Modeling of levofloxacin adsorption to goethite and the competition with phosphate.

Interaction between various compounds in natural systems may influence the adsorption of these species and their environmental fate. In this work, we studied the interactions between a widely used antibiotic levofloxacin (LEV) and phosphate at the surface of goethite (α-FeOOH), which was important to better understand the competitive adsorption of antibiotics and oxyanions in natural systems. The presence of phosphate decreased LEV adsorption to goethite significantly over the whole pH range. The other way around, LEV had a little influence on phosphate adsorption. Eight types of LEV-goethite complexes were proposed and modeled in our study. Electrostatic competition was the main reason for the competition of binary components (LEV and phosphate) to goethite surface. Adsorption of single component (LEV or phosphate) to goethite was well predicted using the CD-MUSIC (Charge Distribution Multi-Site Complexation) model. In competition experiments, phosphate adsorption was still predicted well, but LEV adsorption was overestimated in model calculations. This is because less negative charge of LEV is located at outer electrostatic plane in our study, which decreases their electrostatic competition to goethite surface.