Sulfamethazine degradation and copper transformation in Cu(II)/PMS system: In-depth investigation of the interaction between intermediates and copper

The Cu(II)/Peroxymonosulfate (PMS) system demonstrates significant efficacy in the degradation of sulfonamide antibiotics. Nevertheless, a comprehensive understanding of the mechanism and copper transformation within this Fenton-like system has frequently been disregarded. This study aims to investigate the degradation mechanism of Sulfamethazine (SMZ) in the Cu(II)/PMS system and establish the correlations among Cu(II), SMZ, PMS, and the resulting degradation products. Firstly, the use of complexation assays and analog computations revealed the existence of weak complexation between Cu(II) and SMZ under neutral conditions, leading to the proposition of complexation structures. Secondly, the degradation of SMZ in the Cu(II)/PMS system is limitedly affected by common ions and organic matter. Electron Spin Resonance (ESR) and quenching experiments revealed the primary mechanisms responsible for SMZ degradation, highlighting the important roles of hydroxyl radicals (OH) and direct oxidation of PMS. Moreover, Hirshfeld charges in conjunction with Density Functional Theory (DFT) calculations were utilized to identify potential reaction sites, bond cleavages, and pathways of intermediate products, showing good agreement with Mass spectrometry (MS) results. Finally, the quantification of copper concentrations with varying valences revealed a progressive increase in Cu(I) and Cu(III), accompanied by a decline in Cu(II) throughout the SMZ degradation. It was observed that hydroxylation intermediates may contribute to the reduction of copper, and this study proposes potential electron transfer pathways to explain this phenomenon. This study significantly contributes to our comprehension of the degradation process of SMZ in the Cu(II)/PMS system, as well as the intricate interactions between Cu(II), SMZ, and the intermediates involved.