Adsorption behavior and mechanism of NH2-MIL-101(Cr)@COFs@SA composite adsorbent for tetracycline removal

Abstract

Herein, we employed a rational approach to develop a hydrogel composite material by encapsulating NH2-MIL-101(Cr)/covalent organic frameworks (COFs) in sodium alginate (SA) to effectively capture of tetracycline (TC). Experimental tests and various characterizations (including Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron spectroscopy (SEM), Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy (XPS)) confirmed that the NH2-MIL-101(Cr)@COFs@SA composite exhibited a more robust, multilayer pore structure with abundant active functional groups. Under conditions of 298 K and pH = 7, the NH2-MIL-101(Cr)@COFs@SA adsorbent demonstrated remarkable TC adsorption capability, achieving a removal rate of 96.38 % in 120 min and a qmax of 252.6 mg/g at 298 K by Langmuir model. Kinetic analysis indicated that the interaction between TC and NH2-MIL-101(Cr)@COFs@SA follows a pseudo-second-order model, suggesting that chemisoption governs the process. The Langmuir model and thermodynamic analysis suggested that TC adsorption follows a monolayer sorption pattern and is spontaneous and exothermic. Even in the presence of other ions, NH2-MIL-101(Cr)@COFs@SA maintained high efficiency for TC adsorption, demonstrating superior selectivity. NH2-MIL-101(Cr)@COFs@SA demonstrated remarkable recyclability, with only a minimal reduction in the removal efficiency (85.5 % and 142 mg/g) after 10 cycles of adsorption and regeneration. Various analytical techniques, including FTIR spectroscopy, SEM, EDX, XPS, and density functional theory (DFT) calculations of adsorption energy were used to elucidate the TC adsorption mechanisms by NH2-MIL-101(Cr)@COFs@SA. The adsorption process primarily involved π-π stacking, hydrogen bonding, electrostatic interactions, and complexation.