Photo-oxidation degradation of metronidazole in hematite/sulfite system: Mechanism, kinetic, degradation pathway

Abstract

In this study, the decomposition of metronidazole (MTZ) in the hematite (α-Fe2O3)/sulfite system and in the presence of ultraviolet light was investigated. Based on transmission electron microscopy (TEM) and field scanning electron microscopy (SEM) analysis, hematite nanoparticles were included an average diameter of 80 nm. The results showed the process performance under the acidic and natural conditions was very high, as the destructive performances of MTZ (after 5 min of reaction), at pHs of 3 and 7.5 were 80 % and 92 %, respectively. The efficiency of each of the UV/sulfite (US) and UV/hematite (UH) systems in the degradation of metronidazole was 30 % and 27 %, respectively, which indicated the very low efficiency of these systems when each was used alone. Enhanced performance was observed when both hematite/sulfite agents were simultaneously exposed to UV, so that more than 92 % of metronidazole was degraded under the selected conditions (sulfite/hematite/MTZ molar ratio of 1:3: MTZ concentration and pH of 7.5 after 5 min of reaction). The rate of MTZ destruction in one min (robs (mg L−1 min)) indicated the supremacy of UHS process over UH and US processes. It was found the anions in the water matrix affect the efficiency of the system. Tests to investigate the active radical species in the reaction medium proved the importance of the role of the two hydroxyl and sulfate radicals in MTZ decomposition. A comparison of the total cost, energy consumed, and robs between the different systems, revealed UHS system can be considered as an economically acceptable and efficient process in the destructive of MTZ. Based on analysis done, the final products of MTZ degradation using UHS process were characterized to be Acetaldehyde, 1H-imidazole, Hydroxy acetonitrile, Aminoacetonitrile, Glycine, Isoamyl benzyl ether, Alanine and Acetic acids.