Selective adsorption of cationic dyes from aqueous solution by polyoxometalate-based metal–organic framework composite

Abstract

A novel environmental friendly adsorbent H6P2W18O62/MOF-5 was synthesized by a simple one-step reaction under solvothermal conditions and characterized by XRD, FTIR, thermogravimetric analyses (TGA) and N2 adsorption–desorption isotherms. The removal rate of H6P2W18O62/MOF-5 was quite greater (85%) than that of MOF-5 (almost zero), showing that the adsorption performance of porous MOF-5 can be improved through the modification of H6P2W18O62. Further study revealed that H6P2W18O62/MOF-5 exhibited a fast adsorption rate and selective adsorption ability towards the cationic dyes in aqueous solution. The removal rate was up to 97% for cationic dyes methylene blue (MB) and 68% for rhodamine B(Rhb) within 10min. However, anionicdye methyl orange(MO) can only reach to 10%. The influences including initial concentration, contact time, initial solution pH and temperature of MB adsorption onto H6P2W18O62/MOF-5 were investigated in detail. The kinetic study indicated that the adsorption of MB onto H6P2W18O62/MOF-5 followed the pseudo second-order model well. The isotherm obtained from experimental data fitted the Langmuir model, yielding maximum adsorption capacity of 51.81mg/g. The thermodynamic parameters analysis illustrated that the MB adsorption onto H6P2W18O62 immobilized MOF-5 was spontaneous and endothermic process. Besides, these results implied that designing a novel material polyoxometalate-based metal–organic frameworks is great potential for removing cationic organic pollutants and even extended to improve other specific application.