Degradation of tetracycline by peroxymonosulfate activated with MnFeO-CNTs: Key role of singlet oxygen.

Tetracycline (TC) is a kind of electron-rich organic, and singlet oxygen (O) oxidative pathway-based advanced oxidation processes (AOPs) have represented outstanding selective degradation to such pollutants. In this paper, an excellent prepared strategy for O dominated catalyst was adopted. A catalyst composed of non-stoichiometric doping Mn-Fe bimetallic oxide supported on CNTs (0.3-MnFeO-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 MnFeO-CNTs (0.4 g/L) and PMS (0.8 mM). Through SEM and XRD characterization, MnFeO-CNTs was a hybrid of cubic MnFeO uniformly dispersing on CNTs. By the characterization of XPS and FT-IR, more CO bonds and low-valent Mn (II) & Fe (II) appeared in MnFeO-CNTs. Reactive oxygen species (ROS) was determined by radical quenching experiments and electron spin resonance (EPR) spectroscopy, and O 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 & SO), while the reaction between PMS and the enhanced CO bond on MnFeO-CNTs played a crucial part in the generation of O. In addition, through the comparative degradation of four different organics with distinct charge densities, the excellent selectivity of O-based oxidative pathway to electron-rich pollutants was found. This paper supplied a good strategy to prepare catalyst for PMS activation to form a O-dominated oxidative pathway.