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Microscopic observations of reductive manganite dissolution under oxic conditions
Young-Shin Jun1, Scot T Martin
1Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
Environmental Science & Technology
|July 2, 2003
Summary
Manganese release from manganite (gamma-MnOOH) is controlled by dissolution and precipitation. Lower pH and higher ascorbic acid boost dissolution, while oxygen drives Mn(III)-oxide precipitation at pH > 5.
Area of Science:
- Geochemistry
- Environmental Science
- Surface Chemistry
Background:
- Manganese release from (hydr)oxide minerals is crucial at oxic/anoxic transition zones.
- This release involves a balance between mineral dissolution (aided by organic reductants) and Mn2+(aq) oxidation/precipitation (aided by oxygen).
Purpose of the Study:
- To investigate the reductive dissolution of manganite (gamma-MnOOH) surfaces.
- To understand the influence of pH and ascorbic acid concentration on dissolution and precipitation processes.
- To compare the catalytic activity of manganite with iron oxyhydroxides.
Main Methods:
- Utilized a flow-through atomic force microscope reactor (AFM-R).
- Investigated manganite (gamma-MnOOH) [010] surface dissolution across varying pH and ascorbic acid concentrations.
- Characterized precipitates using X-ray photoelectron spectroscopy (XPS).
Main Results:
- Apparent dissolution rate increased with higher ascorbic acid concentrations and lower pH.
- Dissolution occurred via etching at low pH and concurrent precipitation at high pH.
- Identified Mn(III)-oxide as the precipitate, forming at pH > 5 due to O2 oxidation of dissolved Mn2+.
- Precipitation rate reached up to 71% of the intrinsic dissolution rate.
- Manganite (gamma-MnOOH) demonstrated a 10^8 times higher surface catalytic activity for Mn2+ oxidation compared to gamma-FeOOH.
Conclusions:
- Reductive dissolution of manganite is thermodynamically favorable under all tested conditions.
- Mn(III)-oxide precipitation is thermodynamically possible only at pH > 5.
- The interplay between dissolution and precipitation significantly affects manganese release rates.
- Manganite surfaces are highly effective catalysts for manganese oxidation.
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