Degradation of tetracycline by peroxymonosulfate activated with Mn0.85Fe2.15O4-CNTs: Key role of singlet oxygen

Tetracycline (TC) is a kind of electron-rich organic, and singlet oxygen (1O2) oxidative pathway-based advanced oxidation processes (AOPs) have represented outstanding selective degradation to such pollutants. In this paper, an excellent prepared strategy for 1O2 dominated catalyst was adopted. A catalyst composed of non-stoichiometric doping Mn–Fe bimetallic oxide supported on CNTs (0.3-Mn0.85Fe2.15O4-CNTs) was synthesized and optimized by regulating the non-stoichiometric doping ratio of Mn & Fe and the loading amount of CNTs. Through optimization and control experiments, the optimized catalyst represented 94.9% of TC removal efficiency within 60 min in neutral condition under relatively low concentrations of Mn0.85Fe2.15O4-CNTs (0.4 g/L) and PMS (0.8 mM). Through SEM and XRD characterization, Mn0.85Fe2.15O4-CNTs was a hybrid of cubic Mn0.85Fe2.15O4 uniformly dispersing on CNTs. By the characterization of XPS and FT-IR, more CO bonds and low-valent Mn (II) & Fe (II) appeared in Mn0.85Fe2.15O4-CNTs. Reactive oxygen species (ROS) was determined by radical quenching experiments and electron spin resonance (EPR) spectroscopy, and 1O2 was verified to be the dominated ROS. The mechanism for PMS' activation was speculated, and more low-valent Mn (II) and Fe (II) contributed to the production of free-radical (•OH & SO4•-), while the reaction between PMS and the enhanced CO bond on Mn0.85Fe2.15O4-CNTs played a crucial part in the generation of 1O2. In addition, through the comparative degradation of four different organics with distinct charge densities, the excellent selectivity of 1O2-based oxidative pathway to electron-rich pollutants was found. This paper supplied a good strategy to prepare catalyst for PMS activation to form a 1O2-dominated oxidative pathway.