Adsorption characteristics of tetracycline hydrochloride and oxytetracycline by a MOF-525(Co) metal organic framework

Metal organic frameworks (MOFs) has attracted significant scientific interest due to the advantages of high porosity, special porous structure, large specific surface area, open metal sites and modifiable pore size. In this study, the adsorptive performances of tetracycline hydrochloride (TCH) and oxytetracycline (OTC) are evaluated with three Zr-MOFs (MOF-545, MOF-525(Zn), and MOF-525(Co)). Results show that MOF-525(Co) is the optimal adsorbent when the initial antibiotic concentration of 100 mg L-1. The removal efficiency for TCH and OTC is 89.4% and 84.2% at 15 min; moreover, 100% for TCH or OTC is obtained at 60 min. The optimal adsorption pH is 9.0 for TCH and 5.0 for OTC, and the adsorption capacity of TCH by MOF-525(Co) is decreased while that of OTC is increased with the increasing of NaCl concentration from 0 to 1.0 mol L-1. It is revealed that the pseudosecond-order kinetic model (PSOK) fits much better for describing the antibiotics adsorption processes; It is also further suggested that Langmuir is better fitted for TCH, while Freundlich is better fitted for OTC; the adsorption process of both TCH and OTC can be illustrated as spontaneous, endothermic, and with positive entropy by thermodynamics data. Furthermore, the spectroscopy results (including X-rayphotoelectron spectra (XPS) and fourier transform infrared spectroscopy (FT-IR)) indicate that the adsorption mechanisms of both TCH and OTC by MOF-525(Co) are belonging to the 7C-7C interactions, electrostatic interactions, hydrogen bonds, and pore filling. These results may provide new insights into the potential application of MOF-525(Co) as adsorbent for simultaneous removal of TCH and OTC from aqueous solution.