Adsorption of Norfloxacin on Urban River Sediments: Site Energy Distribution and Effects of Coexisting Ions

The environmental fate of fluoroquinolone antibiotics in urban rivers is controlled by sediment characteristics and water chemistry, yet how these factors modulate adsorption site energetics remains insufficiently quantified. In this study, norfloxacin (NOR) adsorption on urban river sediments from Wuhan, China, was investigated using adsorption isotherms and site energy distribution analysis. All sediments followed the Langmuir model (R2 = 0.981–0.998) with saturated adsorption capacities of 17.932–35.476 mg/g, indicating pronounced sediment-dependent retentions. Site energy distribution analysis revealed clear differences in both characteristic adsorption energy E0* and the peak of frequency distribution curves F(E0*) between the sediments. Higher adsorption capacities were found not necessarily linked to higher adsorption energies, but rather to higher frequencies of effective adsorption sites. Coexisting ions exerted ion-specific effects on NOR adsorption. Common electrolytes suppressed adsorption and shifted site energy distributions toward lower-energy regions, with Na2SO4 exerting a stronger effect than NaCl due to its higher ionic strength. Ca2+ had little effect, whereas Mg2+ slightly enhanced adsorption. Fe3+ caused a slight enhancement, while Cu2+ markedly increased NOR adsorption and shifted the site energy distributions toward higher-energy regions. These findings demonstrate that site energy distribution analysis provides a useful framework for interpreting ion-modulated antibiotic adsorption on urban river sediments.