Efficient degradation of ciprofloxacin by peroxymonosulfate activated using red mud-based ZIF-67 composite as a heterogeneous catalyst

The red mud-based ZIF-67 composite (ZIF-67/RM) was synthesized by a simple coprecipitation method and employed as a heterogeneous catalyst for activating peroxymonosulfate (PMS) to degrade ciprofloxacin (CIP). The optimum preparation conditions of ZIF-67 were as follows: Co(NO3)2·6H2O dosage 10 mmol, 2-MeIM dosage 80 mmol, RM dosage 3 g, stirring time 3 h, stirring temperature 60 °C, stirring speed 640 rpm. Under the conditions of ZIF-67/RM dosage 0.1 g/L, PMS concentration 0.2 mM, CIP concentration 20 mg/L, and pH = 7, the maximal removal rate of CIP could reach about 82.4 % at the reaction time of 30 min. The SEM and BET characterization results showed that the rhombohedral dodecahedral ZIF-67 could fully fill the large pores of RM and form additional pore structures on the surface of RM due to its small particle size and regular shape. This phenomenon was conducive to the increase of specific surface area and the provision of more active sites for ZIF-67/RM composites. The comparative experiments revealed that Fe in RM and Co in ZIF-67 exhibited a positive synergistic catalytic effect for CIP removal. Quenching experiment, electron paramagnetic resonance (EPR) and high performance liquid chromatography (HPLC) analysis showed that Fe(IV) and Co(IV) played a leading role in the ZIF-67/RM/PMS system. At the same time, a large number of reactive oxygen species (ROS) were also produced to participate in the oxidative degradation process. The chronoamperometry experiment showed that electron transfer had little effect on CIP degradation. Furthermore, the catalytic mechanism of the PMS activated by ZIF-67/RM was proposed. DFT calculations and LC-MS analysis deduced the degradation pathway of CIP. Using the principles of quantitative structure–activity relationship (QSAR), the ecotoxicity of the degradation intermediates was analyzed. The recycling experiment, FT-IR, and XRD demonstrated that the presence of Fe-O-Co bonds prevented the leaching of Co and Fe ions, thus ZIF-67/RM exhibiting good stability. In this work, novel insights are provided into the design of composite materials based on RM and metal-organic frameworks (MOFs) as heterogeneous catalysts for the effective treatment of antibiotic wastewater through PMS activation.