Highly Efficient Degradation of Emerging Contaminants with Sodium Bicarbonate-Enhanced Mn(II)/Peracetic Acid Process: Formation and Contribution of Mn(V)

Organic ligands have been extensively used to enhance the catalytic performance of manganese ion (Mn(II)) for peracetic acid (PAA). In this study, sodium bicarbonate (NaHCO₃), an economical and eco-friendly inorganic ligand, was introduced to enhance the degradation of emerging contaminants (ECs) in the Mn(II)/PAA process. NaHCO₃ could significantly improve the oxidizing ability of the Mn(II)/PAA process over the initial pH range of 3.0–11.0. Mn(V) was identified as the primary reactive species for degrading naproxen in the NaHCO₃/Mn(II)/PAA process. HCO₃^– could complex with Mn(II) to generate Mn(II)-HCO₃^–, which has a lower redox potential to enhance the catalytic activity of Mn(II). Mn(II)-HCO₃^– reacted with PAA to produce Mn(III)-HCO₃^– and CH₃C(O)O^•. Mn(V)-HCO₃^– was generated via two-electron transfer between Mn(III)-HCO₃^– and PAA. Although organic radicals were detected in the NaHCO₃/Mn(II)/PAA process, naproxen was mainly degraded by Mn(V)-HCO₃^– via one-electron transfer along with the formation of MnO₂. Notably, the coexisting hydrogen peroxide was vital in the reduction of MnO₂ to Mn(II/III), thereby enhancing the continuous generation of Mn(V)-HCO₃^–. NaHCO₃/Mn(II)/PAA process exhibited exceptional oxidation performance in actual water samples. This study proposed a strategy utilizing an eco-friendly inorganic ligand to address the inherent drawbacks of organic ligand-enhanced Mn(II)/PAA processes and highlighted its potential applications in the removal of ECs.