Simple synthesis, characterization and mechanism of Fe/Zr bimetallic-organic framework for Cr (VI) removal from wastewater

Iron-zirconium bimetallic metal-organic frameworks (MOFs) materials were synthesized by a one-step solvothermal method with abundant active sites for the efficient removal of Cr (VI) ions from wastewater. The as-prepared Fe/Zr-MOFs composites were analyzed by the XRD, FTIR, SEM, TEM, and XPS. The effects of temperature, pH, initial Cr (VI) ion concentration, and co-existing ions on the adsorption properties of the Fe/Zr-MOFs composites were verified. The best removal efficiency of the Fe/Zr-MOFs composites was observed at pH = 5, with the concentration of Cr (VI) ion 20 mg/L under 303 K. In addition, the adsorption process of Cr (VI) by Fe/Zr-MOFs materials consisted with the Langmuir isothermal adsorption model, indicating that the adsorption process is a predominantly uniform monolayer adsorption. Kinetic modelling confirmed that the adsorption process of the material is dual physical and chemical adsorption. The thermodynamic analysis confirmed that the adsorption of Cr (VI) by Fe/Zr-MOFs was a spontaneous feasible, heat-absorbing process. The removal rate was up to 82% after five cycle reused experiments, suggesting the potential practical application of the material. The outperformance of the as-prepared materials was attributed to the combination of the advantages of Fe-MOF and Zr-MOF. Of note, a possible adsorption mechanism was proposed. That is, the adsorption process of the material is a dual physical and chemical adsorption, which mainly includes electrostatic interactions, Chemical coordination, and redox reactions.