Mechanistic Insights into the Oxidizing Reactivity of Birnessite from Electron Affinity

Birnessite is a layered manganese oxide that acts as a strong oxidant, but the electronic reason for this behavior is not fully defined. We tested nine natural and synthetic birnessites with different interlayer cations and Mn(III)/Mn(IV) ratios to evaluate how electron affinity (EA) affects oxidation. Ultraviolet photoelectron spectroscopy, supported by density functional theory, gave EA values of 5.56–6.08 eV. Batch tests at pH 7.0 and 25 °C showed pseudo-second-order kinetics for Fe(II) and As(III) oxidation, with k2 rising from 7.5×10−4 to 3.4×10−3 g mg⁻¹ min⁻¹ for Fe(II) and from 4.1×10−4 to 2.1×10−3 g mg⁻¹ min⁻¹ for As(III); the k2–EA fit was exponential with R2=0.92. The apparent activation energy decreased from 48±3 to 29±2 kJ mol⁻¹ as EA increased. Post-reaction XPS showed less MnOOH on high-EA samples, which kept 82–88% of their initial activity after five cycles, compared with 52–60% for low-EA samples. These results show that electron affinity is a useful, quantitative factor for predicting oxidizing strength in birnessite and can guide the design of stable manganese oxide catalysts for pollutant removal.