Effective Removal of Humic Acid from Aqueous Solution in an Al-Based Metal–Organic Framework

The adsorption of humic acid (HA) onto an Al³⁺-based metal–organic framework (MIL-68­(Al)) was investigated. MIL-68­(Al) was synthesized and characterized using powder X-ray diffraction pattern, Fourier transform infrared spectroscopy, N₂ adsorption–desorption, scanning electronic microscopy, and X-ray photoelectron spectroscopy. Adsorption of HA onto MIL-68­(Al) was investigated systematically under various conditions. Experimental results indicate that the adsorption kinetics follow a pseudo-second-order model and adsorption can reach equilibrium at ∼15 min. The adsorption isotherm can be well-fitted with Freundlich model, and the adsorption capacity (115.5 mg g^–1) is much higher than those of chitosan, fly ash, activated carbon, etc. Thermodynamic study indicates that the adsorption of HA can be enhanced under lower temperature based on the negative value of adsorption enthalpy (−10.52 kJ mol^–1). Besides, MIL-68­(Al) can exhibit a high adsorption capacity of 28.53 mg g^–1 in a real water sample from Fenhe River. The adsorption mechanism can be well-explained by electrostatic interaction and the H-bond interaction. Therefore, this work may provide a guideline to construct MOF-based adsorbents for the high-efficiency capture of HA.