Guanidinium-functionalized ionic covalent organic framework for selective and efficient recovery of palladium(II) from metallurgical wastewater

Efficient and selective recovery of palladium from acidic metallurgical wastewater is of great economic and environmental importance but remains challenging. Here, two novel ionic covalent organic frameworks (iCOFs), Tp-DGCl and BT-DGCl, were synthesized by reaction of 1,3-diaminoguanidine hydrochloride with 1,3,5-triformy-phloroglucinol and benzene-1,3,5-triscarbaldehyde, respectively, and then applied to the recovery of Pd(II) from metallurgical wastewater. The maximum adsorption capacities of Tp-DGCl and BT-DGCl at pH 2.0 and 298.15 K were 342.1 mg g-1 and 231.1 mg g-1, which are higher than the majority of similar reported adsorbents. The adsorption isotherms of Pd(II) on iCOFs were highly consistent with the Langmuir isotherm model. Furthermore, the kinetics of the adsorptions by Tp-DGCl and BT-DGCl conformed to the quasi-second-order kinetic model, indicating that the controlling step of adsorption is chemical adsorption. Thermodynamic studies revealed that elevated temperatures are beneficial for the recovery of Pd(II). At the same time, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, zeta potential, and density functional theory computational analysis confirmed that electrostatic attraction and coordination are the main mechanisms for Pd(II) adsorption on iCOFs. This work not only highlights that the prepared iCOFs can be used as effective reusable adsorbents to selectively adsorb Pd(II) but also opens the door for the practical application of iCOFs in recovering palladium from complex metallurgical wastewater.