A new insight into the mechanism of carbamazepine oxidation by MnO2: Crystalline structure versus Mn(III)

Highlights: • CBZ oxidation by dissolved Mn(III) alone is impossible. • Mn(III) does not play the most critical role in the oxidation of CBZ by MnO₂. • CBZ is oxidized through surface reaction by MnO₂ following the order: δ>> > α ≈ γ > β. • The crystalline plane of δ-MnO₂ significantly contributed to the oxidation of CBZ. Mn(III) has been regarded as the origin of oxidative reactivity of MnO₂ recently, however this remains controvertible. Herein, carbamazepine (CBZ), a typical refractory pharmaceutical, was treated by δ-, α-, β-, and γ-MnO₂ and the role of Mn(III) was investigated. After the removal of Mn(III) by pyrophosphate washing, the δ-MnO₂ exhibited a higher kinetics rate (0.180 min⁻¹) than the sample before washing (0.075 min⁻¹). Dissolved Mn(III) in the forms of acetate-complex Mn(III), newly acid-dissolved Mn(III) from MnO₂ solid, and in-situ generated Mn(III) showed negligible oxidative reactivity towards the oxidation of CBZ. These evidenced that Mn(III) did not play a critical role in the oxidation of CBZ. The oxidative reactivity of MnO₂ with different structures for the oxidation of CBZ followed the order: δ-MnO₂ >> > α-MnO₂ ≈ γ-MnO₂ > β-MnO₂. Density functional theory calculations suggested that the crystalline plane of δ-MnO₂ significantly contributed to the oxidation of CBZ, thus leading to the superior performance of δ-MnO₂. A new surface reaction dominated mechanism was proposed, which implies that the oxidative reactivity of MnO₂ may not result from Mn(III) as previously believed. These findings could shed light on the understanding of MnO₂-involved oxidation in water treatment and natural processes.