Monoethanolamine modulated Cu(II)/PMS system for sulfamethoxazole degradation: The role of Cu(II)-OOSO3- metastable complex and rapid copper species cycling

Abstract

The activation of persulfate (PMS) by copper ions has been demonstrated, yet the precise mechanism of this process still needs to be conclusive. Additionally, the sluggish Cu(I)/Cu(II)/Cu(III) cycle and stringent operational constraints impose significant limitations on its widespread utilization. In this study, introducing the complexing agent monoethanolamine (MEA) into the Cu(II)/PMS system significantly improved the system’s oxidative properties. It is based on the stable complexation of Cu(II)-MEA, which accelerates the Cu(I)/Cu(II)/Cu(III) cycle by metal-to-ligand charge transfer. The Cu(II)-MEA and PMS can secondary coordinate to form the highly catalytically active metastable structure Cu(II)-MEA-OOSO₃^-, which optimizes the activation pathway of PMS. It provides the Cu(II)/MEA/PMS system with a sustained high oxidation capacity, achieving 90% degradation of Sulfamethoxazole (SMX) within just 5 min, which is 19.56 times more effective than the traditional Cu(II)/PMS system. A variety of reactive species were generated in the system, with Cu(III), SO₄^•-, and direct oxygen atom/single electron transfer in the metastable structure playing a dominant role, together with the presence of a certain amount of ¹O₂, O₂^•-, ^•OH. The complexation with MEA renders the Cu(II)/PMS process highly adaptable to typical aqueous matrices and inorganic anions. Moreover, it demonstrates stability across a wide pH range of 5 to 9. In the treatment of halogenated wastewater, the addition of MEA inhibited the formation of halogenated by-products. In conclusion, the utilization of MEA to regulate Cu(II)-activated PMS for the degradation of SMX in water is feasible and provides novel insights into the Cu(II)/PMS system in the presence of organic ligands.