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Redox Reactions between Mn(II) and Hexagonal Birnessite Change Its Layer Symmetry
Huaiyan Zhao1, Mengqiang Zhu2, Wei Li3,4
1Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtse River), Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University , Wuhan 430070, China.
Environmental Science & Technology
|January 9, 2016
Summary
Hexagonal birnessite transforms to orthogonal birnessite when reacting with manganese(II) at pH 8 or higher. This structural change, influenced by pH and particle characteristics, impacts environmental manganese oxide reactivity.
Area of Science:
- Environmental Science
- Geochemistry
- Materials Science
Background:
- Birnessite, a common manganese oxide, influences contaminant and nutrient behavior in natural environments.
- Two birnessite structures, hexagonal (HexLayBir) and orthogonal (OrthLayBir), possess different layer vacancies, Mn(III) content, and reactivity.
- The transformation from OrthLayBir to HexLayBir is known, but the reverse process remains poorly understood.
Purpose of the Study:
- To investigate the conditions and mechanisms of HexLayBir transformation into OrthLayBir.
- To determine the role of aqueous manganese(II) and environmental factors in this structural transformation.
Main Methods:
- Experimental reaction of hexagonal birnessite (HexLayBir) with aqueous manganese(II) at controlled molar ratios and pH.
- Analysis of structural changes using techniques sensitive to layer symmetry and vacancy ordering.
- Investigation of factors influencing transformation rates, including pH, particle size, and stacking disorder.
Main Results:
- Hexagonal birnessite (e.g., δ-MnO2, acid birnessite) transforms to orthogonal birnessite (OrthLayBir) under reaction with aqueous Mn(II) at low molar ratios (5-24%) and pH ≥ 8.
- The transformation is accelerated by higher pH, smaller particle size, and greater stacking disorder of the initial HexLayBir.
- The mechanism involves Mn(III) formation through comproportionation and ordered migration into birnessite vacancies.
Conclusions:
- Aqueous manganese(II) and pH are critical environmental factors governing birnessite structural transformations.
- The study elucidates the reverse transformation pathway from HexLayBir to OrthLayBir, previously poorly understood.
- Understanding these transformations is key to predicting the environmental fate and reactivity of manganese oxides.
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