Role of dissolved organic matter from natural biofilms in oxytetracycline photodegradation

Dissolved organic matter (DOM) is one of the most important factors that influence the photodegradation of antibiotics in the aquatic environment. However, the mechanisms by which DOM produced by natural biofilms (BDOM) influences photodegradation are poorly understood. Here, the influences of BDOM and fulvic acid (FA) as model DOM on the photodegradation of the antibiotic oxytetracycline (OTC) were investigated, and the differences between the characteristics of BDOM and FA were determined by ultraviolet, infrared, and three-dimensional fluorescence spectrum analyses. In addition, an experiment on the scavengers of reactive oxygen species (ROS) was also conducted to explore related mechanisms. The results indicated that the aromaticity degree and molecular weight of BDOM were lower than those of FA. The OTC photodegradation rates increased from 9.7 × 10−2 to 19.9 × 10−2 min−1 with increasing BDOM concentration, while they decreased from 8.9 × 10−2 to 5.6 × 10−2 min−1 with increasing FA concentration. Excited triplet-state BDOM (3BDOM*) and singlet oxygen (1O2) simultaneously promoted the photodegradation of H2OTC and HOTC−. However, FA inhibited OTC photodegradation through strong light-shielding effects. These results are beneficial for understanding the effects of natural biofilms on the antibiotic photodegradation in the aquatic environment.