Light-source-dependent role of nitrate and humic acid in tetracycline photolysis: Kinetics and mechanism

•Tetracycline (TC) photolysis followed pseudo-first-order kinetics.•Light-source-dependent TC photolysis was promoted or inhibited by NO3- or HA.•TC photolysis involved O2--mediated self-sensitized photolysis.•TC photolysis pathways involved hydroxylation and loss of some groups.

To elucidate the environmental fate of tetracycline (TC), we reported the light-source-dependent dual effects of humic acid (HA) and NO3- on TC photolysis. TC photolysis rate was highly pH- and concentration-dependent, and was especially enhanced at higher pH and lower initial TC concentrations. Under UV-254 and UV-365 irradiation, HA inhibited TC photolysis through competitive photoabsorption or reactive oxygen species (ROS) quenching with TC; under solar and xenon lamp irradiation, TC photolysis was enhanced at low HA concentration due to its photosensitization, whereas was suppressed at high HA concentration due to competitive photoabsorption or ROS quenching with TC. Similarly, the effect of NO3- on TC photolysis varied with light irradiation conditions. Even under the same light irradiation conditions, the effects of HA or NO3- on TC photolysis varied with their concentrations. The electron spin resonance spectrometer and ROS scavenger experiments demonstrated that TC photolysis was involved in O2--mediated self-sensitized photolysis. The photolysis pathways were involved in hydroxylation and loss of some groups. More toxic intermediates than TC were generated under different light irradiation conditions. These results can provide insight into the potential fate and transformation of TC in surficial waters.