Acceleration of Fe$^{3+}$/Fe$^{2+}$ cycle in garland-like MIL-101(Fe)/MoS$_2$ nanosheets to promote peroxymonosulfate activation for sulfamethoxazole degradation

Iron-based molybdenum disulfide (Fe-MoS$_2$) has emerged as a Fenton-like catalyst for the highly efficient degradation of antibiotics, but the structure-activity relationship remains elusive. Herein, garland-like MIL-101(Fe)/MoS$_2$ nanosheets (MMS) with dual metal active sites (Fe and Mo) and rich sulfur vacancies were fabricated to directly activate peroxymonosulfate (PMS) for fast degradation of different organic pollutants (phenols, dyes and drugs), even in real water bodies. The MMS exhibited extremely fast catalytic rate constant of 0.289 min$^{-1}$ in the degradation of sulfamethoxazole (SMX), which was about 36 and 29 times that of single MoS$_2$ (0.008 min$^{-1}$) and MIL-101(Fe) (0.01 min$^{-1}$). Moreover, MMS with good stability and reusability could reach 92% degradation of SMX after 5 cycles. Quenching experiments and electron spin resonance (ESR) tests revealed that hydroxyl radicals (.OH) and singlet oxygen ($^1$O$_2$) were the dominant reactive oxygen species (ROS) for SMX degradation. The integration of experimental works, characterization techniques and density functional theory (DFT) calculations unraveled that the formation of sulfur vacancies in MMS catalyst could expose more Mo sites, improve the charge density and boost the electron transfer, which was conducive to accelerating the Fe$^{3+}$/Fe$^{2+}$ cycle for enhancing the activation of PMS. Finally, the C-N, N-O, S-N, C-O and C-S bonds of SMX were easily attacked by ROS to generate the nontoxic intermediates in the MMS/PMS/SMX system. This study offers a new approach to designing high-performance Fe-MoS$_2$ catalysts for the removal of organic pollutants.