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^[rad]−) were the dominant radicals in the systems, while hydroxyl radicals (^[rad]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.