Oxidative degradation kinetics and products of chlortetracycline by manganese dioxide

Abstract

This study investigated the abiotic transformation kinetics of chlortetracycline (CTC) by synthesized δ-MnO 2 under conditions of different solutions. CTC was rapidly oxidized by δ-MnO 2, with the generation of Mn 2+. The measured CTC transformation rate increased considerably with an increase in initial δ-MnO 2 concentration but it decreased as the initial CTC concentration increased. Both the measured CTC transformation rate and the amount of Mn 2+ generated decreased with increasing pH. The CTC transformation rate rose with an increase in temperature. The apparent activation energy (45 ± 14 kJ mol −1) was consistent with a surface-controlled reaction. Dissolved Mn 2+ and Zn 2+, as background cations, and substituted phenols, as co-solutes, remarkably decreased the transformation rate of CTC. Liquid chromatography–tandem mass spectrometry (LC–MS-MS) was used to identify oxidation products, which include iso-CTC, 4-epi-CTC, anhydro-CTC and 4-epi-anhydro-CTC, keto-CTC, 4-epi-keto-CTC, N-demethyl-CTC, 4-epi- N-demethyl-CTC, N-didemethyl-CTC and 4-epi- N-didemethyl-CTC. Product identification together with Fourier transform-infrared (FTIR) spectra suggested that the hydroxyl groups at C6 and C12 and the dimethylamine group of CTC reacted with the Mn–OH groups on the δ-MnO 2 surface. Thus, δ-MnO 2 in the soils most probably plays an important role in the abiotic transformation of tetracycline antibiotics.