Mechanisms of Cu(II) enhancing photolysis of tetracycline under UV irradiation

Tetracycline (TC) and Cu(II) have been frequently detected in natural and engineering waters, especially in aquaculture water. However, literatures involving in the influence of Cu(II) on the transformation of TC are still insufficient. In this study, we investigated the photolytic fate of TC induced by Cu(II) under UV irradiation. Cu(II) significantly promoted the removal of TC with photolysis rate constant of 0.467 h −1 at TC:Cu(II) molar ratio = 1, which was 49.04% higher than that without Cu(II) (0.238 h −1 ). Cu(II) exhibited multiple influences on both direct and indirect photolysis of TC. Singlet oxygen ( 1 O 2 ) and hydroxyl radicals (·OH) made a major contribution to Cu(II)-induced TC photolysis, with contribution rates of 24.69% and 12.96%, respectively. Besides, Cu(II) preferred to complex between O20-O21 or O26-O27 to form TC-Cu(II) complexes with complexation ratio of 1. The Condensed Fukui Function and Condensed Double Descriptors values of TC-Cu(II) complexes were calculated for the first time with density functional theory. It showed that TC-Cu(II) complexes were more vulnerable to ROS attack, thus promoting TC degradation. Moreover, Cu(II) greatly changed the photolysis path of TC, mainly through hydroxylation, hydrogen extraction, and deamidation reaction into new byproducts. Furthermore, Cu(II) changed the toxicity of the products via changing the original TC photolysis products. This study provides new insights for the environmental fate of antibiotics, and also has far-reaching implications for wastewater treatment in pharmaceutical and aquaculture industry.