Covalent Modification of MIL-101 for Enhanced Tetracycline Adsorption from Aqueous Solutions
The pervasive presence of tetracycline (TC) in aquatic environments poses significant ecological and public health risks. This study reports the synthesis of MIL-101-Bim, a covalently modified metal-organic framework (MOF), via a pre-modification strategy that introduces formyl groups into the MIL-101(Cr) framework (MIL-101-CHO), followed by Schiff base condensation and NaBH4 reduction to graft benzimidazole moieties. Powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) confirmed retention of the parent MIL-101(Cr) topology. Fourier-transform infrared (FT-IR) spectroscopy verified successful functionalization. 1H NMR analysis of digested MIL-101-Bim revealed a benzimidazole modification degree of 41%, with 32% of formyl groups reduced to hydroxymethyl and 27% remaining unreacted. Thermogravimetric analysis (TGA) demonstrated good thermal stability. Nitrogen adsorption-desorption measurements showed a specific surface area of 1361 m2·g−1 and pore sizes ranging from 1 to 2.3 nm. Adsorption kinetics for TC on both materials followed a pseudo-second-order model, and isotherm data fitted the Langmuir model. The theoretical maximum adsorption capacity of MIL-101-Bim for TC was 86.31 mg·g−1, significantly higher than that of MIL-101-CHO (39.56 mg·g−1). Zeta potential measurements indicated optimal adsorption performance at pH 5–8. X-ray photoelectron spectroscopy (XPS) provided evidence of hydrogen bond formation during adsorption. The adsorption mechanism involves both physical adsorption (pore filling, electrostatic interactions, π-π stacking) and chemical adsorption (weak hydrogen bonding). Regeneration studies showed that MIL-101-Bim retained an adsorption capacity of 46.93 mg·g−1 after five cycles, demonstrating promising reusability.