Enhancement capture capability of cadmium ions using a MOF-in-MOF composite

Abstract

Due to the limited Cd(II) absorption capacity and stability of single and simple metal-organic frameworks (MOFs), combining two MOF building blocks to create a core-shell MOF-in-MOF composite material offers a promising approach for efficiently capturing Cd(II) from aqueous solutions. Utilizing the epitaxial growth method, we successfully fabricated a core-shell structured (NH₂-MIL-125)-in-(ZIF-67) (M-in-Z) composite material. The material underwent comprehensive characterization employing SEM, XRD, FT-IR, N₂ adsorption-desorption, and diverse testing methods to evaluate its Cd(II) adsorption and removal capabilities in water environments. The Cd(II) adsorption capacities exhibited the sequence NH₂-MIL-125 < ZIF-67 < M-in-Z(54). Adsorption isotherm results adhered to the Langmuir model, indicating a relatively ideal single-molecule layer adsorption process for Cd(II) on M-in-Z(54). The adsorption kinetics conformed to the pseudo-second-order model, indicating that equilibrium was reached in 30 min. Thermodynamic studies unveiled the spontaneous, exothermic nature of the M-in-Z(54) adsorption process, associated with an increase in degrees of freedom. Physical adsorption emerged as the primary driving force, complemented by chemical adsorption. Following five adsorption cycles, M-in-Z(54) sustained its Cd(II) adsorption performance at 92.56 % of the initial capacity, showcasing outstanding regeneration capability. Moreover, the structure and morphology of M-in-Z(54) remained intact after regeneration, demonstrating superior stability compared to the core-satellite structure. This conclusion highlights the promising potential of M-in-Z(54) as an environmentally friendly material for efficient Cd(II) removal.