| Tetracycline antibiotics (TCs) are widely used for treating human diseases and preventing diseases and promoting growth in livestock animals and aquaculture. Due to the large quantity of usage and sales and the limited removal efficiency by Municipal Wastewater Treatment Plant, highly residues of TCs and their metabolites were detected in water, soil and sediment environment, leading to potentially negative impact on aquatic and terrestrial organisms. Thus, it is important to better understand the fate and transformation of TCs in the environment and to explore efficient treatment methods for TCs removal.TCs have a strong tendency to complex with metals, which has great impact on the transport and transformation of TCs in the soil and aqueous environment; on the other hand, Ultraviolet (UV) based processes have been considered as efficient and practical methods of treating PPCPs in water over the past decade. Thus, the objective of this study was to systematically investigate the effect Fe(Ⅱ/Ⅲ) on the degradation and photodegradation of three TCs (Tetracycline, Oxytetracycline and Chlortetracycline).The study of complexation mechanism between Fe(Ⅱ/Ⅲ) and TCs showed that Fe(Ⅱ/Ⅲ) likely complexes with TCs at their A ring. Fe(III)-TC complexation likely reached a saturation [Fe(Ⅲ)]:[TC] ratio of 1:1,1:1 and 2:1 for TTC, OTC and CTC, respectively; and Fe(Ⅱ)-TTC complexation reached a saturation ratio at 1:1, the Job’s methods also verified the 1:1 Fe(Ⅱ)-TTC was the major complex.The presence of Fe(Ⅲ) promoted the degradation of TCs in most experimental pH (5.0,7.0 and 9.0) except at pH 9.0 for CTC, and also promoted degradation of TCs in real surface water and wastewater matrices. Degradation rate constants of TTC, OTC and CTC reached maximum when Fe(Ⅲ):TC molar ratio reached the saturated ratio. The Fe(Ⅲ)-promoted degradation of TCs starts from strong complexation between Fe(Ⅲ) ions and TCs, likely via the A ring’s C4 dimethylamino group, and is followed by oxidation of TCs by Fe(Ⅲ) to generate TC oxidation products and Fe(Ⅱ)The promoted degradation likely occurred via complexation of TCs and subsequent oxidation by Fe(Ⅲ). The degradation of TCs in the presence of Fe(Ⅲ) involved only breakage of certain chemical bonds and structural alternations. Toxicity of the complexes evaluated was increased after several hours of reaction, suggesting the transformation products may exert a stronger toxicity than parent TCs.Experiments with Fe(Ⅱ) and TCs showed that complexation of Fe(Ⅱ) with TTC, OTC or CTC could lead to accelerated oxidation of Fe(Ⅱ), and Fe(Ⅱ) oxidation rate increased at higher TCs concentration and higher pH. The complexation equilibrium constant of FeⅡ-H2L(TC0) and FeⅡ-HL(TC) complexes were 2.39×103 and 2.05×104, and the oxidation rate constants of the two Fe(II) species were 0.269 min-1 and 1.511 min-1, respectively. TCs degradation were also promoted by the ROS (primarily·OH) formed during Fe(Ⅱ) oxidation. These findings indicate the mutually influenced environmental transformation of TCs and Fe(Ⅱ)/Fe(Ⅲ) induced by their complexation. These newly identified reactions established a new cycling of Fe(Ⅱ)/Fe(Ⅲ) in TCs-contaminated water and soil systems.Fe(Ⅲ) could promote TTC degradation under UV 254 nm by complexation and subsequent oxidation. In contrast, Fe(Ⅱ) facilitated TTC degradation under VUV 185 nm due to the additional · OH radicals formed by potential participation of the fenton process. The Electrical Energy per Order (EE/O) was also evaluated and suggest that VUV was a more efficient way of degrading and mineralizing TTC, and the combination with Fe(II) could further promote the efficiency.The photodegradation of TCs were enhanced by increasing ZnO dosage and solution pH under UV/ZnO process. TCs degradation mechanism followed the order of ZnO adsorption, bond breaking of TCs BCD ring, bond breaking of TCs A ring and mineralization. TCs degradation under VUV (+Fe(Ⅱ)) process was more efficient than that under UV/ZnO.Overall, the complexation between Fe(Ⅱ/Ⅲ) and TCs could not only affect TCs transformation and degradation, but also influenced Fe(Ⅱ/Ⅲ) redox cycle with or without UV irradiation. This study may shed new insights of TCs transformation in the environment and provide new efficient treatment methods for TCs removal, as well as improved the knowledge basis to better predict the environmental fate of TC contaminants. |
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