Preparation of metal-organic framework composite beads for selective adsorption and separation of palladium: Properties, mechanism and practical application

Separation and recovery of palladium from industrial waste is an environmental way to utilize the valuable secondary resources. Metal-organic framework (MOF) has a great potential for the adsorption of Pd(II) from aqueous solutions, but the tiny powder form of MOF restricts its practical applications. In this work, a feasible and green method is developed to prepare novel MOF composite beads (B-MOF) with a stable cross-linked network. The adsorption performance of B-MOF for Pd(II) was evaluated in acidic solutions. Comprehensive characterization results show that the structure of B-MOF is stable throughout the adsorption process. At pH 3.0, the adsorption capacity of Pd(II) on B-MOF was high up to 334.68 mg.g(-1), exhibiting superiority to most of the other congeneric adsorbents. The adsorption behavior accorded with the pseudo-second-order kinetic model and the Langmuir adsorption isotherm, which was a spontaneous and endothermic chemisorption process. Moreover, the adsorption performance of B-MOF persisted almost unchanged even after 12 regeneration cycles. B-MOF also demonstrated high selectivity to Pd(II) in the presence of interfering ions, including Na(I), K(I), Ni(II), Mg(II), Cu (II), Zn(II), Co(II), Ca(II) and Al(III). Mechanism studies reveal that Pd(II) can be adsorbed on B-MOF by the inner-sphere complexation to Zr-nodes and the electrostatic attraction with protonated amine groups. The recovery of Pd from a real industrial waste was successfully achieved with a recovery rate of 92.5 %. This study provides an effective method to improve the practicability of MOF-based materials and may open a new way for the disposal of industrial waste containing palladium.