Kinetics and mechanistic study on degradation of prednisone acetate by ozone

Prednisone acetate (PNSA) is one of the regular glucocorticoid medicines that have been detected in surface water. In this work, the removal of PNSA by ozone was systematically studied under various conditions, and degradation intermediates and reaction pathways were proposed. The results showed that aqueous ozonation was able to remove PNSA effectively, and low pH favored this reaction. The addition of tertiary butanol did not inhibit the oxidation of PNSA by ozone, suggesting that the degradation was caused mainly by the direct oxidation effect of ozone molecules. Moreover, the presence of carboxylated or hydroxylated multiwalled carbon nanotubes can enhance the removal efficiency of PNSA by ozone. Under neutral and acidic conditions, the degradation of PNSA followed pseudo-first-order reaction. Seven intermediates were detected via liquid chromatography-mass spectrometry, and the degradation pathways were then proposed by considering the relative charge density of the frontier orbitals calculated with the Gaussian program. The electrophilic reaction and the Criegee mechanism were the primary reaction mechanisms in the degradation of PNSA by ozone. Formic acid, acetic acid, and oxalic acid were detected as the final reaction products via ion chromatography. Additionally, the aquatic toxicity of the ozonation products was predicted using ECOSAR method. The biodegradation potentials of the pollutant and the ozonation products were estimated using BIOWIN^TM, suggesting that O₃ treatment could significantly enhance the biodegradable potentials of PNSA and its transformation intermediates in the biological post-treatment process. This work can provide useful information for the treatment of PNSA-containing wastewaters.