Adsorption of levofloxacin onto an iron-pillared montmorillonite (clay mineral): Kinetics, equilibrium and mechanism

Abstract

Antibiotics have been recognized as a class of emerging pollutant. In this study, Fe-pillared montmorillonite (Fe-P-Mt) was prepared and characterized by XRF, XRD, BET, and UV–vis-DRS analyses. Adsorption of levofloxacin (LVX), a fluoroquinolone antimicrobial agent, onto Fe-P-Mt was investigated as a function of contact time, LVX concentration, pH, temperature, and ionic strength by using a batch adsorption method. FT-IR spectroscopy was used to reveal the interaction of LVX and Fe-P-Mt at the molecular level. The obtained results showed that the adsorption of LVX on Fe-P-Mt followed the pseudo-second-order kinetics, and the adsorption isotherms conformed to the Langmuir model with a maxima adsorption capacity of 48.61mg·g−1 at 25°C. The solution pH exerted a strong influence on LVX adsorption. The maximum adsorption was found to occur around pH7. The addition of NaCl had a minor effect on LVX adsorption, whereas the presence of NaH2PO4 depressed LVX adsorption due to its stronger affinity to Fe species. The thermodynamic parameters indicated that the adsorption process was endothermic and spontaneous. The mean free energies of adsorption calculated from a D–R model, on the other hand, suggested a chemisorption process. FT-IR spectroscopic analysis further revealed that LVX was adsorbed onto Fe-P-Mt via the coordination of ketone and carboxylate functional groups with an Fe atom to form a mononuclear bidentate complex. This study indicates that pillaring montmorillonite with iron species could significantly influence its adsorption behavior and mechanism toward fluoroquinolone antimicrobial agents.