Microscopic Observations of Reductive Manganite Dissolution under Oxic Conditions

At oxic/anoxic transition zones, manganese release from (hydr)oxide minerals into aqueous solution is a dynamic balance between mineral dissolution and Mn2+(aq) oxidation and precipitation, which are processes respectively promoted by organic reductants and molecular oxygen. We employ a flow-through atomic force microscope reactor (AFM-R) to investigate the reductive dissolution of the {010} surface of manganite (γ-MnOOH) across a range of pH values and ascorbic acid concentrations in aqueous solutions equilibrated with atmospheric CO2 and O2. The apparent dissolution rate increases with higher ascorbic acid concentrations and lower pH values. Concurrent changes in surface morphology show that dissolution proceeds at low pH via etching and step retreat, while at high pH dissolution is concurrent with precipitation. The precipitates are characterized ex situ by X-ray photoelectron spectroscopy (XPS) and found to be MnIII-oxide. The onset of precipitation is consistent with an analysis of the thermodynamic driving forces for the reactions of a two-step mechanism. In the first step, Mn2+ is released to aqueous solution by reduction of γ-MnOOH in reaction with ascorbic acid. This step is thermodynamically favorable under all conditions employed. In the second step, which leads to precipitation, surface adsorbed Mn2+ is oxidized by O2 to yield a MnIII-oxide precipitate. This step is thermodynamically possible only at pH > 5 for our experimental conditions. When the second step is active, the apparent dissolution rate equals the intrinsic dissolution rate minus the precipitation rate. Analysis of the growth rates observed in AFM indicates the precipitation rate reaches 71% of the intrinsic dissolution rate under some reactor conditions. Comparison of our γ-MnOOH results to literature reports for Mn2+ oxidation on γ-FeOOH indicates γ-MnOOH is a more effective surface catalyst by a factor of 108.