Ozonation of chlortetracycline in the aqueous phase: Degradation intermediates and pathway confirmed by NMR

Chlortetracycline (CTC) degradation mechanism in aqueous phase ozonation was evaluated for degradation mechanism and its correlation with the biodegradability and mineralization. CTC was removed within 8 and 4 min of ozonation at pH 2.2 and 7.0, respectively. At pH 2.2, HPLC–triple quadrupole mass spectrometry (MS) detected 30 products. The structures for some of these products were proposed on the basis of ozonation chemistry, CTC structure and MS data; these structures were then confirmed by nuclear magnetic resonance (NMR) spectra. Double bond cleavages, dimethyl amino group oxidation, opening and removal of the aromatic ring and dechlorination, mostly direct ozonation reactions, gave products with molecular weights (m.w.) 494, 510, 524, 495 and 413, respectively. Subsequent degradations gave products with m.w. 449, 465, 463 and 415. These products were arranged into a degradation pathway. At pH 7.0, the rate of reaction was increased, though the detected products were similar. Direct ozonation at pH 2.2 increased the biodegradability by altering the structures of CTC and its products. Nevertheless, direct ozonation alone remained insufficient for the mineralization, which was efficient at pH 7.0 due to the production of free radicals. [Display omitted] •Ozonation pathway of aqueous CTC solutions tested at pH 2.2 and 7.0.•30 products detected and major ones arranged in production and degradation pattern.•Confirmed structures of some products by NMR spectra.•Math fitting by Matlab predicted the rate constants for major ozonation reactions.•Ozonation at pH 2.2 changed the CTC structure while at pH 7.0 mineralized it.