Encapsulation of tungstophosphoric acid into harmless MIL-101(Fe) for effectively removing cationic dye from aqueous solution

Abstract

A Keggin-type polyoxometalate H₃PW₁₂O₄₀ (PW₁₂) was firstly incorporated into the harmless porous metal organic framework MIL-101(Fe) resulting in POM@MOF composite material (denoted as PW₁₂@MIL-101). The material was synthesized by a one-pot solvothermal reaction of H₃PW₁₂O₄₀, FeCl₃·6H₂O and terephthalic acid (H₂bdc). The composite was characterized by FTIR, XRD, thermogravimetric analysis (TGA), inductively coupled plasma (ICP) spectrometry, SEM, EDS, ³¹P NMR and nitrogen adsorption–desorption isotherms. These results demonstrated the successful insertion of H₃PW₁₂O₄₀ within the cavities of MIL-101(Fe). The adsorption rate of the isolated MIL-101 was up to 30.7% and 28.3% for cationic dyes methylene blue (MB) and rhodamine B (RhB), and 82% for anionic dye methyl orange (MO) within 30 min. However, the dye adsorption capacity of MIL-101 has greatly changed by introducing highly electronegative polyoxoanions into the cages of MIL-101. The adsorption rate of PW₁₂@MIL-101 composite was able to reach 99% and 96% for cationic dyes MB and RhB, and 16% for anionic dye MO in the initial 5 min. Surprisingly, the composite not only exhibited a large-scale adsorption capacity of 473.7 mg g⁻¹ for MB, but also could quickly remove 97.3% MB from the dye solution with 1 ppm (1 mg L⁻¹). Furthermore, this material could be easily desorbed for reuse, and the structure of the composite was intact during the adsorption experiment. Thus, it is a promising and environmentally friendly adsorbent for removing cationic organic pollutants in dye-wastewater.