Removal of fluoroquinolone from aqueous solution using graphene oxide: experimental and computational elucidation

The environmental risks of antibiotics have attracted increasing research attention due to their prevalence and persistence in the aquatic environment. In this study, oxygen functionalized graphene, namely, graphene oxide (GO), was synthesized by modified Hummer's and Offeman's method and used as potential effective absorbent for the removal of fluoroquinolones (FQs), i.e., ciprofloxacin (CIP), norfloxacin (NOR), and ofloxacin (OFL), from aqueous solution. The as-synthesized GO was characterized by Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), and high-resolution transmission electron microscopy (HRTEM). Out of various factors that were taken to consideration while studying the adsorption process, it was found that pH of antibiotic solution is more crucial than the other experimental parameters such as initial antibiotic concentration, contact time, and adsorbent dosage and has significant impact on FQ adsorption via the GO adsorbent. The maximum removal of FQ was observed at pH 7 for CIP and NOR, while adsorption was maximum at pH 4 for OFL. Experimental data best fitted to the pseudo-second-order model as compared to the pseudo-first-order kinetic adsorption model. Best fitting of the equilibrium experimental data to Langmuir isotherm compared to Freundlich isotherm models established that FQ adsorbs over the GO in monolayer manner. Density functional theory (DFT) calculations performed at B3LYP/6-31G(d) level of theory in order to elucidate the thermodynamic feasibility of adsorption process and nature of interactions of antibiotic molecules with the GO adsorbent.