High-quality Al@Fe-MOF prepared using Fe-MOF as a micro-reactor to improve adsorption performance for selenite

High-quality Al@Fe-MOF was prepared by in situ modification of Fe-MOF with Al³⁺ to improve the adsorption performance for selenite (Se(Ⅳ)). The structures and properties of Al@Fe-MOF were characterized by powder X-ray diffraction, high resolution transmission electron microscope, X-ray photoelectron spectroscopy (XPS), nitrogen isothermal adsorption-desorption measurement and zeta potential. The adsorption performance of Al@Fe-MOF for Se(Ⅳ) was studied by batch adsorption experiments. A large number of pores in Al@Fe-MOF were filled by AlOOH and some bayerite formed on the surfaces. Compared with those of Fe-MOF, the specific surface area (SSA) and microporosity of Al@Fe-MOF decreased to 1368 m²/g and 38.5%, respectively. Hydrolysis occurred at pH > 5.0 for Fe-MOF, but did not for Al@Fe-MOF at the pH range of 3.0-7.0. Compared with in Fe-MOF, the adsorption capacity and efficiency of SSA for Se(Ⅳ) were increased by 77% and 112%, and the average free energy of adsorption was increased to 11.62 kJ/mol in Al@Fe-MOF. Besides, the Se(Ⅳ) adsorption amount of Al@Fe-MOF was almost not influenced by the pH from 3.0 to 7.0. The high resolution XPS (HR-XPS) and pH analysis indicated that Al species in Al@Fe-MOF could significantly increase the density of adsorption sites to improve its adsorption capacity for Se(Ⅳ).