Highly efficient peroxymonosulfate activation by Ni-Co bimetallic modified halloysite nanotube with enriched oxygen vacancies for norfloxacin degradation

In this work, NiCo₂O₄/halloysite with abundant oxygen vacancies (OVs-NiCo₂O₄/halloysite) was prepared by simple water bath-calcination and etching of NaBH₄. The oxygen vacancies (OVs) in composite could promote the activation with PMS (peroxymonosulfate) by improving the electron transfer efficiency and then contributed to the production of ¹O₂ species with stronger environmental adaptability and other radicals. As a result, the OVs-NiCo₂O₄/halloysite/PMS system exhibited highly-efficient removal efficiency of 94.3 % in 40 min with a reaction rate constant of 0.4010 min⁻¹ compared to that in the NiCo₂O₄/PMS (0.0343 min⁻¹) and NiCo₂O₄/halloysite/PMS (0.1670 min⁻¹). Moreover, the degradation efficiency of NOR (Norfloxacin) in OVs-NiCo₂O₄/halloysite/PMS system remains above 80 %, which exhibited exceptional durability after four cycles. The presence of OVs on the surface of OVs-NiCo₂O₄/halloysite-30 % was verified by X-ray photoelectron spectroscopy (XPS) and Electron paramagnetic resonance spectroscopy (EPR). Electrochemical impedance spectra (EIS) and radical quenching experiments proved that the formation of abundant OVs facilitated charge transport behaviors in OVs- NiCo₂O₄/halloysite-30 %, thereby accelerating the redox cycles of Ni (II)/Ni (III) and Co (II)/Co (III). Additionally, this process generated a higher concentration of ¹O₂, which is the primary active species responsible for NOR degradation. Two potential NOR degradation pathways were inferred from the analysis of high performance liquid chromatography-mass spectrometry (HPLC-MS). Overall, our study provides a promising strategy for significantly improving PMS activation performance and applying in pollutant degradation through the introduction of oxygen vacancies and natural mineral carrier.