Chromate(VI)-induced homogeneous oxidation and photolysis of aqueous tetracycline: Kinetics and mechanism

Tetracycline (TC) and chromate (Cr(VI)) are common organic or inorganic pollutants in the environment, and the interactions between them may mutually alter environmental behaviors and risks. However, the mutual effect between TC and Cr(VI) in aqueous solutions remains unclear. In this study, kinetic experiments, complexation and quenching experiments, electron spin resonance (ESR) analysis, density functional theory (DFT) calculation and identification of products, were performed to investigate the effects and underlying mechanism of Cr(VI) on oxidation and photolysis of TC. The results revealed that Cr(VI) could induce oxidation and photolysis of TC with or without light irradiation and the curve was well fitted by a pseudo first-order kinetic model. Moreover, the change of reaction conditions (such as initial concentration and pH) had great effect on oxidation or photolysis of TC. When Cr(VI) oxidized TC to form Cr(III) and TC radical, Cr(III) complexed with TC to accelerate the oxidation. In addition, oxidation still occurred under simulated irradiation. O-1(2) which promoted photolysis of TC increased upon irradiation at higher pH or with the increase of Cr(VI). The participation of Cr(VI) affected self-sensitization and product formation of TC. Therefore, an electron-transfer interaction for photolysis among O-1(2) arising from self-sensitization, Cr(VI)/Cr(III) and TC was first proposed. The transformation pathway of TC mainly included five reactions: hydrolysis, hydroxylation, deamination, dehydration and demethylation only for photolysis. This study identified new oxidation and photolysis of TC induced by Cr(VI), which will be helpful to further understand the fate of heavy metals and antibiotics in the natural aquatic environment.