Effects of MnO2 of different structures on activation of peroxymonosulfate for bisphenol A degradation under acidic conditions
Abstract MnO 2 with various structures, including three tunnel structures (α-, β-, γ-MnO 2 ) and a layered structure (δ-MnO 2 ), were synthesized and investigated for peroxymonosulfate (PMS) activation. The effects of different structured MnO 2 on PMS activation in contaminant degradation, as quantified by the pseudo-first order rate constants of bisphenol A (BPA) oxidation, followed the order: α-MnO 2  > γ-MnO 2  > β-MnO 2  > δ-MnO 2 . Results showed that under acidic conditions, BPA was degraded by both catalytic oxidation by PMS-MnO 2 and direct oxidation by MnO 2 , and the relative importance of the two mechanisms differed for different MnO 2 . The direct oxidation accounted for 25.2, 7.4, 34.1, and 94.5% of the total reactivity of α-, β-, γ-, and δ-MnO 2 , respectively. Physicochemical properties of MnO 2 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 MnO 2 was the dominant factor in the catalytic reactivity, resulting in the lowest reactivity for the least crystalline δ-MnO 2 . For the crystalline MnO 2 , 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 (SO 4 − ) were the dominant radicals in the systems, while hydroxyl radicals ( OH) played a minor role. In addition, nonradical mechanisms such as singlet oxygen ( 1 O 2 ) also contributed to the BPA degradation, especially when δ-MnO 2 was the catalyst. These findings offered new insights into the contaminant degradation mechanisms in PMS-MnO 2 and provided guidance to develop cost-effective catalysts for water/wastewater treatment.