Degradation of clofibric acid by UV, O₃ and UV/O₃ processes: Performance comparison and degradation pathways

In this study, ultraviolet (UV) irradiation, ozonation (O₃) and their combination (UV/O₃) were used to decompose clofibric acid (CA). The results show that UV system exhibited a very high CA removal rate (0.20 min^-1) but the lowest mineralization (14.8%) accompanied by the formation of more toxic products. Ozonation achieved a much lower removal rate (0.05 min^-1) but a higher mineralization efficiency (22.7%) in comparison with UV photolysis. The introduction of UV irradiation into O₃ system significantly enhanced the removal rate (0.21 min^-1) and the mineralization efficiency (68.2%) of CA. The acute toxicity of the reaction solution to Daphnia magna in the UV/O₃ process increased during the first 20 min and then decreased, which illustrates that UV/O₃ is an effective and safe method for the removal of CA. The intermediate products were identified by LC-MS analysis and the degradation pathways for all the three processes were proposed. The direct photolysis and hydrous electron reduction contributed to the CA elimination in UV alone process. In O₃ alone system, the removal of CA occurred via direct ozone oxidation and indirect free radical oxidation. The free radical, ozone, hydrous electron and direct photolysis were involved in the degradation of CA in the UV/O₃ process.