Oxygen vacancies-rich α@δ-MnO2 mediated activation of peroxymonosulfate for the degradation of CIP: The role of electron transfer process on the surface

The environmental-friendly manganese oxides play an important role in the degradation of organic pollutants through radical or non-radical pathway. In this work, the oxygen vacancies (OVs)-rich α@δ-MnO2 (OVs-α@δ-MnO2) was synthesized via a one-step hydrothermal method and used to activate peroxymonosulfate (PMS) for the degradation of antibiotic ciprofloxacin (CIP). OVs-α@δ-MnO2 displayed remarkable catalytic activity with almost 100 % elimination of 20 mg/L CIP within 15 min, which presented better catalytic performance than δ-MnO2 or α-MnO2. OVs-α@δ-MnO2 acts as a shuttle for electron transport from CIP to PMS. The CIP degradation was almost inhibited completely by the addition of 1 mM HPO42-, indicating that the reaction only occurred on the surface of catalyst. The quenching experiments, analysis of electron paramagnetic resonance (EPR), N2 pumping experiments and the comparison of the furfuryl alcohol (FFA) degradation suggested that the contributions of OH, SO4−, O2− and 1O2 could be negligible. The introduction of OVs increases the Mn3+ content in catalyst, improves the ability of combination with PMS and accelerates the reduction of part of Mn4+ in the reactions. OVs-α@δ-MnO2 exhibits excellent catalytic capability in various water conditions including different pH and coexisting anions based on the non-radical electron transfer mechanism. The CIP degradation gradually decreased over the fourth cycles due to the increase of Mn valence states and the coverage of OVs on the surface, but it could be regenerated by a simple NaBH4 treatment in alkaline condition. The intermediate products and possible degradation pathway were identified by UPLC-MS/MS and proposed based on the degradation mechanisms. This study provides a simple method to synthesize OVs-based MnO2 material and presents deep insights into the electron transfer mechanisms in PMS activation processes.