Complexation-assisted regulatory mechanism of Cu(II)/H2O2 process for tetracycline degradation: Insights into microscale interactions and macroscale oxidation kinetics

The formation of antibiotic-metal complexes (AMCs) via organic pollutant-metal ion binding is ubiquitous in transition metal-based Fenton-like processes. However, the regulatory mechanisms of binding interactions on oxidative degradation remain underexplored. In this study, tetracycline (TET) was selected as a model organic pollutant, and the impact of TET-Cu(II) micro-complexation on macroscale degradation in the Cu(II)/H ₂ O ₂ system was systematically elucidated, integrating in-situ spectroscopy, ex-situ characterization, hydrogen nuclear magnetic resonance ( ¹ H NMR), electrochemistry, and electron paramagnetic resonance (EPR) techniques. Results suggested that Cu(II) rapidly formed a TET-Cu(II) complex with TET’s dimethylamino, amino, and hydroxyl groups at a 1:1 ratio, with a complexation constant of 98036.7 L⋅mol ^− 1 , which had been confirmed to significantly promoted the macroscopic degradation of TET. Additionally, the bound TET was proved degraded preferentially over free TET, and the dissociation rates significantly exceeded binding rates during oxidation via time-resolved fluorescence scan analysis. The binding interaction promoted the formation of an TET ^± -Cu(II)-OOH⁻ intermediate, which further enhanced Cu(II)/Cu(I) cycling and accelerated the generation of the dominant reactive species (hydroxyl radicals (•OH)), while O ₂ inhibited reactions by consuming Cu(I) to generate superoxide radicals (•O ₂ ⁻). Based on the Benesi-Hildebrand equation, the bound fractions of both TET and Cu(II) were accurately calculated, indicating that the bound TET in the system accounted for more than 90%. Furthermore, the binding model was constructed to accurately predict the degradation rate constants ( k _obs ), with an R ² of 0.96 against experimental values. This study uncovered the intrinsic link between microscale complexation and macroscale degradation, providing theoretical support for efficient composite pollution treatment.