Outstanding performance of thiophene-based metal-organic frameworks for fluoride capture from wastewater

Worsening fluoride pollution in water poses a great threat to human life. Because of the unique properties of the 2, 5-thiophenedicarboxylate (H2TDC) ligand, a series of TDC-MOFs were prepared by altering the central metal ions (i.e., Al3+, Ce4+ and Zr4+) and they were subsequently used to adsorb fluoride from aqueous solution. Characterization of the materials (SEM, XRD, BET, and TGA) showed that TDC-MOFs had high thermal stability, well-defined microstructure and high specific surface area. The maximum adsorption capacities of Al-TDC, CeTDC, and Zr-TDC at 298 K were 107.5 mg g(-1), 94.9 mg g(-1), and 97.0 mg g(-1), these were superior to the values for most adsorbents reported in the literature. Adsorption of fluoride on TDC-MOFs accorded with the Langmuir adsorption isotherm model, and the quasi-second-order kinetic model, in which chemisorption was the kinetic control step. The optimized parameters (including pH, time, interfering ions, and ionic strength) indicated that TDC-MOFs were suitable for fluoride removal over a broad pH range (4.0-10.0) without interference. Fourier transform infrared spectroscopy (FTIR), Zeta potential, and X-ray photoelectron spectroscopy (XPS) analyses confirmed that the main mechanisms of fluoride adsorption onto TDC-MOFs were the combination of electrostatic attraction and ligand exchange. Fluoride-loaded on TDC-MOF can be eluted with dilute NaOH solution, and the adsorbent can be used for at least 4 cycles of adsorption/desorption. These results indicated that the prepared TDC-MOFs were promising candidate materials for fluoride removal from actual and complex water.