| Summary: | As the dominant manganese oxide mineral phase in terrestrial and aquatic environments, birnessite plays an important role in many biogeochemical processes. The coexistence of birnessite with aqueous Mn<sup>2+</sup> is commonly found in the subsurface environments undergoing Mn redox cycling. This study investigates the change in Mn average oxidation state (AOS) of birnessite after reaction with 0.1–0.4 mM Mn<sup>2+</sup> at pH 4.5–6.5, under conditions in which phase transformation of birnessite by Mn<sup>2+</sup> was not detectable. The amount of Mn<sup>2+</sup> uptake by birnessite and the equilibrium concentration of Mn(III) proportionally increased with the initial concentration of Mn<sup>2+</sup>. The Mn AOS of birnessite particles became 3.87, 3.75, 3.64, and 3.53, respectively, after reaction with 0.1, 0.2, 0.3, and 0.4 mM Mn<sup>2+</sup> at pH 5.5. Oxidation potentials (<i>E<sub>h</sub></i>) of birnessite with different AOS values were estimated using the equilibrium concentrations of hydroquinone oxidized by the birnessite samples, indicating that <i>E<sub>h</sub></i> was linearly proportional to AOS. The oxidation kinetics of bisphenol A (BPA), a model organic pollutant, by birnessite suggest that the logarithms of surface area-normalized pseudo-first-order initial rate constants (log <i>k<sub>SA</sub></i>) for BPA degradation by birnessite were linearly correlated with the <i>E<sub>h</sub></i> or AOS values of birnessite with AOS greater than 3.64.
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