Enhanced cadmium ions capture performance with a dual-zinc metal-organic framework composite

The structure and properties of basic metal-organic frameworks (MOFs) continue to exhibit low Cd(II) uptake and selectivity. Assembly of same zinc-based MOF building blocks into a dual-zinc MOF@MOF composite could offer a promising avenue for efficiently capturing Cd(II) from aqueous solutions. This paper reports a dual-zinc MOF@MOF (Zn-BTC@ZIF-8, DZM) adsorbent, which was constructed for boosting the performances of adsorption and separation. The adsorption behavior of Cd(II) onto DZM from aqueous solution is thoroughly examined at room temperature. Results indicates that the DZM demonstrates significant capability in capturing Cd(II), attributed to the synergistic effects of Zn-BTC and ZIF-8 with diverse active sites, appropriate pore sizes and shapes, stable dual zinc bonding interaction, and more. The maximum Cd(II) uptake capacity reached 150 mg/g, with rapid attainment of adsorption equilibrium within just 20 min, surpassing the performance of other reported adsorbents in the literature. The adsorption data align well with both the Freundlich and pseudosecond-order kinetic models. The process of adsorption is a process of spontaneous heat absorption and increasing degrees of freedom. Particularly noteworthy is the sustained effectiveness of DZM in capturing Cd(II) even after the fifth cycle. This research introduces a fresh approach to crafting desirable MOF@MOF configurations, thereby advancing their potential utility in Cd(II) removal from water.