The Influence of Humification Degree of Humic Acid on Its Sorption of Norfloxacin During Sewage Sludge Composting

Norfloxacin (NOR), an antibiotic widely used in livestock and poultry production, has become ubiquitous in the aquatic and terrestrial ecosystems as a result of veterinary excretion of the parent compound or its active metabolites. The sorption of NOR onto humic acid (HA) may influence the fate of NOR in the environment. In the present study, HA was extracted from sewage sludge in different composting stages of days 0, 10, 30, and 70 to investigate the sorption of NOR onto HA as affected by the humification degree of HA. The results of elemental and Fourier transform infrared (FTIR) spectrum analyses showed that the contents of aromatic and carboxylic groups in HA increased with composting time, indicating an increase of humification degree. The result of sorption experiments demonstrated that the HA had a high sorption capacity for NOR with strong nonlinearity. A two-stage sorption was observed in the sorption process with an equilibration time of 48 h. Both the Freundlich model (Adj. R² range 0.988–0.994) and Langmuir model (Adj. R² range 0.917–0.928) fitted well with all sorption isotherms of the HA samples of different humification degrees. Moreover, the increase of sorption distribution coefficient (K_d) value with composting time indicated that the sorption affinity of HA for NOR increased with increasing humification degree of HA. The major sorption mechanism was the interaction between NOR and rich aromatic moieties and carboxylic group in the HA.