Abstract
Two series of lignin (LN)-based adsorbents, namely, cross-linked lignin (LNEs) with different crosslinking densities and carboxymethyl cross-linked lignin (LNECs) with various degrees of carboxymethyl substitution, were prepared to remove ofloxacin (OFL), a popular fluoroquinolone (FQ) antibiotic, from water. LNEs and LNECs exhibited satisfactory performance in OFL adsorption. Both of them had high adsorption capacity (the maximum contribution of 0.828 mmol/g), good anti-interference to some inorganic salts, and efficient regeneration and reuse performance. The crosslinking density and degree of carboxymethyl substitution strongly affected the content and distribution of oxygen-containing groups in these LN-based adsorbents, which played important roles in OFL adsorption. The pH dependencies of the adsorption performance of LNEs and LNECs indicated the involvement of multiple adsorption mechanisms, including hydrogen bond, electrostatic attraction, π-π electron–donor–acceptor interactions, and negative charge-assisted hydrogen bond. Different mechanisms were dominant under various pH levels, in a near neutral pH, the synergistic effect of electrostatic attraction and π-π interaction allows LINEs and LINECs to reach maximum adsorption capacity. Five FQs with similar structures and their two sub structural analogs were compared in terms of adsorption behavior and electrostatic potential by density functional theory using quantum chemical calculation. FQs with secondary amino groups and low π electron cloud density readily bound to LN-based adsorbents. Hence, LNEs and LNECs were efficient and environment-friendly adsorbents.
Highlights
Over the past few decades, fluoroquinolones (FQs) have gained increasing attention for treatment of broad-spectrum bacterial infections in humans and animals[1]
Three LNEs with different crosslinking densities and five LNECs with various degrees of carboxymethyl substitution were obtained according to the inset of Fig. 1 by controlling the fed amounts of crosslinking agent (EPI) and etherification agent (CA), respectively, which were described in detail in the experimental section
The surface morphologies of LNEs and LNECs samples turned rougher and more compact owing to modifications in comparison with smooth surface of pure LN (Supporting Information Fig. S2)
Summary
Over the past few decades, fluoroquinolones (FQs) have gained increasing attention for treatment of broad-spectrum bacterial infections in humans and animals[1]. As effective adsorbents due to their having abundant oxygen-containing functional groups, lignin-based materials were employed to remove various contaminants in water such as heavy metals[25,26] and cationic dyes[27,28] in previous study. The adsorption properties of the four other FQs with similar structures to OFL [norfloxacin (NOR), ciprofloxacin (CIP), enrofloxacin (ENR), and fleroxacin (FLE)] and their two sub structural analogs [flumequine (FLU) and 1-(2-Fluorophenyl) (FPP)] as molecular probes were compared. Their electrostatic potentials (ESP) were estimated by density functional theory (DFT) using quantum chemical calculation to further explore the adsorption mechanism
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