Effects of water matrix components on degradation efficiency and pathways of antibiotic metronidazole by UV/TiO2 photocatalysis

Abstract

In this study, the effects of water matrix components such as anions (Cl−, SO42−, NO3−, HCO3−, and H2PO4−); cations (Ca2+, Mg2+, and Fe3+); natural organic matter (humic acid); and pharmaceutical excipient (glucose) on the photocatalytic degradation of metronidazole (MNZ) by UV/TiO2 were investigated. Degradation of MNZ noticeably decreased in the presence of H2PO4−, Fe3+, and humic acid (HA). However, the addition of glucose tripled the rate of MNZ degradation. The transformation products formed during photocatalysis were detected and identified using Waters UPLC-QTof/MS (ultra-performance liquid chromatography–quadrupole time-of-flight mass spectrometry) and UPLC-MS/MS (ultra-performance liquid chromatography–tandem mass spectrometry) instruments. The enhanced degradation of MNZ in the presence of glucose was due to the side reactions of MNZ and its intermediates with other organics released when glucose was degraded. HA could activate charge transfer steps, resulting in different photodegradation products. Iron(III) ions competed with MNZ under light adsorption and reacted with organic intermediates, which hindered MNZ degradation. The presence of H2PO4− ions seemed to have no effect on the degradation pathways of MNZ but only slowed down the removal of MNZ and its intermediates by interacting with TiO2. These results indicate that the presence of water matrix components significantly changed the degradation pathways and hence affected the degradation efficiency.