Ozonation of diclofenac in the aqueous solution: Mechanism, kinetics and ecotoxicity assessment

The pharmaceutical and personal care products (PPCPs) in aquatic environment have aroused more interest recently. Many of them are hard to degrade by the typical biological treatments. Diclofenac (DCF), as a significant anti-inflammatory drug, is a typical PPCP and widely existed in water environment. It is reported that DCF has adverse effects on aquatic organisms. This work aims to investigate the mechanism, kinetics and ecotoxicity assessment of DCF transformation initiated by O₃ in aqueous solution, and provide a solution to the degradation of DCF. The O₃-initiated oxidative degradations of DCF were performed by quantum chemical calculations, including thirteen primary reaction pathways and subsequent reactions of the Criegee intermediates with H₂O, NO and O₃. Based on the thermodynamic data, the kinetic parameters were calculated by the transition state theory (TST). The total reaction rate constant of DCF initiated by O₃ is 2.57 × 10³ M^-1 s^-1 at 298 K and 1 atm. The results show that the reaction rate constants have a good correlation with temperature. The acute and chronic toxicities of DCF and its degradation products were evaluated at three different trophic levels by the ECOSAR program. Most products are converted into less toxic or harmless products. Oxalaldehyde (P3) and N-(2,6-dichlorophenyl)-2-oxoacetamide (P6) are still harmful to the three aquatic organisms, which should be paid more attention in the future.