An iron-based metal-organic framework with strong water stability and effective adsorption of methylene blue from wastewater

Abstract

The stability of iron-based metal-organic frameworks (MOFs) in water affects their performance in adsorbing pollutants from wastewater. In this paper, an ionic iron-based metal-organic framework {[NH₂(CH₃)₂][Fe₆(O)₂(HCOO)(CO₃)(L)₆(H₂O)₂]·17(H₂O)·13(DMF)}_n (Fe₆-MOF) and its nano adsorbents (Fe₆-MOF-N) are synthesized by FeSO₄·7H₂O and 4,4′-biphenyldicarboxylic acid (H₂L) as raw materials. X-ray single crystal diffraction shows that L²⁻ and HCOO⁻ in Fe₆-MOF take the form of μ₄-η¹:η¹:η¹:η¹ and μ₂-η¹:η¹ to bridge adjacent iron ions to form a three-dimensional structure with pores (18.96 × 19.89 Å), respectively. The particles of Fe₆-MOF-N are approximately 200 nm and they have broad pH (3.00–9.00) stability. The maximum equilibrium adsorption capacity of Fe₆-MOF-N adsorbs methylene blue (MB) in wastewater under optimal conditions (pH = 6.00, T = 25 °C) is 162.46 mg/g, which is larger than most MOF adsorbents. This may be because Fe₆-MOF-N can adsorbs MB through strong electrostatic attraction and hydrogen bonds. Adsorption thermodynamic shows Fe₆-MOF-N adsorbs MB conforms to the pseudo-first-order adsorption kinetic equation. Moreover, adsorption kinetic also shows that Fe₆-MOF-N adsorbs MB at 25–45 °C is the process of decreasing Gibbs free energy (△G < 0) and enthalpy change (△H < 0). This study provides an example for the synthesis of iron-based MOFs with good thermal stability, water stability, and effectively adsorbs MB.