Fast recovery of Au (III) and Ag(I) via amine-modified zeolitic imidazolate framework-8

In this paper, zeolitic imidazolate framework-8 modified by the ethanediamine (NH₂-ZIF-8) was employed for adsorbing Au (III) and Ag(I) from aqueous solutions. The adsorption capacities of NH₂-ZIF-8 towards Au (III) and Ag(I) were found to be significantly affected by the pH values of the solution. The adsorption kinetics studies show that NH₂-ZIF-8 presents a fast adsorption property towards metals, attaining 93% of adsorption equilibrium uptake for Au (III) within the first 30 min. This phenomenon can be ascribed to the coordination interaction between the amino group and Au (III). The thermodynamic data suggest that the adsorption of NH₂-ZIF-8 towards Au (III) is endothermic process, while that for Ag(I) is exothermic. The maximum adsorption capacities of NH₂-ZIF-8 toward Au (III) and Ag(I) can be achieved to 357 mg·g^?1 and 222.25 mg·g^?1, respectively. The metal ions interference results show that Cu (II) and Ni (II) hardly have no interference on Au (III) adsorption in e-waste containing 1500 mg·l^?1 Cu (II),100 mg·l^?1 Ni (II) and 10 mg·l^?1 Au (III); while for Ag(I), Cd (II) and Zn (II) have little interference on Ag(I) adsorption in the hybrid solutions containing Ag(I), Ni (II), Cd (II) and Zn (II) with equal concentration (50 mg·l^?1), but Ni (II) interference most. The XPS study shows that partial Au (III) was reduced to Au(I), and that Ag(I) was completely reduced to Ag(0) during the adsorption process. The abundant of active sites of NH₂-ZIF-8 containing C=N, N-H, and Zn-OH groups play a key role in the adsorption of Au (III) and Ag(I). In addition, electrostatic interaction can be responsible for the adsorption of Au (III) by NH₂-ZIF-8. The regeneration experiments results show that the adsorption capacities of NH₂-ZIF-8 towards Au (III) and Ag(I) can maintain after three cycles. This work provides a reliable method to improve the adsorption kinetics for metal ions.