Mechanistic insight into the degradation of ibuprofen in UV/H₂O₂ process via a combined experimental and DFT study

The study investigated the degradation kinetic and transformation mechanism of ibuprofen (IBP) in UV/H₂O₂ process from both experimental and theoretical aspects. Impacts of H₂O₂ dosage, solution pH, quenching agent, and concentration of nitrite (NO₂^-) on IBP degradation in UV/H₂O₂ process were evaluated. Both experimental results and theoretical calculations indicated that ^•OH played an important role in the degradation of IBP and its transformation products. The second-order rate constants of ^•OH and ^•NO₂ with IBP were calculated as 3.93 × 10⁹ M^-1 s^-1 and 5.59 × 10^-3 M^-1 s^-1, based on the transition state theory, which explained the phenomenon that addition of NO₂^- inhibited IBP degradation. Further, according to the results of ultra-high-resolution mass and density functional theory calculations, mechanisms of a detailed degradation pathway for IBP were clarified. Namely, the detailed mechanistic formation pathways for hydroxylated and keto-based products were proposed. Then, possible active sites of the keto-based products, as well as the corresponding subsequent products were predicted by Condensed Fukui Function. Our study can broaden the knowledge of the reactions of emerging contaminants with ^•OH, and provide theoretical foundation for the optimization of UV/H₂O₂ process.