Structure-controlled recovery of palladium(II) from acidic aqueous solution using metal-organic frameworks of MOF-802, UiO-66 and MOF-808

For sustainable development and environmental protection, “urban mining” for the recovery of critical metals, especially for precious metals (PMs), from urban solid wastes, has recently attracted substantial attention. Here, metal-organic frameworks (MOFs) of MOF-802, UiO-66 and MOF-808 with significantly different structures, such as various porous properties and Zr₆-node connectivities, were fabricated and tested for the recovery of PMs of palladium(II) (PdCl₄²⁻) from acidic aqueous solutions. Adsorption kinetic and isotherm studies showed structure-controlled recovery performance of Pd(II). Among these MOFs, MOF-808 showed the most rapid Pd(II) uptake and the highest adsorption capacity (the maximum adsorption capacity of 163.9 mg g⁻¹) with achieving the equilibrium within 3 min, which could be attributed to the most convenient diffusion of PdCl₄²⁻ through the largest pores of MOF-808, and the most abundant binding sites in the 6-connected Zr₆-nodes, as well as the adequate inner space for accommodating PdCl₄²⁻, respectively. However, the MOF-802 exhibited the slowest uptake yet highest affinity for Pd(II) with the best adsorption performance in low concentration Pd(II) solutions. This is likely because of the extremely high binding energy (E_bind) between protonated amine groups of MOF-802 and the PdCl₄²⁻. Furthermore, the high adsorption selectivity for Pd(II) and good regeneration performance of MOF materials were observed, showing their potential for further large-scale applications.