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)-O-center dot and solvated electrons (e(aq)(-)). The first step is to evaluate the bimolecular rate constants for the reaction of (OH)-O-center dot and e(aq)(-). Transient absorption spectra for the intermediates formed by the reaction of (OH)-O-center dot were also measured over the time period of 1-250 mu s 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)-O-center dot were (6.3 +/- 0.1) x 10(9), (5.2 +/- 0.2) x 10(9), (5.6 +/- 0.1) x 10(9), and (7.6 +/- 0.1) x 10(9) M-1 s(-1), and for e(aq)(-) were (2.2 +/- 0.1) x 10(10) (1.3 +/- 0.2) x 10(10), (2.3 +/- 0.1) x 10(10), and (2.5 +/- 0.1) x 10(10) M-1 s(-1), respectively. The efficiencies for (OH)-O-center dot reaction with the four tetracyclines ranged from 32% to 60%. The efficiencies for e(aq)(-) 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 e(aq)- 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. (C) 2009 Elsevier Ltd. All rights reserved.