Effect of MnO2 Phase Structure on the Oxidative Reactivity toward Bisphenol A Degradation

Manganese dioxides (MnO2) are among important environmental oxidants in contaminant removal; however, most existing work has only focused on naturally abundant MnO2. We herein report the effects of different phase structures of synthetic MnO2 on their oxidative activity with regard to contaminant degradation. Bisphenol A (BPA), a frequently detected contaminant in the environment, was used as a probe compound. A total of eight MnO2 with five different phase structures (α-, β-, γ-, δ-, and λ-MnO2) were successfully synthesized with different methods. The oxidative reactivity of MnO2, as quantified by pseudo-first-order rate constants of BPA oxidation, followed the order of δ-MnO2-1 > δ-MnO2-2 > α-MnO2-1 > α-MnO2-2 ≈ γ-MnO2 > λ-MnO2 > β-MnO2-2 > β-MnO2-1. Extensive characterization was then conducted for MnO2 crystal structure, morphology, surface area, reduction potential, conductivity, and surface Mn oxidation states and oxygen species. The results showed that the MnO2 oxidative reactivity correlated highly positively with surface Mn(III) content and negatively with surface Mn average oxidation state but correlated poorly with all other properties. This indicates that surface Mn(III) played an important role in MnO2 oxidative reactivity. For the same MnO2 phase structure synthesized by different methods, higher surface area, reduction potential, conductivity, or surface adsorbed oxygen led to higher reactivity, suggesting that these properties play a secondary role in the reactivity. These findings provide general guidance for designing active MnO2 for cost-effective water and wastewater treatment.