Sorption of norfloxacin onto humic acid extracted from weathered coal

Norfloxacin (NOR), is an ionizable and polar antimicrobial compound, and it may enter the environment in substantial amounts via the application of manure or sewage as a fertilizer. Sorption of NOR onto humic acid (HA) may affect its environmental fate. In this study, HA extracted from weathered coal was used to investigate the sorption of NOR at different solution chemistry conditions (pH, ionic strength) and temperatures. The sorption of NOR onto HA showed a two-stage sorption process with an equilibration time of 48 h. The sorption kinetic curve fitted well with a pseudo second-order kinetic model. Thermodynamic characteristics demonstrated that the sorption of NOR onto HA was a spontaneous and exothermic process predominated by physical sorption. All sorption isotherms fitted well with the Freundlich and Langmuir models and they were highly nonlinear with values of n between 0.4 and 0.5, suggesting the high heterogeneity of HA. Increasing Ca2+ concentration resulted in a considerable reduction in the Kd values of NOR, hinting that Ca2+ had probably competed with NOR+,0 for the cation exchange sites on the surfaces of HA. The sorption reached a maximum at pH 6.0 over the pH range of 2.0–8.0, implying that the primary sorption mechanism was cation exchange interaction between NOR+,0 species and the negatively charged functional groups of HA. Spectroscopic evidence demonstrated that the piperazinyl moiety of NOR was responsible for sorption onto HA, while the carbonyl group and the aromatic structure of HA participated in adsorbing NOR.

► The sorption kinetic curve fitted well with a pseudo second-order kinetic model.
► All sorption isotherms were fitted well by Freundlich and Langmuir models.
► The sorption of NOR onto HA was a spontaneous and exothermic process.
► The sorption reached a maximum at pH 6.0 over the pH range of 2.0–8.0.
► The primary sorption mechanism was cation exchange interaction between NOR and HA.