characterized using atomic absorption spectroscopy (AAS), Fourier-transform infra-red (FTIR) spectroscopy, X-ray diffraction (XRD) spectroscopy, Brunauer-Emmett-Teller (BET) analysis and transmission electron microscopy (TEM). The surface area of the synthesized catalyst reached 98.7 m2/g with an average particle size of 18 nm. The half-life of metronidazole reached almost 100 min at pH 7; the calculated kinetic rate constant kc was 0.033 mg/L.min-1 for initial concentration C0 = 20, 30 and 40 mg/L. The generation of •OH radical species was responsible for a series of hydrogen radical abstraction and •OH additions to form highly oxidized MNZ intermediates; this is in conjunction with the presence of surface hydroxyl groups on the catalyst material which gave it a hydrophilic character that can further enhance the interaction between the catalyst and adsorbate. Liquid-chromatography mass-spectrometry (LC-MS) analysis of an intermediate solution showed the presence of four key intermediates with m/z 171.9, 171.9, 214.7 and 185.8, prompting the formulation of a plausible photodegradation pathway for metronidazole.