Analysis of microstructure properties of Ni-Fe MOFs and their influence on selective recognition of single and binary dye systems

Abstract

To address the environmental threats posed by dye pollution, it is crucial to develop adsorbents with high selectivity and adsorption capacity for the removal of chemical dye pollutants. In this study, five types of Ni/Fe-MOFs were synthesized via a one-pot solvothermal method by varying the Ni/Fe molar ratio. Their physicochemical structures, surface morphologies, and compositions were characterized through a series of techniques, and their adsorption performances for different dyes were preliminarily investigated. The results demonstrated that the Ni/Fe ratio significantly affects the microstructure and adsorption performance of the MOFs. As the molar ratio of Ni/Fe increased, the structures of the five materials transformed from nanoflowers_(Fe-MOFs) to nanospheres_{(Ni/Fe(3:1)MOFs)}, and eventually to rhombic blocks_(Ni-MOFs). The specific surface area decreased from 281.3 m²/g_(Fe-MOFs) to 67.11 m²/g_{(Ni/Fe(1:1)MOFs)} and further to 0.5369 m²/g_(Ni-MOFs). XPS analysis confirmed the synergistic coordination between Ni and Fe, and FT-IR spectra revealed characteristic peaks for hydroxyl groups on the MOF surface and carboxylate ligands. XRD analysis indicated that materials with higher Ni content were more prone to ligand decomposition, which is consistent with the weight loss observed in the TG-DSC between 230 and 350 °C. The materials exhibited selective adsorption for different dyes. Under optimal conditions, the maximum adsorption capacities for Eosin Y (EY), Neutral Red (NR), and Acid Fuchsin (AF) were 54.92 mg/g_(Fe-MOFs), 636.31 mg/g_(Ni-MOFs), and 79.30 mg/g_{(Ni/Fe(1:1)MOFs)}, respectively. The adsorption kinetics followed a pseudo-second-order model, and the adsorption isotherms conformed to the Langmuir model, while Fe-MOFs and Ni/Fe(1:1) MOFs also exhibited compatibility with the Freundlich model. The adsorption mechanism primarily involved monolayer adsorption as the dominant process, with localized multilayer adsorption, as well as chemisorption, hydrogen bonding, hydroxyl interactions, and electrostatic interactions. These MOFs demonstrated high dye removal efficiency in actual water samples and fruit and vegetable juices, exhibiting excellent reusability and anti-interference capabilities. Importantly, compared with the single system, Fe-MOFs and Ni/Fe(1:1)MOFs exhibit competitive adsorption, and Ni-MOFs exhibit synergistic adsorption in the binary system. This study confirms that by adjusting the Ni/Fe molar ratio, MOFs with high selectivity and adsorption performance for different dyes can be designed, providing new theoretical insights and technical support for environmental remediation and food safety applications.