Exploring the degradation of ofloxacin in sewer overflows by Fe(Ⅵ)/PMS, Fe(Ⅵ)/PDS, and Fe(Ⅵ)/SPC: Overlooked synergistic effect of oxidation and in-situ coagulation

Sewer overflows are a potential source of emerging contaminants to urban waters, posing a threat to ecosystems and human health. Herein, the performance and mechanism of ferrate(Ⅵ) (Fe(Ⅵ))/peroxymonosulfate (PMS), Fe(Ⅵ)/peroxydisulfate (PDS), and Fe(Ⅵ)/percarbonate (SPC) for the degradation of ofloxacin (OFL) in overflows were comparatively investigated. These systems achieved efficient degradation of OFL and the removal of conventional pollutants. Particularly, Fe(Ⅵ)/PMS showed better degradation performance for OFL with a degradation efficiency of 98.8 %. The dominant reactive oxygen species for OFL degradation in the Fe(Ⅵ)/PMS, Fe(Ⅵ)/PDS, Fe(Ⅵ)/SPC systems were singlet oxygen (¹O₂), sulfate radical (SO₄·^-), and hydroxyl radical (·OH), respectively. High-valent iron species played an important role in the Fe(Ⅵ)/PMS and Fe(Ⅵ)/PDS systems. Notably, the synergistic effect of oxidation and in-situ coagulation played a key role in OFL degradation, which determined the superior performance of Fe(Ⅵ)/PMS. The formed flocs with Fe-O-P bond acted as a highway to promote the electron transfer from OFL to PMS, resulting in the efficient degradation of OFL in Fe(Ⅵ)/PMS system. Moreover, a same degradation pathway of OFL was found, and the toxicity of the degradation products was reduced, especially in the Fe(Ⅵ)/PMS system. This study provided a new strategy for overflows treatment.