Significant role of UV and carbonate radical on the degradation of oxytetracycline in UV-AOPs: Kinetics and mechanism.

Carbonate radical (CO3(•-)), a selective oxidant, reacts readily with electron-rich compounds through electron transfer and/or hydrogen abstraction. In this study, the role of CO3(•-) in degrading oxytetracycline (OTC) by UV only, UV/H2O2 and UV/persulfate (UV/PS) advanced oxidation processes (AOPs) in the presence of HCO3(-) or CO3(2-) was investigated. For UV only process, the presence of photosensitizers, i.e., nitrate (NO3(-)) and natural organic matter (NOM), had different impacts on OTC degradation, i.e., an enhancing effect by NO3(-) due to the generation of HO(•) and a slight inhibiting effect by NOM possibly due to a light scattering effect. Differently for UV/H2O2 and UV/PS processes, the presence of NO3(-) hardly influenced the destruction of OTC. Generation of CO3(•-) presented a positive role on OTC degradation by UV/NO3(-)/HCO3(-). Such influence was also observed in the two studied AOPs in the presence of both bicarbonate and other natural water constituents. When various natural water samples from different sources were used as reaction matrices, UV only and UV/H2O2 showed an inhibiting effect while UV/PS demonstrated a comparable or even promoting effect in OTC decomposition. After elucidating the potential contribution of UV direct photolysis via excited state OTC* at an elevated reaction pH condition, putative OTC transformation byproducts via CO3(•-) reaction were identified by ultra-high definition accurate-mass quadrupole time-of-flight tandem mass spectrometry (QTOF/MS). Five different reaction pathways were subsequently proposed, including hydroxylation (+16 Da), quinonization (+14 Da), demethylation (-14 Da), decarbonylation (-28 Da) and dehydration (-18 Da). The significant role of UV at high pH and CO3(•-) on OTC removal from contaminated water was therefore demonstrated both kinetically and mechanistically.