Kinetics, electron-donor-acceptor interactions, and site energy distribution analyses of norfloxacin adsorption on pretreated barley straw

Occurrence of pharmaceuticals, particularly antibiotics, in water bodies and water streams has raised increasing worries about the potential risks to human health and ecosystems. Norfloxacin is a popularly used fluoroquinolone antibiotic and considered as an emerging pharmaceutical contaminant. In this paper, adsorption of norfloxacin from artificial wastewater was done by the pretreated barley straw (PBS) that was made from raw barley straw with H3PO4 impregnation and microwave heating. The experimentally maximum norfloxacin adsorption capacity of PBS was 349 +/- 17 mg/g at 298.15 K and pH 6.96. The adsorption kinetics and isotherms with respect to temperature were evaluated. The results indicated that adsorption of norfloxacin on PBS was an endothermic process. The adsorption site energy and its distribution based on the Langmuir-Freundlich model were determined to describe the adsorption mechanism. Furthermore, the percentage of adsorption sites, the site energy of which are greater than or equal to a specific energy E*, was estimated. The proposed dominant driving forces, electron-donor-acceptor interactions (n-pi and pi-pi) between the polarizable PBS (carboxyl and aromatic groups, as n- and pi-electron-donors) and norfloxacin (carbon atom in benzene ring attached to F, as a pi-electron-acceptor), were investigated by the C and O K-edge X-ray absorption near-edge structure spectroscopy.