Effects of bile salts and divalent cations on the adsorption of norfloxacin by agricultural soils

The effects of bile salts (sodium cholate and sodium deoxycholate, 0–20 mmol/L), divalent cations (Ca²⁺, Mg²⁺, Cu²⁺ and Zn²⁺, 0–20 mmol/L) or pH (3.0–10.0) on the adsorption of norfloxacin by three selected soils (Paddy_H, Paddy_G and Red_J) were systematically studied. Soil adsorption of norfloxacin follows a pseudo second-order kinetics model, and the maximum adsorption capacity has been determined from the nonlinear fit of the Langmuir isotherm model to be 88.8, 88.1 and 63.0 μmol/g for the adsorption onto Paddy_H, Paddy_G and Red_J, respectively. The results indicate that norfloxacin has a high adsorption affinity for the agricultural soils tested and that the organic content of these soils have at least a slight influence on this adsorption. The adsorption of norfloxacin to soils was strongly dependent on pH and exhibited a maximum at approximately pH 6. The presence of divalent cations prominently suppressed the adsorption of norfloxacin by paddy soils, which followed an order of Cu²⁺ > Mg²⁺ > Ca²⁺ > Zn²⁺, and by red soil, which followed an order of Cu²⁺ > Zn²⁺ > Ca²⁺ > Mg²⁺. The adsorption of norfloxacin (by the soils studied) sharply decreased as the amount of bile salts was increased. For uncharged norfloxacin at environmentally relevant pH values, such factors as soil type, exogenous divalent cations and macromolecules significantly altered the environmental fate and transport of norfloxacin between aquatic and soil interfaces.