Photolysis mechanisms of tetracycline under UV irradiation in simulated aquatic environment surrounding limestone.

As the most typical geological environment, limestone landforms are widespreading in the world and affect the waters that flow around them, which may also change the fate of organic contaminants in these waters. In this study, aquatic environment surrounding limestone was simulated with calcium carbonate, and the photolysis of tetracycline was evaluated under UV irradiation (30 μW/cm). More tetracycline (up to 98%) was removed in 4 h in the presence of calcium carbonate while only 50% of tetracycline was eliminated in control experiment. The removal of tetracycline was greatly enhanced due to the major roles of alkaline pH and minor roles of Ca and HCO/CO. In alkaline pH, tetracycline existed as TCs with higher electronic density in the ring structures, which was more easily attacked by OH. Besides, it could also change the bond orbital energy to facilitate tetracycline absorbing more photon. Moreover, alkaline pH was beneficial to generate more OH and thus promote the indirect photolysis. In addition, alkaline pH also changed the degradation path of tetracycline and rapidly convert tetracycline to the byproducts with m/z 457 via hydroxylation and hydrogen abstraction. This work provides not only better understanding about the fate of tetracycline in aquatic environments but also new insights into the treatment of antibiotic-contaminated water.