Removal of methylene blue from aqueous solutions by an adsorbent based on metal-organic framework and polyoxometalate

A metal-organic framework (Cu₃(BTC)₂, BTC = 1,3,5-benzenetricarboxylate) composite based on polyoxometalate (H₆P₂W₁₈O₆₂) was synthesized by a simple one-pot solvent-thermal method and applied as an adsorbent to remove methylene blue (MB) from aqueous solution. The chemical structure, morphology and thermostability of the composite were characterized by Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Thermogravimetric Analysis (TG) and N₂ adsorption–desorption isotherms. The removal rate of the composite H₆P₂W₁₈O₆₂@Cu₃(BTC)₂ was greater than that of the pure Cu₃(BTC)₂, especially at higher initial concentrations, showing that the adsorption performance of porous Cu₃(BTC)₂ can be improved through the modification of H₆P₂W₁₈O₆₂. The effect factors containing the initial concentration, contact time, initial solution pH and temperature of MB adsorption onto the composite were systematically explored. The experimental isotherm data was found to fit the Freundlich model well and the process of MB adsorption onto H₆P₂W₁₈O₆₂@Cu₃(BTC)₂ was controlled by the pseudo-second-order kinetic model. The thermodynamic parameters illustrated that the adsorption was spontaneous and exothermic process. These results show that designing a metal-organic framework composite is a quite promising strategy to achieve extreme application for metal-organic framework. - Graphical abstract: Comparison of removal rate for MB in pure Cu₃(BTC)₂ and H₆P₂W₁₈O₆₂@Cu₃(BTC)₂ at different initial concentration. - Highlights: • Metal-organic framework (MOF) composite in the adsorption application was proposed. • The adsorption rate of MOF was improved by introducing polyoxometalates. • The adsorption isotherm and kinetic was used to describe the adsorption mechanism. • The thermodynamic parameters on the composite were thoroughly investigated