MnO₂ Nanosheet Nets on Carbon Fiber as Fixed Catalysts for a Highly Efficient, Nonradical Fenton-like Process in Wastewater Refractory Pollutant Treatment

The development of highly efficient fixed catalysts is a significant concern in practical wastewater treatment using the peroxymonosulfate (PMS)-based Fenton-like process. In this study, we successfully synthesized a three-dimensional (3D) δ-MnO 2 nanosheet (MNSN) net grown on a carbon-fiber (CF) sheet (3D-MNSN-CF) through a simple hydrothermal strategy. This novel catalyst demonstrated exceptional efficiency and stability in activating PMS for degrading refractory organics. The 3D-MNSN-CF catalyst was composed of wrinkled, ultrathin δ-MnO 2 nanosheets (∼4.3 nm) grown on CF, forming a uniform 3D net structure with a covering thickness of approximately 7.6 μm (mass ratio ∼ 2.17%). This unique morphology provided a fixed Mn-based catalyst with a highly exposed (001) facet, intrinsic Mn 3+ /Mn 4+ redox pair, and a high ratio of oxygen vacancies (O V s). These features enabled the 3D-MNSN-CF/PMS system to exhibit a remarkable removal ratio of approximately 100% for tetracycline hydrochloride (TC) degradation. The system also showed a high rate constant ( k value) of approximately 0.15 min –1 and a specific activity (ε) of 3.46 L min –1 g –1 based on the MnO 2 ratio, surpassing many reported Mn-based catalysts. Moreover, the 3D-MNSN-CF catalyst maintained excellent performance over a wide pH range from 2 to 11. Furthermore, we discovered that electron-rich oxygen-containing groups exhibited an inhibition effect by competing adsorption with PMS, hindering the generation of radicals. Additionally, the results indicated that singlet oxygen ( 1 O 2 ) was the main reactive species responsible for TC removal, while only a small amount of radicals contributed to the process. The catalyst’s excellent performance was also demonstrated in treating various refractory organics and real wastewater, and it exhibited splendid stability for recycling use.