3D covalent organic framework with crown ether and POSS motifs for selective recovery of Au( <scp>iii</scp> ) and Pd( <scp>ii</scp> ) from highly acidic wastewater

The recovery of Au(III) and Pd(II) from wastewater is crucial for both metal recycling and environmental protection. Three-dimensional covalent organic frameworks (3D COFs) have emerged as promising materials for the selective adsorption and recovery of metal ions from dilute solutions. Herein, we overcame the challenge of constructing crystalline 3D COFs with flexible units and successfully synthesized POSS-18-C-6-COF by solvent optimization, incorporating two flexible units, OAPS and B18C6. The crystallinity of the COF was confirmed by XRD, distinguishing it from an amorphous polymeric network. This flexible 3D COF, with a "scu" topology, exhibited excellent adsorption capacities for Au(III) (425.9 mg g^-1) and Pd(II) (178.4 mg g^-1) in 2 mol L^-1 HNO₃, along with high selectivity for Au(III) and Pd(II) over various interfering ions, such as Pb(II), Cu(II), Al(III) and Fe(III). Stability tests showed that more than 80% of the adsorption efficiency for Au(III) and Pd(II) was retained after five cycles, highlighting the COF's potential for adsorption in highly acidic environments. Mechanism analysis revealed that the nitrogen sites in the COF material are responsible for coordinating with Au(III) and Pd(II). This study provides a new example of synthesizing flexible 3D COFs and offers a promising approach for the recovery of precious metals from wastewater.