Selective degradation of tetracyclines in the Mn(II)/H₂O₂/HCO₃process with low peroxide dosage: The neglected role of complexation-mediated electron transfer

The Mn(II)/H₂O₂/HCO₃ process shows great potential for the removal of organic contaminants. Nevertheless, the use of excess H₂O₂ (mM) not only increases the operational cost of this process but also poses risks to subsequent biological treatment processes. In this study, we found that tetracyclines (TCs) could be selectively removed in the Mn(II)/H₂O₂/HCO₃ process under alkaline conditions at low concentrations of H₂O₂ (sub-mM levels), which can be ascribed to the complex interaction between Mn(II), H₂O₂, HCO₃, and TCs. Mechanistic investigation showed that the active oxidants, including Mn(III), MnO₂, HO•, O₂•, and CO₃•, did not contribute to the selective oxidation of TCs in the Mn(II)/H₂O₂/HCO₃ process. Instead, multiple lines of evidence indicated that the Mn(II)-HCO₃-TCs ternary complex could be formed due to the coexistence of Mn(II), HCO₃, and TCs and this complex facilitated direct electron transfer from H₂O₂ to TCs, enabling the selective elimination of TCs in the Mn(II)/H₂O₂/HCO₃ process. Additionally, the addition of typical water matrix components had negligible effects on TCs degradation, underscoring the potential of this process for the practical application. The findings of this study will provide guidance for designing low-peroxide advanced oxidation processes based on the actual scenario and the properties of organic contaminants.