Interfacial CoAl2O4 from ZIF-67@γ-Al2O3 pellets toward catalytic activation of peroxymonosulfate for metronidazole removal

Metal organic frameworks (MOFs)-based catalysts are rising stars in advanced oxidation processes (AOPs) to generate reactive oxygen species (ROS). Nevertheless, constructing an affordable and recyclable MOFs-derived catalyst with high reactivity and stability is highly desirable but still challenging. Herein, we unprecedentedly designed a novel CoAl2O4@gamma-Al2O3 pellet (CoAl2O4@AP) as a heterogeneous catalyst to activate peroxymonosulfate (PMS) toward ROS generation for metronidazole (MNZ) removal. A CoAl2O4 shell was formed on the pellet surface by direct calcination of the surface-nucleated ZIF-67 on gamma-Al2O3 pellets. The strong and unique interactions between the Co species of ZIF-67 and gamma-Al2O3 resulted in the formation of interfacial CoAl2O4. The CoAl2O4@AP exhibited a remarkably higher efficacy for PMS activation than ZIF-67-derived Co3O4 and commercial Co3O4. The effects of several reaction parameters (i.e., PMS dosage, reaction temperature, initial pH, and background inorganic ions) on MNZ removal were comprehensively investigated and the degradation pathways of MNZ were elucidated. The Co2+/Co3+ and oxygen vacancies of CoAl2O4@AP were the intrinsic active centers for PMS activation to simultaneously generate a diversity of ROS, among which SO4 center dot- and O-1(2) played the decisive roles in MNZ decomposition. After the reaction, CoAl2O4@AP can be easily separated from the bulk solution and regenerated via a PMS-assisted cleaning process, making the composites highly appealing for practical applications. This pioneering work on constructing microscopic CoAl2O4-based composites expands the application of MOFs-based catalysts in novel AOPs systems for the treatment of persistent emerging pollutants.