Effect of phosphate on the adsorption of antibiotics onto iron oxide minerals: Comparison between tetracycline and ciprofloxacin

With the broadly application of antibiotics to treat infectious diseases in humans and animals, antibiotic contaminants such as tetracycline (TC) and ciprofloxacin (CIP) have been detected in soil environments, where iron oxide minerals and phosphate are ubiquitous. To date, the influence of phosphate on the adsorption behaviors of TC/CIP onto iron oxides is still poorly understood. In this study, the effects of phosphate on the adsorptions of TC and CIP onto iron oxide minerals were investigated. Adsorption isotherms showed that the adsorption affinities of TC and CIP onto the three iron oxide minerals were in the order of goethite > hematite > magnetite with or without phosphate, the trend was dominated by different surface area and amount of surface hydroxyl groups of iron oxide minerals. Meanwhile, TC contains more functional groups than CIP for bonding, which resulted in greater adsorption affinity of three iron oxides to TC than that to CIP. Interestingly, phosphate weakened TC adsorption, while enhanced CIP adsorption, on the three iron oxides. This observation was ascribed to that phosphate anion enhanced the surface negative charge of iron oxides, which reinforced the electrostatic repulsion between iron oxides and negatively charged TC, also reinforced the electrostatic attraction between iron oxides and positively charged CIP. Furthermore, the inhibitory effect of phosphate on TC adsorption was dramatically enhanced at high pH, while the promoting effect of phosphate on CIP adsorption was slightly changed with various pH. Our results highlight the importance of phosphate in exploring the environmental fate of antibiotics in natural environment.