MnO2, which is ubiquitous in soil and sediment in natural water environments, may play an important role in the photolysis of contaminants by sunlight, but the interactions between MnO2 and contaminants in aqueous environments under sunlight irradiation have not been investigated. In this study, the simultaneous presence of sunlight and MnO2 significantly enhanced the degradation efficiency of methotrexate (MTX). Accordingly, we hypothesized that the overall enhancement of this synergistic reaction is due to the additional production of Mn(III) via MTX self-sensitized photolysis. The pseudo-first-order kinetic model for the photoreaction of MTX with MnO2 (Light/MTX+MnO2) during the initial reaction kinetics (0-2 h) revealed a rate constant of 0.43 h1 ([MTX] = 20 μM, [MnO2] = 200 μM, and pH = 7), which is faster than that obtained with sunlight alone (0.14 h1) or MnO2 alone; Mn(II) and Mn(III) were formed at concentrations of 24.3 ± 1.0 μM and 14.8 ± 1.4 μM, respectively. Dissolved Mn(III) species were identified as the main oxidant species responsible for the degradation of MTX. Two reaction pathways for the production of Mn(III) through Light/MTX+MnO2 were proposed; MTX acts as a photosensitizer to produce 3MTX* responsible for the reduction of MnO2 to Mn(III), whereas O2• participates in the oxidation of Mn(Ⅱ) to Mn(Ⅲ). Byproduct analysis demonstrated that the Mn(III) generated in the Light/MTX+MnO2 system enhances C－N bond cleavage, ketonization, and hydrolysis pathways in the MTX transformation.
Keywords: Methotrexate; Mn(III); MnO(2); Self-photosensitized photodegradation; Triplet excited state.
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