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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-secondorder 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+ .

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Three-dimensional (3D) double-network graphene oxide/alginate-polyacrylonitrile (GO/Ca-Alg2-PAN) composite hydrogels were synthesized via surface functionalization of GO to activate adsorption sites. The morphology and structure of the GO/Ca-Alg2-PAN were analyzed by scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA-DSC). The results of the physicochemical analyses indicated that GO/Ca-Alg2-PAN was successfully synthesized by the combination of a 2D-structured graphene oxide with the alginate which was functionalized with the PAN polymer to generate the 3D double network composites. This functionalization approach contributed to an increase in Cu²⁺ ion adsorption capacity. The maximum adsorption capacity of the GO/Ca-Alg2-PAN for Cu²⁺ was 5.99 mmol/g. The results of adsorption kinetic experiments indicated that the GO/Ca-Alg2-PAN reached adsorption equilibrium within 147 mins at 2 mM Cu²⁺ in accordance with a pseudo-second-order model.

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