Efficient Capture of Pb (II) Ions From Aqueous Solutions Using Metal Organic Frameworks@Covalent Organic Framework Composites With Abundant Carboxyl Groups

Abstract

The development of efficient adsorbents with high selectivity is essential for mitigating environmental pollution. This study introduces a novel adsorbent comprising a metal–organic framework (i.e., MIL-101(Fe)-NH₂) and a covalent organic framework (COF) enriched with carboxyl groups, which is designated MIL-101(Fe)-NH₂/COF-COOH. The adsorbent is synthesized through a Schiff base reaction followed by carboxymethylation and is subsequently employed for Pb (II) adsorption. The structure of MIL-101(Fe)-NH₂/COF-COOH is confirmed using various characterization techniques, including Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, and Brunauer, Emmett, and Teller surface area analysis. The adsorption capacity (q_e) value of MIL-101(Fe)-NH₂/COF-COOH is estimated through batch adsorption experiments^. The effects of several factors, including pH, adsorbent dosage, contact time, Pb (II) concentration, solution ionic strength, and temperature, on the adsorption process are systematically assessed. MIL-101(Fe)-NH₂/COF-COOH achieves the highest adsorption of 184 mg/g at pH 6. The adsorption kinetics, isotherms, and thermodynamics indicate that the adsorption process occur through a spontaneous monolayer chemical process. Moreover, MIL-101(Fe)-NH₂/COF-COOH displays remarkable anti-interference capability, and its q_e decreases by only 10% after 10 cycles. Based on the characterization results, the Pb (II) adsorption mechanism is determined to be primarily electrostatic and chelation interactions between Pb (II) and O- and N-containing functional groups. Thus, this highly efficient and recyclable adsorbent exhibits considerable potential for removing Pb (II) from aquatic ecosystems.