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−, SO 4 2−, NO 3 −, HCO 3 −, and H 2 PO 4 −); cations (Ca2+, Mg2+, and Fe3+); natural organic matter (humic acid); and pharmaceutical excipient (glucose) on the photocatalytic degradation of metronidazole (MNZ) by UV/TiO 2 were investigated. Degradation of MNZ noticeably decreased in the presence of H 2 PO 4 −, 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 H 2 PO 4 − 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 TiO 2. These results indicate that the presence of water matrix components significantly changed the degradation pathways and hence affected the degradation efficiency. Graphical abstract The effect of the presence of water matrix components on photodegradation removal of antibiotic metronidazole (MNZ) using UV/TiO 2 catalysts was comprehensively examined. These components not only impact on degradation efficiency but also cause changes in degradation pathways by forming new transformation products. These products were detected and identified by both UPLC®-QTof/MS and UPLC®-MS/MS. Among a variety of water matrix components assessed, it was concluded that the existence of the popular natural organic matter humic acid (HA) and common pharmaceutical excipient glucose will significantly change the degradation pathways. Unlabelled Image Highlights • TiO 2 photocatalysis of MNZ in the presence of anions, cations, humic acid, and glucose is studied. • Transformation products formed in photocatalysis are identified by LC-QTof/MS and LC-MS/MS. • The presence of glucose causes side-reactions that promotes degradation efficiency. • Humic acid will activate electron transfer that results in the formation of new intermediates. • The presence of Fe3+ ions hinders UV photocatalysis of MNZ by a complexation reaction. [ABSTRACT FROM AUTHOR]