Tunable Mn Oxidation State and Redox Potential of Birnessite Coexisting with Aqueous Mn(II) in Mildly Acidic Environments

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(2+)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(2+)at pH 4.5-6.5, under conditions in which phase transformation of birnessite by Mn(2+)was not detectable. The amount of Mn(2+)uptake by birnessite and the equilibrium concentration of Mn(III) proportionally increased with the initial concentration of Mn2+. 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(2+)at pH 5.5. Oxidation potentials (E-h) of birnessite with different AOS values were estimated using the equilibrium concentrations of hydroquinone oxidized by the birnessite samples, indicating thatE(h)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 (logk(SA)) for BPA degradation by birnessite were linearly correlated with theE(h)or AOS values of birnessite with AOS greater than 3.64.