Effect of humic substances derived from pastoral areas in Zoige Plateau on photodegradation of sulfamethoxazole and ciprofloxacin

Sulfonamides and quinolones are widely used antibiotics. Some end up discharged into aqueous environments where they can seriously affect human health and safety. The photochemical degradation of sulfamethoxazole and ciprofloxacin with the various concentrations was analyzed in different pH natural waters. Meanwhile, the effect of natural humic and fulvic acids on the photodegradation was tracked. The photolysis curves were perfectly consistent with the first-order kinetics. High concentration of antibiotics was not conducive to photodegradation. Photodegradation of ciprofloxacin was generally occur in neutral and alkaline environments, while photodegradation of sulfamethoxazole was generally in lower acidic environments. Persistent free radicals including reactive oxygen species were found in the photolytic reactions. They were found to arise both from auto-decay of the antibiotics and from decay of the humic substances. According to EPR test,·OH and 1O2 both have a good promotion effect on the photodegradation of antibiotics, and the contribution rate of 1O2 (50.63%) is greater than that of •OH (25.56%), indicating that 1O2 is the main active species in the photodegradation of sulfamethoxazole and ciprofloxacin, and the contribution rate of 1O2 produced by humic acids to the photodegradation of antibiotics is greater than that of fulvic acids. Humic substances produced a strong single signal with good symmetry in darkness and light, indicating that the free radicals produced by humic substances were EPFRs with stability and persistence. However, the amount of EPFRs produced by humic acids were more than that of fulvic acids, indicating that the free radicals produced by humic acids were the carbon center free radicals containing oxygen functional groups. Results of this study will provide further comprehensive fundamental data for risk assessment and control of antibiotics in farmland ecosystems in pastoral areas of Zoige Plateau.