Simultaneous removal of mixed contaminants, copper and norfloxacin, from aqueous solution by ZIF-8

Many wastewaters commonly contain complex mixtures of both antibiotics and heavy metals, where the simultaneous removal of these mixed contaminants is still a challenge. In this paper, ZIF-8, a metal-organic framework material, was proposed as a potential absorbent to simultaneously remove norfloxacin and copper from wastewater. Batch adsorption experiments showed that ZIF-8 could simultaneously remove 95.4% of Cu and 80.3% of norfloxacin. ZIF-8 was characterized before and after the removal of the mixed contaminants. X-ray powder diffraction (XRD) showed that ZIF-8 was synthesized successfully and maintained a stable structure after contaminant adsorption. Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) both suggested that contaminants were partially adsorbed on the surface of ZIF-8. X-ray photoelectron spectroscopy (XPS) also indicated the presence of both carboxyl and Cu on the ZIF-8 surface confirming that a mixture of contaminants was adsorbed. For both contaminants, adsorption followed pseudo-second-order kinetics and best fit the Freundlich adsorption model with correlation coefficients (R-2) of 0.973 and 0.994, for copper and norfloxacin, respectively. Finally, removal mechanisms for copper and norfloxacin by ZIF-8 were proposed. For Cu, the adsorption process on ZIF-8 was mainly due to ion exchange, whereas electrostatic interactions and pi-pi stacking may be responsible for norfloxacin adsorption. Overall this study demonstrated that ZIF-8 was a promising material for the treatment of wastewater containing mixtures of organic and metallic contaminants.