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

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-, ß-, 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 ≈ Î³-MnO2 > ß-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.