Adsorption of fluoroquinolone antibiotics onto ferrihydrite under different anionic surfactants and solution pH

To date, little information is available regarding the impacts of the widespread anionic surfactants on the adsorption behaviors of antibiotics onto typical iron oxides. Herein, we have investigated the effects of two typical surfactants (sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS)) on the adsorption of two widely used antibiotics (i.e., levofloxacin (LEV) and ciprofloxacin (CIP)) onto ferrihydrite. Results of kinetic experiments showed that the adsorption of antibiotics was well fitted by the pseudo-second-order kinetic models, indicating that the adsorption process might be controlled by chemisorption. The affinity of ferrihydrite toward CIP was greater than that toward LEV, which was ascribed to the higher hydrophobicity of CIP than LEV. Both surfactants enhanced antibiotic adsorption owing to SDS or SDBS molecules as bridge agents between ferrihydrite particles and antibiotics. Interestingly, the extent of the enhanced effects of surfactants on antibiotic adsorption declined as the background solution pH increased from 5.0 to 9.0, which was mainly due to the weaker hydrophobic interactions between antibiotics and the adsorbed surfactants on the iron oxide surfaces as well as the greater electrostatic repulsion between the anionic species of antibiotics and the negatively charged ferrihydrite particles at higher pH. Together, these findings emphasize the importance of widespread surfactants for illustrating the interactions between fluoroquinolone antibiotics and iron oxide minerals in the natural environment.