Multiple roles of humic acid in the photolysis of sulfamethoxazole: kinetics and mechanism

Photolysis is considered to be an important elimination process for antibiotics in nature. Humic acids (HA) are ubiquitous in aquatic environments and inevitably encounter antibiotics, which may interfere with their photolysis and thus alter their environmental fate. In this study, we chose sulfamethoxazole (SMX) as a typical antibiotic to assess the effect of HA on photolysis. HA at 1 mg L-1 greatly promoted the photolysis of SMX by 98.8%. However, high-concentration HA inhibited the photolysis, and the photolysis rate decreased by 92.9% in the presence of 100 mg L-1 HA. Furthermore, HA exhibited different effects on direct and indirect photolysis. Low-concentration HA induced the production of OH to promote indirect photolysis, whereas high-concentration HA could compete with SMX for photons and decrease light penetration via light shielding to inhibit direct photolysis greatly. Based on density functional theory (DFT) calculation, the Na site in HA could bind with the N-O and -SO2-NH- on the isoxazole ring of SMX to form complexes. The active sites of SMX and SMX-HA complexes were also identified, and the active sites increased after complexation. Moreover, low-concentration HA exhibited little influence on the degradation pathway of SMX due to little complexation degree while SMX was degraded via different paths in the presence of high-concentration HA, which was mainly attributed to the complexation of SMX and HA. Besides, the toxicity of most intermediates were lower than that of SMX, but some intermediates in new degradation pathways tended to accumulate during photolysis, resulting in higher potential environmental risk. Humic acid exhibited multiple roles in the photolysis of sulfamethoxazole via distributing ROS generation and complexing with sulfamethoxazole, leading to different degradation paths and intermediates.