Long-term transformation of oxolinic acid in water from an eel pond

Oxolinic acid (OA) is one of the widely used antibiotics in aquaculture. Its residue in the aquatic environment is often detected globally and has raised environmental and public health concerns. To understand its transformation in water, samples obtained from an eel pond was added at three OA concentrations (5, 10, 20 mg l− 1) and then incubated under a combination of two illumination regimes (light-5800 lux, dark) and two oxygen profiles (aerobic, anaerobic) for 691 d. OA was highly resistant to transformation and its residue at the end of the experiment still ranged from 1.1 to 6.6 mg l− 1. Illumination regime played the key role among the three factors used in the study. OA decayed faster under light than in the dark, with a mean half-life of 298 d and 509 d, respectively. High added OA concentration resulted in faster transformation. No difference in OA transformation was found between aerobic and anaerobic incubation. Further research on transformation of OA under light of higher intensity and/or ultraviolet wavelength is recommended to resolve the slow OA transformation problem.