Oxidation of tetracycline antibiotics induced by Fe(III) ions without light irradiation

The presence of Fe(III) ions was found to induce degradation of three tetracycline antibiotics (TCs), tetracycline (TTC), oxytetracycline (OTC) and chlorotetracycline (CTC), in aqueous solutions without light. 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. Degradation rate constants of TTC, OTC and CTC reached maximum ((6.2±0.5)×10(-3) h(-1), (10.6±0.1)×10(-3) h(-1) and (15.9±0.5)×10(-3) h(-1) at pH 7.0, 20 °C) when Fe(III):TC molar ratio was 1:1, 1:1 and 2:1, respectively. Such metal-to-ligand ratios agreed well with the most favorable complexation between Fe(III) and each TC. Compared to without metals, Fe(III) enhanced the degradation rate of TTC, OTC and CTC by up to 20.67, 7.07 and 2.30 times, respectively, in clean water matrix, and also promoted degradation of TCs in real surface water and wastewater matrices. The promoted degradation likely occurred via complexation of TCs and subsequent oxidation by Fe(III). Degradation results of CTC versus 4-epi-CTC suggested Fe(III) likely binds to TCs' C4 dimethylamino group. Toxicity of the complexes evaluated using Photobacterium phosphoreum T3 was increased after several hours of reaction, suggesting the transformation products may exert a stronger toxicity than parent TCs. This study identifies new oxidative transformation of TCs induced by Fe(III) ions without light irradiation, further supporting the important role of iron species in the environmental fate of TCs.