Synthesis of Metal–Organic Framework ZnO _x -MOF@MnO ₂ Composites for Selective Removal of Strontium Ions from Aqueous Solutions

Abstract

A Zn(II)-based metal–organic framework (MOF) compound and MnO ₂ were used to prepare ZnO _x -MOF@MnO ₂ composites for selective Sr ²⁺ removal in aqueous solutions. The ZnO _x -MOF@MnO ₂ composites were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, and Brunauer–Emmett–Teller surface area analysis. The functional groups, morphologies, thermal stabilities, and specific surface areas of the composites were suitable for Sr ²⁺ adsorption. A maximum adsorption capacity of 147.094 mg g ^–1 was observed in batch adsorption experiments, and the sorption isotherms were fit well by the Freundlich model of multilayer adsorption. Adsorption reached equilibrium rapidly in kinetic experiments and followed the pseudo-second-order kinetic model. The adsorption capacity of the ZnO _x -MOF@MnO ₂ composite with the highest MnO ₂ content was high over a wide pH range, and the composite was highly selective toward Sr ²⁺ in solutions containing coexisting competing ions. Also, it has a good reusability for removing Sr ²⁺.