Effects of MnO2 of different structures on activation of peroxymonosulfate for bisphenol A degradation under acidic conditions

MnO2 with various structures, including three tunnel structures (α-, β-, γ-MnO2) and a layered structure (δ-MnO2), were synthesized and investigated for peroxymonosulfate (PMS) activation. The effects of different structured MnO2 on PMS activation in contaminant degradation, as quantified by the pseudo-first order rate constants of bisphenol A (BPA) oxidation, followed the order: α-MnO2 > γ-MnO2 > β-MnO2 > δ-MnO2. Results showed that under acidic conditions, BPA was degraded by both catalytic oxidation by PMS-MnO2 and direct oxidation by MnO2, and the relative importance of the two mechanisms differed for different MnO2. The direct oxidation accounted for 25.2, 7.4, 34.1, and 94.5% of the total reactivity of α-, β-, γ-, and δ-MnO2, respectively. Physicochemical properties of MnO2 including crystal structure, morphology, surface Mn oxidation states, surface area, oxygen species and conductivity were characterized and correlated with the catalytic reactivity. The results demonstrated that the crystallinity of MnO2 was the dominant factor in the catalytic reactivity, resulting in the lowest reactivity for the least crystalline δ-MnO2. For the crystalline MnO2, the catalytic reactivity linearly correlated with Mn average oxidation state, Mn(III) content, and conductivity. Electron spin resonance (ESR) and quenching experiments with ethanol and tert-butanol suggested that sulfate radicals (SO4−) were the dominant radicals in the systems, while hydroxyl radicals (OH) played a minor role. In addition, nonradical mechanisms such as singlet oxygen (1O2) also contributed to the BPA degradation, especially when δ-MnO2 was the catalyst. These findings offered new insights into the contaminant degradation mechanisms in PMS-MnO2 and provided guidance to develop cost-effective catalysts for water/wastewater treatment.