Influence of MnO2 on the transformation of oxy- and chlortetracycline in pond water

The antibiotics oxytetracycline (OT) and chlortetracycline (CT) underwent non-photolytic abiotic transformation in a natural surface water (pH 8) in the absence and presence of suspended manganese dioxide (MnO2), a solid-phase oxidant naturally occurring in soils and sediments. In the absence of MnO2, the CT concentration declined by >60% after 20 d, while OT did not degrade appreciably. Adding MnO2 increased CT transformation and resulted in the formation of epi-iso-chlortetracycline and the appearance of a hydroxylated product with a chemical structure differing substantially from the parent compound. Approximately 40% of the CT was converted to iso-chlortetracycline and epi-iso-chlortetracycline regardless of the presence or absence of MnO2. In the presence of MnO2, 91% of the OT and 79% of the CT were transformed after 20 d. The initial rate of CT degradation exceeded that of OT. The dominant OT transformation product, N-desmethyl-oxytetracycline, accounted for up to 10% of the OT loss. The major transformation products are expected to possess reduced biological activity.