Insight into the effects of hydroxyl groups on the rates and pathways of tetracycline antibiotics degradation in the carbon black activated peroxydisulfate oxidation process

Highlights: • Carbon black activate peroxydisulfate to generate radicals of ·O₂^- and ¹O₂. • TCs degradation rates and pathways varied with hydroxyl groups on different sites. • DFT calculation reveals the effect of hydroxyl groups on TCs degradation. • Different degradation pathways adversely affect the degradation rates of TCs. To investigate the effects of hydroxyl groups on the degradation of tetracycline antibiotics (TCs), three kinds of TCs [tetracycline (TC), oxytetracycline (OTC), and doxycycline (DTC)] were chosen as the target molecules and then degraded in the carbon black (CB)-activated peroxydisulfate (PDS) oxidation process. The degradation ratios of the TC, OTC, and DTC in the CB/PDS oxidation process reached 52%, 60%, and 87% within 40 min, respectively, with the degradation rate following the order of DTC > OTC > TC. According to the density functional theory calculations, these three TCs have different charge distributions, electrostatic potential distributions and average local ionization energy, which are caused by the distinct hydroxyl group position, thus contribute to the different degradation ratios and reaction rate constants. The hydrogenation of TC was slower special degradation pathway relative to those of OTC and DTC, while the decarbonylation of OTC was slower special degradation pathway relative to that of DTC, which adversely resulted in the degradation rate following the order of DTC > OTC > TC. This presentation gives a knowledge that the position and numbers of hydroxyl groups are pivotal to the degradation efficiency toward TCs.