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 (R2) 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 π-π 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.