Red mud-activated peroxymonosulfate process for the removal of fluoroquinolones in hospital wastewater

In this study, a novel peroxymonosulfate (PMS) activation method, which combines a solid waste (i.e., red mud, RM) and a reducing agent (i.e., hydroxylamine, HA), for the oxidative degradation of fluoroquinolones (FQs; i.e., flumequine (FLU) and ciprofloxacin (CIP)) in hospital wastewater (HW) was developed. The addition of HA into the PMS/RM suspension significantly enhanced FLU removal, owing to its ability to enhance the Fe(III)/Fe(II) cycle on the RM surface. The results of the quenching experiments suggested the predominance of SO₄^•- over ^•OH in the PMS/RM/HA system. Moreover, owing to the greater reactivity between CIP and SO₄^•-, CIP removal was more effective than FLU removal. Additionally, the liquid chromatography-mass spectroscopy (LC-MS) analysis revealed that the oxidation of CIP and FLU by PMS/RM/HA occurred via sequential and separate processes, involving ring cleavage, hydroxylation, decarbonylation, and defluorination. Surprisingly, the wastewater components exhibited contrasting effects on FLU removal in HW. Natural organic matter, nitrate and sulfate showed a slight impact on the removal performance of FLU, whereas chloride improved the oxidation extent. However, phosphate significantly inhibited the FLU removal because of its competitive binding at the RM surface and its scavenging effect towards SO₄^•-. This inhibitory effect was overcome by increasing the PMS concentration and its sequential addition, thus guaranteeing successful mineralization of FLU in HW. These results show that the RM/HA system can be utilized to activate PMS for the removal of antibiotics in wastewater.