Characterizing the Interaction between Antibiotics and Humic Acid by Fluorescence Quenching Method

The method of Three-Dimensional Excitation Emission Matrix Fluorescence Spectrophotometer was used to identify the interaction mechanism between humic acid (HA) and antibiotics. The effects of antibiotic concentration, temperature, and pH on the bonding strength between HA and antibiotics were investigated. The results showed that with the increase of antibiotics concentration, the quenching effect on HA was enhanced. The quenching of HA by both oxytetracycline (OTC) and sulfadiazine (SD) is a single static quenching of the complexes, and the interaction forces are mainly a hydrogen bond and the van der Waals force. The quenching constant K(sv)OTC > K(sv)SD and the binding constant K(b)OTC > K(b)SD, indicates that HA has a more obvious quenching effect on OTC. The K(b) of HA by OTC ranged from 3.223 × 10(3) to 12.784 × 10(3) L·mol(−1) in the range of 298 K to 313 K, while the K(b) of HA by SD ranged from 2.051 × 10(3) to 5.533 × 10(3) L·mol(−1). With the increase of temperature, the quenching constant K(sv) and binding constant K(b) of both OTC and SD by HA gradually decrease, and the low temperature is beneficial to the interaction. The composite of OTC and HA is more stable than SD. Under neutral alkaline conditions, both OTC and SD had the strongest quenching effect on HA, and the resulting complex was the most stable. However, the K(sv) and Kb of HA by OTC were greater than SD in the pH range of the experiment and the pH effect on quenching of HA by OTC (K(sv)) was greater than that of SD.