Degradation mechanisms of carbamazepine by δ-MnO2: Role of protonation of degradation intermediates

Highlights: • The degradation pathways of CBZ by δ-MnO₂ are clarified. • The protonation of degradation intermediates is found to play an important role. • CBZ degradation kinetics is via a diffusion and subsequent rapid reaction model. Carbamazepine (CBZ), a widely used antiepileptic drug, is refractory to biological wastewater treatment. Rapid removal of CBZ is possible using synthetic manganese oxide (δ-MnO₂) but the removal mechanisms require further investigation. In this study, CBZ degradation by δ-MnO₂ was carried out at different pH to further explore the degradation mechanisms. Results show that CBZ degradation by δ-MnO₂ was highly pH dependent, and rapid degradation occurred when pH < 2.8. Based on the density functional theory calculations, increasing [H⁺] not only increased the reactivity of δ-MnO₂, but also enhanced the secondary reactions of the intermediates. During the degradation process, protonation of CBZ degradation intermediates, instead of CBZ, played an important role. The overall kinetics of CBZ degradation was then described by the retarded first-order model. The initial rate (r_init) in the model between pH 2.0 and 6.2 was determined to be r_init = (2.41 ± 0.51) × 10⁻³[CBZ]^1.21[MnO₂]^1.07[H⁺]^1.41. This is the first report revealing that protonation of intermediates from CBZ degradation can improve the CBZ oxidation by δ-MnO2. The pathways of CBZ degradation by δ-MnO₂ were also proposed. The results of this study provide a new insight into the processing mechanism.