An understanding of the interaction mechanisms of antibiotics with environmentally relevant sorbents is important to determine the environmental fate of antibiotics and to develop wastewater treatment strategies. Magnetite (Fe3O4(s)) is ubiquitous in the environment and occurs as a secondary corrosion product of iron nanoparticles that are commonly used as a remediation material. In this study, we aimed to assess the sorption mechanisms of ciprofloxacin (CIP), an important class of fluoroquinolone antibiotics, with magnetite nanoparticles using a combination of wet chemical and in situ ATR-FTIR spectroscopic measurements. Ciprofloxacin sorption was characterized as a function of pH (3.4-8.0), CIP concentration (1-500 mu M), ionic strength (0.5, 0.1, and 0.01 M NaCl), and competing anion such as phosphate (0.1 mM) to cover a broad range of environmentally relevant geochemical conditions. Results indicated a bell-shaped sorption envelop where sorption of CIP on nano-Fe3O4(s) increased from 45% to 80% at pH 3.44-5.97; beyond that sorption gradually decreased to a value of 25% at pH 8.39. Phosphate had negligible effect on CIP sorption. In situ ATR-FTIR results indicated inner-sphere coordination of CIP at the magnetite surface mediated by carboxylic acid groups. Results suggest that nano-Fe3O4(s) has the potential to remove CIP from wastewater effectively. (C) 2012 Elsevier B.V. All rights reserved.