Cadmium(II) adsorption with ZIF-8 nanocrystals of different morphology

The food safety and human health have been seriously threatened by water pollution with heavy metals. Metal-organic framework (MOF) is considered as one of the promising adsorbents for decontamination of heavy-metal-containing water owing to ordered structure with high porosity. In this work, ZIF-8 nanocrystals with different morphologies (ZIF-RD, ZIF-TD and ZIF-L) were obtained by simply adjusting the synthesis process and precisely characterized via XRD, BET, SEM, FT-IR and XPS. The as-synthesized samples were used for adsorption of Cd2+ to measure the adsorption behavior. The largest Cd2+ adsorption capacity was achieved on truncated-dodecahedral ZIF-8 sample with the qe of 73.8 mg/g, whereas the rhombic dodecahedral ZIF-8 with the largest surface area showed the poorest adsorption performance. Adsorption mechanism study revealed that the Cd2+ adsorption on three ZIF-based materials involves chemical adsorption process. Moreover, ZIF-8 chelates with Cd2+ ions primarily through N-containing groups on the MOF skeleton. Our findings also proved that the specific area of adsorbents does not necessarily ensure the efficiency for removing hazardous heavy metal ions.