A convenient antibiotic indicator in the ozone treatment of wastewaters. An experimental and theoretical study

N-Acetyl-oxazolidin-2-one has been found to be a convenient indicator in the removal of nitroimidazole-type antibiotic contaminants in wastewaters by ozone since it is the unique product generated in the treatment of waters containing metronidazole, and it is stable against further ozonation in the experimental conditions used. The oxidative conditions imposed by the reaction do not allow isolation of any by-products, nor other stable subproducts. High-performance liquid chromatography coupled with electrospray ionization-time-of-flight mass spectrometry (HPLC-ESI-TOF(MS)) revealed the presence of simple derivatives of both metronidazole and N-acetyl-oxazolidin-2-one, thus indicating that both products are unambiguously connected and that, possibly, a unique reaction pathway would take place in this case. A feasible mechanism for this transformation has then been postulated, which is supported by several experimental findings such as TOC analysis and nitrate titration, both correlating well with the proposed transformation. Furthermore, this mechanism has been evaluated by DFT/B3LYP calculations, showing that the observed product is readily obtained thanks to the high exothermicity displayed by the metronidazole ozonation process.