Sorption of the fluoroquinolone antibiotic ofloxacin by aquatic sediments: influence of biofilm development at the sediment-water interface

Abstract

Sorption to sediments and biofilms is thought to be a crucial mechanism controlling the fate and transport of emerging contaminants. Biofilm growth changes the properties of the sediments, which may further influence the sorption mechanism of emerging contaminants to sediments. This study is focused on the effects of biofilms on the linear and nonlinear sorption of fluoroquinolone antibiotic ofloxacin (OFL) by sediments. The top 5 cm of sediments and the overlying water containing natural microorganisms were collected from a shallow lake in summer. They were cultivated in the laboratory for the development of biofilms on the surface of sediments. Batch sorption experiments of OFL by original sediments and biofilm-coated sediments were conducted, and infrared spectrometry was used to obtain the main functional groups involved in sorption. Extracellular polymeric substances (EPS) were extracted from biofilms to investigate their interaction with OFL through three-dimensional excitation-emission matrix fluorescence spectroscopy and UV-Vis spectroscopies. The results showed that linear partition and nonlinear adsorption were simultaneously involved in the sorption process. The linear partition coefficients of OFL in sediments decreased by 50% and 60%, whereas the nonlinear adsorption capacities increased by 1.7 and 2.0 times after the biofilms colonized the sediment surface for 30 and 45 days, respectively. The decreased linear partition coefficients of OFL were related to the barrier created by biofilm coatings affecting hydrophobic interaction, whereas the increased nonlinear adsorption capacities were due to the increase in cation exchange capacities and the formation of hydrogen bonds between fluorine atoms in OFL and –OH groups in sediment. In addition, OFL and protein-like substances contained in EPS from biofilms could form complexes that affect the sorption processes. The present study reveals that biofilms can inhibit hydrophobic interaction but facilitate cation exchange and hydrogen binding between OFL and sediments. Our work yields new insights into the interaction of aquatic solid sorbents, which is significant to understanding the transport and fate of organic contaminants in natural waters.