Hydrazide-functionalized covalent organic frameworks exhibit ultra-high palladium adsorption capacity and excellent selectivity

Palladium is a rare resource and is found in extremely low concentration in the earth's crust. Therefore, the efficient recovery of palladium from wastewater is significant but remains challenging. Herein, two acyl hydrazone-functionalized covalent organic frameworks (DMTH-FTD and DHTH-FTD) were synthesized and their adsorption properties for Pd(II) in water were investigated. The DHTH-FTD has an exceptional adsorption capacity of 714.1 mg‧g−1 for Pd(II) at pH 1.0, which is the highest value among all the crystalline porous adsorbents that have been reported so far. At a concentration of 50 mg‧L−1, 96.3 % of Pd(II) can be removed in a short period of time, which still maintains up to 92.2 % after five cycles. Both DMTH-FTD and DHTH-FTD both demonstrate excellent Pd(II) selectivity, with removal rates of 93.4 % and 99.6 % of Pd(II) from 17 coexisting competing ions. The FT-IR spectroscopy, point zero charge (PZC), XPS and DFT calculations reveal that the mechanisms behind the efficient adsorption of Pd(II) by DHTH-FTD is attributed to the chelation between O and N atoms and Pd(II). This work not only provides a new research idea for the efficient adsorption and recovery of Pd(II), but also contributes to the development of covalent organic materials.