Degradation of tetracycline antibiotics: Mechanisms and kinetic studies for advanced oxidation/reduction processes

This study involves elucidating the destruction mechanisms of four tetracyclines via reactions with OH and solvated electrons (eaq-). The first step is to evaluate the bimolecular rate constants for the reaction of OH and eaq-. Transient absorption spectra for the intermediates formed by the reaction of OH were also measured over the time period of 1–250 μs to assist in selecting the appropriate wavelength for the absolute bimolecular reaction rate constants. For these four compounds, tetracycline, chlortetracycline, oxytetracycline, and doxycycline, the absolute rate constants with OH were (6.3 ± 0.1) × 109, (5.2 ± 0.2) × 109, (5.6 ± 0.1) × 109, and (7.6 ± 0.1) × 109 M−1 s−1, and for eaq- were (2.2 ± 0.1) × 1010, (1.3 ± 0.2) × 1010, (2.3 ± 0.1) × 1010, and (2.5 ± 0.1) × 1010 M−1 s−1, respectively. The efficiencies for OH reaction with the four tetracyclines ranged from 32% to 60%. The efficiencies for eaq- reaction were 15–29% except for chlortetracycline which was significantly higher (97%) than the other tetracyclines in spite of the similar reaction rate constants for eaq- in all cases. To evaluate the use of advanced oxidation/reduction processes for the destruction of tetracyclines it is necessary to have reaction rates, reaction efficiencies and destruction mechanisms. This paper is the first step in eventually realizing the formulation of a detailed kinetic destruction model for these four tetracycline antibiotics.